Patent Publication Number: US-7217641-B2

Title: Pendeoepitaxial methods of fabricating gallium nitride semiconductor layers on sapphire substrates, and gallium nitride semiconductor structures fabricated thereby

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of and claims priority from U.S. patent application Ser. No. 10/404,616, now issued U.S. Pat. No. 6,686,261, filed Apr. 1, 2003, which in turn is a continuation application of, and claims priority from, U.S. patent application Ser. No. 09/899,586 filed Jul. 3, 2001, now issued U.S. Pat. No. 6,545,300, which in turn is a divisional application and claims priority from, U.S. patent application Ser. No. 09/441,753, filed Nov. 17, 1999, now issued U.S. Pat. No. 6,521,514, the entire disclosures of which are incorporated herein by reference. 
    
    
     FEDERALLY SPONSORED RESEARCH 
     This invention was made with Government support under Office of Naval Research Contract Nos. N00014-96-1-0765, N00014-98-1-0384, and N00014-98-1-0654. The Government may have certain rights to this invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to microelectronic devices and fabrication methods, and more particularly to gallium nitride semiconductor devices and fabrication methods therefor. 
     BACKGROUND OF THE INVENTION 
     Gallium nitride is being widely investigated for microelectronic devices including but not limited to transistors, field emitters and optoelectronic devices. It will be understood that, as used herein, gallium nitride also includes alloys of gallium nitride such as aluminum gallium nitride, indium gallium nitride and aluminum indium gallium nitride. 
     A major problem in fabricating gallium nitride-based microelectronic devices is the fabrication of gallium nitride semiconductor layers having low defect densities. It is known that one contributor to defect density is the substrate on which the gallium nitride layer is grown. Accordingly, although gallium nitride layers have been grown on sapphire substrates, it is known to reduce defect density by growing gallium nitride layers on aluminum nitride buffer layers which are themselves formed on silicon carbide substrates. Notwithstanding these advances, continued reduction in defect density is desirable. 
     It also is known to produce low defect density gallium nitride layers by forming a mask on a layer of gallium nitride, the mask including at least one opening therein that exposes the underlying layer of gallium nitride, and laterally growing the underlying layer of gallium nitride through the at least one opening and onto the mask. This technique often is referred to as “Epitaxial Lateral Overgrowth” (ELO). The layer of gallium nitride may be laterally grown until the gallium nitride coalesces on the mask to form a single layer on the mask. In order to form a continuous layer of gallium nitride with relatively low defect density, a second mask may be formed on the laterally overgrown gallium nitride layer, that includes at least one opening that is offset from the opening in the underlying mask. ELO then again is performed through the openings in the second mask to thereby overgrow a second low defect density continuous gallium nitride layer. Microelectronic devices then may be formed in this second overgrown layer. ELO of gallium nitride is described, for example, in the publications entitled  Lateral Epitaxy of Low Defect Density GaN Layers Via Organometallic Vapor Phase Epitaxy  to Nam et al., Appl. Phys. Lett. Vol. 71, No. 18, Nov. 3, 1997, pp. 2638–2640; and  Dislocation Density Reduction Via Lateral Epitaxy in Selectively Grown GaN Structures  to Zheleva et al, Appl. Phys. Lett., Vol. 71, No. 17, Oct. 27, 1997, pp. 2472–2474, the disclosures of which are hereby incorporated herein by reference. 
     It also is known to produce a layer of gallium nitride with low defect density by forming at least one trench or post in an underlying layer of gallium nitride to define at least one sidewall therein. A layer of gallium nitride is then laterally grown from the at least one sidewall. Lateral growth preferably takes place until the laterally grown layers coalesce within the trenches. Lateral growth also preferably continues until the gallium nitride layer that is grown from the sidewalls laterally overgrows onto the tops of the posts. In order to facilitate lateral growth and produce nucleation of gallium nitride and growth in the vertical direction, the top of the posts and/or the trench floors may be masked. Lateral growth from the sidewalls of trenches and/or posts also is referred to as “pendeoepitaxy” and is described, for example, in publications entitled  Pendeo - Epitaxy: A New Approach for Lateral Growth of Gallium Nitride Films  by Zheleva et al., Journal of Electronic Materials, Vol. 28, No. 4, February 1999, pp. L5–L8; and  Pendeoepitaxy of Gallium Nitride Thin Films  by Linthicum et al., Applied Physics Letters, Vol. 75, No. 2, July 1999, pp. 196–198, the disclosures of which are hereby incorporated herein by reference. 
