1. Department of Crystal Physics and Technology, Scientific Research Company ``Electron-Carat'', 202 Stryjska Str., 79031 Lviv, Ukraine
2. Ya.S. Pidstryhach Institute for Applied Problems of Mechanics and Mathematics NASU, 3b Naukova Str., 79031 Lviv, Ukraine
3. Department of Semiconductor Electronics, Lviv Polytechnic National University, 12 Bandery Str., 79013 Lviv, Ukraine
4. Department of Electrical Engineering, Indian Institute of Technology-Madras, 600 036 Chennai, India