JP2018140914A - Laminate structure, and light-emitting diode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laminate structure showing high luminous efficiency when used as a light-emitting element.SOLUTION: A laminate structure 100 has a m-face interlayer 20 containing ZnOM(0<X≤1;M is S, Se, Te or Po. When M is S, 0.5<X≤1.) and a m-face ZnSO(0.5≤Y≤1) 30 laminated in that order on a m-face sapphire substrate 10.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a laminated structure and a light emitting diode.

  2. Description of the Related Art In recent years, lighting fixtures (LED lighting) using light emitting diodes (LEDs) have become widespread as new lighting fixtures that can replace incandescent bulbs and fluorescent lamps. LED lighting has various features such as energy saving compared to existing light sources, long life, small size, and light weight.

The current mainstream white LED has a structure in which a yellow LED is contained in a blue LED chip using indium-doped gallium nitride (InGaN) as a light emitting layer. White light is obtained by mixing two colors of blue and yellow, which are complementary colors.
This InGaN includes rare metals such as In and Ga. In addition, a metal organic chemical vapor deposition (MOCVD) method using a high-purity gas or a vacuum process is generally used for forming a thin film of InGaN.

In light-emitting elements such as LEDs, alternative materials that are rare metals and do not contain expensive In and Ga are desired. On the other hand, zinc sulfide (ZnS) is one of inexpensive alternative materials currently being studied. This ZnS has a direct transition type band structure and a wide band gap of 3.5 to 3.8 eV, and blue light emission due to sulfur defects has also been confirmed. For this reason, ZnS is expected to be applied to blue LEDs.
For example, Non-Patent Document 1 discloses an a-plane sapphire substrate / c-plane ZnO / c-plane ZnS, which is a structure in which a ZnO film strongly oriented in the c-axis and a ZnS film are stacked on an a-plane sapphire substrate. Yes. In this structure, light emission in the visible region based on a mixed crystal of ZnO and ZnS has been confirmed.

Koike, K., et al., “Preparation and interface reaction of ZnS / ZnO heterojunction by molecular beam epitaxy”, J. Vac. Soc. Jpn. (Vacuum) Vol. 48, no. 3, 2005, P127-129.

A light-emitting layer in a light-emitting element such as an LED needs to be a highly crystalline and homogeneous layer from the viewpoint of light emission efficiency. On the other hand, in the structure described in Non-Patent Document 1, the light emission efficiency is still insufficient.
This invention is made | formed in view of the said situation, and makes it a subject to provide the laminated structure which can improve luminous efficiency more, when it utilizes for a light emitting element.

When the ZnS layer is stacked on the polar surface (c-plane) of the sapphire substrate, spontaneous polarization electric fields and piezoelectric fields are generated along the c-axis (stacking direction) inside the ZnS layer, and various adverse effects are caused. And the luminous efficiency is lowered. In order to solve this, the present inventors examined a method of epitaxially growing a ZnS crystal on a nonpolar surface (m-plane) of a sapphire substrate.
And it discovered that the m-plane zinc oxide crystal layer which is a nonpolar surface could be formed in m surface of a sapphire substrate by mist CVD method.
On the other hand, the zinc sulfide has a zinc blende structure (cubic) and a wurtzite structure (hexagonal), while zinc oxide has a wurtzite structure. As a result of repeated studies focusing on the fact that both zinc sulfide and zinc oxide have a wurtzite structure, the present inventors have further found zinc sulfide epitaxy on zinc oxide crystals.
That is, the inventors have created a technique for growing m-plane zinc sulfide crystals on m-plane zinc oxide on an m-plane sapphire substrate, thereby completing the present invention.

In the laminated structure according to the first aspect of the present invention, ZnO _X M _1-X (0 <X ≦ 1; M is S, Se, Te, or Po) on an m-plane sapphire substrate. In the case of S, an m-plane intermediate layer containing 0.5 <X ≦ 1) and an m-plane ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1) layer were laminated in this order. It is characterized by that.

As the laminated structure according to the first aspect, for example, the m-plane intermediate layer is formed by laminating a first m-plane intermediate layer and a second m-plane intermediate layer, and the first m-plane intermediate layer. The layer is adjacent to the m-plane sapphire substrate and is made of a polycrystal, and the second m-plane intermediate layer is the m-plane ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1). Suitable examples include those that are adjacent to the layer and are composed of a single crystal.

  A light-emitting diode according to a second aspect of the present invention includes the multilayer structure according to the first aspect.

  According to the laminated structure of the present invention, since a nonpolar surface is used when used in a light emitting device, an electric field is not applied to the quantum well layer, which is a light emitting layer, and the light emission efficiency is further improved.

