WO2011149148A1

WO2011149148A1 - Hexagonal crystal structure and light-emitting diode using the same

Info

Publication number: WO2011149148A1
Application number: PCT/KR2010/005064
Authority: WO
Inventors: Ki-Seok Kim; Gun-Young Jung
Original assignee: Gwangju Institute Of Science And Technology
Priority date: 2010-05-28
Filing date: 2010-08-02
Publication date: 2011-12-01

Abstract

The present invention provides hexagonal zinc oxide structures having various shapes. The zinc oxide may be formed into a crystalline rod, a tree structure, and a hemispherical structure. These zinc oxide structures can be applied to a light-emitting diode. The light-emitting diode has excellent light extraction efficiency due to the zinc oxide structure applied thereto.

Description

HEXAGONAL CRYSTAL STRUCTURE AND LIGHT-EMITTING DIODE USING THE SAME

The present invention relates to a hexagonal zinc oxide (ZnO) structure and a light-emitting diode using the same, more particularly, to hemispherical, crystalline rod-like, and tree-like ZnO structures, and light-emitting diodes including the same.

A hexagonal crystal system means a structure in which the components of a crystal are arranged in a lattice pattern with respect to four axes. Three of the four axes are intercepted at 120°from each other and have the same length. C-axis as the fourth axis is arranged perpendicularly to the plane defined by the three axes. Especially, ZnO, ZnSe, ZnS, and CdSe have a wurtzite structure, in which the bonds between metals and nonmetals form a tetrahedral structure and the metal and nonmetal are stacked in a metal-nonmetal-metal-metal-nonmetal pattern.

Zinc oxide is a group II-VI semiconductor and has a hexagonal wurtzite structure, which is crystallographically the same as GaN, ZnSe, ZnS, etc. The zinc oxide has a lattice mismatch of only 1.98% and thus is most likely to be applied to a different stack structures.

Moreover, the zinc oxide has an optical band gap of 3.37 eV, which is similar to 3.4 eV of GaN, and thus it can be used as a light source of the near ultraviolet region. Besides, the zinc oxide has higher defect formation energy, and thus, when it is formed into an optical device, it may have good properties. Moreover, the zinc oxide has an exciton binding energy of 60 meV, which is about three times higher than 24 meV of GaN and 19 meV of ZnSe. Therefore, an optical device based on the exciton has high optical efficiency.

Moreover, while the GaN requires a high crystal growth temperature of about 1,030℃, the zinc oxide can be epitaxially grown at a temperature of about 450℃ to 750℃. Accordingly, the zinc oxide has the advantage of preventing defect formation.

On the contrary, the zinc oxide may have a crystal structure distorted by an intrinsic defect caused by vacancies and invasive defects or an extrinsic defect caused by injected impurities during single crystal formation. Especially, a p-type zinc oxide has poor single crystal forming ability because of the difficult in crystal growth and the capability of oxygen substitution of dopants.

In spite of the optical advantages of the zinc oxide, the zinc oxide cannot be used as a material for the optical device due to the above-described problems.

Since a silicon carbide light-emitting diode was first developed, technological advances in its structure and fabrication method have progressed. Especially, a nitride light-emitting diode and various fluorescent materials have been developed in recent years, thereby implementing white light-emitting diode. The implemented white light-emitting diode is applied to various lighting devices and its application field is expanded. A complicated manufacturing process is required for the formation of nitrides, which is because the light-emitting diodes are based on compound semiconductors and an epitaxial process is employed. Recently, the light-emitting diode products are required to have high brightness characteristics. That is, the light-emitting diode should have high brightness and thermal stability so as to be used as a lighting device.

A metal organic chemical vapor deposition (MOCVD) has been used to solve these problems. A stack structure of high-purity nitride single crystals is realized by the MOCVD. Moreover, various techniques have been introduced to improve the light efficiency of the light emitting diode during packaging.

The implementation of a multi-quantum well (MQW) structure is facilitated by the MOCVD, and the efficiency of the light emitted from a light-emitting layer is increased by the MQW structure, which is related to the improvement of internal quantum efficiency. For the improvement of internal quantum efficiency, a crystal structure formed by epitaxial growth is required to have few defects.

Moreover, it is very important that the light emitted from the light-emitting layer is efficiently extracted to the outside, which is defined as the improvement of external quantum efficiency. For the improvement of external quantum efficiency, a concave-convex structure is formed on a p-type GaN layer arranged in a direction in which the light is extracted. However, it is difficult to maintain a constant shape of the concave-convex structure according to the fabrication method, and the ohmic characteristics are deteriorated. Besides, a technique for forming an optical crystal structure by forming a predetermined pattern on a sapphire substrate has been used. The optical crystal structure uses the resonance of light of a particular wavelength, and the light extraction efficiency may be increased by the optical crystal structure. However, the technique for forming a pattern on the substrate may cause a defect in a GaN single crystal.

Therefore, it is necessary to provide a technique capable of improving the light extraction efficiency without damaging a layer and a substrate, which constitute the existing light-emitting layer.

Therefore, it is a first object of the present invention to provide a hexagonal crystal structure including a hemisphere.

It is a second object of the present invention to provide a method of fabricating a hexagonal crystal structure including a hemisphere.

It is a third object of the present invention to provide a light-emitting diode including a hemispherical structure formed of zinc oxide.

It is a fourth object of the present invention to provide a method of fabricating a light-emitting diode used to achieve the third object of the present invention.

It is a fifth object of the present invention to provide a hexagonal crystal structure oriented perpendicularly to a substrate or having various shapes.

It is a sixth object of the present invention to provide a method of fabricating a hexagonal crystal structure used to achieve the fifth object of the present invention.

It is a seventh object of the present invention to provide a light-emitting diode using a crystalline rod formed of zinc oxide capable of improving light extraction efficiency.

It is an eighth object of the present invention to provide a method of fabricating a light-emitting diode used to achieve the seventh object of the present invention.

It is a ninth object of the present invention to provide a method of fabricating a zinc oxide tree structure.

It is a tenth object of the present invention to provide a zinc oxide structure having a tree structure obtained by the achievement of the ninth object of the present invention.

It is an eleventh object of the present invention to provide a light-emitting diode using a tree structure formed of zinc oxide.

It is a twelfth object of the present invention to provide a method of fabricating a light-emitting diode for achieving the eleventh object of the present invention.

To achieve the first object, the present invention provides a hexagonal crystal structure comprising: a seed layer formed on a substrate; and a hemisphere formed on the seed layer and having a (0001) plane as a main surface.

To achieve the second object, the present invention provides a method of fabricating a hexagonal crystal structure, the method comprising: forming a seed layer on a substrate; forming an urchin-like structure of hexagonal crystals on the seed layer; and forming a hemisphere by lateral growth of the urchin-like structure.

To achieve the third object, the present invention provides a light-emitting diode comprising: a light-emitting structure for emitting light; and a hemispherical structure formed of zinc oxide and arranged in a direction in which the emitted light is extracted.

The third object of the present invention can also be achieved by providing a light-emitting diode comprising a hemispherical structure, which comprises a plurality of ZnO rods and is arranged in a direction in which light emitted from a light-emitting structure is extracted.

To achieve the fourth object, the present invention provides a method of fabricating a light-emitting diode, the method comprising: forming a seed layer on a light-emitting structure for emitting light; forming a urchin-like structure of ZnO rods on the seed layer; and forming a hemispherical structure from the urchin-like structure of ZnO rods.

To achieve the fifth object, the present invention provides a hexagonal crystal structure comprising: a seed layer formed on a seed layer; and a crystalline rod formed on the seed layer, formed of zinc oxide, and oriented perpendicularly to the substrate.

The fifth object of the present invention can also be achieved by providing a hexagonal crystal structure comprising: a seed layer formed on a substrate; and a crystalline rod formed on the seed layer by erosion of a zinc oxide crystal.

To achieve the sixth object, the present invention provides a method of fabricating a hexagonal crystal structure, the method comprising: forming a seed layer on a lower substrate; forming a growth guiding layer on the seed layer; and forming a crystalline rod of zinc oxide oriented perpendicularly to the lower substrate on the seed layer exposed by the growth guiding layer.

To achieve the seventh object, the present invention provides a light-emitting diode comprising: a light-emitting structure for emitting light; and a crystalline rod formed of zinc oxide and arranged in a direction in which the light is extracted from the light-emitting structure.

To achieve the eighth object, the present invention provides a method of fabricating a light-emitting diode, the method comprising: forming a seed layer on a light-emitting diode structure; and forming a crystalline rod of zinc oxide on the seed layer.

To achieve the ninth object, the present invention provides a method of fabricating a zinc oxide tree structure, the method comprising: forming a zinc oxide seed layer on a substrate; forming a crystalline rod on the seed layer; and forming a crystalline branch using a zinc oxide branch seed precipitated on the side of the crystalline rod.

To achieve the tenth object, the present invention provides a zinc oxide tree structure comprising: a zinc oxide seed layer formed on a substrate; a crystalline rod formed from the seed layer; and a crystalline branch formed from a branch seed precipitated on the side of the crystalline rod.

To achieve the eleventh object, the present invention provides a light-emitting diode comprising: a light-emitting structure for emitting light; and a tree formed of zinc oxide in a direction in which the light is extracted from the light-emitting structure.

To achieve the twelfth object, the present invention provides a method of fabricating a light-emitting diode, the method comprising: a zinc oxide seed layer on a light-emitting structure; forming a crystalline rod on the seed layer; and forming a crystalline branch using a zinc oxide branch seed precipitated on the side of the crystalline rod.

According to the present invention, an urchin-like structure is formed on a seed layer, and the (0001) surface of hexagonal crystalline rods containing zinc oxide, which constitute the urchin-like structure, is capped with an anionic polymer. Therefore, the growth in the [0001] direction is suppressed and the lateral growth is promoted. A hemispherical structure is formed of zinc oxide by the lateral growth.

According to the present invention, a hemispherical structure is arranged in a direction in which light emitted from a light-emitting structure is extracted. Each of the ZnO rods that constitute the hemispherical structure functions as an optical waveguide and improves light extraction efficiency.

According to the present invention, crystalline rods having various shapes are provided. The crystalline rods can be used as optical devices, solar cells, biochips, etc. according to their shapes. Moreover, it is possible to maintain high stability since the fabrication process is performed at a temperature lower than the existing process, and it is further possible to ensure excellent single crystallinity using a growth method such as a hydrothermal synthesis method.

According to the present invention, a crystalline rod structure is formed of zinc oxide on the surface of a light-emitting structure for emitting light. The crystalline rod is arranged in a direction in which the emitted light is extracted. The arranged crystalline rod functions as an optical waveguide and prevents light from leaking to the side of the waveguide, thereby achieving high light extraction efficiency.

According to the present invention, a tree structure is provided on a substrate. The tree structure formed of zinc oxide comprises a crystalline rod grown from the substrate and a crystalline branch formed on the side of the crystalline rod. A branch seed is precipitated on the side of the crystalline rod for the formation of the crystalline branch. A ZnO seed crystal is partially precipitated on the side of the crystalline rod using the intrinsic polarity of the zinc oxide for the formation of the crystalline branch. The zinc oxide tree structure formed by the above-described process can be applied to various optical devices. For example, the zinc oxide tree structure having a nano-size may function as an optical waveguide. Moreover, it can be used as a fast charge carrier using one-dimensional structure having a large surface area in a solar cell.

According to the present invention, a zinc oxide tree structure is provided on the surface of a light-emitting structure for emitting light. The tree structure is arranged in a direction in which the emitted light is extracted. The arranged tree functions as an optical waveguide and prevents light from leaking to the side of the waveguide, thereby achieving high light extraction efficiency.

FIGS. 1 to 4 are cross-sectional views illustrating a method of fabricating a hexagonal crystal structure including a hemisphere in accordance with a first embodiment of the present invention.

FIG. 5 is a scanning electron microscope (SEM) image showing a zinc oxide structure including a zinc oxide hemisphere formed by the first embodiment of the present invention.

FIG. 6 is an SEM image showing the zinc oxide structure of FIG. 5, in which a part is destroyed.

FIGS. 7 to 11 are cross-sectional views illustrating a method of fabricating a light-emitting diode including a hemispherical zinc oxide structure in accordance with a second embodiment of the present invention.

FIG. 12 is a cross-sectional view of another light-emitting diode including a hemispherical structure in accordance with the second embodiment of the present invention.

FIG. 13 is an SEM image showing a plurality of hemispherical structures formed on a light-emitting diode in accordance with the second embodiment of the present invention.

FIG. 14 is a graph illustrating EL properties of the light-emitting diode including the hemispherical structure of FIG. 13.

FIGS. 15 to 18 are cross-sectional views illustrating a method of fabricating a crystalline rod-like hexagonal crystal structure in accordance with a third embodiment of the present invention.

FIG. 19 is an SEM image showing a plurality of crystalline rod-like hexagonal crystal structures in accordance with the third embodiment of the present invention.

FIG. 20 is an SEM image showing a plurality of pencil-like hexagonal crystal structures in accordance with the third embodiment of the present invention.

FIG. 21 is another SEM image showing the hexagonal crystal structures of FIG. 20.

FIG. 22 is an SEM image showing a plurality of needle-like hexagonal crystal structures in accordance with the third embodiment of the present invention.

FIG. 23 is an SEM image showing a plurality of tube-like hexagonal crystal structures in accordance with the third embodiment of the present invention.

FIG. 24 is a conceptual view showing zinc oxide crystal structures in accordance with the third embodiment of the present invention.

FIGS. 25 to 29 are cross-sectional views illustrating a method of fabricating a light-emitting diode including a zinc oxide crystalline rod in accordance with a fourth embodiment of the present invention.

FIG. 30 is a graph illustrating EL properties of the light-emitting diode including the zinc oxide crystalline rod in accordance with the fourth embodiment of the present invention.

FIG. 31 is a cross-sectional view of another light-emitting diode including a crystalline rod in accordance with the fourth embodiment of the present invention.

FIGS. 32 to 35 are cross-sectional views illustrating a method of fabricating a zinc oxide tree structure in accordance with a fifth embodiment of the present invention.

FIG. 36 is an SEM image showing a plurality of zinc oxide tree structures in accordance with the fifth embodiment of the present invention.

FIGS. 37 to 40 are cross-sectional views illustrating a method of fabricating a light-emitting diode including a zinc oxide tree structure in accordance with a sixth embodiment of the present invention.

FIG. 41 is a graph illustrating EL properties of the light-emitting diode including the zinc oxide tree structure in accordance with the sixth embodiment of the present invention.

The present invention can be variously modified and have several embodiments, and some exemplary embodiments are illustrated in the accompanying drawings and will be described in detail in the specification. However, the present invention is not limited to the specific embodiments and should be construed as including all the changes, equivalents, and substitutions included in the spirit and scope of the present invention. In the following description and drawings, like components refer to like reference numerals.

Unless indicated otherwise, it is to be understood that all the terms used in the specification including technical and scientific terms has the same meaning as those that are understood by those who skilled in the art. It must be understood that the terms defined by the dictionary are identical with the meaning of the context of the related art, and they should not be ideally or excessively defined formally unless the context clearly dictate otherwise.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

First Embodiment: Formation of hexagonal crystal structure with hemisphere

A hexagonal crystal structure provided in the present embodiment includes a hemisphere formed on a substrate. The formation of the hemisphere is achieved by forming a seed layer on the substrate and inducing the formation of an urchin-like structure having a plurality of crystalline rods and the lateral growth of the urchin-like structure from the seed layer.

Especially, the crystalline rods of the urchin-like structure for forming the hemisphere comprise ZnO, ZnSe, ZnS, or CdSe. Moreover, the (0001) plane of the crystalline rods, which constitute the urchin-like structure, as an end surface is treated with a vertical growth inhibitor, and the urchin-like structure is treated for the lateral growth with a growth solution containing a central metal ion donor and a coordinate covalent bond ion donor. In the hexagonal crystal structure, the central metal ion donor provides Zn²⁺ or Cd²⁺, and the coordinate covalent bond ion donor provides O^2-, Se^2-, or S^2-.

The hexagonal crystal structure comprises ZnO, ZnSe, ZnS, or CdSe. Especially, FIGS. 1 to 4 illustrate a method of fabricating a ZnO structure as one of the hexagonal crystal structure.

Referring to FIG. 1, a seed layer 110 is formed on a substrate 100. The substrate 100 may be a substrate for forming an optical device or any layer for supporting the seed layer 110.

The formation of the seed layer 110 may be achieved by various known methods.

For example, the seed layer 110 may be formed by preparing a solution by mixing ZnO powder and a surfactant and spin-coating the solution on the substrate 100.

Moreover, if zinc metal is deposited on the substrate 100 and the resulting substrate 100 is immersed in a solution having a pH above 10, the zinc metal is bonded to oxygen contained in the solution and converted into ZnO, which can be used as the seed layer 110.

Besides, the seed layer 110 may be formed on the substrate 100 by sputtering using a zinc oxide target.

Moreover, the seed layer 110 may be formed by a hydrothermal synthesis method.

A seed growth solution is prepared for the formation of the seed layer 110 by the hydrothermal synthesis method. The seed growth solution is prepared by dissolving a zinc salt and a precipitator in a polar solvent.

The zinc salt comprises zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride. The precipitator comprises NaOH, Na₂CO₃, LiOH, H₂O₂, KOH, hexamethylenetetramine (HMTA), or NH₄OH. The polar solvent may comprise water, alcohol, or an organic solvent. Preferably, the polar solvent may contain both water and alcohol.

ZnO particles are formed by applying heat energy to the seed growth solution. The application of heat energy may be performed in the temperature range of 50℃ to 100℃ at atmospheric pressure.

The formation of the ZnO particles can be represented by the following formulas:

[Formula 1]

Zn²⁺ + 2OH^- → ZnO + H₂O

[Formula 2]

Zn²⁺ + 2OH^- ↔ Zn(OH)₂

[Formula 3]

Zn(OH)₂ + 2OH^- → Zn(OH)₄ ^2-

[Formula 4]

Zn(OH)₄ ^2- → ZnO + H₂O + 2OH^-

In formulas 1 and 2, Zn²⁺ is supplied from the zinc salt and OH^- is supplied from the precipitator. The cation and anion are reacted together to form ZnO or Zn(OH)₂ as an intermediate.

Moreover, in formula 3, the intermediate, Zn(OH)₂, reacts with OH^- to form Zn(OH)₄ ^2- as a ZnO growth factor, which forms ZnO in formula 4.

An overgrowth inhibitor may be used to control the size of the ZnO particles. The overgrowth inhibitor is added to the seed growth solution in which the ZnO particles are formed. The overgrowth inhibitor may be a cationic polymer. In detail, the cationic polymer may be a polymer having an amine group such as polyethyleneimine (PEI) having high solubility in a polar solvent. The ZnO growth factor, Zn(OH)₄ ^2-, is bonded to a cation of the cationic polymer and does not participate in the growth of zinc oxide. As a result, the size of the ZnO particles can be controlled by the overgrowth inhibitor.

Subsequently, the thus formed ZnO particles are separated, and the separated ZnO particles are dispersed in a solvent and formed into the seed layer 110 by spin coating.

As mentioned above, the formation of the seed layer 110 may be achieved by various methods. That is, the seed layer 110 may be formed by evaporation, metal organic chemical vapor deposition (MOCVD), sputtering or coating using a brush.

Besides, the seed layer 110 may be formed by depositing or dispersing the ZnO particles.

Moreover, the seed layer 110 may be formed as a layer having a relatively uniform thickness on the substrate 100 and may have a regular pattern.

The seed layer 110 does not have a predetermined orientation in a direction perpendicular to the substrate 100. Therefore, the seed layer 110 formed on the substrate 100 may have various orientations other than the direction perpendicular to the substrate 100.

Referring to FIG. 2, a growth guiding layer 120 is formed on the substrate 100, on which the seed layer 110 is formed. The growth guiding layer 120 partially covers the seed layer 110 from the outside. Therefore, if it is intended to induce the growth of zinc oxide from the seed layer 110 using a predetermined process, the growth of zinc oxide occurs partially on a surface of the seed layer 110 exposed by the growth guiding layer 120.

IF the seed layer 110 has a regular pattern, the formation of the growth guiding layer 120 may be omitted.

