WO2018080300A1

WO2018080300A1 - Method for producing a non-polar a-plane gallium nitride (gan) thin film on an r-plane sapphire substrate

Info

Publication number: WO2018080300A1
Application number: PCT/MY2017/050066
Authority: WO
Inventors: Ahmad Shuhaimi BIN ABU BAKAR; Mohd Adreen Shah BIN AZMAN SHAH; Omar Ayad Fadhil AL-ZUHAIRI; Anas BIN KAMARUNDZAMAN
Original assignee: Universiti Malaya
Priority date: 2016-10-31
Filing date: 2017-10-27
Publication date: 2018-05-03
Also published as: MY190127A

Abstract

A method for producing a non-polar a-plane gallium nitride (GaN) thin film on an r-plane sapphire substrate using metal organic chemical vapour deposition (MOCVD) technique, the method comprising the steps of: annealing the r-plane sapphire substrate using a hydrogen cleaning process; depositing a GaN nucleation layer in the presence of vaporized sources of gallium and nitrogen on the substrate; depositing and growing a GaN buffer layer atop the nucleation layer; and laterally growing a GaN overgrowth layer atop the buffer layer, wherein the step of depositing and growing of the layers are conducted under a standardized volumetric flow rate of the nitrogen source within a range of 0.5 to 2.1 slm. Particularly, the method also comprises controlling V/III ratio, reaction temperature and flow rate of reactants during growth of GaN layers. It was found that such method could result in reduced lattice mismatch between each layer.

Description

METHOD FOR PRODUCING A NON-POLAR A-PLANE GALLIUM NITRIDE (GAN) THIN FILM ON AN R-PLANE SAPPHIRE SUBSTRATE

FIELD OF TECHNOLOGY

The present invention relates to semiconductor devices, materials used in production of the devices and techniques for processing the materials for fabrication of the devices. More particularly, the invention relates to a method of producing a non-polar a-plane gallium nitride (GaN) film on an r-plane sapphire using metal-organic chemical vapour deposition (MOCVD).

BACKGROUND OF THE INVENTION

Gallium nitride (GaN)-based materials have recently gained interest due to their potential in optoelectronic devices. Growth of GaN epilayers on sapphire substrates using metalorganic chemical vapor deposition (MOCVD) and a two-step growth technique is commonly applied to produce the aforementioned GaN-based materials. The two-step technique generally involve the steps of: (1) depositing a nitride-based buffer layer on an annealed substrate; and (2) growing GaN epitaxy layer on the buffer layer.

Mostly, GaN-based materials are prepared by growing GaN along the polar c-direction [0001] of its wurtzite crystal structure. Nevertheless, as the wurtzite structure is non- centrosymmetric, optoelectronic devices obtained on the basis of c-plane nitrides suffer from piezoelectric polarization. To eliminate such polarization effects, growth along non- polar direction is desired. United States Patent No. US 7091514 B2 disclosed a method for growing a non-polar a- plane GaN thin film on an r-plane sapphire substrate through MOCVD. Apparently, a two-step approach was adopted by the method disclosed in US 7091514 B2, where a nucleation layer was formed on the substrate, followed by growth of another GaN layer atop the nucleation layer. More particularly, the formation of the nucleation layer (which act as a buffer layer) was conducted at temperature between 400 to 900 °C and atmospheric pressure to initiate GaN growth on the r-plane sapphire. Further, the formation of non-polar a-plane GaN layer was conducted at temperature of approximately 1100 °C under low pressure (0.2 atmosphere or less) in order to produce a planar film.

As an alternative, this invention provides a three-step approach for growing a non-polar a-plane GaN thin film on an r-plane sapphire substrate through MOCVD. Particularly, the method adopts suitable V/III ratio(s) under a constant pressure.

SUMMARY OF THE INVENTION

This invention is a three-step process where a GaN nucleation layer is first deposited on r-plane of a sapphire substrate, followed by growth of a-plane GaN in accordance to Volmer- Weber mode and further lateral growth of a GaN in accordance to Frank-van-der Merwe mode.

