1. Institut für Physik, Montanuniversität Leoben, Franz-Josef-Straße 18, Leoben A-8700, Austria
2. V. Lashkaryov Institute of Semiconductor Physics of NAS of Ukraine, pr. Nauki 41, Kyiv 03028, Ukraine
3. Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, Kosice 04001, Slovakia