Moreover, the growth guiding layer 120 may be formed by forming a photoresist layer on the seed layer 110 and then patterning the photoresist layer by photolithography. Besides, the growth guiding layer 120 may be formed by nanoimprint lithography or laser interference lithography. Moreover, any material having an etch selectivity with respect to the substrate 100 and the seed layer 110 may be used.

The growth guiding layer 120 may be omitted according to the number of ZnO hemispheres and their use.

Referring to FIG. 3, an urchin-like or flower-like ZnO structure 130 is formed from the seed layer 110. The urchin-like structure 130 includes a plurality of ZnO crystalline rods 135 formed radially from the center. The urchin-like structure 130 may be formed after the zinc oxide fills the region exposed by the growth guiding layer 120. Especially, the zinc oxide that fills the exposed region may have a polycrystal structure. Therefore, the region exposed by the growth guiding layer 120 is filled with the ZnO polycrystal structure, and the urchin-like structure 130 is formed based on the ZnO polycrystal structure.

In detail, the substrate 100 including the seed layer 110 and the growth guiding layer 120 is immersed in a first growth solution such that the urchin-like structure 130 grows.

The first growth solution contains a first zinc ion donor, a first hydroxyl group donor, and a solvent. The first zinc ion donor acts as the central metal ion donor and the first hydroxyl group donor acts as the coordinate covalent bond ion donor. That is, during the formation of the urchin-like structure 130 including the ZnO crystalline rods 135, the first zinc ion donor provides zinc ions and the first hydroxyl group donor provides oxygen ions.

The first zinc ion donor comprises zinc nitrate, zinc sulfate, or zinc chloride. The first hydroxyl group donor comprises NaOH, Na₂CO₃, LiOH, H₂O₂, KOH, hexamethylenetetramine (HMTA), or NH₄OH. The solvent may be a polar solvent comprising water, alcohol, or an organic solvent.

The urchin-like ZnO structure 130 is formed on the exposed seed layer 110 of the substrate 100 immersed in the first growth solution, which results from the characteristics of the seed layer 110 from which a particular orientation is excluded. That is, the ZnO crystalline rods 135 are formed from the surface of the exposed seed layer 110 having higher surface energy. The size of the crystalline rods 135 is determined by the concentration and temperature of the first zinc ion donor and the first hydroxyl group donor.

Therefore, the crystalline rods 135 grows into the urchin-like structure 130, and isotropic growth occurs on the surface of the exposed seed layer 110. Therefore, the plurality of crystalline rods 135 are formed from a growth factor into the urchin-like structure 130. Each of the crystalline rods 135 grows under the same chemical conditions, and thus the difference in length and thickness is insignificant.

The urchin-like ZnO structure 130 may be formed by other methods.

That is, zinc particles are dispersed on the substrate 100 and heated at a temperature of about 500℃. The heated zinc particles are formed into a micro- or nano-sized zinc agglomerate. The zinc agglomerate has an approximately hemispherical shape. Then, the zinc agglomerate is heated at a temperature of about 600℃ in a reactor, and oxygen is introduced into the reactor to form a ZnO core on the surface of the zinc agglomerate. The ZnO core is formed uniformly over the entire surface of the hemispherical zinc agglomerate. Subsequently, if the oxygen is continuously supplied to the reactor at the same temperature, the ZnO core grows into the urchin-like structure. The reason for this is that the hemispherical zinc agglomerate maintains the spherical shape by the cohesive force even at the melting point and reacts with oxygen to form nanorods on the ZnO core.

As such, the urchin-like ZnO structure 130 can be formed by various methods.

Referring to FIG. 4, the urchin-like structure grows into a hemisphere 140 of zinc oxide. The substrate 100 is immersed into a second growth solution to allow the urchin-like structure to grow into the hemisphere 140.

The growth guiding layer 120 shown in FIG. 3 may be removed before the formation of the hemisphere 140.

The second growth solution contains a second zinc ion donor, a second hydroxyl group donor, and a vertical growth inhibitor. The second zinc ion donor acts as the central metal ion donor and the second hydroxyl group donor acts as the coordinate covalent bond ion donor. That is, during the formation of the hemisphere 140 through the lateral growth of the urchin-like structure 130, the second zinc ion donor provides zinc ions and the second hydroxyl group donor provides oxygen ions.

The second zinc ion donor comprises zinc nitrate, zinc sulfate, or zinc chloride. The second hydroxyl group donor comprises Na₂CO₃, LiOH, H₂O₂, NH₄OH, NaOH, HMTA(C₆H₁₂N₄), or KOH. Preferably, the second hydroxyl group donor may be C₆H₁₂N₄. The C₆H₁₂N₄ can produce NH₄ ⁺ and OH^-, which are the growth factors for forming the zinc oxide, at the same time, can control the reaction rate since the growth rate and the OH^- concentration can be easily controlled.

The vertical growth inhibitor comprises an anionic polymer, a citrate anion, or an R-O anion, where R is an alkyl group and O is an oxygen atom.

The second growth solution may be applied to the substrate 100 in various ways, instead of immersing the substrate 100 in the second growth solution. For example, the second growth solution may be applied to the substrate 100 by a spray process, and it is possible to expose the seed layer 110 on the substrate 100 to the second growth solution by any solution process.

The second growth solution induces the lateral growth of the ZnO crystalline rods 135. That is, the growth of the ZnO crystal structure in the c-axis direction is suppressed, and the lateral growth is promoted. For this purpose, the growth using the second growth solution is performed above the isoelectric point of the zinc oxide.

The isoelectric point of the zinc oxide represents a particular pH, at which the partial polarization of the ZnO crystal occurs in the solution. That is, the zinc oxide shows the positive polarity of Zn²⁺ on the (0001) plane of the crystal and the negative polarity of O^2- on the side above the isoelectric point. The isoelectric point of the ZnO crystalline rods 135 has a pH of 9.7. Moreover, other hexagonal crystal structures such as ZnSe, ZnS, and CdSe have a pH of 8.5, 8.7, and 7.2, respectively.

Therefore, when the hemisphere 140 is formed from the urchin-like structure 130, the pH of the second growth solution is adjusted to above 9.7. The second growth solution may contain an anionic material as the vertical growth inhibitor. The anionic material is adhered to a cationic zinc atom located at the end of the ZnO crystal structure and caps it. Therefore, the growth of ZnO crystal structure in the c-axis direction is suppressed, and the lateral growth is promoted.

The lateral growth occurs to fill the gap between the ZnO crystalline rods 135 of the urchin-like structure 130, which can be represented by the following formulas.

Hexamine (C₆H₁₂N₄) used as the second hydroxyl group donor can produce NH₄ ⁺ and OH^- by the following formulas 5 and 6. Moreover, Zn(NO₃)₂ used as the second zinc ion donor can produce zinc ions by the following formula 7.

[Formula 5]

C₆H₁₂N₄ + 6H₂O ↔ 6CH₂O + 4NH₃

[Formula 6]

NH₃ + H₂O ↔ NH₄ ⁺ + OH^-

[Formula 7]

Zn(NO₃)₂ → Zn²⁺ + 2NO₃ ^-

The 4NH₃, 4OH^-, and Zn²⁺ produced by the above formulas 5 to 7 can produce Zn(NH₃)₄ ²⁺ and Zn(OH)₄ ^2-, which are the growth factors of the zinc oxide, by the following formulas 8 and 9.

[Formula 8]

Zn²⁺ + 4NH₃ → Zn(NH₃)₄ ²⁺

[Formula 9]

Zn²⁺ + 4OH^- → Zn(OH)₄ ^2-

The growth factor, Zn(NH₃)₄ ²⁺, produced by the above formula 8 can produce the ZnO crystal represented by the following formula 10 by the reaction with OH^- as a reaction factor, and the growth factor, Zn(OH)₄ ^2-, produced by the above formula 9 can produce the ZnO crystal by the following formula 11.

[Formula 10]

Zn(NH₃)₄ ²⁺ + 2OH^- → ZnO + 4NH₃ + H₂O

[Formula 11]

Zn(OH)₄ ^2- → ZnO + H₂O + 2OH^-

Especially, the growth of the ZnO crystal structure, which fills the gap between the ZnO crystalline rods 135 of the urchin-like ZnO structure 130, occurs predominantly by formula 10. That is, the ZnO crystal structure is filled to the side of the ZnO crystalline rods 135, from which it can be seen that the growth of the ZnO crystal structure occurs in the lateral direction.

The end of the urchin-like structure 130 is capped with the vertical growth inhibitor. Therefore, after the lateral growth occurs continuously, the growth of the ZnO crystalline rods 135 does not exceed the urchin-like structure 130, and thus forming the ZnO hemisphere 140.

Moreover, the second growth solution may contain a second zinc ion donor and a second hydroxyl group donor. The substrate 100 including the ZnO crystalline rods 135 of the urchin-like structure 130 may be treated with the vertical growth inhibitor before it is immersed in the second growth solution. That is, the substrate 100 including the ZnO crystalline rods 135 capped with the vertical growth inhibitor may be immersed into the second growth solution later, and the ZnO crystalline rods 135 may grow in the lateral direction.

The growth mechanism of the ZnO hemisphere 140 can be explained by Oswald ripening. That is, a particle of a larger size has a low surface area, compared to its volume, and thus has low surface energy. Material systems, in which synthesis or growth occurs, generally tend to enter a lower energy state. Therefore, the particle of a smaller size tends to be attached to a particle of a larger size. As a result, the number of particles of a smaller size is reduced, and the size of the particles of a larger size is gradually increased.

Moreover, the outer surface of the ZnO hemisphere 140 has the (0001) plane of the zinc oxide. The reason for this is considered that the ZnO crystal structure, which grows toward the side of the ZnO crystalline rods 135 of the urchin-like structure 130, enters the lower energy state to form a stable crystal structure. That is, to follow the orientation of the crystal is the way to enter the lowest energy state, and thus the ZnO crystal structure is filled to the side of the ZnO crystalline rods 135 of the urchin-like structure 130 and has the same orientation as the ZnO crystalline rods 135.

The growth of the ZnO crystal structure, which fills the gap between the ZnO crystalline rods 135 of the urchin-like ZnO structure 130 at the beginning of the synthesis, forms a grain boundary where the gap between the adjacent ZnO crystalline rods 135 is filled. Therefore, when viewed from the outside of the hemisphere 140, the (0001) plane is formed preferentially, and the growth of fan-shaped nanorods is shown in the interior of the hemisphere 140. Moreover, the grain boundary is formed between the ZnO crystalline rods 135. As a result, the plurality of ZnO crystalline rods 135 are bonded to the ZnO hemisphere 140 of the present invention, and a polycrystal structure, in which a partial mismatch of crystals is caused, is formed between adjacent bonded ZnO crystalline rods 135.

Moreover, although the hemisphere 140 may be a complete hemisphere, it may be smaller or greater than a hemisphere. That is, the surface of the hemisphere has an approximately circular shape, and the center of the circle may be located in the interior of the hemisphere 140 or on the seed layer 110 or its lower layer.

The hemisphere 140 may have an approximately elliptical shape. That is, a partial surface of the hemisphere 140 may have a predetermined curvature radius.

FIG. 5 is an SEM image showing a zinc oxide structure including a zinc oxide hemisphere formed by a preferred embodiment of the present invention.

Referring to FIG. 5, the seed layer on the sapphire substrate is formed by patterning the ZnO nanoparticles. To form an urchin-like structure, zinc nitrate is used as the first zinc ion donor, and HMTA is used as the first hydroxyl group donor. An urchin-like ZnO structure is formed in the first growth solution by heating at 65℃ for 1 hour.

The substrate, on which the urchin-like structure is formed, is capped with citrate anions as an anionic polymer to form a hemisphere. Then, the resulting substrate is immersed in the second growth solution. The second growth solution contains zinc nitrate and HMTA. The substrate immersed in the second growth solution is heated at 95℃ for 3 hours, thus obtaining a ZnO hemisphere.

Referring to FIG. 6, a plurality of ZnO rods, which grow into single crystals filing the gap between the ZnO rods in the lateral direction from the urchin-like structure, is observed. The adjacent ZnO rods are visually divided from each other by the mismatch of crystal structures. That is, the urchin-like structure is formed radially from the lower center of the hemisphere and grows in the lateral direction. The ZnO nanorods grow in the lateral direction to come in contact with adjacent ZnO nanorods, thereby forming the hemisphere.

The ZnO nanorods forming the hemisphere extend in a fan-shape from the approximate center of the hemisphere to the surface of the hemisphere.

Moreover, the hexagonal crystal structure provided by the present invention is formed by a coordinate covalent bond between a metal ion and a nonmetal ion. That is, the metal ions having a positive charge are Zn and Cd, and the nonmetal ions having a negative charge such as O, S, and Se have unshared electron pairs to form a covalent bond. During the formation of the covalent bond, the covalent bond is formed by the unshared electron pairs.

The hexagonal crystal structure including the ZnO hemisphere can be applied to a light-emitting diode, a solar cell, etc. Especially, when the hexagonal crystal structure including the ZnO hemisphere is applied to the light-emitting diode, it is possible to efficiently extract the light emitted from a light-emitting layer to the outside. Moreover, since the hemisphere has a lens shape, it can be used as an optical device such as a micro-lens without particular limitations.

Second Embodiment: light-emitting diode including hemispherical structure and fabrication method thereof

Referring to FIG. 7, a light-emitting structure 210 is provided on a substrate 200.

The light-emitting structure 210 may comprise a group III nitride or a group II oxide. A ZnO hemispherical structure can be applied regardless of the type and shape of the light-emitting structure 210.

Therefore, the light-emitting structure 210 may be provided in various types such as a normal type light-emitting structure, a flip-chip type light-emitting structure, and a vertical type light-emitting structure.

The normal type light-emitting structure comprises an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer, which are formed on a sapphire substrate. A buffer layer may be interposed between the substrate and the n-type semiconductor layer. Electrodes are formed on an exposed region of the n-type semiconductor layer and on the p-type semiconductor layer, respectively. Moreover, a current spreading layer may be provided on the p-type semiconductor layer to uniformly spread the current to the entire surface of the light-emitting structure. Typically, the current spreading layer may comprise ITO.

The flip-chip type light-emitting structure has substantially the same structure as the normal type. However, the flip-chip type light-emitting structure has a different structure in which the light emitted from a light-emitting layer is extracted toward the substrate. Therefore, if the flip-chip type light-emitting structure is mounted on a printed circuit board, for example, two electrodes are bonded to the printed circuit board without any wire bonding.

The vertical type light-emitting structure employs an acceptor substrate, which is required by the fabrication process, separately from a substrate. Moreover, a lift-off process of separating the substrate is employed during the fabrication process. The vertical type light-emitting structure has a typical structure in which a p-type semiconductor layer, a light-emitting layer, and an n-type semiconductor layer, which are sequentially provided on the acceptor substrate. Moreover, electrodes are formed on the n-type semiconductor layer and on the rear surface of the acceptor substrate facing the p-type semiconductor layer. A reflective layer formed of a metal material may be interposed between the acceptor substrate and the p-type semiconductor layer. Moreover, the n-type semiconductor layer may be first provided on the acceptor substrate according to the fabrication method.

For the convenience of description and better understanding of the present invention, the normal type light-emitting structure will be hereinafter exemplarily used as the light-emitting structure in this embodiment shown in FIG. 7. Therefore, the light-emitting structure 210 has a structure in which an n-type semiconductor layer 213, a light-emitting layer 215, and a p-type semiconductor layer 217 are sequentially formed on the substrate 200. Moreover, an electrode 230 is provided on the p-type semiconductor layer 217. A current spreading layer 219 as a transparent conductor may be further interposed between the p-type semiconductor layer 217 and the electrode 230. A buffer layer 211 may be further interposed between the substrate 200 and the n-type semiconductor layer 213 to reduce the lattice mismatch.

If the light-emitting structure is the flip-chip type, the substrate is disposed on the light emitting structure.

Moreover, if the light-emitting structure is the vertical type, the electrode is disposed on the n-type semiconductor layer. According to the embodiments, the p-type semiconductor layer and the electrode may be provided on the light-emitting structure.

That is, the light-emitting structure 210 may be designed differently according to its type in this embodiment. However, the ZnO hemispherical structure is arranged in a direction in which the light emitted from the light-emitting layer 215 is extracted. Particularly, the ZnO hemispherical structure is not interposed between the semiconductor layers 213 and 217 and the light-emitting layer 215 but is provided on the outside of the semiconductor layers 213 and 217 or the light-emitting layer 215 according to the light extraction direction. In the case of the flip-chip type light-emitting structure, the ZnO hemispherical structure may be formed on the substrate.

Referring to FIG. 8, a seed layer 250 is formed on the light-emitting structure 210. Moreover, the current spreading layer 219 is further provided on the light-emitting structure 210. It is described that the seed layer 250 is formed on the current spreading layer 219. The seed layer 250 may be formed by various methods.

For example, the seed layer 250 may be formed by preparing a solution by mixing ZnO powder and a surfactant and spin-coating the solution on the light-emitting structure 210.

Moreover, if zinc metal is deposited on the light-emitting structure 210 and the resulting light-emitting structure 210 is immersed in a solution having a pH above 10, the zinc metal is bonded to oxygen contained in the solution and converted into ZnO, which can be used as the seed layer 250.

Besides, the seed layer 250 may be formed by a hydrothermal synthesis method.

A seed growth solution is prepared for the formation of the seed layer 250 by the hydrothermal synthesis method. The seed growth solution is prepared by dissolving a zinc salt and a precipitator in a polar solvent.

The formation of the ZnO particles by the hydrothermal synthesis method can be represented by formulas 1 to 4 of the first embodiment.

Subsequently, the thus formed ZnO particles are separated, and the separated ZnO particles are dispersed in a solvent and formed into the seed layer 250 by spin coating.

As mentioned above, the formation of the seed layer 250 may be achieved by various methods. That is, the seed layer 250 may be formed by evaporation, metal organic chemical vapor deposition (MOCVD), sputtering or coating using a brush.

Besides, the seed layer 250 may be formed by depositing or dispersing the ZnO particles.

Moreover, the seed layer 250 may be formed as a layer having a relatively uniform thickness on the light-emitting structure 210 and may have a regular pattern.

The seed layer 250 does not have a predetermined orientation in a direction perpendicular to the substrate 200. Therefore, the seed layer 250 formed on the substrate 200 may have various orientations other than the direction perpendicular to the substrate 200.

Especially, if the electrode 230 is formed on the light-emitting structure 210, the seed layer 250 may be formed in a region other than the electrode 230. It is necessary to cover the electrode 230 from the outside to form the seed layer 250 in a region other than the electrode 230. For this purpose, the region where the electrode 230 is formed may be covered with a photoresist pattern by nanoimprint lithography or laser interference lithography. Alternatively, a ZnO solution in a sol state may be patterned by contact printing.

Referring to FIG. 9, a growth guiding layer 260 is formed on the seed layer 250. The growth guiding layer 260 may be formed by forming a photoresist layer on the seed layer 250 and then patterning the photoresist layer by photolithography. A partial surface of the seed layer 250 is exposed by a patterned growth guiding layer 260.

If the electrode 230 is formed on the light-emitting structure 210, the growth guiding layer 260 may cover the electrode 230.

Referring to FIG. 10, a plurality of ZnO rods 275 of an urchin-like structure 270 are formed on the seed layer 250 exposed by the growth guiding layer 260.

The substrate 200 including the seed layer 250 and the growth layer 260 is immersed in a first growth solution such that the urchin-like structure 270 grows. In detail, the region exposed by the growth guiding layer 260 is filled with the ZnO crystal structure, and the urchin-like structure 270 grows from the ZnO crystal structure.

The first growth solution contains a first zinc ion donor, a first hydroxyl group donor, and a solvent.

The urchin-like ZnO structure 270 is formed on the exposed seed layer 250 of the substrate 200 immersed in the first growth solution, which results from the characteristics of the seed layer 250 from which a particular orientation is excluded. That is, the ZnO rods 275 are formed from the surface of the exposed seed layer 250 having higher surface energy. The size of the crystalline rods 275 is determined by the concentration and temperature of the first zinc ion donor and the first hydroxyl group donor.

