Self-absorption effect in soft X-ray emission spectra utilized for bandgap evaluation of semiconductors

The self-absorption effects observed in the background intensity just above the Si L-emission spectra of Si and beta-Si3N4, and the C K-emission spectra of diamond and graphite were examined. Based on comparisons with reported results, the energy positions of absorption edges - representing the bottom of conduction bands (CBs) - were assigned. The self-absorption profiles in the background intensities were consistent with previously reported data. The simultaneous observation of valence band and CB edges allowed the determination of a bandgap energy of 1.1 eV for Si, which agrees with the indirect bandgap energy of Si. For beta-Si3N4, the bandgap energy was evaluated as 5.1 eV. For diamond, the edge positions were matched with reported values, and the bandgap energy was calculated to be 5.0 eV, slightly smaller than the optical gap of 5.5 eV. These observations suggest that both edges can be expected for semiconductors in principle. On the other hand, C K-emission spectrum of graphite, a semimetal also showed an edge structure, which was assigned to the self-absorption edge due to the transitions from 1s to sigma* antibonding state of sp2 bonding.