On the limit of defect doping in transition metal oxides

Transition metal oxides are being increasingly used in many applications like nonvolatile memory, contacts to transition metal dichalcogenide transistors and photovoltaics, and thin-film transistors, to name a few, because the conductivity can be tuned by defect doping. The mechanism of conduction through substoichiometric oxides is however not well understood. Earlier studies attributed the conduction in substoichiometric oxides to Poole–Frenkel emission. But the assumptions underlying the Poole–Frenkel model break down in thin dielectrics and when a broad range of temperature is considered. The authors model the conduction through substoichiometric nickel oxide (NiO_x) using a kinetic Monte-Carlo framework based on trap-assisted tunneling (TAT), by studying devices made of metal/NiO_x/Si stacks. Modeling the temperature dependence of I–V characteristics enables the extraction of the trap parameters, like trap ionization energy and trap relaxation energy. The authors study the effects of the UV/ozone treatment, which has been shown to reduce the resistivity of NiO_x by orders of magnitude, as well as the choice of metal electrode on the trap properties. The high trap relaxation energy (∼1.6 eV) is identified as an important factor in limiting the effectiveness of defect doping in NiOx, because it hinders the carrier emission step of the TAT process. The relaxation energy is another design knob that can be used when screening oxide candidates for various applications.