     ELO and pendeoepitaxy can provide relatively large, low defect gallium nitride layers for microelectronic applications. However, a major concern that may limit the mass production of gallium nitride devices is the growth of the gallium nitride layers on a silicon carbide substrate. Notwithstanding silicon carbide&#39;s increasing commercial importance, silicon carbide substrates still may be relatively expensive. Moreover, it may be difficult to use silicon carbide substrates in optical devices, where back illumination may be desired, because silicon carbide is opaque Accordingly, the use of an underlying silicon carbide substrate for fabricating gallium nitride microelectronic structures may adversely impact the cost and/or applications of gallium nitride devices. 
     SUMMARY OF THE INVENTION 
     The present invention pendeoepitaxially grows sidewalls of posts in an underlying gallium nitride layer that itself is on a sapphire substrate, by treating the underlying gallium nitride layer and/or the sapphire substrate to prevent vertical growth of gallium nitride from the trench floor from interfering with the pendeoepitaxial growth of the gallium nitride sidewalls of the posts. Thus, widely available sapphire substrates may be used for pendeoepitaxial of gallium nitride, to thereby allow reduced cost and/or wider applications for gallium nitride devices. 
     More specifically, gallium nitride semiconductor layers may be fabricated by etching an underlying gallium nitride layer on a sapphire substrate, to define at least one post in the underlying gallium nitride layer and at least one trench in the underlying gallium nitride layer. The at least one post includes a gallium nitride top and a gallium nitride sidewall. The at least one trench includes a trench floor. The gallium nitride sidewalls are laterally grown into the at least one trench, to thereby form a gallium nitride semiconductor layer. However, prior to performing the laterally growing step, the sapphire substrate and/or the underlying gallium nitride layer is treated to prevent growth of gallium nitride from the trench floor from interfering with the lateral growth of the gallium nitride sidewalls of the at least one post into the at least one trench. 
     The sapphire substrate may be etched beneath the at least one trench sufficiently deep to create a sapphire floor and to prevent vertical growth of gallium nitride from the sapphire floor from interfering with the lateral growth of the gallium nitride sidewalls of the at least one post into the at least one trench. Alternatively or in addition, the trench floor may be masked with a mask. In yet other alternatives, the underlying gallium nitride layer is selectively etched to expose the sapphire substrate and create a sapphire floor. The gallium nitride post tops also may be masked to reduce nucleation of gallium nitride thereon, compared to on gallium nitride. Following growth, at least one microelectronic device may be formed in the gallium nitride semiconductor layer. 
     Even more specifically, an underlying gallium nitride layer on a sapphire substrate is etched to selectively expose the sapphire substrate and define at least one post and at least one trench in the underlying gallium nitride layer. The at least one post each includes a gallium nitride top and a gallium nitride sidewall. The at least one trench includes a sapphire floor. The gallium nitride sidewall of the at least one post is grown laterally into the at least one trench, to thereby form a gallium nitride semiconductor layer. 
     Preferably, when etching the underlying gallium nitride layer on the sapphire substrate, the sapphire substrate is etched as well, to define at least one post in the underlying gallium nitride layer and in the sapphire substrate, and at least one trench in the underlying gallium nitride layer and in the sapphire substrate. The at least one post each includes a gallium nitride top, a gallium nitride sidewall and a sapphire sidewall. The at least one trench includes a sapphire floor. More preferably, the sapphire substrate is etched sufficiently deep to prevent vertical growth of gallium nitride from the sapphire floor from interfering with the step of laterally growing the gallium nitride sidewalls of the at least one post into the at least one trench. For example, the sapphire sidewall height to sapphire floor width ratio exceeds about ¼. In another embodiment, the sapphire floor is masked with a mask that reduces nucleation of gallium nitride thereon compared to on sapphire. 
     In yet other embodiments, the sapphire substrate includes an aluminum nitride buffer layer thereon. During the etching step, the gallium nitride layer and the aluminum nitride buffer layer both are etched to selectively expose the sapphire substrate. In other embodiments, the sapphire substrate also is selectively etched so that the trenches extend into the sapphire substrate. 
