KR100970179B1

KR100970179B1 - Method of forming a thin film and luminescence device

Info

Publication number: KR100970179B1
Application number: KR1020080071016A
Authority: KR
Inventors: 신병철; 이원재
Original assignee: 동의대학교 산학협력단
Priority date: 2008-07-22
Filing date: 2008-07-22
Publication date: 2010-07-14
Also published as: KR20100010153A

Abstract

The present invention relates to a method for forming a thin film and a light emitting device, and forms a sodium-doped ZnO thin film by pulse laser deposition using a sodium-doped ZnO target, and uses the same as a transparent electrode to form oxygen vacancies by doping the ZnO thin film. This can be prevented to prevent deep level emission, thereby improving the luminous efficiency and life of the UV optical device.

Sodium, Doped, ZnO, PLD, Deep Level Release

Description

Method of forming a thin film and luminescence device

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of forming a thin film, and more particularly, to a method of depositing a sodium-doped ZnO thin film using a pulse laser deposition (PLD) and a light emitting device.

GaN is one of very useful materials used in optical devices such as light emitting diodes and laser diodes. ZnO is a semiconductor material with a wide bandgap and has properties similar to GaN. For example, GaN has a bandgap of 3.4 eV and ZnO has a bandgap of 3.3 eV. In addition, ZnO has the same structure as GaN. Due to the similar properties of GaN and ZnO, ZnO is spotlighted as the most promising material for photonic devices within the ultraviolet range. ZnO is widely used as a transparent conductive thin film, a solar cell window, a bulk acoustic wave device, and the like. In addition to the similarity between GaN and ZnO, ZnO has an exciton binding energy greater than that of GaN at room temperature. That is, ZnO has an exciton binding energy of 60 meV, but GaN has an exciton binding energy of 25 meV. Thus, ZnO can be used in place of GaN in exciton-related devices.

ZnO is formed by methods such as molecular beam epitaxy (MBE), chemical vapor deposition (CVD), sputtering, pyrolysis and pulsed laser deposition (PLD). Among them, pulsed laser deposition is widely used to form metal oxide thin films and related materials. Pulsed laser deposition allows the deposition of metal oxide thin films in a very simple manner, and atomic layer control is possible by controlling the laser fluence and pulse rate. In addition, a multi-layer hetero structure may be formed in-situ by using a multi target carrousel.

In order to implement an optical device using ZnO, simultaneous growth of n-type and p-type ZnO is required. Most semiconductors, including ZnO, naturally have n-type conductivity. The n-type ZnO is easy to manufacture, and the conductivity can be controlled by the doping and doping levels of other materials. However, p-type ZnO is not easy to manufacture, and it is reported that p-type ZnO has been obtained by many research results. However, some results are only repetition of research results. In addition, the characteristics of the p-type ZnO are difficult to use in the device.

The present invention provides a thin film forming method and a light emitting device for forming a p-type ZnO thin film by doping sodium.

The present invention provides a thin film forming method and a light emitting device for forming a sodium doped ZnO thin film using a pulse laser deposition method.

According to one or more exemplary embodiments, a method of forming a thin film includes: loading a substrate after loading a sodium doped ZnO target into a pulse laser deposition chamber; And irradiating a pulse laser to the target to form a sodium doped ZnO thin film on the substrate.

The sodium is contained 0.1 to 0.5 at.%.

The target is produced by mixing N ₂ O powder and ZnO powder by using a plastic container having a ball in a planetary milling system, followed by uniaxial pressurization and normal temperature integral molding.

The target is sintered for 3-5 hours in a furnace at 1000-1300 ° C.

The chamber maintains a pressure of 50 to 200 mTorr and an oxygen atmosphere and a substrate temperature of 100 to 600 ° C.

A light emitting device according to another aspect of the present invention includes an anode, a light emitting layer and a cathode sequentially formed in a predetermined region on the substrate, the anode is formed of a ZnO thin film doped with sodium.

