CA2331893C - Fabrication of gallium nitride semiconductor layers by lateral growth from trench sidewalls - Google Patents
Fabrication of gallium nitride semiconductor layers by lateral growth from trench sidewalls Download PDFInfo
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- CA2331893C CA2331893C CA002331893A CA2331893A CA2331893C CA 2331893 C CA2331893 C CA 2331893C CA 002331893 A CA002331893 A CA 002331893A CA 2331893 A CA2331893 A CA 2331893A CA 2331893 C CA2331893 C CA 2331893C
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- H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
- H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic System or AIIIBV compounds with or without impurities, e.g. doping materials
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- H01L21/02365—Forming inorganic semiconducting materials on a substrate
- H01L21/02612—Formation types
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- H01L21/02636—Selective deposition, e.g. simultaneous growth of mono- and non-monocrystalline semiconductor materials
- H01L21/02647—Lateral overgrowth
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- H01L21/02365—Forming inorganic semiconducting materials on a substrate
- H01L21/02612—Formation types
- H01L21/02617—Deposition types
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Abstract
A sidewall (105) of an underlying gallium nitride layer (106) is laterally grown into a trench (107) in the underlying gallium nitride layer, to thereby form a lateral gallium nitride semiconductor layer ( 108a).
Microelectronic devices may then be formed in the lateral gallium nitride layer. Dislocation defects do not significantly propagate laterally from the sidewall into the trench in the underlying gallium nitride layer, so that the lateral gallium nitride semiconductor layer is relatively defect free. Moreover, the sidewall growth may be accomplished without the need to mask portions of the underlying gallium nitride layer during growth of the lateral gallium nitride layer. The defect density of the lateral gallium nitride semiconductor layer may be further decreased by growing a second gallium nitride semiconductor layer from the lateral gallium nitride layer.
Microelectronic devices may then be formed in the lateral gallium nitride layer. Dislocation defects do not significantly propagate laterally from the sidewall into the trench in the underlying gallium nitride layer, so that the lateral gallium nitride semiconductor layer is relatively defect free. Moreover, the sidewall growth may be accomplished without the need to mask portions of the underlying gallium nitride layer during growth of the lateral gallium nitride layer. The defect density of the lateral gallium nitride semiconductor layer may be further decreased by growing a second gallium nitride semiconductor layer from the lateral gallium nitride layer.
Description
FABRICATION OF GALLIUM NITRIDE SEMICONDUCTOR LAYERS BY LATERAL GROWTH FROM
TRENCH SIDEWALLS
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 is also known to fabricate gallium nitride structures through openings in a mask. For example, in fabricating field emitter arrays, it is known to selectively grow gallium nitride on stripe or circular patterned substrates. See, for example, the publications by Nam et al. entitled "Selective Growth of GaN and AIo,~Gao.BN
on GaNlAINl6H SiC(0001) Multilayer Substrates Via Organometallic Vapor Phase Epitaxy", Proceedings of the Materials Research Society, December 1996, and "Growth of GaN and AIo.~Gao,&lV on Patterened Saebstrates via Organometallic Vapor Phase Epitaxy", Japanese Journal of Applied Physics., Vol. 36. Part 2, No. ~A, May 1997, pp. L532-L535. As disclosed in these publications, undesired ridge growth or lateral overgrowth may occur under certain conditions.
European Patent Application 0 551 721 A2 describes a gallium nitride type semiconductor device that comprises a silicon substrate, an intermediate layer consisting of a compound containing at least aluminum and nitrogen and formed on the silicon substrate, and a crystal layer of (Ga~_XAIX)1_yInyN (0<_x<_l, 0<_y<_l, excluding the case of x = 1 and y = 0). The aluminum/nitrogen intermediate layer suppresses the occurrence of crystal defects and thus the (Ga~_XAIX)~_ylnyN
layer has very high crystallization and flatness. In a method of fabrication a silicon single crystal substrate is kept at a temperature of 400 to 1300°C and is held in an atmosphere of a metalorganic compound containing at least aluminum and a nitrogen containing compound to form a thin intermediate layer containing at least aluminum and nitrogen on a part or on the entirety of the surface of the since crystal substrate.
At least one layer or multiple layers of a single crystal of (Gal_XAIX)1_yInyN
are then formed on the intermediate layer.
A publication entitled Lateral Epitaxy Of Low Defect Density GaN Layers Yia Organometallic Yapor Phase Epitaxy to Nam et al., Applied Physics Letters, Vol. 71, No. 18, November 3, 1997, pp. 2638-2640, reports organometallic vapor phase lateral epitaxy and coalescence of GaN layers originating from GaN stripes deposited within 3-p,m-wide windows spaced 3 pm apart and contained in Si02 masks on GaN/AlN/6H-SiC(0001) substrates. The extent and microstructural characteristics of the lateral overgrowth were a strong function of stripe orientation. A high density of threading dislocations, originating from the interface of the underlying GaN
with the A1N buffer layer, were contained in the GaN grown in the widow regions. The overgrowth regions, by contrast, contained a very low density of dislocations.
The coalesced layers had a rms surface roughness of 0.25 nm.
Summary of the Invention It is therefore an object of an aspect of the present invention to provide improved methods of fabricating gallium nitride semiconductor layers, and improved gallium nitride layers so fabricated.
It is another object of an aspect of the invention to provide methods of fabricating gallium nitride semiconductor layers that can have low defect densities, and gallium nitride semiconductor layers so fabricated.
-2a-These and other objects are provided, according to the present invention by laterally growing a sidewall of an underlying gallium nitride layer into a trench in the underlying gallium nitride layer, to thereby form a lateral gallium nitride layer.
Microelectronic devices may then be formed in the lateral gallium nitride layer.
It has been found, according to the present invention, that dislocation defects do not significantly propagate laterally from the sidewall into the trench in the underlying gallium nitride layer, so that the lateral gallium nitride semiconductor layer is relatively defect free. The sidewall growth may be accomplished without the need to mask portions of the underlying gallium nitride layer during growth of the lateral gallium nitride layer.
According to another aspect of the present invention, a pair of sidewalk of the underlying gallium nitride layer are laterally grown into a trench in the underlying gallium nitride layer between the pair of sidewalk until the grown sidewalls coalesce in the trench. The lateral gallium nitride semiconductor layer may be laterally grown using metalorganic vapor phase epitaxy (MOVPE). For example, the lateral gallium nitride layer may be laterally grown using triethylgallium (TEG) and ammonia (NH3) precursors at 1000-1100°C and 45 Torr. Preferably, TEG at 13-39~mo1/min and NH3 at 1500 scan are used in combination with 3000 scan HZ diluent. Most preferably, TEG at 26~mo1/min, NH3 at 1 S00 scan and HZ at 3000 scan at a temperature of 1100°C and 45 Torr are used. The underlying gallium nitride layer preferably is formed on a substrate such as 6H-SiC(0001), which itself includes a buffer layer such as aluminum nitride thereon. Other substrates such as sapphire, and other buffer WO 99!65068 PCT/US99/12967 _ J
layers such as low temperature gallium nitride, may be used. Multiple substrate layers and buffer layers also may be used.
The underlying gallium nitride layer including the sidewall may be formed by forming the trench in the underlying gallium nitride layer, such that the trench includes the sidewall. Alternatively, the sidewall may be formed by forming a post on the underlying gallium nitride layer, the post including the sidewall and defining the trench. A series of alternating trenches and posts is preferably formed to form a plurality of sidewalls. Trenches and/or posts may be formed by selective etching, selective epitaxial growth, combinations of etchings and growth. or other techniques.
The trenches may extend into the buffer layer and into the substrate.
The sidewall of the underlying gallium nitride layer is laterally grown into the trench, to thereby form the lateral gallium nitride layer of lower defect density than the defect density of the underlying gallium nitride layer. Some vertical growth may also occur. The laterally grown gallium nitride layer is vertically grown while propagating the lower defect density. Vertical growth may also take place simultaneous with the lateral growth.
The defect density of the overgrown Gallium nitride semiconductor layer may be further decreased by growing a second gallium nitride semiconductor layer from the lateral gallium nitride layer. In one embodiment, the lateral gallium nitride layer is masked with a mask that includes an array of openings therein. The lateral gallium nitride layer is grown through the array of openings and onto the mask, to thereby form an overgrown gallium nitride semiconductor layer. In another embodiment, the lateral gallium nitride layer is grown vertically. A plurality of second sidewalls are formed in the vertically grown lateral gallium nitride layer to define a plurality of 2~ second trenches. The plurality of second sidewalls of the vertically grown lateral gallium nitride layer are then laterally grown into the plurality of second trenches, to thereby form a second lateral gallium nitride layer. Microelectronic devices are then formed in the gallivun nitride semiconductor layer. The plurality of sidewalls of the underlying gallium nitride layer may be grown using metalorganic vapor phase epitaxy as was described above. The second sidewalls may be grown by etching and/or selective epitaxial growth of trenches and/or posts, as was described above.
Gallium nitride semiconductor structures according to the invention comprise .
an underlying gallium nitride layer including a trench having a sidewall, and a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench. A vertical gallium nitride layer extends from the lateral gallium nitride layer. A plurality of microelectronic devices are included in the vertical gallium nitride layer. A series of alternating trenches and posts may be provided to define a plurality of sidewalk. The underlying gallium nitride layer includes a predetermined defect density, and the lateral gallium nitride layer includes a predetermined defect density, and the lateral gallium nitride layer is of lower defect density than the predetermined defect density.
