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 w hen comparing different series of wafers produced in two separate. but basi cally similar, home-made physical vapor transport (PVT) reactors. Looking i n detail for the origin of this phenomenon, we have found a strong influenc e of the residual purity of the graphite material used to manufacture the c rucibles. After proper optimization, a second effect has been found. It man ifests when the residual level of impurities in the seed material is high a nd provides evidence for in situ auto-doping. Finally, quantitative analyse s of C(V) characteristics and Raman spectra have been done. In this way we follow the trend in residual carrier concentration and mobility. (C) 1999 E lsevier Science S.A. All rights reserved.