Thermal Conductivity Measurements of IV-VI Semiconductor Nanostructures

This dissertation describes photoluminescence (PL) measurement and finite element thermal modeling to quantify optically induced surface heating of lead chalcogenide (IV-VI) semiconductor thin films and extract thermal conductivity for different superlattice (SL) materials. The results provide the first experimental evidence on the role of PbSe and Pb0.85Sn0.15Se SL layer thicknesses in modifying thermal transport properties in the temperature range from 300 K to 90 K. Low temperature data at 90 K indicated a reduction in lattice thermal conductivity by a factor of 9, from 4.0 Wm-1K-1 to 0.45 Wm-1K-1, for 1.2 nm thick SL layers as compared to bulk PbSe. This dissertation also contains characterization data for the electrical conductivity, σ, and Seebeck coefficient, S, for lightly doped SL materials. The performance of these materials was estimated using the thermoelectric (TE) figure of merit ZT ≡ S2σT/k. The data indicate SL materials with an optimal dopant concentration of 3 x 1018 cm-3 at 300 K can be fabricated with σ = 1000 S/cm, S = -190 µV/K and a cross-plane thermal conductivity k = 1.0 Wm-1K-1 which would result in a ZT ≥ 1.0. The same SL will have a ZT = 0.20 at 100 K, much better than bulk PbSe, which has a ZT = 0.05 at 100 K. These results show that IV-VI SL materials can enable development of next generation TE devices for cooling applications. The first in-depth analysis of phonon wave theory for the IV-VI semiconductor system is also presented to estimate the potential for further reduction of cross-plane thermal conductivity from interface reflections. The standard transfer matrix method for optical distributed Bragg reflectors (DBRs) adapted to acoustic waves was implemented to calculate the thickness of SL layers and the number of mirror pairs required for reflectance of different phonon energies