Polarization-dependent spectroscopy of the near-bandgap excitonic emission in free standing GaN
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Y6.1.1
Polarization-dependent spectroscopy of the near-bandgap excitonic emission in free standing GaN
P. P. Paskov, T. Paskova, P. O. Holtz, B. Monemar Department of Physics and Measurement Technology, Linköping University, S-581 83 Linköping, Sweden
ABSTRACT We report a comparative study of the exciton emission in free standing HVPE layer for all polarization configurations. A noticeable difference between the emission spectra polarized perpendicular and parallel to the c-axis of the crystal is observed. The spectra for E⊥c and EIIc are found to be dominated by the emissions of the donor-bound exciton and exciton-polariton both arising from the A and B valence band, respectively, which clearly reveals the optical selection rules in wurtzite GaN. The temperature evolution of the emission spectra is also examined and the thermal redistribution of the excitons at different polarization is discussed.
INTRODUCTION It is well known that the near-bandgap optical properties of GaN are governed by the exciton transitions. The low temperatures emission spectra of undoped material is usually dominated by the peaks of excitons bound to shallow donors and acceptors, while at room temperature the emission arises from the free exciton recombination. The emission properties of heteroepitaxial GaN layers have been extensively studied over the past decade, but the interpretation of the spectra often leads to confusing conclusions because the energy position, relative intensity and linewidth of the emission peaks are strongly influenced by the residual strain and crystalline defects. Only recently, with the availability of high quality free standing and homoepitaxial GaN layers, an accurate study and unambiguously identifications of the emission peaks become possible [1-3]. In wurtzite GaN, the valence band is split by the crystal field and spin-orbit interaction which leads to three free exciton series labelled A, B and C. Each of these excitons can be bound to shallow impurities or defects giving rise of a large number of bound exciton transitions. On the other hand all dipole allowed exciton states are strongly coupled with photons and form mixed exciton-polariton states with two-branch dispersion curves. As a result, emission from the different polariton branches could be observed depending on the excitation photon energy, excitation intensity, temperature etc. In common experiments, however, the population in the B and C valence bands is rather low and mainly transitions connected with the A exciton are observed, especially at low temperatures. Since the A exciton is dipole forbidden for light polarized parallel to the c-axis of the crystal (EIIc), polarization-dependent spectroscopy could be a powerful technique for studying the exciton transitions related to the B and C valence bands. So far, there are only few reports on the polarized photoluminescence in GaN [4-7].
Y6.1.2
Although the dissimilarity between the spectra polarized parallel and perpendicular to the c-axis has been observed, the identification of the emis
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