Avinash K. Gupta

SUBLIMATION GROWTH OF SIC SINGLE CRYSTALS

The technological potential of silicon carbide (SiC) single crystals for highpower, high-temperature, and high-frequency electronic devices has been recognized for several decades; however, such applications have been greatly hindered by problems related to bulk crystal growth. SiC bulk crystal growth technology has recently achieved drastic improvement and enabled the growth of large high-quality single crystals. This chapter overviews the recent achievements in SiC bulk crystal growth aimed at producing high-quality largediameter crystals, highlighting the improvement of the crystal diameter enlargement process and the reduction of crystallographic defects in SiC crystals.

Status of Large Diameter SiC Single Crystals

Large-diameter SiC single crystals are grown at II-VI by the sublimation technique. 100mm substrates of semi-insulating 6H SiC and n-type 4H SiC are produced as commercial products; in development, diameter expansion to 125mm has been achieved. Over the last two years, significant improvements have been made in crystal quality. The values of FWHM of x-ray rocking curves are typically 20-40 arc-seconds for 6H SI wafers and 12-30 arc-seconds for 4H n+ wafers. Micropipe density is less than 3 cm-2, and less than 0.1 cm-2 in best substrates. Electrical resistivity of SI substrates is, typically, of 1011 Ω•cm or above. For 4H n+ substrates, the typical dislocation density is about 9×103 cm-2 and the typical BPD density is less than 1×103 cm-2.

Microstructure and Interdiffusion Phenomena in YSZ-CGO Composite Electrolyte

In order to investigate the co-firing feasibility of a double-layer ceria-zirconia electrolyte for low-temperature Solid Oxide Fuel Cells (SOFCs), interdiffusion phenomena between yttria-stabilized zirconia (YSZ) and cerium-gadolinium oxide (CGO) were studied in the temperature range 1200°C to 1500°C with the aid of SEM/EDX-WDS. Significant solid state reaction and interdiffusion phenomena across the YSZ-CGO interface were found to occur already at 1200°C. The extent of the interdiffusion zone formed at the interface as well as the elemental distributions in this zone showed no dependence on sintering temperature and holding time. The difference of the diffusivity of Ce 4+ and Gd 3+ into YSZ and Y 3+ and Zr 4+ into CGO resulted in porosity emerging at the YSZ/CGO interface. The use of a microstructure with a graded YSZ/CGO 0.5 YSZ 0.5 /CGO composition was found to reduce the cation diffusion rates across the interface. However, the low conductivity of the CGO 0.5 YSZ 0.5 solid solution phase used requires further optimization of the composition of the interlayer to be used.

Advanced PVT Growth of 2 & 3-Inch Diameter 6H SiC Crystals

Abstract. The Wide Bandgap Materials Group of II-VI Inc., develops, manufactures and markets n+ and semi-insulating (SI) 6H SiC crystals, including vanadium-compensated and V-free. The PVT growth process is tuned to produce high-quality semi-insulating 6H SiC boules with micropipe densities below 15 cm (for 2-inch wafers) and below 70 cm (for 3-inch wafers). Room temperature resistivity for 2-inch and 3-inch SI V-doped wafers is greater than 10 Ω·cm and 10 Ω·cm, respectively. A novel synthesis process is used for the production of high-purity polycrystalline SiC source, yielding a material in which most impurities are below their GDMS detection limits. An advanced PVT process (APVT) has been developed for the growth of V-free SI 6H SiC crystals. These APVT SiC crystals contain boron below 6.2·10cm, nitrogen below 4.0·10cm and demonstrate semi-insulating behavior with ρ between 10 and 10 Ω·cm. Photoluminescence and EPR of V-free 6H SiC has been studied and EPR data have been assigned to native point defects (C vacancy, Si antisite and CVAC-CSi pair).

Growth and Characterization of Large Diameter 6H and 4H SiC Single Crystals

Over the past year, II-VI has transitioned from 2” to 3” commercial SiC substrates. Large-diameter semi-insulating 6H-SiC and n-type 4H-SiC single crystals are grown using the Advanced PVT growth process. Expansion of boule diameter from 2 to 3 and up to 4.25 inches has been carried out using a specially designed growth technique. Stable semi-insulating properties in 6H-SiC are achieved by precise vanadium compensation. The technique of compensation is optimized to produce a controlled and spatially uniform distribution of vanadium and high and spatially uniform electrical resistivity reaching 10 10 – 1011 ·cm. N-type 3-inch 4H-SiC crystals are grown using doping with nitrogen, and 3-inch 4H-SiC substrates show uniform resistivity of about 0.018 ·cm. The best quality semiinsulating (SI) 3” 6H-SiC substrates demonstrate micropipe density of 3 cm-2, and n-type 3” 4H-SiC substrates - about 1 cm-2. X-ray rocking curve topography of the produced 3” SiC substrates is used for evaluation of their crystal quality.

