Fermi level control and deep levels in semi-insulating 4H-SiC

Temperature dependent Hall effect, optical admittance spectroscopy, and optical absorption measurements of semi-insulating bulk 4H-SiC are reported. Both intentionally vanadium doped material and commercial grade semi-insulating material were investigated. The carrier concentration versus inverse temperature results from Hall effect measurements up to 1000 K indicated the samples were dominated by one of two deep levels near midgap. In addition to the deep donor level of substitutional vanadium, E-c-1.6 eV, we observed another level at E-c-1.1 eV in some samples, indicating that levels other than the vanadium donor can pin the Fermi level in semi-insulating SiC. Optical admittance measurements on the semi-insulating material indicate the presence of levels at E-c-1.73 and 1.18 eV that were previously observed in conducting samples with this technique and we attribute these levels to the same defects producing the 1.1 and 1.6 eV levels seen by Hall effect. Secondary ion mass spectroscopy measurements of dopant and impurity concentrations are reported. Even though vanadium is present in all of these samples, along with other impurities we are at present unable to definitively identify the 1.1 eV level. (C) 1999 American Institute of Physics. [S0021- 8979(99)03621-X].