Temperature-dependent Hall-effect, optical admittance spectroscopic, and optical absorption studies were made of semi-insulating bulk 4H-type material. Both intentionally V-doped material and commercial-grade semi-insulating samples were used. Carrier concentration versus inverse temperature results, from Hall-effect measurements which were performed at temperatures of up to 1000K, indicated that the samples were dominated by one of 2 deep levels near to the mid-gap. In addition to the deep donor level of substitutional V, Ec - 1.6eV, another level was observed at Ec - 1.1eV in some samples. This indicated that levels other than the V donor could pin the Fermi level in semi-insulating material. Optical admittance measurements of the semi-insulating material indicated the presence of levels at Ec - 1.73 and 1.18eV that had previously been observed in conducting samples by using this technique. These levels were attributed to the same defects which produced the 1.1 and 1.6eV levels that were detected by using the Hall effect. Although V was present in all of the samples, together with other impurities, it was not possible to identify the 1.1eV level conclusively.

Fermi Level Control and Deep Levels in Semi-Insulating 4H-SiC. W.C.Mitchel, R.Perrin, J.Goldstein, A.Saxler, M.Roth, S.R.Smith, J.S.Solomon, A.O.Evwaraye: Journal of Applied Physics, 1999, 86[9], 5040-4