# Patent

## Title
Growth of non-polar M-plane III-nitride film using metalorganic chemical vapor deposition (MOCVD)

## Abstract

A method of growing non-polar m-plane III-nitride film, such as GaN, AlN, AlGaN or InGaN, wherein the non-polar m-plane III-nitride film is grown on a suitable substrate, such as an m-SiC, m-GaN, LiGaO2 or LiAlO2 substrate, using metalorganic chemical vapor deposition (MOCVD). The method includes performing a solvent clean and acid dip of the substrate to remove oxide from the surface, annealing the substrate, growing a nucleation layer, such as aluminum nitride (AlN), on the annealed substrate, and growing the non-polar m-plane III-nitride film on the nucleation layer using MOCVD.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of and claims the benefit under 35 U.S.C. Section 120 of co-pending and commonly-assigned U.S. Utility patent application Ser. No. 11/870,115, filed on Oct. 10, 2007, by Bilge M. Imer, James S. Speck, Steven P. Denbaars, and Shuji Nakamura, entitled “GROWTH OF NON-POLAR M-PLANE III-NITRIDE FILM USING METALORGANIC CHEMICAL VAPOR DEPOSITION (MOCVD),” which application is a continuation of U.S. Utility patent application Ser. No. 11/444,083, filed on May 31, 2006, now U.S. Pat. No. 7,338,828, issued Mar. 4, 2008, by Bilge M. Imer, James S. Speck, Steven P. Denbaars, and Shuji Nakamura, entitled “GROWTH OF PLANAR NON-POLAR {1 -1 0 0} M-PLANE GALLIUM NITRIDE WITH METALORGANIC CHEMICAL VAPOR DEPOSITION (MOCVD),” which application claims the benefit under 35 U.S.C. Section 119(e) of U.S. Provisional Patent Application Ser. No. 60/685,908, filed on May 31, 2005, by Bilge M. Imer, James S. Speck and Steven P. Denbaars, entitled “GROWTH OF PLANAR NON-POLAR {1-1 0 0} M-PLANE GALLIUM NITRIDE WITH METALORGANIC CHEMICAL VAPOR DEPOSITION (MOCVD)”;
all of which applications are incorporated by reference herein.
This application is related to the following co-pending and commonly-assigned applications:
U.S. Utility patent application Ser. No. 11/444,084, filed on May 31, 2006, now U.S. Pat. No. 7,361,576, issued Apr. 22, 2008, by Bilge M. Imer, James S. Speck and Steven P. DenBaars, entitled “DEFECT REDUCTION OF NON-POLAR AND SEMI-POLAR III-NITRIDES WITH SIDEWALL LATERAL EPITAXIAL OVERGROWTH (SLEO),” which application claims the benefit under 35 U.S.C. Section 119(e) of U.S. Provisional Patent Application Ser. No. 60/685,952, filed on May 31, 2005 by Bilge M. Imer, James S. Speck and Steven P. DenBaars, entitled “DEFECT REDUCTION OF NON-POLAR GALLIUM NITRIDE WITH SINGLE-STEP SIDEWALL LATERAL EPITAXIAL OVERGROWTH”;
U.S. Utility patent application Ser. No. 10/537,385, filed Jun. 3, 2005, now U.S. Pat. No. 7,427,555, issued Sep. 23, 2008, by Benjamin A. Haskell, Paul T. Fini, Shigemasa Matsuda, Michael D. Craven, Steven P. DenBaars, James S. Speck, and Shuji Nakamura, entitled “GROWTH OF PLANAR, NON-POLAR A-PLANE GALLIUM NITRIDE BY HYDRIDE VAPOR PHASE EPITAXY,” which application claims priority to International Patent Application No. PCT/US03/21916, filed Jul. 15, 2003, by Benjamin A. Haskell, Paul T. Fini, Shigemasa Matsuda, Michael D. Craven, Steven P. DenBaars, James S. Speck, and Shuji Nakamura, entitled “GROWTH OF PLANAR, NON-POLAR A-PLANE GALLIUM NITRIDE BY HYDRIDE VAPOR PHASE EPITAXY,” which application claims priority to U.S. Provisional Patent Application Ser. No. 60/433,844, filed Dec. 16, 2002, by Benjamin A. Haskell, Paul T. Fini, Shigemasa Matsuda, Michael D. Craven, Steven P. DenBaars, James S. Speck, and Shuji Nakamura, entitled “TECHNIQUE FOR THE GROWTH OF PLANAR, NON-POLAR A-PLANE GALLIUM NITRIDE BY HYDRIDE VAPOR PHASE EPITAXY”; and
