Electronic properties of amorphous zinc tin oxide films by junction capacitance methods

Amorphous zinc tin oxides have recently shown sufficiently high mobilities that fully transparent electronic devices with these materials are now considered promising. We report the first detailed characterization of the electronic properties of these materials using the methods of admittance spectroscopy, drive-level capacitance profiling (DLCP), and transient photocapacitance (TPC) spectroscopy. We have examined a series of 1–3 μm thick zinc tin oxide films (at a composition of 1:1 ZnO:SnO2) incorporated into metal–insulator–semiconductor (MIS) devices. These were annealed at 400, 500, and 600 °C. The DLCP measurement is largely insensitive to interface defects and thus provides a good measurement of the free carrier and defect densities within the bulk region of these films. For the best samples (annealed at 600 °C), our results indicate a free carrier density of 5 × 1014 cm−3 plus a deep defect density 1.5 × 1015 cm−3. The TPC spectra disclose optical transitions between defect levels and the conduction and/or valence bands. They reveal two primary features in these materials: A dominant gaussian shaped deep defect band with an optical threshold near 1.6 eV relative to the conduction band, and an exponential band-tail with a characteristic (Urbach) energy near 120 meV. The Urbach energy indicates the degree of disorder in semiconducting materials, and is directly correlated to the anneal temperature for this series of samples.