The authors are grateful to Professor T. Mat-sumoto of the Faculty of Engineering;
Yamanashi University for his advice on this study and also the support of photoluminescence measurement;
to Professor S. Tanaka of the Electrical and Electronic Engineering Department of Tottori University for his advice on this paper;
and to Mr. Taniguchi of Fuji Electric Corporate Research and Development;
Ltd;
for his SEM measurements;