Other embodiments of gallium nitride semiconductor structures according to the invention comprise a mask including an array of openings therein on the lateral gallium nitride layer and a vertical gallium nitride layer that extends from the lateral gallium nitride layer through the openings and onto the mask. Alternatively, a vertical gallium nitride layer extends from the lateral gallium nitride layer and includes a plurality of second sidewalk therein. A second lateral gallium nitride layer extends from the plurality of second sidewalk. Microelectronic devices are included in the second gallium nitride layer. Accordingly, low defect density gallium nitride semiconductor layers may be produced, to thereby allow the production of high performance microelectronic devices.
In accordance with an aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the gallium nitride layer includes a first defect density, and wherein the laterally growing a sidewall of a gallium nitride layer into a trench in the gallium nitride layer to thereby form a lateral gallium nitride layer comprises:
laterally growing the sidewall of the gallium nitride layer to thereby form the lateral gallium nitride layer of lower defect density than the first defect density; and vertically growing the lateral gallium nitride layer while propagating the lower defect density.
In accordance with another aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride -4a-layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing is preceded by forming the gallium nitride layer including the sidewall on a substrate;
wherein the forming comprises forming the trench in the gallium nitride layer, the trench including the sidewall;
wherein the trench forming comprises selectively etching the gallium nitride layer to form the trench that includes the sidewall.
In accordance with a further aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises:
laterally growing a plurality of sidewalls of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the laterally growing is followed by:
masking the lateral gallium nitride layer with a mask that includes an array of openings therein; and growing the lateral gallium nitride layer through the array of openings and onto the mask, to thereby form an overgrown gallium nitride semiconductor layer.
In accordance with another aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises:
laterally growing a plurality of sidewalk of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the laterally growing is followed by:
vertically growing the lateral gallium nitride layer;
-4b-forming a plurality of second sidewalk in the vertically grown lateral gallium nitride layer to define a plurality of second posts and a plurality of second trenches;
and laterally growing the plurality of second sidewalls of the vertically grown lateral gallium nitride layer into the plurality of second trenches at a rate faster than vertically growing the vertically grown lateral gallium nitride layer on the second post tops, to thereby form a second lateral gallium nitride semiconductor layer.
In accordance with a further aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises:
laterally growing a plurality of sidewalk of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the gallium nitride layer includes a first defect density, and wherein the laterally growing a plurality of sidewalls of the gallium nitride layer into the plurality of trenches in the gallium nitride layer to thereby form a lateral gallium nitride layer comprises:
laterally growing the plurality of sidewalk of the gallium nitride layer into the plurality of trenches to thereby form a lateral gallium nitride semiconductor layer of lower defect density than the first defect density; and vertically growing the laterally gallium nitride layer while propagating the lower defect density.
In accordance with another aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the trench includes a trench floor and wherein the laterally growing comprises laterally growing the sidewall of the gallium nitride layer into the trench, - 4c -spaced apart from the trench floor, to thereby form a cantilevered lateral gallium nitride semiconductor layer.
In accordance with a further aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises growing a pair of sidewalk of the gallium nitride layer into a trench in the gallium nitride layer between the pair of sidewalk until the grown pair of sidewalk coalesce in the trench;
wherein the trench includes a trench floor and wherein the laterally growing comprises laterally growing the pair of sidewalls of the gallium nitride layer into the trench, spaced apart from the trench floor.
In accordance with another aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer wherein the laterally growing comprises:
laterally growing a plurality of sidewalk of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the trenches include trench floors and wherein the laterally growing comprises laterally growing the plurality of sidewalls of the gallium nitride layer into the plurality of trenches, spaced apart from the trench floors, to thereby form cantilevered lateral gallium nitride semiconductor layers.
In accordance with a further aspect of the present invention, there is provided a gallium nitride semiconductor structure comprising:
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and -4d-a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer;
a second vertical gallium nitride layer that extends from the lateral gallium nitride layer, and that has lower defect density than the vertical gallium nitride layer.
In accordance with another aspect of the present invention, there is provided a gallium nitride semiconductor structure comprising:
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer;
wherein the underlying gallium nitride layer including a plurality of posts having a plurality of sidewalls and tops and defining a plurality of trenches;
and wherein the lateral gallium nitride layer extends from the plurality of sidewalk of the underlying gallium nitride layer into the plurality of trenches; and wherein the vertical gallium nitride layer is on the plurality of post tops.
In accordance with another aspect of the present invention, there is provided a gallium nitride semiconductor structure comprising:
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer;
wherein the trench includes a trench floor and wherein the lateral gallium nitride layer is a cantilevered lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench and is spaced apart from the trench floor.
In accordance with a further aspect of the present invention, there is provided a gallium nitride semiconductor structure comprising:
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
- 4e -a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer.
wherein the trenches include trench floors and wherein the lateral gallium nitride layer is a cantilevered lateral gallium nitride layer that extends from the plurality of sidewalk of the underlying gallium nitride layer into the trenches and is spaced apart from the trench floors.
Brief Description of the Drawings Figures 1-5 are cross-sectional views of first embodiments of gallium nitride semiconductor structures during intermediate fabrication steps according to the present invention.
Figures 6-10 are cross-sectional views of second embodiments of gallium nitride semiconductor structures during intermediate fabrication steps according to the present invention.
Figures 11-15 are cross-sectional views of third embodiments of gallium nitride semiconductor structures during intermediate fabrication steps according to the present invention.
Detailed Description of Preferred Embodiments The present invention now will be described more fully hereina$er with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different -$-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 Figures 1-5, methods of fabricating gallium nitride semiconductor structures according to a first embodiment of the present invention will now be described. As shown in Figure 1, an underlying gallium nitride layer 104 is grown on a substrate 102. The substrate 102 may include a 6H-SiC(0001) substrate 102a and an aluminum nitride buffer layer 102b. 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 1.0 and 2.O~m thick, and may be grown at 1000°C on a high temperature (1100°C) aluminum nitride buffer layer 102b that was deposited on the 6H-SiC substrate 102a in a cold wall vertical and inductively heated metalorganic vapor phase epitaxy system using triethylgallium at 26~mo1/min, ammonia at 1500 scan and 3000 scan hydrogen diluent. Additional details of this growth technique may be found in a publication by T.W. Weeks et al. entitled "GaN Thin Films Deposited Via Organometallic Vapor Phase Epitaxy on ~x(6H)-SiC(0001) Using High-Temperature Monocrystalline AZN
Buffer Layer ", Applied Physics Letters, Vol. 67, No. 3, July 17, 1995, pp.
401-403.
Other substrates, with or without buffer layers, may be used.
Still referring to Figure 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 sidewalk 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 sidewalk I05 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. It will be understood that the posts 106 and the trenches 107 that define the sidewalls 10~ may be fabricated by selective etching and/or selective epitaxial growth andJor other conventional techniques. Moreover, it will also be understood that the sidewalls need not be orthogonal to the substrate 102, but S rather may be oblique thereto. Finally, it will also be understood that although the sidewalls 105 are shown in cross-section in Figure l, the posts 106 and trenches 107 may define elongated regions that are straight, V-shaped or have other shapes.
As shown in Figure l, the trenches 107 may extend into the buffer layer 102b and into the substrate 102a, so that subsequent gallium nitride growth occurs preferentially on the sidewalk 10~ rather than on the trench floors. In other embodiments, the trenches may not extend into the substrate 102a, and also may not extend into buffer layer 102b, depending, for example, on the trench geometry and the lateral versus vertical growth rates of the gallium nitride.
Referring now to Figure 2, the sidewalls 105 of the underlying gallium nitride layer 104 are laterally grown to form a lateral gallium nitride layer 108a 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~.mol/min and NH3 at 1 X00 sccm may be used in combination with a 3000 sccm Hz 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 sidewalk 10~. It will also be understood that some vertical growth on the posts 106 may also take place during the lateral growth from sidewalls 10~. As used herein, the term "vertical" denotes a directional parallel to the sidewalls 105.
Referring now to Figure 3, continued growth of the lateral gallium nitride layer 108a causes vertical growth onto the underlying gallium nitride layer 104, specifically onto the posts 106, to form a vertical gallium nitride layer 108b. Growth conditions for vertical growth may be maintained as was described in connection with Figure 2. As also shown in Figure 3, continued vertical growth into trenches 107 may take place at the bottom of the trenches.
Referring now to Figure 4, growth is allowed to continue until the lateral growth fronts coalesce in the trenches 107 at the interfaces 108c, to form a continuous gallium nitride semiconductor layer in the trenches. The total growth time may be_ approximately 60 minutes. As shown in Figure ~, microelectronic devices 110 may _7_ then be formed in the lateral gallium nitride semiconductor layer l OSa.
Devices may also be formed in vertical gallium nitride layer 108b.
Accordingly, in Figure 5, gallium nitride semiconductor structures 100 according to a first embodiment of the present invention are illustrated. The gallium nitride structures 100 include the substrate 102. The substrate may be sapphire or gallium nitride or other conventional substrates. However, preferably, the substrate includes the 6H-SiC(0001) substrate I02a and the aluminum nitride buffer layer 102b on the silicon carbide substrate 102a. The aluminum nitride buffer layer 102b may be 0.1 pm thick.
The fabrication of the substrate I02 is well known to those having skill in the art and need not be described further. Fabrication of silicon carbide substrates are described, for example, in U.S. Patents 4,865,685 to Palmour; Re 34,861 to Davis et al.; 4,912,064 to Kong et al. and 4,946,547 to Palmour et a1.
