The effect of quantum confinement on the thermoelectric figure of merit

Abstract

The thermoelectric figure of merit (Z) determines the usefulness of a material for thermoelectric energy conversion applications. Since the 1960's, the best thermoelectric material has been Bi2Te3 alloys, with a ZT of 1.0 at a temperature ofT = 300 K. The advancement of nano-scale technologies has opened up the possibility of engineering materials at nano-scale dimensions to achieve low-dimensional thermoelectric structures which may be superior to their bulk forms. In this thesis, I established the basis of the low dimensional thermoelectric transport principle in the Si/Si1-xGex quantum well superlattice (two-dimensional) system and in the Bi quantum wire (one-dimensional) system. In bulk form, Si1_xGex is a promising thermoelectric material for high temperature applications. The Si/Si1 _xGex quantum well superlattice structures are studied based on their electronic band structures using semiclassical transport theory. Detailed subband structures are considered in an infinite series of finite height quantum wells and barriers. A significant enhancement of the thermoelectric figure of merit is expected. Based on my calculations, experimental studies are designed and performed on MBE grown Si/Sii -xGex quantum well superlattice structures. The experimental results are found to be consistent with theoretical predictions and indicate a significant enhancement of Z within the quantum wells over bulk values. The bismuth quantum wire system is a one-dimensional (ID) thermoelectric system. Bismuth as a semimetal is not a good thermoelectric material in bulk form becamm of the approximate cancellation between the electron and hole contributions to the Seebeck coefficient. However, quantum confinement can be introduced by making Bi nanowires to yield a ID semiconductor. ID transport properties are calculated along the principal crystallographic directions. By carefully tailoring the Bi wire size and carrier concentration, substantial enhancement in Z is expected. A preliminary experimental study of Bi nanowire arrays is also presented.