Photoluminescence Excitation Spectroscopy of Carbon-Doped Gallium Nitride
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Downloaded from https://www.cambridge.org/core. IP address: 212.119.44.49, on 08 Sep 2020 at 11:18:24, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300002738
specific associations of these levels with proposed models for the YL have not been definitively proven. It is important to note that the sapphire substrates often used for growing GaN usually contain Cr3+ impurities which contribute to a broad PL emission from the substrate in the same spectral region as the YL from GaN. Sapphire containing Cr3+ also exhibits a sharp line emission at 1.79 eV when pumped in the broad Cr3+ absorption bands centered at 2.2 eV and 3.0 eV [15]. When luminescence is excited by photons with energy below the bandgap of GaN from samples consisting of thin (several micron thick) GaN epilayers grown on thick sapphire substrates containing Cr3+, the luminescence observed can be a mixture of emission from the substrate and the GaN and in some cases may be primarily from the substrate. Ideally PL and photoluminescence excitation (PLE) spectroscopy excited with photon energies below the GaN bandgap would be performed on free-standing GaN epilayers removed from their substrates. However, the substrate removal is difficult and PL and PLE measurements are routinely performed on thin films still mounted on their substrates[3,17]. Some workers have pointed out that even when PL is pumped with photon energies above the GaN bandgap, it is possible for intense GaN PL in the blue spectral range to excite Cr3+ luminescence from the sapphire substrate [15]. Although the excitation of substrate emission is even more probable for direct optical pumping below the GaN bandgap, several PLE investigations of GaN in the below-gap spectral range have been reported in which 3-5 micron thick epitaxial layers of GaN on sapphire substrates were employed [3,17]. These publications make no mention of precautions to avoid distortion of the PLE spectra by substrate emission which complicates the interpretation of their reported PLE lineshapes and low energy thresholds. The present work demonstrates the effectiveness of PLE spectroscopy in the investigation of optical absorption and emission below the bandgap of GaN. PLE spectroscopy involves detecting luminescence intensity at a selected wavelength within a luminescence band as a function of the wavelength of the exciting light, thus isolating one luminescence band and providing information about the absorption energies required to excite that luminescence band. Above-gap excitation of GaN typically produces broad, featureless, mid-gap PL spectra comprising multiple, overlapping YL and RL bands from deep defect or impurity levels [2, 9, 11]. PLE spectroscopy in the below-gap spectral range can detect extrinsic absorption bands which selectively and separately excite these overlapping PL bands. In the work presented here, the selective excitation isolates a broad RL band associated with C-doping, enabling its PL and PLE spectra to be o
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