Active solid-state devices (e.g. – transistors – solid-state diode – Physical configuration of semiconductor – Mesa structure
Reexamination Certificate
2001-10-09
2003-09-16
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Physical configuration of semiconductor
Mesa structure
62, 62, 62, 62, 62, 62, 62, 62, 62
Reexamination Certificate
active
06621148
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 also is known to produce low defect density gallium nitride layers by forming a mask on a layer of gallium nitride, the mask including at least one opening therein that exposes the underlying layer of gallium nitride, and laterally growing the underlying layer of gallium nitride through the at least one opening and onto the mask. This technique often is referred to as “Epitaxial Lateral Overgrowth” (ELO). The layer of gallium nitride may be laterally grown until the gallium nitride coalesces on the mask to form a single layer on the mask. In order to form a continuous layer of gallium nitride with relatively low defect density, a second mask may be formed on the laterally overgrown gallium nitride layer, that includes at least one opening that is offset from the opening in the underlying mask. ELO then again is performed through the openings in the second mask to thereby overgrow a second low defect density continuous gallium nitride layer. Microelectronic devices then may be formed in this second overgrown layer. ELO of gallium nitride is described, for example, in the publications entitled
Lateral Epitaxy of Low Defect Density GaN Layers Via Organometallic Vapor Phase Epitaxy
to Nam et al., Appl. Phys. Lett. Vol. 71, No. 18, Nov. 3, 1997, pp. 2638-2640; and
Dislocation Density Reduction Via Lateral Epitaxy in Selectively Grown GaN Structures
to Zheleva et al, Appl. Phys. Lett., Vol. 71, No. 17, Oct. 27, 1997, pp. 2472-2474, the disclosures of which are hereby incorporated herein by reference.
It also is known to produce a layer of gallium nitride with low defect density by forming at least one trench or post in an underlying layer of gallium nitride to define at least one sidewall therein. A layer of gallium nitride is then laterally grown from the at least one sidewall. Lateral growth preferably takes place until the laterally grown layers coalesce within the trenches. Lateral growth also preferably continues until the gallium nitride layer that is grown from the sidewalls laterally overgrows onto the tops of the posts. In order to facilitate lateral growth and produce nucleation of gallium nitride and growth in the vertical direction, the top of the posts and/or the trench floors may be masked. Lateral growth from the sidewalls of trenches and/or posts also is referred to as “pendeoepitaxy” and is described, for example, in publications entitled
Pendeo
-
Epitaxy: A New Approach for Lateral Growth of Gallium Nitride Films
by Zheleva et al., Journal of Electronic Materials, Vol. 28, No. Feb. 4, 1999, pp. L5-L 8; and
Pendeoepitaxy of Gallium Nitride Thin Films
by Linthicum et al., Applied Physics Letters, Vol. 75, No. 2, July 1999, pp. 196-198, the disclosures of which are hereby incorporated herein by reference.
Unfortunately, both ELO and pendeoepitaxy may use one or more masks to mask portions of an underlying gallium nitride layer during ELO and/or pendeoepitaxy. These masks may complicate the fabrication process. Moreover, multiple growth steps of gallium nitride may be needed with mask formation therebetween. These multiple growth steps also may complicate the fabrication processes, because the structures may need to be removed from the gallium nitride growth chamber in order to form the mask or masks. Accordingly, notwithstanding the recent advances in ELO and pendeoepitaxy, there continues to be a need for methods of fabricating gallium nitride semiconductor layers that do not need masking layers and/or need not interrupt the gallium nitride growth process.
SUMMARY OF THE INVENTION
The present invention provides a substrate including non-gallium nitride posts that define trenches therebetween, wherein the non-gallium nitride posts include non-gallium nitride sidewalls and non-gallium nitride tops, and the trenches include non-gallium floors. These substrates also may be referred to herein as “textured” substrates. Then, gallium nitride is grown on the non-gallium nitride posts, including on the non-gallium nitride tops. Preferably, gallium nitride pyramids are grown on the non-gallium nitride tops and gallium nitride then is grown on the gallium nitride pyramids. The gallium nitride pyramids preferably are grown at a first temperature and the gallium nitride preferably is grown on the pyramids at a second temperature that is higher than the first temperature. The first temperature preferably is about 1000° C. or less and the second temperature preferably is about 1100° C. or more. However, other than temperature, the same processing conditions preferably are used for both growth steps. The grown gallium nitride on the pyramids preferably coalesces to form a continuous gallium nitride layer.
Accordingly, gallium nitride may be grown on a textured substrate, without the need to provide masks during the gallium nitride growth process. Moreover, the gallium nitride growth may be performed using the same processing conditions other than temperatures changes. Accordingly, uninterrupted gallium nitride growth may be performed. Simplified processing conditions therefore may be employed to grow a gallium nitride layer having low defect densities, for example defect densities of less than about 10
5
cm
−2
.
During growth of the gallium nitride pyramids on the non-gallium nitride tops, gallium nitride pyramids also may be simultaneously grown on the non-gallium nitride floors. Moreover, a conformal gallium nitride layer also may be formed simultaneously on the sidewalls, between the gallium nitride pyramids on the non-gallium nitride tops and on the non-gallium nitride floors. Upon growing the gallium nitride on the pyramids, the trenches also may be simultaneously filled with gallium nitride. A conformal buffer layer may be formed on the substrate including on the non-gallium nitride sidewalls, the non-gallium nitride tops and the non-gallium nitride floors, prior to growing the gallium nitride pyramids. For example, a conformal layer of aluminum nitride may be used.
Gallium nitride semiconductor structures therefore may be fabricated, according to the present invention, by providing a textured substrate, including a plurality of non-gallium nitride posts that define trenches therebetween, wherein the non-gallium nitride posts include non-gallium nitride sidewalls and non-gallium nitride tops, and the trenches include non-gallium nitride floors. The substrate preferably is free of masking materials on the non-gallium nitride floors and on the non-gallium nitride tops. Gallium nitride then is grown at a first temperature and the growth of gallium nitride then is continued at a second temperature that is higher than the first temperature. Growth at the second temperature preferably continues until the gallium nitride forms a continuous gallium nitride layer on the substrate.
Gallium nitride semiconductor structures according to the present invention preferably comprise a textured substrate includin
Davis Robert F.
Gehrke Thomas
Linthicum Kevin J.
Flynn Nathan J.
Mandala Jr. Victor A.
Moore & Van Allen PLLC
North Carolina State University
Phillips Steven B.
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