According to the present invention, by forming a sodium-doped ZnO thin film by pulse laser deposition using a sodium-doped ZnO target, and using it as a transparent electrode, oxygen vacancies are suppressed by doping of the ZnO thin film to prevent deep level emission. Can be. Therefore, the luminous efficiency and lifetime of a UV optical element can be improved.

Hereinafter, with reference to the accompanying drawings will be described an embodiment of the present invention; However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various forms, and only the embodiments are intended to complete the disclosure of the present invention and to those skilled in the art. It is provided for complete information.

1 is a schematic cross-sectional view of a pulse laser deposition apparatus used for thin film deposition according to an embodiment of the present invention.

As shown, the present invention is irradiated with a pulse laser to the target 20 installed in the chamber 10 of the pulse laser deposition apparatus, the target 20 on the substrate 30 disposed at a position opposite to the target 20 A thin film of approximately the same composition is deposited. The chamber 10 includes a target holder (not shown) for fixing the target 20 and a substrate holder (not shown) for fixing the substrate 30. The target holder (not shown) and the substrate holder (not shown) are provided. For example, four and one is provided, respectively.

A window 40 is formed at one side of the chamber 10, and the pulse laser is irradiated to the target 20 through the window 40. Here, the focal length and the degree of condensing of the pulse laser are adjusted by the lens 50 so that the pulse laser is irradiated to a predetermined position on the target 20 exactly. The laser irradiated onto the target 20 generates a strong plasma plume on the surface of the target 20, and when the plasma plume reaches the substrate 30, a thin film having a composition substantially the same as that of the target 20 is formed on the substrate ( 30).

As the pulse laser, for example, a KrF excimer laser or the like which outputs a wavelength in the ultraviolet region is used. KrF excimer laser (λ = 248 nm, τ = 25 μs) is used for ablation of the target 20 at an energy density of about 1.5 J / cm ² .

As the target 20, a ZnO target doped with sodium is used. The target 20 is manufactured using, for example, Aldrich Co.'s high purity ZnO powder (99.99%) and Na ₂ O powder (99.99%). The content of Na ₂ O is 0.1 to 0.5 at.%, Preferably 0.12 at.%. In order to dope Na ₂ O to the desired amount, high purity Na ₂ O powder and ZnO powder are mixed for 10 hours using a plastic container with balls, for example in a milling system.

In addition, the target 20 is produced in the shape of a disk of 1 inch diameter, for this purpose by uniaxial pressing at 700kg / ㎠ and then produced by cold iso-static press of 24000㎏ / ㎠ do. The denser the target 20 is, the higher the quality of plasma can be obtained. Therefore, in order to produce the dense target 20, the disk-shaped target 20 is sintered for 3 to 5 hours in the furnace of 1000-1300 degreeC. Preferably it is sintered for 4 hours in a furnace at 1200 ℃. The target 20 is uploaded to the chamber after being attached to the target holder (not shown). The target 20 is disposed to maintain a distance of about 35 mm from the substrate 30.

In addition, the chamber 10 should be maintained at a predetermined atmosphere, pressure and temperature while the thin film is deposited, and maintains an oxygen atmosphere of 50 to 200 mTorr, preferably 100 mTorr and a substrate temperature of 200 to 600 ° C. Then, the repetition frequency of the pulse laser is 5 kHz, and a thin film is deposited to a thickness of about 300 nm by performing a deposition process for about 40 minutes. After deposition, the thin film is naturally cooled at room temperature for various measurements.

Properties of the thin film deposited under the above conditions are observed by various methods. That is, the crystal structure of the thin film was observed using an X-ray diffractometer (D / MAX 2100H, Rigagu, 400 kV, 30 mA) having CuKα1 emission λ = 1.5405 Hz, and the surface shape of the thin film was atomic microscope (atomic). It is observed with a scanning probe microscope (SPM) in force microscope (AFM) mode. In addition, the electrical properties of the sodium-doped ZnO thin film are measured by the van der pauw method in a hall-effect measurement system. The transmittance of the thin film is measured with a UV-Vis-IR spectrophotometer (Vary-5 Australia) at a wavelength in the range of 300 to 700 nm. The excitation source used for photoluminescent (hereinafter referred to as "PL") measurement is a He-Cd laser operated at 325 nm with an output power of 30 mW. In this case, a cutoff filter is used to suppress dispersion of the laser. The cutoff wavelength at the ultraviolet side of the cutoff filter is about 340 nm.