It is sectional drawing about the laminated structure of this embodiment. It is sectional drawing about the laminated structure of other embodiment. It is a schematic diagram which shows one Embodiment of a mist CVD apparatus (parallel supply system). It is a schematic diagram which shows one Embodiment of a mist CVD apparatus (high-speed rotation vertical supply system). It is a figure which shows the result of the X-ray-diffraction measurement ((theta) -2 (theta) method) of the zinc oxide crystal layer (film-forming temperature 700 degreeC) formed into a film using a different zinc oxide precursor and a board | substrate. It is a figure (Drawing 6 (a)) which shows the state of m plane sapphire crystal, and a figure (Drawing 6 (b)) which shows the state of m plane zinc oxide crystal. It is a figure which shows the result of the X-ray-diffraction measurement ((theta) -2 (theta) method) about the zinc oxide crystal layer (film-forming temperature 600 degreeC and 650 degreeC, substrate position 25mm) of an Example. It is a figure which shows the film-forming temperature dependence of the result of the X-ray-diffraction measurement ((theta) -2 (theta) method) in the ZnS thin film on a single crystal ZnO layer. It is a figure (narrow scan curve) which shows the carrier gas flow rate dependence of the result of the X-ray-diffraction measurement ((theta) -2 (theta) method) in the ZnS thin film on a single crystal ZnO layer. It is a figure which shows the X-ray (phi) scan measurement result [a surface diffraction] in the ZnS thin film on a single crystal ZnO layer.

(Laminated structure)
FIG. 1 shows a cross-sectional view of the laminated structure of this embodiment.
A laminated structure 100 shown in FIG. 1 is obtained by laminating an intermediate layer 20 and a surface layer 30 on a substrate 10 in this order.

<Board>
As the substrate 10 of the laminated structure 100, an m-plane sapphire substrate in which the crystal plane on the surface in contact with the intermediate layer 20 is a nonpolar m-plane is used.
The shape of the substrate 10 is not particularly limited, and examples thereof include a wafer shape. When the substrate 10 has a wafer shape, the diameter is preferably 10 to 300 mm.
The thickness of the substrate 10 is, for example, 400 to 1000 μm.

<Intermediate layer>
The intermediate layer 20 of the laminated structure 100 has crystallinity continuous with the surface of the substrate 10 (m-plane of the sapphire substrate), and the crystal plane of the surface in contact with the surface layer 30 is a non-polar m-plane. That is, the intermediate layer 20 is an m-plane intermediate layer.
The thickness of the intermediate layer 20 is, for example, 0.1 to 3 μm.

The intermediate layer 20 is ZnO _X M _1-X (0 <X ≦ 1; M is S, Se, Te or Po. However, when M is S, 0.5 <X ≦ 1) Containing.
The intermediate layer 20 may be formed from only one layer, or may be formed from two or more layers. Further, the composition of the intermediate layer 20 may change continuously or stepwise from the substrate 10 side toward the surface layer 30 side.
As the crystallinity of the intermediate layer 20 is improved, the crystallinity of the surface layer 30 is also improved. Considering this, it is preferable that the crystallinity and flatness of the surface of the intermediate layer 20 in contact with the surface layer 30 are high.
The intermediate layer _20, a layer containing _{ZnO X M 1-X (0.5} <X ≦ 1) is _preferably a layer consisting of _{ZnO X M 1-X (0.5} <X ≦ 1) is more preferred, A layer made of ZnO is more preferable.

<Surface>
The surface layer 30 of the laminated structure 100 has crystallinity continuous with the surface of the intermediate layer 20 (the m-plane of ZnO _X M _1-X ).
The thickness of the surface layer 30 is, for example, 0.1 to 5 μm.

The surface layer 30 is composed of an m-plane ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1) layer.
The surface layer 30 may be formed from only one layer, or may be formed from two or more layers. In addition, the composition of the surface layer 30 may be changed continuously or stepwise from the intermediate layer 20 side toward the surface 30 s side of the surface layer 30 facing the intermediate layer 20.
As the crystallinity of the surface layer 30 is improved, the light emission efficiency is increased when it is used in a light emitting element. Considering this, it is preferable that the crystallinity on the surface 30s side of the surface layer 30 and the flatness of the surface 30s are high.
The _{ZnS Y O 1-Y (0.5} ≦ Y ≦ 1) constituting the surface layer 30, preferably as Y value is larger, made of ZnS layer is more preferable.

[Manufacturing method of laminated structure]
In the laminated structure 100 of the present embodiment, for example, ZnO _X M _1-X (0 <X ≦ 1; M is S, Se, Te) on a sapphire substrate (substrate 10) whose surface crystal plane is m-plane. Or a Po (provided that 0.5 <X ≦ 1 when M is S), and an m-plane intermediate layer (intermediate layer 20) is formed, and the intermediate ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1) is crystal-grown on the layer 20 to form an m-plane ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1) layer (surface layer 30). It can manufacture using the manufacturing method which has process (II) to do.

In the following description, “forming a layer by crystal growth” may be expressed as “forming a film”.
The “mist CVD method (mist chemical vapor deposition method)” is a method in which a solution as a raw material is made into a mist (mist) by a predetermined method, transported to a reaction part by a carrier gas, and deposited by a thermochemical reaction. .

