A study of V 3+ and the Vanadium acceptor level in semi-insulating 6H-SiC
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E1.3.1
A study of V3+ and the vanadium acceptor level in semi-insulating 6H-SiC Wonwoo Lee and Mary E. Zvanut University of Alabama at Birmingham 1300 University Blvd. Birmingham AL 35292, U.S.A. ABSTRACT Infrared absorption (IR) and electron paramagnetic resonance (EPR) spectroscopies are used to study the V3+ impurity and the vanadium acceptor level in 6H semi-insulating SiC. IR and EPR data obtained from samples cut from the same wafer support the assignment of the 0.60 and 0.62 eV IR absorption lines to substitutional V3+. Photo-induced EPR measurements reveal identical photo-thresholds for V3+ and V4+ ions. A peak at 0.8 eV, where the intensity of the three plus charge state decreases and the four plus charge state increases by an equal amount, is thought to represent excitation of an electron from V3+ to the conduction band edge. The 0.8 eV peak is therefore attributed to the V3+/4+ level. The difference between the optically measured value reported here and that measured previously using temperature dependent techniques is attributed thermal relaxation.
INTRODUCTION Semi-insulating SiC has unique physical and electrical properties that make it one of the materials of choice for the fabrication of high power, high temperature, and high frequency devices. Vanadium doping in SiC produces both deep donor and acceptor levels, which act as compensating sites for residual shallow impurities. The defect levels of vanadium doped 6H-SiC are shown in Figure 1. The compensation leads to resistivities as high as 1010 ohm-cm at room temperature and produces semi-insulating (SI) material.
Ec
EA
ED Ev
Figure 1. The defect levels of vanadium doped 6H-SiC. Energy bandgap of 6H-SiC is approximately 3.02 eV (Ec – Ev). Ec and Ev represent the conduction band edge and valence band edge, respectively. ED and EA indicate the vanadium donor level (V4+/5+) and acceptor level (V3+/4+), respectively.
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The charge states of vanadium in SiC are detected by several different techniques including infrared absorption (IR) and electron paramagnetic resonance (EPR) spectroscopies [1-6]. In past years, Lauer and coworkers attributed absorption peaks of vanadium-doped 6H-SiC at 0.893 eV and 0.917 eV to the two cubic sites of V4+ and one at 0.948 eV to the hexagonal site of V4+. In addition, the IR line detected at 0.62 eV was suggested to be an inter-shell transition of the V3+ charge state [3]. The EPR signatures of V4+ and V3+ in 6H-SiC were reported by Maier and coworkers [4]. Armed with these results, Kunzer and coworkers employed magnetic circular dichroism-detected EPR to investigate the absorption band at 0.62 eV. The measurements showed that the defect responsible for the band possesses a nuclear spin of 7/2 and ground state total electron spin of 1, consistent with the earlier assignment as substitutional vanadium. However, Jenny and coworkers suggest that polarization studies contradict Kunzer’s conclusions and attribute the 0.62 eV absorption band to a vanadium complex [1, 2]. The vanadium defect levels are critical to c
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