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

摘要

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. To take advantage of these recent advancements, use is made of high quality 4H SiC boule and epitaxial samples to study the material's 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 region of interest here (~3500--4000 A), the dominant absorption and emission processes are characterized by 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 for polarized illumination of the sample is consistent with a splitting of the free exciton ground state. 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 A (~3.4--3.6 eV) shows evidence of free exciton absorption processes due either to the exciton's hole occupying a lower-lying valence band (separated from the topmost by the crystal field splitting) or the exciton's electron occupying the second lowest conduction band minimum. The locations of these features in the absorption spectrum are consistent with the calculated positions of these band extrema in the electronic band structure, for which there still remains little experimental evidence even today.