U.S. Utility patent application Ser. No. 10/413,691, filed Apr. 15, 2003, by Michael D. Craven and James S. Speck, entitled “NON-POLAR A-PLANE GALLIUM NITRIDE THIN FILMS GROWN BY METALORGANIC CHEMICAL VAPOR DEPOSITION,” which application claims priority to U.S. Provisional Patent Application Ser. No. 60/372,909, filed Apr. 15, 2002, by Michael D. Craven, Stacia Keller, Steven P. DenBaars, Tal Margalith, James S. Speck, Shuji Nakamura, and Umesh K. Mishra, entitled “NON-POLAR GALLIUM NITRIDE BASED THIN FILMS AND HETEROSTRUCTURE MATERIALS”;
all of which applications are incorporated by reference herein.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the growth of non-polar m-plane III-nitride film using metalorganic chemical vapor deposition (MOCVD).
2. Description of the Related Art
Gallium nitride (GaN) and its ternary and quaternary compounds are prime candidates for fabrication of visible and ultraviolet high-power and high-performance optoelectronic devices and electronic devices. These devices are typically grown epitaxially by growth techniques including molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), or hydride vapor phase epitaxy (HVPE).
The selection of substrate is critical for achieving the desired GaN growth orientation. Some of the most widely used substrates for III-N growth include SiC, Al2O3, and LiAlO2. Various crystallographic orientations of these substrates are commercially available.
FIGS. 1(a) and 1(b) are schematics of crystallographic directions and planes of interest in hexagonal GaN. Specifically, these schematics show the different crystallographic growth directions and also the planes of interest in the hexagonal wurtzite GaN structure, wherein FIG. 1(a) shows the crystallographic directions a1, a2, a3, c, <10-10> and <11-20>, and FIG. 1(b) shows planes a (11-20), m (10-10) and r (10-12). The fill patterns of FIG. 1(b) are intended to illustrate the planes of interest, but do not represent the materials of the structure.
It is relatively easy to grow planar c-plane GaN due to its large growth stability window. Therefore, nearly all GaN-based devices are grown parallel to the polar c-axis. However, as a result of c-plane growth, each material layer suffers from separation of electrons and holes to opposite faces of the layers. Furthermore, strain at the interfaces between adjacent layers gives rise to piezoelectric polarization, causing further charge separation.
FIGS. 2(a) and 2(b), which are schematics of band bending and electron hole separation as a result of polarization, show this effect, wherein FIG. 2(a) is a graph of energy (eV) vs. depth (nm) and represents a c-plane quantum well, while FIG. 2(b) is a graph of energy (eV) vs. depth (nm) and represents a non-polar quantum well.
Such polarization effects decrease the likelihood of electrons and holes recombining, causing the final device to perform poorly. One possible approach for eliminating piezoelectric polarization effects in GaN optoelectronic devices is to grow the devices on non-polar planes of the crystal such as a-{11-20} and m-{1-100} planes family of GaN. Such planes contain equal numbers of Ga and N atoms and are charge-neutral.
Planar {1-100} m-plane GaN growth has been developed by HVPE and MBE methods. However, prior to the invention described herein, planar m-plane GaN growth had not been accomplished with MOCVD.
SUMMARY OF THE INVENTION
The general purpose of the present invention is to grow non-polar m-plane III-nitride film using MOCVD. The method includes performing a solvent clean and acid dip of an m-SiC substrate to remove oxide from the surface of the substrate ex situ prior to growth, in situ annealing of the substrate, growing a nucleation layer on the annealed substrate, and growing the non-polar m-plane III-nitride film on the nucleation layer using MOCVD. The present invention takes advantage of non-polar nature of the m-plane III-nitride film to eliminate polarization fields, and gives rise to flexibility in growth variables, such as temperature, pressure and precursor flows, utilizing the advantage of the m-plane III-nitride film's stability during growth.



BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the drawings in which like reference numbers represent corresponding parts throughout:
FIGS. 1(a) and 1(b) are schematics of crystallographic directions and planes of interest in hexagonal GaN.
FIGS. 2(a) and 2(b) are schematics of band bending and electron hole separation as a result of polarization.
FIG. 3 provides a structural characterization of non-polar planar m-plane GaN on m-plane SiC, from top to bottom, wherein the crystal plane of interest is shown in a unit cell/
FIG. 4 is a 5 μm×5 μm atomic force microscopy (AFM) surface image with a surface roughness value 2.54 nm.
FIG. 5 is a graph that illustrates the xray diffraction rocking curves for on-axis and off-axis.
FIG. 6 is a flowchart that illustrates the processing steps for growing planar m-plane III-Nitrides using MOCVD according to the preferred embodiment of the present invention.
FIG. 7 further illustrates the results of the processing steps of FIG. 6 according to the preferred embodiment of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
In the following description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.
Overview
The growth of (Ga, In, Al, B)N materials in the polar [0001] c-direction causes lower performance in optical devices due to polarization fields causing charge separation along the primary conduction direction. Therefore, recent research has been conducted focusing on non-polar direction growth along a-[11-20] and m-[1-100] directions of these materials to eliminate such effects and so to improve the device performance significantly. While both a-plane and m-plane growth of GaN has been explored by HVPE and MBE, only non-polar a-{11-20} plane growth of GaN has been demonstrated by MOCVD. However, it has been found that the growth window for planar a-plane GaN is very small and this specific orientation is very sensitive to changes in growth variables such as pressure and precursor flows. This resulted in the exploration of a new non-polar orientation in GaN growth with MOCVD. However, for m-plane growth, substrate availability has been a problem due to high growth temperatures required in MOCVD. Commercially available substrates such as γ-LiAlO2 have melting points lower than the temperature required for MOCVD growth. With the emergence of commercially obtainable m-SiC substrates, which are stable during MOCVD growth, the current invention was made possible. The present invention is the first ever successful growth of m-{1-100} plane GaN on m-SiC by MOCVD.
Technical Description
The m-plane SiC substrate is annealed in hydrogen prior to growth. An AlN layer is formed as a nucleation layer before GaN film growth. Finally, a GaN layer is grown by MOCVD. FIG. 3 illustrates the non-polar m-plane GaN (1-100) crystal plane of interest in the unit cell.
To accomplish the optimum quality m-plane GaN, VIII ratios of 400-5500 and 200-3000, growth pressures varying in between 50-760 Torr, and temperature series of 1100° C.-1275° C. and 1000° C.-1160° C. for AlN and GaN layers were tested, respectively. The m-plane, for both AlN and GaN, was stable over this wide range of temperatures, reactor pressures, and precursor flows.
The optimum AlN nucleation layers, leading to best quality GaN films, were realized at temperatures over 1175° C., at relatively low pressures, and VIII ratio of  ̃3500 with the nucleation layer thickness below 150 nm.
For GaN layer epitaxy, the most favorable conditions were realized at low pressures, such as below 100 Torr, at temperatures in the range of 1100° C.-1160° C., and at VIII ratios below 700 with low NH3 vapor pressure.
A 5 μm×5 μm atomic force microscopy (AFM) surface image of the resulting m-plane GaN material is shown in FIG. 4. The grains are oriented along the <11-20> direction and the surface roughness value (root mean square) is  ̃2.54 nm for a 5 μm×5 μm scan.
FIG. 5 is a graph of omega (degrees) vs. counts/second showing the x-ray diffraction rocking curves on-axis and off-axis. As can been seen from Table 1 below, on-axis (1-100) full width at half max (FWHM) values are measured as low as 0.22° and 1.2°, for a-mosaic and c-mosaic, respectively, and the off-axis (10-12) reflection has FWHM value of 0.38°. These roughness and FWHM values were found to not change significantly by changing growth conditions of the nucleation layer and epitaxial GaN film itself.







