Sub-bandgap optical properties of wide bandgap semiconductors

Abstract

Wide bandgap semiconductors are being extensively investigated over the world for electronic and optoelectronic devices with higher efficiency, greater power, higher temperature operation, smaller size, and lower cost due to their large bandgap and other outstanding properties. To explore improved applications of wide bandgap semiconductors, more comprehensive and better understanding of their fundamental properties is needed. Although plenty of experiments have been conducted on wide bandgap semiconductors, some basic phenomena remain to be clarified and explained. This thesis thus attempts to illustrate the sub-bandgap optical properties of wide bandgap semiconductors including GaN, AlN, ZnO, Ga2O3 and diamond. The first phenomenon treated in this thesis study was the electric fields induced by ionized impurities and their influence on the sub-bandgap optical absorption in GaN. For the electric fields caused by ionized dopants, their strengths were estimated as a function of doping concentration. Based on Bayes’ rule in mathematic statistics, the strength probability distribution of dopant-induced local electric fields was derived. In particular, the influence of the dopant-induced electric fields on the sub-bandgap optical absorption coefficient of GaN was calculated without adjustable parameter under the Franz-Keldysh mechanism. Good agreement between calculation and experiment is achieved, leading to a self-consistent and quantitative explanation to the sub-bandgap optical absorption of GaN. The second phenomenon discussed in the thesis was the frequently observed variable-period oscillations (VPOs) in optical spectra in the sub-bandgap wavelength region in various materials including nitrides, oxides, silicides, sulfides and perovskites. To interpret the phenomenon, a new dispersion with frictional components was proposed for the sub-bandgap refractive index. By utilizing this newly proposed dispersion relationship, a set of analytical formulae were developed for simulating the VPOs pattern in the four fundamental types of optical spectra such as reflection, transmission, absorption and even photoluminescence. It is firmly shown that the major physical mechanism causing the VPOs could be the rapid peculiar change of refractive index below the band gap. In addition, such peculiar sub-bandgap dispersion containing frictional components is argued to be associated with the unavoidable defects and imperfections in solids. The dopant-induced electric field effect and variable-period oscillations are jointly observed in the sub-bandgap transmission spectra of Si-doped β-Ga2O3 thin films grown on Si substrate. By applying above approaches, the VPO pattern was successfully simulated such that the effective optical bandgap values, refractive index, and thickness were determined for the thin films. It is found that the effective optical bandgap decreases with increasing the doping concentration. The third phenomenon investigated in the thesis was the temperature dependence of the zero-phonon luminescence (ZPL) lines of NV centers in CVD-grown diamond. The ZPL lines of the NV center with neutral (denoted as NV0) and negative charge (NV-1) states were measured in the temperature range of 5-300 K. It is shown that the experimental data, including the temperature induced broadening, peak shift and even intensity deterioration of the two sharp ZPL lines of NV0 and NV−1, all on the whole can be interpreted with a generalized quantum theory in which Franck–Condon harmonic and Debye approximations were assumed. In particular, the thermal deterioration tendency of the integrated intensities can be described by Debye–Waller factor in the theory of the Mössbauer effect.