1. Institute of Technical Physics, Faculty of Materials Science and Applied Chemistry, Riga Technical University, 3/7 Paula Valdena Str., LV-1048 Riga, Latvia
2. Institute of Applied Research, Vilnius University, Sauletekio Ave. 3, Vilnius 10222, Lithuania
3. Department of Semiconductor Physics, Vilnius University, Sauletekio Ave. 3, Vilnius 10222, Lithuania
4. Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., 1063 Riga, Latvia
5. Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu 432-8011, Japan