High surface quality GaN wafer and method of fabricating same

Details High surface quality GaN wafer and method of fabricating same High surface quality GaN wafer and method of fabricating same

2001-06-08

2002-12-03

Hiteshew, Felisa (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Processes of growth with a subsequent step acting on the...

C117S001000, C117S090000, C117S092000, C117S097000, C117S106000, C117S109000

Reexamination Certificate

active

06488767

ABSTRACT:

BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates to an Al
x
Ga
y
In
z
N (wherein 0<y≦1 and x+y+z=1) semiconductor wafer having superior surface quality at its Ga-side, and to a method of fabricating such E wafer.
II. Description of the Related Art
GaN and related GaN-like III-V nitride crystal films, represented by the general formula Al
x
Ga
y
In
z
N, wherein 0<y≦1 and x+y+z=1, are useful materials in various applications, such as high temperature electronics, power electronics, and optoelectronics (e.g., light emitting diodes (LEDs) and blue light laser diodes (LDs)). Blue light emitting diodes (LED's) and lasers are an enabling technology, allowing much higher storage density in magneto-optic memories and CDROM's and the construction of full color light emitting displays. Blue light emitting diodes may replace today's incandescent light bulbs in road and railway signals etc., where they promise very substantial cost and energy savings.
Currently, Al
x
Ga
y
In
z
N films are grown on non-native substrates such as sapphire or silicon carbide due to unavailability of high quality Al
x
Ga
y
In
z
N substrates. However, differences in thermal expansion and lattice constants between such foreign substrates and the Al
x
Ga
y
In
z
N crystals epitaxially grown thereon cause significant thermal stress and internal stress in the grown Al
x
Ga
y
In
z
N crystals. The thermal stress and internal stress cause micro-cracks, distortions, and other defects in the Al
x
Ga
y
In
z
N crystals, and make such Al
x
Ga
y
In
z
N crystals easy to break. Growing on lattice non-matched foreign substrates causes high density of lattice defects, leading to poor device performance.
In order to reduce the deleterious thermal stress and high defect density in the grown Al
x
Ga
y
In
z
N crystals, it is desirable to provide high quality freestanding Al
x
Ga
y
In
z
N wafers as film-growing substrates in place of the above-mentioned foreign substrates.
U.S. Pat. No. 5,679,152 entitled “Method for Making a Single Crystal Ga*N Article” and U.S. Pat. No. 5,679,153 entitled “Bulk Single Crystal Gallium Nitride and Method of Making Same” disclose a hydride vapor phase epitaxy (HVPE) process for fabricating freestanding Al
x
Ga
y
In
z
N crystals, which may advantageously be used as crystal-growing substrates for homoepitaxial growth of Al
x
Ga
y
In
z
N crystals thereon.
Since quality of a subsequently grown Al
x
Ga
y
In
z
N crystal is directly correlated to the quality of the substrate surface and near surface region on which the Al
x
Ga
y
In
z
N crystal is grown, it is important to provide a highly smooth initial substrate surface without any surface and subsurface damage.
However, after mechanical polishing, the Al
x
Ga
y
In
z
N crystals typically have very poor surface quality, with substantial surface and subsurface damage and polishing scratches. Additional wafer finish processing therefore is necessary to further enhance the surface quality of the freestanding Al
x
Ga
y
In
z
N crystal, so that it is suitable for high-quality epitaxial growth and device fabrication thereon.
Crystalline Al
x
Ga
y
In
z
N generally exists in a chemically stable wurtzite structure. The most common crystallographic orientation of Al
x
Ga
y
In
z
N compounds has two polar surfaces perpendicular to its c-axis: one side is N-terminated, and the other one is Ga-terminated (Ga hereinafter in the context of the Ga-side of the crystal structure being understood as generally illustrative and representative of alternative Group III (Al
x
Ga
y
In
z
) crystalline compositions, e.g., of a corresponding Ga
x
In
y
-side in Ga
x
In
y
N crystals, of a corresponding Al
x
Ga
y
In
z
-side in Al
x
Ga
y
In
z
N crystals, and of a corresponding Al
x
Ga
y
-side in Al
x
Ga
y
