AlGaN AlGaN/GaN AlGaN/GaN heterostructures AlN GaN GaN HEMT HVPE Kelvin probe force microscopy LEDs LTE MISHEMT MOVPE ammonothermal method conductance-frequency crystal growth diffusion diffusion coefficients edge effects effective diffusion length gallium nitride gallium nitride nanowires growth polarity interface state density ion implantation laser diode microwave power amplifier molecular beam epitaxy n/a nanowires nitrides