A Study of the Semiconductor and Thermoelectric Properties of Sputtered Mixed Metal-Oxide Thin Films

Abstract

The effects of stoichiometric ratio, oxygen (both during sputtering and annealing), and annealing on the transistor and thermoelectric properties of zinc indium oxide (ZIO) and zinc tin oxide (ZTO) were studied. Thin films of (ZnO)x(In2O3)1−xand (ZnO)x(SnO2)1−xwere fabricated via combinatorial RF sputtering, nominally varying from 0 > x > 1. The films were post-annealed under either argon or oxygen, or left as-deposited. Films sputtered under an argon/oxygen mixture were used in the construction of thin-film transistors. ZIO TFTs annealed under oxygen at 300◦C exhibited field-effect electron mobilities of 30 cm2/V·s around x = 0.7, with on/off ratios as high as 109, and inverse sub-threshold slopes of less than 1 V/dec. ZTO TFTs annealed at 600◦C under oxygen exhibited mobilities of 10 cm2/V·s at x = 0.25 and x = 0.80, with inverse sub-threshold slopes of 1.5 V/dec and on/off ratios of 106. For the determination of thermoelectric properties, a room-temperature value of ZT was calculated for each fabrication condition with all measurements from a single fabrication run. Films were post-annealed under either argon or oxygen. ZIO was found to be highly sensitive to oxygen during sputtering, with effective Lorenz numbers (Leff) for un-annealed films four orders of magnitude lower when sputtered under argon, close to the ideal Lorenz number, indicating oxygen vacancies as the primary electron donor. Thermal conductivity was generally found to rise toward the edges of the stoichiometric range, with a low of 2 W/m·K. ZT was highest in ZIO at 0.02±0.01 for x near 0.5 in films sputtered under partial oxygen and annealed at 300◦C under oxygen. Un-annealed ZTO films were also sensitive to oxygen during sputtering, but Leff for argon-sputtered films was on the order of 10−4W·Ω/K2, indicating semiconductor-like behaviour, while partial-oxygen sputtered ZTO was highly resistive. Thermal conductivity was low (2 to 5 W/m·K) for most fabrication conditions except near the zinc-rich end of the range. For ZTO, the highest ZT was 0.004±0.002 for argon-sputtered, un-annealed films, at zinc-rich stoichiometries of x > 0.9.