The Potential Impact of Confined Acoustic Phonons in Soi-based Nanoelectronics

We have recently observed and simulated confined acoustic phonons in both thin silicon-on insulator films and in SOI membranes [1]. Since progress in silicon device technology is going through increasing use of silicon-on-insulator (SOI) as a strategic device material to increase the operation speed of integrated circuits [2] modifications in the phonon spectrum are of strategic interest to device engineers and scientist. There are two main reasons with origin in the electronacoustic phonon scattering, namely, the thermal conduction [3] and the charge carrier mobility. Here we present our work on off-resonance room temperature low frequency Raman scattering in undoped membranes [1] and thin SOI films [4]. The SOI layers studied have typically a thickness below 50 nm. In this size regime the crystalline silicon is a slab where electronic transport is subjected to several scattering mechanisms. Moreover, in thin slabs phonons are expected to be confined due to the acoustic mismatch of sound velocity in the slab and surrounding as well as the different densities of the materials [2]. In turn, confined phonons are expected to have a smaller group velocity than bulk phonons impairing heat dissipation. Thus, low frequency acoustic phonons are relevant to the understanding and modelling of novel nanoelectronic devices, such as CMOS based on SOI. In a series of Silicon membranes of thickness between 29 and 31 nm, Raman scattering spectra show the presence of confined acoustic phonons as a series of oscillations between 3 and 40 cm. The frequency of these phonons is inversely proportional to the membrane thickness. Depending on the interplay of the phonon cavity confinement strength and the electromagnetic cavity, adjacent peaks in the Raman spectra are seen to vary in intensity quite dramatically in the backand forward scattering configuration. This behaviour has been simulated with a model describing the phononelectron-photon interactions including the photo-elastic effect [5].