Walter M. Klahold



Silicon carbide is a semiconductor material that holds special interest for its use in electronics meant to withstand harsh environments and demanding operating conditions. Its technological importance has been recognized over the course of a relatively long history of research and development, but only recently have growth and production capabilities allowed this material to become a viable platform for industrial electronics. This is especially true of the 4H polytype of silicon carbide, which occupies the sole focus of the work to be presented here. Use is made of high quality 4H SiC boule and epitaxial sample material to study the photoluminescence and wavelength modulated absorption (WMA) spectra at 2 K or lower in temperature. The main results have been obtained in the WMA measurements, which have been performed in this work at higher resolution than in previous studies. In the wavelength regions of interest here (∼3500–4000 Å), the dominant absorption and emission processes are characterized in terms of excitation or recombination of free excitons whose electron and hole occupy the valence and conduction band extrema.

            Four known features of the WMA spectrum have been found to reproducibly exhibit 0.7±0.1 meV splittings that have not been resolved previously. These have been observed for samples taken from two independently grown 4 cm diameter boules, and have the spectral profile characteristic of free exciton absorption. The multiplicity of these splittings and their behavior under polarized illumination of the sample are consistent with a splitting of the free exciton ground state, and this will be shown to be connected to the fundamental properties of the electronic band structure. Several other features which clearly do not have the shape expected for free exciton absorption will be shown to originate from an avoided crossing between the topmost valence bands. A preliminary examination of the WMA spectrum in the range ∼3500–3660 Å (∼3.4–3.6 eV) will also be given which shows evidence of free exciton absorption processes offset to higher photon energies, with this offset being due either to the hole occupying a lower-lying valence band (separated from the topmost by the crystal field splitting) or the electron occupying the second lowest conduction band minimum. These features are significantly broader and weaker than those appearing at the lower energy portion of the absorption spectrum, and are attributed to enhanced lifetime broadening for these excited states of the exciton.

Co-Advisors:  Wolfgang J. Choyke and Robert P. Devaty



Energy Level Structure of Free Excitons in 4H SiC from Wavelength Modulated Absorption Spectroscopy and Low Temperature Photoluminescence





Graduate Advisor

Wolfgang J. Choyke