Thermoelectric properties of PbTe thin films prepared by gas evaporation method
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Thermoelectric properties of PbTe thin films prepared by gas evaporation method Masatoshi Ito, Won-Son Seo, and Kunihito Koumotoa) Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan (Received 1 August 1997; accepted 16 March 1998)
PbTe thin films with fine grains were successfully fabricated by the gas evaporation method. Thermoelectric properties, i.e., Seebeck coefficient and electrical conductivity, both decreased with decreasing grain size. This was attributed to the decrease in carrier mobility exceeding the increase in carrier concentration with decreasing grain size. It was clarified that the effects of grain boundaries and of oxidation on carrier mobility are considerably large.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
In recent years, studies of PbTe have been pursued due to their device applications1–3 in many fields like IR detectors, photoconductors, etc. and there has been extensive work on thermoelectric properties, in the bulk state4 and the film state,5 –7 for the purpose of improving thermoelectric energy conversion efficiency. The performance of thermoelectric materials can be evaluated by the figure of merit defined as Z a 2 syk, where a is the Seebeck coefficient, s is the electrical conductivity, and k is the total thermal conductivity. Thermal conductivity is the sum of the contributions arising from the lattice thermal conductivity (phonon) k1 , and the electronic thermal conductivity ke , i.e., k k1 1 ke . In semiconductors, thermal conduction is mainly caused by phonons. In recent years some efforts have offered a possibility of reducing the lattice thermal conductivity to improve Z 8,9 by means of the scattering of phonons at grain boundaries. In fact, it is reported that Z in highly disordered alloys of PbTe with a mean grain size of ,1 mm is calculated to be about 10% higher than equivalent single crystal values.10 Materials with fine grains are easily prepared by many techniques such as gas evaporation method,11 colloidal method,12 and alkoxide method,13 and recently investigations of ultrafine particulate films have produced many results.14,15 Many results have also been reported concerning morphology and structure16 and optical properties17 of ultrafine particles produced mostly by the gas evaporation technique. However, until now little has been known about how fine grains affect thermoelectric properties. In the present paper we have prepared PbTe films with fine grains and studied the microstructural effects on their thermoelectric properties.
PbTe films were deposited on clean glass substrates (Corning #7059) held at 323 K by the gas evaporation technique, using PbTe powder (99.9% purity) as a source. In this case, He gas was introduced into the chamber and the control of grain size in films was attempted by changing the He gas pressure. The lateral dimensions of the films were 2.5 3 2.5 cm. The thickness of the films and the rate of deposition were
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