However, the ZnO rods 275 grows into the urchin-like structure 270, and isotropic growth occurs on the surface of the exposed seed layer 250. Therefore, the plurality of ZnO rods 275 are formed from a growth factor into the urchin-like structure 270. Each of the ZnO rods 275 grows under the same chemical conditions, and thus the difference in length and thickness is insignificant.

The urchin-like ZnO structure 270 may be formed by other methods.

That is, zinc particles are dispersed on the substrate 200 and heated at a temperature of about 500℃. The heated zinc particles are formed into a micro- or nano-sized zinc agglomerate. The zinc agglomerate has an approximately hemispherical shape. Then, the zinc agglomerate is heated at a temperature of about 600℃ in a reactor, and oxygen is introduced into the reactor to form a ZnO core on the surface of the zinc agglomerate. The ZnO core is formed uniformly over the entire surface of the hemispherical zinc agglomerate. Subsequently, if the oxygen is continuously supplied to the reactor at the same temperature, the ZnO core grows into the urchin-like structure. The reason for this is that the hemispherical zinc agglomerate maintains the spherical shape by the cohesive force even at the melting point and reacts with oxygen to form nanorods on the ZnO core.

As such, the urchin-like ZnO structure 270 can be formed by various methods.

Referring to FIG. 11, the urchin-like ZnO structure 270 formed on the light-emitting structure 210 grows into a ZnO hemispherical structure 280.

If the growth guiding layer 260 is formed as shown in FIG. 10, the growth guiding layer 260 may be removed for the formation of the hemispherical structure 280.

The light-emitting structure 210 including the ZnO rods 275 of the urchin-like structure 270 is immersed in a second growth solution.

The second growth solution contains a second zinc ion donor, a second hydroxyl group donor, and a vertical growth inhibitor.

The second growth solution may be applied to the light-emitting structure 210 in various ways, instead of immersing the light-emitting structure 210 in the second growth solution. For example, the second growth solution may be applied to the light-emitting structure 210 by a spray process, and it is possible to expose the urchin-like structure 270 on the light-emitting structure 210 to the second growth solution by any solution process.

The second growth solution induces the lateral growth of the ZnO crystalline rods 275. That is, the growth of the ZnO crystal structure in the c-axis direction is suppressed, and the lateral growth is promoted. For this purpose, the growth using the second growth solution is performed above the isoelectric point of the zinc oxide.

The isoelectric point of the zinc oxide represents a particular pH, at which the partial polarization of the ZnO crystal occurs in the solution. That is, the zinc oxide shows the positive polarity Zn²⁺ on the (0001) plane of the crystal and the negative polarity of O^2- on the side above the isoelectric point. The isoelectric point of the ZnO crystalline rods 275 has a pH of 9.7.

Therefore, the pH of the second growth solution is adjusted to above 9.7. The second growth solution may contain an anionic material as the vertical growth inhibitor. The anionic material is adhered to a cationic zinc atom located at the end of a polar ZnO crystal structure and caps it. Therefore, the growth of ZnO crystal structure in the c-axis direction is suppressed, and the lateral growth is promoted.

The lateral growth occurs to fill the gap between the ZnO rods 275 of the urchin-like structure 270. Especially, the growth of ZnO crystal structure by the hexamine (C₆H₁₂N₄) used as the second hydroxyl group donor can be represented by formula 5 to 11 of the first embodiment.

The growth of the ZnO crystal structure, which fills the gap between the ZnO rods 275 of the urchin-like ZnO structure 270, occurs predominantly by formula 10. That is, the ZnO crystal structure is filled to the side of the ZnO rods 275, from which it can be seen that the growth of the ZnO crystal structure occurs in the lateral direction.

The end of the urchin-like structure 270 is capped with the vertical growth inhibitor. Therefore, after the lateral growth occurs continuously, the growth of the ZnO rods 275 does not exceed the urchin-like structure 270, and thus forming a ZnO hemisphere 280.

Moreover, the second growth solution may contain a second zinc ion donor and a second hydroxyl group donor. The light-emitting structure 210 including the ZnO rods 275 of the urchin-like structure 270 may be treated with the vertical growth inhibitor before it is immersed in the second growth solution. That is, the light-emitting structure 210 including the ZnO rods 275 capped with the vertical growth inhibitor may be immersed into the second growth solution later, and the ZnO rods 275 may grow in the lateral direction.

The growth mechanism of the ZnO hemispherical structure 280 can be explained by Oswald ripening. That is, a particle of a larger size has a low surface area, compared to its volume, and thus has low surface energy. Typically, a material system, in which a material is being synthesized or growing, tends to enter a lower energy state. Therefore, the particle of a smaller size tends to be attached to a particle of a larger size. As a result, the number of particles of a smaller size is reduced, and the size of the particles of a larger size is gradually increased.

Moreover, the outer surface of the ZnO hemispherical structure 280 has the (0001) plane of the zinc oxide. The reason for this is considered that the ZnO crystal structure, which grows toward the side of the ZnO rods 275 of the urchin-like structure 270, enters the lower energy state to form a stable crystal structure. That is, to follow the orientation of the crystal is the way to enter the lowest energy state, and thus the ZnO crystal structure is filled to the side of the ZnO rods 275 of the urchin-like structure 27 and has the same orientation as the ZnO rods 275.

The growth of the ZnO crystal structure, which fills the gap between the ZnO rods 275 of the urchin-like ZnO structure 270 at the beginning of the synthesis, forms a grain boundary where the gap between the adjacent ZnO rods 275 is filled. Therefore, when viewed from the outside of the hemispherical structure 280, the (0001) plane is formed predominantly, and the growth of fan-shaped nanorods is shown in the interior of the hemispherical structure 280. Moreover, the grain boundary is formed between the ZnO rods 275. As a result, the plurality of ZnO rods 275 are bonded to the ZnO hemispherical structure 280 of the present invention, and a polycrystal structure, in which a partial mismatch of crystals is caused, is formed between adjacent bonded ZnO rods 275.

The light-emitting structure including the ZnO hemispherical structure 280 has high light extraction efficiency, which can be described as follows.

If the ZnO hemispherical structure 280 is formed on the current spreading layer 219 such as an ITO layer, the light emitted from the light-emitting structure 210 is integrated into the ZnO hemispherical structure 280 through the ITO layer and is then emitted to the outside through the nanorods of the ZnO hemispherical structure 280. It is required for the zinc oxide to have a refraction index similar to that of the ITO layer such that the light passing through the ITO layer is effectively integrated into the ZnO hemispherical structure 280. Actually, the refraction index of the ITO layer is 2.06 and that of the zinc oxide is 2.04. Therefore, even if the light emitted from the ITO layer has a higher incident angle, it can be easily integrated into the hemispherical structure 280. Moreover, the light emitted from the light-emitting layer 215 is easily incident through the ZnO polycrystal structure, which fills the region exposed by the growth guiding layer. Each of the ZnO rods that constitute the hemispherical structure functions as an excellent optical waveguide. Therefore, the light emitted from the light-emitting layer 215 is easily incident to the ZnO polycrystal structure and then incident to each of the ZnO rods that constitute the hemispherical structure 280. The incident light propagates into the crystal structure of each nanorod functioning as an optical waveguide, thereby minimizing the light loss with respect to the outside.

Moreover, although the hemispherical structure 280 may be a complete hemisphere, it may be smaller or greater than a hemisphere. That is, the surface of the hemispherical structure 280 has an approximately circular shape, and the center of the circle may be located in the interior of the hemispherical structure 280 or on the seed layer 250 or its lower layer.

The hemispherical structure 280 may have an approximately elliptical shape. That is, a partial surface of the hemispherical structure 280 may have a predetermined curvature radius.

The light integrated into the ZnO hemispherical structure 280 can be easily emitted to the outside even at a large incident angle. The reason for this is that the plurality of ZnO rods that constitute the hemispherical structure 280 have an orientation perpendicular to the surface of the ZnO hemispherical structure 280. That is, the light incident from the ITO layer at the bottom of the hemispherical structure 280 enters the plurality of ZnO rods that constitute the hemispherical structure 280. The nanoscale rods grow radially from the center of the hemispherical structure 280 to the surface of the hemispherical structure 280. Each of the ZnO rods that constitute the hemispherical structure 280 functions as an optical waveguide. Therefore, the interior of the hemispherical structure 280 has a structure in which a plurality of fine waveguides are formed such that the light incident to the hemispherical structure 280 propagates in a direction perpendicular to the surface of the hemispherical structure 280 through the plurality of waveguides. When the light propagates through the ZnO rods as nanoscale waveguides, the light reaching the surface of the hemispherical structure 280 can be easily emitted to the outside. Therefore, the light-emitting structure 210 including the hemispherical structure 280 has high light extraction efficiency.

Referring to FIG. 12, a light-emitting structure 210 is provided on a substrate 200. The light-emitting structure 210 comprises an n-type semiconductor layer 213, a light-emitting layer 215, and a p-type semiconductor layer 217.

A ZnO hemispherical structure 280 is provided on the light-emitting structure 210. Moreover, a current spreading layer 219 and an electrode 230 are provided on the hemispherical structure 280.

The formation of the hemispherical structure 280 can be achieved by the method shown in FIGS. 7 to 11. That is, a seed layer 250 is formed on the light-emitting structure 210, and the hemispherical structure 280 is formed on the seed layer 250.

The hemispherical structure 280 is formed from the urchin-like ZnO structure grown from the seed layer 250. The growth of ZnO rods, which constitute the urchin-like structure, in the [0001] direction is suppressed by the vertical growth inhibitor, and the lateral growth is promoted. As a result, the urchin-like structure formed on the seed layer 250 grows into the hemispherical structure 280.

The formation of the seed layer 250, the urchin-like structure, and the hemispherical structure can be achieved by the method shown in FIGS. 7 to 11.

Subsequently, the current spreading layer 219 fills the gap between the hemispherical structures 280. The current spreading layer 219 is formed of a transparent conductive material, and thus the current spreading layer 219 may comprise ITO. The electrode 230 is provided on the current spreading layer 219.

When current is applied through the electrode 230, the current is applied to the light-emitting structure 210 through the current spreading layer 219, and the light-emitting structure 210 performs light emitting operation. The light emitted from the light-emitting structure 210 is integrated into the hemispherical structure 280 at the top of the light-emitting structure 210 and then extracted in a direction approximately perpendicular to the surface of the hemispherical structure 280.

FIG. 13 is an SEM image showing a plurality of hemispherical structures formed on a light-emitting diode by the method shown in FIGS. 7 to 11 in accordance with the second embodiment of the present invention.

The hemispherical structures are formed on a light-emitting structure provided on a sapphire substrate. The sapphire substrate is patterned in the form of a plurality of balls, and the ball pitch is 600 nm. A GaN buffer layer having a thickness of 3 um is formed on the substrate. An n-type GaN layer is formed on the GaN buffer layer. The n-type GaN layer has a thickness of 2.5 um, and Si is used as a dopant. A light-emitting layer is formed on the n-type GaN layer. The light-emitting layer has a multi-quantum well (MQW) structure and comprises a ternary system such as InGaN. Moreover, the light-emitting layer is configured to have a five-layered structure of barrier layers and well layers. A p-type GaN layer is formed on the light-emitting layer. The p-type GaN layer has a thickness of 0.14 um, and Mg is used as a dopant. Moreover, an ITO layer as the current spreading layer is provided on the p-type GaN layer. The light-emitting structure is in a normal type bare wafer state (in which the substrate is not yet separated) and has a chip size of 300 um x 300 um.

Moreover, the pitch between the hemispherical structures formed on the ITO layer as the current spreading layer is 400 um, and each of the hemispherical structures has a diameter of 250 um. The hemispherical structure is fabricated by the following process.

In detail, the seed layer on the sapphire substrate is formed by patterning the ZnO nanoparticles. To form an urchin-like structure, zinc nitrate is used as the first zinc ion donor, and HMTA is used as the first hydroxyl group donor. An urchin-like ZnO structure is formed in the first growth solution by heating at 65℃ for 1 hour.

The substrate, on which the urchin-like structure is formed, is capped with citrate anions as an anionic polymer to form a hemispherical structure. Then, the resulting substrate is immersed in the second growth solution. The second growth solution contains zinc nitrate and HMTA. The substrate immersed in the second growth solution is heated at 95℃ for 3 hours, thus obtaining a ZnO hemisphere.

Referring to FIG. 14, when a current of 110 mA is applied through electrodes, a light-emitting diode having no hemispherical structure has an intensity of 8000 a.u. at a wavelength of about 450 nm. Moreover, the light-emitting diode having the hemispherical structure shown in FIG. 13 has an intensity of 6800 a.u. at the same wavelength, from which it can be seen that the ZnO hemispherical structure is applied to the top of the light-emitting structure, it has light extraction efficiency about 7 times higher than the existing light-emitting diode.

The reason for this is considered that the hemispherical structure transmits the light, which passes through the current spreading layer, to the outside through the plurality of optical waveguides provided therein, thus minimizing the light loss.

Third Embodiment: Fabrication of ZnO crystalline structure

[Fabrication of flat-top type crystalline rods]

Referring to FIG. 15, a seed layer 310 is formed on a lower substrate 300.

The lower substrate 300 may be a glass substrate, a sapphire substrate, an ITO substrate, a silicon substrate, a GaN substrate, a SiC substrate, a ZnO substrate, a GaAs substrate, an InP substrate, an AlN substrate, a ScAlMgO₄ substrate, or a LiNbO₃ substrate.

Moreover, the lower substrate 300 may not be a physical substrate, but may be the same as or different from a layer, which will be formed in the following process.

The seed layer 310 formed on the lower substrate 300 may be a ZnO particle layer containing ZnO particles having a predetermined size. If the material of the lower substrate 300 is the same as the seed layer 310, the formation of the seed layer 310 may be omitted. Therefore, if the lower substrate 300 is a ZnO substrate and the seed layer 310 to be formed comprises zinc oxide, the formation of the seed layer 310 may be omitted.

The seed layer 310 may be formed by various methods. That is, any method that can arrange a plurality of crystalline particles in a direction perpendicular to the lower substrate 300 to induce the growth of crystalline rods may be used.

For example, the seed layer 310 may be formed by a sol-gel method.

In detail, a hydrate containing zinc salt is dissolved in a solvent to prepare a first solution. The zinc salt may comprise zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride. Moreover, the solvent may be a polar solvent. For example, the first solution may be prepared using ethanol as the solvent and ZnC₄H₁₀O₆·6H₂O as the hydrate.

Subsequently, the first solution is heated to be in a sol state. The heating temperature may be about 60℃ to 100℃. If the heating temperature is less than 60℃, the dissociation of the hydrate does not take place, whereas, if the heating temperature is more than 100℃, the ZnO crystals are not formed and overgrowth occurs in a dissociated state. The hydrate containing zinc salt is dissociated in the sol state. For example, if the ZnC₄H₁₀O₆·6H₂O is used as the hydrate, Zn²⁺, CH₃COO^-, and H₂O are produced.

Next, a surfactant is added to the solution in the sol state. Any surfactant that has no reactivity with ions and various compounds remaining in the sol state and can increase the viscosity of the solution in the sol state and the dispersion of ion species may be used. Therefore, the surfactant may be polyethylene glycol (PEG) or hydroxypropyl cellulose (HPC).

Then, the surfactant and the solution in the sol state are heated to be mixed together, thus preparing a second solution. The heating temperature may vary according to the type of the surfactant. For example, if the PEG is used as the surfactant, the heating temperature may be about 40℃ to 80℃.

The second solution is spin-coated on the lower substrate 300 and heated to be in a gel state. The heating temperature may be about 200℃ to 1,000℃. If the heating temperature is less than 200℃, by-products other than ZnO contained in the second solution are not completely removed, whereas, if the heating temperature is more than 1,000℃, the crystallinity of the formed seed layer 310 may be damaged. If the ZnC₄H₁₀O₆·6H₂O is used as the hydrate, for example, the seed layer 310 is formed by applying heat after spin-coating, and this reaction can be represented by the following formula 12:

[Formula 12]

Zn²⁺ + H₂O + 2CH₃COO^- → ZnO + 2CH₃COOH

The seed layer 310 in the gel state has a preferred orientation in the c-axis direction. That is, the ZnO particles formed when entering the gel state grow in the c-axis direction during the heating process after the spin-coating, which is the intrinsic characteristic of the ZnO crystal structure. In other words, the ZnO crystals have a high growth rate in the [0001] direction and have a low growth rate in the lateral direction. Moreover, partial polarization of ZnO occurs on the (0001) plane, but does not occur in the lateral direction. Therefore, the ZnO particles in the seed layer 310 have crystalline properties that can grow in a direction perpendicular to the lower substrate 300 during the heating process after the spin-coating.

Alternatively, the seed layer 310 may be formed by a hydrothermal synthesis method.

For the formation of the seed layer 310 by the hydrothermal synthesis method, a seed growth solution containing a first zinc salt and a first precipitator is prepared.

The seed growth solution may be prepared by dissolving the first zinc salt and the first precipitator in a polar solvent, respectively. The first zinc salt may comprise zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride, and the first precipitator may comprise Na₂CO₃, LiOH, H₂O₂, NH₄OH, NaOH, HMTA(C₆H₁₂N₄), or KOH. The polar solvent may comprise water, alcohol, or an organic solvent. Preferably, the polar solvent may contain both water and alcohol.

The ZnO particles may be formed by applying heat energy to the seed growth solution. The application of heat energy may be performed in the temperature range of 50℃ to 100℃ at atmospheric pressure for 1 to 2 hours.

The reaction mechanism of the ZnO particles can be represented by the following formulas 13 to 17. In detail, when the first zinc salt solution and the first precipitation solution are mixed together, Zn²⁺ in the first zinc salt solution and OH^- in the first precipitation solution can produce Zn(OH)₂ as an intermediate by the following formula 13. When heat is applied to the mixed solution, the Zn(OH)₂ may be decomposed into Zn²⁺ and OH^- by the following formula 14.

When the concentration of Zn²⁺ and OH^- is increased by continuous decomposition, a ZnO core may be formed by a condensation reaction represented by the following formula 15. At the same time, a ZnO growth factor, Zn(OH)₄ ^2-, can be produced by the following formula 16. Subsequently, the ZnO growth factor, Zn(OH)₄ ^2-, may react with the ZnO core to produce a ZnO particle by the following formula 17.

[Formula 13]

Zn²⁺ + 2OH^- ↔ Zn(OH)₂

[Formula 14]

Zn(OH)₂ ↔ Zn²⁺ + 2OH^-

[Formula 15]

Zn²⁺ + 2OH^- → ZnO + H₂O

[Formula 16]

Zn(OH)₂ + 2OH^- → Zn(OH)₄ ^2-

[Formula 17]

Zn(OH)₄ ^2- → ZnO + H₂O + 2OH^-

Moreover, a first overgrowth inhibitor is further added to the solution containing the ZnO particles, and the resulting solution is refluxed with a rotary evaporator to suppress the overgrowth of the ZnO particles.

The first overgrowth inhibitor may comprise a cationic polymer. In detail, the cationic polymer may be a polymer having an amine group such as polyethyleneimine (PEI) having high solubility in a polar solvent, for example. The cationic polymer may have a hyperbranched structure. Therefore, the growth factors containing anions, Zn(OH)₄ ^2-, are bonded to the cations present in the branches of the polymer and do not participate in the growth of the ZnO cores, thereby preventing the ZnO particles from overgrowing.

The diameter of the ZnO particles can be controlled by adjusting the concentration of the first overgrowth inhibitor. That is, when the concentration of the first overgrowth inhibitor is increased, the diameter of the ZnO particles may be reduced.

The ZnO particles may be separated from the solutions. The ZnO particles may be separated by a centrifugal separator, and the separated ZnO particles may be washed with alcohol. The resulting ZnO particles are dried to yield the final ZnO particles. The drying may be carried out at a temperature of about 70℃.

The ZnO particles prepared in the above manner are prevented from overgrowing by the first overgrowth inhibitor, and thus it is possible to produce the ZnO particles having a uniform shape. The ZnO particles may have a nano-size, for example, a size of 3nm to 5 nm.

The thus prepared ZnO particles are dispersed in a solvent and used to form the seed layer 310. The seed layer 310 may be formed by a solution process such as spin-casting. The solvent may be a polar solvent. The polar solvent may be ethanol, isopropyl alcohol, water, or distilled water. Preferably, the polar solvent may contain both water and ethanol.

