OPTICAL CHARACTERIZATION OF WIDE ENERGY AGP GROUP III-NITRIDE SEMICONDUCTORS

Abstract

This thesis documents the findings of the optical characterization of wide-band gap group III nitrides. Several optical spectroscopic techniques were applied to GaN and related materials grown by metalorganic chemical vapor deposition (MOCVD). Variable-temperature photoluminescence (PL) spectra were measured for a series of undoped GaN and n-type GaN thin films as well as InGaN alloys of low indium compositions grown on (1000)-plane sapphire substrates. Free exciton transitions, bound exciton transitions and longitudinal optical (LO) phonon-assisted exciton transitions were observed in undoped GaN samples. By applying Permogorov's theory, we gave a reasonable explanation to the temperature dependence of the LO phonon replicas, and made tentative assessment of the concentration and recombination lifetime of free excitons in GaN. In then-type GaN samples, temperature-dependent behaviour of the near-band edge band was studied. Band-gap renormalization effect and the doping induced broadening of the PL line width were observed and analyzed in detail. In low-indium composition InGaN alloy samples, the PL mechanism was studied by monitoring the PL peak energy and intensity as a function of temperature. An involvement of band-tail state in the radiative recombination was assumed to interpret the experimental data and good agreement was obtained. Photovoltaic spectroscopy (PV) was used to study the properties of band edge of undoped GaN, Si-eloped n-type GaN, and InGaN/GaN multi-quantum-well (MQW) structures. Clearly resolved exciton resonance and its polarization nature were observed in undoped GaN. In n-GaN free excitons were found to be quenched by carrier screening due to the heavily n-type doping. For InGaN/GaN MQWs, phase sensitive characteristics of PV spectra were adopted to identify the complex PV spectra and findings. Photoreflectance (PR) and contactless electroreflectance (CER) were employed to study the undoped GaN, n-type GaN, Au-Schottky contacts on GaN, and piezoelectric effects in AIGaN/InGaN/GaN heterostructures. With the assistance of the PV spectra, we found that both the free exciton transitions and the band-to-band transitions contributed to the PR signals of undoped GaN, instead of the free exciton transitions or the band-to-band transitions alone reported by other authors. In n-GaN, we have found that the eloping concentration dependence of PR magnitude follows the relationship of log10 [|?R/R|]?-(1.2±0.2)log10[ND]+constant. We have developed a theoretical model based on the assumption of low-density surface states existing at the GaN surface, which can give a consistent interpretation of the experimental finding above. Our result suggests that low-density surface states exist at GaN surface, and is important to the understanding of the Schottky contact characteristics on GaN. Furthermore, PR spectra were measured to study the Au-Schottky contacts on n-GaN. Not only the significant spectral difference caused by the formation of the AuSchottky contacts was observed, but the Schottky barrier height was also given out by analyzing the Franz-Keldysh oscillations (FKOs) in the PR spectra of GaN. Finally, strain-induced piezoelectric effects in AlGaN/InGaN/GaN heterostructures were investigated by CER. Through the FKO analysis, the strengths of the piezoelectric fields were estimated in GaN and AIGaN layers, respectively.