This invention is a method which provides suitable lattice-matching condition between layers of the thin film. The layers being a nucleation layer and the substrate, as well as between a buffer layer and the nucleation layer, and between the buffer layer and an overgrowth layer. Such condition is responsible for difference in crystal quality of a-plane GaN film. At least one of the preceding objects is met, in whole or in part, by the present invention, in which one of the embodiments of the present invention is a method for producing a non-polar a-plane gallium nitride (GaN) thin film on an r-plane sapphire substrate using metal organic chemical vapour deposition (MOCVD) technique, the method comprising the steps of: annealing the r-plane sapphire substrate using a hydrogen cleaning process; depositing a GaN nucleation layer in the presence of vaporized sources of gallium and nitrogen on the substrate; depositing and growing a GaN buffer layer atop the nucleation layer; and laterally growing a GaN overgrowth layer atop the buffer layer, wherein the step of depositing and growing of the layers are conducted under a standardized volumetric flow rate of the nitrogen source within a range of 0.5 to 2.1 slm.

In the preferred embodiment, in-plane orientations of the GaN film with respect to the r- plane substrate are [ll-20]GaN//[l-102] _sapphire, [0001]GaN//[-H01]_sapphire and/or [1- 100]_GaN//[ll-20]_Sapphire.

Preferably, the step of depositing the nucleation layer is conducted with a first V/III ratio in the range of 550 to 750 and at temperature of 500 to 600 °C, wherein the first V/III ratio is maintained by controlling the standardized volumetric flow rate of the nitrogen source within a range of 0.5 to 1.5 slm.

Preferably, the step of depositing the buffer layer is conducted with a second V/III ratio in the range of 1817 to 4326 and at temperature of 1000 to 1100 °C, wherein the second V/III ratio is maintained by controlling the standardized volumetric flow rate of the nitrogen source within a range of 2.1 to 5 slm.

Preferably, the step of depositing the overgrowth layer is conducted with a third V/III ratio in the range of 550 to 750 and at temperature of 1000 to 1100 °C, wherein the third V/III ratio is maintained by controlling the standardized volumetric flow rate of the nitrogen source within a range of 0.5 to 1.5 slm. Preferably, the GaN nucleation layer has a thickness of 50 to 60 nm.

Preferably, the GaN buffer layer has a thickness of 100 to 110 nm.

Preferably, the GaN overgrown layer has a thickness of 100 nm to 2 um.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawing the preferred embodiments from an inspection of which when considered in connection with the following description, the invention, its construction and operation and many of its advantages would be readily understood and appreciated.

Fig. la to If are Field Emission Scanning Electron Microscope (FESEM) images of a non-polar a-plane GaN thin film grown according to exemplary embodiments of this invention (Sample A to F of Table 1).

Fig. 2 shows X-ray Diffraction (XRD) Spectra with ω-2θ scan for a non-polar a-plane GaN thin film grown on an r-plane (1-102) sapphire substrate grown according to exemplary embodiments of this invention.

Fig. 3 shows full width at half maximum (FWHM) of X-ray rocking curves of (11-20) GaN thin film produced according to embodiments of this invention measured at X-ray incident beam parallel to c- and m-GaN directions.

Fig. 4 shows schematic diagram of in-plane orientations of (11-20) GaN thin film grown on r-plane (1-102) sapphire substrate grown according to exemplary embodiments of this invention. Fig. 5 a to 5f are Atomic Force Microscopy (AFM) images of a non-polar a-plane GaN thin film grown according to exemplary embodiments of this invention (Sample A to F of Table 1).

Fig. 6 shows a growth rate of a non-polar a-plane GaN thin film grown according to exemplary embodiments of this invention.

BRIEF DESCRIPTION OF THE INVENTION

This invention is a method for growing a non-polar a-plane GaN thin film on an r-plane sapphire substrate. Particularly, it is a three-step approach for producing a non-polar a- plane GaN thin film on an r-plane sapphire substrate through MOCVD. Exemplary, non- limiting embodiments of the method will be disclosed with reference to Fig. 1 to 6. However, it is to be understood that limiting the description to the preferred embodiments of the invention is merely to facilitate discussion of the present invention and it is envisioned that those skilled in the art may devise various modifications without departing from the scope of the appended claim.