TABLE 1







Rocking curve FWHM values








On-Axis Values










a-mosaic
c-mosaic
Off-Axis












0.22° 
1.2° 
0.38° 









Process Steps
FIG. 6 is a flowchart that illustrates the processing steps for growing a planar non polar {1-100} m-plane III-Nitride epitaxial film using MOCVD according to the preferred embodiment of the present invention, wherein the planar non polar m-plane III-Nitride epitaxial film may comprise a planar m-plane GaN epitaxial layer. FIG. 7 further illustrates the results of each of the processing steps of FIG. 6.
Block 600 represents a solvent clean and acid dip of a suitable substrate (700), for example, in a 1:10 diluted BHF:DI solution, to remove oxide (702) from the substrate (700) surface before loading the substrate (700) into a reactor for the growth step. (Although this step is recommended, its omission would not significantly alter the results.) The substrate (700) may comprise an m-SiC or any substrate that is suitable for non-polar m-plane III-Nitride growth.
Block 602 represents in situ annealing of the substrate (700), for example, in hydrogen, prior to the growth step. (Although this step is recommended, its omission would not significantly alter the results.)
Block 604 represents growing a nucleation layer (704) on the substrate (700). The nucleation layer (704) typically comprises an aluminum nitride (AlN) nucleation layer or interlayer, but may comprise any nucleation layer (704) that is appropriate for non-polar m-plane III-Nitride growth. Moreover, the nucleation layer (704) may be grown after the annealing step, and prior to the non polar m-plane III-Nitride growth.
Block 606 represents growing the non-polar m-plane III-Nitride epitaxial layer (706) using MOCVD. The non-polar m-plane III-Nitride epitaxial layer (706) typically comprises a non-polar m-plane GaN epitaxial layer, but may comprise other non-polar m-plane III-Nitride epitaxial layers as well. Moreover, the non-polar m-plane III-Nitride epitaxial layer (706) may be grown on the nucleation layer (704), or on the substrate (700) itself.
Preferably, the end result is a device, or a free standing wafer, or a substrate, or a template, having a planar epitaxial layer of the non-polar m-plane III-Nitride.
Possible Modifications and Variations
Although the preferred embodiment describes the MOCVD growth of non-polar m-GaN on m-SiC using an AlN interlayer, alternative suitable substrates, on which the non-polar m-plane III-Nitride epitaxial film could be formed, include, but are not limited to, 6H or 4H m-plane SiC, freestanding m-GaN, LiGaO2 and LiAlO2.
Prior to growth, the suitable substrate can be treated in many different ways in-situ or ex-situ, or it may not be treated at all.
The non-polar epitaxial film can be nucleated and grown over different nucleation layers, such as GaN or AlN grown at various conditions and methods, or over a bare substrate.
The epitaxial film can be any non-polar m-plane III-Nitride material including, but not limited to, GaN, AlN, AlGaN and InGaN with various thicknesses.
The growth parameters required for the growth of non-polar m-plane III-Nitride material may vary from reactor to reactor.
Finally, it is understood that processing steps may be omitted, added or rearranged as desired.
Such variations do not fundamentally alter the general practice of this invention.
Advantages and Improvements
The growth of m-{1-100} plane GaN has been successfully demonstrated by HVPE and MBE. However, the present invention is the first-ever successful demonstration of high-quality planar non-polar m-{1-100} plane GaN growth by MOCVD.
Growth of planar m-plane GaN has an advantage over growth of planar a-{11-20} GaN with MOCVD in terms of its stability with a large growth window. This was shown when growth variables such as temperature, pressure and precursor flows for AlN nucleation layer and GaN epitaxial film were changed.
To accomplish the optimum quality m-plane GaN, VIII ratios of 400-5500 and 200-3000, growth pressures varying in between 50-760 Torr, and temperature series of 1100° C.-1275° C. and 1000° C.-1160° C. for AlN and GaN layers were tested, respectively. Alterations in such conditions did not affect the crystal and surface quality significantly unlike the planar non-polar a-plane GaN films in which crystal and surface quality are extremely susceptible to change in growth conditions and constrained with small growth window.
The growth stability advantage combined with the non-polar nature of m-GaN brings new possibilities in III-Nitride non-polar device research.
REFERENCES
The following reference is incorporated by reference herein:
1. “Molecular-beam epitaxy of GaN/AlxGa1-xN multiple quantum wells on R-plane (10-12) sapphire substrates,” H. M. Ng, Appl. Phys. Lett. 80, 4369 (2002)
CONCLUSION
This concludes the description of the preferred embodiment of the present invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching, such as additional adjustments to the process described herein, without fundamentally deviating from the essence of the present invention. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.

## Claims

1. A III-nitride crystal, comprising:
a top surface having an area of least 5 micrometers by 5 micrometers, wherein:
the top surface has a non-polar m-plane orientation, and
a surface roughness of the top surface, as grown, is not more than 2.54 nanometers at least for the area of 5 micrometers by 5 micrometers, and
the III-nitride crystal has a crystalline quality characterized by an off-axis rocking curve having an off-axis full width at half maximum (FWHM) of no more than 0.38°, as measured by X-ray diffraction.

2. The III-nitride crystal of claim 1, wherein:
the III-nitride crystal has a crystalline quality characterized by an on-axis rocking curve having an on-axis (1-100) full width at half maximum (FWHM) of:
no more than 0.22°, for an a-mosaic of the III-nitride crystal, or
no more than 1.2°, for a c-mosaic of the III-nitride crystal or
no more than 0.22°, for an a-mosaic of the III-nitride crystal and no more than 1.2°, for a c-mosaic of the III-nitride crystal, and
the rocking curves are measured by X-ray diffraction.