The laminated structure 100 can be manufactured using, for example, a mist CVD apparatus.
The mist CVD method does not require a special apparatus or a vacuum atmosphere, can be formed under atmospheric pressure, and has an advantage that it is a safe and inexpensive process, energy saving, and a small environmental load.
Examples of the mist CVD apparatus include a parallel supply film forming apparatus as shown in FIG. 3 and a high-speed rotation vertical supply film forming apparatus as shown in FIG.

FIG. 3 is a schematic view showing an embodiment of a mist CVD apparatus (parallel supply method).
A film forming apparatus 300 shown in FIG. 3 includes a mist generator 310 and a chamber 320. The mist generator 310 and the chamber 320 are connected via a supply pipe 330.
The mist generator 310 has a substantially cylindrical shape, includes an ultrasonic transducer 312 at the bottom, a mist outlet 314 near the center of the upper surface, and a carrier gas inlet 316 near the center of one side surface. Yes.
The chamber 320 is formed of a substantially rectangular parallelepiped quartz furnace having a long axis as a horizontal direction. In the furnace, a concave substrate placing part 322 and a flow path 324 provided so that mist supplied into the furnace flows horizontally above the substrate placing part 322 and is discharged out of the furnace. And are formed. A plurality of heaters 326 are disposed on the bottom surface of the chamber 320.
In FIG. 3, the raw material solution is accommodated in the mist generator 310 and mist is generated, and the mist generated in the mist generator 310 flows through the supply pipe 330 and is supplied into the chamber 320.

FIG. 4 is a schematic view showing an embodiment of a mist CVD apparatus (high-speed rotation vertical supply system).
A film forming apparatus 400 shown in FIG. 4 includes a mist generator 410 and a chamber 420 disposed above the mist generator 410. The mist generator 410 and the chamber 420 are connected via a supply pipe 430.
The mist generator 410 has a substantially cylindrical shape, includes an ultrasonic transducer 412 at the bottom, a mist outlet 414 near the center of the upper surface, and a carrier gas inlet 416 near the center of one side surface. Yes.
In the chamber 420, a nozzle 424 protruding into the chamber 420 is provided near the center of the bottom surface so as to be connected to the supply pipe 430. In the chamber 420, a rotation stage 422 is disposed near the center of the upper surface facing the nozzle 424.
The chamber 420 has a substantially rectangular parallelepiped shape with the long axis as the horizontal direction, and exhaust ports 428a and 428b are provided above a pair of opposing side surfaces, respectively.
In FIG. 4, the raw material solution is accommodated in the mist generator 410 and mist is generated, and the mist generated in the mist generator 410 flows through the supply pipe 430 and is supplied into the chamber 420.

Step (I):
In step (I), ZnO _X M _1-X is grown on the substrate 10 to form the intermediate layer 20.
When film formation is performed using the film formation apparatus 300 (parallel supply method), first, the raw material solution is accommodated in the mist generator 310, and the substrate placement unit in the chamber 320 (quartz furnace) under atmospheric pressure. The substrate 10 is placed on 322. Next, an ultrasonic wave is applied to the raw material solution using the ultrasonic vibrator 312 while heating the substrate 10 to a predetermined temperature by the heater 326. Thus, raw material solution was atomized _mist is generated containing ZnO _{X M 1-X} precursor. At the same time, the carrier gas flows into the mist generator 310 from the inlet 316. Then, the generated mist is carried on the gas flow of the carrier gas that has flowed into the mist generator 310 and sent onto the substrate 10 in the chamber 320. As a result, the mist undergoes a thermochemical reaction with the surface of the substrate 10, and a crystal layer containing ZnO _X M _1-X is formed on the surface of the substrate 10.

In the case of the film forming apparatus 300 shown in FIG. 3, since the mist flows along with the carrier gas along the surface of the substrate 10, it is possible to ensure a relatively long time for the thermal decomposition product of the raw material to contact the substrate 10. As a _{result, ZnO X M 1-X} of ZnO _{X M 1-X} precursor was formed by thermal chemical reaction, crystal growth under the influence on the crystallinity of the substrate 10 surface, m surface middle layer 20 is formed The

When film formation is performed using the film formation apparatus 400 (high-speed rotation vertical supply method), first, the raw material solution is accommodated in the mist generator 410 and the substrate 10 is rotated while being held on the rotation stage 422. Next, ultrasonic waves are applied to the raw material solution using the ultrasonic transducer 412 while heating the substrate 10 to a predetermined temperature by the heater 426. Thus, raw material solution was atomized _mist is generated containing ZnO _{X M 1-X} precursor. At the same time, the carrier gas flows into the mist generator 410 from the inlet 416. Then, the generated mist is put on the gas flow of the carrier gas that has flowed into the mist generator 410 and sprayed onto the substrate 10 that rotates in the chamber 420. As a result, the mist undergoes a thermochemical reaction with the surface of the substrate 10, and the m-plane intermediate layer 20 containing ZnO _X M _1-X is formed on the surface of the substrate 10.

