K. Rothe, M. Stordeur, H. S. Leipner
Power factor anisotropy of p-type and n-type conductive thermoelectric Bi-Sb-Te thin films.
J. Electron. Mater. 39, 9 (2010), 1395-1398
The best films for thermoelectric applications near room temperature are based on the compounds Bi2Te3, Sb2Te3, and Bi2Se3, which as single crystals have distinct anisotropy in their electrical conductivity sigma regarding the trigonal c-axis, whereas the Seebeck coefficient S is nearly isotropic. For p- and n-type alloys, P_perpendicular c > P_||c, and the power factors P_perpendicular c of single crystals are always higher compared with polycrystalline films, where the power factor is defined as P = S ^2 sigma, perpendicular c and ||c are the direction perpendicular and parallel to the c-axis, respectively. For the first time in sputter-deposited p-type (Bi0.15Sb0.85)2Te3 and n-type Bi2(Te0.9Se0.1)3 thin films, the anisotropy of the electrical conductivity has been measured directly as it depends on the angle phi between the electrical current and the preferential orientation of the polycrystals (texture) using a standard four-probe method. The graphs of sigma(phi) show the expected behavior, which can be described by a weighted mixture of sigma_perpendicular c and sigma_||c contributions. Because (sigma_perpendicular/sigma_||c) p < (sigma_perpendicular/sigma_||c) n , the n-type films have stronger anisotropy than the p-type films. For this reason, the angular weighted contributions of P ||c lead to a larger drop in the power factor of polycrystalline n-type films compared with p-type films.
Keywords: anisotropy, doping, conduction, thermoelectrics, sputter