Band structure properties, phonons, and exciton fine structure in 4H -SiC measured by wavelength-modulated absorption and low-temperature photoluminescence

Abstract

Owing to its hexagonal symmetry, indirect band gap, and relatively large unit cell, the electronic band structure of $4H$-SiC is comprised of a complicated series of anisotropic valence and conduction band extrema even very near to the uppermost valence band maximum and lowest conduction band minimum. This has presented a difficult challenge to those experiments which have attempted to resolve the small energy separations between these band extrema. To overcome this challenge, we have measured the wavelength-modulated absorption (WMA) spectrum of $4H$-SiC over a broader wavelength range (3500--3800 \AA{}) and at a higher resolution ($l0.1$ \AA{}) than in previous work. By comparing these measurements with the low-temperature photoluminescence spectrum in ultrapure $4H$-SiC, we have identified several features, which we attribute to a $56\ifmmode\pm\else\textpm\fi{}3$ meV crystal-field splitting of the valence band maximum or a $136\ifmmode\pm\else\textpm\fi{}3$ meV separation between the two lowest conduction band minima. We also show that the spin-orbit split-off valence band, which has been observed in previous measurements of $4H$-SiC, contributes to nonparabolic dispersion near the valence band maximum, and this is responsible for several previously misidentified features in the WMA spectrum. Finally, we report the first experimental measurement of fine structure splittings in the free exciton ground state, which manifests as four small ($0.7\ifmmode\pm\else\textpm\fi{}0.1$ meV) splittings in the WMA spectrum due to mass anisotropy and electron-hole exchange interaction.