“…[1][2][3][4][5] In addition, when varying the Al content, the bandgap of AlGaN varies from 3.4 eV for GaN to 6.2 eV for AlN, covering a broad spectral range. [5][6][7][8] This makes AlGaN and their based heterostructures very promising for the realization of a number of electronic and optoelectronic devices, such as high electron mobility transistors (HEMT), [9][10][11] metal-oxide semiconductor (MOS) HEMT, 12 MOS heterostructures field-effect transistor, 13,14 ultra-violate (UV) light-emitting diodes (LEDs), [15][16][17][18] and solar-blind UV photodetectors. [19][20][21] Yet, the performance of AlGaN-based devices has been limited by some challenging issues connected to the AlGaN layer quality, 4,8,22 e.g., the relatively low internal quantum efficiency, the poor p-type doping efficiency, and hardly achievable n-and p-type good ohmic contacts.…”