MOCVD is a technique whereby sources of reactants are introduced as vapor phase constituents into a reaction chamber at room temperature and are thermally decomposed at elevated temperatures by a hot susceptor and substrate to form a desired film in the reaction chamber. Particularly, the vapor phase reactants are thermally decomposed at elevated temperatures to form a non-volatile product which is deposited on the substrate and the susceptor, while other volatile product is carried away by a hydrogen carrier gas to an exhaust. This invention manipulates the MOCVD technique by way of controlling growth conditions in the reactant chamber thereby producing a non-polar a-plane GaN thin film (being the non-volatile product) on an r-plane sapphire substrate. The growth conditions include but not limited to growth temperature, partial pressure of reactants, V/III ratio, carrier gas and pressure. Preferably, the method uses hydrogen as carrier gas. Alternatively, purified nitrogen gas can be used. The pressure inside the MOCVD reaction chamber is preferably maintained at 13 kPa to 15 kPa, more preferably at 13.3 kPa. Further, the source of reactant, particularly gallium, is preferably trimethylgallium. The source of nitrogen is preferably ammonia.

In the preferred embodiment, the r-plane sapphire substrate is thermally treated with ambient hydrogen at temperature 1100 to 1200 °C and pressure of 13 to 15 kPa prior to growth of any GaN layer. In this way, the substrate is treated to remove any contamination thereon and being surface activated to facilitate growing step thereinafter.

In accordance to the preceding description, this invention is a method which produces GaN thin film on the substrate in three main steps. The steps comprises: (1) forming GaN nucleation layer on the r-plane sapphire substrate; (2) depositing GaN buffer layer on the nucleation layer; and (3) growing epilayer of GaN on the buffer layer. Compared to the two-step approach as mentioned in the background, the method in the present invention deposits an additional layer (known as 'buffer layer' herein) between the nucleation layer (the layer which has direct contact with substrate) and the overgrowth layer (the layer grows on top of the buffer layer). The present invention preferably produces GaN nucleation layer instead of A1N nucleation layer to eradicate imperfections that may result from island coalescence. Further, the present invention preferably produces a sandwiched buffer layer so as to facilitate lateral overgrowth of the GaN overgrown layer, more particularly avoiding misfit dislocation between the overgrown layer and the substrate, thereby enhancing quality of the thin film. Nevertheless, each layer is preferably deposited or grown under suitable growth condition(s) so as to reduce lattice mismatch between one another layer. In the present invention, a combination of the first, second and/or third V/III ratios may provide GaN thin film of different surface morphology. In the step of depositing and growing GaN buffer layer, the preferred second V/III ratio is able to encourage growth of GaN in a single direction, i.e. a-direction. More particularly, the preferred second V/III ratio shall be able cause reduction or even avoid V-pits formation in a GaN thin film. Besides, the step of depositing and growing the GaN overgrowth layer preferably uses a third V/III ratio which is able to encourage lateral growth (in c- and m-direction) of GaN on the buffer layer, thereby producing a thin film having smooth surface. Accordingly, it is preferred that a combination of the second and third V/III ratios used could encourage production of GaN with Ga-facet that is superior to a GaN with N-facet. According to Fig. 4, in-plane orientations of the GaN thin film with respect to the r-plane substrate are

[l l-20]GaN//[l-102]_sapphire, [0001]_ΟΟΝ//[- 1101] _sapphire and [ 1- 100] GaN//[l 1-20] sapphire-

