Subject Number :S-17-MS-2009
Support Type : Common use (including technical support necessary for the training)
Proposal Title (English) : In-situ studied the oxygen 2p-vacancy in the TaOx interlayers of resistive switching materials
Username (English) : J. W. Chiou1, Y. F. Wang2, W. F. Pong2, J. S. Chen3, T. Ohigashi4 and N. Kosugi4
Affiliation (English) : 1Department of Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan
2Department of Physics, Tamkang University, Tamsui 251, Taiwan
3Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan
4UVSOR Synchrotron, Institute for Molecular Science, Okazaki 444-8585, Japan

Resistance random access memory (ReRAM) using the change in resistance between crystalline and amorphous states of a chalcogenide compound has attracted a great deal of attention for use as next-generation nonvolatile memories. The ReRAM memory cell has a capacitor-like structure composed of insulating or semiconducting materials sandwiched between two metal electrodes. Due to its simple structure, highly scalable cross-point and multilevel stacking memory structures have been proposed. In the resistive switching phenomenon, A. Sawa [1] reports that a large change in resistance (>1000%) occurs on applying pulsed voltages and the resistance of the cell can be set to a desired values by applying the appropriate voltage pulse.
In this work, we have in-situ studied the electronic structure and the location of oxygen 2p-vacancy in the TaOx interlayers and identified the type of conducting path by scanning transmission x-ray microscopy (STXM) and x-ray absorption near-edge structure (XANES). During the STXM measurements, a various voltage, 0, +50, -50, 0 volts, has exerted on both ends of the Ta/TaOx/Pt thin film, respectively, to specify the electronic structure associated with the conducting path. The amorphous TaOx thin film of thickness of ~75 nm was deposited on Pt/SiO2/Si substrate by electron beam evaporation method. Active electrodes of Ta were thermally evaporated respectively on the TaOx thin film with an equivalent thickness of ~130 nm. As shown in Figure 1, the STXM stack mapping displays the cross-sectional views of the Ta/TaOx/Pt thin film under various applied voltages. The experiments were performed at the 4U beamline. The corresponding O K-edge STXM-XANES spectra of the Ta/TaOx/Pt thin film were recorded at TaOx interlayers, as shown in Figure 2. According to the dipole-transition selection rule, the features at ~530-545 eV are attributed to the electron excitations from O 1s¬ core level to O 2p-derived states, which are approximately proportional to the density of the unoccupied O 2p-derived states. The intensities of the O K-edge STXM-XANES spectra are significantly lower as a voltage applied on both ends of the Ta/TaOx/Pt thin film, which reflects the decrease in the number of unoccupied O 2p-derived states. In other words, the STXM-XANES results demonstrate that the population of defects at the O sites in the TaOx interlayers and intensely support the phenomena that the existence of O 2p-vacancy in the TaOx interlayers plays a main role for the mechanics of charge transfer in resistive switching materials.

Fig. 1. The STXM stack mapping obtained in applying (a) 0, (b) +50, (c) -50, and (d) 0 volts on both ends of the Ta/TaOx/Pt thin film, respectively.

Fig. 2. The corresponding O K-edge STXM-XANES spectra were recorded at TaOx interlayers under various applying voltages.

[1] A. Sawa, Materials Today 11 (2008) 28-36.

Subject Number :S-17-MS-2009-2
Support Type :Common use (including technical support necessary for the training)
Proposal Title (English) : Probing the Electronic Structure of BiVO4 Coated ZnO Nanodendrite Core – Shell Nanocomposite Using Spatially Resolved Scanning Transmission X-ray Microscopy Studies
Username (English) : Mandar M. Shirolkar1, Jau–Wern Chiou2, Takuji Ohigashi3, Nobuhiro Kosugi3 and Way-Faung Pong1
Affiliation (English) : 1Department of Physics, Tamkang University, Tamsui 251, Taiwan
2Department of Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan
3Institute for Molecular Science, Okazaki 444-8585, Japan

Recently, it has been reported that ZnO nanodendrites (NDs) coated with BiVO4 nanolayers core-shell nanocomposite thin film structure forms a multiple-level hierarchical heterostructure, which is remarkably beneficial for light absorption and charge carrier separation for the competent photoelectrochemical (PEC) properties compared to their individual counterparts [1]. As PEC properties are essentially a function of band alignment and atomic – electronic structures, we studied the nanocomposite thin film using various X-ray spectroscopic and microscopy studies. We have probed the origin and nature of localized electron states in core-shell structures using valance band spectroscopy, X-ray absorption spectroscopies and spatially resolved scanning transmission X-ray microscopy (STXM). The valance band studies show band alignment at the core-shell interface, which allows efficient charge transfer between heterostructure. While, X-ray absorption studies essentially show that in core-shell structure tetrahedral environment of V5+ in BiVO4 is unusually distorted and the inter-band gap states related to another valence state of vanadium, namely, V4+ is present. The presence of multiple valance states of vanadium gives favorable conditions for small polaron formation in BiVO4, thereby degrading PEC activities. However, it was observed that V4+ coordinates with ZnO lattice thereby giving rise to conditions similar to vanadium doped ZnO. These studies reveal electronic structure over few micrometers length scale within the thin film, which contains hundreds of nanostructures and precise locations of the coordination within individual nanostructure remain unrevealed. Moreover, high- resolution transmission electron microscopy – electron energy loss spectroscopy could not effectively resolve above-mentioned coordination features.
On the other hand, STXM is powerful technique and retains the ability to efficiently resolve the observed coordination features efficiently within single or cluster of nanocomposite structure because of its high spatial resolution features [2]. Figure 1 depicts STXM results obtained on the core – shell nanostructure. With STXM measurements, we able to probe core-shell coordination and the sites in ZnO NDs contributing to V4+ doped ZnO within single nanocomposite structure. Consequently, our STXM investigations strongly support our other synchrotron measurements.

[1] J. – S. Yang and J. –J. Wu, Nano Energy 32 (2017), 232.