     Lateral growth preferably proceeds pendeoepitaxially by laterally overgrowing the gallium nitride sidewall onto the gallium nitride top, to thereby form a gallium nitride semiconductor layer. Prior to pendeoepitaxial growth, the gallium nitride top may be masked with a mask that reduces nucleation of gallium nitride thereon compared to on gallium nitride. 
     According to another aspect of the present invention, the trench floor may be masked with a mask, thereby obviating the need to expose the sapphire substrate. Specifically, an underlying gallium nitride layer on a sapphire substrate may be etched to define at least one post in the underlying gallium nitride and at least one trench in the underlying gallium nitride layer. The at least one post includes a top and a sidewall and the at least one trench includes a trench floor. The at least one floor is masked with a mask, and the sidewall of the at least one post is laterally grown into the at least one trench, to thereby form a gallium nitride semiconductor layer. As was described above, the post tops also may be masked. Preferably, the at least one floor and the at least one top are masked simultaneously, for example by performing a directional deposition that forms a mask on the lateral tops and floors, but not on the sidewalls. As also was described above, when an aluminum nitride buffer layer is present, it may be etched to define the posts and trenches, or the mask may be formed on the aluminum nitride buffer layer. In another alternative, the trench floor may be located in the gallium nitride layer itself, and the gallium nitride trench floor may be masked as was described above. 
     Embodiments of gallium nitride semiconductor structures according to the present invention can include a sapphire substrate and an underlying gallium nitride layer on the sapphire substrate. The underlying gallium nitride layer includes therein at least one post and at least one trench. The at least one post each includes a gallium nitride top and a gallium nitride sidewall. The at least one trench includes a sapphire floor. A lateral gallium nitride layer extends laterally from the gallium nitride sidewall of the at least one post into the at least one trench. In a preferred embodiment, the at least one trench extends into the sapphire substrate such that the at least one post each includes a gallium nitride top, a gallium nitride sidewall and a sapphire sidewall and the at least one trench includes a sapphire floor. The sapphire floor preferably is free of a vertical gallium nitride layer thereon and the sapphire sidewall height to sapphire floor width ratio may exceed about ¼. A mask may be included on the sapphire floor and an aluminum nitride buffer layer also may be included between the sapphire substrate and the underlying gallium nitride layer. A mask also may be included on the gallium nitride top. The mask on the floor and the mask on the top preferably comprise same material. 
     Other embodiments of gallium nitride semiconductor structures according to the present invention also can include a sapphire substrate and an underlying gallium nitride layer on the sapphire substrate. The underlying gallium nitride layer includes therein at least one post and at least one trench. The at least one post includes a gallium nitride top and a gallium nitride sidewall, and the at least one trench includes a trench floor. A mask is included on the at least one trench floor, and the gallium nitride layer extends laterally from the gallium nitride sidewall of the at least one post into the at least one trench. In a preferred embodiment, the trench floor is a sapphire floor. A mask may be provided on a gallium nitride top that preferably comprises the same material as the mask on the trench floor. An aluminum nitride buffer layer also may be provided, as was described above. At least one microelectronic device may be formed in the gallium nitride semiconductor layer. 
     Accordingly, sapphire may be employed as a substrate for growing gallium nitride semiconductor layers that can have low defect densities. Low cost and/or high availability gallium nitride devices thereby may be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1–5  are cross-sectional views of first gallium nitride microelectronic structures during intermediate fabrication steps, according to the present invention. 
         FIGS. 6–10  are cross-sectional views of other gallium nitride microelectronic structures during intermediate fabrication steps, according to the present invention. 
         FIGS. 11–16  are cross-sectional views of yet other gallium nitride microelectronic structures during intermediate fabrication steps, according to the present invention. 
         FIGS. 17–22  are cross-sectional views of still other gallium nitride microelectronic structures during intermediate fabrication steps, according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. It will be understood that when an element such as a layer, region or substrate is referred to as being “on” or “onto” another element, it can be directly on the other element or intervening elements may also be present. Moreover, each embodiment described and illustrated herein includes its complementary conductivity type embodiment as well. 
