Direct measurement of density-of-states effective mass and scattering parameter in transparent conducting oxides using second-order transport phenomena

Abstract

The Boltzmann transport equation can be solved to give analytical solutions to the resistivity, Hall, Seebeck, and Nernst coefficients. These solutions may be solved simultaneously to give the density-of-states effective mass (md*), the Fermi energy relative to either the conduction or valence band, and a scattering parameter that is related to a relaxation time and the Fermi energy. The Nernst coefficient is essential for determining the scattering parameter and, thereby, the effective scattering mechanism(s). We constructed equipment to measure these four transport coefficients simultaneously over a temperature range of 30–350 K for thin, semiconducting films deposited on insulating substrates. We measured these coefficients for rf magnetron-sputtered zinc oxide, both doped and undoped, with aluminum with carrier concentrations in the range of 1×1019–5×1020 cm−3. The (md*) was not constant over this carrier concentration range: varying from 0.3 to 0.48 me, leading us to conclude that zinc oxide has a nonparabolic conduction band. Conductivity effective mass values for zinc oxide matched our md* values, revealing a single valley, nearly spherical, constant energy surface for zinc oxide. The measured scattering parameter changed from close to zero to 1.5 as the carrier concentrations increased. The scattering parameter, Seebeck coefficient, and mobility versus temperature data support neutral impurity scattering in the undoped material and ionized impurity scattering in the Al-doped ZnO. The transport theory also allows an extrapolation for an md* value at the bottom of the conduction band, which was found to be 0.27 me.