Nitridation of GaN surface for power device application: A first-principles study

The nitridation effects on GaN surface are dissected by first-principles calculations and manifested by photoemission (XPS/UPS) measurements. Two surface bands (upper- and lower-band) are found within the bandgap for several surface configurations. With sufficient nitridation, the energy levels of both bands are shifted towards the valence band, leading to a merge of the lower band with the valence band. The modification to the energy levels of the surface bands is experimentally verified by XPS/UPS analysis performed on GaN surface treated by low-energy N₂ plasma. The surface state modification explains the significantly improved interface quality in GaN MIS- and MOS-structures featuring nitridation interfacial layer, and also supports a physical model that explains the GaN band-edge emission in forward biased metal-AlGaN/GaN Schottky heterojunction.