Resistive switching of amorphous non-stoichiometric gallium oxide thin films

Amorphous, non-stoichiometric gallium oxide (a-GaOx, x < 1.5) is a promising material for resistive switching memory and logic, especially for neuromorphic computing. However, despite considerable research efforts in recent years, its resistive switching mechanism is still far from understood. A wide range of different switching patterns has been reported, without consensus in the interpretations. This work aims at a deeper understanding of the resistive switching processes in a-GaOx, by focusing on the electron transport and the ways it can be influenced by ion migration. The resistive switching measurements not only confirm counter-8, figure-8 and abnormal bipolar resistive switching, but also unveil new links between these switching patterns. The surface oxidation of a-GaOx, which is quantified for the first time, and the voltage amplitude seem to play a decisive role. The highlight of this work is the discovery of variable-range hopping (VRH) as the dominating electron transport mechanism in a-GaOx at room temperature. This has never been reported before, as all previous works explicitly or implicitly assumed band conduction. The measured conductivity closely follows the exponential T^(-1/4)-dependency of Mott's law for VRH. The presence of localized electronic states near the Fermi level is confirmed by photoemission spectroscopy and density functional theory. The electron hopping between the localized states is simulated with the kinetic Monte Carlo (KMC) method, in what are likely the first VRH simulations with an ab-initio density of states of an amorphous material. It is demonstrated that the KMC-VRH model can reproduce the conductivity measurements quantitatively. Experiment and simulation also agree that the conductivity decreases non-linearly with increasing O/Ga ratio. At high electric field strength, the simulations yield an elevated electron temperature and an exponential increase of the conductivity, in accordance with VRH theory and with notable resemblance to the resistive switching curves. Finally, the consequences of VRH on the device scale, particularly for metal|a-GaOx Schottky barriers, are investigated by finite element modeling. The device simulations reveal low-conductive regions near both interfaces, which typically dominate the total resistance. The electric field dependence of the VRH conductivity in these regions leads to symmetric, S-shaped current-voltage curves that are comparable to the experiments. Resistive switching thus occurs most likely via changes of the O/Ga ratio in these regions. Based on this conclusion, a new unified switching mechanism is devised that would explain the various experimental observations.