Optical study of isotopic effects in the sulfur deep level in silicon

Abstract

The optical absorption spectrum of silicon diffusion doped with sulfur has been studied in the region near 2 μm. Samples containing either 32s or 34s were examined to attempt to detect an isotopic shift of the energy level of the deepest sulfur center. Only a very small shift of 0.14 meV was observed; this is two orders of magnitude smaller than that found for the center studied by deep-level spectroscopy. Analysis of the literature indicates that both measurements are in fact on the same center. The optical spectrum is interpreted as superimposed spectra of a group of slightly differing sulfur centers. If these perturbed centers were formed in different concentrations in different samples, an apparent isotope shift could easily be inferred in the deep-level measurement. The interpretation of multiple centers for the sulfur deepest donor is analogous to the well-known X levels found for several acceptors in silicon. These results may have general applicability toward an understanding of reported differences between various experiments on other deep-level systems.