The characteristics of the sodium-doped ZnO thin film formed by the pulse laser deposition according to the present invention measured by the above method are as follows. Hereinafter, the characteristics of the ZnO thin film doped with sodium 0.12at.% Will be described.

FIG. 2 is an image showing the surface structure of a sodium doped ZnO thin film deposited on a sapphire substrate by pulsed laser deposition at various temperatures, and scanning probe microscope (SPM) in AFM mode (SPA-400, Seiko Instruments). ) Is a photograph observed. The scanning area is 2 μm × 2 μm, and the grain size, mean diameter, and root-mean-square (RMS) roughness observed by AFM are shown in [Table 1].

Growth temperature (℃) Grain size (× 10 ⁴ nm ² ) Average diameter (nm) RMS (nm) 200 1.74 149 3.19 300 2.57 181 5.76 400 2.74 187 2.58 500 2.52 179 6.26 600 2.81 189 6.15

As can be seen in Figure 2 and Table 1, grain size increases with growth temperature. In other words, the grain size increases as the growth temperature increases. However, thin films deposited at 400 ° C. have the smallest grain size and the smallest surface roughness.

3 shows an X-ray diffraction pattern of a sodium doped ZnO thin film deposited on a sapphire substrate by pulsed laser deposition at various temperatures. Here, the x axis represents an angle and the y axis represents an intensity expressed in logarithmic scale.

As shown, all thin films are observed to exhibit increased (002) and (004) orientations and (002) and (004) peak intensities as the growth temperature increases. As the growth temperature increases above 300 ° C., one or more (100) ZnO peaks appear. Increasing the peak intensity means that the film quality improves with the growth temperature. The temperature dependence of the film quality can be explained by the mobility of the particles at different growth temperatures. That is, the low mobility of particles grown at low temperatures interferes with the crystallization of the thin film. This weakens the intensity of the peak. On the other hand, no new phases related to sodium are observed in all thin films, which means that when the amount of impurities is small, the doping level does not change the structure of the ZnO thin film.

Hall effect measurement is performed at room temperature to measure the electrical properties of the thin film. Table 2 shows the results measured using the Van der Pau method using four probes. However, this method does not measure the electrical properties of the thin film grown at 300 ℃.

Temperature (℃) Resistivity (Ω㎝) Mobility (㎠V ^-1 s ^-1 ) Carrier Density (cm ^-3 ) Hall Coefficient (㎡C ^-1 ) 200 0.16 8.51 -4.58 × 10 ¹⁸ -1.36 300 N / A N / A N / A N / A 400 9.84 2.06 -4.80 × 10 ¹⁷ -13.01 500 6.32 3.52 -2.89 × 10 ¹⁷ -21.6 600 1.10 1.39 -3.78 × 10 ¹⁸ -1.65

As can be seen from Table 3, all the thin films measured are n-type semiconductors. This experiment demonstrates that ZnO thin films doped with sodium doping levels of 0.12 at.% Are n-type semiconductors. The resistivity of the sodium doped ZnO thin film is measured to be high. As can be seen in Table 3, the resistivity of thin films deposited at 200 ° C. and 600 ° C. is lower than the resistivity of thin films deposited at 400 ° C. and 500 ° C. The carrier density of all thin films is in the range of 10 ¹⁷ to 10 ¹⁸ cm ^-3 . In addition, the hole mobility of all the thin films is in the range of ^{1 to 9} cm 2 V ^-1 s ^-1 .

