Piezoreflectance study of short-period strained Si-Ge superlattices grown on (001) Ge

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Abstract
We have studied the stress-modulated reflectivity at 77 K of a series of short-period strained Si-Ge superlattices grown on (001) Ge substrates. These samples are similar (or identical) to those measured using electroreflectance (40 K) by Pearsall et al. [Phys. Rev. Lett. 63, 2104 (1989)]. We have detected several low-energy features just above the direct band gap of Ge. The observed spacer-layer-thickness dependence of the transition energies and comparison to a calculation clearly show that they are due to quantum-confined direct transitions in the Ge spacer region, not pseudodirect transitions in the short-period Si-Ge portion of the samples.