W. vonAmmon, Yu.Gelfgat, L.Gorbunov, A.Mühlbauer, A.Muiznieks, Y.Makarov, J. Virbulis, G.Müller 1 Siltronic AG, 24 Johannes Hess Str., 84489 Burghausen, Germany (wilfried.ammon@siltronic.com) 2 Institute of Physics, University of Latvia, 32 Miera, LV-2169, Salaspils, Latvia 3 Institute for Electroheat, University of Hanover, 4 Wilhlem-Busch-Str., D-30167 Hanover, Germany 4 Faculty of Physics and Mathematics, University of Latvia, 8 Zellu street, LV-1002 Riga, Latvia 5 Semiconductor Technology Research, Inc., P.O.Box 70604, Richmond, VA 23255-0604, U.S.A. 6 PAIC, Riga, LV-1002, 8 Zellu street, Latvia 7 Department of Material Science (WW6), Crystal Growth Laboratory, University of Erlangen-Nürnberg, 7 Martensstr., D-91058 Erlangen, Germany