Cross-plane thermal conductivity temperature dependence for PbSnSe/PbSe thin film superlattice material from 100K to 300
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Cross-plane thermal conductivity temperature dependence for PbSnSe/PbSe thin film superlattice material from 100K to 300K James D. Jeffers, Leonard Olona, Zhihua Cai, Khosrow Namjou, and Patrick J. McCann School of Electrical and Computer Engineering, University of Oklahoma, Norman, OK 73019 ABSTRACT The temperature dependence of cross-plane lattice thermal conductivity for thin film IV-VI semiconductors grown by molecular beam epitaxy was measured. Samples consisting of PbSe/PbSrSe multiple quantum wells (MQWs) on PbSe/PbSnSe superlattices (SLs) were grown with variations in SL layer thickness and the number of SL pairs. Localized lattice temperatures within the MQW layers were extracted from analysis of continuous wave photoluminescence (PL) emission spectra at heat sink temperatures between 100 K and 250 K. These data, finite element analysis, and electrical characterization were used to determine cross-plane lattice thermal conductivity of two different SL materials. A SL material with three different PbSe/PbSnSe thicknesses (1.2/1.2, 1.8/1.8, and 2.4/2.4 nm) exhibited a fairly constant lattice thermal conductivity from 1.2 to 1.3 W/mK as the sample was cooled from 250 K to 100 K. Another SL material with five different PbSe/PbSnSe thicknesses (0.5/0.5, 1.0/1.0, 1.6/1.6, 2.1/2.1, and 2.6/2.6 nm) exhibited very low lattice thermal conductivities from 0.46 to 0.47 W/mK 250 K to 100 K. These results are consistent with reflection of low energy heat transporting acoustic phonons within the SL material. INTRODUCTION Research employing various nanostructure designs has shown significant improvements in the ZT figure of merit for thermoelectric materials [1,2]. It has been proposed that reduction in lattice thermal conductivity (κ) resulting from quantum dots and superlattices (SLs) is due to increased phonon scattering at material boundaries. For example, Heremans et al. [2] showed an increase in ZT for Pb nanoparticles in PbTe at temperatures from 400 K to 100 K. However, κ increased at lower temperatures similar to bulk materials as the phonon mean free path increased. By contrast, investigations into SL heating effects [3] have shown a dissimilar thermal conductivity temperature trend compared to bulk materials, affected not only by the number of interfaces but on layer thickness. Similar results from other researchers have suggested that nanostructured materials with specific compositional periodicities result in phonon reflection in addition to scattering [4]. In this work we report on the molecular beam epitaxial (MBE) growth, characterization, and analysis of different IV-VI semiconductor SL materials. Photoluminescence (PL) measurements were used to determine total cross-plane thermal conductivity, and Hall effect measurements were used to calculate electronic thermal conductivity. The obtained lattice thermal conductivities clearly show that SL structures can have significantly lower lattice thermal conductivities than binary bulk material and that this reduction is maintained at low lattice temperatures. The
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