The underlying gallium nitride layer 104 is also included on the buffer layer 102b opposite the substrate 102a. The underlying gallium nitride layer 104 may be between about 1.0 and 2.O~m 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 10g and 101°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 102b 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 Figure 5, the underlying gallium nitride layer 104 includes the plurality of sidewalk 105 that may be defined by the plurality of pedestals 106 and/or the plurality of trenches 107. As was described above, the sidewalls may be oblique and of various elongated shapes.
Continuing with the description of Figure 5, the lateral gallium nitride layer 108a extends from the plurality of sidewalls 105 of the underlying gallium nitride layer 104. The lateral gallium nitride layer 108a may be formed using metalorganic vapor phase epitaxy at about 1000-1100°C and 45 Torr. Precursors of triethygallium (TEG) at 1 3-39p.mol/min and ammonia (NH3) at 1500 seem may be used in _g_ combination with a 3000 sccm H~ diluent, to form the lateral gallium nitride layer 108a.
Still continuing with the description of Figure 5, the gallium nitride semiconductor structure 100 also includes the vertical gallium nitride layer 108b that extends vertically from the posts 106.
As shown in Figure 5, the lateral gallium nitride layer 108a coalesces at the interfaces 108c to form a continuous lateral gallium nitride semiconductor layer 108a 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 10~
with the same density as vertically from the underlying gallium nitride layer 104.
Thus, the lateral gallium nitride layer 108a can have a relatively low defect density, for example less that 10~' cm-'. Accordingly, the lateral gallium nitride layer 108b may form device quality gallium nitride semiconductor material. Thus, as shown in Figure 5, microelectronic devices 110 may be formed in the lateral gallium nitride semiconductor layer 108a. It will also be understood that a mask need not be used to fabricate the gallium nitride semiconductor structures 100 of Figure 5, because lateral growth is directed from the sidewalls 10~.
Referring now to Figures 6-10, second embodiments of gallium nitride semiconductor structures and fabrication methods according to the present invention will now be described. First, gallium nitride semiconductor structures of Figure 4 are fabricated as was already described with regard to Figures 1-4. Then, referring to Figure 6, the posts 106 are masked with a mask 206 that includes an array of openings therein. The mask may comprise silicon dioxide at thickness of 1000th and may be deposited using low pressure chemical vapor deposition at 410°C. Other masking materials may be used. The mask may be patterned using standard photolithography techniques and etched in a buffered HF solution. In one embodiment, the openings are 3~m-wide openings that extend in parallel at distances of between 3 and 40~cm and that are oriented along the < 1 100 > direction on the lateral gallium nitride layer 108a. Prior to further processing, the structure may be dipped in a 50%
hydrochloric acid (HC1) solution to remove surface oxides. It will be understood that although the mask 206 is preferably located above the posts 106, it can also be offset therefrom.
Referring now to Figure 7, the lateral gallium nitride semiconductor layer -108a is grown through the array of openings to form a vertical gallium nitride layer 208a in the openings. Growth of gallium nitride may be obtained, as was described in connection with Figure 2.
It will be understood that growth in two dimensions may be used to form an overgrown gallium nitride semiconductor layer. Specifically, the mask 206 may be patterned to include an array of openings that extend along two orthogonal directions such as < 1100 > and < 1120 > . Thus, the openings can form a rectangle of orthogonal striped patterns. In this case, the ratio of the edges of the rectangle is preferably proportional to the ratio of the growth rates of the {1120} and {1101;
facets, for example, in a ratio of 1.4:1. The openings can be equitriangular with respect to directions such as < 1100 > and < 1 i20 > .
Referring now to Figure 8, continued growth of the vertical gallium nitride layer 208a causes lateral growth onto the mask 206, to form a second lateral gallium nitride layer 208b. Conditions for overgroWh may be maintained as was described in connection with Figure 7.
Referring now to Figure 9, lateral overgrowth is allowed to continue until the lateral growth fronts coalesce at the second interfaces 208e on the mask 206 to form a continuous overgrown gallium nitride semiconductor layer 208. The total growth time may be approximately sixty minutes. As shown in Figure 10, microelectronic devices 210 may then be formed in the second lateral gallium nitride layer 208b. The microelectronic devices may also be formed in the vertical gallium nitride layer 208a.
Accordingly, by providing the second lateral growth layer 208b, defects that were present in continuous gallium nitride semiconductor layer 108 may be reduced even further, to obtain device quality gallium nitride in the gallium nitride semiconductor structure 200.
Referring now to Figures 11-15, third embodiments of gallium nitride semiconductor structures and fabrication methods according to the present invention will now be described. First, gallium nitride semiconductor structures of Figure 4 are fabricated as was already described in connection with Figures 1-4. Then, a plurality of second sidewalls 305 are formed. The second sidewalls 305 may be formed by selective epitaxial growth of second posts 306 by etching second trenches 307 in the first posts 106 and/or combinations thereof. As was already described, the second sidewalls 30~ need not be orthogonal to substrate 102. but rather may be oblique. The second trenches 307 need not be directly over the first posts 106, but may be laterally offset therefrom. The second trenches are preferably deep so that lateral growth preferentially occurs on the sidewalls 30~ rather than on the bottom of second trenches 306.
Referring now to Figure 12. the second sidewalls 30~ of the second posts 306 and/or the second trenches 307 are laterally grown to form a second lateral gallium nitride layer 308a in the second trenches 307. As was already described, lateral growth of gallium nitride may be obtained at 1000-1100°C and 45 Torr.
The precursors TEG at 13-39~mo1/min and NHS at 1500 sccm may be used in combination with a 3000 scan H~ diluent. If gallium nitride alloys are formed, additional conventional precursors of aluminum or indium, for example, may also be used. It will also be understood that some vertical growth may take place on the second posts 306 during the lateral growth from the second sidewalls 30~.
Referring now to Figure 13, continued growth of the second lateral gallium nitride layer 308a causes vertical growth onto the second posts 306, to foml a second vertical gallium nitride layer 308b. As also shown, vertical growth from the floors of the second trenches and from the tops of the second posts may also take place.
Growth conditions for vertical growth may be maintained as was described in connection with Figure 12.
Referring now to Figure 14, growth is allowed to continue until the lateral growth fronts coalesce in the second trenches 307 at the second interfaces 308c to form a second continuous gallium nitride semiconductor layer 308. The total growth time may be approximately sixty minutes. As shown in Figure 15, microelectronic devices 310 may then be formed in the second continuous gallium nitride semiconductor layer 308.
Accordingly, third embodiments of gallium nitride semiconductor structures 300 according to the present invention may be formed without the need to mask gallium nitride for purposes of defining lateral growth. Rather, lateral growth from first and second sidewalls may be used. By performing two separate lateral growths, the defect density may be reduced considerably.
Additional discussion of methods and structures of the present invention will now be provided. The first and second trenches 107 and 307 and the openings in the mask 206 are preferably rectangular trenches and openings that preferably extend along the < 1120 > and/or < 1 100 > directions on the underlying gallium nitride layer 104 or the first lateral gallium nitride layer 108a. Tnmcated triangular stripes having (1 I O1) slant facets and a narrow (0001 ) top facet may be obtained for trenches and/or mask openings along the < 1120 > direction. Rectangular stripes having a (0001) top facet, (1120) vertical side faces and (1 1 O1) slant facets may be grown along the < 1 100 > direction. For growth times up to 3 minutes, similar morphologies may be obtained regardless of orientation. The stripes develop into different shapes if the growth is continued.
The amount of lateral growth generally exhibits a strong dependence on trench and/or mask opening orientation. The lateral growth rate of the < 1 I 00 >
oriented trenches and/or mask openings is generally much faster than those along < I
120 > .
Accordingly, it is most preferred to orient the trenches and/or mask openings, so that they extend along the < 1 100 > direction of the underlying gallium nitride layer 104 or the first lateral gallium nitride layer 108a.
The different morphological development as a function of trench and/or mask opening orientation appears to be related to the stability of the crystallographic planes in the gallium nitride structure. Trenches and/or mask openings oriented along < 1120 > may have wide (1 100) slant facets and either a very nan:ow or no (0001 ) top facet depending on the growth conditions. This may be because (1 101) is the most stable plane in the gallium nitride wurtzite crystal structure, and the growth rate of this plane is lower than that of others. The ~1 101} planes of the < 1 100 >
oriented trenches and/or mask openings may be wavy, which implies the existence of more than one Miller index. It appears that competitive growth of selected {1 1 O1) planes occurs during the deposition which causes these planes to become unstable and which causes their growth rate to increase relative to that of the (I 1 O1) of trenches and/or mask openings oriented along < 1120 > .
The morphologies of the gallium nitride layers selectively grown from trenches and/or mask openings oriented along < 1 100 > are also Denerally a strong function of the growth temperatures. Layers grown at 1000°C may possess a truncated triangular shape. This morphology may gradually change to a rectangular cross-section as the growth temperature is increased. This shape change may occur as a WO 99/65068 PCT/US99,~12967 result of the increase in the diffusion coefficient and therefore the flux of the gallium species along the (0001) top plane onto the {1 1 O1} planes with an increase in growth temperature. This may result in a decrease in the ;rowth rate of the (0001 ) plane and an increase in that of the {1 1 O1} . This phenomenon has also been observed in the selective growth of gallium arsenide on silicon dioxide. Accordingly, temperatures of 1100°C appear to be most preferred.
The morphological development of the gallium nitride regions also appears to depend on the flow rate of the TEG. An increase in the supply of TEG generally increases the growth rate in both the lateral and the vertical directions.
However, the lateral/vertical growth rate ratio decrease from 1.7 at the TEG flow rate of l3~mol/min to 0.86 at 39pmol.min. This increased influence on growth rate along <0001 > relative to that of < 1120 > with TEG flow rate may be related to the type of reactor employed, wherein the reactant gases flow vertically and perpendicular to the substrate. The considerable increase in the concentration of the gallium species on the surface may sufficiently impede their diffusion to the {I 1 O1; planes such that chemisorption and gallium nitride growth occur more readily on the (0001) plane.