     Referring now to  FIGS. 1–5 , methods of fabricating gallium nitride semiconductor structures according to embodiments of the present invention now will be described. As shown in  FIG. 1 , an underlying gallium nitride layer  104  is grown on a substrate  102 . The substrate  102  includes a sapphire (Al 2 O 3 ) substrate  102   a , preferably with (0001) (c-plane) orientation, and also preferably includes an aluminum nitride and/or gallium nitride buffer layer  102   b . The crystallographic designation conventions used herein are well known to those having skill in the art, and need not be described further. The gallium nitride layer  104  may be between 0.5 and 2.0 μm thick, and may be grown at 1000° C. on a low temperature (600° C.) aluminum nitride buffer layer and/or a low temperature (500°) gallium nitride buffer layer  102   b  that was deposited on the sapphire substrate  102   a  in a cold wall vertical and inductively heated metalorganic vapor phase epitaxy system using triethylgallium at 26 μmol/min, ammonia at 1500 sccm and 3000 sccm hydrogen diluent. The growth of a gallium nitride layer on a sapphire substrate including an aluminum nitride buffer layer is described in publications entitled  Improvements on the Electrical and Luminescent Properties of Reactive Molecular Beam Epitaxially Grown GaN Films by Using AlN - Coated Sapphire Substrates  to Yoshida et al., Appl. Phys. Lett. 42(5), Mar. 1, 1983, pp. 427–429 ; Metalorganic Vapor Phase Epitaxial Growth of a High Quality GaN Film Using an AlN Buffer Layer  to Amano et al., Appl. Phys. Lett., 48(5), February 1986, pp. 353–355 ; Influence of Buffer Layers on the Deposition of High Quality Single Crystal GaN Over Sapphire Substrate  to Kuznia et al., J. Appl. Phys. 73(9), May 1, 1993, pp. 4700–4702;  GaN Growth Using GaN Buffer Layer  to Nakamura, Japanese Journal of Applied Physics, Vol. 30, No. 10A, October 1991, pp. L1705–L1707;  The Effect of GaN and AlN Buffer Layers on GaN Film Properties Grown on Both C - Plane and A - Plane Sapphire  to Doverspike et al., Journal of Electronic Materials, Vol. 24, No. 4, 1995, pp. 269–273, the disclosures of which are hereby incorporated herein by reference. 
     Still referring to  FIG. 1 , the underlying gallium nitride layer  104  includes a plurality of sidewalls  105  therein. It will be understood by those having skill in the art that the sidewalls  105  may be thought of as being defined by a plurality of spaced apart posts  106 , that also may be referred to as “mesas”, “pedestals” or “columns”. The sidewalls  105  may also be thought of as being defined by a plurality of trenches  107 , also referred to as “wells” in the underlying gallium nitride layer  104 . The sidewalls  105  may also be thought of as being defined by a series of alternating trenches  107  and posts  106 . Moreover, a single post  106  may be provided, that may be thought of as being defined by at least one trench  107  adjacent the single post. It will be understood that the posts  106  and the trenches  107  that define the sidewalls  105  may be fabricated by selective etching ad/or selective epitaxial growth and/or other conventional techniques. Moreover, it will also be understood that the sidewalls need not be orthogonal to the substrate  102 , but rather may be oblique thereto. Finally, it will also be understood that although the sidewalls  105  are shown in cross-section in  FIG. 1 , the posts  106  and trenches  107  may define elongated regions that are straight, V-shaped or have other shapes. As shown in  FIG. 1 , the trenches  107  preferably extend into the buffer layer  102   b  and into the substrate  102   a , so that subsequent gallium nitride growth occurs preferentially on the sidewalls  105  rather than on the trench floors. 
     Referring now to  FIG. 2 , the sidewalls  105  of the underlying gallium nitride layer  104  are laterally grown to form a lateral gallium nitride layer  108   a  in the trenches  107 . Lateral growth of gallium nitride may be obtained at 1000–1100° C. and 45 Torr. The precursors TEG at 13–39 μmol/min and NH 3  at 1500 sccm may be used in combination with a 3000 sccm H 2  diluent. If gallium nitride alloys are formed, additional conventional precursors of aluminum or indium, for example, may also be used. As used herein, the term “lateral” means a direction that is orthogonal to the sidewalls  105 . It will also be understood that some vertical growth on the posts  106  may also take place during the lateral growth from sidewalls  105 . As used herein, the term “vertical” denotes a directional parallel to the sidewalls  105 . 
