Optical transitions in strained Ge/Si superlattices

Abstract

Band-to-band infrared absorption has been studied in a series of Sn1Gem (m=11, 15, and 21) strained layer superlattices for the first time. The absorption coefficient shows a quadratic increase with two onsets, characteristic for indirect transitions. From a numerical fit to the absorption spectra, band gap energies are determined in the energy range between 0.55 and 0.75 eV, in accordance with theoretical calculations. The temperature dependence of the band gaps show the usual behavior, at low temperatures quadratic and at higher temperatures linear decrease with increasing temperature. Applied Physics Letters is copyrighted by The American Institute of Physics.We present spectroellipsometric measurements of the linear optical response of ultrathin Ge/Si strained-layer superlattices (SLSs) grown by molecular beam epitaxy with varying strain state and periodicity. The experiments were performed at 300 K in the energy range 1.6–5.7 eV.

As predicted from ab initio calculations, the E1 transitions of the bulk split into various components in the SLSs. From the fitted critical point (CP) energy of the most pronounced of these components, the composition of the SLSs can be estimated. Compared with alloys of the same composition, the onset of the absorption in the SLSs is shifted towards lower energies than in compositionally equivalent alloys.

The E2 transitions can be fitted to two CPs. Both exhibit a shift due to the hydrostatic component associated with the internal strain that is about one-half of what would be expected from bulk properties. We also present evidence of confinement effects.