Polarity Control in GaN Epilayers Grown by Metalorganic Chemical Vapor Deposition

Abstract

Polarity control of gallium nitride (GaN) on c-plane sapphire substrate was studied via low pressure Metalorganic Chemical Vapor Deposition (MOCVD). Under mass-transport-limited growth regime with a given process supersaturation, the polarities of GaN thin films (i.e. gallium (Ga) and nitrogen (N)-polarities) depended on specific treatments of the sapphire substrate prior to GaN deposition, in addition, identical growth rates for both polar films were obtained. This ability made the fabrication of lateral polar junction (LPJ) GaN structures possible. New designs of novel device structures utilizing the resulting polarity control scheme were developed.

N-polar films were consistently obtained after exposing a H2-annealed sapphire substrate to an ammonia atmosphere at temperature above 950°C. Ga-polar films were obtained either by preventing any exposure of the substrate to ammonia prior to deposition or by depositing the film on a properly annealed low temperature aluminum nitride nucleation layer (LT-AlN NL) deposited on a previously ammonia annealed sapphire substrate. As-grown Ga-polar films were generally insulating and smooth surface morphology while N-polar films exhibited n-type conductivity with carrier concentration approaching 1x1019 cm-3 and a rougher surface morphology.

Following the established polarity control scheme for GaN films, LPJ structures consisting Ga-polar and N-polar domains side-by-side on a single sapphire wafer were achieved by utilizing a prior patterned AlN⁄bare sapphire template. The two regions were separated by an inversion domain boundary (IDB), which did not hinder the current flow across it, i.e. no energy barrier for the charge carriers. This in principle showed the possibility for the fabrication of lateral junctions and lateral based devices within the GaN technology exploiting polar doping selectivity. Understanding the doping selectivity of the two different polar domains allowed us to fabricate a lateral p⁄n junction in GaN by the simultaneous growth of the p- and n-type regions. Identifying the basic characteristics of a p⁄n junction demonstrated that the fabricated structure was a functional p/n diode. For GaN based junctions, these characteristics were: current rectification, electroluminescence and the photovoltaic effect under UV excitation.