     When the sapphire substrate is exposed to the gas phase during growth of gallium nitride, it has been found that gallium nitride can nucleate on the sapphire. Thus, vertical growth of gallium nitride may take place from the sapphire trench floors, that can interfere with lateral growth of the gallium nitride sidewalls into the at least one trench. Alternatively, because of the presence of ammonia, the exposed areas of the surface of the sapphire may be converted to aluminum nitride. Unfortunately, gallium nitride can nucleate well on aluminum nitride, and thereby allow vertical growth of the gallium nitride from the trench floor, which can interfere with the lateral growth of the gallium nitride sidewalls. 
     The conversion of the exposed areas of the surface of the sapphire to aluminum nitride may be reduced and preferably eliminated by using a high growth temperature for growing the gallium nitride. For example, a temperature of about 1100° C. may be used rather than a conventional temperature of about 1000° C. However, this still may not prevent the nucleation of gallium nitride on the floor of the sapphire substrate. 
     Referring again to  FIG. 2 , according to the present invention, the sapphire substrate  102   a  is etched sufficiently deep to prevent vertical growth of gallium nitride from the sapphire trench floor  107   a  from interfering with the step of laterally growing the gallium nitride sidewalls of the at least one post into the at least one trench. For example, the ratio of the sapphire sidewall height y to the sapphire floor width x may be at least ¼. Other ratios may be used depending on the vertical to lateral growth rate ratio during gallium nitride growth. Under the conditions described below, the lateral growth rate of gallium nitride can be faster than the vertical growth rate. Under these conditions, and with sufficiently deep trenches, the sidewall growth from the posts can coalesce over the trenches before the vertical gallium nitride growth in the trenches that results from nucleation of gallium nitride on the sapphire substrate can interfere with the lateral growth. 
     Referring now to  FIG. 3 , continued growth of the lateral gallium nitride layer  108   a  causes vertical growth onto the underlying gallium nitride layer  104 , specifically onto the posts  106 , to form a vertical gallium nitride layer  108   b . Growth conditions for vertical growth may be maintained as was described in connection with  FIG. 2 . As also shown in  FIG. 3 , continued vertical growth into trenches  107  may take place at the bottom of the trenches. A void  109  preferably remains between the lateral gallium nitride layer  108   a  and the trench floor  107   a.    
     Referring now to  FIG. 4 , growth is allowed to continue until the lateral growth fronts coalesce in the trenches  107  at the interfaces  108   c , to form a continuous gallium nitride semiconductor layer in the trenches. The total growth time may be approximately 60 minutes. As shown in  FIG. 5 , microelectronic devices  110  may then be formed in the lateral gallium nitride semiconductor layer  108   a . Devices may also be formed in vertical gallium nitride layer  108   b.    
     Accordingly, in  FIG. 5 , gallium nitride semiconductor structures  100  according to embodiments of the present invention are illustrated. The gallium nitride structures  100  include the substrate  102 . The substrate includes the sapphire substrate  102   a  and the aluminum nitride buffer layer  102   b  on the sapphire substrate  102   a . The aluminum nitride and/or gallium nitride buffer layer  102   b  may be about 200–300 Å thick. 
     The underlying gallium nitride layer  104  is also included on the buffer layer  102   b  opposite the substrate  102   a . The underlying gallium nitride layer  104  may be between about 0.5 and 2.0 μm thick, and may be formed using metalorganic vapor phase epitaxy (MOVPE). The underlying gallium nitride layer generally has an undesired relatively high defect density. For example, dislocation densities of between about  10   8  and  10   10 cm −2  may be present in the underlying gallium nitride layer. These high defect densities may result from mismatches in lattice parameters between the buffer layer  102   b  and the underlying gallium nitride layer  104 , and/or other causes. These high defect densities may impact the performance of microelectronic devices formed in the underlying gallium nitride layer  104 . 
     Still continuing with the description of  FIG. 5 , the underlying gallium nitride layer  104  includes the plurality of sidewalls  105  that may be defined by the plurality of posts  106  and/or the plurality of trenches  107 . As was described above, the sidewalls may be oblique and of various elongated shapes. The posts  106  include a gallium nitride top, a gallium nitride sidewall and a sapphire sidewall, and the at least one trench includes a sapphire floor  107   a . The sapphire floor  107   a  preferably is free of a vertical gallium nitride layer thereon. The sapphire sidewall height to sapphire floor width ratio preferably is at least ¼. 
