Thermoelectric performance of Si0.8Ge0.2/Si heterostructures synthesized by MBE and sputtering

Abstract

The electronic properties of bulk materials are altered when they are incorporated into quantum wells. These wells may or may not impart beneficial electronic properties to an ensemble of such interfaces. Two‐dimensional quantum wells (2D QWS) have been synthesized by alternating layers of Si and Si0.8Ge0.2. Such nanostructures are being investigated as a candidate thermoelectric material for high figures of merit (Z). The predicted enhancement is attributed to the confined motion of charge carriers in the 2D QW and can be translated into a large increase in conversion efficiency. This combination of materials is of interest since Si0.8Ge0.2 is the preferred thermoelectric material for high temperature applications. Other heterostructures are also being investigated. Two techniques have been used to prepare these multilayer Si/Si0.8Ge0.2 films, high‐rate dual‐magnetron sputtering and molecular beam epitaxy (MBE). Films have been deposited on single‐crystal silicon and sapphire substrates. Substrate heating was used as a means of controlling film structure and electronic properties. Both types of films have been characterized with transmission electron microscopy (TEM) and selected area transmission electron diffraction. The thermoelectric properties of these films have also been determined over a broad range of temperature, extending from the boiling point of liquid helium to 1200 K. The performance of the MBE and sputtered films have been systematically compared. Efforts have been made to correct the electronic measurements of all thin films for the effects of their respective substrates. Preliminary thermoelectric measurements of these multilayer structures, especially those produced with MBE, lead us to believe that significant gains in the thermoelectric figure of merit (Z) may be possible with this approach.