Thermal Conductivity Of Bi/Sb Superlattice
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Thermal Conductivity Of Bi/Sb Superlattice D. W. Song and G. Chen Mechanical and Aerospace Engineering Department University of California at Los Angeles, Los Angeles, CA 90025 S. Cho, Y. Kim, and J. B. Ketterson Department of Physics and Astronomy Northwestern University, Evanston, IL 60208
ABSTRACT The temperature-dependent cross-plane thermal conductivity of a 1-µm thick 50Å Bi / 50Å Sb superlattice on a (111) CdTe substrate was measured, using a differential 3-ω method. This method uses the temperature difference between the superlattice sample and a reference sample to calculate its cross-plane thermal conductivity. However, the substrate thermal conductivity is comparable to or smaller than the superlattice thermal conductivity near room temperature. This results in a very small or negative temperature difference, making the existing data reduction method inapplicable. Based on an improved model, the temperature-dependent thermal conductivity of the Bi/Sb superlattice is obtained and is about half of the literature value of Bi0.5Sb0.5 bulk alloy. INTRODUCTION BiSb alloys have long been studied as a possible thermoelectric material.1-10 Some have observed that high thermoelectric efficiencies can be obtained in a BiSb alloys, especially at low temperatures and in a magnetic field.2-9 Since a reduction of the thermal conductivity, to below that of the corresponding alloy, has been reported in several superlattice systems, including GaAs/AlAs,11-14 Si/Ge,15 Bi2Te3/Sb2Te3,16 and CoSb3/IrSb3,17 it is of interest to investigate the thermal conductivity of Bi/Sb superlattices, particularly since both Bi and Sb are semimetals. We report in this paper an experimental study on the thermal conductivity of a 50Å/50Å Bi/Sb superlattice grown on a (111) CdTe substrate using the 3-ω method. At near room temperature, the superlattice has a thermal conductivity value close to that of the substrate, rendering the usual way of obtaining the thermal conductivity based on one-dimensional steadystate heat conduction inapplicable. Based on a new data analysis scheme, we deduced the thermal conductivity of the superlattice and found that it is about half that of the alloy. EXPERIMENTS A 1-µm thick Bi/Sb superlattice film consisting of alternating layers of 50 Å Bi and 50 Å Sb was grown by molecular beam epitaxy18 on a (111)B CdTe substrate. On top of the Bi/Sb superlattice film, an 800 Å layer of ZnTe was also deposited, to electrically insulate the Bi/Sb superlattice. However, the ZnTe film was not sufficiently insulating for the required measurements, and therefore an additional 1100 Å layer of SixNy was deposited onto the ZnTe surface by plasma-enhanced chemical vapor deposition. A 10 µm wide, 2 mm long heater was
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then deposited on the SixNy surface by conventional microfabrication t
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