The effects of localized tail states on charge transport mechanisms in amorphous zinc tin oxide Schottky diodes

Temperature-dependent current–voltage measurements were performed on vertical Schottky diodes made with solution-processed amorphous zinc tin oxide (a-ZTO) semiconductor and palladium rectifying contacts. Above 260 K, forward bias electron transport occurs via thermionic emission over an inhomogeneous, voltage-dependent Schottky barrier with ${\overline{\varphi }}_{b0}$ = 0.72 eV, σ ₀ = 0.12 eV, and A* = 44 A cm⁻² K⁻², where ${\overline{\varphi }}_{b0}$ and ${\sigma }_0$ are the mean potential barrier and its standard deviation at zero bias, respectively, and A* is Richardson's constant. For large currents, the series ohmic resistance of the bulk semiconductor dominates. At temperatures below 260 K, less carriers are excited from localized states below the conduction band edge, and space-charge-limited current (SCLC) dominates. The exponential tail density of states parameters extracted for a-ZTO are g _tc = 1.34 × 10¹⁹ cm⁻³ eV⁻¹ and kT _t = 26 meV. The intermediate tail state density in a-ZTO, less than that of amorphous silicon and greater than that of amorphous indium gallium zinc oxide, allows for experimental observation of a temperature-dependent transition of bulk charge transport mechanisms in strong forward bias from semiconductor-like ohmic conduction near room temperature to insulator-like SCLC at lower temperatures. In reverse bias, the same tail states lead to modified Poole–Frenkel emission, reducing the leakage current. The frequency response of a half-wave rectifier and diode impedance spectroscopy confirm that the Schottky diode cut-off frequency is above 1 MHz. (letter)