Photoluminescence Excitation Study of Lo-Phonon Assisted Excitonic Transitions in GaN
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performed using a He-Cd laser (325 nm) as an excitation source and a 0.85-meter double-grating monochromator, in conjunction with a photon counting apparatus as a detection system. For PLE measurements, a wavelength-tunable quasimonochromatic light source consisting of a 0.25-meter monochromator and a xenon arc lamp was used for excitation. RESULTS AND DISCUSSIONS
The luminescence spectra observed fromthe t GaN samples are dominated by strong and sharp
GNspphi
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GaN/sapphire P
Fx' 8
FX 10K K
220K
0
30K
C_
50K
near band-edge emission lines resulting from the radiative recombination of bound-excitons and free70K excitons. Figure 1 shows a set of near band-edge PL spectra from a 2.5-jim GaN grown on a sapphire at 90K various temperatures. on the temperature depedene thir oBased misson ntesitesthe 3.44 3.45 3.46 3.47 3.48 3.49 3.50 3.51 3.52 dependence of their emission intensities, the Photon Energy (eV) strongest peak can be identified as emission associated with bound-excitons (BX). The second strongest peak and weak emission at higher energies Fig. 1. Photoluminescence spectra from at the 10K spectrum can be attributed to transitions 2.5-/um GaN epilayer on sapphire at of free-excitons A (FXA) and B (FX 5 ). The various temperatures. spectrally well-resolved luminescence structures allow us to make relatively unambiguous study of the intrinsic free-exciton transitions in GaN using PLE measurements. It is known that although free-excitons can be formed by photo-generated electron-hole pairs in the absorption continuum through Coulomb interaction, phonon-assisted exciton formation is a very significant process in polar materials.7- 8 In such an indirect phonon-assisted exciton formation process, when the energy of incident light is E=Eo+nhwLo, where E0 is the exciton energy at K=0 and n is an integer, the photons create excitons of energy (nf-l)hoLO in the n=l free-exciton band with the simultaneous emission of a LO-phonon. The excitons quickly lose their kinetic energy by emitting LO-phonons with an appropriate wave vector K before reaching the minimum at K=0 of the exciton band, where they annihilate and emit light with an energy of E0. When the excitation photon energy is E=Eo+nhboL++AE, where AE
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