Bias dependent NDR in TiO2/NiO heterojunction diodes

TiO2/NiO thin film heterojunction diodes are fabricated using electron beam evaporation and DC sputtering techniques. Three different thicknesses of NiO (58, 82, and 160 nm) are combined with a TiO2 film of about 80 nm thick. The common device configuration is FTO/TiO2/NiO/Au with a 4 mm sq. device area. The thickness dependent diode I-V characteristics were analyzed; with the increase in thickness, the diode knee voltage, series resistance, and ideality factor have reduced. All the devices exhibit a high diode ideality factor, which means the diode currents are not limited by drift, diffusion, or recombination at the space charge region. The high ideality factor points to the presence of interface states, spatial inhomogeneities, or tunneling in the devices. Further, the bias dependent negative differential resistance (NDR) in I-V characteristics and the presence of an anomalous peak in C-V characteristics were analyzed, which claimed the presence of interfacial trap states, defects, or tunneling in the diodes. The Fowler-Nordheim tunnel model was applied to the devices and the field enhancement factors were calculated as -14.3, -9.3, and -9.2 for the diodes in the increasing order of thickness. The tunneling phenomenon was confirmed as trap-assisted indirect tunneling. Also, the temperature dependence of NDR was studied.