Alternatively, the seed layer 310 may be formed by producing the ZnO particles by a reduction method and spin-casting the ZnO particles in a solvent. Moreover, the seed layer 310 may be formed by evaporation, metal organic chemical vapor deposition (MOCVD), sputtering or coating using a brush.

Referring to FIG. 16, a growth guiding layer 320 having a plurality of holes may be formed on the seed layer 310. Any material that is different from that of the seed layer 310 and can ensure chemical and thermal stability during the following process of forming crystalline rods may be used to form the growth guiding layer 320. Preferably, the growth guiding layer 320 may be a photoresist pattern. The photoresist pattern is formed by forming a photoresist layer on the seed layer 310 and then patterning the photoresist layer by photolithography.

Therefore, the photoresist layer may be formed by spin-coating and patterned by nanoimprint lithography, laser interference lithography, electron beam lithography, ultraviolet lithography, holographic lithography, or immersion lithography.

The growth guiding layer 320 may have various patterns. The shape of the crystalline rods, which will be formed in the following process, may be determined by the pattern of the growth guiding layer 320. The reason for this is that the crystalline growth does not proceed any longer in the seed layer 310 covered by the growth guiding layer 320 and the crystalline growth only proceed in the seed layer 310 exposed by the growth guiding layer 320. Therefore, the hole areas exposed by the growth guiding layer 320 may have a rod or line shape spaced from each other.

Referring to FIG. 17, crystalline rods 330 are formed in the holes of the growth guiding layer 320. Preferably, a crystalline rod 330 may be formed in each hole of the growth guiding layer 320. Therefore, the crystalline rod 330 may have a nano-size.

The crystalline rods 330 are formed on the seed layer 310 formed on the lower substrate 300. The crystalline rods 330 may be formed of the same material as the seed layer 310. Each of the crystalline rods 330 has a structure, in which the crystalline growth occurs predominantly in a specific direction, and the structure may partially have an amorphous structure in which the crystallinity is reduced, but the crystalline growth is the main factor in forming crystalline rods 330. Therefore, if the seed layer 310 comprises zinc oxide, the crystalline rods 330 may comprise zinc oxide.

Moreover, the crystalline rods 330 are grown in a direction substantially perpendicular to the lower substrate 300 and have a regular or irregular arrangement with respect to adjacent crystalline rods 330.

The diameter of the crystalline rods 330 formed by the described method may vary according to the fabrication method. That is, the degree of the growth may vary according to the chemical environment, processing time, temperature or pressure. Moreover, the shape and size of the crystalline rods 330 may be determined by the preferred growth direction. For example, if the crystalline growth in a direction perpendicular to the lower substrate 300 is much larger than the crystalline growth in the lateral direction, the crystalline growth in the lateral direction is significantly suppressed, and thus the diameter of the crystalline rods 330 may be smaller than the opposite case. Therefore, the crystalline rods 330 may have a micro- or nano-size.

Moreover, the shape of the crystalline rods 330 may be determined by the shape of the growth guiding layer 320. Therefore, the crystalline rods 330 may have a rod shape separated from each other or a line shape having a predetermined interval. In the following embodiments, the crystalline rods 330 may also have a rod shape separated from each other or a line shape having a predetermined interval. Moreover, the crystalline rods 330 may have various shapes such as a tube. The reason for this is that any shape may be used as long as the crystalline rods 330 of the present invention have excellent crystallinity, can contribute to the formation of layers, which will be formed in the following process, and can grow from the seed layer 310 on the substrate.

The crystalline rods 330 may be formed by the hydrothermal synthesis method. For example, if the crystalline rods 330 comprising zinc oxide are formed by the hydrothermal synthesis method, the crystalline rods 330 may be formed using a rod growth solution containing a hydrate containing a second zinc salt, a second precipitator, and a second overgrowth inhibitor. The use of the second overgrowth inhibitor may be omitted, if necessary.

The hydrate containing the second zinc salt may comprise zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride, and the second precipitator may comprise Na₂CO₃, LiOH, H₂O₂, NH₄OH, NaOH, HMTA(C₆H₁₂N₄), or KOH. The C₆H₁₂N₄ can produce NH₄ ⁺ and OH^-, which are the growth factors for forming the crystalline rods 330 comprising zinc oxide and, at the same time, can control the reaction rate since the growth rate and the OH^- concentration can be easily controlled.

The second overgrowth inhibitor may comprise a cationic polymer. In detail, the cationic polymer may be a polymer having an amine group such as polyethyleneimine (PEI) having high solubility in a polar solvent.

In detail, after the lower substrate 300 including the growth guiding layer 320 is immersed in the rod growth solution, heat energy is applied thereto. The application of heat energy may be performed in the temperature range of 50℃ to 100℃ at atmospheric pressure. If the heating temperature is less than 50℃, the growth of the crystalline rods 330 slows down, which makes it difficult to achieve a substantial growth of the crystalline rods 330, whereas, if the heating temperature is more than 100℃, the crystallinity of the crystalline rods 330 is damaged by an unexpected reaction between ion species in the rod growth solution.

The growth mechanism of the crystalline rods 330 comprising zinc oxide can be represented by the following formulas 18 to 24.

Hexamine (C₆H₁₂N₄) used as the second precipitator can produce NH₄ ⁺ and OH^- by the following formulas 18 and 19. Moreover, Zn(NO₃)₂ used as the second zinc salt can produce zinc ions by the following formula 20.

[Formula 18]

C₆H₁₂N₄ + 6H₂O ↔ 6CH₂O + 4NH₃

[Formula 19]

NH₃ + H₂O ↔ NH₄ ⁺ + OH^-

[Formula 20]

Zn(NO₃)₂ → Zn²⁺ + 2NO₃ ^-

The 4NH₃, 4OH^-, and Zn²⁺ produced by the above formulas 18 to 20 can produce Zn(NH₃)₄ ²⁺ and Zn(OH)₄ ^2-, which are the growth factors of the ZnO crystalline rods 330, by the following formulas 21 and 22.

[Formula 21]

Zn²⁺ + 4NH₃ → Zn(NH₃)₄ ²⁺

[Formula 22]

Zn²⁺ + 4OH^- → Zn(OH)₄ ^2-

The growth factor, Zn(NH₃)₄ ²⁺, produced by the above formula 21 can produce the ZnO crystalline rods 330 represented by the following formula 23 by the reaction with OH^- as a reaction factor, and the growth factor, Zn(OH)₄ ^2-, produced by the above formula 22 can produce the ZnO crystalline rods 330 by the following formula 24.

[Formula 23]

Zn(NH₃)₄ ²⁺ + 2OH^- → ZnO + 4NH₃ + H₂O

[Formula 24]

Zn(OH)₄ ^2- → ZnO + H₂O + 2OH^-

However, when the cationic polymer as the second overgrowth inhibitor is added to the rod growth solution, Zn(OH)₄ ^2-, which is one of the growth factors, is adhered to the cationic polymer such that the Zn(OH)₄ ^2- cannot participate in the growth of the crystalline rods 330 comprising zinc oxide. The Zn(OH)₄ ^2- is known as a factor that allows the ZnO crystals to grow into an urchin-like structure.

Therefore, the cationic polymer prevents the Zn(OH)₄ ^2- from participating in the growth of the ZnO particles, and thus the ZnO crystals are prevented from growing into the urchin-like structure. Especially, the zinc oxide has a crystalline structure with a preferential growth in the c-axis direction. Partial polarization of zinc and oxygen occurs in the c-axis direction, i.e., the [0001] direction, but does not occur in the lateral direction. Therefore, the growth of the crystalline rods 330 may be predominant in the c-axis direction, i.e., the [0001] direction, even in a state where there is no particular inhibition to the growth. Furthermore, the growth in the lateral direction may also occur continuously.

If the second overgrowth inhibitor is used, the cationic polymer is adhered to the Zn(OH)₄ ^2- and further caps the anionic O^2- exposed to the side of the already formed ZnO crystal structure, thus suppressing the growth in the lateral direction. Therefore, the second overgrowth inhibitor suppresses the growth of the crystalline rods 330 comprising zinc oxide in the lateral direction. Moreover, the crystalline rods 330 may grow in a direction perpendicular to the lower substrate 300 by the control of the growth factor.

In addition, it is possible to form one crystalline rod 330 in each hole of the growth guiding layer 320 by adjusting the concentration of the cationic polymer. For example, the cationic polymer may be added in an amount of 0.5 M to 1 M with respect to 1 M of the second zinc salt.

Meanwhile, the rod growth solution may have a pH of 9 to 11. If the growth solution has a pH above 11, the ZnO crystalline rods 330 may be damaged by excessive erosion. Therefore, the rod growth solution may have a pH of 10. For this purpose, an alkaline solution such as ammonia water may be added to the rod growth solution.

The excess of OH^- contained in the rod growth solution may erode the ZnO crystalline rods, thereby producing a by-product, Zn(OH)₂, as represented by the following formula 25. As a result, each of the ZnO crystalline rods 330 may have a pointed end like a pencil.

[Formula 25]

ZnO + 3OH^-→ Zn(OH)₂ + H₂O

However, the growth reaction of the zinc oxide represented by the above formula 23 may continue along with the erosion. Referring to formula 23, the OH^- is consumed as the ZnO crystals grow, and thereby the pH of the rod growth solution may be reduced. As a result, the growth reaction occurs more preferentially than the erosion, which results in the formation of ZnO crystalline rods 330.

Referring to FIG. 18, the growth guiding layer 320 formed on the seed layer 310 is removed. Therefore, the crystalline rods 330 formed based on the growth guiding layer 320 are shown on the seed layer 310 formed on the substrate 300.

The crystalline rods 330 are oriented perpendicularly to the surface of the substrate 300 by the growth factor which promotes the vertical growth rather than the lateral direction. Moreover, the crystalline rods 330 are formed by the predominant growth in the c-axis direction perpendicular to the plane, which is formed by zinc atoms or oxygen atoms.

Referring to FIG. 19, a silicon substrate is used as a substrate. The silicon substrate has a (100) plane as a main surface. A seed layer is formed by the sol-gel method, and the substrate including the seed layer is immersed in a rod growth solution for 30 minutes, thereby forming ZnO crystalline rods each having a hexagonal prism shape.

0.55 M zinc acetate is dissolved in ethanol to be in a sol state by the sol-gel method. Especially, the mixed solution is heated at 60℃ for 30 minutes to promote the dissolution, thereby preparing a solution in a sol state. Subsequently, a surfactant such as polyethylene glycol (PEG) or hydroxypropyl cellulose (HPC) is added to the solution in a sol state to facilitate the spin coating of the solution.

After spin-coating the solution on the substrate, the resulting solution is heated at 350℃ for 40 minutes to form a seed layer in a gel state. The height of the seed layer is 90 nm.

Moreover, a growth guiding layer is formed by laser interference lithography. The pitch between holes of the growth guiding layer is 400 nm, and each hole has a circular shape.

It is necessary to apply hydrophilic treatment to the top of the seed layer exposed by the growth guiding layer. When the solution process is used for the hydrophilic treatment, 10 wt% hydrogen peroxide is mixed with water as a solvent.

The substrate including the seed layer is treated with the solution containing hydrogen peroxide at 40℃ for 10 minutes. Alternatively, the hydrophilic treatment may be achieved by plasma treatment using a gas phase process. In this case, oxygen gas having a purity of 99% may be used as a plasma gas, the partial pressure of oxygen supplied may be 25 sccm, the pressure in a chamber may be 20 mtorr, and the plasma treatment may be performed at room temperature.

The reason for the hydrophilic treatment is to facilitate the flow of the growth solution through the holes of the growth guiding layer having a small diameter and a high selectivity, thus improving the growth efficiency. Moreover, it is another reason that the oxygen atom vacancies formed by the thus formed ZnO crystals facilitate the control the structure by a secondary process.

Moreover, the rod growth solution containing 70 mM zinc nitrate, 65 nm HMTA, and DI water as a solvent is prepared for the formation of the crystalline rods. Moreover, 40 mM polyethyleneimine (PEI) is used as the second overgrowth inhibitor.

The growth reaction of the rod growth solution in which the seed layer is immersed is performed at 93℃ for 6 hours, thereby forming the crystalline rods.

In FIG. 19, the ZnO crystalline rods are grown vertically and, since the crystalline rods are formed under the same solution process conditions, the crystalline rods have the same height and size as adjacent crystalline rods. As shown in FIG. 19, the crystalline rods have a height of 1.5 um.

If a process of forming oxygen vacancies is added after the formation of the growth guiding layer, the oxygen vacancies are formed in the whole or part of the crystalline rod due to a defect caused by oxygen vacancies at the bottom of the crystalline rod. That is, each of the crystalline rods may have a defect-containing structure.

In addition to the above-described methods, the crystalline rods may be formed by various methods. For example, the crystalline rods may be formed by chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD), in which a source gas is supplied to the top of the exposed seed layer and the growth of crystals is used.

Each of the crystalline rods formed by the present embodiment has an approximately flat-top shape, which will be called a flat-top type in the present invention.

[Fabrication of pencil-like crystalline rods]

The flat-top type crystalline rods prepared in this embodiment may be formed into pencil-like hexagonal crystal structures by a secondary process. That is, the hexagonal crystal structure having a crystalline rod-like ZnO structure may have a pencil-like ZnO structure by the secondary process.

Moreover, if a process of binding an oxygen ligand to the seed layer is performed before the formation of the crystalline rods as described with reference to FIG. 19 of the present embodiment, the formation of the pencil-like hexagonal crystal structures by the secondary process can be facilitated.

That is, the flat-top type crystalline rods are immersed in a first erosion solution having a pH of 9.7 to 10.2. If the pH is below 9.7, the partial polarization of the crystalline rods does not occur and the erosion is not expected to occur. If the pH is above 10.2, the pencil-like crystalline rods are not formed by excessive erosion. The excess of OH^- erodes the corners of the flap-top type crystalline rods under the pH conditions, which can be represented by the following formula 26:

[Formula 26]

ZnO + OH^-→ Zn(OH)₂ ^2- + H₂O

The above-described process is performed above the isoelectric point of the ZnO crystalline rods.

That is, the isoelectric point of the ZnO crystalline rods represents a particular pH, at which the polarization of the ZnO crystal occurs in the solution. That is, the zinc oxide shows the positive polarity Zn² ⁺ on the (0001) plane of the crystal and the negative polarity of O^2- on the side above the isoelectric point. The isoelectric point of the ZnO crystalline rods is at a pH of 9.7.

Therefore, the ZnO crystal with a wurtzite structure has high crystallinity in the c-direction. Above the isoelectric point, the zinc oxide has unstable Zn²⁺ ions on the (0001) plane and maintains high reactivity with OH^- compared to the relative non-polar side plane. Moreover, the zinc atoms at the corners of the topmost layer of the flat-top type crystalline rod have the highest reactivity. Thereby, the pencil-like hexagonal crystal structure is formed by the erosion at the corners.

The pencil-like hexagonal crystal structure may be formed using a growth solution in which the growth and erosion of the crystalline rod occur at the same time under particular conditions. For example, if polyethyleneimine (PEI) of high concentration is contained in the rod growth solution used in the formation process of the flat-top type crystalline rods, the imine group (-NH) of the PEI produces a precursor, Zn(NH₃)₄ ²⁺ which is shown in formula 21, as a growth factor of the ZnO crystalline rod. Moreover, the precursor Zn(NH₃)₄ ²⁺ reacts with water in the rod growth solution to produce ammonium hydroxide. The ammonium hydroxide is dissolved into NH₄ ⁺ and OH^-, which can be represented by the following formula 27:

[Formula 27]

Zn(NH₃)₄ ²⁺ + 3H₂O → ZnO + 2NH₄ ⁺ + 2NH₃·H₂O

The concentration of OH^- in the rod growth solution is increased in the above reaction and, if the pH is above the isoelectric point, the erosion occurs predominantly, and thus the pencil-like hexagonal crystal structure is formed.

As mentioned above, it can be seen that the growth and erosion of the crystalline rod occurring in the same solution can be achieved by controlling the concentration of the second overgrowth inhibitor contained in the rod growth solution used in the formation process of the flat-top type crystalline rods. That is, if the concentration of the second overgrowth inhibitor is high, the second overgrowth inhibitor is adhered to the Zn(OH)₄ ^2- shown in formula 24, and the excess of the second overgrowth inhibitor may induce the above-described erosion.

Referring to FIG. 20, the rod growth solution used in the this embodiment contained 50 mM zinc nitrate, 65 mM HMTA, and 95 mM PEI as the second overgrowth inhibitor. Moreover, the substrate and the seed layer are formed by the same method as described with reference to FIG. 19 of this embodiment.

As shown in formula 27, the growth and erosion of the crystalline rod in the rod growth solution compete with each other based on the isoelectric point. That is, if the pH of the rod growth solution is below the isoelectric point, the growth reaction occurs predominantly, which results in the growth of the crystalline rods. If the pH of the rod growth solution is above the isoelectric point by the second overgrowth inhibitor of high concentration, the erosion reaction occurs predominantly, which results in the formation of the pencil-like hexagonal crystal structure.

Moreover, if the process of bonding the oxygen ligand to the seed layer is performed before the formation of the crystalline rods, the thus formed pencil-like hexagonal crystal structures of FIG. 20 may have oxygen vacancies.

Therefore, heat treatment may be performed with oxygen gas to remove the oxygen vacancies in the following process. In this case, the heat treatment may be carried out at an oxygen partial pressure of 25 sccm, a chamber pressure of 25 mtorr, and a calcining temperature of 350℃ to 550℃ for 1 hour. Thereby, oxygen atoms are supplied to the pencil-like hexagonal crystal structures to remove the oxygen vacancies.

It can be seen from FIG. 20 that the pencil-like hexagonal crystal structures having a regular arrangement are formed by the erosion at the corners of the topmost layer of the flap-top type crystalline rods.

Moreover, FIG. 21 is another SEM image showing the hexagonal crystal structures of FIG. 20.

It can be seen from FIG. 21 that each of the pencil-like hexagonal crystal structure is formed of zinc oxide, has a regular arrangement with respect to adjacent pencil-like hexagonal crystal structures, and has the same shape.

[Fabrication of needle-like crystalline rods]

The flat-top type crystalline rods prepared in this embodiment may be formed into needle-like hexagonal crystal structures by high erosion under specific pH conditions. That is, the flat-top type crystalline rods prepared in this embodiment are immersed in a second erosion solution having a high pH using the erosion described in the above fabrication method of pencil-like crystalline rods.

During the immersion, the erosion occurs rapidly on the corners of the crystalline rods, and thereby the needle-like hexagonal crystal structures, which means that each of the ZnO crystalline rods formed in this embodiment has a needle shape.

The second erosion solution used in this embodiment may have a pH of 11 to 12.4, for example. If the pH of the second erosion is below 11, it is difficult to form the needle shape due to a low reaction rate, whereas, if the pH of the second erosion is above 12.4, the pencil-like crystalline rods are not formed by excessive erosion.

Therefore, the erosion is performed under the pH conditions of the second erosion solution, and the erosion on the flat-top type crystalline rods can be represented by the above formula 26.

The needle-like hexagonal crystal structure may be formed using a growth solution in which the growth and erosion of the crystalline rod occur at the same time under particular conditions. For example, if polyethyleneimine (PEI) of high concentration is contained in the rod growth solution used in this embodiment, the imine group (-NH) of the PEI produces a precursor, Zn(NH₃)₄ ²⁺ which is shown in formula 21, as a growth factor of the ZnO crystalline rod. Moreover, the precursor Zn(NH₃)₄ ²⁺ reacts with water in the rod growth solution to produce ammonium hydroxide. The ammonium hydroxide is dissolved into NH₄ ⁺ and OH^- as shown in the above formula 27.

The concentration of OH^- in the rod growth solution is increased in the above reaction and, if the pH is above the isoelectric point and is adjusted to 11 to 12.4, the erosion occurs predominantly, and thus the pencil-like hexagonal crystal structure is formed.

Referring to FIG. 22, the rod growth solution used in the this embodiment contained 50 mM zinc nitrate, 65 mM HMTA, 120 mM PEI as the overgrowth inhibitor, and water as the solvent. Moreover, the substrate and the seed layer are formed by the same method as described with reference to FIG. 19 of this embodiment. The rod growth solution has a pH of 11 to 12.4, and thus the erosion occurs predominantly, which results in the formation of the needle-like hexagonal crystal structures.

