1. Department of Physical Chemistry, Institute of Chemical Technology, Prague, Technická 5, CZ-166 28 Prague 6, Czech Republic, Department of Semiconductors, Institute of Physics, Academy of Sciences of the Czech Republic, v. v. i., Cukrovarnická 10, 162 00 Prague 6, Czech Republic, and SAFC Hitech, Power Road, Bromborough, Wirral, Merseyside CH62 3QF, United Kingdom