The Effect of Annealing on High-resistivity and Semi-insulating 4H-SiC
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The Effect of Annealing on High-resistivity and Semi-insulating 4H-SiC S.R. Smitha, A.O. Evwarayeb, and W.C. Mitchel Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLPS, Wright-Patterson Air Force Base, OH 45433-7707 a
University of Dayton Research Institute, 300 College Park, Dayton, OH 45469-0178
b
University of Dayton, Physics Department, 300 College Park, Dayton, OH 45469-2314
ABSTRACT We have examined specimens of high-resistivity, and semi-insulating, 4H-SiC before and after thermal annealing at 1600 ºC, using Optical Admittance Spectroscopy. We have found enhanced ultraviolet response in most specimens. Enhanced activation of previously undetected V impurities has also been observed. Peaks believed to be attributable to complex V-related defects were greatly reduced by annealing. The annealing was in addition to a thermal oxidation at 1150 ºC for 4 hours. The purpose of the oxidation was to remove surface-related deep levels known to be present in polished SiC. Transition metal impurities in these bulk specimens were quantified by SIMS. In specimens where Ti was not detected by SIMS, no further activation of Ti centers was detected by Optical Admittance Spectroscopy.
INTRODUCTION High-power, microwave applications of semiconductors require a stable semiinsulating (si) substrate. This substrate should conduct heat very well while remaining a poor electrical conductor, even at elevated temperatures. Semi-insulating 4H-SiC is such a material. SiC can be made semi-insulating by very closely compensating residual impurities, or, by intentionally adding elements or intrinsic defects that give rise to electrical levels near the middle of the bandgap. These deep levels must be added in sufficient quantities to pin the Fermi level. For our purposes, high-resistivity samples are those whose room temperature resistivity was between 1 x 103 Ω-cm and 1 x 107 Ω-cm. Semi-insulating refers to those specimens with a room temperature resistivity greater than 107 Ω-cm. One of the most popular elements for this purpose is the transition metal vanadium. Vanadium is universally believed to add a donor level near midgap. However, it is difficult to add just V because there are usually impurities present in the graphite parts of the growth chamber comprised of other transition metals (e.g. Ti, Cr, or Zr). If the concentration is small enough, they are of little consequence, but if the concentration becomes large, the impurities may affect the electrical characteristics of the semiconductor as much as the intentional dopant. For the most part these impurities have been assumed to be electrically insignificant. However, Ti has been reported to give rise to an isoelectronic center in SiC, as well as forming Ti-N pairs [1]. Others have reported
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deep acceptor-like levels for Ti near the conduction band [2, 3]. Cr has been identified with a deep level in 6H-SiC [4], and in 4H-SiC [3]. EXPERIMENT Optical Admittance Spectroscopy (OAS) measures the response of a transparent Schottky diod
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