Density-of-States Effective Mass and Scattering Parameter Measurements on Transparent Conducting Oxides Using Second-Order Transport Phenomena

Abstract

AbstractTransparent conducting oxides (TCO) have relatively low mobilities, which limit their performance optically and electrically, and which limit the techniques that may be used to explore their band structure via the effective mass. We have used transport theory to directly measure the density-of-states effective mass and other fundamental electronic properties of TCO films. The Boltzmann transport equation may be solved to give analytic solutions to the resistivity, Hall, Seebeck, and Nernst coefficients. In turn, these may be solved simultaneously to give the density-of-states effective mass, the Fermi energy relative to either the conduction or valence band, and a scattering parameter, s, which characterizes the relaxation time dependence on the carrier energy and can serve as a signature of the dominate scattering mechanism. The little-known Nernst effect is essential for determining the scattering parameter and, thereby, the effective scattering mechanism(s). We constructed equipment to measure these four transport coefficients on the same sample over a temperature range of 30 – 350 K for thin films deposited on insulating substrates. We measured the resistivity, Hall, Seebeck, and Nernst coefficients for rf magnetron-sputtered aluminum-doped zinc oxide. We found that the effective mass for zinc oxide increases from a minimum value of 0.24me, up to a value of 0.47me, at a carrier density of 4.5 × 1020 cm−3, indicating a nonparabolic conduction energy band. In addition, our measured density-of-states effective values are nearly equal to conductivity effective mass values estimated from the plasma frequency, denoting a single energy minimum with a nearly spherical, constant-energy surface. The measured scattering parameter, mobility vs. temperature, along with Seebeck coefficient values, characterize ionized impurity scattering in the ZnO:AI and neutral impurity scattering in the undoped material.