Polarity control of GaN films grown by metal organic chemical vapor deposition on (0001) sapphire substrates

The polarity control of GaN films grown on c-plane sapphire substrates by low pressure metal organic chemical vapor deposition (MOCVD) was achieved by using NZ as a diluent and transport gas. The type of polarity was governed by the substrate treatment prior to the GaN growth. N-face (-c) GaN films were only obtained by pre-nitridation of the sapphire substrate after a H-2 anneal, while Ga-face (+c) GaN films were grown directly on the substrates or on properly annealed AIN buffer layers. In addition, GaN films on improperly annealed AIN buffer layers, that is, under- or over-annealed buffer layers, yielded films with mixed polarity. Smooth N-face GaN films with 2.5 nm RMS roughness, as determined by atomic force microscopy (AFM), were obtained with shorter nitridation times (less than 2 min). Wet chemical etching in an aqueous solution of potassium hydroxide (KOH) was used to determine the polarity type.