In the case of the film forming apparatus 400 shown in FIG. 4, by rotating the substrate 10 while flowing mist vertically from the lower side with respect to the center of the substrate 10, film quality and film thickness unevenness due to the position in the surface of the substrate 10 are reduced. As a result, the thickness of the thin film formed on the substrate 10 becomes more uniform. In particular, by adjusting the concentration of the raw material solution, the flow rate of the carrier gas, and the position of the nozzle through which the mist is introduced, a more uniform film formation (within ± 5% of film thickness and film quality) can be easily realized.
Further, since the film forming apparatus 400 supplies the mist to the substrate 10 from the lower side to the upper side by the nozzle 424 in the vertical direction, the problem that the mist supply becomes unstable and the problem that the liquid dripping from the nozzle 424 does not occur easily. .
In the film formation apparatus 400, it is preferable that the distance between the nozzle 424 and the substrate 10 can be changed. When the distance between the substrate 10 and the nozzle 424 is variable, the distance between the substrate 10 and the nozzle 424 can be easily adjusted. When the distance between the substrate 10 and the nozzle 424 is short, the growth rate of the formed oxide film increases, but the film thickness distribution in the substrate 10 tends to increase. When the distance between the substrate 10 and the nozzle 424 is long, the growth rate of the oxide film decreases, but the film thickness distribution in the substrate 10 tends to decrease. Therefore, the distance between the substrate 10 and the nozzle 424 may be determined in consideration of the balance between the growth rate of the oxide film and the in-plane uniformity.

The conditions for the film formation temperature of the film containing ZnO _X M _1-X on the substrate 10 are preferably in the range of 550 to 650 ° C., more preferably in the range of 590 to 610 ° C.
The condition for the film formation time of the film containing ZnO _X M _1-X on the substrate 10 is preferably 1 hour or less, and more preferably in the range of 20 minutes to 40 minutes.

As the solvent in the raw material solution, a solvent having a high solubility in the ZnO _X M _1-X precursor to be used is selected. That is, a suitable solvent may be appropriately selected from polar solvents such as water, methanol, and ethanol; and nonpolar solvents such as acetone and toluene, depending on the type of the ZnO _X M _1-X precursor. Among these, it is preferable that the solvent in the raw material solution is water or a solvent mainly composed of water in that the contamination in the apparatus is difficult to occur, the environmental load can be reduced, and the cost is low. Is particularly preferred.

Any ZnO _X M _1-X precursor may be used as long as it can generate ZnO _X M _1-X by a thermochemical reaction. That is, it includes both a compound that can generate a ZnO _X M _1-X precursor by thermal decomposition and a compound that can generate ZnO _X M _1-X by reacting with a carrier gas. Specifically, it is roughly classified into inorganic precursors such as chlorides, acetates, nitrates and sulfates; organic precursors such as dimethylzinc and diethylzinc. When water or a solvent mainly composed of water is used as the solvent in the raw material solution, the ZnO _X M _1-X precursor is preferably a water-soluble salt. In this embodiment, zinc chloride is one of the preferred precursors because it can form a zinc oxide crystal layer with high crystallinity.

The concentration of the ZnO _X M _1-X precursor in the raw material solution is appropriately determined in consideration of the types of the precursor and the solvent, various conditions such as the substrate temperature, the thickness of the target intermediate layer, necessary crystallinity, etc. It is determined. For example, when the precursor is zinc chloride and the solvent is water, the amount is usually 0.01 to 0.5 mol / L.
The raw material solution may contain a substance other than the ZnO _X M _1-X precursor for the purpose of changing the physical properties of the crystal layer.

The carrier gas is a gas for transporting the mist generated by atomizing the raw material solution onto the substrate 10 of the mist CVD apparatus, and is appropriately determined in consideration of the types of the precursor and the solvent. Specifically, an inert gas such as argon, an oxidizing gas such as oxygen or water vapor, nitrogen, or a mixed gas of these gases is used as the carrier gas.
The flow rate of the carrier gas is appropriately determined in consideration of various conditions such as the type of the precursor and the solvent, the substrate temperature, the target intermediate layer thickness, the required crystallinity, and the like. / Min, preferably 7.5 to 8.5 L / min.

Step (II):
In step (II), ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1) is grown on the intermediate layer 20 to form the surface layer 30.
The film formation of ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1) on the intermediate layer 20 can be used in the step (I), or the film formation apparatus 300 shown in FIG. 3 or the film formation shown in FIG. The same can be done using the apparatus 400.

The condition of the film formation temperature of ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1) on the intermediate layer 20 is preferably in the range of 400 to 600 ° C., more preferably in the range of 400 to 450 ° C., The inside of the range of 420-430 degreeC is further more preferable.
When the film forming temperature is equal to or higher than the lower limit of the above preferable range, the film thickness can be easily maintained, and the flatness of the film is improved. On the other hand, if it is not more than the upper limit of the above preferred range, the crystallinity of the thin film is easily enhanced.

