Electrical and chemical analysis of zinc oxide interfaces with high dielectric constant barium tantalate and aluminum oxide in metal-insulator-semiconductor structures fabricated at Low temperatures

Zinc oxide (ZnO) was incorporated into metal-insulator-semiconductor (MIS) structures featuring high dielectric constant (high-κ) barium tantalate (BaTa₂O₆)or alumina (Al₂O₃)as the insulator, and the structures were electrically evaluated for potential applications in transparent thin film transistors. The ZnO films were deposited by radio-frequency magnetron sputtering at 100 °C whereas the dielectric films were deposited by the same method at room temperature. The leakage currents of both the BaTa₂O₆ and Al₂O₃ structures were on the order of 10⁻⁷A/cm². The trap density and trapped charge concentration at the BaTa₂O₆/ZnO interface were determined to be 6.18 × 10¹¹ eV⁻¹ cm⁻²and 5.82 × 10¹¹ cm⁻² from conductance-voltage and capacitance-voltage measurements. At the Al₂O₃/ZnO interface the trap density and trapped charge were more than an order of magnitude smaller at 1.09 × 10¹⁰ eV⁻¹ cm⁻²and 1.04 × 10¹⁰ cm⁻² respectively. The BaTa₂O₆ structures had significantly larger frequency dispersions due to the larger number of interface traps. Chemical analysis using X-ray photoelectron spectroscopy with depth profiling indicates that acceptor type defects associated with a deficiency of oxygen are related to the observed electron trapping in the BaTa₂O₆MIS structure. Overall, the results indicate that Al₂O₃ would be better suited for transparent thin film transistors deposited at low temperature or without substrate heating.