Continuous 2~m thick gallium nitride semiconductor layers may be obtained using 3pm wide trenches and/or mask openingTs spaced 7~m apart and oriented alone < 1 100 > , at 1100°C and a TEG flow rate of 26ymol/min. The continuous gallium nitride semiconductor layers may include subsurface voids that form when two growth fronts coalesce. These voids may occur most often using lateral growth conditions wherein rectangular trenches and/or mask openings having vertical {1120}
side facets developed.
The continuous gallium nitride semiconductor layers may have a microscopically flat and pit-free surface. The surfaces of the laterally grown gallium nitride layers may include a terrace structure having an average step height of 0.32nm.
This terrace structure may be related to the laterally grown gallium nitride, because it is generally not included in much larger area films grown only on aluminum nitride buffer layers. The average RMS roughness values may be similar to the values obtained for the underlying gallium nitride layer 104.
Threading dislocations, originating from the interface between the underlying .
gallium nitride layer 104 and the buffer layer 102b, appear to propagate to the top surface of the underlying gallium nitride layer 104. The dislocation density within these regions is approximately 109 cm-'. By contrast. threading dislocations do not appear to readily propagate laterally. Rather, the lateral gallium nitride regions 108a and 308a contain only a few dislocations. These few dislocations may be formed parallel to the (0001 ) plane via the extension of the vertical threading dislocations after a 90° bend in the regrown region. These dislocations do not appear to propagate to the top surface of the overgrown gallium nitride layer.
As described, the formation mechanism of the selectively grown gallium nitride layers is lateral epitaxy. The two main stages of this mechanism are vertical growth and lateral growth. During vertical growth through a mask, the deposited gallium nitride grows selectively within the mask openings more rapidly than it grows on the mask, apparently due to the much higher sticking coefficient. s, of the gallium atoms on the gallium nitride surface (s=1 ) compared to on the mask (s«1 ).
Since the SiOz bond strength is 799.6 kJ/mole and much higher than that of Si-N (439 kJ/mole), Ga-N (103 kJ/mole), and Ga-O (353.6 kJ/mole), Ga or N atoms should not readily bond to the mask surface in numbers and for a time sufficient to cause gallium nitride nuclei to form. They would either evaporate or diffuse along the mask surface to the opening in the mask or to the vertical gallium nitride surfaces which have emerged.
During lateral growth, the gallium nitride grows simultaneously both vertically and laterally.
Surface diffusion of gallium and nitrogen on the gallium nitride may play a role in gallium nitride selective growth. The major source of material appears to be derived from the gas phase. This may be demonstrated by the fact that an increase in the TEG flow rate causes the growth rate of the (0001 ) top facets to develop faster than the (1 1 O1) side facets and thus controls the lateral growth.
The laterally grown gallium nitride bonds to the underlying mask sufficiently strongly so that it generally does not break away on cooling. However, lateral cracking within the SiO~ mask may take place due to thermal stresses generated on cooling. The viscosity (p) of the SiO~ at 1050°C is about 10'''' poise which is one order of magnitude greater than the strain point (about 10~~~' poise) where stress relief in a bulk amorphous material occurs within approximately six hours. Thus, the Si02 mask may provide limited compliance on cooling. As the atomic arrangement on the amorphous SiO~ surface is quite different from that on the GaN surface, chemical bonding may occur only when appropriate pairs of atoms are in close proximity.
Extremely small relaxations of the silicon and oxygen and gallium and nitrogen atoms on the respective surfaces and/or within the bulk of the SiO~ may accommodate the gallium nitride and cause it to bond to the oxide. Accordingly, the embodiments of Figures I-S and 11-1~, which need not employ a mask, may be particularly advantageous.
In conclusion, lateral epitaxial overgrowth may be obtained from sidewalls of an underlying gallium nitride layer via MOVPE. The growth may depend strongly on the sidewall orientation, growth temperature and TEG flow rate. Coalescence of overgrown gallium nitride regions to form regions with both extremely low densities of dislocations and smooth and pit-free surfaces may be achieved through 3~.m wide trenches between 7~m wide posts and extending along the < I I 00 > direction, at 1100°C and a TEG flow rate of 26~mo1/min. The lateral overgrowth of gallium nitride from sidewalls via MOVPE may be used to obtain low defect density regions for microelectronic devices, without the need to use masks.
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.
TRENCH SIDEWALLS
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 is also known to fabricate gallium nitride structures through openings in a mask. For example, in fabricating field emitter arrays, it is known to selectively grow gallium nitride on stripe or circular patterned substrates. See, for example, the publications by Nam et al. entitled "Selective Growth of GaN and AIo,~Gao.BN
on GaNlAINl6H SiC(0001) Multilayer Substrates Via Organometallic Vapor Phase Epitaxy", Proceedings of the Materials Research Society, December 1996, and "Growth of GaN and AIo.~Gao,&lV on Patterened Saebstrates via Organometallic Vapor Phase Epitaxy", Japanese Journal of Applied Physics., Vol. 36. Part 2, No. ~A, May 1997, pp. L532-L535. As disclosed in these publications, undesired ridge growth or lateral overgrowth may occur under certain conditions.
European Patent Application 0 551 721 A2 describes a gallium nitride type semiconductor device that comprises a silicon substrate, an intermediate layer consisting of a compound containing at least aluminum and nitrogen and formed on the silicon substrate, and a crystal layer of (Ga~_XAIX)1_yInyN (0<_x<_l, 0<_y<_l, excluding the case of x = 1 and y = 0). The aluminum/nitrogen intermediate layer suppresses the occurrence of crystal defects and thus the (Ga~_XAIX)~_ylnyN
layer has very high crystallization and flatness. In a method of fabrication a silicon single crystal substrate is kept at a temperature of 400 to 1300°C and is held in an atmosphere of a metalorganic compound containing at least aluminum and a nitrogen containing compound to form a thin intermediate layer containing at least aluminum and nitrogen on a part or on the entirety of the surface of the since crystal substrate.
At least one layer or multiple layers of a single crystal of (Gal_XAIX)1_yInyN
are then formed on the intermediate layer.
A publication entitled Lateral Epitaxy Of Low Defect Density GaN Layers Yia Organometallic Yapor Phase Epitaxy to Nam et al., Applied Physics Letters, Vol. 71, No. 18, November 3, 1997, pp. 2638-2640, reports organometallic vapor phase lateral epitaxy and coalescence of GaN layers originating from GaN stripes deposited within 3-p,m-wide windows spaced 3 pm apart and contained in Si02 masks on GaN/AlN/6H-SiC(0001) substrates. The extent and microstructural characteristics of the lateral overgrowth were a strong function of stripe orientation. A high density of threading dislocations, originating from the interface of the underlying GaN
with the A1N buffer layer, were contained in the GaN grown in the widow regions. The overgrowth regions, by contrast, contained a very low density of dislocations.
The coalesced layers had a rms surface roughness of 0.25 nm.
Summary of the Invention It is therefore an object of an aspect of the present invention to provide improved methods of fabricating gallium nitride semiconductor layers, and improved gallium nitride layers so fabricated.
It is another object of an aspect of the invention to provide methods of fabricating gallium nitride semiconductor layers that can have low defect densities, and gallium nitride semiconductor layers so fabricated.
-2a-These and other objects are provided, according to the present invention by laterally growing a sidewall of an underlying gallium nitride layer into a trench in the underlying gallium nitride layer, to thereby form a lateral gallium nitride layer.
Microelectronic devices may then be formed in the lateral gallium nitride layer.
It has been found, according to the present invention, that dislocation defects do not significantly propagate laterally from the sidewall into the trench in the underlying gallium nitride layer, so that the lateral gallium nitride semiconductor layer is relatively defect free. The sidewall growth may be accomplished without the need to mask portions of the underlying gallium nitride layer during growth of the lateral gallium nitride layer.
According to another aspect of the present invention, a pair of sidewalk of the underlying gallium nitride layer are laterally grown into a trench in the underlying gallium nitride layer between the pair of sidewalk until the grown sidewalls coalesce in the trench. The lateral gallium nitride semiconductor layer may be laterally grown using metalorganic vapor phase epitaxy (MOVPE). For example, the lateral gallium nitride layer may be laterally grown using triethylgallium (TEG) and ammonia (NH3) precursors at 1000-1100°C and 45 Torr. Preferably, TEG at 13-39~mo1/min and NH3 at 1500 scan are used in combination with 3000 scan HZ diluent. Most preferably, TEG at 26~mo1/min, NH3 at 1 S00 scan and HZ at 3000 scan at a temperature of 1100°C and 45 Torr are used. The underlying gallium nitride layer preferably is formed on a substrate such as 6H-SiC(0001), which itself includes a buffer layer such as aluminum nitride thereon. Other substrates such as sapphire, and other buffer WO 99!65068 PCT/US99/12967 _ J
layers such as low temperature gallium nitride, may be used. Multiple substrate layers and buffer layers also may be used.
The underlying gallium nitride layer including the sidewall may be formed by forming the trench in the underlying gallium nitride layer, such that the trench includes the sidewall. Alternatively, the sidewall may be formed by forming a post on the underlying gallium nitride layer, the post including the sidewall and defining the trench. A series of alternating trenches and posts is preferably formed to form a plurality of sidewalls. Trenches and/or posts may be formed by selective etching, selective epitaxial growth, combinations of etchings and growth. or other techniques.
