Optical etalon effects and electronic structure in silicon-germanium 4 monolayer: 4 monolayer strained layer superlattices

Si/Ge short period superlattices containing four monolayers of Si and four monolayers of Ge have been grown on Si(100) substrates by molecular beam epitaxy. The superlattices have been investigated structurally by secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and Raman spectroscopy, and electronically by electroreflectance (ER), Resonant Raman (RR) and photo current (PC) spectroscopies. TEM and Raman measurements show good layer quality, and also that strain is clearly present in the Ge layers. While ER is a technique that has been widely used in the literature, the authors show by comparison with SIMS and TEM observations that such ER measurement may be complicated by an optical etalon effect when the epitaxial layer is sufficiently thick. Allowing for this by a suitable design of sample, they are able to observe the critical points in the superlattice at 2.4 and 2.8 eV, which they have confirmed with RR measurements. The RR also shows clearly that the critical points must be superlattice related. Optical transitions too weak to observe using ER or RR can be observed with PC below the bandgap of silicon, and are related to weak absorption at the superlattice bandgap of approximately 0.8 eV. The observed electronic transitions in this and the 2.4-2.8 eV energy range are compared in energy and oscillator strength with theoretical predictions.