We report on the emission properties of nonpolar a-plane GaN layers grown on r-plane sapphire. Temperature-, excitation-density-, and polarization-dependent photoluminescences and spatially resolved microphotoluminescence and cathodoluminescence are employed in order to clarify the nature of the different emission bands in the 3.0–3.5eV spectral range. In the near band-edge region the emission lines of the donor-bound excitons (3.472eV) and free excitons (3.478eV) are resolved in the polarized low-temperature spectra, indicating a good quality of the layers. At low energies two other emissions bands with intensity and shape varying with the excited area are observed. The 3.42eV emission commonly attributed to the excitons bound to basal plane stacking faults shows thermal quenching with two activation energies (7 and 30meV) and an S-shaped temperature dependence of the peak position. This behavior is analyzed in terms of hole localization in the vicinity of the stacking faults. The emission band that peaked at 3.29eV is found to blueshift and saturate with increasing excitation intensity. The spatially resolved cathodoluminesence measurements show that the emission is asymmetrically distributed around the triangular-shaped pits occurring at the surface. The 3.29eV emission is suggested to involve impurities, which decorate the partial dislocation terminating the basal stacking faults.

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The optical signatures of Mg-related acceptors in GaN have been revisited in samples specifically grown on bulk GaN templates, to avoid strain broadening of the optical spectra. Bound-exciton spectra can be studied in these samples for Mg concentrations up to [Mg] approximately 2 x 10(19) cm(-3). Contrary to previous work it is found that instabilities in the photoluminescence spectra are not due to unstable shallow donors, but to unstable Mg-related acceptors. Our data show that there are two Mg-related acceptors simultaneously present: the regular (stable) substitutional Mg acceptor, and a complex acceptor which is unstable in p-GaN.

Nanoscience by an authorized administrator of DigitalCommons@University of Nebraska -Lincoln. Darakchieva, V.; Paskova, T.; Schubert, Mathias; Arwin, H.; Paskov, P.P.; Monemar, B.; Hommel, D.; Heuken, M.; Off, J.; Scholz, F.; Haskell, B.A.; Fini, P.T.; Speck, J.S.; and Nakamura, S., "Anisotropic strain and phonon deformation potentials in GaN" (2007

Thick GaN bars with ͓112 គ 0͔ orientation have been sliced from GaN boules grown on freestanding films by hydride vapor phase epitaxy ͑HVPE͒ in the ͓0001͔ direction. High-resolution x-ray diffraction and transmission electron microscopy have been used to study the structural quality and defect distribution in the material in comparison to heteroepitaxially grown thick HVPE-GaN films grown in the ͓112 គ 0͔ direction on ͑11 គ 02͒-plane sapphire. It is demonstrated that while the heteroepitaxial material possesses a high density of stacking faults and partial dislocations, leading to anisotropic structural characteristics, the ͑112 គ 0͒-plane bulk GaN, sliced from boules, exhibits low dislocation density and narrow rocking curves with isotropic in-plane character.