1. Stepanov Institute of Physics, National Academy of Sciences, F. Skaryna Ave. 68, 220072 Minsk, Belarus
2. Minsk Research Institute of Radiomaterials, Minsk, Belarus
3. AIXTRON AG, Kackertstr. 15–17, 52072 Aachen, Germany
4. Institut für Theoretische Elektrotechnik, RWTH Aachen, Germany