以調制光譜、拉曼光譜及X-ray繞射研究砷銻化鎵、氮化銦和氮化銦/氮化鎵多層量子井的光學特性

Abstract

This dissertation explores the electro-optical properties of GaAs1-xSbx, InN and InN/GaN multiple-quantum-well light-emitting diodes. The main focus of the dissertation is divided into four parts. First, The bandgap, surface Fermi level, and surface state density of a series of GaAs1-xSbx surface intrinsic-n+ structures with GaAs as substrate are determined for various Sb mole fractions x by the photoreflectance (PR) modulation spectroscopy. The dependence of the bandgap on the mole composition x is in good agreement with previous measurements as well as predictions calculated using the dielectric model of Van Vechten and Bergstresser in Phys. Rev. B 1, 3551 (1970). For a particular composition x, the surface Fermi level is always strongly pinned within the bandgap of GaAs1-xSbx and we find its variation with composition x is well described by a function EF = 0.70-0.192x for 0≦x≦0.35, a result which is notably different from that reported by Chouaib et al. [Appl. Phys. Lett. 93, 041913 (2008)]. Our results suggest that the surface Fermi level is pinned at the midgap of GaAs and near the valence band of the GaSb. PR is then applied to study InN films with In and N polarities grown by molecular beam epitaxy. No PR feature is observed at 293 K. At 50 K, for N-polar InN, a broad PR feature with Franz-Keldysh oscillations (FKOs) is observed. The surface electric field (312 kV/cm) and band gap (0.682 eV) are deduced from analyzing FKO extremes. However, some narrow PR features are observed for In-polar InN and three transition energies are obtained, but no FKO is observed. These indicate that the surface electric field (or surface band bending) of In-polar InN is smaller than that of N-polar InN. In the third section of this work, the photoelastic effect is observed in indium nitride (InN) nanocones with high surface to volume ratio grown by Metal-organic Chemical Vapor Deposition (MOCVD). Photoluminescence (PL), Raman spectroscopy and atomic force microscopy (AFM) are employed to characterize the effect. With an increase in the optical excitation intensity, it is observed that the E2(high) mode exhibits a redshift in frequency. Through a detailed analysis of the frequency shifts of Raman peaks, the variations of the strain in InN nanocones are deduced. Besides, the reduction of the band-gap energies due to the varied-strain with increasing excitation intensity is also observed by PL measurements. By comparing the change of the PL peak energies under high and low excitation intensities with the corresponding redshift of E2(high) phonon mode, the band gap of InN is found to redshift 19 meV as the in-plane compressive strain reduces 0.1%. All our results can be accounted for by the photoelastic effect, in which the built-in surface electric field is screened by photoexcited carriers. InN/GaN multiple-quantum-well (MQW) light-emitting diodes, with around one-nanometer-thick InN wells, are grown by metal-organic chemical vapor deposition. The high-resolution x-ray diffraction measurement and high-resolution transmission electron microscopy indicate that the InN well layers are characterized by abrupt interfaces and uniform thickness. The photoluminescence (PL) peak energies, internal quantum efficiency (IQE) and quantum confined Stark effect (QCSE) are investigated by PL measurements. The higher IQE with increasing well width up to 12.2 % is found due to deeper confined states as well as the absence of QCSE. The comparison of PL spectra with the calculated transition energies is also taken into account. It suggests that indium surface segregation in InN/GaN MQWs plays an important role in emission energies.