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Deep level defects in nonpolar m-plane GaN grown by ammonia-based molecular beam epitaxy were characterized using deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) and compared with polar c-plane GaN that was grown simultaneously in the same growth run. Significant differences in both the levels present and their concentrations were observed upon comparison of both growth orientations. DLTS revealed electron traps with activation energies of 0.14 eV, 0.20 eV, and 0.66 eV in the m-plane material, with concentrations that were ∼10-50 × higher than traps of similar activation energies in the c-plane material. Likewise, DLOS measurements showed ∼20 × higher concentrations of both a CN acceptor-like state at EC - 3.26 eV, which correlates with a high background carbon concentration observed by secondary ion mass spectroscopy for the m-plane material [A. Armstrong, A. R. Arehart, B. Moran, S. P. DenBaars, U. K. Mishra, J. S. Speck, and S. A. Ringel, Appl. Phys. Lett. 84, 374 (2004)], and the VGa-related state level at EC - 2.49 eV, which is consistent with an enhanced yellow luminescence observed by photoluminescence. The findings suggest a strong impact of growth dynamics on the incorporation of impurities and electrically active native point defects as a function of GaN growth plane polarity. [ABSTRACT FROM AUTHOR]

High external quantum efficiency (EQE) p-i-n heterojunction solar cells grown by NH3-based molecular beam epitaxy are presented. EQE values including optical losses are greater than 50% with fill-factors over 72% when illuminated with a 1 sun AM0 spectrum. Optical absorption measurements in conjunction with EQE measurements indicate an internal quantum efficiency greater than 90% for the InGaN absorbing layer. By adjusting the thickness of the top p-type GaN window contact layer, it is shown that the short-wavelength (<365 nm) quantum efficiency is limited by the minority carrier diffusion length in highly Mg-doped p-GaN. [ABSTRACT FROM AUTHOR]

Deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) were utilized to investigate the behavior of deep states in m-plane, n-type GaN grown by ammonia-based molecular beam epitaxy (NH3-MBE) as a function of systematically varied V/III growth flux ratios. Levels were detected at EC - 0.14 eV, EC - 0.21 eV, EC - 0.26 eV, EC - 0.62 eV, EC - 0.67 eV, EC - 2.65 eV, and EC - 3.31 eV, with the concentrations of several traps exhibiting systematic dependencies on V/III ratio. The DLTS spectra are dominated by traps at EC - 0.14 eV and EC - 0.67 eV, whose concentrations decreased monotonically with increasing V/III ratio and decreasing oxygen impurity concentration, and by a trap at EC - 0.21 eV that revealed no dependence of its concentration on growth conditions, suggestive of different physical origins. Higher concentrations of deeper trap states detected by DLOS with activation energies of EC - 2.65 eV and EC - 3.31 eV in each sample did not display measureable sensitivity to the intentionally varied V/III ratio, necessitating further study on reducing these deep traps through growth optimization for maximizing material quality of NH3-MBE grown m-plane GaN. [ABSTRACT FROM AUTHOR]