The trenches may extend into the buffer layer and into the substrate.
The sidewall of the underlying gallium nitride layer is laterally grown into the trench, to thereby form the lateral gallium nitride layer of lower defect density than the defect density of the underlying gallium nitride layer. Some vertical growth may also occur. The laterally grown gallium nitride layer is vertically grown while propagating the lower defect density. Vertical growth may also take place simultaneous with the lateral growth.
The defect density of the overgrown Gallium nitride semiconductor layer may be further decreased by growing a second gallium nitride semiconductor layer from the lateral gallium nitride layer. In one embodiment, the lateral gallium nitride layer is masked with a mask that includes an array of openings therein. The lateral gallium nitride layer is grown through the array of openings and onto the mask, to thereby form an overgrown gallium nitride semiconductor layer. In another embodiment, the lateral gallium nitride layer is grown vertically. A plurality of second sidewalls are formed in the vertically grown lateral gallium nitride layer to define a plurality of 2~ second trenches. The plurality of second sidewalls of the vertically grown lateral gallium nitride layer are then laterally grown into the plurality of second trenches, to thereby form a second lateral gallium nitride layer. Microelectronic devices are then formed in the gallivun nitride semiconductor layer. The plurality of sidewalls of the underlying gallium nitride layer may be grown using metalorganic vapor phase epitaxy as was described above. The second sidewalls may be grown by etching and/or selective epitaxial growth of trenches and/or posts, as was described above.
Gallium nitride semiconductor structures according to the invention comprise .
an underlying gallium nitride layer including a trench having a sidewall, and a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench. A vertical gallium nitride layer extends from the lateral gallium nitride layer. A plurality of microelectronic devices are included in the vertical gallium nitride layer. A series of alternating trenches and posts may be provided to define a plurality of sidewalk. The underlying gallium nitride layer includes a predetermined defect density, and the lateral gallium nitride layer includes a predetermined defect density, and the lateral gallium nitride layer is of lower defect density than the predetermined defect density.
Other embodiments of gallium nitride semiconductor structures according to the invention comprise a mask including an array of openings therein on the lateral gallium nitride layer and a vertical gallium nitride layer that extends from the lateral gallium nitride layer through the openings and onto the mask. Alternatively, a vertical gallium nitride layer extends from the lateral gallium nitride layer and includes a plurality of second sidewalk therein. A second lateral gallium nitride layer extends from the plurality of second sidewalk. Microelectronic devices are included in the second gallium nitride layer. Accordingly, low defect density gallium nitride semiconductor layers may be produced, to thereby allow the production of high performance microelectronic devices.
In accordance with an aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the gallium nitride layer includes a first defect density, and wherein the laterally growing a sidewall of a gallium nitride layer into a trench in the gallium nitride layer to thereby form a lateral gallium nitride layer comprises:
laterally growing the sidewall of the gallium nitride layer to thereby form the lateral gallium nitride layer of lower defect density than the first defect density; and vertically growing the lateral gallium nitride layer while propagating the lower defect density.
In accordance with another aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride -4a-layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing is preceded by forming the gallium nitride layer including the sidewall on a substrate;
wherein the forming comprises forming the trench in the gallium nitride layer, the trench including the sidewall;
wherein the trench forming comprises selectively etching the gallium nitride layer to form the trench that includes the sidewall.
In accordance with a further aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises:
laterally growing a plurality of sidewalls of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the laterally growing is followed by:
masking the lateral gallium nitride layer with a mask that includes an array of openings therein; and growing the lateral gallium nitride layer through the array of openings and onto the mask, to thereby form an overgrown gallium nitride semiconductor layer.
In accordance with another aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises:
laterally growing a plurality of sidewalk of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the laterally growing is followed by:
vertically growing the lateral gallium nitride layer;
-4b-forming a plurality of second sidewalk in the vertically grown lateral gallium nitride layer to define a plurality of second posts and a plurality of second trenches;
and laterally growing the plurality of second sidewalls of the vertically grown lateral gallium nitride layer into the plurality of second trenches at a rate faster than vertically growing the vertically grown lateral gallium nitride layer on the second post tops, to thereby form a second lateral gallium nitride semiconductor layer.
In accordance with a further aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises:
laterally growing a plurality of sidewalk of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the gallium nitride layer includes a first defect density, and wherein the laterally growing a plurality of sidewalls of the gallium nitride layer into the plurality of trenches in the gallium nitride layer to thereby form a lateral gallium nitride layer comprises:
laterally growing the plurality of sidewalk of the gallium nitride layer into the plurality of trenches to thereby form a lateral gallium nitride semiconductor layer of lower defect density than the first defect density; and vertically growing the laterally gallium nitride layer while propagating the lower defect density.
In accordance with another aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the trench includes a trench floor and wherein the laterally growing comprises laterally growing the sidewall of the gallium nitride layer into the trench, - 4c -spaced apart from the trench floor, to thereby form a cantilevered lateral gallium nitride semiconductor layer.
In accordance with a further aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises growing a pair of sidewalk of the gallium nitride layer into a trench in the gallium nitride layer between the pair of sidewalk until the grown pair of sidewalk coalesce in the trench;
wherein the trench includes a trench floor and wherein the laterally growing comprises laterally growing the pair of sidewalls of the gallium nitride layer into the trench, spaced apart from the trench floor.
In accordance with another aspect of the present invention, there is provided a method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer wherein the laterally growing comprises:
laterally growing a plurality of sidewalk of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the trenches include trench floors and wherein the laterally growing comprises laterally growing the plurality of sidewalls of the gallium nitride layer into the plurality of trenches, spaced apart from the trench floors, to thereby form cantilevered lateral gallium nitride semiconductor layers.
In accordance with a further aspect of the present invention, there is provided a gallium nitride semiconductor structure comprising:
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and -4d-a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer;
a second vertical gallium nitride layer that extends from the lateral gallium nitride layer, and that has lower defect density than the vertical gallium nitride layer.
In accordance with another aspect of the present invention, there is provided a gallium nitride semiconductor structure comprising:
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer;
wherein the underlying gallium nitride layer including a plurality of posts having a plurality of sidewalls and tops and defining a plurality of trenches;
and wherein the lateral gallium nitride layer extends from the plurality of sidewalk of the underlying gallium nitride layer into the plurality of trenches; and wherein the vertical gallium nitride layer is on the plurality of post tops.
In accordance with another aspect of the present invention, there is provided a gallium nitride semiconductor structure comprising:
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer;
wherein the trench includes a trench floor and wherein the lateral gallium nitride layer is a cantilevered lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench and is spaced apart from the trench floor.
In accordance with a further aspect of the present invention, there is provided a gallium nitride semiconductor structure comprising:
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
- 4e -a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer.
wherein the trenches include trench floors and wherein the lateral gallium nitride layer is a cantilevered lateral gallium nitride layer that extends from the plurality of sidewalk of the underlying gallium nitride layer into the trenches and is spaced apart from the trench floors.
Brief Description of the Drawings Figures 1-5 are cross-sectional views of first embodiments of gallium nitride semiconductor structures during intermediate fabrication steps according to the present invention.
Figures 6-10 are cross-sectional views of second embodiments of gallium nitride semiconductor structures during intermediate fabrication steps according to the present invention.
Figures 11-15 are cross-sectional views of third embodiments of gallium nitride semiconductor structures during intermediate fabrication steps according to the present invention.
Detailed Description of Preferred Embodiments The present invention now will be described more fully hereina$er with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different -$-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 Figures 1-5, methods of fabricating gallium nitride semiconductor structures according to a first embodiment of the present invention will now be described. As shown in Figure 1, an underlying gallium nitride layer 104 is grown on a substrate 102. The substrate 102 may include a 6H-SiC(0001) substrate 102a and an aluminum nitride buffer layer 102b. 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 1.0 and 2.O~m thick, and may be grown at 1000°C on a high temperature (1100°C) aluminum nitride buffer layer 102b that was deposited on the 6H-SiC substrate 102a in a cold wall vertical and inductively heated metalorganic vapor phase epitaxy system using triethylgallium at 26~mo1/min, ammonia at 1500 scan and 3000 scan hydrogen diluent. Additional details of this growth technique may be found in a publication by T.W. Weeks et al. entitled "GaN Thin Films Deposited Via Organometallic Vapor Phase Epitaxy on ~x(6H)-SiC(0001) Using High-Temperature Monocrystalline AZN
Buffer Layer ", Applied Physics Letters, Vol. 67, No. 3, July 17, 1995, pp.
401-403.
Other substrates, with or without buffer layers, may be used.
Still referring to Figure 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 sidewalk 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 sidewalk I05 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. It will be understood that the posts 106 and the trenches 107 that define the sidewalls 10~ may be fabricated by selective etching and/or selective epitaxial growth andJor other conventional techniques. Moreover, it will also be understood that the sidewalls need not be orthogonal to the substrate 102, but S rather may be oblique thereto. Finally, it will also be understood that although the sidewalls 105 are shown in cross-section in Figure l, the posts 106 and trenches 107 may define elongated regions that are straight, V-shaped or have other shapes.
As shown in Figure l, the trenches 107 may extend into the buffer layer 102b and into the substrate 102a, so that subsequent gallium nitride growth occurs preferentially on the sidewalk 10~ rather than on the trench floors. In other embodiments, the trenches may not extend into the substrate 102a, and also may not extend into buffer layer 102b, depending, for example, on the trench geometry and the lateral versus vertical growth rates of the gallium nitride.