When forming the surface layer 30 on the intermediate layer 20, the flow rate of the carrier gas is preferably 10 L / min or less, more preferably 8 L / min or less, and preferably 0.2 to 5 L / min. More preferably, it is particularly preferably 0.5 to 3 L / min.
When the flow rate of the carrier gas is less than or equal to the upper limit value of the preferable range, the film thickness can be easily maintained, and the flatness of the film is improved. On the other hand, when it is at least the lower limit value of the above preferred range, the crystallinity of the thin film is easily enhanced.

The condition for the film formation time of ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1) on the intermediate layer 20 is preferably 2 hours or less, more preferably 1 hour or less, and 20 minutes. More preferably, it is within the range of 40 minutes.

In the laminated structure 100 of the present embodiment, an m-plane intermediate layer containing ZnO _X M _1-X as the intermediate layer 20 is laminated on the m-plane sapphire substrate as the substrate 10, and on the m-plane intermediate layer, As the surface layer 30, an m-plane ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1) layer is laminated.
That is, the intermediate layer 20 is formed by epitaxially growing ZnO _X M _1-X on the m-plane (nonpolar plane) of the sapphire substrate, and the surface layer 30 is formed on the m-plane (nonpolar plane) of the intermediate layer 20 with ZnS _Y O. It is formed by epitaxially growing _1-Y . Since both the intermediate layer 20 and the surface layer 30 are formed on the m-plane, there is no internal electric field in the quantum well structure that is indispensable for carrier confinement of electrons and holes (the use of a nonpolar plane makes it possible to emit light). Since the electric field is not applied to the quantum well layer, which is a layer), the probability of coupling of electrons and holes is increased.
In addition, according to the composition of sapphire substrate / ZnO _X M _1-X layer / ZnS _Y O _1-Y layer, the lattice mismatch between the ZnO _X M _1-X crystal and the sapphire crystal is small, The lattice mismatch between the ZnS _Y O _1-Y crystal and the ZnO _X M _1-X crystal is smaller than the lattice mismatch between the ZnS _Y O _1-Y crystal and the sapphire crystal. Furthermore, the structures of the ZnS _Y O _1-Y crystal and the ZnO _X M _1-X crystal are both considered to be a wurtzite structure (hexagonal system). Therefore, the crystallinity of the ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1) layer can be improved.
Therefore, according to the laminated structure 100 of the present embodiment, the light emission intensity can be increased, and the light emission efficiency can be further increased when used in the light emitting element.

By the way, the current LED is manufactured based on a gallium nitride material and uses a rare metal such as gallium or indium, and generally requires a high purity gas or vacuum atmosphere called a metal organic chemical vapor deposition (MOCVD) method. Since the film forming method is used, it is expensive overall.
In the laminated structure 100 of the present embodiment, inexpensive sapphire can be used as the material of the substrate 10, and abundant and inexpensive zinc sulfide can be used as the material of the surface layer 30. Further, for manufacturing the laminated structure 100, a mist CVD method capable of forming a film at atmospheric pressure is used.
Therefore, by applying the laminated structure 100 of the present embodiment, a significant cost reduction can be achieved as compared with the conventional case.

  Furthermore, it is known that zinc sulfide emits blue light due to a defect level, and the present inventors have already confirmed blue light emission by photoexcitation fluorescence (photoluminescence). Therefore, if a pn junction is formed in the laminated structure 100 using zinc sulfide for the surface layer 30, a low-cost blue LED can be manufactured.

[Other embodiments]
In the laminated structure 100 of the present embodiment, the intermediate layer is illustrated as having a single layer, but the present invention is not limited to this, and may be an intermediate layer having two or more layers as described above.

FIG. 2 shows a cross-sectional view of a laminated structure according to another embodiment.
A laminated structure 200 shown in FIG. 2 is obtained by laminating an intermediate layer 50 and a surface layer 60 in this order on a substrate 40. The intermediate layer 50 is configured by laminating a first intermediate layer 51 and a second intermediate layer 52.
As the substrate 40, an m-plane sapphire substrate in which the crystal plane on the surface in contact with the first intermediate layer 41 is a nonpolar m-plane is used. The description of the substrate 40 is the same as the description of the substrate 10 in FIG.
The first intermediate layer 51 is adjacent to the substrate 40 and has crystallinity that is continuous with the surface of the substrate 40 (m-plane of the sapphire substrate). The first intermediate layer 51 is a layer composed of a polycrystalline body (polycrystalline layer).
The second m-plane intermediate layer 52 is adjacent to the surface layer 60 and is a layer composed of a single crystal (single crystal layer).
The surface layer 60 is composed of an m-plane ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1) layer. The description of the surface layer 60 is the same as the description of the surface layer 30 in FIG.

 In the laminated structure 200, the crystallinity of the second m-plane intermediate layer 52 is further improved by forming the intermediate layer 50 as two layers (polycrystalline layer / single crystal layer) having different crystal states. Along with this, the crystallinity of the surface layer 60 is further improved. As a result, the luminous efficiency is further enhanced when used in a light emitting device.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited by these examples.

