Oxygen doping of HTSC and resistive switching in HTSC-based heterostructures
© Tulina et al.; licensee Springer. 2013
Received: 27 June 2013
Accepted: 10 August 2013
Published: 15 August 2013
The studies of the bipolar resistive switching effect in thin film heterojunctions (YBa2Cu3O7−δ /Ag) and (Nd 2−x Ce x CuO4−y /Ag) have exhibited the role of oxygen as a doping element in hole- and electron-doped HTSC compounds.
Most HTSC materials are hole-doped compounds. Nd2−x Ce x CuO 4−y (NCCO), Pr2−x Ce x CuO 4−y (PCCO), as well as Ba1−x K x BiO 3−y and BaPb1−x Bi x O 3−y are few representatives of electron-doped ones. The hole- and electron-doped compounds differ by their physical properties and their phase diagrams (Dagotto1994). Nevertheless, they have a feature in common. It is strong dependence of the properties on oxygen stoichiometry, since oxygen plays significant role in formation of the basic (ground) state in these compounds. Influence of oxygen is well studiled for p-type HTSC, it is important as hole dopant. However, questions still remain concerning the role of oxygen in electron-doped HTSC cuprates (Orenstein and Wishwanath2010). The studies of Hall effect (Jiang et al.1994; Mao et al.1995) in oxygen-reduced NCCO and PCCO revealed presence of holelike states in these n-type materials. The question arises whether the presence of p-type carriers is crucial for superconductivity in HTSC.
The effect of bipolar resistive switching (BRS) observed in oxide-based heterostructures, including based on strongly correlated electron systems (SCES), such as HTSC (see, for instance, (Waser and Aono2007; Tulina2007; Sawa2008; Kang et al.2007;Yang et al.2008; Inoue et al.2008)), is sensitive to type of charge carriers in SCES. BRS is exhibited in SCES-normal metal heterocontacts at specific polarity of the electric field as a change of the phase composition of the SCES surface layer at the nanosize level. As a result, the metastable high resistance (OFF) and low resistance (ON) states of the heterocontact are realized leading to colossal electrical resistance (CER)(Tulina2007). CER is the ratio of the resistance on OFF state to the resistance in ON state and characterizes the memory effect. CER = ΔR/R = (R OFF (V = 0) − R ON (V = 0))/R ON (V = 0). Intensive fundamental and application studies of this phenomenon are underway since BRS-based devices are regarded as a new generation of alternative nonvolatile memory (Meijer2008). Besides, BRS effect allows to study the microscopic nature of SCES underlying such devices.
Recent studies of the BRS effect in various compounds revealed the essential role of the oxygen diffusion processes. Electric field of different polarity induces two resistive states of the degraded HTSC surface which is indicative of its separation into conducting and dielectric phases. The switching process gives rise to metastable states with different oxygen content in the interface area of the compound. It is this area that determines the resistive properties of the structure (Tulina et al.2001; Tulina and Sirotkin2004). Therefore, the BRS effect involves reproducible oxygen doping of a specific sample area. In (Tulina and Klinkova2007) we observed BRS inversion in heterojunctions based on electron-doped BaK0,6 Bi 0,4 O 3−y : the switching to the high resistance metastable phase takes place when the HTSC crystal has a negative potential with respect to the normal electrode. In this case the current field (J = σ E) is directed to the surface in contrast to switching observed in hole-doped compounds.
Results of NCCO films structure study were reported elsewhere (Tulina et al.2011). There were detected reflexes of NCCO and Nd0.5 Ce 0.5 O 1.75(NCO) phase. Both phases were epitaxial and (001)-oriented. The obtained results are consistent with other evidences of two phases coexistence after oxygen reducing even in perfect single crystals (Mang et al.2004). Along with the main phase, the second phase can affect the behavior of the interface of the studied heterojunctions and lead to changes in the type of conductivity. Photoemission studies provided support for the X-ray data. After the film surface was milled with nitrogen ions at depth of 50 nm, it was shown using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy that the film surface was metallic and had an entire set of elements (Figure1). We thus obtained heterojunctions of two types on NCCO films: (a) with a surface buffer layer formed by NCO (Figure1a; see the XPS data above) and (b) without the NCO oxide layer, which was removed by ion milling to the main phase NCCO under photoemission control (Figure1).
The bipolar resistive switching has been experimentally observed in NCCO- and YBCO-based film heterojunctions.
It has been shown that the surface layer of the studied HTSC films is oxygen degraded and can serve as an interface in HTSC perovskite-based heterostructures to create memory elements based on resistive switching.
The switching polarity and the BRS effect value in the electron- and hole-doped compounds reflect the pattern of their oxygen doping.
The work was financially supported by the Program of the Division of Physical Sciences RAS "Physics of Novel Materials and Structures", the program of the Presidium RAS "Quantum Condensed Matter Physics" and the Russian Foundation for Basic Research (project 11-02-01174a).
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