Low temperature photoluminescence spectra of synthetic PbS crystals having the natural isotopic abundance ͑95% 32 S͒ are compared with crystals grown using 99% 34 S. An anomalous decrease in the band gap energy of 6.5± 2 cm −1 with increasing S mass was observed. The unusual sign of this isotope shift can be related to the temperature dependence of the band gap energy, which opposite to the behavior seen for most common semiconductors, increases strongly between 2 K and 320 K. This temperature dependence was measured with improved accuracy using absorption spectroscopy, and a fit to this data suggests that there should be virtually no net renormalization of the band gap energy due to zero point motion at low temperatures. This must result from a cancellation of the contributions from S and Pb, but the predicted "normal" isotope shift of the band gap energy with Pb mass is too small to be measured.
We have studied the dependence of the lattice parameter of silicon on isotopic mass, using high-resolution photoluminescence spectroscopy to detect splittings of the shallow donor bound exciton transitions in epitaxial layers of either isotopically enriched 28 Si or 30 Si grown on silicon substrates of natural isotopic composition. The slight lattice parameter mismatch between the isotopically enriched epitaxial layer and the natural silicon substrate induces a biaxial strain in the epitaxial layer, which results in a splitting of the hole states in the bound exciton. This can be detected with remarkable precision, especially in the highly enriched 28 Si epilayers, where the bound exciton lines are extremely sharp.
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