A Study On The Resistive Switching Effect Of Strontium Titanate Surface

Developing fast, dense, non-volatile memories with low power consumption hasbecome one of the main directions of studies on information storage devices. ResistiveRandom Access Memory (RRAM) is a new type of non-volatile random access memorythat is based on resistive switching (RS) effect. RRAM has attracted tremendousattention due to its simple structure and excellent performance. A large number ofmaterials showing RS effect have been demonstrated in the literature. Due to thedifficulties of bulk measurements and complexity of the intrinsic defects, themechanism of the RS effect is still unclear. Understanding the physical mechanism ofthe RS effect is vital for its application. In recent years, the RS effect has been observedat the surface of SrTiO3(STO) and Nb doped SrTiO3(NSTO) single crystal. Because ofthe lack of defects and the ease of observation, the surface becomes a good system forthe investigation of RS mechanism. In this thesis, the electric transport properties andthe mechanism of the RS effect of the surfaces of STO and NSTO are investigated.The DC and AC electric properties of as-prepared and annealed Au/NSTOheterojunctions are comparatively investigated. The annealed junction behaves like aconventional semiconductor Schottky contact with negligible resistive switchingphenomena. The as-prepared junction has hysteretic IV and CV characteristics, showingsignificant resistive switching effects. Based on equivalent circuit analysis and thenon-hysteretic CV curve of the annealed junction, we reproduced successfully themeasured hysteretic CV curve of the as-prepared junction. We demonstrated that the CVhysteresis is resulted from the leakage current and the nonnegligible serial resistance.Our results suggest that during the resistive switching, the Schottky barrier profile iskept unchanged, and the conductive filaments play a vital role.Based on the macroscopic results, we investigated the microscopic mechanism ofthe surface RS effect of NSTO using the conductive atomic force microscopy (CAFM)and Kelvin probe microscopy (KPM). In high vacuum, the NSTO surface can beswitched between high resistance state (HRS) and low resistance state (LRS). The HRSregion shows low Fermi level while the LRS region shows high Fermi level. Both theRS effect and the Fermi level shift were suppressed in the oxygen atmosphere.Considering the RS polarity, Fermi level shift and oxygen dependence, we propose that the RS effect is caused by surface electrochemical reaction. Based on first principlecalculations, a double well model is proposed to account for the macroscopic andmicroscopic results consistently.A quasi two dimensional conductive layer was prepared at the STO surface usingCAFM tip in high vacuum. The temperature dependence of the conductivity of thislayer shows a semiconducting thermal activation behavior. The fitted activation energyis similar to the first ionization energy of the oxygen vacancy. The conductivity wassuppressed by oxygen atmosphere, suggesting the oxygen vacancy origin of theconductive layer. Based on first principle calculation and analysis of atom orbital andelectric field of oxygen vacancy, the oxygen vacancy originated metallic andsemiconductive layer are explained consistently.