AVS 46th International Symposium
    Thin Films Division Wednesday Sessions
       Session TF-WeA

Paper TF-WeA10
Direct Measurement of Density-of-States Effective Mass and Scattering Mechanisms in Transparent Conducting Oxides Using Second-Order Transport Phenomena

Wednesday, October 27, 1999, 5:00 pm, Room 615

Session: Transparent Conductive Oxides
Presenter: D.L. Young, National Renewable Energy Laboratory
Authors: D.L. Young, National Renewable Energy Laboratory
T.J. Coutts, National Renewable Energy Laboratory
V.I. Kaydanov, Colorado School of Mines
W.P. Mulligan, Sunpower Inc.
Correspondent: Click to Email
TCOs have relatively low mobilities, which limit the techniques that may be used to explore their band structure via the effective mass, and limit the performance of the materials optically and electrically. The de Haas-van Alphen and other resonance techniques used to characterize the Fermi surface are not appropriate for TCOs that have a rather short relaxation time (i.e., low mobility). We have used transport theory to directly measure the effective mass and other fundamental properties of TCO films.@footnote 1@ The Boltzmann transport equation can be solved to give analytical 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 the scattering parameter, s, which is related to the relaxation time and the Fermi energy. The little-known Nernst-Ettingshausen effect 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 the resistivity, Hall, Seebeck, and Nernst coefficients for rf magnetron-sputtered cadmium stannate (CTO) films@footnote 2@ with carrier concentrations in the range of 2-7x10@super 20@ cm@super -3@. We found that CTO is a highly degenerate semiconductor with a parabolic conduction band in this range of carrier concentration and that the density-of-states effective mass is 0.29±0.04 m@sub e@. This value agrees well with earlier studies of CTO@footnote 3,4@ but is, to our knowledge, the first direct measurement of both m@super *@ and s. Optical modeling of the effective mass agrees well with our directly measured value. Spectrophotometric analysis, resistance as a function of frequency, and mobility as a function of carrier concentration all indicate that grain-boundary scattering plays only a minor role in degenerate CTO. Early results indicate that the mobility reaches a maximum of nearly 80 cm@super 2@ V@super -1@ s@super -1@ for a carrier concentration of about 5x10@super 20@ cm@super -3@ when s approaches zero. The transition in the dominant scattering mechanism is indicated by a change in the sign of the Nernst voltage. @FootnoteText@ @footnote 1@ I.A. Chernik, V.I. Kaydanov, M.I. Vinogradova, and N.V. Kolomoets: Soviet Physics - Semiconductors, Vol. 2, No. 6 (1968) 645. @footnote 2@ X. Wu, W.P. Mulligan, and T.J. Coutts: Thin Solid Films, 286 (1996) 274. @footnote 3@ G. Haacke: Applied Physics Letters, Vol. 28, No. 10 (1976) 622. @footnote 4@ W. Mulligan: Ph.D. Thesis, Colorado School of Mines, Golden, CO (1997).