N crystals).
Crystal polarity strongly influences the growth morphology and chemical stability of the crystal surface. It has been determined that the N-side of the Al
x
Ga
y
In
z
N crystal is chemically reactive with KOH or NaOH-based solutions, whereas the Ga-side of such crystal is very stable and not reactive with most conventional chemical etchants. The N-side can therefore be easily polished, using an aqueous solution of KOH or NaOH, to remove surface damage and scratches left by the mechanical polishing process and to obtain a highly smooth surface.
The Ga-side (Al
x
Ga
y
In
z
side) of the Al
x
Ga
y
In
z
N crystal, on the other hand, remains substantially the same after contacting the KOH or NaOH solution, with its surface damage and scratches unaltered by such solution. See Weyher et al., “Chemical Polishing of Bulk and Epitaxial GaN”, J. CRYSTAL GROWTH, vol. 182, pp. 17-22, 1997; also see Porowski et al. International Patent Application Publication No. WO 98/45511 entitled “Mechano-Chemical Polishing of Crystals and Epitaxial Layers of GaN and Ga
1−x−y
Al
x
In
y
N”.
However, it has been determined that the Ga-side of the Al
x
Ga
y
In
z
N crystal is a better film-growing surface than the N-side. See Miskys et al., “MOCVD-Epitaxy on Free-Standing HVPE-GaN Substrates”, PHYS. STAT. SOL. (A), vol. 176, pp. 443-46, 1999. It therefore is important to provide a wafer finish process that is particularly effective for preparing the Ga-side of the Al
x
Ga
y
In
z
N crystal to make it suitable for subsequent crystal growth thereupon.
Reactive ion etching (RIE) recently has been used to remove a layer of surface material from the Ga-side of an Al
x
Ga
y
In
z
N wafer to obtain smoother wafer surface. See Karouta et al., “Final Polishing of Ga-Polar GaN Substrates Using Reactive Ion Etching”, J. ELECTRONIC MATERIALS, vol. 28, pp. 1448-51, 1999. However, such RIE process is unsatisfactory because it is ineffective for removing deeper scratches, and it introduces additional damage by ion bombardment and additional surface irregularities by concomitant contamination, which in turn requires additional cleaning of the GaN wafer in an O
2
plasma.
It is therefore advantageous to provide an Al
x
Ga
y
In
z
N wafer with high surface quality on its Ga-side, with substantially no or little surface and subsurface damage or contamination. It is also desirable that such Al
x
Ga
y
In
z
N wafer is prepared by a surface polishing process that is both economic and effective, and requires no cumbersome cleaning process during or after polishing.
SUMMARY OF THE INVENTION
The present invention generally relates to an Al
x
Ga
y
In
z
N (wherein 0<y≦1 and x+y+z=1) wafer having superior surface quality at its Ga-side, and to a method of fabricating such wafer.
One aspect of the present invention relates to a high quality Al
x
Ga
y
In
z
N wafer of such type, wherein the wafer has a surface roughness characterized by a root means square (RMS) roughness of less than 1 nm in a 10×10 &mgr;m
2
area at its Ga-side.
In ranges of progressively increasing preference, the RMS surface roughness of such wafer at its Ga-side is within the following ranges: (1) less than 0.7 nm in a 10×10 &mgr;m
2
area; (2) less than 0.5 nm in a 10×10 &mgr;m
2
area; (3) less than 0.4 nm in a 2×2 &mgr;m
2
area; (4) less than 0.2 nm in a 2×2 &mgr;m
2
area; and (5) less than 0.15 nm in a 2×2 &mgr;m
2
area.
AI
x
Ga
y
In
z
N wafers according to the present invention preferably are characterized by a regular step structure at the Ga-side thereof when observed by atomic force microscope.
Al
x
Ga
y
In
z
N wafers according to the present invention preferably are characterized by that the crystal defects of the Al
x
Ga
y
In
z
N wafer at its Ga-side constitute small pits with diameters of less than 1 &mgr;m. Small pits of such size are readily visible by both atomic force microscope (AFM) and scanning electron microscope (SEM) techniques, while at the same time these pits do not constitute significant damage of the Al
x
Ga
y
In
z
N wafer surface and therefore do not impair quality of Al
x
Ga
y
In
z
N crystals subsequently grown thereon.
Such high quality Al
x
Ga
y
In
z
N crystal wafers are readily ma

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