Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
2001-08-01
2003-11-18
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S094000, C117S095000, C117S105000, C117S910000, C117S952000
Reexamination Certificate
active
06648966
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of semiconductors, and, more particularly, to methods for making single crystal wafers and the wafers produced thereby.
BACKGROUND OF THE INVENTION
Gallium nitride (GaN) is a highly desirable material for making many types of electronic devices. GaN has a wide bandgap of about 3.4 eV and is a direct-transistion type of semiconductor, and is thus attractive for light-emitting devices. It also has a high breakdown voltage, good transport properties and the ability to form high quality heterostructures. Accordingly, GaN is also attractive for high power, high temperature applications, such as high power amplifiers.
The desirable starting material for a GaN-based device would preferably be a bulk crystal or wafer form of GaN, on which various doped layers could be epitaxially grown. Unfortunately, GaN in wafer form is not producible using conventional melt pulling crystal growth techniques, as are silicon wafers, for example. Accordingly, approaches have been pursued for producing single crystal GaN films on growth substrates which remain attached to the GaN film to further serve as support or which are later removed.
For example, U.S. Pat. No. 5,625,202 to Chai discloses forming GaN on various substrate materials to produce light emitting devices. These substrate materials are described as modified wurtzite structure oxide compounds and include Lithium Aluminum Oxide, Sodium Aluminum Oxide, Lithium Gallium Oxide, Sodium Gallium Oxide, Lithium Germanium Oxide, Sodium Germanium Oxide, Lithium Silicon Oxide, Silicon Oxide, Lithium Phosphor Oxide, Lithium Arsenic Oxide, Lithium Vanadium Oxide, Lithium Magnesium Germanium Oxide, Lithium Zinc Germanium Oxide, Lithium Cadmium Germanium Oxide, Lithium Magnesium Silicon Oxide, Lithium Zinc Silicon Oxide, Lithium Cadmium Silicon Oxide, Sodium Magnesium Germanium Oxide, Sodium Zinc Germanium Oxide, and Sodium Zinc Silicon Oxide. The GaN layer remains on the growth substrate.
U.S. Pat. No. 6,156,581 to Vaudo et al. discloses growing one of a gallium, aluminum, or indium (Ga, Al, In) nitride layer on a substrate for subsequent fabrication, by metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) of a microelectronic device thereon. Vapor-phase (Ga, Al, In) chloride is reacted with a vapor-phase nitrogenous compound in the presence of a substrate to form (Ga, Al, In) nitride. The thickness of the base layer may be on the order of 2 microns and greater, and the defect density may be on the order of 10
8
cm
−2
or lower. The patent provides a laundry list of proposed foreign substrates including sapphire, silicon, silicon carbide, diamond, lithium gallate, lithium aluminate, zinc oxide, spinel, magnesium oxide, ScAlMgO4, gallium arsenide, silicon-on-insulator, carbonized silicon-on-insulator, carbonized silicon-on-silicon, gallium nitride, etc., including conductive as well as insulating and semi-insulating substrates, twist-bonded substrates (i.e., where the substrate of crystalline material is bonded to another single crystal substrate material with a finite angular crystallographic misalignment), and compliant substrates of the type disclosed in U.S. Pat. No. 5,563,428 to Ek et al., etc. The patent further discloses that in some embodiments, the substrate can be removed leaving a free-standing wafer. Unfortunately, the patent provides specific growth information only for sapphire.
U.S. Pat. No. 6,139,628 to Yuri et al. discloses forming GaN on sapphire by first heating the substrate in gas atmosphere including gallium, forming a first gallium nitride on the substrate, and forming a second gallium nitride on the first gallium nitride at a higher temperature than the temperature when the first gallium nitride was formed. A product of the gas including mono-atomic metal gallium is bonded to the surface of the substrate homogeneously and in a high density by applying heat to the substrate in the gas atmosphere including gallium. The first gallium nitride is formed at the low temperature by using the bonded metal gallium so that gallium nitride can complete its initial growth flat and homogeneously without re-vaporization. Further, after forming the first gallium nitride, the second gallium nitride is formed at the higher temperature, whereby the crystallinity of not only the first gallium nitride but also the second gallium nitride are improved through the heat treatment with the high temperature. After the third step is completed, the substrate may be removed.
