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Optical absorption measurements for diamond cubic SnₓGe_1-x alloy films indicate strong interband transitions with a change in direct energy gap of 0.35<Eg<0.80eV for $0.15>x>0$. The optical energy gap undergoes an indirect to direct transition in this composition range and decreases much faster with Sn content than predicted by tight binding and pseudopotential calculations in the virtual crystal approximation.
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He et al. (1997) studied this question.
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