1. Institute of High Pressure Physics, Polish Academy of Sciences, ul. Sokołowska 29/37, 01-142 Warsaw, Poland
2. Top-GaN Ltd., ul. Sokołowska 29/37, 01-142 Warsaw, Poland
3. University of Montpellier and CNRS, UMR 5221, Laboratoire Charles Coulomb (L2C), Pl. E. Bataillon, 34095 Montpellier, France
4. Ioffe Institute, Russian Academy of Sciences, Politekhnicheskaya ul., 26, 194021 St. Petersburg, Russia
5. National Research University of Information Technologies, Mechanics, and Optics, St. Petersburg 197101, Russia