Insulator/GaN heterostructures of low interfacial density of states

A review is given on insulators (oxides and nitrides) which have been deposited on GaN to form metal-insulator (oxides and nitrides)-semiconductor (MOS or MIS) diodes with a low interfacial density of states (D_it). These insulators include AlN, SiO₂, Si₃N₄, SiO₂/Ga₂O₃, and Ga₂O₃(Gd₂O₃). Techniques for depositing these insulators and methods for cleaning GaN surfaces prior to the insulator deposition are discussed. Recent progress on GaN MOSFET's (with SiO₂/Ga₂O₃, and Ga₂O₃(Gd₂O₃) as gate dielectrics) and MISFET's (with AlN as a gate dielectric) is also reviewed. When exposed to room air, GaN surface is not as robust as previously thought. Therefore, preparation of a clean GaN surface for deposition of oxides and nitrides is necessary to achieve a low D_it. By heating GaN samples in UHV to clean the surfaces followed by deposition of Ga₂O₃(Gd₂O₃) and SiO₂, we have achieved a low D_it with negligible hysteretic loops in the capacitance-voltage curves.