Impact of oxygen stoichiometry on electroforming and multiple switching modes in TiN/TaO_x/Pt based ReRAM

摘要

We have investigated the material and electrical properties of tantalum oxide thin films (TaO_x) with engineered oxygen contents grown by RF-plasma assisted molecular beam epitaxy. The optical bandgap and the density of the TaO_x films change consistently with oxygen contents in the range of 3.63 to 4.66 eV and 12.4 to 9.0g/cm~3, respectively. When exposed to atmosphere, an oxidized Ta_2O_(5-y) surface layer forms with a maximal thickness of 1.2 nm depending on the initial oxygen deficiency of the film. X-ray photoelectron spectroscopy studies show that multiple sub-stoichiometric compositions occur in oxygen deficient TaO_x thin films, where all valence states of Ta including metallic Ta are possible. Devices of the form Pt/Ta_2O_(5-y)/TaO_x/TiN exhibit highly tunable forming voltages of 10.5 V to 1.5 V with decreasing oxygen contents in TaO^. While a stable bipolar resistive switching (BRS) occurs in all devices irrespective of oxygen content, unipolar switching was found to coexist with BRS only at higher oxygen contents, which transforms to a threshold switching behaviour in the devices grown under highest oxidation.