Referring now to Figure 2, the sidewalls 105 of the underlying gallium nitride layer 104 are laterally grown to form a lateral gallium nitride layer 108a 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~.mol/min and NH3 at 1 X00 sccm may be used in combination with a 3000 sccm Hz 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 sidewalk 10~. It will also be understood that some vertical growth on the posts 106 may also take place during the lateral growth from sidewalls 10~. As used herein, the term "vertical" denotes a directional parallel to the sidewalls 105.
Referring now to Figure 3, continued growth of the lateral gallium nitride layer 108a causes vertical growth onto the underlying gallium nitride layer 104, specifically onto the posts 106, to form a vertical gallium nitride layer 108b. Growth conditions for vertical growth may be maintained as was described in connection with Figure 2. As also shown in Figure 3, continued vertical growth into trenches 107 may take place at the bottom of the trenches.
Referring now to Figure 4, growth is allowed to continue until the lateral growth fronts coalesce in the trenches 107 at the interfaces 108c, to form a continuous gallium nitride semiconductor layer in the trenches. The total growth time may be_ approximately 60 minutes. As shown in Figure ~, microelectronic devices 110 may _7_ then be formed in the lateral gallium nitride semiconductor layer l OSa.
Devices may also be formed in vertical gallium nitride layer 108b.
Accordingly, in Figure 5, gallium nitride semiconductor structures 100 according to a first embodiment of the present invention are illustrated. The gallium nitride structures 100 include the substrate 102. The substrate may be sapphire or gallium nitride or other conventional substrates. However, preferably, the substrate includes the 6H-SiC(0001) substrate I02a and the aluminum nitride buffer layer 102b on the silicon carbide substrate 102a. The aluminum nitride buffer layer 102b may be 0.1 pm thick.
The fabrication of the substrate I02 is well known to those having skill in the art and need not be described further. Fabrication of silicon carbide substrates are described, for example, in U.S. Patents 4,865,685 to Palmour; Re 34,861 to Davis et al.; 4,912,064 to Kong et al. and 4,946,547 to Palmour et a1.
The underlying gallium nitride layer 104 is also included on the buffer layer 102b opposite the substrate 102a. The underlying gallium nitride layer 104 may be between about 1.0 and 2.O~m 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 10g and 101°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 102b 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 Figure 5, the underlying gallium nitride layer 104 includes the plurality of sidewalk 105 that may be defined by the plurality of pedestals 106 and/or the plurality of trenches 107. As was described above, the sidewalls may be oblique and of various elongated shapes.
Continuing with the description of Figure 5, the lateral gallium nitride layer 108a extends from the plurality of sidewalls 105 of the underlying gallium nitride layer 104. The lateral gallium nitride layer 108a may be formed using metalorganic vapor phase epitaxy at about 1000-1100°C and 45 Torr. Precursors of triethygallium (TEG) at 1 3-39p.mol/min and ammonia (NH3) at 1500 seem may be used in _g_ combination with a 3000 sccm H~ diluent, to form the lateral gallium nitride layer 108a.
Still continuing with the description of Figure 5, the gallium nitride semiconductor structure 100 also includes the vertical gallium nitride layer 108b that extends vertically from the posts 106.
As shown in Figure 5, the lateral gallium nitride layer 108a coalesces at the interfaces 108c to form a continuous lateral gallium nitride semiconductor layer 108a 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 10~
with the same density as vertically from the underlying gallium nitride layer 104.
Thus, the lateral gallium nitride layer 108a can have a relatively low defect density, for example less that 10~' cm-'. Accordingly, the lateral gallium nitride layer 108b may form device quality gallium nitride semiconductor material. Thus, as shown in Figure 5, microelectronic devices 110 may be formed in the lateral gallium nitride semiconductor layer 108a. It will also be understood that a mask need not be used to fabricate the gallium nitride semiconductor structures 100 of Figure 5, because lateral growth is directed from the sidewalls 10~.
Referring now to Figures 6-10, second embodiments of gallium nitride semiconductor structures and fabrication methods according to the present invention will now be described. First, gallium nitride semiconductor structures of Figure 4 are fabricated as was already described with regard to Figures 1-4. Then, referring to Figure 6, the posts 106 are masked with a mask 206 that includes an array of openings therein. The mask may comprise silicon dioxide at thickness of 1000th and may be deposited using low pressure chemical vapor deposition at 410°C. Other masking materials may be used. The mask may be patterned using standard photolithography techniques and etched in a buffered HF solution. In one embodiment, the openings are 3~m-wide openings that extend in parallel at distances of between 3 and 40~cm and that are oriented along the < 1 100 > direction on the lateral gallium nitride layer 108a. Prior to further processing, the structure may be dipped in a 50%
hydrochloric acid (HC1) solution to remove surface oxides. It will be understood that although the mask 206 is preferably located above the posts 106, it can also be offset therefrom.
Referring now to Figure 7, the lateral gallium nitride semiconductor layer -108a is grown through the array of openings to form a vertical gallium nitride layer 208a in the openings. Growth of gallium nitride may be obtained, as was described in connection with Figure 2.
It will be understood that growth in two dimensions may be used to form an overgrown gallium nitride semiconductor layer. Specifically, the mask 206 may be patterned to include an array of openings that extend along two orthogonal directions such as < 1100 > and < 1120 > . Thus, the openings can form a rectangle of orthogonal striped patterns. In this case, the ratio of the edges of the rectangle is preferably proportional to the ratio of the growth rates of the {1120} and {1101;
facets, for example, in a ratio of 1.4:1. The openings can be equitriangular with respect to directions such as < 1100 > and < 1 i20 > .
Referring now to Figure 8, continued growth of the vertical gallium nitride layer 208a causes lateral growth onto the mask 206, to form a second lateral gallium nitride layer 208b. Conditions for overgroWh may be maintained as was described in connection with Figure 7.
Referring now to Figure 9, lateral overgrowth is allowed to continue until the lateral growth fronts coalesce at the second interfaces 208e on the mask 206 to form a continuous overgrown gallium nitride semiconductor layer 208. The total growth time may be approximately sixty minutes. As shown in Figure 10, microelectronic devices 210 may then be formed in the second lateral gallium nitride layer 208b. The microelectronic devices may also be formed in the vertical gallium nitride layer 208a.
Accordingly, by providing the second lateral growth layer 208b, defects that were present in continuous gallium nitride semiconductor layer 108 may be reduced even further, to obtain device quality gallium nitride in the gallium nitride semiconductor structure 200.
Referring now to Figures 11-15, third embodiments of gallium nitride semiconductor structures and fabrication methods according to the present invention will now be described. First, gallium nitride semiconductor structures of Figure 4 are fabricated as was already described in connection with Figures 1-4. Then, a plurality of second sidewalls 305 are formed. The second sidewalls 305 may be formed by selective epitaxial growth of second posts 306 by etching second trenches 307 in the first posts 106 and/or combinations thereof. As was already described, the second sidewalls 30~ need not be orthogonal to substrate 102. but rather may be oblique. The second trenches 307 need not be directly over the first posts 106, but may be laterally offset therefrom. The second trenches are preferably deep so that lateral growth preferentially occurs on the sidewalls 30~ rather than on the bottom of second trenches 306.
Referring now to Figure 12. the second sidewalls 30~ of the second posts 306 and/or the second trenches 307 are laterally grown to form a second lateral gallium nitride layer 308a in the second trenches 307. As was already described, lateral growth of gallium nitride may be obtained at 1000-1100°C and 45 Torr.
The precursors TEG at 13-39~mo1/min and NHS at 1500 sccm may be used in combination with a 3000 scan H~ diluent. If gallium nitride alloys are formed, additional conventional precursors of aluminum or indium, for example, may also be used. It will also be understood that some vertical growth may take place on the second posts 306 during the lateral growth from the second sidewalls 30~.
Referring now to Figure 13, continued growth of the second lateral gallium nitride layer 308a causes vertical growth onto the second posts 306, to foml a second vertical gallium nitride layer 308b. As also shown, vertical growth from the floors of the second trenches and from the tops of the second posts may also take place.
Growth conditions for vertical growth may be maintained as was described in connection with Figure 12.
Referring now to Figure 14, growth is allowed to continue until the lateral growth fronts coalesce in the second trenches 307 at the second interfaces 308c to form a second continuous gallium nitride semiconductor layer 308. The total growth time may be approximately sixty minutes. As shown in Figure 15, microelectronic devices 310 may then be formed in the second continuous gallium nitride semiconductor layer 308.
Accordingly, third embodiments of gallium nitride semiconductor structures 300 according to the present invention may be formed without the need to mask gallium nitride for purposes of defining lateral growth. Rather, lateral growth from first and second sidewalls may be used. By performing two separate lateral growths, the defect density may be reduced considerably.
Additional discussion of methods and structures of the present invention will now be provided. The first and second trenches 107 and 307 and the openings in the mask 206 are preferably rectangular trenches and openings that preferably extend along the < 1120 > and/or < 1 100 > directions on the underlying gallium nitride layer 104 or the first lateral gallium nitride layer 108a. Tnmcated triangular stripes having (1 I O1) slant facets and a narrow (0001 ) top facet may be obtained for trenches and/or mask openings along the < 1120 > direction. Rectangular stripes having a (0001) top facet, (1120) vertical side faces and (1 1 O1) slant facets may be grown along the < 1 100 > direction. For growth times up to 3 minutes, similar morphologies may be obtained regardless of orientation. The stripes develop into different shapes if the growth is continued.
The amount of lateral growth generally exhibits a strong dependence on trench and/or mask opening orientation. The lateral growth rate of the < 1 I 00 >
oriented trenches and/or mask openings is generally much faster than those along < I
120 > .