Especially, if the process of bonding the oxygen ligand to the seed layer is performed before the formation of the crystalline rods, the thus formed pencil-like hexagonal crystal structures of FIG. 20 have oxygen vacancies.

[Fabrication of tube-like crystalline rods]

The crystalline rods prepared in this embodiment may be formed into tube-like crystalline rods using an etching solution under specific pH conditions. That is, each of the ZnO crystalline rods prepared in this embodiment has a hollow tube shape.

A third erosion solution used in this embodiment has a pH of 6 to 8 at which the polarization of the ZnO crystalline rods does not occur.

In the third erosion solution in which the salt is dissolved in an aqueous solution, the salt is dissociated into ions. The dissociated ions react with zinc and oxygen to erode the ZnO crystalline rods. For example, in the case where the erosion solution contains potassium chloride, the reaction can be represented by the following formula 28:

[Formula 28]

ZnO + 2KCl + 2H₂O → ZnCl₂ + 2KOH +H₂O

The reaction of formula 28 is an endothermic reaction, and thus it is possible to heat the third erosion solution, in which the crystalline rods are immersed, to a predetermined temperature, thus controlling the reaction rate.

Referring to FIG. 23, the tube-like hexagonal crystal structures are formed using an erosion solution having a KCl concentration of 4.5 M and a pH 7, and the erosion solution is heated at 65℃ for 5 hours.

It can be seen from FIG. 23 that the erosion occurs only in the center of each crystalline rod, and thus the tube shape is formed. The present inventors propose the following hypothesis concerning the above phenomenon.

The space between the crystalline rods is very small, and the diffusion or convection of the solution is very limited in such a small space. When the erosion of the zinc oxide first occurs, the concentration of KCl is reduced in the space and does not erode ZnO. Therefore, the erosion does not continue in the space between the crystalline rods.

However, the erosion occurs in the center of the upper surface of each crystalline rod, where the contact area with the erosion solution is large. Although the concentration of KCl is locally reduced by the erosion, the KCl in the center of the upper surface has a concentration that can cause the erosion by the diffusion or convection.

Moreover, if the corrosion proceeds and a hole is formed in the center, the partial polarization of zinc and oxygen occurs in the hole in various directions. Especially, various polarizations occur in the vertical direction and in similar directions. Therefore, the erosion in a direction perpendicular to the (0001) plane of the crystal occurs more preferentially than the (0001) plane. The tube-like hexagonal crystal structures are formed by the above-described process.

Besides, the above-described phenomenon may result from the local difference in polarity due to the orientation of the crystal.

Referring to FIG. 24, there is no difference between the (0001) plane and the

plane that intersect the c-axis, which means that the chemical erosion in the horizontal direction does not easily occur.

On the contrary, there is a local difference in polarity in a direction parallel to the

plane or the

plane. That is, there is a difference in polarity in a direction perpendicular to the crystalline rods, which facilitates the chemical erosion in the vertical direction of the crystal.

As result, the tube-like hexagonal crystal structure, in which the erosion occurs predominantly in the vertical direction in the center, is formed.

The above-described embodiment of the present invention provides the crystalline rod, the pencil-like hexagonal crystal structure, the needle-like hexagonal crystal structure, and the tube-like hexagonal crystal structure, which are arranged in a direction perpendicular to the substrate.

The vertically arranged crystalline rod can be used in various applications such as an optical waveguide, and the pencil-like hexagonal crystal structure can be used as a light concentrator and an anti-reflection layer in an optical device such as a laser diode and can be used as a lower electrode in an organic/inorganic hybrid solar cell.

Moreover, the needle-like hexagonal crystal structure can also be used as a light concentrator and an anti-reflection layer in an optical device such as a laser diode and can be used as a lower electrode in an organic/inorganic hybrid solar cell. Besides, the needle-like zinc oxide structure can be used as a tissue culture layer in a biochip.

Furthermore, the tube-like hexagonal crystal structure can be applied to a solid-state dye-sensitized solar cell. For example, a tube in which a quantum dot is formed can be used as a current collector. Moreover, the tube-like hexagonal crystal structure can be used as a tissue culture layer in a biochip and an optical waveguide in an optical device such as a light-emitting diode.

Although the present embodiment provides various shapes of the hexagonal crystal structure formed using zinc oxide, these structures can be formed using ZnSe, ZnS, or CdSe, which can from similar hexagonal crystal structures. This results from the fact that the above-described zinc oxide structures have the same wurtzite structure as ZnSe, ZnS, or CdSe. However, the isoelectric points of these materials are different from each other, and thus the hexagonal crystal structures having various shapes can be formed by controlling the pH of the growth solution and the erosion solution.

Moreover, the hexagonal crystal structures having various shapes can be used in various applications such as optical techniques, solar cell technologies, bio technologies, etc.

Forth Embodiment: Light-emitting diode including ZnO crystalline rods

FIGS. 25 to 29 are cross-sectional views illustrating a method of fabricating a light-emitting diode including a ZnO crystalline rod in accordance with a fourth embodiment of the present invention.

Referring to FIG. 25, a light-emitting structure 410 is provided on a substrate 400.

The light-emitting structure 410 may comprise a group III nitride or a group II oxide. A ZnO crystalline rod can be applied regardless of the type and shape of the light-emitting structure 410.

Therefore, the light-emitting structure 410 may be provided in various types such as a normal type, a flip-chip type, and a vertical type.

The vertical type light-emitting structure employs an acceptor substrate, which is required by the fabrication process, separately from a substrate. Moreover, a lift-off process of separating the substrate is employed during the fabrication process. The vertical type light-emitting structure has a typical structure in which a p-type semiconductor layer, a light-emitting layer, and an n-type semiconductor layer, which are sequentially provided on the acceptor substrate. Moreover, electrodes may be formed on the n-type semiconductor layer and on the rear surface of the acceptor substrate facing the p-type semiconductor layer. A reflective layer formed of a metal material may be interposed between the acceptor substrate and the p-type semiconductor layer. Moreover, the n-type semiconductor layer may be first provided on the acceptor substrate according to the fabrication method.

For the convenience of description and better understanding of the present invention, the normal type light-emitting structure will be hereinafter exemplarily used as the light-emitting structure in this embodiment shown in FIG. 25. Therefore, the light-emitting structure 410 has a structure in which an n-type semiconductor layer 413, a light-emitting layer 415, and a p-type semiconductor layer 417 are sequentially formed on the substrate 400. Moreover, an electrode 430 is provided on the p-type semiconductor layer 417. A current spreading layer 419 as a transparent conductor may be further interposed between the p-type semiconductor layer 417 and the electrode 430. A buffer layer 411 may be further interposed between the substrate 400 and the n-type semiconductor layer 413 to reduce the lattice mismatch.

That is, the light-emitting structure 410 may be designed differently according to its type in this embodiment. However, the ZnO crystalline rods are arranged in a direction in which the light emitted from the light-emitting layer 415 is extracted. Particularly, the ZnO crystalline rods are not interposed between the semiconductor layers 413 and 417 and the light-emitting layer 415 but are provided on the semiconductor layers 413 and 417 or the light-emitting layer 415. In the case of the flip-chip type light-emitting structure, the ZnO crystalline rods may be formed on the substrate.

Referring to FIG. 26, a seed layer 450 is formed on the light-emitting structure 410. The seed layer 450 may be formed by various methods.

Moreover, the seed layer 450 may have a regular or irregular orientation.

For example, the seed layer 450 may be formed by preparing a solution by mixing ZnO powder and a surfactant and spin-coating the solution on the light-emitting structure 410.

Moreover, if zinc metal is deposited on the light-emitting structure 410 and the resulting light-emitting structure 410 is immersed in a solution having a pH above 10, the zinc metal is bonded to oxygen contained in the solution and converted into ZnO, which can be used as the seed layer 450.

Besides, the seed layer 450 may be formed by a hydrothermal synthesis method.

A seed growth solution is prepared for the formation of the seed layer 450 by the hydrothermal synthesis method. The seed growth solution is prepared by dissolving a first zinc salt and a first precipitator in a polar solvent.

The first zinc salt comprises zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride. The first precipitator comprises NaOH, Na₂CO₃, LiOH, H₂O₂, KOH, hexamethylenetetramine (HMTA), or NH₄OH. The polar solvent may comprise water, alcohol, or an organic solvent. Preferably, the polar solvent may contain both water and alcohol.

The formation of the ZnO particles can be represented by formulas 13 to 17 of the third embodiment.

In formulas 13 and 15, Zn²⁺ is supplied from the first zinc salt and OH^- is supplied from the first precipitator. The cation and anion are reacted together to form ZnO or Zn(OH)₂ as an intermediate.

Moreover, in formula 16, the intermediate, Zn(OH)₂, reacts with OH^- to form Zn(OH)₄ ^2- as a ZnO growth factor, which forms ZnO in formula 17.

A first overgrowth inhibitor may be used to control the size of the ZnO particles. The first overgrowth inhibitor is added to the seed growth solution in which the ZnO particles are formed. The first overgrowth inhibitor may be a cationic polymer. In detail, the cationic polymer may be a polymer having an amine group such as polyethyleneimine (PEI) having high solubility in a polar solvent. The ZnO growth factor, Zn(OH)₄ ^2-, is adhered to a cation of the cationic polymer and does not participate in the growth of zinc oxide. As a result, the size of the ZnO particles can be controlled by the first overgrowth inhibitor.

Subsequently, the thus formed ZnO particles are separated, and the separated ZnO particles are dispersed in a solvent and formed into the seed layer 450 by spin coating.

As mentioned above, the formation of the seed layer 450 may be achieved by various methods. That is, the seed layer 450 may be formed by evaporation, metal organic chemical vapor deposition (MOCVD), sputtering or coating using a brush.

Besides, the seed layer 450 may be formed by depositing or dispersing the ZnO particles.

The seed layer 450 may be formed by a sol-gel method. In detail, a hydrate containing zinc salt is dissolved in a solvent to prepare a first solution. The zinc salt may comprise zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride. Moreover, the solvent may be a polar solvent. For example, the first solution may be prepared using ethanol as the solvent and ZnC₄H₁₀O₆·6H₂O as the hydrate.

The second solution is spin-coated on the light-emitting structure 410 on the substrate 400 and heated to be in a gel state. The heating temperature may be about 200℃ to 1,000℃. If the heating temperature is less than 200℃, by-products other than ZnO contained in the second solution are not completely removed, whereas, if the heating temperature is more than 1,000℃, the crystallinity of the formed seed layer 450 may be damaged. The seed layer 450 is formed by applying heat after spin-coating, and this reaction can be represented by formula 12 of the third embodiment.

The seed layer 450 in the gel state has a preferred orientation in the c-axis direction. That is, the ion species dissolved when entering the sol state are formed into ZnO particles during the heating process after the spin-coating, and the ZnO particles grow in the c-axis direction, which is the intrinsic characteristic of the ZnO crystal structure. In other words, the ZnO crystals have a high growth rate in the [0001] direction and have a low growth rate in the lateral direction. Moreover, partial polarization of ZnO occurs on the (0001) plane, but does not occur in the lateral direction. Therefore, the ZnO particles in the seed layer 450 have crystalline properties that can grow in a direction perpendicular to the lower substrate 400 during the heating process after the spin-coating.

Moreover, the seed layer 450 may be formed as a layer having a relatively uniform thickness on the light-emitting structure 410 and may have a regular pattern.

Especially, if the electrode 430 is formed on the light-emitting structure 410, the seed layer 450 may be formed in a region other than the electrode 430 (e.g., on the p-type semiconductor layer 417 or the current spreading layer 419). It is necessary to cover the electrode 430 from the outside to form the seed layer 450 in a region other than the electrode 430. For this purpose, the region where the electrode 430 is formed may be covered with a photoresist pattern, which may be formed by forming a photoresist layer on the seed layer 450 and then patterning the photoresist layer by photolithography.

Referring to FIG. 27, a growth guiding layer 460 is formed on the seed layer 450. The growth guiding layer 460 is provided to induce the vertical growth of the ZnO crystalline rods.

The growth guiding layer 460 may be formed by typical photolithography or by other processes such as laser interference lithography, nanoimprint lithography, electron beam lithography, ultraviolet lithography, holographic lithography, or immersion lithography.

The growth guiding layer 460 may have a regular pattern and partially exposes the seed layer 450. That is, the holes of the growth guiding layer 460 partially expose the seed layer 450.

Referring to FIG. 28, crystalline rods 470 are formed on the seed layer 450. The crystalline rods 470 comprise zinc oxide. The major growth factor of the crystalline rods 470 results from the growth of the crystal based on the seed layer 450, and the growth in the [0001] direction occurs predominantly. The crystalline rods 470 may have a regular arrangement with respect to adjacent crystalline rods 470 and may grow only from the seed layer 450 exposed by the growth guiding layer 460.

Moreover, the crystalline rods 470 may grow in a direction perpendicular to the upper plane formed by the light-emitting structure 410. The direction that the crystalline rods 470 grow is based on the orientation of the ZnO particles that constitute the seed layer 450.

The crystalline rods 470 may be formed by various methods such as evaporation, chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD).

The crystalline rods 470 may have a micro- or nano-size. Moreover, the crystalline rods 470 may be formed into single crystals, but their crystallinity may be damaged by other factors generated during the growth into the crystals. However, it is apparent that the predominant factor in the formation and growth of the crystalline rods 470 is the single crystal growth.

Especially, the crystalline rods 470 may be formed by the hydrothermal synthesis method. That is, the crystalline rods 470 may be formed by immersing the substrate 400 including the seed layer 450 in a rod growth solution prepared in an aqueous solution phase and applying heat energy thereto.

In detail, the rod growth solution comprises a hydrate containing a second zinc salt, a second precipitator, and a second overgrowth inhibitor. The use of the second overgrowth inhibitor may be omitted, if necessary.

The second zinc salt acts as a zinc ion donor and the second precipitator acts as a hydroxyl group donor.

After the substrate 400 including the seed layer 450 is immersed in the rod growth solution, heat energy is applied thereto. The application of heat energy may be performed in the temperature range of 50℃ to 100℃ at atmospheric pressure. If the heating temperature is less than 50℃, the growth of the crystalline rods 470 slows down, which makes it difficult to achieve a substantial growth of the crystalline rods 470, whereas, if the heating temperature is more than 100℃, the crystallinity of the crystalline rods 470 is damaged by an unexpected reaction between ion species in the rod growth solution.

The hydrate containing the second zinc salt may comprise zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride. The second precipitator may comprise NaOH, Na₂CO₃, LiOH, H₂O₂, KOH, hexamethylenetetramine (HMTA), or NH₄OH. Preferably, the second precipitator may be C₆H₁₂N₄. The C₆H₁₂N₄ can produce NH₄ ⁺ and OH^-, which are the growth factors for forming the crystalline rods 470 comprising zinc oxide, at the same time, can control the reaction rate since the growth rate and the OH^- concentration can be easily controlled.

The growth mechanism of the crystalline rods 470 comprising zinc oxide can be represented by formulas 18 to 24 of the third embodiment. Hexamine (C₆H₁₂N₄) used as the second precipitator can produce NH₄ ⁺ and OH^-. Moreover, Zn(NO₃)₂ used as the second zinc salt can produce zinc ions.

The 4NH₃, 4OH^-, and Zn²⁺ produced by the formulas 18 to 20 of the third embodiment can produce Zn(NH₃)₄ ²⁺ and Zn(OH)₄ ^2-, which are the growth factors of the ZnO crystalline rods 470 by formulas 21 and 22 of the third embodiment.

The growth factor, Zn(NH₃)₄ ²⁺, produced by formula 21 of the third embodiment can produce the ZnO crystalline rods 470 represented by above formula 23 by the reaction with OH^- as a reaction factor, and the growth factor, Zn(OH)₄ ^2-, produced by the above formula 22 can produce the ZnO crystalline rods 470 by the above formula 24.

The zinc oxide has a crystalline structure with a preferential growth in the c-axis direction. Partial polarization of zinc and oxygen occurs in the c-axis direction, i.e., the [0001] direction, but does not occur in the lateral direction. Therefore, the growth of the crystalline rods 470 may be predominant in the c-axis direction, i.e., the [0001] direction, even in a state where there is no particular inhibition to the growth. As a result, the growth in the lateral direction may also occur continuously.

However, when the cationic polymer as the second overgrowth inhibitor is added to the rod growth solution, Zn(OH)₄ ^2-, which is one of the growth factors, is adhered to the cationic polymer such that the Zn(OH)₄ ^2- cannot participate in the growth of the crystalline rods 470 comprising zinc oxide.

If the second overgrowth inhibitor is used, the cationic polymer is adhered to the Zn(OH)₄ ^2- and further caps the anionic O^2- exposed to the side of the already formed ZnO crystal structure, thus inhibiting the growth in the lateral direction. Therefore, the second overgrowth inhibitor suppresses the growth of the crystalline rods 470 comprising zinc oxide in the lateral direction.

Meanwhile, the rod growth solution may have a pH of 9 to 11. If the growth solution has a pH above 11, the ZnO crystalline rods 470 may be damaged by excessive erosion. Therefore, the rod growth solution may have a pH of 10. For this purpose, an alkaline solution such as ammonia water may be added to the rod growth solution.

Especially, when the seed layer 450 is formed by the sol-gel method, the crystalline rods 470 have an orientation substantially perpendicular to the light-emitting structure 410. The reason for this is that the ZnO particles are not attached to the surface of the light-emitting structure 410, but are naturally formed on the surface of the light-emitting structure 410 when entering the sol state.

In order that a particular material is formed on another layer and enters a stable state, it is more advantageous that the material is received on the layer at the bottom, rather than that it is floating in the solution. Especially, in the case of the zinc oxide, the adjacent ZnO particles tend to be agglomerated together in the same orientation. Moreover, the orientation of the (0001) plane on the surface of the lower layer has the most stable property. Therefore, when the seed layer 450 is formed by the sol-gel method, the majority of crystalline rods 470 formed thereafter have the growth orientation perpendicular to the upper surface of the light-emitting structure 410.

Referring to FIG. 29, the growth guiding layer 460 remaining on the light-emitting structure 410 is removed, and thereby a plurality of crystalline rods 470 remain on the light-emitting structure 410.

The ZnO crystalline rods 470 are formed on the light-emitting structure 410 by the above-described process. Each of the crystalline rods 470 functions as a waveguide of the light emitted from the light-emitting structure 410.

FIG. 30 is a graph illustrating EL properties of the light-emitting diode including the ZnO crystalline rod formed in accordance with the fourth embodiment of the present invention.

Referring to FIG. 30, an ITO layer as the current spreading layer is formed on the normal type light-emitting structure. The ZnO crystalline rods are formed on the ITO layer.

Subsequently, the EL properties of a conventional normal type light-emitting diode and those of a conventional normal type light-emitting diode including the crystalline rods of the present invention are compared with each other.

A conventional normal type light-emitting diode is formed on a sapphire substrate. The sapphire substrate includes a pattern having a pitch of 600 nm. The pattern has an approximately circular shape. A GaN layer as a buffer layer is formed on the substrate. The buffer layer is not doped and has a thickness of 3 um. An n-type GaN layer is formed on the buffer layer. Si is used as a dopant to form the n-type GaN layer having a thickness of 2.5 um. Moreover, a light-emitting layer having a multi-quantum well MQW) structure is formed on the n-type GaN layer. The (MQW structure comprises a ternary system such as InGaN. Moreover, the light-emitting layer is configured to have a five-layered structure of barrier layers and well layers. A p-type GaN layer is formed on the light-emitting layer. The p-type GaN layer has a thickness of 0.14 um, and Mg is used as a dopant.

Moreover, the ITO layer as the current spreading layer is provided on the p-type GaN layer and has a thickness of 250 nm. A partial surface of the n-type GaN layer is exposed by etching. The light-emitting structure has a chip size of 300 um x 300 um. The chip described in FIG. 30 of the present embodiment is not yet packaged.

When a current of 5 mA is applied through electrodes formed on the ITO layer, the conventional normal type light-emitting diode has an intensity of 3000 a.u. at a wavelength of about 450 nm, which is shown by the dotted line in FIG. 30.

