Structural, Optical, and Electron Transport Qualities of Zinc-Stannate Thin Films
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Structural, Optical, and Electron Transport Qualities of Zinc-Stannate Thin Films David L. Young,1 Timothy J. Coutts,1 and Don L. Williamson2 1 National Renewable Energy Laboratory, Golden, CO 80401, U.S.A. 2 Colorado School of Mines, Golden, CO 80401, U.S.A. ABSTRACT Single-phase, spinel zinc stannate (ZTO = Zn2SnO4) thin films were grown by rf magnetron sputtering onto glass substrates. Uniaxially oriented films with resistivities of 10-2 - 10-3 Ωcm, mobilities of 16 - 26 cm2/V-s, and n-type carrier concentrations in the low 1019 cm-3 range were achieved. X-ray diffraction peak intensity studies established the films to be in the inverse spinel configuration. 119Sn Mössbauer studies identified two octahedral Sn sites, each with a unique quadrupole splitting, but with a common isomer shift consistent with Sn+4. A pronounced Burstein-Moss shift moved the optical bandgap from 3.35 eV to as high as 3.89 eV. Density-of-states effective mass, relaxation time, mobility, Fermi energy level, and a scattering parameter were calculated from transport data. Effective-mass values increased with carrier concentration from 0.16 to 0.26 me as the Fermi energy increased from 0.2 to 0.9 eV above the conduction-band minimum. First-order nonparabolic conduction-band theory was applied to extrapolate a bottom-of-the-band effective mass of 0.15 me. Calculated scattering parameters and temperature-dependent transport measurements correlated well with ionized impurity scattering with screening by free electrons for highly degenerate films. INTRODUCTION Transparent conducting oxide (TCO) thin films are used in many large-volume applications such as flat-panel displays, thin-film solar cells, and low-e architectural window coatings. Indium tin oxide (ITO) is one the most widely used TCO materials because of its high conductivity, low optical absorptance in the visible spectrum, and its ability to be deposited at low temperatures. Cadmium stannate (CTO) exhibits some the highest mobility values of any TCO material, and thus, has both excellent optical transparency and electrical conductivity [1]. Both of these materials, though outstanding TCOs, have serious drawbacks for future, largevolume applications. ITO uses rare and expensive indium, and CTO requires the toxic element cadmium. Zinc stannate (ZTO = Zn2SnO4) is a logical material choice for future TCO development because of its low cost, nontoxicity, and similar spinel crystal structure to that of CTO. ZTO has been shown to exhibit TCO qualities by several groups [2-4], but low electrical conductivity dampened further efforts. Wu et al.[5] used the high transparency and low conductivity in ZTO to form a transparent, high-resistivity buffer layer in CdTe/CdS thin-film solar cells [6]. To understand the limiting conduction mechanism at work in ZTO and to advance our understanding of its basic material properties, a thorough investigation was completed on the structural, optical, and electron-transport properties of rf magnetron sputter-deposited ZTO films onto glass substrates. F3.8.1
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