<Manufacture of laminated structure>
≪Formation of zinc oxide (ZnO) crystal layer on substrate≫
In order to select a zinc oxide precursor and a substrate useful for forming a single crystal zinc oxide thin film, the following experiment was conducted.

A zinc oxide (ZnO) crystal layer was formed on the substrate using a mist CVD apparatus having the configuration shown in FIG. Zinc acetate and zinc chloride (ZnCl ₂ ) were used as the zinc oxide precursor.
In the mist CVD apparatus, a zinc acetate aqueous solution having a concentration of 0.1 mol / L or an aqueous zinc chloride (ZnCl ₂ ) solution having a concentration of 0.1 mol / L is atomized by an ultrasonic vibrator, and the generated mist is nitrogen gas (flow rate) 20 L / min), each film was transferred onto an a-plane sapphire substrate (2-inch substrate) and an m-plane sapphire substrate (2-inch substrate) and subjected to thermochemical reaction to form a film.
The substrate temperature was measured at the center position of the substrate. The film formation time is all 1 hour, and no buffer layer is used on the substrate. The substrate temperature during film formation was set to 700 ° C.
The crystallinity of the zinc oxide crystal layer formed on each substrate was evaluated by X-ray diffraction measurement (XRD) (θ-2θ method) at the center position of the substrate. The result is shown in FIG.

FIG. 5 is a diagram showing the results of X-ray diffraction measurement (θ-2θ method) of zinc oxide crystal layers (film formation temperature 700 ° C.) formed with different zinc oxide precursors and substrates.
From FIG. 5, it was found that when using an aqueous zinc chloride solution, the crystal grain size of zinc oxide tends to be larger than when using an aqueous zinc acetate solution. That is, zinc chloride is more preferable as the zinc oxide precursor.
Further, FIG. 5 shows that when the a-plane sapphire substrate is used, the zinc oxide thin film grows in two directions, ie, the (0002) plane and the (10-11) plane. On the other hand, when the m-plane sapphire substrate is used, it can be seen that the (10-10) plane grows as a single unit.

FIG. 6 is a diagram showing a state of an m-plane sapphire crystal (FIG. 6A) and a diagram showing a state of an m-plane zinc oxide crystal (FIG. 6B).
From the result of X-ray diffraction measurement using the φ scan method by in-plane rotation of the substrate, it is confirmed that the formed m-plane zinc oxide crystal layer has the same crystal axis as that of the m-plane sapphire substrate. It was done. From this, it was found that the substrate was epitaxy (see FIGS. 6A and 6B). That is, it was shown that the single crystal ZnO thin film was formed in at least a part of the area within the 2-inch m-plane sapphire substrate.
Here, when an LED is formed by growing a crystal in the [0002] direction, the wave function of carriers shifts due to the effect of spontaneous polarization, and the probability of recombination of electrons and holes decreases, that is, quantum efficiency may deteriorate. is there. Zinc oxide grown in the [10-10] direction is perpendicular to the [0002] direction and can avoid the influence of spontaneous polarization, and thus is suitable for forming an LED with good quantum efficiency.
From the above results, although a crystalline zinc oxide layer could be formed on either the a-plane or the m-plane sapphire substrate, the m-plane sapphire substrate is more suitable for the formation of LEDs with good quantum efficiency. I can say that.

[Evaluation of film formation temperature dependence of zinc oxide crystals]
In the mist CVD apparatus having the configuration shown in FIG. 3, an aqueous zinc chloride solution having a concentration of 0.1 mol / L is atomized by an ultrasonic vibrator, and the generated mist is nitrogen gas (flow rate 20 L / min) to form an m-plane sapphire substrate. The film was transferred onto a (2 inch substrate) and subjected to a thermochemical reaction to form a film.
The substrate temperature was measured at the center position of the substrate. The film formation time is all 1 hour, and no buffer layer is used on the substrate. The substrate temperature during film formation was 600 ° C. to 850 ° C., and the film thickness of the zinc oxide crystal layer (sample) formed at each temperature was evaluated using a step gauge.

It was confirmed that the film thickness of the sample changed continuously with respect to the substrate position and was not constant. This is because in the mist CVD apparatus, the mist flows in one direction toward the heat source, so that the deposition rate of zinc oxide varies depending on the position of the substrate due to the progress of heating or decomposition of the mist.
When the substrate temperature was 600 ° C. to 750 ° C., the film thickness increased as the substrate position was rearward. This is presumably because the mist was heated as it was behind the substrate, the temperature of the mist increased, and the reaction became easier. However, at a substrate temperature of 800 ° C. to 850 ° C., the film thickness decreased as the substrate position was rearward. This is believed to be due to re-evaporation of zinc oxide or consumption of feedstock in front of the substrate.

