Thermoelectric efficiency of IV-VI multiple quantumwell and superlattice systems

In this thesis the thermoelectric properties of PbTe-based multiple quantum well (MQW) and superlattice (SL) structures grown by molecular beam epitaxy (MBE) are investigated. The two approaches for an enhancement of the dimensionless thermoelectric figure of merit ZT = σS2T/λ (σ: electric conductivity, S: Seebeck coefficient, λ: thermal conductivity, T: temperature) in low dimensional structures are discussed: an enhancement of the two dimensional (2D) power factor σS22D due to carrier confinement or a reduction of the thermal conductivity due to a modified phonon dispersion and stronger phonon scattering in layered structures. The novel PbTe/Pb1-xSrxTe-MQW system is investigated for a rise of the 2D power factor σS22D. A detailed analysis of optical and electronic parameters based on infrared transmission spectra is performed. Only a small rise of the 2D power factor σS22D for the MQW's compared to homogeneous epitaxial PbTe-layers is found. This smaller σS22D-rise compared to first predictions for ideal MQW's is related to the finite barrier height and effects like lifting of the valley degeneracy or enhanced carrier scattering. However, a strong reduction of the thermal conductivity is found in MQW structures resulting in 2D ZT values between 0.5 and 1.0 at T = 300 K and up to 1.6 at T = 550 K. For the use in technical applications the three dimensional (3D) figure of merit of the whole structure ZT3D must be raised. Different novel PbTe-based superlattice structures are investigated with respect to a 3D ZT-increase due to thermal conductivity reduction. ZT3D-values of 0.33 for p-type PbTe-doping superlattices and of 0.45 at T = 300 K for PbTe/PbSe20Te80-SL's are determined (T = 300 K; n-PbTe: ZTmax = 0.36, p-PbTe: ZTmax = 0.30). Maximum ZT-values up to 1.25 for PbTe/PbSe20Te80-SL's and 1.20 for PbTe-doping SL's are estimated for temperatures between 450 K and 550 K. These values show the potential of these structures for applications in this temperature range, where no well-suited thin film material providing high thermoelectric efficiency is known. For higher temperatures possible applications are limited because of material interdiffusion and chalcogen out-gassing. Conclusively the results show that for PbTe-based materials the approach of a reduction of the thermal conductivity is more effective for a ZT-enhancement than the MQW approach using carrier quantum confinement. (author)