Crystalline rods are applied to the top of the above-described conventional normal type light-emitting diode.

In detail, a seed layer is formed on the ITO layer to form the crystalline rods. The seed layer is formed by the sol-gel method using zinc acetate as a zinc ion donor and ethanol as a solvent. The solution is heated at 65℃ for 30 minutes to form a first solution in a sol state. Subsequently, a surfactant is used to form a second solution. The second solution is coated on the ITO layer and then heated at 350℃ for 1 hour to be in a gel state, thereby forming the seed layer.

A growth guiding layer is formed on the seed layer. The growth guiding layer has a regular pattern. That is, a photoresist pattern as the growth guiding layer is formed on the seed layer such that the crystalline rods, which will be formed later, have a regular arrangement with respect to adjacent crystalline rods. The pattern has a plurality of circular holes that expose the surface of the seed layer. The pitch between adjacent circular holes is 400 nm and each of the holes has a diameter of 150 nm.

The hydrothermal synthesis method is used for the growth of the crystalline rods. Zinc nitrate is used as the second zinc salt, a zinc ion donor, and HMTA is used as the second precipitator, a hydroxyl group donor. Moreover, polyethyleneimine (PEI) is used as the second overgrowth inhibitor, a cationic polymer.

When the same current is applied, the light-emitting diode including the ZnO crystalline rods has an intensity of 6000 a.u. at a wavelength of about 450 nm as shown by the solid line in FIG. 30.

This means that the EL properties of the light-emitting diode including the ZnO crystalline rods are increased about two times. The reason for this is considered that each of the ZnO crystalline rods functions as a waveguide of the light emitted from the light-emitting structure. The waveguide extends in the [0001] direction of the ZnO crystal and prevents the emitted light from leaking to the outside. Therefore, the unnecessary scattering of the emitted light is minimized, and thus the light can be effectively transmitted to the outside.

Referring to FIG. 31, the seed layer 450 and the crystalline rods 470 are provided on the p-type semiconductor layer 417. The formation of the seed layer 450 and the crystalline rods 470 can be achieved by the method shown in FIGS. 25 to 29. However, there is a difference in that the seed layer 450 and the crystalline rods 470 are provided before the formation of the current spreading layer 419. Therefore, the crystalline rods 470 are provide on the light-emitting structure 410, and the current spreading layer 419 and an electrode 430 are formed on the crystalline rods 470.

Moreover, the current spreading layer 419 covers the crystalline rods 470. The current spreading layer 419 may be formed of any transparent conductive material.

The crystalline rods 470 are arranged in a direction in which the light emitted from the light-emitting layer 415 is extracted. Moreover, the crystalline rods 470 have an orientation perpendicular to the substrate 400, which results from the preferential growth in the [0001] direction.

Moreover, each of the crystalline rods 470 functions as a waveguide of the light emitted from the light-emitting layer 415 to minimize the light loss, thereby achieving excellent light extraction efficiency.

The present embodiment has been described by way of an example where the formation of the crystalline rods is made on the normal type light-emitting structure. Therefore, if the crystalline rods of the present invention are applied to the flip-chip type light-emitting structure, the crystalline rods may be formed on the substrate. Moreover, if the crystalline rods of the present invention are applied to the vertical type light-emitting structure, the crystalline rods may be formed on the n-type semiconductor layer.

Fifth Embodiment: Zinc oxide tree structure

Referring to FIG. 32, a seed layer 520 is formed on a substrate 500.

The substrate 500 may be a substrate used in a process of fabricating a semiconductor process, a solar cell, or a light-emitting diode or may be a layer that constitute an optical device.

The seed layer 520 may be formed by various methods. Moreover, the seed layer 520 may have a regular or irregular orientation. That is, the growth of the seed layer 520 may proceed in a regular or irregular direction.

For example, the seed layer 520 may be formed by preparing a solution by mixing ZnO powder and a surfactant and spin-coating the solution on the substrate 500.

Moreover, if zinc metal is deposited on the substrate 500 and the resulting substrate 500 is immersed in a solution having a pH above 10, the zinc metal is bonded to oxygen contained in the solution and converted into ZnO, which can be used as the seed layer 520.

Besides, the seed layer 520 may be formed by a hydrothermal synthesis method.

A seed growth solution is prepared for the formation of the seed layer 520 by the hydrothermal synthesis method. The seed growth solution is prepared by dissolving a first zinc salt and a first precipitator in a polar solvent. The first zinc salt acts as a zinc ion (Zn²⁺) donor and the first precipitator acts as a hydroxyl group (OH^-) donor.

In the above formulas, Zn²⁺ is supplied from the first zinc salt and OH^- is supplied from the first precipitator. The cation and anion are reacted together to form ZnO or Zn(OH)₂ as an intermediate.

Moreover, the intermediate, Zn(OH)₂, reacts with OH^- to form Zn(OH)₄ ^2- as a ZnO growth factor, which forms ZnO.

A first overgrowth inhibitor may be used to control the size of the ZnO particles. The first overgrowth inhibitor is added to the seed growth solution in which the ZnO particles are formed. The first overgrowth inhibitor may be a cationic polymer. In detail, the cationic polymer may be a polymer having an amine group such as polyethyleneimine (PEI) having high solubility in a polar solvent. The ZnO growth factor, Zn(OH)₄ ^2-, is bonded to a cation of the cationic polymer and does not participate in the growth of zinc oxide. As a result, the size of the ZnO particles can be controlled by the first overgrowth inhibitor.

Subsequently, the thus formed ZnO particles are separated, and the separated ZnO particles are dispersed in a solvent and formed into the seed layer 520 by spin coating.

As mentioned above, the formation of the seed layer 520 may be achieved by various methods. That is, the seed layer 520 may be formed by evaporation, metal organic chemical vapor deposition (MOCVD), sputtering or coating using a brush.

Besides, the seed layer 520 may be formed by depositing or dispersing the ZnO particles.

The seed layer 520 may be formed by a sol-gel method. In detail, a hydrate containing zinc salt is dissolved in a solvent to prepare a first solution. The zinc salt may comprise zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride. Moreover, the solvent may be a polar solvent. For example, the first solution may be prepared using ethanol as the solvent and ZnC₄H₁₀O₆·6H₂O as the hydrate.

The second solution is spin-coated on the substrate 500 and heated to be in a gel state. The heating temperature may be about 200℃ to 1,000℃. If the heating temperature is less than 200℃, by-products other than ZnO contained in the second solution are not completely removed, whereas, if the heating temperature is more than 1,000℃, the crystallinity of the formed seed layer 520 may be damaged. The seed layer 520 is formed by applying heat after spin-coating, and this reaction can be represented by formula 12 of the third embodiment.

The seed layer 520 in the gel state has a preferred orientation in the c-axis direction. That is, the ion species dissolved when entering the sol state are formed into ZnO particles during the heating process after the spin-coating, and the ZnO particles grow in the c-axis direction, which is the intrinsic characteristic of the ZnO crystal structure. In other words, the ZnO crystals have a high growth rate in the [0001] direction and have a low growth rate in the lateral direction. Moreover, partial polarization of ZnO occurs on the (0001) plane, but does not occur in the lateral direction. Therefore, the ZnO particles in the seed layer 520 have crystalline properties that can grow in a direction perpendicular to the substrate 500 during the heating process after the spin-coating.

Moreover, the seed layer 520 may be formed as a layer having a relatively uniform thickness on the substrate 500 and may have a regular pattern.

Referring to FIG. 33, crystalline rods 540 are formed on the seed layer 520. The crystalline rods 540 may grow in a direction perpendicular to the substrate 500 or in a random direction according to the arrangement of the seed layer 520. Moreover, the crystalline rods 540 may be formed by various methods such as evaporation, chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD).

The crystalline rods 540 may have a micro- or nano-size. Moreover, the crystalline rods 540 may be formed into single crystals, but their crystallinity may be damaged by other factors generated during the growth into the crystals. However, it is apparent that the predominant factor in the formation and growth of the crystalline rods 540 is the single crystal growth.

Especially, the crystalline rods 540 may be formed by the hydrothermal synthesis method. That is, the crystalline rods 540 may be formed by immersing the substrate 500 including the seed layer 520 in a rod growth solution prepared in an aqueous solution phase and applying heat energy thereto.

After the substrate 500 including the seed layer 520 is immersed in the rod growth solution, heat energy is applied thereto. The application of heat energy may be performed in the temperature range of 50℃ to 100℃ at atmospheric pressure. If the heating temperature is less than 50℃, the growth of the crystalline rods 540 slows down, which makes it difficult to achieve a substantial growth of the crystalline rods 540, whereas, if the heating temperature is more than 100℃, the crystallinity of the crystalline rods 540 is damaged by an unexpected reaction between ion species in the rod growth solution.

The hydrate containing the second zinc salt may comprise zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride. The second precipitator may comprise NaOH, Na₂CO₃, LiOH, H₂O₂, KOH, hexamethylenetetramine (HMTA), or NH₄OH. Preferably, the second precipitator may be C₆H₁₂N₄. The C₆H₁₂N₄ can produce NH₄ ⁺ and OH^-, which are the growth factors for forming the crystalline rods 540 comprising zinc oxide, at the same time, can control the reaction rate since the growth rate and the OH^- concentration can be easily controlled.

The growth mechanism of the crystalline rods 540 comprising zinc oxide can be represented by formulas 18 to 24 of the third embodiment. Hexamine (C₆H₁₂N₄) used as the second precipitator can produce NH₄ ⁺ and OH^-. Moreover, Zn(NO₃)₂ used as the second zinc salt can produce zinc ions.

The 4NH₃, 4OH^-, and Zn²⁺ produced by the formulas 18 to 20 of the third embodiment can produce Zn(NH₃)₄ ²⁺ and Zn(OH)₄ ^2-, which are the growth factors of the ZnO crystalline rods 540 by formulas 21 and 22 of the third embodiment.

The growth factor, Zn(NH₃)₄ ²⁺, produced by formula 21 of the third embodiment can produce the ZnO crystalline rods 540 represented by above formula 23 by the reaction with OH^- as a reaction factor, and the growth factor, Zn(OH)₄ ^2-, produced by the above formula 22 can produce the ZnO crystalline rods 540 by the above formula 24.

However, when the cationic polymer as the second overgrowth inhibitor is added to the rod growth solution, Zn(OH)₄ ^2-, which is one of the growth factors, is adhered to the cationic polymer such that the Zn(OH)₄ ^2- cannot participate in the growth of the crystalline rods 540 comprising zinc oxide. The Zn(OH)₄ ^2- is known as a factor that allows the ZnO crystals to grow into an urchin-like structure.

Especially, the zinc oxide has a crystalline structure with a preferential growth in the c-axis direction. Partial polarization of zinc and oxygen occurs in the c-axis direction, i.e., the [0001] direction, but does not occur in the lateral direction. Therefore, the growth of the crystalline rods 540 may be predominant in the c-axis direction, i.e., the [0001] direction, even in a state where there is no particular inhibition to the growth. As a result, the growth in the lateral direction may also occur continuously.

If the second overgrowth inhibitor is used, the cationic polymer is adhered to the Zn(OH)₄ ^2- and further caps the anionic O^2- exposed to the side of the already formed ZnO crystal structure, thus inhibiting the growth in the lateral direction. Therefore, the second overgrowth inhibitor suppresses the growth of the crystalline rods 540 comprising zinc oxide in the lateral direction.

Meanwhile, the rod growth solution may have a pH of 9 to 11. If the growth solution has a pH above 11, the ZnO crystalline rods 540 may be damaged by excessive erosion. Therefore, the rod growth solution may have a pH of 10. For this purpose, an alkaline solution such as ammonia water may be added to the rod growth solution.

The excess of OH^- contained in the rod growth solution may erode the ZnO crystalline rods, thereby producing a by-product, Zn(OH)₂, as represented by the following formula 25. As a result, each of the ZnO crystalline rods 540 may have a pointed end like a pencil.

However, the growth reaction of the zinc oxide may continue along with the erosion. The OH^- is consumed as the ZnO crystals grow, and thereby the pH of the rod growth solution may be reduced. As a result, the growth reaction occurs more preferentially than the erosion, which results in the formation of ZnO crystalline rods 540.

Referring to FIG. 34, a branch seed 560 is formed on the side of the crystalline rod 540. The branch seed 560 has the same chemical composition as the ZnO crystalline rod 540. However, the branch seed 560 is formed on the side of the crystalline rod 540.

The substrate 500 including the crystalline rod 540 is immersed in a seed forming solution for the formation of the branch seed 560. The seed forming solution comprises a zinc salt, a cationic polymer, and a solvent.

The zinc salt comprises zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride. There are no particular limitations on the type of cationic polymer, and polyethyleneimine (PEI) may be used. Moreover, the solvent may be a polar solvent.

The zinc salt contained in the solution tends to be adhered to the cationic polymer. That is, the zinc salt is adhered to the cationic polymer and bonded to the chain of the cationic polymer.

For example, the PEI has a secondary amine structure in which an amine group is bonded to two alkyl groups. The nitrogen atom in the secondary amine structure has unshared electron pairs and has a polarity. The zinc salt itself or a zinc atom is adhered or bonded to the PEI by the polarity of the PEI. Moreover, the cationic polymer is adhered to the side of the crystalline rod 540, which results from the ionic bond with O^2- produced on the side of the crystalline rod 540.

Preferably, the ionic bond of the cationic polymer and the adhesion or bond of the zinc/zinc oxide may be performed above the isoelectric point. The isoelectric point of the ZnO crystalline rod 540 represents a particular pH, at which the partial polarization of the ZnO crystal occurs in the solution. That is, the zinc oxide shows the positive polarity Zn²⁺ on the (0001) plane of the crystal and the negative polarity of O^2- on the side above the isoelectric point. The isoelectric point of the ZnO crystalline rods 540 has a pH of 9.7.

Therefore, the pH of the seed forming solution is adjusted to above 9.7.

Subsequently, the substrate 500 is heated at 200℃ to 500℃, and thus the ZnO branch seed 560 is formed on the side of the crystalline rod 540. The heating is performed to remove any polymer or organic material. The zinc salt is precipitated as zinc oxide by the heating, and the ZnO branch seed 560 is formed on the side surface of the crystalline rod 540. The reason that the zinc oxide is precipitated is that the formation on the ZnO crystalline rod 540 of the same material is most stable.

Referring to FIG. 35, the ZnO branch seeds 560 grow from the side of the crystalline rod 540 into a tree structure. That is, crystalline branches 580 are formed on the side of the crystalline rod 540, and the crystalline branches 580 grow from the branch seeds 560.

The crystalline branches 580 may be formed by various methods. Especially, the crystalline branches 580 may be formed by the hydrothermal synthesis method, which does not cause thermal damage to the substrate 500.

A branch growth solution is prepared to employ the hydrothermal synthesis method.

The branch growth solution contains a third zinc salt and a third precipitator. The third zinc salt acts as a zinc ion donor and the third precipitator acts as a hydroxyl group donor.

In detail, to form the crystalline branches 580 in the branch growth solution, the substrate 500 is immersed in the rod growth solution and heat energy is applied thereto. The application of heat energy may be performed in the temperature range of 50℃ to 100℃ at atmospheric pressure. If the heating temperature is less than 50℃, the growth of the crystalline branches 580 slows down, which makes it difficult to achieve a substantial growth of the nano-sized crystalline branches 580, whereas, if the heating temperature is more than 100℃, the crystallinity of the crystalline branches 580 is damaged by an unexpected reaction between ion species in the branch growth solution.

The third zinc salt may comprise zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride. The third precipitator may comprise NaOH, Na₂CO₃, LiOH, H₂O₂, KOH, hexamethylenetetramine (HMTA), or NH₄OH. Preferably, the third precipitator may be C₆H₁₂N₄. The C₆H₁₂N₄ can produce NH₄ ⁺ and OH^-, which are the growth factors for forming the crystalline branches 580 comprising zinc oxide, at the same time, can control the reaction rate since the growth rate and the OH^- concentration can be easily controlled.

The growth mechanism of the crystalline branches 580 comprising zinc oxide can be represented by formulas 18 to 24 of the third embodiment. That is, the crystalline branches 580 grow from the branch seeds 560 formed on the side of the crystalline rod 540.

The branch growth solution may further contain a third overgrowth inhibitor. The third overgrowth inhibitor may be a cationic polymer. When the third overgrowth inhibitor is added to the branch growth solution, Zn(OH)₄ ^2-, which is one of the growth factors, is adhered to the cationic polymer such that the Zn(OH)₄ ^2- cannot participate in the growth of the crystalline branches 580 comprising zinc oxide.

Especially, the zinc oxide has a crystalline structure with a preferential growth in the c-axis direction. Partial polarization of zinc and oxygen occurs in the c-axis direction, i.e., the [0001] direction, but does not occur in the lateral direction. Therefore, the growth of the crystalline branches 580 may be predominant in the c-axis direction, i.e., the [0001] direction, even in a state where there is no particular inhibition to the growth. As a result, the growth in the lateral direction may also occur continuously.

If the third overgrowth inhibitor is used, the cationic polymer is adhered to the Zn(OH)₄ ^2- and further caps the anionic O^2- exposed to the side of the already formed ZnO crystal structure, thus inhibiting the growth in the lateral direction. Therefore, the third overgrowth inhibitor suppresses the growth of the crystalline branches 580 comprising zinc oxide in a direction other than the [0001] direction.

In the present embodiment, an additional process is required to allow the crystalline rod 540 to have a regular arrangement with respect to adjacent crystalline rods 540. That is, a regular pattern, which can induce the growth of the crystalline rods 540, is provided on the seed layer 520 such that the crystalline rods 540 can grow only from the surface exposed by the pattern. In the case where the pattern is provided, a partial surface of the seed layer 520 covered by the pattern prevents the crystalline rods 540 from growing, and thus the crystalline rods 540 can grow only from the surface exposed by the pattern. The pattern may be a photoresist pattern which can be formed by typical photolithography, nanoimprint lithography, or laser interference lithography. Preferably, the formation of the pattern may be performed after the formation of the seed layer 520 and before the growth of the crystalline rods 540.

Moreover, the crystalline rods 540 may be arranged in a direction perpendicular to the substrate 500 or in a random direction from the substrate 500.

A zinc oxide tree structure is formed on the substrate 500 by the above-described process.

FIG. 36 is an SEM image showing the zinc oxide tree structures formed by the above-described process in accordance with the fifth embodiment of the present invention.

Referring to FIG. 36, the crystalline branches are formed on the side of each crystalline rod, which has grown in a direction perpendicularly to the substrate.

In the zinc oxide tree structure shown in FIG. 36, the seed layer is formed by the sol-gel method using zinc acetate as a zinc salt and ethanol as a solvent. The solution is heated at 65℃ for 30 minutes to form a first solution in a sol state. Subsequently, polyethylene glycol (PEG) as a surfactant is used to form a second solution. The second solution is coated on a sapphire substrate and then heated at 350℃ for 1 hour to be in a gel state, thereby forming the seed layer.

A regular pattern including a plurality of circular holes is formed on the substrate including the zinc oxide seed layer by photolithography. The pitch between adjacent circular holes is 600 nm and each of the holes has a diameter of 150 nm.

Subsequently, the growth of the crystalline rods is made on the substrate including the seed layer. The hydrothermal synthesis method is used for the growth of the crystalline rods. Zinc nitrate is used as the zinc salt, and hexamethylenetetramine (HMTA) is used as the precipitator. Moreover, polyethyleneimine (PEI) is used as the overgrowth inhibitor, a cationic polymer.

The branch seeds are formed on the side of each crystalline rod using a seed forming solution containing zinc acetate as a zinc salt and PEI as a cationic polymer. The seed forming solution has a pH of 10. The substrate is immersed in the seed forming solution and heated at 90℃ for 40 minutes, and thereby the branch seeds are precipitated on the side of the crystalline rod.

Zinc nitrate is used as the zinc salt and HMTA is used as the precipitator such that the crystalline branches grow from the branch seeds.

The zinc oxide tree structure formed by the above-described process can be applied to various optical devices. For example, the zinc oxide tree structure having a nano-size may function as an optical waveguide. Moreover, it can be used as a charge carrier in a solar cell.