As a result of SEM surface observation, it was confirmed that a zinc oxide crystal layer was formed on the m-plane sapphire substrate under all temperature conditions of 600 to 850 ° C.
In addition, from the X-ray diffraction measurement using the θ-2θ method, it was confirmed that the zinc oxide layer formed under all temperature conditions of 600 to 850 ° C. is a single crystal grown epitaxially on the m-plane sapphire substrate. It was done.
As a representative example, the X-ray diffraction measurement result (θ-2θ method, substrate position 25 mm) at a substrate temperature of 600 ° C. is shown in FIG.

In FIG. 7, a single peak indicating m-plane zinc oxide was observed, and it was confirmed that an m-plane zinc oxide crystal layer which is a nonpolar plane was formed.
Regarding the plane orientation, when an LED is manufactured on c-plane zinc oxide, the internal electric field generated in the c-axis direction spatially separates electrons and holes in the light-emitting layer, and thus the luminous efficiency deteriorates. There is. If an LED is manufactured using the m-plane zinc oxide formed this time, it is non-polar so that an improvement in luminous efficiency can be expected without generating an internal electric field.

<< Formation of a zinc sulfide (ZnS) crystal layer on a zinc oxide (ZnO) crystal layer >>
A zinc sulfide (ZnS) crystal layer was formed on a zinc oxide (ZnO) crystal layer using a mist CVD apparatus having the configuration shown in FIG. An ultrasonic vibrator having an oscillation frequency of 2.4 MHz was used as an ultrasonic vibrator for generating mist, and nitrogen (N ₂ ) gas was used as a carrier gas for pushing the mist.

Formation of a zinc oxide (ZnO) crystal layer on a substrate:
A zinc chloride (ZnCl ₂ ) aqueous solution having a concentration of 0.1 mol / L was used as a raw material, the film formation temperature was set to 600 ° C., the carrier gas flow rate was set to 8 L / min, and the film formation time was 30 minutes. A zinc oxide (ZnO) crystal layer was formed. As a result, a ZnO crystal was epitaxially grown on the m-plane sapphire substrate to form an m-plane single crystal ZnO layer.

Formation of a zinc sulfide (ZnS) crystal layer on a zinc oxide (ZnO) crystal layer:
Regarding the formation of the zinc sulfide (ZnS) crystal layer, film formation was performed by changing the film formation temperature and the carrier gas flow rate in order to obtain the optimum film formation conditions.
In any case, the evaluation of the film formation temperature dependency and the carrier gas flow rate dependency during the formation of the ZnS crystal layer is performed by evaluating the crystallinity in the stacking direction by X-ray diffraction measurement (XRD) (θ-2θ method), SEM The surface shape was evaluated by observation, and the film thickness was measured by a step gauge. For samples formed under optimum film forming conditions, in-plane orientation evaluation by X-ray φ scan measurement was also performed.

[Evaluation of film formation temperature dependence of zinc sulfide crystals]
In this evaluation, a zinc sulfide (ZnS) crystal layer was formed by dissolving 0.1 mol / L ZnCl ₂ aqueous solution as a zinc source and thiourea (CH ₄ N ₂ S) powder as a sulfur source in pure water. A mixed solution of 0.5 mol / L aqueous solution was used. At this time, the concentration ratio (molar ratio, atomic ratio of Zn and S) of ZnCl ₂ and CH ₄ N ₂ S in the mixed solution was set to 1: 1.
Evaluation was performed by changing the deposition temperature of the ZnS crystal layer at 400 to 600 ° C. The evaluation was performed by setting the carrier gas to nitrogen gas, the carrier gas flow rate to 8 L / min, and the deposition time of the ZnS crystal layer to 30 minutes. The result is shown in FIG.

FIG. 8 is a diagram showing the film formation temperature dependence of the result of X-ray diffraction measurement (θ-2θ method) in a ZnS thin film on a single crystal ZnO layer.
Strong diffraction peaks can be confirmed around 28 ° and 56 ° at all film forming temperatures. This is a peak derived from the m-plane of ZnS having a wurtzite structure (wurtzite type), and thus suggests that the m-plane of wurtzite ZnS is growing in the stacking direction.
When the film formation temperature is 450 ° C. or higher, a diffraction peak due to the (10-13) plane of wurtzite ZnS can be confirmed around 52 °, and the intensity of the diffraction peak increases as the film formation temperature rises. Yes. From this, it can be seen that the increase in the film formation temperature promotes the growth of the (10-13) plane of wurtzite ZnS and contributes to the deterioration of the orientation in the stacking direction of the ZnS thin film on the single crystal ZnO layer.
However, at a film forming temperature of 425 ° C. or lower, no peak can be confirmed on the (10-13) plane of the wurtzite type ZnS, and only a peak attributed to the m plane of the wurtzite type ZnS. This suggests that if a ZnS thin film is formed at a film forming temperature of 425 ° C. or less on a single crystal ZnO layer, the m-plane of wurtzite ZnS can be selectively grown in the stacking direction. doing.

