Photoluminescence study of deep-level defects in undoped GaN

We studied photoluminescence (PL) and PL excitation (PLE) spectra in a large number of undoped GaN layers grown on sapphire by molecular beam epitaxy (MBE). metal-organic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy, (HVPE). The HVPE-grown GaN layers with thickness of similar to200 mum were separated from the sapphire substrate by laser lift-off and represented bulk freestanding templates of very high quality. Identical position and shape of the YL band were reproduced in many samples grown by MBE and MOCVD: maximum at similar to2.23 eV and full width at half maximum (FWHM) of about 460 meV at room temperature. However. in some samples the band maximum was observed at about 2.0 eV. The freestanding templates reveal a broad band (FWHM=530-680 meV) whose position depends on excitation energy and intensity. varying from 2.22 eV to 2.47 eV, PLE spectra taken from various samples represented a broad band with apparent maximum at about 3.3 eV. For below-gap excitation, tile intensity of tile YL band was independent of temperature except for the one in the freestanding template. The latter was temperature independent above 60 K, however at lower temperatures the PL intensity decreased by 5 times. Al activation energy of 15 meV has been determined that is related to a barrier in the adiabatic potential in the excited state of the defect.