New Technique for Testing Performance of Thermoelectric Quantum Well Materials

Abstract

A new test technique and apparatus have been developed for measuring the thermoelectric (TE) performance of the quantum well (QW) thin films. Innovative, nanotechnology Si/SiGe QW TE thin film materials have been developed that appear to demonstrate significantly higher Seebeck coefficients and lower electrical resistivities that show the power factor, Seebeck coefficient squared divided by resistivity, to be many times higher than for the state-of-the-art TE materials such as Bi2Te3, PbTe, TAGS or SiGe bulk materials. The power factor values were derived from QW films deposited on very electrically resistive Si substrates. Since the electrical resistance of the Si substrate is so high (> 100 times the QW film sample resistance) it acts like an insulator and the Seebeck and resistivity values that are measured are essentially those of the QW films. The measurement of thermal conductivity of QW films to obtain efficiency and the Figure of Merit, ZT, is much more difficult to measure and spurred this new experimental approach. This test was designed to determine if the QW materials are significantly better in ZT and efficiency than state-of-the-art TE materials such as Bi2Te3. As with the Seebeck and resistivity measurements, the presence of the Si substrate complicates the performance analysis. This test setup was designed to minimize the influence of the substrate. The technique developed allows the N or P sample to be measured as a thermoelectric couple with a copper wire as the other leg. During the test, the electrical output of the test sample and the imposed temperature difference are recorded simultaneously. The measured temperature difference, along with measured electrical properties at the steady-state conditions, is used to calculate the conversion efficiency by two different methods. Three separate QW samples were tested in the new test apparatus. A bulk Bi2Te3 sample was also tested to compare the QW performance with a state-of-the-art bulk TE material. From the experimental data, it was found that the QW samples exhibited conversion efficiencies which were approximately three times higher than the efficiency of the bulk Bi2Te3 material. Also, the experimentally measured properties of the Bi2Te3 sample were in good agreement with the published properties of the material, thus providing additional confirmation of this new test technique. Another confirmation of a higher ZT is that maximum efficiency and maximum power peaks exist at considerably different loads, whereas these peaks both occur near matched loads for Bi2Te3 alloys. The new test apparatus has been used effectively to measure the power factor of the QW thin films deposited on silicon substrates, and this power factor is significantly higher than for the state-of-the-art bulk TE materials.