Active solid-state devices (e.g. – transistors – solid-state diode – Specified wide band gap semiconductor material other than...
Reexamination Certificate
2003-05-02
2004-10-12
Dang, Phuc T. (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Specified wide band gap semiconductor material other than...
C257S190000, C257S201000
Reexamination Certificate
active
06803602
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electronic device structures and fabrication methods, and more particularly to Group III-nitride semiconductor structures and methods of fabrication by pendeo- and lateral epitaxial overgrowth.
BACKGROUND
Gallium nitride (GaN) is a wide-bandgap semiconductor material widely known for its usefulness as an active layer in blue light emitting diodes. GaN is also under investigation for use in other microelectronic devices including laser diodes and high-speed, high power transistor devices. As used herein, “gallium nitride” or “GaN” refers to gallium nitride and III-nitride alloys thereof, including aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and aluminum indium gallium nitride (AlInGaN).
High-quality bulk crystals of GaN are currently unavailable for commercial use. Thus, GaN crystals are typically fabricated as heteroepitaxial layers on underlying non-GaN substrates. Unfortunately, GaN has a considerable lattice mismatch with most suitable substrate crystals. For example, GaN has a 15% lattice mismatch with sapphire and a 3.5% lattice mismatch with silicon carbide. Lattice mismatches between a substrate and an epitaxial layer cause threading dislocations which may propagate through the growing epitaxial layer. Even when grown on silicon carbide with an aluminum nitride buffer layer, a GaN epitaxial layer exhibits dislocation densities estimated to be in excess of 10
8
/cm
2
. Such defect densities limit the usefulness of GaN in highly sensitive electronic devices such as laser diodes.
Lateral Epitaxial Overgrowth (LEO) of GaN has been the subject of considerable interest since it was first introduced as a method of reducing the dislocation densities of epitaxially grown GaN films. Essentially, the technique consists of masking an underlying layer of GaN with a mask having a pattern of openings and growing the GaN up through and laterally onto the mask. It was found that the portion of the GaN layer grown laterally over the mask exhibits a much lower dislocation density than the underlying GaN layer or the portion of the GaN layer above the mask openings. As used herein, “lateral” or “horizontal”,refers to a direction generally parallel to the surface of a substrate, while the term “vertical” means a direction generally orthogonal to the surface of a substrate.
One drawback to conventional LEO techniques is that separate process steps are required for growing the underlying GaN layer, masking the GaN layer and then growing the lateral layer. Early embodiments of LEO did not place the mask directly on the non-GaN substrate because unwanted nucleation would occur on the mask during nucleation of the GaN layer at low temperatures, preventing adjacent laterally-grown regions from coalescing (or otherwise from growing laterally a desired distance if coalescence is not required). When the mask is placed directly on a GaN layer, unwanted nucleation on the mask is typically not a problem since low temperature nucleation is not required and the growth temperature of GaN is very high, typically above 1000° C. During high temperature growth, unwanted nucleation does not occur on the mask due to the much higher sticking coefficient of gallium atoms on the gallium nitride surface as compared to the mask.
This drawback is addressed with some success by a “single step” process for LEO. Shealy et al. disclosed a process whereby an underlying SiC or sapphire substrate was masked with silicon nitride. The process is referred to as “single step” because it does not require growth of an intermediate layer of GaN between the substrate and the mask. Shealy found that minimizing nucleation on the silicon nitride mask permitted growth of a relatively defect free layer of laterally-grown GaN over the masks. However, under certain circumstances it is desirable to avoid having to minimize nucleation on the mask, yet still be able to grow a relatively defect free layer of GaN in a single step process.
OBJECT AND SUMMARY OF THE INVENTION
Accordingly, there is a need in the art for a method of fabricating a relatively defect-free single crystal film of gallium nitride in a single step process without having to minimize nucleation on the mask layer.
Moreover, there is a need in the art for a method of fabricating a relatively defect-free single crystal film of gallium nitride in a single step process without having to minimize nucleation on the mask layer which provides a conductive buffer layer to permit electrical communication between a conductive substrate and an epitaxial layer of gallium nitride.
It is an object of the present invention to provide a method of fabricating a relatively defect-free single crystal film of gallium nitride in a single step process without having to minimize nucleation on the mask layer.
It is a further object of the present invention to provide a method of fabricating a relatively defect-free single crystal film of gallium nitride in a single step process without having to minimize nucleation on the mask layer which provides a conductive buffer layer to permit electrical communication between a conductive substrate and an epitaxial layer of gallium nitride.
The foregoing and other objects are achieved by a method of fabricating a gallium nitride-based semiconductor structure on a substrate. The method includes the steps of forming a mask having at least one opening therein directly on the substrate, growing a buffer layer through said at least one opening, and growing a layer of gallium nitride upwardly from said buffer layer and laterally across said mask. During growth of the gallium nitride from the mask, the vertical and horizontal growth rates of the gallium nitride layer are maintained at rates sufficient to prevent polycrystalline material nucleating on said mask from interrupting the lateral growth of the gallium nitride layer.
In an alternative embodiment, the method includes forming at least one raised portion defining adjacent trenches in the substrate and forming a mask on the substrate, the mask having at least one opening over the upper surface of the raised portion. A buffer layer may be grown from the upper surface of the raised portion. The gallium nitride layer is then grown laterally by pendeoepitaxy over the trenches.
In another embodiment, the present invention provides a gallium nitride-based semiconductor structure on a substrate. The structure includes a substrate and a mask having at least one window therein applied directly on the upper surface of the substrate. An overgrown layer of gallium nitride extends upwardly from the mask window and laterally across the mask, on which polycrystalline material has nucleated and grown.
In another embodiment, the substrate includes at least one raised portion defining adjacent trenches. A mask structure overlays the substrate and a window in the mask exposes at least a portion of the upper surface of the raised portion. An overgrown layer of gallium nitride extends upwardly from the mask window and laterally over the trench and across the mask, on which polycrystalline material has nucleated and grown.
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Smart, et al., “Single step process for epitaxial l
Edmond John Adam
Emerson David Todd
Haberern Kevin Ward
Kong Hua-Shuang
Cree Inc.
Dang Phuc T.
Summa & Allan P.A.
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