G. Springholz, M. Pinczolits, G. Bauer, Institut für Halbleiterphysik, Johannes Kepler Universität Linz, 4040 Linz, Austria. V. Holy, Laboratory of Thin Films and Nanostructures, Faculty of Science, Masaryk University, Kotlarska 2, 611 37 Brno, Czech Republic. (At 1998-10-23)