Growth of Undoped (Vanadium-Free) Semi-Insulating 6H-SiC Single Crystals

II-VI has developed an Advanced PVT (APVT) process for the growth of nominally undoped (vanadium-free) semi-insulating 2” and 3” diameter 6H-SiC crystals with room temperature resistivity up to 1010 W·cm. The process utilizes high-purity SiC source and employs special measures aimed at the reduction of the impurity background. The APVT-grown material demonstrates concentrations of B and N reduced to about 2·1015cm-3. Wafer resistivity has been studied and correlated with Schottky barrier capacitance, yielding the density of deep compensating centers in 6H-SiC in the low 1015 cm-3 range for both ntype and p-type material. The nearly equal density of deep donors and deep acceptors ndicates that the centers responsible for the intrinsic compensation can be amphoteric. TheEPR density of spins from free carbon vacancies is about 1014 cm-3. It is also hypothesized that impurity-vacancy complexes can be present in the undoped material and participate in compensation.

Growth of Large Diameter 6H SI and 4H n+ SiC Single Crystals

SiC single crystals are grown at II-VI by the seeded sublimation technique. The process has been scaled up and optimized to support commercial production of high-quality 100 mm diameter, Semi-Insulating (SI) 6H substrates and 100 mm 4H n+ substrates. The growth process incorporates special elements aimed at achieving uniform sublimation of the source, steady growth rate, uniform doping and reduced presence of background impurities. Semi-insulating 6H substrates are produced using precise vanadium compensation. Vanadium doping is optimized to yield SI material with very high resistivity and low capacitance. Crystal quality of the substrates is evaluated using a wide variety of techniques. Specific defects, their interaction and evolution during growth are described with emphasis on micropipes and dislocations. The current quality of the 6H SI and 4H n+ crystals grown at II-VI is summarized.

Characterization of SiC Substrates Using X-Ray Rocking Curve Mapping

SiC substrates produced at II-VI, Inc. have been characterized using x-ray rocking curve mapping (topography). The rocking curves have been measured in the -scan mode for the (0006) Bragg reflection of 6H and the (0004) reflection of 4H SiC substrates. The maps contain information extracted from the rocking curves, such as the peak angle () and the rocking curve broadening (FWHM). In the case when lattice distortion is present due to the elastic or plastic deformation, the peak angle () changes gradually upon scanning, with the d/dx gradient proportional to the lattice curvature in the plane of diffraction. Multi-peak reflections and/or sharp change in the value of indicate the presence of misoriented grains. X-ray rocking curve mapping of SiC substrates yields excellent measures of crystalline quality that contain important information on the lattice strain and sub-grain misorientation.

Growth of Large Diameter SiC Crystals by Advanced Physical Vapor Transport

Semi-insulating 6H SiC substrates, 2”, 3” and 100mm in diameter, and n+ 4H SiC substrates, 2” and 3” in diameter, are grown at II-VI using the Advanced Physical Vapor Transport (APVT) technique [1]. The process utilizes high-purity SiC source and employs special measures aimed at the reduction of background contamination. Semi-insulating properties are achieved by precise vanadium compensation, which yields substrates with stable and uniform electrical resistivity reaching ~ 1011 Ω-cm and higher. Conductive n+ 4H SiC crystals with the spatially uniform resistivity of 0.02 W-cm are grown using nitrogen doping. Crystal quality of the substrates, their electrical properties and low temperature photoluminescence are discussed.

Status of Large Diameter SiC Single Crystals at II-VI

II-VI is developing large-diameter SiC crystals to be used as lattice-matched, high thermal conductivity substrates for new generation GaN-based and SiC-based semiconductor devices. Large-diameter 6H SiC single crystals are grown at II-VI using our Advanced PVT sublimation growth process. Stable SI properties are achieved by compensation with vanadium, which results in high and spatially uniform resistivity, on the order of 1011 Ohm-cm. The quality of the presently grown 100 mm 6H SI substrates has been dramatically improved [1], and they are free of edge defects. Micropipe density in the 100 mm 6H SI substrates ranges from 2 to 8 cm-2 and dislocation density from 3·104 to 6·104 cm-2. X-ray rocking curves measured on as-sawn 100 mm 6H wafers showed edge-to-edge lattice curvature () between 0.1° and 0.3° and FWHM of the rocking curve between 50 and 100 arc-seconds