In the present invention, the V/III ratios can be achieved by adjusting or maintaining volumetric flow rate of the nitrogen source, while keeping the flow of gallium source, carrier gas, and pressure constant through that three steps. It is preferred that the step of depositing and growing the GaN nucleation layer and overgrowth layer are conducted under low or minimal volumetric flow rate of the nitogen source throughout the MOCVD. More preferably in a range of 0.5 to 2.1 slm. In conjunction with the V/III ratios, temperature plays an important role in deposition and growth of each of the layers as well. In accordance to the preceding description, the method in the present invention comprises the steps of: (1) depositing a GaN nucleation layer in the presence of vaporized sources of gallium and nitrogen on the substrate with a first V/III ratio in the range of 550 to 750 and at temperature of 500 to 600 °C, wherein the first V/III ratio is maintained by controlling the standardized volumetric flow rate of the nitrogen source within a range of 0.5 to 1.5 slm; (2) depositing a GaN buffer layer atop the nucleation layer with a second V/III ratio in the range of 1817 to 4326 and at temperature of 1000 to 1100 °C, wherein the second V/III ratio is maintained by controlling the standardized volumetric flow rate of the nitrogen source within a range of 2.1 to 5 slm; and (3) laterally growing a GaN overgrowth layer atop the buffer layer with a third V/III ratio in the range of 550 to 750 and at temperature of 1000 to 1100 °C, wherein the third V/III ratio is maintained by controlling the standardized volumetric flow rate of the nitrogen source within a range of 0.5 to 1.5 slm. It is preferred that the nucleation layer has a thickness in the range of 50 to 60 nm. More preferably, 50 nm. Such thickness enables the nucleation layer to act as seed for GaN growth. In the event where GaN nucleation layer has a thickness which exceeds 60 nm, the seed of GaN at the nucleation layer will not promote further a-GaN crystal growth. Instead, such high thickness induces lateral growth thereby affecting overgrowth along the desired direction, hence resulting in high defect densities due to lattice mismatch between a-GaN and r-sapphire. In order to control thickness of the nucleation layer, the step of depositing the GaN nucleation layer is preferably conducted for 90 to 108 seconds (based on growth rate). More preferably, 90 seconds under a growth rate of 0.56 nm/sec. The growth rate may be regulated by growth pressure and flow rate of the precursor.

Preferably, the GaN buffer layer has a thickness of 100 to 110 nm. More preferably, 100 nm. It was found that satisfactory three-dimensional growth up to 110 nm can produce buffer layer having smooth surfaces. In an event where the growth exceeds 110 nm in thickness, the buffer layer has undesired surface roughness which renders coalescence of overgrowth thereafter. In order to achieve such thickness for the buffer layer, the step of depositing the GaN buffer layer is preferably conducted for 180 to 198 (based on growth rate) seconds. More preferably, 180 seconds under a growth rate of 0.56 nm/sec. The growth rate may be regulated by growth pressure and flow rate of the precursor.

Preferably, the GaN overgrowth layer has a thickness of 100 nm to 2 μπι. More preferably, 100 nm. In this regard, the step of depositing the GaN overgrowth layer is preferably conducted for 90 minutes based on the growth rate of 0.45 μΓη/min. The thickness of the GaN overgrowth layer can vary according to application. GaN overgrowth layer having thickness from 200 nm to 1 μπι can be applied in light emitting diode. GaN overgrowth layer having thickness from 1 μπι to 2 μπι is preferably applied to laser diode devices as such thickness of undoped GaN overgrowth layer offers sufficient crystalline quality. For certain application, the GaN overgrowth layer can be doped or used in its un-doped form.

EXAMPLE

An r-plane sapphire substrate was first thermally treated with ambient hydrogen at 1200 °C at 13.3 kPa before subjecting to growth of GaN in an MOCVD reactor. During the growth, hydrogen was used as the carrier gas. Trimethylgallium (TMGa) was used as source material of gallium while ammonia (NH₃) was used as source of nitrogen. Thereinafter, the r-plane sapphire substrate, being placed inside a reactant chamber used for MOCVD, is exposed to vaporized flow of TMGa and NH₃ under conditions specified in Table 1 and Fig. 6. More particularly, the step of depositing GaN nucleation layer was conducted for 90 s and obtained a thickness of 50 nm, the step of depositing and growing GaN buffer layer was conducted for 180 s and has a thickness of 100 nm, and the step of growing GaN overgrowth layer was conducted for 90 mins and has a thickness of 500 nm. Table 1. Growth conditions of the three-step approach in Sample A to F.

