Abstract:
A method of growing planar non-polar m-plane III-Nitride material, such as an m-plane gallium nitride (GaN) epitaxial layer, wherein the III-Nitride material is grown on a suitable substrate, such as an m-plane silicon carbide (m-SiC) 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 an aluminum nitride (AIN) on the annealed substrate, and growing the non-polar m-plane III-Nitride epitaxial layer on the nucleation layer using MOCVD.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit under 35 U.S.C. Section 19(e) of the following co-pending and commonly-assigned U.S. patent application:  
         [0002]     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 {1-100} M-PLANE GALLIUM NITRIDE WITH METALORGANIC CHEMICAL VAPOR DEPOSITION (MOCVD),” attorneys&#39; docket no. 30794.136-US-P1 (2005-566);  
         [0003]     which application is incorporated by reference herein.  
         [0004]     This application is related to the following co-pending and commonly-assigned applications:  
         [0005]     U.S. Utility Patent Application Ser. No. ______, filed on same date herewith, 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),” attorneys&#39; docket no. 30794.135-US-U1 (2005-565), 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,” attorneys&#39; docket no. 30794.135-US-P1 (2005-565);  
         [0006]     U.S. Utility Patent Application Ser. No. 10/537,385, filed Jun. 3, 2005, 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,” attorneys docket number 30794.094-US-Wo (2002-225-2), 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,” attorneys docket number 30794.94-WO-U1 (2003-225-2), 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,” attorneys docket number 30794.94-US-P1 (2003-225-1); and  
         [0007]     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,” attorneys docket number 30794.100-US-U1 (2002-294-2), 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,” attorneys docket number 30794.95-US-P1 (2002-294/301/303);  
         [0008]     all of which applications are incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0009]     1. Field of the Invention  
         [0010]     The present invention relates to the growth of planar non-polar {1-1 0 0} m-plane gallium nitride (GaN) with metalorganic chemical vapor deposition (MOCVD).  
         [0011]     2. Description of the Related Art  
         [0012]     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).  
         [0013]     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, Al 2 O 3 , and LiAlO 2 . Various crystallographic orientations of these substrates are commercially available.  
         [0014]     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 a 1 , a 2 , a 3 , c, &lt;10-10&gt; and &lt;11-20&gt;, 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.  
         [0015]     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.  
         [0016]     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.  
         [0017]     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.  
         [0018]     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  
       [0019]     The general purpose of the present invention is to grow planar non-polar m-{1-100} plane GaN material 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 an aluminum nitride (AIN) nucleation layer on the annealed substrate, and growing the non-polar m-plane GaN epitaxial layer on the nucleation layer with MOCVD. The present invention takes advantage of non-polar nature of m-plane GaN to eliminate polarization fields, and gives rise to flexibility in growth variables, such as temperature, pressure and precursor flows, utilizing the advantage of m-GaN stability during growth. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     Referring now to the drawings in which like reference numbers represent corresponding parts throughout:  
         [0021]     FIGS.  1 ( a ) and  1 ( b ) are schematics of crystallographic directions and planes of interest in hexagonal GaN.  
         [0022]     FIGS.  2 ( a ) and  2 ( b ) are schematics of band bending and electron hole separation as a result of polarization.  
         [0023]      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/  
         [0024]      FIG. 4  is a 5 μm×5 μm atomic force microscopy (AFM) surface image with a surface roughness value 2.54 nm.  
         [0025]      FIG. 5  is a graph that illustrates the xray diffraction rocking curves for on-axis and off-axis.  
         [0026]      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.  
         [0027]      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  
       [0028]     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.  
         [0029]     Overview  
         [0030]     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-LiAlO 2  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.  
         [0031]     Technical Description  
         [0032]     The m-plane SiC substrate is annealed in hydrogen prior to growth. An AIN 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.  
         [0033]     To accomplish the optimum quality m-plane GaN, V/III 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 AIN and GaN layers were tested, respectively. The m-plane, for both AIN and GaN, was stable over this wide range of temperatures, reactor pressures, and precursor flows.  
         [0034]     The optimum AIN nucleation layers, leading to best quality GaN films, were realized at temperatures over 1175° C., at relatively low pressures, and V/III ratio of ˜3500 with the nucleation layer thickness below 150 nm.  
         [0035]     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 V/III ratios below 700 with low NH 3  vapor pressure.  
         [0036]     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 &lt;11-20&gt; direction and the surface roughness value (root mean square) is ˜2.54 nm for a 5 μm×5 μm scan.  
         [0037]      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.  
                                                           Rocking curve FWHM values                On-Axis Values                a-mosaic   c-mosaic   Off-Axis               0.22°   1.2°   0.38°                    
         [0038]     Process Steps  
         [0039]      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 .  
         [0040]     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.  
         [0041]     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.)  
         [0042]     Block  604  represents growing a nucleation layer ( 704 ) on the substrate ( 700 ). The nucleation layer ( 704 ) typically comprises an aluminum nitride (AIN) 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.  
         [0043]     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.  
         [0044]     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.  
         [0045]     Possible Modifications and Variations  
         [0046]     Although the preferred embodiment describes the MOCVD growth of non-polar m-GaN on m-SiC using an AIN interlayer, alternative suitable substrates, on which the non-polar m-plane III-Nitride epitaxial film could be formed, include, but are not limited to,  6 H or 4H m-plane SiC, freestanding m-GaN, LiGaO 2  and LiAlO 2 .  
         [0047]     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.  
         [0048]     The non-polar epitaxial film can be nucleated and grown over different nucleation layers, such as GaN or AIN grown at various conditions and methods, or over a bare substrate.  
         [0049]     The epitaxial film can be any non-polar m-plane III-Nitride material including, but not limited to, GaN, AIN, AlGaN and InGaN with various thicknesses.  
         [0050]     The growth parameters required for the growth of non-polar m-plane III-Nitride material may vary from reactor to reactor.  
         [0051]     Finally, it is understood that processing steps may be omitted, added or rearranged as desired.  
         [0052]     Such variations do not fundamentally alter the general practice of this invention.  
         [0053]     Advantages and Improvements  
         [0054]     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.  
         [0055]     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.  
         [0056]     To accomplish the optimum quality m-plane GaN, V/III 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 AIN 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.  
         [0057]     The growth stability advantage combined with the non-polar nature of m-GaN brings new possibilities in III-Nitride non-polar device research.  
       REFERENCES  
       [0058]     The following reference is incorporated by reference herein:  
         [0059]     1. “Molecular-beam epitaxy of GaN/Al x Ga 1-x N multiple quantum wells on R-plane (10-12) sapphire substrates,” H. M. Ng, Appl. Phys. Lett. 80, 4369 (2002)  
         [0060]     Conclusion  
         [0061]     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.