Accordingly, it is most preferred to orient the trenches and/or mask openings, so that they extend along the < 1 100 > direction of the underlying gallium nitride layer 104 or the first lateral gallium nitride layer 108a.
The different morphological development as a function of trench and/or mask opening orientation appears to be related to the stability of the crystallographic planes in the gallium nitride structure. Trenches and/or mask openings oriented along < 1120 > may have wide (1 100) slant facets and either a very nan:ow or no (0001 ) top facet depending on the growth conditions. This may be because (1 101) is the most stable plane in the gallium nitride wurtzite crystal structure, and the growth rate of this plane is lower than that of others. The ~1 101} planes of the < 1 100 >
oriented trenches and/or mask openings may be wavy, which implies the existence of more than one Miller index. It appears that competitive growth of selected {1 1 O1) planes occurs during the deposition which causes these planes to become unstable and which causes their growth rate to increase relative to that of the (I 1 O1) of trenches and/or mask openings oriented along < 1120 > .
The morphologies of the gallium nitride layers selectively grown from trenches and/or mask openings oriented along < 1 100 > are also Denerally a strong function of the growth temperatures. Layers grown at 1000°C may possess a truncated triangular shape. This morphology may gradually change to a rectangular cross-section as the growth temperature is increased. This shape change may occur as a WO 99/65068 PCT/US99,~12967 result of the increase in the diffusion coefficient and therefore the flux of the gallium species along the (0001) top plane onto the {1 1 O1} planes with an increase in growth temperature. This may result in a decrease in the ;rowth rate of the (0001 ) plane and an increase in that of the {1 1 O1} . This phenomenon has also been observed in the selective growth of gallium arsenide on silicon dioxide. Accordingly, temperatures of 1100°C appear to be most preferred.
The morphological development of the gallium nitride regions also appears to depend on the flow rate of the TEG. An increase in the supply of TEG generally increases the growth rate in both the lateral and the vertical directions.
However, the lateral/vertical growth rate ratio decrease from 1.7 at the TEG flow rate of l3~mol/min to 0.86 at 39pmol.min. This increased influence on growth rate along <0001 > relative to that of < 1120 > with TEG flow rate may be related to the type of reactor employed, wherein the reactant gases flow vertically and perpendicular to the substrate. The considerable increase in the concentration of the gallium species on the surface may sufficiently impede their diffusion to the {I 1 O1; planes such that chemisorption and gallium nitride growth occur more readily on the (0001) plane.
Continuous 2~m thick gallium nitride semiconductor layers may be obtained using 3pm wide trenches and/or mask openingTs spaced 7~m apart and oriented alone < 1 100 > , at 1100°C and a TEG flow rate of 26ymol/min. The continuous gallium nitride semiconductor layers may include subsurface voids that form when two growth fronts coalesce. These voids may occur most often using lateral growth conditions wherein rectangular trenches and/or mask openings having vertical {1120}
side facets developed.
The continuous gallium nitride semiconductor layers may have a microscopically flat and pit-free surface. The surfaces of the laterally grown gallium nitride layers may include a terrace structure having an average step height of 0.32nm.
This terrace structure may be related to the laterally grown gallium nitride, because it is generally not included in much larger area films grown only on aluminum nitride buffer layers. The average RMS roughness values may be similar to the values obtained for the underlying gallium nitride layer 104.
Threading dislocations, originating from the interface between the underlying .
gallium nitride layer 104 and the buffer layer 102b, appear to propagate to the top surface of the underlying gallium nitride layer 104. The dislocation density within these regions is approximately 109 cm-'. By contrast. threading dislocations do not appear to readily propagate laterally. Rather, the lateral gallium nitride regions 108a and 308a contain only a few dislocations. These few dislocations may be formed parallel to the (0001 ) plane via the extension of the vertical threading dislocations after a 90° bend in the regrown region. These dislocations do not appear to propagate to the top surface of the overgrown gallium nitride layer.
As described, the formation mechanism of the selectively grown gallium nitride layers is lateral epitaxy. The two main stages of this mechanism are vertical growth and lateral growth. During vertical growth through a mask, the deposited gallium nitride grows selectively within the mask openings more rapidly than it grows on the mask, apparently due to the much higher sticking coefficient. s, of the gallium atoms on the gallium nitride surface (s=1 ) compared to on the mask (s«1 ).
Since the SiOz bond strength is 799.6 kJ/mole and much higher than that of Si-N (439 kJ/mole), Ga-N (103 kJ/mole), and Ga-O (353.6 kJ/mole), Ga or N atoms should not readily bond to the mask surface in numbers and for a time sufficient to cause gallium nitride nuclei to form. They would either evaporate or diffuse along the mask surface to the opening in the mask or to the vertical gallium nitride surfaces which have emerged.
During lateral growth, the gallium nitride grows simultaneously both vertically and laterally.
Surface diffusion of gallium and nitrogen on the gallium nitride may play a role in gallium nitride selective growth. The major source of material appears to be derived from the gas phase. This may be demonstrated by the fact that an increase in the TEG flow rate causes the growth rate of the (0001 ) top facets to develop faster than the (1 1 O1) side facets and thus controls the lateral growth.
The laterally grown gallium nitride bonds to the underlying mask sufficiently strongly so that it generally does not break away on cooling. However, lateral cracking within the SiO~ mask may take place due to thermal stresses generated on cooling. The viscosity (p) of the SiO~ at 1050°C is about 10'''' poise which is one order of magnitude greater than the strain point (about 10~~~' poise) where stress relief in a bulk amorphous material occurs within approximately six hours. Thus, the Si02 mask may provide limited compliance on cooling. As the atomic arrangement on the amorphous SiO~ surface is quite different from that on the GaN surface, chemical bonding may occur only when appropriate pairs of atoms are in close proximity.
Extremely small relaxations of the silicon and oxygen and gallium and nitrogen atoms on the respective surfaces and/or within the bulk of the SiO~ may accommodate the gallium nitride and cause it to bond to the oxide. Accordingly, the embodiments of Figures I-S and 11-1~, which need not employ a mask, may be particularly advantageous.
In conclusion, lateral epitaxial overgrowth may be obtained from sidewalls of an underlying gallium nitride layer via MOVPE. The growth may depend strongly on the sidewall orientation, growth temperature and TEG flow rate. Coalescence of overgrown gallium nitride regions to form regions with both extremely low densities of dislocations and smooth and pit-free surfaces may be achieved through 3~.m wide trenches between 7~m wide posts and extending along the < I I 00 > direction, at 1100°C and a TEG flow rate of 26~mo1/min. The lateral overgrowth of gallium nitride from sidewalls via MOVPE may be used to obtain low defect density regions for microelectronic devices, without the need to use masks.
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.
Claims (21)
1. A method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the gallium nitride layer includes a first defect density, and wherein the laterally growing a sidewall of a gallium nitride layer into a trench in the gallium nitride layer to thereby form a lateral gallium nitride layer comprises:
laterally growing the sidewall of the gallium nitride layer to thereby form the lateral gallium nitride layer of lower defect density than the first defect density; and vertically growing the lateral gallium nitride layer while propagating the lower defect density.
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the gallium nitride layer includes a first defect density, and wherein the laterally growing a sidewall of a gallium nitride layer into a trench in the gallium nitride layer to thereby form a lateral gallium nitride layer comprises:
laterally growing the sidewall of the gallium nitride layer to thereby form the lateral gallium nitride layer of lower defect density than the first defect density; and vertically growing the lateral gallium nitride layer while propagating the lower defect density.
2. A method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing is preceded by forming the gallium nitride layer including the sidewall on a substrate;
wherein the forming comprises forming the trench in the gallium nitride layer, the trench including the sidewall;
wherein the trench forming comprises selectively etching the gallium nitride layer to form the trench that includes the sidewall.
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing is preceded by forming the gallium nitride layer including the sidewall on a substrate;
wherein the forming comprises forming the trench in the gallium nitride layer, the trench including the sidewall;
wherein the trench forming comprises selectively etching the gallium nitride layer to form the trench that includes the sidewall.
3. A method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises:
laterally growing a plurality of sidewalk of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the laterally growing is followed by:
masking the lateral gallium nitride layer with a mask that includes an array of openings therein; and growing the lateral gallium nitride layer through the array of openings and onto the mask, to thereby form an overgrown gallium nitride semiconductor layer.
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises:
laterally growing a plurality of sidewalk of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the laterally growing is followed by:
masking the lateral gallium nitride layer with a mask that includes an array of openings therein; and growing the lateral gallium nitride layer through the array of openings and onto the mask, to thereby form an overgrown gallium nitride semiconductor layer.
4. A method according to Claim 3 wherein the laterally growing is followed by forming at least one microelectronic device in the overgrown gallium nitride semiconductor layer.
5. A method according to Claim 3 wherein the growing comprises growing the lateral gallium nitride layer through the array of openings and onto the mask until the grown lateral gallium nitride layer coalesces on the mask to form a continuous overgrown gallium nitride semiconductor layer.
6. A method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises:
laterally growing a plurality of sidewalls of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the laterally growing is followed by:
vertically growing the lateral gallium nitride layer;
forming a plurality of second sidewalls in the vertically grown lateral gallium nitride layer to define a plurality of second posts and a plurality of second trenches;
and laterally growing the plurality of second sidewalk of the vertically grown lateral gallium nitride layer into the plurality of second trenches at a rate faster than vertically growing the vertically grown lateral gallium nitride layer on the second post tops, to thereby form a second lateral gallium nitride semiconductor layer.