Each sample was characterised and explained with references to Table 1 and Fig. 1 to 5 hereinafter. As seen in Fig. la to If, which correspond to Sample A to F respectively, surface images for each sample differs in relation to a combination of the second and third V/III ratios. Further referring to Fig. 2, high reflections can be observed from peaks representing sapphire at (1-102), sapphire (2-204) and GaN (11-20) at 25.64°, 52.68° and 57.78°, respectively. A single reflection peak of GaN (11-20) at 57.78° shows epitaxial growth in a single direction, along a-direction [ 11 -20] .

Surface roughness of Sample A to F was determined by Atomic Force Microscopy (APM), their APM images as shown in Fig. 5a to 5f. V-pits appeared in Fig. la to lc and Fig. 5a to 5c are lesser in density than in Fig. Id to If and Fig. 5d to 5f. This showed that lateral growth is favourable at growth of buffer layer using a second V/III ratio of 1817.

The influence of the third V/III ratio towards the crystalline quality of the a-plane GaN thin film was observed based on changes in full width at half maximum (FWHM) value of an X-ray rocking curve (XRC) of GaN (11-20) as shown in Fig. 3. According to Fig. 1 and Fig. 3, as FWHM value decreases from 4128 to 3627, the feather-like defect starts to disappear from the GaN surface. Further, surface roughness of GaN thin film produced under a third V/III ratio of 220, 439 and 659 are 1.31 nm, 2.01 nm and 2.57 nm, respectively. The present disclosure includes as contained in the appended claims, as well as that of the foregoing description. Although this invention has been described in its preferred form with a degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of the combination may be resorted to without departing from the scope of the invention.

Claims

1. A method for producing a non-polar a-plane gallium nitride (GaN) thin film on an r-plane sapphire substrate using metal organic chemical vapour deposition (MOCVD) technique, the method comprising the steps of: annealing the r-plane sapphire substrate using a hydrogen cleaning process; depositing a GaN nucleation layer in the presence of vaporized sources of gallium and nitrogen on the substrate; depositing and growing a GaN buffer layer atop the nucleation layer; and laterally growing a GaN overgrowth layer atop the buffer layer, wherein the step of depositing and growing of the layers are conducted under a standardized volumetric flow rate of the nitrogen source within a range of 0.5 to 2.1 slm.

2. A method according to claim 1, wherein in-plane orientations of the GaN film with respect to the r-plane substrate are [l l-20]GaN//[l-102] _Sap_Phire, [0001]GaN//[-

1101]_sapphire and/θΓ [l-100]GaN//[l l-20]sapphire.

3. A method according to claim 1 or 2, wherein the step of depositing the nucleation layer is conducted with a first V/III ratio in the range of 550 to 750 and at temperature of 500 to 600 °C, wherein the first V/III ratio is maintained by controlling the standardized volumetric flow rate of the nitrogen source within a range of 0.5 to 1.5 slm.

4. A method according to any one of claims 1 to 3, wherein the step of depositing the buffer layer is conducted with a second V/III ratio in the range of 1817 to 4326 and at temperature of 1000 to 1100 °C, wherein the second V/III ratio is maintained by controlling the standardized volumetric flow rate of the nitrogen source within a range of 2.1 to 5 slm.

5. A method according to any one of claims 1 to 4, wherein the step of depositing the overgrowth layer is conducted with a third V/III ratio in the range of 550 to 750 and at temperature of 1000 to 1100 °C, wherein the third V/III ratio is maintained by controlling the standardized volumetric flow rate of the nitrogen source within a range of 0.5 to 1.5 slm.

6. A method according to any one of claims 1 to 5, wherein the GaN nucleation layer has a thickness of 50 to 60 nm.

7. A method according to any one of claims 1 to 6, wherein the GaN buffer layer has a thickness of 100 to 110 nm.

8. A method according to any one of claims 1 to 7, wherein the GaN overgrown layer has a thickness of 100 nm to 2 um.