[Evaluation of carrier gas flow rate dependence in the formation of zinc sulfide crystals]
In this evaluation, a zinc sulfide (ZnS) crystal layer was formed by dissolving 0.1 mol / L ZnCl ₂ aqueous solution as a zinc source and thiourea (CH ₄ N ₂ S) powder as a sulfur source in pure water. A mixed solution of 0.5 mol / L aqueous solution was used. At this time, the concentration ratio (molar ratio, atomic ratio of Zn and S) of ZnCl ₂ and CH ₄ N ₂ S in the mixed solution was set to 1: 1.
Evaluation was performed with the deposition temperature of the ZnS crystal layer fixed at 425 ° C. The evaluation was carried out with nitrogen gas as the carrier gas, the carrier gas flow rate varied from 1 to 8 L / min, and the deposition time of the ZnS crystal layer as 30 minutes. The result is shown in FIG.

FIG. 9 is a diagram (narrow scan curve) showing the carrier gas flow rate dependence of the result of X-ray diffraction measurement (θ-2θ method) in a ZnS thin film on a single crystal ZnO layer.
FIG. 9 shows that the diffraction peak intensity derived from m-plane ZnS increases as the carrier gas flow rate decreases. This suggests that the film thickness of the ZnS thin film increases as the carrier gas flow rate decreases.

From the evaluation results of the film formation temperature dependency and the carrier gas flow rate dependency in the film formation of the zinc sulfide crystal described above, the optimum film formation conditions for the ZnS thin film on the single crystal ZnO layer were set as follows.
Raw material: A mixed solution of 0.1 mol / L ZnCl ₂ aqueous solution and 0.5 mol / L thiourea (CH ₄ N ₂ S) aqueous solution. The concentration ratio of ZnCl ₂ and CH ₄ N ₂ S in the mixed solution (molar ratio, atomic ratio of Zn and S) is 1: 1.
Carrier gas: Nitrogen gas Flow rate of carrier gas: 1 L / min
Deposition temperature: 425 ° C
Deposition time: 30 minutes

Next, the crystal state of the ZnS thin film on the single crystal ZnO layer formed under the above film forming conditions was evaluated. The result is shown in FIG.
FIG. 10 is a diagram showing an X-ray φ scan measurement result in the ZnS thin film on the single crystal ZnO layer. The diffractive surface is the a (11-20) plane.

The diffraction peaks of the sapphire substrate and the ZnO layer are both symmetrical twice, and have a repeated pattern whose positions are shifted by 90 °.
This shows that the ZnO crystal is laminated on the sapphire substrate so that the sapphire crystal c-axis and the ZnO c-axis are orthogonal to each other. This is because the lattice mismatch with the sapphire crystal is reduced when four ZnO crystals are rotated 90 ° with respect to the sapphire crystal and arranged.

  The diffraction peak of the ZnS layer is two-fold symmetrical and appears at the same position as the ZnO layer. This suggests that ZnS on the ZnO layer is a single crystal and is oriented in the same direction as ZnO in the in-plane direction.

  From the results shown in FIG. 10, it can be seen that the m-plane ZnS thin film was epitaxially grown on the single-crystal m-plane ZnO layer in the deposition under the above-described deposition conditions. That is, a laminated structure in which an m-plane single crystal ZnS layer is formed by epitaxially growing ZnO on an m-plane sapphire substrate to form an m-plane ZnO layer and epitaxially growing ZnS on the m-plane sapphire substrate using mist CVD. It was confirmed that

  The laminated structure of the present invention is a material useful for a wide band gap semiconductor device, and can be used for lighting equipment, other LED-related products, power transistors, and the like. Such a laminated structure is useful as a material for a light-emitting device that operates in the visible to ultraviolet region at room temperature, for example. Application of the laminated structure may provide a blue LED with high light emission efficiency and reduce the material and manufacturing cost thereof.

10 substrate, 20 intermediate layer, 30 surface layer, 40 substrate, 50 intermediate layer, 51 first intermediate layer, 52 second intermediate layer, 60 surface layer, 100 laminated structure, 200 laminated structure, 300 film forming apparatus, 310 Mist generator, 312 ultrasonic vibrator, 320 chamber, 330 supply pipe, 400 film forming apparatus, 410 mist generator, 412 ultrasonic vibrator, 420 chamber, 422 rotary stage, 430 supply pipe.

Claims (3)

On m-plane sapphire substrate,
M _- plane containing ZnO _X M _1-X (0 <X ≦ 1; M is S, Se, Te or Po. However, when M is S, 0.5 <X ≦ 1). The middle layer,
an m-plane ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1) layer;
Are stacked structures in this order. The m-plane intermediate layer is formed by laminating a first m-plane intermediate layer and a second m-plane intermediate layer,
The first m-plane intermediate layer is a layer that is adjacent to the m-plane sapphire substrate and is made of a polycrystal.
2. The second m-plane intermediate layer according to claim 1, wherein the second m-plane intermediate layer is a layer that is adjacent to the m-plane ZnS _Y O _1-Y (0.5 ≦ Y ≦ 1) layer and is made of a single crystal. Laminated structure.   A light emitting diode comprising the laminated structure according to claim 1.