Optical assessment of purity improvement effects in bulk 6H and 4H-SiC wafers grown by physical vapor transport

We report on purity improvement effects which recently have been observed when comparing different series of wafers produced in two separate, but basically similar, home-made physical vapor transport (PVT) reactors. Looking in detail for the origin of this phenomenon, we have found a strong influence of the residual purity of the graphite material used to manufacture the crucibles. After proper optimization, a second effect has been found. It manifests when the residual level of impurities in the seed material is high and provides evidence for in situ auto-doping. Finally, quantitative analyses of C(V) characteristics and Raman spectra have been done. In this way we follow the trend in residual carrier concentration and mobility.