Photoluminescence study of deep-level defects in undoped GaN
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Photoluminescence study of deep-level defects in undoped GaN M. A. Reshchikov and H. Morkoç Virginia Commonwealth University, Richmond, VA 23284, U.S.A. S. S. Park and K. Y. Lee Samsung Advanced Institute of Technology, P.O.Box 111, Suwon, Korea 440-600 ABSTRACT 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 HVPEgrown GaN layers with thickness of ~200 µm 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 ~2.23 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 belowgap excitation, the intensity of the 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. An 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. INTRODUCTION Identification of defects responsible for the omnipresent yellow luminescence (YL) in GaN remains a challenging problem. While it seems to be unambiguously proved that this band is related to transitions from the conduction band or a shallow donor to a deep acceptor [1,2,3], it is still not clear if only one or several different acceptors contribute to the broad luminescence band. The issue of whether the YL is related to a point defect with strong electron-phonon coupling [1,4,5] or to a distribution of states in the gap [6] is also an open question. Regarding its origin, the YL band has been repeatedly attributed to gallium vacancies (VGa) [1,3,7,8]. Theoretical calculations predict low formation energies and deep-level states for the isolated VGa and its complexes with oxygen (VGaON) [7,8]. It has also been demonstrated that the formation of these defects is much more favorable at the threading-edge dislocations [9]. Position of the YL band varies slightly in different works (for example, 2.15 eV in C-doped GaN [1], 2.21 eV [5] and 2.3 eV [10] in undoped GaN at low temperature). The scattering may be due to physical reasons (e.g., existence of different defects with slightly different energy levels [9], different residual strains, different doping levels, different excitation conditions, etc.), as well as due to lack of proper correction of PL spectrum for the response of the optical system. There is little information
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