Size and temperature effects on the thermoelectric power and electrical resistivity of bismuth telluride thin films

Thermoelectric power and electrical resistivity measurements have been carried out as a function of temperature in the range 300–470 K on well-annealed thin films of ${\mathrm{Bi}}_2$ ${\mathrm{Te}}_3$ of various thicknesses in the range 400–1900 Å. The films of a given thickness for both the measurements have been prepared simultaneously in a single evaporation so that the results of the thermoelectric power and the electrical resistivity measurements can be combined to evaluate useful material parameters. Annealed ${\mathrm{Bi}}_2$ ${\mathrm{Te}}_3$ thin films of all thicknesses exhibit semiconducting behavior, viz., an exponential decrease of resistivity with increasing temperature and a mildly temperature-dependent thermoelectric power, the latter’s magnitude increasing with increasing temperature. (The thermoelectric power of all the ${\mathrm{Bi}}_2$ ${\mathrm{Te}}_3$ thin films is negative, indicating that the majority of carriers are electrons.) The effective-mean-free-path model and least-squares fitting by local (spline) functions have been used to analyze the thickness dependence of thermoelectric power and electrical resistivity of ${\mathrm{Bi}}_2$ ${\mathrm{Te}}_3$ thin films. Both are found to be linear functions of inverse thickness. By combining the results of analyses of electrical resistivity data and thermoelectric power data, material parameters like mean free path, carrier concentration, their effective mass, and Fermi energy have been evaluated.