J. Appl. Phys. 112, 023703 (2012); http://dx.doi.org/10.1063/1.4733999 Scanning Tunneling Microscopy/Spectroscopy Studies of Resistive Switching in Nb-doped SrTiO3 Published by : Y. L. Chen, J. Wang, C. M. Xiong, R. F. Dou, J. Y. Yang, and J. C. Nie J. Appl. Phys. 112, 023703 (2012); http://dx.doi.org/10.1063/1.4733999 Present by : Panithan Sriboriboon , Peerawat Lorhana, and Wipakorn Rittisut

Motivation In recent years, the observations of reversible resistive switching (RS) effect in large variety of transition metal oxides, RS have attracted much attention due to their promising for next generation memories. Some models have been proposed to explain the RS effect and a complete investigation by various experimental techniques, is still required.

What is Resistive Switching? Resistive switching refers to the physical phenomena where a dielectric suddenly changes its resistance under the action of a strong electric field or current. The change of resistance is non-volatile and reversible Dependence of direct-current-voltage characteristics of Al-Al2O3-Au sandwich on aluminum oxide thickness. J. Appl. Phys. 33, 2669 (1962); doi: 10.1063/1.1702530

Resistive Switching Measurement Appl. Phys. Lett. 77, 139 (2000); http://dx.doi.org/10.1063/1.126902 Applied Physics Letters 105, 183103 (2014); doi: 10.1063/1.4901053 1. Sandwich electrode 2. AFM electrode

Principle of Resistive Switching Waser et al., Adv. Mater. 21, 2632 (2009) Electrode Electrode The electrode contact effects might have an influence on the RS, which makes the study complicated (https://en.wikipedia.org/wiki/Schottky_barrier) Band diagram for n-type semiconductor Schottky barrier at zero bias (equilibrium) with graphical definition of the Schottky barrier height, ΦB, as the difference between the interfacial conduction band edge EC and EV.

Scanning Tunneling Microscopy/Spectroscopy Studies of Resistive Switching in Nb-doped SrTiO3 STM tip does not touch the sample in our measurement. In this case, the electrode contact effect can be definitely excluded, and then the bulk-related effect might play an important role Tip Nb-doped SrTiO3 A Y. L. Chen Journal of Applied Physics 112, 023703 (2012); doi: 10.1063/1.4733999

Scanning Tunneling Microscopy Image Tip Nb-dopeSrTiO3 A The OFF and ON areas of blue dashed square shown in these images were obtained by processing this area with writing scan (set point: 0.5 nA at V > 2.0 V) and reversal scan (set point: 0.5 nA at V < -2.0 V), respectively. Tunneling I-V curves of STON Y. L. Chen Journal of Applied Physics 112, 023703 (2012); doi: 10.1063/1.4733999

Scanning Tunneling Spectroscopy The valence band maximum (VBM) and the conduction band minimum (CBM) is defined as the initial upturning point in the negative and positive bias part, respectively. As can be seen, the energy gap between VBM and CBM is about 3.2 eV VBM CBM LRS HRS Y. L. Chen Journal of Applied Physics 112, 023703 (2012); doi: 10.1063/1.4733999

Scanning Tunneling Spectroscopy Oxygen can produce a donor-like level near the conduction band, while the presence of VSr or VTi will induce an acceptor-like level near the valence band. XPS confirmed the migration oxygen during the RS process. Y. L. Chen Journal of Applied Physics 112, 023703 (2012); doi: 10.1063/1.4733999

Scanning Tunneling Spectroscopy The oxygen anions at the interface near the STM tip were oxidized into oxygen molecule and left the lattice. Simultaneously, VO diffused into the sample, accompanied with the increase of the Ti3+ content. The increment of donor-like level induced by VO would cause distortions in LDOS near conduction band and enhance sample’s conducting. In contrast, when the positive voltage was applied, the oxygen anions returned into the sample and tuned Ti3+ to Ti4+. Then, the influence of the donor-like level became weak and the conductivity decreased. LRS HRS Y. L. Chen Journal of Applied Physics 112, 023703 (2012); doi: 10.1063/1.4733999

Scanning Tunneling Spectroscopy TaOx Thin Film LRS STM images taken with VTip = -3.0 V and IT = 0.5 nA of the TaOx sample HRS Marco Moors, ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b07020

Scanning Tunneling Spectroscopy Cobalt nanoparticles embedded in TiO2 matrix Topography Conductivity map LRS HRS STM and tunnel conductivity map of a Co NP embedded in TiO2 matrix The curve after conditioning was limited in the range 2.5 V–1 V in order to prevent out of range of the STM current. Alessandro Gambardella 2014 , DOI: 10.1038/srep04196

Conclusions STM can be a useful tool to control and detect the RS effect in STON STS, combined with the XPS investigation, revealed that the oxygen migration mechanism plays a dominant role in the variation of LDOS and thus the change of resistance during the RS process. These results have implications for understanding the RS effect (no electrode contact effects).

Thank you for attention