Confinement and zone folding in the E₁-like optical transitions of Ge/Si quantum wells and superlattices

We report low-temperature (77 K) electroreflectance and photoreflectance measurements of the E₁-like optical transitions on a series of Ge/Si heterostructures (superlattices and quantum wells). For single and double quantum wells of any thickness, as well as for thick-layer superlattices (period ∼100 Å) the spectra can be understood in terms of quantum-confined bulk-Ge E₁ states. For thin (∼10 Å) multiple quantum wells and superlattices, zone-folding effects dominate the spectra. Actual confinement of the electronic states is determined by measuring the Raman spectrum of each sample in and out of resonance with a given transition.