     Continuing with the description of  FIG. 5 , the lateral gallium nitride layer  108   a  extends from the plurality of sidewalls  105  of the underlying gallium nitride layer  104 . The lateral gallium nitride layer  108   a  may be formed using metalorganic vapor phase epitaxy at about 1000–1100° C. and 45 Torr. Precursors of triethygallium (TEG) at 13–39 μmol/min and ammonia (NH 3 ) at 1500 sccm may be used in combination with a 3000 sccm H 2  diluent, to form the lateral gallium nitride layer  108   a . The gallium nitride semiconductor structure  100  also includes the vertical gallium nitride layer  108   b  that extends vertically from the posts  106 . 
     As shown in  FIG. 5 , the lateral gallium nitride layer  108   a  coalesces at the interfaces  108   c  to form a continuous lateral gallium nitride semiconductor layer  108   a  in the trenches. It has been found that the dislocation densities in the underlying gallium nitride layer  104  generally do not propagate laterally from the sidewalls  105  with the same density as vertically from the underlying gallium nitride layer  104 . Thus, the lateral gallium nitride layer  108   a  can have a relatively low defect density, for example less that 10 4  cm −2 . Accordingly, the lateral gallium nitride layer  108   b  may form device quality gallium nitride semiconductor material. Thus, as shown in  FIG. 5 , microelectronic devices  110  may be formed in the lateral gallium nitride semiconductor layer  108   a . It will also be understood that a mask need not be used to fabricate the gallium nitride semiconductor structures  100  of  FIG. 5 , because lateral growth is directed from the sidewalls  105 . 
       FIGS. 6–10  illustrate other embodiments according to the present invention. As shown in  FIG. 6 , a mask  201  is formed on the trench floors  107   a ′. When forming the mask  201  on the trench floors  107   a ′, the trench need not be etched into the sapphire substrate  102   a . Rather, as shown in  FIG. 6 , the trench may only be etched through the aluminum nitride buffer layer  102   b . However, it will be understood by those having skill in the art that the trench also may be etched into the sapphire substrate  102   a , as was illustrated in  FIG. 1 , and the trench floor  107   a  in the sapphire substrate may be masked with a mask  201 . In still another alternative, the trench may be etched only partially into the aluminum nitride buffer layer  102   b , rather than entirely through the aluminum nitride buffer layer  102   b  as shown in  FIG. 6 . In yet another alternative, the trench need not be etched into the aluminum nitride buffer layer  102   b  at all, but rather the mask  201  may be formed on the exposed portion of the aluminum nitride buffer layer  102   b . In yet another alternative, the trenches may not extend into the aluminum nitride buffer layer, but rather may terminate within the gallium nitride layer  104 , and the mask  201  may be formed on the gallium nitride floor. Finally, it will be understood that although the mask  201  is shown to have the same thickness as the aluminum nitride buffer layer  102   b , it need not have the same thickness. Rather, it can be thinner or thicker. 
     It has been found, according to the present invention, that gallium nitride does not nucleate appreciably on certain amorphous and crystalline materials, such as silicon dioxide, silicon nitride and certain metals such as tungsten. Accordingly, a “line of sight” deposition technique, such as thermal evaporation or electron beam evaporation, may be used to deposit a masking material such as silicon dioxide, silicon nitride and/or tungsten on the trench floors. Since the gallium nitride does not nucleate specifically on the mask, it can be forced to grow off the sidewalls of the posts only. The remaining processing steps of  FIGS. 6–10  correspond to those of  FIGS. 1–5 , and need not be described again herein. 
       FIGS. 11–16  illustrate yet other embodiments according to the present invention. In  FIGS. 11–16 , the sapphire substrate  102   a  is etched sufficiently deep to prevent vertical growth of gallium nitride from the sapphire floor from interfering with the step of laterally growing the gallium nitride sidewalls of the at least one post into the at least one trench, as was described in connection with  FIGS. 1–5 , and need not be described herein again. However, in contrast with  FIGS. 1–5 , in  FIGS. 11–16 , a mask, such as a silicon dioxide, silicon nitride and/or tungsten mask  209  is included on the underlying gallium nitride layer  104 . The mask  209  may have a thickness of about 1000 Å or less and may be formed on the underlying gallium nitride layer  104  using low pressure Chemical Vapor Deposition (CVD) of silicon dioxide and/or silicon nitride. Alternatively, electron beam or thermal evaporation may be used to deposit tungsten. The mask  209  is patterned to provide an array of openings therein, using conventional photolithography techniques. 