3. The III-nitride crystal of claim 2, wherein the off-axis is (10-12).

4. The III-nitride crystal of claim 1, wherein the III-nitride crystal is a Gallium Nitride crystal.

5. The III-nitride crystal of claim 1, wherein the III-nitride crystal comprises AlN, AlGaN or InGaN.

6. The III-nitride crystal of claim 1, further comprising:
a nucleation or first nitride layer grown on a substrate; and
the III-nitride crystal grown on the nucleation layer or the first nitride layer.

7. The III-nitride crystal of claim 6, wherein the nucleation layer or first nitride layer comprises aluminum nitride (AlN).

8. The III-nitride crystal of claim 1, wherein the III-nitride crystal is a device, wafer, substrate, or template.

9. The III-nitride crystal of claim 8, wherein the III-nitride crystal comprises Gallium Nitride.

10. The III-nitride crystal of claim 1, wherein the III-nitride crystal is sufficiently thick to be free-standing.

11. The III-nitride crystal of claim 1, further comprising a non-polar optoelectronic or electronic device grown on the top surface.

12. The III-nitride crystal of claim 1, wherein the III-nitride crystal is grown on a Silicon Carbide substrate.

13. The III-nitride crystal of claim 1, wherein the III-nitride crystal is grown on a Gallium Nitride substrate.

14. The III-nitride crystal of claim 1, wherein the III-nitride crystal is grown on a LiGaO2 or LiAlO2 substrate.

15. The III-nitride crystal of claim 1, wherein individual grains of the III-nitride crystal are not distinguishable based on height measurements of the surface.

16. The III-nitride crystal of claim 1, wherein the surface results from predominantly stepped growth.

17. The crystal of claim 1, wherein the top surface is a largest surface of the III-nitride crystal.

18. The III-nitride crystal of claim 1, wherein the top surface is planar.

19. The III-nitride crystal of claim 1, wherein the III-nitride crystal is an initial III-nitride crystal.

20. A device structure comprising the III-nitride crystal of claim 1, wherein:
the III-nitride crystal is a non-polar m-plane film having a crystalline quality characterized by a rocking curve having a full width at half maximum (FWHM) of no more than 0.22° as measured by X-Ray diffraction, and
the top surface of the non-polar m-plane film is epitaxially grown.

21. The device structure of claim 20, wherein the non-polar m-plane III-nitride film comprises different III-nitride layers.

22. A device structure, comprising:
a non-polar m-plane III-nitride crystal, wherein:
the non-polar m-plane III-nitride crystal is grown on a surface of a substrate,
a subsequent non-polar m-plane III-nitride layer is grown on a top surface of the non-polar m-plane III-nitride crystal,
the top surface includes an area with a root mean square surface roughness of not more than 2.54 nanometers at least for an area of 5 micrometers by 5 micrometers, and

the III-nitride crystal has a crystalline quality characterized by an off-axis rocking curve having an off-axis full width at half maximum (FWHM) of no more than 0.38°, as measured by X-ray diffraction.

23. A method for growing an III-nitride crystal, comprising:
growing a top surface of a III-nitride crystal using growth conditions, wherein:
the top surface has an area of least 5 micrometers by 5 micrometers,
the top surface has a non-polar m-plane orientation,
a surface roughness of the top surface, as grown, is not more than 2.54 nanometers
at least for the area of 5 micrometers by 5 micrometers,
the III-nitride crystal has a crystalline quality characterized by an off-axis rocking
curve having an off-axis full width at half maximum (FWHM) of no more than 0.38°, as measured by X-ray diffraction.

24. The method of claim 23, wherein the growing is by Metal Organic Chemical Vapor Deposition using optimized growth conditions.

25. The method of claim 23, wherein the III-nitride crystal is Gallium Nitride or Aluminum Nitride, and the growing is at:
a V/III ratio between 200-5500,
a growth pressure between 50-760 Torr, and
a growth temperature of at least 1000° C.

26. The method of claim 25, further comprising:
growing an Aluminum Nitride (AlN) layer on the substrate at a temperature over 1175° C. and to a thickness below 150 nanometers; and
growing the III-nitride crystal on the AlN layer.

27. The method of claim 23, wherein the III-nitride crystal is Gallium Nitride and the growing is at a growth pressure below 100 Torr.

28. The method of claim 27, wherein the growing is at a V/III ratio below 700.

29. The method of claim 28, wherein the growing is at a temperature in a range of 1100° C.-1160° C.

30. The method of claim 23, further comprising:
growing a first nitride layer on a substrate at a temperature of at least 1000° C., and growing the non-polar m-plane III-Nitride layer on the first nitride layer, at the temperature of at least 1000° C.

31. The method of claim 30, wherein the first III-nitride layer comprises aluminum nitride (AlN) or Gallium Nitride.