1. Institute of High Pressure Physics, Polish Academy of Sciences, ul. Sokołowska 29/37, 01-142 Warsaw, Poland
2. TopGaN Ltd., ul. Sokołowska 29/37, 01-142 Warsaw, Poland
3. Laboratoire Charles Coulomb (L2C), UMR 5221 CNRS, Montpellier University, Pl. E. Bataillon, 34095 Montpellier, France
4. Department of Electrical Engineering, University of South Carolina, Columbia, South Carolina 29208, USA
5. National Research University of Information Technologies, Mechanics, and Optics, St. Petersburg 197101, Russia