Optical detection of biatomic sheets of silicon in Si/Ge superlattices

Abstract

A distinctive Raman spectrum associated with biatomic sheets of silicon in Si/Ge superlattices has been found in the energy range 370–410 cm−1. This double-peaked structure was obtained over an order of magnitude of germanium layer thickness, but was not found in the alloy control layers or structures with thicker Si layers. It is proposed that the signal is due to modes that are normally forbidden in this scattering configuration. Strong direct optical transitions have been predicted for certain Si/Ge superlattices incorporating biatomic sheets of Si and this distinctive Raman signal could be used to characterize the biatomic sheets of silicon in these structures.

Thermal stability of biatomic Si sheets in Ge

“…The double peak in the calculated phonon DOS in the energy range 380 cm-' to 400 cm-' is due to the non-degenerate transverse interface modes. Since the form of the Raman spectrum obtained for these structures resembles strongly the form of the calculated phonon DOS, we have asserted that the dominant characteristic of the experimentally observed Raman interface peak arises from the transverse interface modes [6,7].…”

“…The relative volume of the superlattice where the Si layers are two atomic layers thick is thus reduced at temperatures lower than would otherwise be expected, and it is this which leads to the double-peaked interfacial Raman signal becoming less distinct for the lower-temperature anneals. Recent theoretical predictions [6] are that Si sheets of 1 ML and 3 ML will not give rise to a double peak in the Raman spectra.…”

“…The superlattice chosen for the work was a 50-period (Si)2/(Ge)21 superlattice grown on a (001)-oriented Ge Thermal stability of biatomic Si sheets in G e substrate; details of the molecular beam epitaxy (MBE) growth procedure have been given elsewhere [6]. This structure was selected because the thickness of the Ge layers meant that neighbouring Si layers were well isolated from each other, thus reducing the chance of cross diffusion effects.…”

“…A similar lineshape profile is also found in superlattices containing biatomic sheets of Ge. A full discussion of such Raman spectra, together with a wide range of experjmental and theoretical results, is given elsewhere [6]. In the present study, this unique Raman lineshape is used to investigate the effects of post-growth annealing.…”

<title>Raman sudy of Ge quantum dots formed by submonolayer Ge coverages in Ge/Si superlattices</title>

“…Previously these hopes were connected with theoretical assumption that stressed ultrathin Ge/Si superlattices (SLs) should have direct band gap structure [1] . Recently, the great excitement of researchers was generated by promising optical properties of self-organizing Ge quantum dots (QDs) embedded in a Si matrix.…”