Modeling Raman and Photoluminescence Spectra of Defective Materials

Abstract

Defects are inevitable in solid materials as a result of fluctuations in thermal equilibrium and their processing kinetics. Depending on the targeted application, they can be either beneficial or detrimental. For example, doping semiconductors by foreign atoms can be used to increase carrier concentrations, but at the same time, the carrier mobilities decrease due to enhanced scattering. Defects can also introduce a two-state light emitter system to compensate for the indirect nature of the electronic band gap. However, it is necessary to elucidate the presence of defects and their influence on the properties of materials to develop an application-specific strategy for defect engineering. Raman spectroscopy can be utilized as a contactless and inexpensive diagnostic tool to characterize materials and evaluate their qualities. Photoluminescence spectroscopy is another non-destructive technique that helps us to understand the behavior of optically active defects in materials. While experimental measurements can be complicated, modern computational platforms possess good predictive power and can give us a deep insight into the relevant physical processes. In the case of defects, one needs high computational resources and an advanced theoretical framework to calculate Raman spectra. These issues need to be tackled in a reliable way before calculations. This thesis focuses on simulating Raman and photoluminescence spectra to investigate defects signatures in materials. An efficient computational method to simulate Raman spectra of large systems, which applies to alloys and systems with a small number of defects is devised. The method is based on the projection of vibrational eigenvectors of the supercell to the eigenvectors from the pristine primitive unit cell and using the Raman tensors calculated for the latter. Using photoluminescence and accounting for electron-phonon interactions, we investigate vibronic structures of color centers and calculate phonon side bands. Through this thesis, the behavior of phonons in several two-dimensional semiconducting alloys is explored. We demonstrate that (i) the so-called mass approximation significantly facilitates the calculation of Raman spectra, (ii) simulating the Raman spectra for defective materials can be done by a combination of empirical potentials and quantum mechanical density functional theory calculations, (iii) the so-called phonon confinement model can be used to capture the asymmetric broadening of the prominent peaks. In the context of photoluminescence, we compare the vibronic structures of different color centers by evaluating several important characteristics that are experimentally achievable.