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises:
laterally growing a plurality of sidewalls of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the laterally growing is followed by:
vertically growing the lateral gallium nitride layer;
forming a plurality of second sidewalls in the vertically grown lateral gallium nitride layer to define a plurality of second posts and a plurality of second trenches;
and laterally growing the plurality of second sidewalk of the vertically grown lateral gallium nitride layer into the plurality of second trenches at a rate faster than vertically growing the vertically grown lateral gallium nitride layer on the second post tops, to thereby form a second lateral gallium nitride semiconductor layer.
7. A method according to Claim 6 wherein the laterally growing the plurality of second sidewalk is followed by forming at least one microelectronic device in the second lateral gallium nitride semiconductor layer.
8. A method according to Claim 6 wherein the laterally growing the plurality of second sidewalls comprises laterally growing the plurality of second sidewalls of the vertically grown lateral gallium nitride layer into the plurality of second trenches until the plurality of laterally grown second sidewalk coalesce in the plurality of second trenches.
9. A method according to Claim 6 wherein the second trenches include second trench floors and wherein the laterally growing comprises laterally growing the plurality of second sidewalk of the vertically grown lateral gallium nitride layer into the plurality of second trenches, spaced apart from the second trench floors, to thereby form a cantilevered second lateral gallium nitride semiconductor layer.
10. A method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises:
laterally growing a plurality of sidewalls of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the gallium nitride layer includes a first defect density, and wherein the laterally growing a plurality of sidewalls of the gallium nitride layer into the plurality of trenches in the gallium nitride layer to thereby form a lateral gallium nitride layer comprises:
laterally growing the plurality of sidewalls of the gallium nitride layer into the plurality of trenches to thereby form a lateral gallium nitride semiconductor layer of lower defect density than the first defect density; and vertically growing the laterally gallium nitride layer while propagating the lower defect density.
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises:
laterally growing a plurality of sidewalls of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the gallium nitride layer includes a first defect density, and wherein the laterally growing a plurality of sidewalls of the gallium nitride layer into the plurality of trenches in the gallium nitride layer to thereby form a lateral gallium nitride layer comprises:
laterally growing the plurality of sidewalls of the gallium nitride layer into the plurality of trenches to thereby form a lateral gallium nitride semiconductor layer of lower defect density than the first defect density; and vertically growing the laterally gallium nitride layer while propagating the lower defect density.
11. A method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the trench includes a trench floor and wherein the laterally growing comprises laterally growing the sidewall of the gallium nitride layer into the trench, spaced apart from the trench floor, to thereby form a cantilevered lateral gallium nitride semiconductor layer.
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the trench includes a trench floor and wherein the laterally growing comprises laterally growing the sidewall of the gallium nitride layer into the trench, spaced apart from the trench floor, to thereby form a cantilevered lateral gallium nitride semiconductor layer.
12. A method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises growing a pair of sidewalk of the gallium nitride layer into a trench in the gallium nitride layer between the pair of sidewalk until the grown pair of sidewalls coalesce in the trench;
wherein the trench includes a trench floor and wherein the laterally growing comprises laterally growing the pair of sidewalls of the gallium nitride layer into the trench, spaced apart from the trench floor.
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer;
wherein the laterally growing comprises growing a pair of sidewalk of the gallium nitride layer into a trench in the gallium nitride layer between the pair of sidewalk until the grown pair of sidewalls coalesce in the trench;
wherein the trench includes a trench floor and wherein the laterally growing comprises laterally growing the pair of sidewalls of the gallium nitride layer into the trench, spaced apart from the trench floor.
13. A method of fabricating a gallium nitride semiconductor layer comprising:
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer wherein the laterally growing comprises:
laterally growing a plurality of sidewalls of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the trenches include trench floors and wherein the laterally growing comprises laterally growing the plurality of sidewalls of the gallium nitride layer into the plurality of trenches, spaced apart from the trench floors, to thereby form cantilevered lateral gallium nitride semiconductor layers.
laterally growing a sidewall of a post in a gallium nitride layer into a trench in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post top to thereby form a lateral gallium nitride semiconductor layer wherein the laterally growing comprises:
laterally growing a plurality of sidewalls of posts in a gallium nitride layer into a plurality of trenches in the gallium nitride layer at a rate faster than vertically growing the gallium nitride layer on the post tops to thereby form a lateral gallium nitride layer;
wherein the trenches include trench floors and wherein the laterally growing comprises laterally growing the plurality of sidewalls of the gallium nitride layer into the plurality of trenches, spaced apart from the trench floors, to thereby form cantilevered lateral gallium nitride semiconductor layers.
14. A gallium nitride semiconductor structure comprising:
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer;
a second vertical gallium nitride layer that extends from the lateral gallium nitride layer, and that has lower defect density than the vertical gallium nitride layer.
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer;
a second vertical gallium nitride layer that extends from the lateral gallium nitride layer, and that has lower defect density than the vertical gallium nitride layer.
15. A gallium nitride semiconductor structure comprising:
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer;
wherein the underlying gallium nitride layer including a plurality of posts having a plurality of sidewalls and tops and defining a plurality of trenches;
and wherein the lateral gallium nitride layer extends from the plurality of sidewalls of the underlying gallium nitride layer into the plurality of trenches; and wherein the vertical gallium nitride layer is on the plurality of post tops.
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer;
wherein the underlying gallium nitride layer including a plurality of posts having a plurality of sidewalls and tops and defining a plurality of trenches;
and wherein the lateral gallium nitride layer extends from the plurality of sidewalls of the underlying gallium nitride layer into the plurality of trenches; and wherein the vertical gallium nitride layer is on the plurality of post tops.
16. A structure according to Claim 15 further comprising:
a mask including an array of openings therein on the lateral gallium nitride layer; and a second vertical gallium nitride layer that extends from the lateral gallium nitride layer, through the openings and onto the mask, and that has lower defect density than the vertical gallium nitride layer.
a mask including an array of openings therein on the lateral gallium nitride layer; and a second vertical gallium nitride layer that extends from the lateral gallium nitride layer, through the openings and onto the mask, and that has lower defect density than the vertical gallium nitride layer.
17. A structure according to Claim 15 further comprising:
a second vertical gallium nitride layer that extends from the lateral gallium nitride layer, wherein the second vertical gallium nitride layer includes a plurality of second sidewalls therein and narrower defect density than the vertical gallium nitride layer; and a second lateral gallium nitride layer that extends from the plurality of second sidewalls.
a second vertical gallium nitride layer that extends from the lateral gallium nitride layer, wherein the second vertical gallium nitride layer includes a plurality of second sidewalls therein and narrower defect density than the vertical gallium nitride layer; and a second lateral gallium nitride layer that extends from the plurality of second sidewalls.
18. A structure according to Claim 16 further comprising:
at least one microelectronic device in the lateral gallium nitride layer.
at least one microelectronic device in the lateral gallium nitride layer.
19. A structure according to Claim 17 further comprising:
at least one plurality of microelectronic device in the second lateral gallium nitride layer.
at least one plurality of microelectronic device in the second lateral gallium nitride layer.
20. A gallium nitride semiconductor structure comprising:
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer;
wherein the trench includes a trench floor and wherein the lateral gallium nitride layer is a cantilevered lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench and is spaced apart from the trench floor.
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer;
wherein the trench includes a trench floor and wherein the lateral gallium nitride layer is a cantilevered lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench and is spaced apart from the trench floor.
21. A gallium nitride semiconductor structure comprising:
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer.
wherein the trenches include trench floors and wherein the lateral gallium nitride layer is a cantilevered lateral gallium nitride layer that extends from the plurality of sidewalk of the underlying gallium nitride layer into the trenches and is spaced apart from the trench floors.
an underlying gallium nitride layer including a post having a sidewall and a top, and defining a trench;
a lateral gallium nitride layer that extends from the sidewall of the underlying gallium nitride layer into the trench; and a vertical gallium nitride layer on the post top, that has higher defect density than the lateral gallium nitride layer.
wherein the trenches include trench floors and wherein the lateral gallium nitride layer is a cantilevered lateral gallium nitride layer that extends from the plurality of sidewalk of the underlying gallium nitride layer into the trenches and is spaced apart from the trench floors.
Applications Claiming Priority (5)
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US09/327,136 | 1999-06-07 | ||
US09/327,136 US6265289B1 (en) | 1998-06-10 | 1999-06-07 | Methods of fabricating gallium nitride semiconductor layers by lateral growth from sidewalls into trenches, and gallium nitride semiconductor structures fabricated thereby |
PCT/US1999/012967 WO1999065068A1 (en) | 1998-06-10 | 1999-06-09 | Fabrication of gallium nitride semiconductor layers by lateral growth from trench sidewalls |
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CA2331893C true CA2331893C (en) | 2007-01-23 |
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CA002331893A Expired - Lifetime CA2331893C (en) | 1998-06-10 | 1999-06-09 | Fabrication of gallium nitride semiconductor layers by lateral growth from trench sidewalls |
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CN (1) | CN1143364C (en) |
AU (1) | AU4556599A (en) |
CA (1) | CA2331893C (en) |
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1999
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WO1999065068A1 (en) | 1999-12-16 |
US20050009304A1 (en) | 2005-01-13 |
CA2331893A1 (en) | 1999-12-16 |
US6897483B2 (en) | 2005-05-24 |
KR20010071417A (en) | 2001-07-28 |
US20010039102A1 (en) | 2001-11-08 |
US6265289B1 (en) | 2001-07-24 |
CN1305639A (en) | 2001-07-25 |
EP1088340A1 (en) | 2001-04-04 |
KR100498164B1 (en) | 2005-07-01 |
JP3950630B2 (en) | 2007-08-01 |
US7195993B2 (en) | 2007-03-27 |
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