Electronic Structure and Temperature Dependence of Excitons in GaN
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ABSTRACT We present an optical study of the excitonic properties of epitaxial GaN using reflectivity and photoluminescence (PL) measurements. The values for the intrinsic exciton energies are found to be dependent on the built in strain developed due to the difference in thermal expansion coefficients between the GaN epilayer and the foreign substrate. For GaN on sapphire the compressive biaxial strain causes an upshift of A and B excitons by typically 15 meV relative to the strain free sample, in accordance with previous data. For GaN on SiC, on the other hand, a downshift - 8 meV in the free exciton energies is observed at 2K. Only two excitonic peaks about 18 meV apart, are resolved in reflectivity spectra for GaN on SiC, probably due to the overlapping of A and B excitons. The suggested explanation implies the reduction of the bandgap energy and of the valence band splitting under the action of a biaxial tensional strain in the GaN layer. For all structures the strain-induced shifts of the bandgap energy are much smaller at elevated temperatures, presumably reflecting the temperature dependence of the accumulated strain energy. The exciton-polariton structure of the GaN is also discussed. The enhanced intensity of the no-phonon (NP) A line compared to its longitudinal (LO) phonon replica is suggested to be partially attributed to strong defect scattering. INTRODUCTION The increasing interest in group-Ill nitrides is partly motivated by the need for optical materials functioning in the blue and ultraviolet spectral region, and partly by their potential application in high power and high frequency electronic devices capable of operating in hostile environments and at high temperatures. These devices recently became closer to reality due to advances in crystal growth techniques, in particular after the discovery of growth conditions providing p-type doping of the material [1]. The epitaxial growth of IM-V nitride structures is mostly performed using foreign substrates due to the lack of high quality bulk GaN. The lattice mismatch and the difference in the thermal expansion coefficient (TEC) between the film and substrate in such structures cause an appearance of built in mechanical strain, which influences the epilayer properties. The effect of the internal stress on the fundamental optical properties of GaN epilayers has been studied only for the layers grown on sapphire substrates [2-41. The compressive biaxial stress field in the plane of the layer was found to be responsible for the increased bandgap and increased A-B-C exciton splitting compared to the case of unstrained bulk material. A different behaviour is expected for GaN grown on SiC substrates, where a tensional biaxial strain caused by the difference in TEC is revealed by the measurements of lattice parameters [5,6]. However, the influence of this built in strain on the GaN electronic structure has not so far been experimentally established. In this paper we study the influence of built in strain on the intrinsic optical properties of GaN epilayers us
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