Papers by Author: Murugesu Yoganathan

Abstract: 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.

Abstract: X-ray rocking curve characterization is a relatively fast and nondestructive technique that can be utilized to evaluate the crystal quality of SiC substrates. The contribution of lattice curvature to rocking curve broadening is estimated, and shown to be the major contribution to the measured broadening (FWHM). The feedback on lattice quality is used to optimize our SiC growth process. In the optimized growth runs, the typical variation in rocking curve sample angle Ω across the entire 3” diameter wafer is about 0.2 degrees. Possible mechanisms leading to changes in the lattice curvature are discussed.

Abstract: Etching of 4H-SiC wafers in molten KOH as a method for micropipe and dislocation density analysis was investigated. The obtained results were correlated with those of the synchrotron white beam x-ray topography. Heavily nitrogen-doped SiC shows a significantly different etching behavior in comparison with the low-doped material. This complicates identification of different types of threading defects. In particular, it is difficult to separate Threading Screw Dislocations (TSD) from Threading Edge Dislocations (TED). Depending on the level of doping and thermal history of the crystal, some of the etch pits emerging due to the 1c screw dislocations can be as large as those due to the micropipes.

Abstract: 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

Abstract: Several morphological defects in 4H SiC epitaxial wafers, including Comets and Triangles, may significantly impact on the yield and reliability of SiC devices. The formation of these epilayer defects is closely related to the substrate quality. This paper focuses on the study of the substrate quality and its relationship with defects in the epilayers. The crystalline quality of 4H n+ substrates has been characterized by x-ray diffraction, and the distribution of dislocations has been determined using etching in molten KOH. The relationship between Comet and Triangle epilayer defects and the dislocations has been established. A 10-fold reduction in the overall dislocation density in the 4H SiC substrates was achieved through technological improvements. The improvement was validated by the reduction in the number of the epilayer defects.

Abstract: 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.

Abstract: A room temperature PL mapping technique was applied to establish the origin of resistivity variation in PVT-grown 6H SiC substrates. A direct correlation between the native defect-related PL and resistivity was found in undoped (V-free) samples. In vanadium-doped samples with low vanadium content, the resistivity showed a good correlation with the total PL signal consisting of contributions from both vanadium and native point defects. Well-known UD1 and UD3 levels were revealed by low-temperature PL spectroscopy. Some correlation was observed between these low-temperature PL signatures and the resistivity distribution.

Abstract: 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.

Abstract: 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.