A number of other approaches have also formed pretreatment layers prior to GaN deposition. For example, U.S. Pat. No. 6,086,673 to Molnar discloses forming a zinc oxide pretreatment layer on sapphire and then subjected to a gaseous environment including HCl and/or NH3 containing gas, that is thermochemically reactive with the zinc oxide. An epitaxial layer of GaN can be grown on the pretreated substrate.
Along these lines, U.S. Pat. No. 6,218,280 B1 to Kryliouk et al. discloses forming a nitrided layer on a lithium gallate substrate, forming a first GaN layer on the nitrided layer by metalorganic chemical vapor deposition (MOCVD), growing a next GaN portion using halide vapor phase epitaxy, and growing a capping GaN layer again using MOCVD. The GaN layers may then be separated from the substrate. The patent lists a number of other proposed substrates in addition to the specifically disclosed lithium gallate. These other substrates include LiAlO
2
, MgAlScO
4
, Al2MgO
4
and LiNdO
2
. Unfortunately, the use of MOCVD results in carbon being incorporated into the GaN wafer. This carbon may be undesirable for many applications where pure GaN is desired.
An article by Naniwae et al. entitled “Growth of Single Crystal GaN substrate Using Hydride Vapor Phase Epitaxy” in Jnl of Crystal Growth, Vol. 99, 1990, pp. 381-384, discloses growth of GaN films on a sapphire substrate. A pretreatment of gallium and HCl without ammonia for 10-20 minutes at 1030° C. is used to pretreat the sapphire surface prior to metalorganic vapor phase epitaxy (MOVPE) of the GaN film.
An article by Xu et al. entitled “&ggr;-LiAlO
2
single crystal: a novel substrate for GaN epitaxy” in the Journal of Crystal Growth, Vol. 193, 1998, pp. 127-132, discloses LiAlO
2
as a substrate for GaN film growth. The substrates were pretreated with ammonia, and thereafter the GaN film was grown using metalorganic chemical vapor deposition. Another article by Xu et al. entitled “MOCVD Growth of GaN on LiAlO
2
Substrates” in Phys. Stat. Sol. (a) Vol. 176 (1999), pp. 589-593 also discloses an LiAlO
2
substrate, an ammonia pretreatment, and MOCVD to form the GaN layer. Unfortunately, the MOCVD process may not be sufficiently fast to produce thicker films. In addition, the precursor gas for deposition is trimethylgallium which results in carbon being undesirably incorporated into the GaN layer.
An article by Waltereit et al. entitled “Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes” in Letters to Nature, Vol. 406, Aug. 24, 2000, pp. 865-868, discloses the epitaxial growth of a thin layer of M-plane GaN on &ggr;-LiAlO
2
using plasma-assisted molecular beam epitaxy. The exposed surface of the thin GaN layer may be bonded to another substrate and the LiAlO
2
layer then selectively removed to form certain types of higher efficiency devices.
An article also by Waltereit et al. entitled “Growth of M-Plane GaN(1{overscore (i)}00): A Way to Evade Electrical Polarization in Nitrides” in Phys. Stat. Sol. (a) Vol. 180 (2000) pp. 133-138, similarly discloses the formation of an M-plane GaN layer on LiAlO
2
substrate. The thin GaN layer (1.5 &mgr;m sample) is grown using molecular beam epitaxy at a relatively slow growth rate of 0.5 &mgr;m/h. The article reports that M-plane GaN is free of electrical polarization, as compared to more convention C-plane GaN, and that this leads to improved electron-hole wavefunction overlap and therefore improved quantum efficiencies. The M-plane GaN quantum wells have a dramatic imp
Chou Mitch M. C.
Gallagher John Joseph
Maruska Herbert Paul
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Crystal Photonics, Incorporated
Kunemund Robert
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