Sixth Embodiment: Light-emitting diode including zinc oxide tree structure

Referring to FIG. 37, a light-emitting structure 610 is provided on a substrate 600.

The light-emitting structure 610 may comprise a group III nitride or a group II oxide. A zinc oxide tree structure can be applied regardless of the type and shape of the light-emitting structure 610.

Therefore, the light-emitting structure 610 may be provided in various types such as a normal type, a flip-chip type, and a vertical type.

The flip-chip type light-emitting structure has substantially the same structure as the normal type. However, the flip-chip type light-emitting structure has a different structure in which the light emitted from a light-emitting layer propagates toward the substrate. Therefore, if the flip-chip type light-emitting structure is mounted on a printed circuit board, for example, two electrodes are bonded to the printed circuit board without any wire bonding.

For the convenience of description and better understanding of the present invention, the normal type light-emitting structure will be hereinafter exemplarily used as the light-emitting structure in this embodiment shown in FIG. 37. Therefore, the light-emitting structure 610 has a structure in which an n-type semiconductor layer 613, a light-emitting layer 615, and a p-type semiconductor layer 617 are sequentially formed on the substrate 600. Moreover, an electrode 630 is provided on the p-type semiconductor layer 617. A current spreading layer 619 as a transparent conductor may be further interposed between the p-type semiconductor layer 617 and the electrode 630. A buffer layer 611 may be further interposed between the substrate 600 and the n-type semiconductor layer 613 to reduce the lattice mismatch.

That is, the light-emitting structure 610 may be designed differently according to its type in this embodiment. However, the zinc oxide tree structure 615 is arranged in a direction in which the light emitted from the light-emitting layer 615 is extracted. Particularly, the zinc oxide tree structure is not interposed between the semiconductor layers 613 and 617 and the light-emitting layer 615 but is provided on the semiconductor layers 613 and 617 or the light-emitting layer 615 according to the light extraction direction. In the case of the flip-chip type light-emitting structure, the zinc oxide tree structure may be formed on the substrate.

Referring to FIG. 38, a seed layer 650 is formed on the light-emitting structure 610. The seed layer 650 may be formed by various methods.

Moreover, the seed layer 650 may have a regular or irregular orientation. That is, the growth of the seed layer 650 may proceed in a regular or irregular direction.

For example, the seed layer 650 may be formed by preparing a solution by mixing ZnO powder and a surfactant and spin-coating the solution on the light-emitting structure 610.

Moreover, if zinc metal is deposited on the light-emitting structure 610 and the resulting light-emitting structure 610 is immersed in a solution having a pH above 10, the zinc metal is bonded to oxygen contained in the solution and converted into ZnO, which can be used as the seed layer 650.

Besides, the seed layer 650 may be formed by a hydrothermal synthesis method.

A seed growth solution is prepared for the formation of the seed layer 650 by the hydrothermal synthesis method. The seed growth solution is prepared by dissolving a first zinc salt and a first precipitator in a polar solvent. The first zinc salt acts as a zinc ion (Zn²⁺) donor and the first precipitator acts as a hydroxyl group (OH^-) donor.

Subsequently, the thus formed ZnO particles are separated, and the separated ZnO particles are dispersed in a solvent and formed into the seed layer 650 by spin coating.

As mentioned above, the formation of the seed layer 650 may be achieved by various methods. That is, the seed layer 650 may be formed by evaporation, metal organic chemical vapor deposition (MOCVD), sputtering or coating using a brush.

Besides, the seed layer 650 may be formed by depositing or dispersing the ZnO particles.

The seed layer 650 may be formed by a sol-gel method. In detail, a hydrate containing zinc salt is dissolved in a solvent to prepare a first solution. The zinc salt may comprise zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride. Moreover, the solvent may be a polar solvent. For example, the first solution may be prepared using ethanol as the solvent and ZnC₄H₁₀O₆·6H₂O as the hydrate.

The second solution is spin-coated on the light-emitting structure 610 on the substrate 600 and heated to be in a gel state. The heating temperature may be about 200℃ to 1,000℃. If the heating temperature is less than 200℃, by-products other than ZnO contained in the second solution are not completely removed, whereas, if the heating temperature is more than 1,000℃, the crystallinity of the formed seed layer 650 may be damaged. The seed layer 650 is formed by applying heat after spin-coating, and this reaction can be represented by formula 12 of the third embodiment.

The seed layer 650 in the gel state has a preferred orientation in the c-axis direction. That is, the ion species dissolved when entering the sol state are formed into ZnO particles during the heating process after the spin-coating, and the ZnO particles grow in the c-axis direction, which is the intrinsic characteristic of the ZnO crystal structure. In other words, the ZnO crystals have a high growth rate in the [0001] direction and have a low growth rate in the lateral direction. Moreover, partial polarization of ZnO occurs on the (0001) plane, but does not occur in the lateral direction. Therefore, the ZnO particles in the seed layer 650 have crystalline properties that can grow in a direction perpendicular to the lower substrate 600 during the heating process after the spin-coating.

Moreover, the seed layer 650 may be formed as a layer having a relatively uniform thickness on the light-emitting structure 610 and may have a regular pattern.

Especially, if the electrode 630 is formed on the light-emitting structure 610, the seed layer 650 may be formed in a region other than the electrode 630 (e.g., on the p-type semiconductor layer 617 or the current spreading layer 619). It is necessary to cover the electrode 630 from the outside to form the seed layer 650 in a region other than the electrode 630. For this purpose, the region where the electrode 630 is formed may be covered with a photoresist pattern, which may be formed by forming a photoresist layer on the seed layer 650 and then patterning the photoresist layer by photolithography.

Referring to FIG. 39, crystalline rods 670 are formed on the seed layer 650. The crystalline rods 670 comprise zinc oxide. The major growth factor of the crystalline rods 670 results from the growth of the single crystal based on the seed layer 650, and the growth in the [0001] direction occurs predominantly. The crystalline rods 670 may have a regular arrangement with respect to adjacent crystalline rods 670 and may grow only from the seed layer 650 exposed by the pattern.

Moreover, the crystalline rods 670 may grow in a direction perpendicular to the upper plane formed by the light-emitting structure 610 or in a random direction from the upper plane formed by the light-emitting structure 610. The direction that the crystalline rods 670 grow is based on the orientation of the ZnO particles that constitute the seed layer 650.

The crystalline rods 670 may be formed by various methods such as evaporation, chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD).

The crystalline rods 670 may have a micro- or nano-size. Moreover, the crystalline rods 670 may be formed into single crystals, but their crystallinity may be damaged by other factors generated during the growth into the crystals. However, it is apparent that the predominant factor in the formation and growth of the crystalline rods 670 is the single crystal growth.

Especially, the crystalline rods 670 may be formed by the hydrothermal synthesis method. That is, the crystalline rods 670 may be formed by immersing the substrate 600 including the seed layer 650 in a rod growth solution prepared in an aqueous solution phase and applying heat energy thereto.

After the substrate 600 including the seed layer 650 is immersed in the rod growth solution, heat energy is applied thereto. The application of heat energy may be performed in the temperature range of 50℃ to 100℃ at atmospheric pressure. If the heating temperature is less than 50℃, the growth of the crystalline rods 670 slows down, which makes it difficult to achieve a substantial growth of the crystalline rods 670, whereas, if the heating temperature is more than 100℃, the crystallinity of the crystalline rods 670 is damaged by an unexpected reaction between ion species in the rod growth solution.

The hydrate containing the second zinc salt may comprise zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride. The second precipitator may comprise NaOH, Na₂CO₃, LiOH, H₂O₂, KOH, hexamethylenetetramine (HMTA), or NH₄OH. Preferably, the second precipitator may be C₆H₁₂N₄. The C₆H₁₂N₄ can produce NH₄ ⁺ and OH^-, which are the growth factors for forming the crystalline rods 670 comprising zinc oxide, at the same time, can control the reaction rate since the growth rate and the OH^- concentration can be easily controlled.

The growth mechanism of the crystalline rods 670 comprising zinc oxide can be represented by formulas 18 to 24 of the third embodiment. Hexamine (C₆H₁₂N₄) used as the second precipitator can produce NH₄ ⁺ and OH^-. Moreover, Zn(NO₃)₂ used as the second zinc salt can produce zinc ions.

The 4NH₃, 4OH^-, and Zn²⁺ produced by the formulas 18 to 20 of the third embodiment can produce Zn(NH₃)₄ ²⁺ and Zn(OH)₄ ^2-, which are the growth factors of the ZnO crystalline rods 670 by formulas 21 and 22 of the third embodiment.

The growth factor, Zn(NH₃)₄ ²⁺, produced by formula 21 of the third embodiment can produce the ZnO crystalline rods 670 represented by above formula 23 by the reaction with OH^- as a reaction factor, and the growth factor, Zn(OH)₄ ^2-, produced by the above formula 22 can produce the ZnO crystalline rods 670 by the above formula 24.

The zinc oxide has a crystalline structure with a preferential growth in the c-axis direction. Partial polarization of zinc and oxygen occurs in the c-axis direction, i.e., the [0001] direction, but does not occur in the lateral direction. Therefore, the growth of the crystalline rods 670 may be predominant in the c-axis direction, i.e., the [0001] direction, even in a state where there is no particular inhibition to the growth. As a result, the growth in the lateral direction may also occur continuously.

However, when the cationic polymer as the second overgrowth inhibitor is added to the rod growth solution, Zn(OH)₄ ^2-, which is one of the growth factors, is adhered to the cationic polymer such that the Zn(OH)₄ ^2- cannot participate in the growth of the crystalline rods 670 comprising zinc oxide.

If the second overgrowth inhibitor is used, the cationic polymer is adhered to the Zn(OH)₄ ^2- and further caps the anionic O^2- exposed to the side of the already formed ZnO crystal structure, thus suppressing the growth in the lateral direction. Therefore, the second overgrowth inhibitor suppresses the growth of the crystalline rods 670 comprising zinc oxide in the lateral direction.

Meanwhile, the rod growth solution may have a pH of 9 to 11. If the growth solution has a pH above 11, the ZnO crystalline rods 670 may be damaged by excessive erosion. Therefore, the rod growth solution may have a pH of 10. For this purpose, an alkaline solution such as ammonia water may be added to the rod growth solution.

The crystalline rods 670 may be grown with a regular arrangement with respect to adjacent crystalline rods 670. A growth guiding layer (not shown) may be provided on the seed layer 650 to allow the crystalline rods 670 to have the regular arrangement. That is, the growth guiding layer having a predetermined pattern can partially expose the seed layer 650. The growth guiding layer may be a photoresist pattern. The crystalline rods 670 can grow only from a surface of the seed layer 650 exposed by the growth guiding layer. If the growth guiding layer having the regular pattern is provided, the crystalline rods 670 can also grow with a regular arrangement corresponding to the pattern of the growth guiding layer.

Especially, when the seed layer 650 is formed by the sol-gel method, the crystalline rods 670 have an orientation substantially perpendicular to the light-emitting structure 610. The reason for this is that the ZnO particles are not attached to the surface of the light-emitting structure 610, but are naturally formed on the surface of the light-emitting structure 610 when entering the sol state.

In order that a particular material is formed on another layer and enters a stable state, it is more advantageous that the material is received on the layer at the bottom, rather than that it is floating in the solution. Especially, in the case of the zinc oxide, the adjacent ZnO particles tend to be agglomerated together in the same orientation. Moreover, the orientation of the (0001) plane on the surface of the lower layer has the most stable property. Therefore, when the seed layer 650 is formed by the sol-gel method, the majority of crystalline rods 670 formed thereafter have the growth orientation perpendicular to the upper surface of the light-emitting structure 610.

Referring to FIG. 40, crystalline branches 680 are formed on the side of each crystalline rod 670.

The crystalline branches 680 are formed by the formation of branch seeds and the growth of crystalline branches based on the formed branch seeds. That is, the branch seeds as the growth factor of the zinc oxide are formed on the side of each crystalline rod 670, and the crystalline branches 680 grow from the branch seeds using a branch growth solution. Especially, the branch seeds are precipitated predominantly on the side of the crystalline rod 670. Therefore, the branch seeds are formed on the side of the crystalline rod 670 in a random distribution, not in a regular arrangement.

Each of the crystalline rods 670 and the crystalline branches 680 forms a waveguide of the light emitted from the light-emitting structure 610.

The substrate 600 including the crystalline rods 670 is immersed in a seed forming solution for the formation of the branch seeds. The seed forming solution comprises a zinc salt, a cationic polymer, and a solvent.

For example, the PEI has a secondary amine structure in which an amine group is bonded to two alkyl groups. The nitrogen atom in the secondary amine structure has unshared electron pairs and has a polarity. The zinc salt itself or a zinc atom is adhered or bonded to the PEI by the polarity of the PEI. Moreover, the cationic polymer is adhered to the side of the crystalline rod, which results from the ionic bond with O^2- produced on the side of the crystalline rod.

Preferably, the ionic bond of the cationic polymer and the adhesion or bond of the zinc/zinc oxide may be performed above the isoelectric point. The isoelectric point of the ZnO crystalline rod 670 represents a particular pH, at which the partial polarization of the ZnO crystal occurs in the solution. That is, the zinc oxide shows the positive polarity Zn²⁺ on the (0001) plane of the crystal and the negative polarity of O^2- on the side above the isoelectric point. The isoelectric point of the ZnO crystalline rods 670 has a pH of 9.7.

Therefore, the pH of the seed forming solution is adjusted to above 9.7.

Subsequently, the substrate 600 is heated at 200℃ to 500℃, and thus the ZnO branch seed is formed on the side of the crystalline rod 670. The heating is performed to remove any polymer or organic material. The zinc salt is precipitated as zinc oxide by the heating, and the ZnO branch seed is formed on the side surface of the crystalline rod 670. The reason that the zinc oxide is precipitated is that the formation on the ZnO crystalline rod 670 of the same material is most stable.

Next, the crystalline branches 680 are formed from the branch seeds, and thereby a zinc oxide tree structure is formed on the light-emitting structure 610.

The crystalline branches 680 may be formed by various methods. Especially, the crystalline branches 680 may be formed by the hydrothermal synthesis method, which does not cause thermal damage to the substrate 600.

In detail, to form the crystalline branches 680 in the branch growth solution, the substrate 600 is immersed in the rod growth solution and heat energy is applied thereto. The application of heat energy may be performed in the temperature range of 50℃ to 100℃ at atmospheric pressure. If the heating temperature is less than 50℃, the growth of the crystalline branches 680 slows down, which makes it difficult to achieve a substantial growth of the nano-sized crystalline branches 680, whereas, if the heating temperature is more than 100℃, the crystallinity of the crystalline branches 680 is damaged by an unexpected reaction between ion species in the branch growth solution.

The third zinc salt may comprise zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride. The third precipitator may comprise NaOH, Na₂CO₃, LiOH, H₂O₂, KOH, hexamethylenetetramine (HMTA), or NH₄OH. Preferably, the third precipitator may be C₆H₁₂N₄. The C₆H₁₂N₄ can produce NH₄ ⁺ and OH^-, which are the growth factors for forming the crystalline branches 680 comprising zinc oxide, at the same time, can control the reaction rate since the growth rate and the OH^- concentration can be easily controlled.

The growth mechanism of the crystalline branches 680 comprising zinc oxide can be represented by formulas 18 to 24 of the third embodiment. That is, the crystalline branches 680 grow from the branch seeds 560 formed on the side of the crystalline rod 670.

The branch growth solution may further contain a third overgrowth inhibitor. The third overgrowth inhibitor may be a cationic polymer. When the third overgrowth inhibitor is added to the branch growth solution, Zn(OH)₄ ^2-, which is one of the growth factors, is adhered to the cationic polymer such that the Zn(OH)₄ ^2- cannot participate in the growth of the crystalline branches 680 comprising zinc oxide.

Especially, the zinc oxide has a crystalline structure with a preferential growth in the c-axis direction. Partial polarization of zinc and oxygen occurs in the c-axis direction, i.e., the [0001] direction, but does not occur in the lateral direction. Therefore, the growth of the crystalline branches 680 may be predominant in the c-axis direction, i.e., the [0001] direction, even in a state where there is no particular inhibition to the growth. As a result, the growth in the lateral direction may also occur continuously.

If the third overgrowth inhibitor is used, the cationic polymer is adhered to the Zn(OH)₄ ^2- and further caps the anionic O^2- exposed to the side of the already formed ZnO crystal structure, thus suppressing the growth in the lateral direction. Therefore, the third overgrowth inhibitor suppresses the growth of the crystalline branches 680 comprising zinc oxide in a direction other than the [0001] direction.

A zinc oxide tree structure is formed on the light-emitting structure 610 by the above-described process. The tree structure functions as a waveguide of the light emitted from the light-emitting structure 610.

Although the light-emitting structure descried with reference to FIGS. 37 to 40 is the normal type, the zinc oxide tree structure according to the present invention can be applied to the vertical type and the flip-chip type. Therefore, in the case of the vertical type light-emitting structure, the zinc oxide tree structure may be formed on the n-type semiconductor layer or a corresponding layer. In the case of the flip-chip type light-emitting structure, the zinc oxide tree structure may be formed on the substrate from which the light is extracted.

Moreover, although the zinc oxide tree structure is formed on the current spreading layer in the normal type light-emitting structure, the tree structure may be first formed on the p-type semiconductor layer and the current spreading layer for covering the tree structure may then be formed. When the current spreading layer covers the tree structure, the electrode is formed on the current spreading layer.

Referring to FIG. 41, an ITO layer as the current spreading layer is formed on the normal type light-emitting structure. The zinc oxide tree structure is formed on the ITO layer.

Subsequently, the EL properties of a conventional normal type light-emitting diode and those of a conventional normal type light-emitting diode including the zinc oxide tree structure of the present invention are compared with each other.

A conventional normal type light-emitting diode is formed on a sapphire substrate. The sapphire substrate includes a pattern having a pitch of 600 nm. The pattern has an approximately circular shape. A GaN layer as a buffer layer is formed on the substrate. The buffer layer is not doped and has a thickness of 3 um. An n-type GaN layer is formed on the buffer layer. Si is used as a dopant to form the n-type GaN layer having a thickness of 2.5 um. Moreover, a light-emitting layer having a multi-quantum well (MQW) structure is formed on the n-type GaN layer. The MQW structure comprises a ternary system such as InGaN. Moreover, the light-emitting layer is configured to have a five-layered structure of barrier layers and well layers. A p-type GaN layer is formed on the light-emitting layer. The p-type GaN layer has a thickness of 0.14 um, and Mg is used as a dopant.

When a current of 20 mA is applied through electrodes formed on the ITO layer, the conventional normal type light-emitting diode has an intensity of 15000 a.u. at a wavelength of about 450 nm, which is shown by the blue solid line in FIG. 41.

A zinc oxide tree structure is applied to the top of the above-described conventional normal type light-emitting diode.

Crystalline rods of zinc oxide are formed on the seed layer. The crystalline rods grow after a photoresist pattern is provided. That is, the photoresist pattern is provided on the seed layer such that the crystalline rods have a regular arrangement with respect to adjacent crystalline rods. The pattern has a plurality of circular holes that expose the surface of the seed layer. The pitch between adjacent circular holes is 400 nm and each of the holes has a diameter of 150 nm.

Then, branch seeds are formed on the side of each crystalline rod using a seed forming solution containing zinc acetate as a zinc salt and PEI as a cationic polymer. The seed forming solution has a pH of 10. The substrate is immersed in the seed forming solution and heated at 90℃ for 40 minutes, and thereby the branch seeds are precipitated on the side of the crystalline rod.

Next, crystalline rods grow from the branch seeds to form the zinc oxide tree structure using a branch growth solution comprising zinc nitrate used as the zinc salt and HMTA as the precipitator.

When the same current is applied, the light-emitting diode including the zinc oxide tree structure has an intensity of 37500 a.u. at a wavelength of about 450 nm as shown by the red solid line in FIG. 41.

This means that the EL properties of the light-emitting diode including the zinc oxide tree structure are increased about 2.5 times. The reason for this is considered that the zinc oxide trees structure functions as a waveguide of the light emitted from the light-emitting structure. The waveguide extends in the [0001] direction of the ZnO crystal and prevents the emitted light from leaking to the outside. Therefore, the unnecessary scattering of the emitted light is minimized, and thus the light can be effectively transmitted to the outside.

The invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

The hexagonal crystal structure including the ZnO hemisphere according to the present invention can be applied to a light-emitting diode, a solar cell, etc. Especially, when the hexagonal crystal structure including the ZnO hemisphere is applied to the light-emitting diode, it is possible to efficiently extract the light emitted from a light-emitting layer to the outside. Moreover, since the hemisphere has a lens shape, it can be used as an optical device such as a micro-lens without particular limitations.

Claims

A hexagonal crystal structure comprising:

a seed layer formed on a substrate; and

a hemisphere formed on the seed layer and having a (0001) plane as a main surface.
The hexagonal crystal structure of claim 1, wherein the hemisphere comprises ZnO, ZnSe, ZnS, or CdSe.
The hexagonal crystal structure of claim 1, wherein the hemisphere comprises a plurality of crystalline rods formed radially from the center, and a grain boundary due to mismatch of crystals is formed between adjacent crystalline rods.
The hexagonal crystal structure of claim 3, wherein the hemisphere comprises ZnO and is formed above the isoelectric point of the crystalline rods.
The hexagonal crystal structure of claim 4, wherein the crystalline rods are formed in a fan shape from the center of the hemisphere to the surface of the hemisphere.
The hexagonal crystal structure of claim 3, wherein the hemisphere comprising ZnO is formed by lateral growth of an urchin-like structure.
A method of fabricating a hexagonal crystal structure, the method comprising:

forming a seed layer on a substrate;

forming an urchin-like structure of hexagonal crystals on the seed layer; and

forming a hemisphere by lateral growth of the urchin-like structure.
The method of claim 7, wherein the hemisphere comprises ZnO, ZnSe, ZnS, or CdSe.
The method of claim 7, wherein the formation of the hemisphere comprises:

treating the substrate with a vertical growth inhibitor; and

immersing the substrate treated with the vertical growth inhibitor in a growth solution containing a central metal ion donor and a coordinate covalent bond ion donor.
The method of claim 7, wherein the formation of the hemisphere comprises immersing the substrate in a growth solution containing a central metal ion donor, a coordinate covalent bond ion donor, and a vertical growth inhibitor to suppress vertical growth of the crystalline rods of the urchin-like structure and to promote lateral growth.
The method of claim 9 or 10, wherein, if the hemisphere comprises ZnO, the central metal ion donor comprises zinc nitrate, zinc sulfate, or zinc chloride, the coordinate covalent bond ion donor comprises Na₂CO₃, LiOH, H₂O₂, NH₄OH, NaOH, HMTA(C₆H₁₂N₄), or KOH, and the vertical growth inhibitor comprises an anionic polymer, a citrate anion, or an R-O anion, where R is an alkyl group and O is an oxygen atom.
The method of claim 7, wherein the formation of the hemisphere is performed above the isoelectric point of the crystalline rods of the urchin-like structure.
A light-emitting diode comprising:

a light-emitting structure for emitting light; and

a hemispherical structure formed of zinc oxide and arranged in a direction in which the emitted light is extracted.
The light-emitting diode of claim 13, wherein the light-emitting structure comprises:

an n-type semiconductor layer formed on a substrate;

a light-emitting layer formed on the n-type semiconductor layer; and

a p-type semiconductor layer formed on the light-emitting diode.
The light-emitting diode of claim 14, wherein the light-emitting structure further comprises a current spreading layer formed on the p-type semiconductor layer.
The light-emitting diode of claim 13, further comprising a current spreading layer, which fills the hemispherical structure.
The light-emitting diode of claim 13, wherein the hemispherical structure is formed on the substrate.
The light-emitting diode of claim 13, wherein the light-emitting structure comprises:

a p-type semiconductor layer formed on a substrate;

a light-emitting layer formed on the p-type semiconductor layer; and

an n-type semiconductor layer formed on the light-emitting diode.
The light-emitting diode of claim 13, wherein the hemispherical structure has a (0001) plane as a main surface.
The light-emitting diode of claim 13, wherein the hemispherical structure comprises a plurality of ZnO rods formed radially from the center.
The light-emitting diode of claim 20, wherein each of the ZnO rods forms an optical waveguide.
A light-emitting diode comprising a hemispherical structure, which comprises a plurality of ZnO rods and is arranged in a direction in which light emitted from a light-emitting structure is extracted.
The light-emitting diode of claim 22, wherein the ZnO rods grow radially from the center of the hemispherical structure.
The light-emitting diode of claim 22, wherein the hemispherical structure has a (0001) plane as a main surface.
A method of fabricating a light-emitting diode, the method comprising:

forming a seed layer on a light-emitting structure for emitting light;

forming a urchin-like structure of ZnO rods on the seed layer; and

forming a hemispherical structure from the urchin-like structure of ZnO rods.
The method of claim 25, wherein the formation of the hemispherical structure comprises immersing the light-emitting structure in a second growth solution to suppress vertical growth of the ZnO rods of the urchin-like structure and to promote lateral growth.
The method of claim 26, wherein the second growth solution comprises a second zinc ion donor, a second hydroxyl group donor, and a vertical growth inhibitor.
The method of claim 27, wherein the second zinc ion donor comprises zinc nitrate, zinc sulfate, or zinc chloride.
The method of claim 27, wherein the second hydroxyl group donor comprises Na₂CO₃, LiOH, H₂O₂, NH₄OH, NaOH, HMTA(C₆H₁₂N₄), or KOH.
The method of claim 25, wherein the vertical growth inhibitor comprises an anionic polymer, a citrate anion, or an R-O anion, where R is an alkyl group and O is an oxygen atom.
The method of claim 25, wherein the formation of the hemispherical structure is performed above the isoelectric point of the ZnO rods of the urchin-like structure.
The method of claim 25, wherein the formation of the hemispherical structure comprises:

treating the urchin-like structure with a vertical growth inhibitor; and

immersing the urchin-like structure treated with the vertical growth inhibitor in a second growth solution containing a second zinc ion donor and a second precipitator.
A hexagonal crystal structure comprising:

a seed layer formed on a seed layer; and

a crystalline rod formed on the seed layer, formed of zinc oxide, and oriented perpendicularly to the substrate.
The hexagonal crystal structure of claim 33, wherein the seed layer is formed by a sol-gel method.
The hexagonal crystal structure of claim 34, wherein the seed layer is formed by heating a first solution, prepared by dissolving a hydrate containing zinc salt, in a solvent to be in a sol state and heating a second solution, prepared by adding a surfactant to the first solution in the sol state, to be in a gel state.
The hexagonal crystal structure of claim 33, wherein the crystalline rod has a (0001) plane as a growth end surface.
The hexagonal crystal structure of claim 33, wherein the crystalline rod has a predetermined distance spaced from an adjacent crystalline rod.
A hexagonal crystal structure comprising:

a seed layer formed on a substrate; and

a crystalline rod formed on the seed layer by erosion of a zinc oxide crystal.
The hexagonal crystal structure of claim 38, wherein the crystalline rod has a pencil shape.
The hexagonal crystal structure of claim 39, wherein the pencil-like crystalline rod is formed by erosion of a crystalline rod having a flat end in an erosion environment at a pH of 9.7 to 10.2.
The hexagonal crystal structure of claim 38, wherein the crystalline rod has a needle shape.
The hexagonal crystal structure of claim 41, wherein the needle-like crystalline rod is formed by erosion of a crystalline rod having a flat end in an erosion environment at a pH of 11 to 12.4.
The hexagonal crystal structure of claim 38, wherein the crystalline rod has a tube shape.
The hexagonal crystal structure of claim 43, wherein the tube-like crystalline rod is formed by erosion of a crystalline rod having a flat end and vertical erosion of the crystalline rod having a flat end occurs more preferentially than horizontal erosion.
The hexagonal crystal structure of claim 43, wherein the tube-like crystalline rod is formed by salt dissolution in an erosion environment at a pH of 6 to 8.
A method of fabricating a hexagonal crystal structure, the method comprising:

forming a seed layer on a lower substrate;

forming a growth guiding layer on the seed layer; and

forming a crystalline rod of zinc oxide oriented perpendicularly to the lower substrate on the seed layer exposed by the growth guiding layer.
The method of claim 46, wherein the formation of the seed layer comprises:

preparing a first solution by dissolving a hydrate containing zinc salt in a solvent;

heating the first solution to be in a sol state;

preparing a second solution by adding a surfactant to the first solution in the sol state; and

coating the second solution on the lower substrate and heating the resulting solution to be in a gel state.
The method of claim 47, wherein the zinc salt comprises zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride.
The method of claim 46, wherein the formation of the crystalline rod comprises:

preparing a rod growth solution containing a hydrate containing a second zinc salt and a second precipitator; and

immersing the lower substrate including the seed layer in the rod growth solution and applying heat energy thereto.
The method of claim 49, wherein the second zinc salt comprises zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride, and the second precipitator comprises Na₂CO₃, LiOH, H₂O₂, NH₄OH, NaOH, HMTA(C₆H₁₂N₄), or KOH.
The method of claim 49, wherein the rod growth solution further comprises a second overgrowth inhibitor containing a cationic polymer.
The method of claim 51, wherein the rod growth solution is adjusted to have a pH of 9.7 to 10.2 by controlling the concentration of the second overgrowth inhibitor, and erosion occurs more preferentially than the growth of a zinc oxide crystal.
The method of claim 51, wherein the rod growth solution is adjusted to have a pH of 11 to 12.4 by controlling the concentration of the second overgrowth inhibitor, and erosion occurs more preferentially than the growth of a zinc oxide crystal.
The method of claim 46, further comprising, before the growth of the crystalline rod, adding an oxygen ligand to the seed layer.
The method of claim 46, further comprising, after the growth of the crystalline rod, applying an erosion solution having a pH above the isoelectric point to the crystalline rod to cause erosion of the crystalline rod.
The method of claim 46, further comprising, after the growth of the crystalline rod, applying an erosion solution having a pH of 6 to 9 to the crystalline rod to cause erosion of the crystalline rod such that the center of the crystalline rod becomes hollow by salt dissolution.
A light-emitting diode comprising:

a light-emitting structure for emitting light; and

a crystalline rod formed of zinc oxide and arranged in a direction in which the light is emitted from the light-emitting structure.
The light-emitting diode of claim 57, wherein the light-emitting structure comprises a current spreading layer and the crystalline rod is formed on the current spreading layer.
The light-emitting diode of claim 58, wherein the current spreading layer comprises an electrode formed thereon and the crystalline rod is formed on the current spreading layer other than the electrode.
The light-emitting diode of claim 57, wherein the crystalline rod is formed on a substrate arranged in a direction in which the emitted light is extracted.
The light-emitting diode of claim 57, wherein the light-emitting structure comprises an n-type semiconductor layer and the crystalline rod is formed on the n-type semiconductor layer of the light-emitting structure arranged in a direction in which the emitted light is extracted.
The light-emitting diode of claim 57, wherein the light-emitting structure comprises a current spreading layer formed of a transparent conductive material and provided on the crystalline rod to cover the crystalline rod.
The light-emitting diode of claim 62, wherein the current spreading layer comprises an electrode formed thereon.
The light-emitting diode of claim 57, wherein the crystalline rod formed of zinc oxide is a waveguide of the light emitted from the light-emitting structure.
The light-emitting diode of claim 57, wherein the crystalline rod has a regular arrangement with respect to adjacent crystalline rods and grows in a direction perpendicular to the surface of the light-emitting structure.
A method of fabricating a light-emitting diode, the method comprising:

forming a seed layer on a light-emitting diode structure; and

forming a crystalline rod of zinc oxide on the seed layer.
The method of claim 66, wherein the formation of the seed layer comprises:

preparing a first solution by dissolving a hydrate containing zinc salt in a solvent;

heating the first solution to be in a sol state;

preparing a second solution by adding a surfactant to the first solution in the sol state; and

coating the second solution on the light-emitting diode structure and heating the resulting solution to be in a gel state.
The method of claim 67, wherein the zinc salt comprises zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride.
The method of claim 66, wherein the formation of the seed layer comprises:

preparing a first growth solution containing a first zinc salt and a first precipitator;

heating the first growth solution to form zinc oxide particles; and

coating the zinc oxide particles on the light-emitting structure.
The method of claim 66, wherein the formation of the crystalline rod comprises:

preparing a rod growth solution containing a hydrate containing a second zinc salt and a second precipitator; and

immersing the light-emitting structure in the rod growth solution and applying heat energy thereto such that the crystalline rod grows from the seed layer.
The method of claim 70, wherein the second zinc salt comprises zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride.
The method of claim 70, wherein the second precipitator comprises NaOH, Na₂CO₃, LiOH, H₂O₂, KOH, hexamethylenetetramine (HMTA), or NH₄OH.
The method of claim 70, wherein the rod growth solution further comprises a second overgrowth inhibitor containing a cationic polymer.
The method of claim 66, further comprising, before the formation of the crystalline rod, forming a growth guiding layer having a regular pattern on the seed layer.
The method of claim 74, wherein the crystalline rod grows through a surface of the seed layer exposed by the growth guiding layer.
The method of claim 66, wherein the crystalline rod grows in the [0001] direction of zinc oxide.
A method of fabricating a zinc oxide tree structure, the method comprising:

forming a zinc oxide seed layer on a substrate;

forming a crystalline rod on the seed layer; and

forming a crystalline branch using a zinc oxide branch seed precipitated on the side of the crystalline rod.
The method of claim 77, wherein the formation of the seed layer comprises:

preparing a first solution by dissolving a hydrate containing zinc salt in a solvent;

heating the first solution to be in a sol state;

preparing a second solution by adding a surfactant to the first solution in the sol state; and

coating the second solution on the substrate and heating the resulting solution to be in a gel state.
The method of claim 78, wherein the zinc salt comprises zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride.
The method of claim 77, wherein the formation of the seed layer comprises:

preparing a first growth solution containing a first zinc salt and a first precipitator;

heating the first growth solution to form zinc oxide particles; and

coating the zinc oxide particles on the substrate.
The method of claim 77, wherein the formation of the crystalline rod comprises:

preparing a rod growth solution containing a hydrate containing a second zinc salt and a second precipitator; and

immersing the substrate in the rod growth solution and applying heat energy thereto such that the crystalline rod grows from the seed layer.
The method of claim 81, wherein the second zinc salt comprises zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride.
The method of claim 81, wherein the second precipitator comprises NaOH, Na₂CO₃, LiOH, H₂O₂, KOH, hexamethylenetetramine (HMTA), or NH₄OH.
The method of claim 77, further comprising, before the formation of the crystalline rod, forming a regular pattern on the seed layer.
The method of claim 84, wherein the crystalline rod grows through a surface of the seed layer exposed by the pattern.
The method of claim 77, wherein the formation of the crystalline branch comprises:

precipitating the branch seed of zinc oxide on the side of the crystalline rod; and

forming the crystalline branch from the branch seed.
The method of claim 86, wherein the precipitation of the branch seed comprises:

applying a seed forming solution containing a zinc salt and a cationic polymer to the substrate; and

heating the resulting substrate such that the branch seed is precipitated on the side of the crystalline rod.
The method of claim 87, wherein the zinc salt of the seed forming solution is adhered or attached to the cationic polymer.
The method of claim 87, wherein the precipitation of the branch seed is performed above the isoelectric point.
The method of claim 77, wherein the crystalline rod or the crystalline branch grows in the [0001] direction of zinc oxide.
A zinc oxide tree structure comprising:

a zinc oxide seed layer formed on a substrate;

a crystalline rod formed from the seed layer; and

a crystalline branch formed from a branch seed precipitated on the side of the crystalline rod.
The zinc oxide tree structure of claim 91, wherein the crystalline rod has a regular arrangement with respect to adjacent crystalline rods and grows predominantly in the [0001] direction.
The zinc oxide tree structure of claim 91, wherein the branch seed is precipitated on the side of the crystalline rod by heating zinc oxide adhered to a cationic polymer.
The zinc oxide tree structure of claim 91, wherein the crystalline branch is formed by a hydrothermal synthesis method.
The zinc oxide tree structure of claim 91, wherein the seed layer is formed by a sol-gel method such that the crystalline branch is oriented perpendicularly to the substrate.
The zinc oxide tree structure of claim 91, wherein the crystalline rod or the crystalline branch is prevented from growing in the lateral side by addition of a cationic polymer.
A light-emitting diode comprising:

a light-emitting structure for emitting light; and

a tree structure formed of zinc oxide in a direction in which the light is emitted from the light-emitting structure.
The light-emitting diode of claim 97, wherein the light emitting structure comprises a current spreading layer and the tree structure is formed on the current spreading layer.
The light-emitting diode of claim 97, wherein the light emitting structure comprises a p-type semiconductor layer and the light-emitting diode further comprises a current spreading layer such that the tree structure is formed on the p-type semiconductor layer and covered with the current spreading layer.
The light-emitting diode of claim 97, wherein the tree structure is formed on a substrate arranged in a direction in which the emitted light is extracted.
The light-emitting diode of claim 97, wherein the light-emitting structure comprises an n-type semiconductor layer and the tree structure is formed on the n-type semiconductor layer of the light-emitting structure arranged in a direction in which the emitted light is extracted.
The light-emitting diode of claim 97, wherein the tree structure formed of zinc oxide is a waveguide of the light emitted from the light-emitting structure.
The light-emitting diode of claim 97, wherein the tree structure comprises:

a seed layer formed on the light-emitting structure;

a crystalline rod formed from the seed layer; and

a crystalline branch formed from a branch seed precipitated on the side of the crystalline rod.
The light-emitting diode of claim 103, wherein the crystalline rod has a regular arrangement with respect to adjacent crystalline rods and grows in a direction perpendicularly to the surface of the light-emitting structure.
The light-emitting diode of claim 103, wherein the branch seed is precipitated on the side of the crystalline rod by heating zinc oxide adhered to a cationic polymer.
The light-emitting diode of claim 103, wherein the seed layer is formed by a sol-gel method.
A method of fabricating a light-emitting diode, the method comprising:

a zinc oxide seed layer on a light-emitting structure;

forming a crystalline rod on the seed layer; and

forming a crystalline branch using a zinc oxide branch seed precipitated on the side of the crystalline rod.
The method of claim 107, wherein the formation of the seed layer comprises:

preparing a first solution by dissolving a hydrate containing zinc salt in a solvent;

heating the first solution to be in a sol state;

preparing a second solution by adding a surfactant to the first solution in the sol state; and

coating the second solution on the light-emitting structure and heating the resulting solution to be in a gel state.
The method of claim 108, wherein the zinc salt comprises zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride.
The method of claim 107, wherein the formation of the seed layer comprises:

preparing a first growth solution containing a first zinc salt and a first precipitator;

heating the first growth solution to form zinc oxide particles; and

coating the zinc oxide particles on the light-emitting structure.
The method of claim 107, wherein the formation of the crystalline rod comprises:

preparing a rod growth solution containing a hydrate containing a second zinc salt and a second precipitator; and

immersing the light-emitting structure in the rod growth solution and applying heat energy thereto such that the crystalline rod grows from the seed layer.
The method of claim 111, wherein the second zinc salt comprises zinc acetate, zinc nitrate, zinc sulfate, or zinc chloride.
The method of claim 112, wherein the second precipitator comprises NaOH, Na₂CO₃, LiOH, H₂O₂, KOH, hexamethylenetetramine (HMTA), or NH₄OH.
The method of claim 107, further comprising, before the formation of the crystalline rod, forming a growth guiding layer having a regular pattern on the seed layer.
The method of claim 114, wherein the crystalline rod grows through a surface of the seed layer exposed by the growth guiding layer.
The method of claim 107, wherein the formation of the crystalline branch comprises:

precipitating the branch seed of zinc oxide on the side of the crystalline rod; and

forming the crystalline branch from the branch seed.
The method of claim 116, wherein the precipitation of the branch seed comprises:

applying a seed forming solution containing a zinc salt and a cationic polymer to the light-emitting structure; and

heating the resulting light-emitting structure such that the branch seed is precipitated on the side of the crystalline rod.
The method of claim 116, wherein the precipitation of the branch seed is performed above the isoelectric point.
The method of claim 107, wherein the crystalline rod or the crystalline branch grows in the [0001] direction of zinc oxide.