Measurement of the Effective Mass of Transparent Conducting films of Cadmium Tin Oxide

Abstract

ABSTRACTWe have prepared transparent conducting films of cadmium stannate (Cd2SnO4 with resistivity as low as 1.5×10-4 Ω cm. The resistivity of these films is low because of their surprisingly high mobility (60 cm2 V-1 sec-1) at high carrier concentration (7×1020cm-3). We conducted an investigation to determine whether the high mobility is due to unusually low carrier effective mass or to a long carrier relaxation time. The conductivity effective mass and relaxation time were estimated by Drude free-electron modeling of reflectance and transmittance spectroscopie measurements. The Drude model appears to represent the behavior of cadmium tin oxide very well. The optical effective mass of the electrons in cadmium stannate is about 0.35 mo, which is similar to that of other transparent conducting oxides. By varying the doping level, we were able to fabricate films with various levels of degeneracy, but these showed no significant difference in effective mass.We were also able to determine the carrier effective mass and the relaxation time by measuring four separate electron transport coefficients: conductivity, Hall, Seebeck, and transverse Nernst-Ettingshausen (Nernst). The Fermi level was found to vary from 0.31 to 0.83 eV above the conduction band minimum, and the conduction band was very close to parabolic in this range, with m* ∼ 0.33 mo, in good agreement with the optical result. We conclude that the effective mass of electrons in cadmium stannate is similar to electrons in other transparent conducting oxides and that high mobility results from a relatively long carrier relaxation time.