Electrical Measurement of the Vanadium Acceptor Level in 4H- and 6H-SiC
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0911-B05-06
Electrical Measurement of the Vanadium Acceptor Level in 4H- and 6H-SiC William C. Mitchel1, William D. Mitchell1, H. E. Smith1,2, M. E. Zvanut3, and Wonwoo Lee3 1 Air Force Research Laboratory, AFRL/MLPS, Wright Patterson AFB, OH, 45433-7707 2 University of Dayton Research Institute, Dayton, OH, 45469 3 Physics Department, University of Alabama at Birmingham, Birmingham, AL, 35294 ABSTRACT Temperature dependent Hall effect, Fourier transform infrared absorption, and electron paramagnetic resonance (EPR) studies have been performed on both 6H and 4H vanadium doped semi-insulating SiC samples grown by the physical vapor transport technique. Nitrogen and boron concentrations have been measured in some samples by secondary ion mass spectrometry (SIMS). Unlike undoped s.i. SiC, where several different thermal ionization energies have been observed, the ionization energies for all of the vanadium doped s.i. samples studied here were found to cluster around only two values for the two polytypes, EC – 0.85 eV and EC – 1.54 eV for 6H and EC – 1.11 eV and EC – 1.57 eV for 4H. SIMS measurements indicate that the nitrogen concentration exceeds the boron concentration in samples with the shallower of two values while the opposite is true for the deeper level samples. EPR detected both V3+ and V4+ in shallower level samples while only V4+ was detected in the deeper level samples. These results indicate that the vanadium acceptor level, V3+/4+, is located at EC – 0.85 eV in 6H-SiC and EC – 1.11 eV in 4H-SiC. However, some EPR results do show a small, unexpected asymmetry in the angular dependence of the V4+ signal, most noticeably in the 4H samples. This suggests that at least some of the vanadium related levels might be complexed with another defect or be under higher local strain than expected.
INTRODUCTION Transition metal doping is a common method for producing the high resistivity in semiinsulating (s.i.) semiconductors and vanadium doping is presently being used to make s.i. SiC. [1,2] The vanadium deep levels compensate shallow level impurities such as residual nitrogen and boron and pin the Fermi level at the deep level. Since vanadium is amphoteric in SiC, the Fermi level can be pinned by either its donor level, V4+/5+, or its acceptor level, V3+/4+, depending on the relative concentration of the shallow donors and acceptors. The vanadium donor level is widely accepted to be located around EC – 1.5 eV in both 6H and 4H SiC. However, there are conflicting reports on the location of the acceptor level. Jenny et al. [3-5] reported EC - 0.66 eV and EC - 0.8 eV for the V3+/4+ level in 6H and 4H material respectively. Dalibor et al. [6] reported 0.63 to 0.74 eV and 0.88 to 0.97 eV for the acceptor energies in 6H and 4H-SiC respectively, while Achtziger et al. [7,8] reported vanadium levels at 0.71 and 0.75 eV for the 6H and 0.97 eV for 4H material. Mitchel et al. [9] reported a level at EC - 1.1 eV in vanadium doped 4H-SiC but attributed it to a vanadium related complex. We report here a study of vanadium doped 6
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