It is well known that gallium nitride (GaN) decomposes at temperatures above 800 C without either a large nitrogen overpressure or thermally stable cap to prevent nitrogen loss. This study looks at electrical and surface topography changes in 10.5-m hydride vapor phase epitaxy (HVPE) GaN on sapphire samples when annealed at four different temperatures and times. HVPE GaN samples were capped with a double layer aluminum nitride (AlN) annealing cap, deposited first by an 80-nm low-temperature metal-organic chemical vapor deposition film and followed by a sputtered 1-m AlN layer. After cap removal, atomic force microscopy (AFM) topographs were used to characterize surface changes. AFM topographs at 1150 and 1200 C showed step growth structures were still visible with slight increases in surface roughness due to polygonization with increased annealing time. Anneals at 1250 C for 4 and 8 min and all 1300 C show increased roughness with time. An as received (AR) GaN control sample and the annealed GaN were then processed into Schottky diodes. Current-voltage andcapacitance-voltage measurements were taken of the diodes. Current density-voltage analysis of AR diodes showed reverse leakage decreases, knee voltage (VK) increases, and conductance (G) decreases dependent on increasing distance from the edge of the sample. b, reverse leakage, VK, and G changes were interpreted as dependent on compressive stress induced piezoelectric effects. Reverse leakage increased with anneal time and temperature. Increased tensile stress and concave bowing with temperature from differential coefficient of thermal expansion between the GaN film and sapphire substrate is believed to contribute to increasing Nd near the surface progressing from edge to center. Increases in Nd is believed to be creation of nitrogen vacancies that act as donors.
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