Spectrophotometers (Cary-5, UV-VIS-NIR) are used to measure the transparency of thin films at room temperature. The transparency of the ZnO thin film doped with sodium at 0.12 at.% Is shown in FIG. 4. Transparency was measured in the UV-visible region with a wavelength of 300-700 nm. All thin films have an optical transparency of 90% or more in the visible region. All thin films have very sharp absorption edges and oscillations. In addition, the bandgap energy of the sodium doped ZnO thin film is calculated by linear fitting of the sharp absorption edges. For example, a method of calculating the band gap energy of a thin film deposited at 500 ° C. is shown in FIG. 5. ZnO is a direct band gap semiconductor and has an absorption coefficient α∝-lnT. Therefore, the photon energy hυ is expressed by [α × (hυ)] ² . Sharp absorption edges can be used to accurately determine high quality thin films using linear adjustments. The bandgap energy of the thin film after calculation is shown in FIG. As shown in FIG. 6, as the growth temperature increases from 200 ° C. to 600 ° C., the bandgap energy of the thin film increases from 3.26 eV to 3.29 eV. This difference is due to the different vapor pressures of sodium and ZnO at different growth temperatures. The bandgap energy of a sodium doped ZnO thin film is about the same as that of a pure ZnO thin film deposited under the same conditions. This implies that the Burstein-Moss effect is not a major cause after doping ZnO thin films doped with sodium at a doping level of 0.12 at.%. ZnO thin films are naturally n-type materials and the Fermi level is inside the conducting band when the sodium is heavily doped. The absorbing edge moves to high energy after doping. This is called the burststein morse effect. By measuring the transparency of the thin film, the visible and infrared wavelength regions of the spectra can be used to calculate the refractive index and thickness of the thin film.

PL measurements are performed at room temperature to study the emission properties of sodium doped ZnO thin films. PL spectra grown at various temperatures using pulsed laser deposition are shown in FIG. 7. Thin films deposited at 200 ° C. were observed to show very weak emission. Thin films deposited at 300 ° C. and 400 ° C. show very weak and wide deep level (DL) emission. As the growth temperature increases, near band edge (NBE) emission appears, and as the growth temperature increases, the intensity of NBE emission increases. Thin films deposited at 600 ° C. showed the strongest NBE emission and the strongest DL emission. The PL results correspond to the XRD results. XRD measurements show that thin films deposited at 200 ° C. exhibit (002) and (004) orientations. However, films deposited at temperatures above 200 ° C. exhibit (100) peaks and (002) and (004) peak intensities that increase with growth temperature. The emission properties of thin films increase with increasing crystalline.

NBE emission and DL emission are always observed in undoped ZnO thin films, where doped ZnO thin films are inhibited from DL emission. DL release comes from oxygen vacancy. Impurities doped in the ZnO thin film inhibit DL release because the impurity fills oxygen vacancies. On the other hand, dopants certainly reduce NBE emissions because they form non-emitting recombination centers in the doped films. This result shows that sodium doped ZnO thin films deposited at 200 ° C. at this doping level are not released. Thin films deposited at temperatures above 300 ° C. show obvious DL emission and NBE emission as the growth temperature increases. This means that at low growth temperatures sodium doped ZnO at a doping level of 0.12 at.% Inhibits DL release and NBE release. This results from the reduction of oxygen vacancies in the thin film and the creation of non-emitting recombination centers. At higher growth temperatures, different vapor pressures of sodium and oxygen create more oxygen vacancies. On the other hand, the crystallinity of thin films grown at high temperatures is better, leading to increased NBE emission.

8 is a cross-sectional view of a light emitting device manufactured using a sodium doped ZnO thin film according to the present invention.

Referring to FIG. 8, the light emitting device according to the exemplary embodiment includes an anode 110, an emission layer 120, and a cathode 130 on the substrate 100. The emission layer 120 may include a hole injection layer 121, a hole transport layer 122, an organic emission layer 123, and an electron injection layer 124. .

