On the preparation of semi-insulating SiC bulk crystals by the PVT technique

Vanadium in SiC can act as a deep acceptor compensating residual nitrogen (DeltaE approximate to 0.8 eV) or as a deep donor compensating p-type (Al/B) impurities (DeltaE approximate to 1.6 eV) leading to semi-insulating behavior. In any case, doping homogeneity is crucial. Additionally, the V solubility limit in SiC must not be exceeded. To determine impurity incorporation, growth of nominally undoped crystals was performed. Here, nitrogen is the residual impurity and the charge carrier concentration n is decreasing exponentially with growth time. Wafers with remaining n = 8 x 10(15) cm(-3) were obtained. During B-doped growth, the hole concentration increases with growth time. The influence of the compensation by nitrogen and the loss of boron content in the source is discussed. While effective boron segregation on the facet during growth is determined to be close to unity, this value is not reached due to evaporation of the B source. V incorporation is related to the partial pressure of the V species in the crucible. Several regions were found in V-doped crystals. V exceeding the solubility limit leads to the formation of V-rich precipitates, V source depletion is indicated by a change to n-type conducting behavior because of residual nitrogen. With temperature-dependent Hall effect measurements, the specific resistivity at room temperature is determined to rho (293K) = 2-5 x 10(11) Omega cm, while resistivity mappings show dopant inhomogeneities. By reducing the V species evaporation rate, bulk SiC crystals exhibiting precipitate-free, semi-insulating behavior were obtained. (C) 2001 Elsevier Science B.V. All rights reserved.