     As shown in  FIG. 11 , the underlying gallium nitride layer is etched through the array of openings to define the plurality of posts  106  in the underlying gallium nitride layer  104  and the plurality of trenches  107  therebetween. The posts each include the sidewall  105  and a top having the mask  209  thereon. It will also be understood that although the posts  106  and trenches  107  are preferably formed by masking and etching as described above, the posts may also be formed by selectively growing the posts from an underlying gallium nitride layer and then forming a capping layer on the tops of the posts. Combinations of selective growth and selective etching also may be used. 
     As shown in  FIG. 12 , the sidewalls  105  of the underlying gallium nitride layer  104  are laterally grown to form a lateral gallium nitride layer  108   a  in the trenches  107 . Lateral growth may proceed as was described above. It will be understood that growth and/or nucleation on the top of the posts  106  is reduced and preferably eliminated by the mask  209 . 
     Referring to  FIG. 13 , continued growth of the lateral gallium nitride layer  108   a  causes vertical growth of the lateral gallium nitride layer  108   a  through the array of openings. Conditions for vertical growth may be maintained as was described in connection with  FIG. 12 . 
     Referring now to  FIG. 14 , continued growth of the lateral gallium nitride layer  108   a  causes lateral overgrowth onto the mask  209 , to form an overgrown lateral gallium nitride layer  108   b . Growth conditions for overgrowth may be maintained as was described in connection with  FIG. 12 . 
     Referring now to  FIG. 15 , growth is allowed to continue until the lateral growth fronts coalesce in the trenches  107  at the interfaces  108   c , to form a continuous lateral gallium nitride semiconductor layer  108   a  in the trenches. 
     Still referring to  FIG. 15 , growth is also allowed to continue until the lateral overgrowth fronts coalesce over the mask  209  at the interfaces  108   d , to form a continuous overgrown lateral gallium nitride semiconductor layer  108   b . The total growth time may be approximately 60 minutes. A single continuous growth step may be used. As shown in  FIG. 16 , microelectronic devices  110  may then be formed in the lateral gallium nitride semiconductor layer  108   a . Microelectronic devices also may be formed in the overgrown lateral gallium nitride layer  108   b.    
     Finally, referring to  FIGS. 17–22 , still other embodiments of the present invention are illustrated.  FIGS. 17–22  combine the mask  201  on the floor of the trenches  107 , as was illustrated in  FIGS. 6–10 , with the mask  209  on the top of the posts  106 , as was illustrated in  FIG. 1 . It will be understood that the mask  201  at the bottom of the trenches, and the mask  209  on the top of the posts  106 , preferably are formed simultaneously and preferably comprise the same material. Accordingly, for example, line of sight of deposition techniques, such as thermal evaporation or electron beam evaporation of masking material such as silicon dioxide, silicon nitride and/or metal such as tungsten may be used. If the mask material is deposited after the etching step, it covers only the vertical surfaces, i.e. the top surfaces of the posts.  106  and the bottom surfaces (floors) of the trenches  107 . The gallium nitride preferably nucleates little, if at all, on the masks  201  and  209 , so that gallium nitride preferably only grows from the sidewalls  105  of the posts. Alternatively, the masks  201  and  209  may comprise different materials and/or be of different thicknesses. The remaining steps of  FIGS. 17–22  are similar to  FIGS. 11–16 , and need not be described again in detail. 
     It will be understood that the masks  201  may be formed on an exposed sapphire floor of the substrate  102   a , on an exposed aluminum nitride floor of layer  102   b , or on an exposed gallium nitride floor in layer  104 . Stated differently, the trenches may be etched partly into gallium nitride layer  104 , fully through gallium nitride layer  104 , partly into aluminum nitride buffer layer  102   b , fully through aluminum nitride layer  102   b , and/or partly into sapphire substrate  102   a . Moreover, the thickness of the mask  201  may be thinner than or thicker than aluminum nitride layer  102   b . Accordingly, sapphire substrates may be used for growth of gallium nitride semiconductor layers, to thereby provide low cost and/or high availability. 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.