The substrate 100 may use an insulating substrate, a semiconductor substrate, or a conductive substrate, that is, a plastic substrate (PE, PES, PET, PEN, etc.), a glass substrate, an Al ₂ O ₃ substrate, a SiC substrate, a ZnO substrate, Si At least one of a substrate, a GaAs substrate, a GaP substrate, a LiAl ₂ O ₃ substrate, a BN substrate, an AlN substrate, an SOI substrate, and a GaN substrate may be used.

The anode 110 is an electrode for hole injection, and is formed using a sodium-doped ZnO thin film according to the present invention. The sodium-doped ZnO thin film is formed by pulsed laser deposition using a sodium-doped ZnO target as described above, and is about 150 nm thick by maintaining an oxygen atmosphere of 50 to 200 mTorr and a substrate temperature of 100 to 600 ° C. To form.

The hole injection layer 121 constituting the light emitting layer 120 may be formed to a thickness of about 20 nm using copper phthaloyanine (CuPc) or the like, and the hole transport layer 122 may include α-NPD. Can be formed to a thickness of about 40 nm. In addition, the organic light emitting layer 123 may be formed to a thickness of about 60 nm using 8-hydroxyquinoline aluminum (Alq ₃ ) or the like, and the electron injection layer 124 may be formed of lithium fluorine (LiF). Or the like to form a thickness of about 0.5 nm.

The cathode 130 is used as the electron injection electrode, any material having electrical conductivity may be used, and the cathode 130 may be formed to a thickness of 20 to 150 nm. The cathode 130 is preferably made of metals such as Al, Ag, Au, Pt, Cu, which have low electrical resistance and excellent interface properties with conductive organic materials. However, it is more preferable to use a metal having a low work function in order to obtain a high current density in electron injection by lowering a barrier formed between the organic layer 120 and Al, which is a material that is relatively stable to air. It is most preferable to use.

1 is a schematic cross-sectional view of a pulse laser deposition equipment for thin film deposition according to an embodiment of the present invention.

2 is an AFM photograph of a sodium doped ZnO thin film deposited on a sapphire substrate by pulsed laser deposition at various temperatures.

3 is an XRD pattern of a sodium doped ZnO thin film deposited on a sapphire substrate by pulsed laser deposition at various temperatures.

4 is a graph of transparency of a sodium doped ZnO thin film deposited on a sapphire substrate by pulsed laser deposition at various temperatures.

5 is a graph for explaining a bandgap energy calculation method by the linear adjustment of the absorption edge.

6 is a graph for explaining the temperature dependence of the bandgap energy of a sodium doped ZnO thin film deposited on a sapphire substrate by pulsed laser deposition at various temperatures.

7 is a PL spectrum of a sodium doped ZnO thin film deposited on a sapphire substrate by pulsed laser deposition at various temperatures.

8 is a cross-sectional view of a light emitting device manufactured using a sodium-doped ZnO thin film according to an embodiment of the present invention.

10 chamber 20 target

30: substrate 40: window

50: Lens

100: substrate 110: anode

120 emitting layer 130 cathode

121: hole injection layer 122: hole transport layer

123: organic light emitting layer 124: electron injection layer

Claims

Loading the substrate after loading the sodium doped ZnO target into a pulsed laser deposition chamber;

Irradiating a pulse laser to the target to form a sodium doped ZnO thin film on the substrate.

The method of claim 1, wherein the sodium is contained in an amount of 0.1 to 0.5 at.%.

The method of claim 1, wherein the target is a thin film forming method of mixing N ₂ O powder and ZnO powder using a plastic container having a ball in a planetary milling system, followed by uniaxial pressurization and normal temperature integral molding. .

The method of claim 3, wherein the target is sintered in a furnace at 1000 to 1300 ° C. for 3 to 5 hours.

The method of claim 1, wherein the chamber maintains a pressure of 50 to 200 mTorr, an oxygen atmosphere, and a substrate temperature of 100 to 600 ° C. 6.

It includes an anode, a light emitting layer and a cathode sequentially formed in a predetermined region on the substrate,

The anode is a light emitting device formed of a sodium doped ZnO thin film.