Optical and electrical properties of solids have a close relationship with their dimensionalities. In this thesis, optical properties of several direct bandgap materials with different dimensionalities were investigated in detail. In three dimensional bulk zinc oxide (ZnO) single crystal, two electron satellite transition (TES) was examined in terms of a radiative Auger effect. On the basis of experimental spectral data, a model was proposed to interpret temperature dependence of its integrated intensity. Meanwhile, the phonon coupling with various excitonic transitions were discussed as a function of temperature in bulk ZnO single crystals. Being as a new family of two dimensional materials, monolayers of transition metal dichalcogenides (TMDCs) have received an increasing research interest in the past few years because they have been proved to be direct bandgap semiconductors. Light emission properties of tungsten sulfide (WS2) monolayers were characterized by using excitation power dependent photoluminescence (PL) technique. Two kinds of emission mechanisms, namely band-edge free excitonic transition and localized-states ensemble emission, were revealed in WS2 monolayers. Meanwhile, PL and Raman mapping for WS2 monolayers were conducted. It was found the relative intensity of inter-plane vibration mode with respect to in-plane mode is critical for monolayer identification with Raman spectroscopic technique and PL mapping can yield more information on the uniformity and quality of the samples. In the third part of this degree research, various low dimensional nanostructures of WS2, ZnO and GaN were studied. WS2 nanotubes were found to show interesting Raman light scattering features, while ZnO nanorods prepared by vapor phase transport method were revealed to have distinctive light emission properties. Finally, by focused ion beam milling, GaN and ZnO nano-array were fabricated. Surface vibration mode was firmly demonstrated to exist in these nanostructures with optical studies and theoretical analysis. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

D.Ing. (Electrical and Electronic Engineering) / A theory was developed for the investigation of the optical properties of inhomogeneous layered media. Reflectivity and transmissivity analysis of multi-layered structures was realized by utilizing flow graph representations and by employing Mason's rule. This study served as a base for the development of analytical expressions in integral form for reflectivity, transmissivity, reflectance, bilinear transformed reflectance and transmittance of materials possessing inhomogeneous refractive index profiles. These proposed formulas were derived for both normal and oblique incidence and contemplate nonabsorbing, as well as, absorbing materials. An ellipsometric expression for inhomogeneous layers was also derived by employing the developed theory. Several hypothetical examples that emulate refractive index profiles in ionimplanted semiconductors were investigated, including a buried layer with a gaussian refractive index profile, and two homogeneous layers with a half-gaussian transition region between them. Curves of reflectance versus wave number were simulated using the derived formulations in two different ways: (i) employing numerical methods (ii) applying analytical solutions. The performance of these simulations was compared to standard techniques such as the matrix method and the Wentzel-Kramers-Brillouin (WKB) approximation. Very good agreement between the proposed theory and the matrix technique was found. The developed formulations were appropriate even at wave numbers where the WKB approximation was not valid. It must be stressed that the analysis of the reflectance at these wave numbers is important in the study of processed semiconductors. In comparison to the matrix technique, the integral formulation led to substantial time saving, which, depending on the particular application, was between one and two orders of magnitude faster. This fact indicated that the developed expressions for reflectance and transmittance can be used to great advantage in least-square curve-fit ...

Typescript (photocopy) / 122 leaves, [7] leaves of plates : ill. ; 27 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.) Dept. of Physics, University of Adelaide, 1977

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

The following is a comprehensive study of transient and steady-state nonlinear optical properties of semiconductor microcrystals embedded in a glass matrix (semiconductor doped glass). Transient thermal effects which give rise to longitudinal excitation discontinuities (i.e., kinks) that arise from partial sample switching in increasing absorption optical bistability are observed in a doped glass. The transient thermal effects occur on time scales of a few hundred milliseconds. Femtosecond and nanosecond laser pulses are employed to measure time-resolved and steady-state transmission and differential transmission spectra. The measured spectra reveal several beautiful effects which are attributed to the many-particle effects of electron-hole plasma. The spectra reveal: bandgap renormalization, broadening of the tail states and screening of the continuum states, state filling (spectral hole burning), thermalization of nonthermal carrier population distributions, band filling due to carrier relaxation of the thermal and nonthermal distributions, direct electron-hole recombination and long lived (>>100 ps) tail states which are attributed to electron trapping. Absorption edge dynamics discussed in this dissertation span 15 orders of magnitude.

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

No description available.

In this thesis we develop a new first-principles method for the calculation of optical absorption spectra and band structures in semiconductors and insulators at finite tem- peratures. The theoretical framework of our methodology originates back to 1950s in two pivotal research papers by F. Williams and M. Lax. Here, we expand the scope of the pioneering works by Williams and Lax, and we present a new theory of phonon- assisted optical absorption and temperature-dependent band structures. We demon- strate that our technique is highly efficient and simple to the point that a single calcu- lation is sufficient to capture temperature-dependent absorption coefficients including the effect of quantum zero-point motion. We report calculations of optical absorption spectra and of direct and indirect band gaps of Si, C, GaAs and MAPbI₃ . We obtain good agreement with experiment and with previous calculations. The approach pro- posed in this thesis is highly versatile, and can straightforwardly be combined with improved descriptions of the dielectric function by including electron-hole effects via the Bethe-Salpeter and GW equations. In this thesis we also investigate the underly- ing mechanisms leading to the "inverse Varshni effect" in materials that belong to the family of metal halide perovskites. We show, using the example of MAPbI 3 , that the vibrational modes with metal-halide-metal bending or rocking character are the major cause of the band gap opening with increasing temperature. To this aim we present an approach for elucidating the physics underpinning the changes of the band gap with vibrational motion. Our methodologies developed in this thesis are simple to imple- ment in any electronic structure package as a post-processing step, having the potential to find broad applications in the ab-initio community. We anticipate that our work will open the way to predictive calculations, as well as will contribute to the better under- standing of the optoelectronic properties of solids at finite temperatures.