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BB3.09 - Electroforming of Resistively Switching Fe:STO Samples Made Visible by Electrocoloration Observed by High Resolution Optical Microscopy 
Date/Time:
April 22, 2014   4:30pm - 4:45pm
 
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Resistively switching devices have attracted great attention for potential use in future nonvolatile information storage. Their resistance can be changed by applying appropriate voltages. This resistive switching effect has been observed in a variety of oxide materials, e.g. SrTiO3, TiOx or TaOx. Prior to actual resistive switching of valence change memory cells an electroforming step is typically required. This initial electroforming step impacts device performance and switching variability. Thus, a thorough understanding of the physical processes involved in the electroforming process is fundamental for device optimization.In this study, the electroforming process in strontium titanate (STO) single crystals is made visible. Transparent samples of Fe doped STO single crystal were put under temperature and longitudinal electrical stress and observed by means of high resolution transmission optical microcopy, whereby the voltage and current were being logged with time and the sample was continuously captured with a camera. For the experiment a custom sample enclosure was constructed. In the electrocoloration process, color changes within the sample are caused by a valence change of the Fe ions, and accordingly by drift of oxygen vacancies. The colorful regions are growing and propagating towards the electrodes. At the anode Fe atoms are reduced and thus form a dark brown region, while at the cathode they are oxidized and the color turns to bright yellow. The redox reaction is therefore an indirect proof of the oxygen vacancy drift-diffusion in the bulk.Time development and behavior of the color areas within the sample are influenced by multiple factors (voltage, current, temperature, dopant and defect concentration, ambient atmosphere, time, etc.). We study the influence of the particular parameters by capturing the progression of electrocoloration at several samples. The measurements show current and resistance change over time along with sample visual mapping.The behavior is to some extend similar to previous published findings [1], [2], but also new phenomena are observed, i.e. the formation of lines and impermanent spots in the bulk. These are analyzed by different ex-situ characterization techniques (EDX, mösbauer spectroscopy, AFM, etc.). Furthermore, 1D drift-diffusion simulations of the temporal evolution of the oxygen vacancy distribution are performed, which represent well the initial progress of the virtual cathode region.[1] J. Blanc, Physical Review B, 4, 3548-3557, 1971[2] R. Waser, Journal of the American Ceramic Society, 6, 1654-1662, 1990
 


 
 
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