Evaluating tantalum oxide stoichiometry and oxidation states for optimal memristor performance

Tantalum oxide has shown promising electrical switching characteristics for memristor devices. Consequently, a number of reports have investigated the electrical behavior of TaOx thin films. Some effort has been made to characterize the composition of the TaOx films and it is known that there must be an optimal stoichiometry of TaOx where forming and switching behavior are optimized. However, many previous reports lack details on the methodology used for identifying the chemistry of the films. X-ray photoelectron spectroscopy has been the most commonly used technique; however, peak fitting routines vary widely among reports and a native surface oxide of Ta2O5 often confounds the analysis. In this report a series of large area TaOx films were deposited via sputtering with controlled O2 partial pressures in the sputtering gas, resulting in tunable oxide compositions. Spectra from numerous samples from each wafer spanning a range of oxide stoichiometries were used to develop a highly constrained peak fitting routine. This procedure allowed for the composition of the TaOx films to be identified with greater detail than elemental ratios alone. Additionally, the peak fitting routine was used to evaluate uniformity of deposition across individual wafers. The appearance of a greater contribution of Ta4+ oxidation states in the oxygen starved films are believed to relate to films with optimal forming characteristics