Methods of fabricating gallium nitride semiconductor layers...

Details Methods of fabricating gallium nitride semiconductor layers... Methods of fabricating gallium nitride semiconductor layers...

1999-06-07

2001-07-24

Christianson, Keith (Department: 2813)

Semiconductor device manufacturing: process

Formation of semiconductive active region on any substrate

Fluid growth from gaseous state combined with preceding...

Reexamination Certificate

active

06265289

ABSTRACT:

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 Al
0.2
Ga
0.8
N on GaN/AlN/
6
H—SiC
(0001)
Multilayer Substrates Via Organometallic Vapor Phase Epitaxy
”, Proceedings of the Materials Research Society, December 1996, and “
Growth of GaN and Al
0.2
Ga
0.8
N on Patterened Substrates via Organometallic Vapor Phase Epitaxy
”, Japanese Journal of Applied Physics., Vol. 36, Part 2, No. 5A, May 1997, pp. L532-L535. As disclosed in these publications, undesired ridge growth or lateral overgrowth may occur under certain conditions.
SUMMARY OF THE INVENTION
It is therefore an object 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 the invention to provide methods of fabricating gallium nitride semiconductor layers that can have low defect densities, and gallium nitride semiconductor layers so fabricated.
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 sidewalls of the underlying gallium nitride layer are laterally grown into a trench in the underlying gallium nitride layer between the pair of sidewalls 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 (NH
3
) precursors at 1000-1100° C. and 45 Torr. Preferably, TEG at 13-39 &mgr;mol/min and NH
3
at 1500 sccm are used in combination with 3000 sccm H
2
diluent. Most preferably, TEG at 26 &mgr;mol/min, NH
3
at 1500 sccm and H
2
at 3000 sccm 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 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 etching 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 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 gallium 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 sidewalls. The underlying 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 sidewalls therein. A second lateral gallium nitride layer extends from the plurality of second sidewalls. Microelectronic devices are included in the second lateral gallium nitride layer. Accordingly, low defect density gallium nitride semiconductor layers may be produced, to thereby allow the production of high perfor

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