Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state
Reexamination Certificate
2001-11-30
2004-02-17
Hiteshew, Felisa Carla (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
C117S089000, C117S095000, C117S097000, C117S106000, C117S952000, C117S953000
Reexamination Certificate
active
06692568
ABSTRACT:
TECHNICAL FIELD
The present invention is generally directed to the production of Group III metal nitride materials for use as free-standing articles as well as substrates for further processes and/or microelectronic and optoelectronic devices. In particular, the present invention is directed to the production of low-defect density single-crystal materials grown from a strain-relieving layer of single-crystal columns, utilizing enhanced sputtering techniques.
BACKGROUND
A wide variety of techniques exist for depositing thin films onto substrates in order to achieve desirable properties which are either different from, similar to, or superior to the properties of the substrates themselves. Thin films are employed in many kinds of optical, electrical, magnetic, chemical, mechanical and thermal applications. Optical applications include reflective/anti-reflective coatings, interference filters, memory storage in compact disc form, and waveguides. Electrical applications include insulating, conducting and semiconductor devices, as well as piezoelectric drivers. Magnetic applications include memory discs. Chemical applications include barriers to diffusion or alloying (e.g., galling), protection against oxidation or corrosion, and gas or liquid sensors. Mechanical applications include tribological (wear-resistant) coatings, materials having desirable hardness or adhesion properties, and micromechanics. Thermal applications include barrier layers and heat sinks.
Bulk materials can be used as substrates upon which microelectronic and optical devices are fabricated. Wide bandgap semiconductor materials such as gallium nitride, aluminum nitride, indium nitride and their alloys are being studied for their potential application in microelectronics and opto-electronics. These materials are particularly well suited for short wavelength optical applications, such as green, blue and UV light emitting devices (LEDs and LDs), and visible and solar-blind UV detectors. The use of UV or blue GaN-based LEDs makes possible the fabrication of solid state white light sources, with higher efficiencies and lifetimes 10 to 100 times longer than conventional sources. Additionally, GaN has a region of negative differential mobility with a high peak electron velocity and high-saturated velocity, which can be used for fabricating high-speed switching and microwave components. P-type doping of GaN and AlGaN with relatively high hole concentrations is now readily achieved, and ohmic and Schottky contacts have been characterized for n- and p-type materials. Thus, many of the above devices have or potentially have large, technologically important markets. Such markets include display technology, optical storage technology, and space-based communications and detection systems. Other applications include high temperature microelectronics, opto-electronic devices, piezoelectric and acousto-optic modulators, negative-electron affinity devices and radiation/EMP hard devices for military and space uses.
Attempts to grow low-defect density gallium nitride (GaN) thin films heteroepitaxially on substrates such as sapphire and silicon carbide (SiC) have had limited success. GaN materials heteroepitaxially grown on these substrates suffer from large concentrations of threading defects, typically on the order of 10
−8
-10
−10
cm
−2
, due to the large lattice mismatch between the film and substrate. Threading defects increase leakage currents in diode and FET structures and act as a significant source of noise in photodetectors. As a result, the operation of high performance devices, such as high-speed, high-sensitivity UV photodetectors, and high power, high frequency microelectronic devices, is presently limited. Buffer layers of AlN, GaN, and other materials have been used to reduce the lattice mismatch. However, threading defects and low angle grain boundaries remain in the films. Differences between the film and substrate thermal expansion coefficients also result in stresses in the films.
Accordingly, homoepitaxial growth of GaN thin films on bulk GaN substrates is of great interest. The use of GaN substrates would eliminate the problems due to lattice mismatch and thermal expansion mismatch. Unfortunately, the availability of GaN substrates has been limited due to conventional processing capabilities. This problem has hindered the development of devices based on GaN and related nitride semiconductors. Several obstacles exist to the successful manufacturing and commercializing of high device-quality Group III nitride-based materials, whether in bulk, single-crystal, polycrystalline or epitaxial form, for electronics and other applications. These obstacles generally include cost, reproducibility, and purity.
For instance, gallium nitride has a high equilibrium vapor pressure of nitrogen that results in its decomposition at elevated temperatures. The solubility of nitrogen in gallium metal at room temperature and pressure is very low. As a result, conventional crystal growth methods to produce GaN are not practical. This has led to the development of several alternate bulk growth methods, including high-temperature, high-pressure (15 kbar) solution growth, evaporation, and sublimation.
Currently, aluminum nitride and gallium nitride exist only as polycrystalline or powder forms, or in thin films. Polycrystalline bulk aluminum nitride can be manufactured using powder processing techniques. This process has not yielded semiconductor-grade single crystal material. Formidable problems are associated with such techniques, beginning with the production of pure aluminum nitride powders and then the sintering of oxygen-free and defect-free aluminum nitride. Some of these problems include the production of both high-purity and uniform particle-size powders. The highest purity powders can contain up to 1% of oxygen and binders, such as Y
2
O
3
, that are needed to produce aluminum nitride with a high density. Therefore, high density is achievable at the expense of contamination. Sintering of these aluminum nitride powders is also a difficult process. The covalent nature of aluminum nitride prevents densification of pure aluminum nitride at low temperatures. Aluminum nitride decomposes at high temperatures, such as above 1600° C., thereby preventing densification. Hence, costly sintering aids such as high pressures and impurities are required for producing high-density material. Other problems associated with powder processing of aluminum nitride include maintaining the purity and integrity of the powder, controlling the environment at high sintering temperatures, and the production of defect-free parts. Aluminum nitride is very difficult to manufacture using powder processing techniques without introducing contamination that will have adverse effects on the optical and thermal properties of the material. These impurities can be present in the crystalline lattice structure, and can migrate to the grain boundaries during sintering, causing the infrared absorbance to be high.
Various masking techniques have been explored in conjunction with lateral epitaxial overgrowth (LEO) and selective area growth (SAG) techniques in search of improved methods for fabricating low-defect density gallium nitride crystal layers. For example, U.S. Pat. No. 6,153,010 discloses a method for growing nitride semiconductor crystals. A nitride buffer layer is first grown on a substrate using gaseous Group III element and nitrogen sources (e.g., MOVPE, MBE, or HVPE). Using a vapor-phase technique and photolithography, an oxide selective growth mask is formed on the underlayer. The mask is configured as discrete stripes so that areas of the buffer layer remain exposed. Nitride semiconductor material portions are then grown on these exposed areas using gaseous Group III element and nitrogen sources. When such growth exceeds the upper ends of the mask stripes, the semiconductor material grows laterally on the mask stripes. Continued vertical growth results in the material portions combining to form an integral nitride semiconductor crystal.
Carlson Eric Porter
Cuomo Jerome J.
Hanser Andrew David
Thomas Darin Taze
Williams N. Mark
Hiteshew Felisa Carla
Jenkins & Wilson & Taylor, P.A.
Kyma Technologies, Inc.
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