Impact of Electrode Oxidation on the Current Transport Properties at Platinum/(Niobium-Doped Strontium-Titanate) Schottky Junctions

Abstract

The current transport properties and resistance-switching (RS) behavior at platinum/niobium-doped strontium-titanate (Pt/NSTO) Schottky junctions were investigated for junctions with different degrees of oxidation (PtO x ) in the Schottky electrode. It was found that the rectifying current-voltage (I-V) profile strongly depends on the heat-treatment of the NSTO crystal and oxidation of the Pt electrode. The hysteresis in the I-V relationships was larger for the PtO x /NSTO junction than for the pure Pt/NSTO junction, and heat-treatment of the NSTO crystal suppresses the hysteresis in I-V and increases the leakage current. The change in the rectifying I-V profiles can be reasonably explained by the increase in the built-in potential caused by partial oxidation of the electrode interface, as determined by soft X-ray photoemission spectroscopy. The experimental results indicate that control of the built-in potential and the partial oxidation of the electrode, which affects the homogeneity of the potential profile, is the key to improving and controlling the rectification and RS properties in Pt/NSTO junctions.Colossal electroresistance (CER) phenomena, i.e., the immense changes in electric resistance induced by applying electric fields, 1 have been observed in many metal/insulator/metal (MIM) structures including titanium oxide sandwiched by metal electrodes 2 and metal/semiconductor (MS) Schottky junction structures such as niobium-doped strontium titanate (NSTO) with a platinum (Pt) electrode. 3 One typical consequence of the CER effect is the so-called resistance-switching (RS) behavior, which is defined as a reversible change in the electronic resistance between the high-resistance state (HRS) and low-resistance state (LRS) upon application of an electric field such as a pulsed bias. What is unique about the RS behavior is that the HRS and LRS remain even after removal of the electric bias applied for switching. For this reason, RS behavior is of interest for the development of nonvolatile random access memory (NVRAM) cells. 2,4 Although the RS behavior of metal/NSTO junctions is interesting in terms of both physics and electronics, several technical problems remain. The most significant problem is poor reproducibility of the properties of metal/NSTO junctions. In particular, the appearance of RS behavior strongly depends on the sample preparation procedures. 5 Obvious hysteresis in the current-voltage (I-V) curves (RS behavior) of metal/NSTO junctions have been reported in some papers, 3,6 but, in contrast, no trace of RS behavior was found at the metal/NSTO junctions reported in other papers. 7,8 Another problem is that the RS speed and resistance retention properties are poor compared with those of conductive-filament-type RS devices. 9,10 In particular, a long pulse voltage is needed to obtain a large resistance change, and the resistance state cannot be memorized for long in metal/NSTO junctions. Therefore, for practical application of RS devices, we need to clarify the material and process parameters gover...

Electric field and temperature dependence of dielectric permittivity in strontium titanate investigated by a photoemission study on Pt/SrTiO3:Nb junctions

“…It is also noteworthy that the spectra for the SrTiO 3 :Nb beneath the Pt layer presented an obvious broadening compared to the spectra from the bare SrTiO 3 :Nb surface. This behavior is very similar to that shown in previously published papers, 5,14,16 indicating that a potential barrier, known as the Schottky barrier, is formed at the Pt/ SrTiO 3 :Nb interface. As seen in the inset of Figure 1, the magnitude of the peak shift and degree of Ti 2p core-level broadening resulting from the application of the Pt electrode remained unchanged with temperature.…”

“…The purity of the Ar gas and that of Pt target used for electrode deposition were 99.999% and 99.9%, respectively. As reported previously, 5 it is evident that the Pt/SrTiO 3 :Nb junctions exhibit enhanced RS behavior when inhomogeneity and non-stoichiometry are introduced into the Pt electrode. For instance, the presence of partially oxidized Pt (PtO x ) in the junction, such as Pt/PtO x /SrTiO 3 :Nb structure, enhances the RS behavior at the junction.…”

“…In contrast, it was inferred that the model was not appropriate for junction behavior at lower temperatures, e.g., <150 K. Therefore, a reconstruction of the phenomenological model is necessary to adequately explain the dielectric properties of SrTiO 3 Recently, junctions made from a metal and niobiumdoped strontium titanate (SrTiO 3 :Nb) have gathered considerable attention because they exhibit behavior known as resistance switching (RS). [1][2][3][4][5][6] For instance, Schottky junctions formed of platinum (Pt) and SrTiO 3 :Nb show a significant hysteresis in their current-voltage (I-V) characterisitcs. 1,3 Such RS behavior has been observed for numerous kinds of metal/SrTiO 3 junctions such as SrRuO 3 /SrTiO 3 :Nb (Ref.…”

“…Note that very homogeneous distribution of / b0 and N D were assumed on analyses of HXPES spectra, although the concept of junction inhomogeneity 18,19 is likely necessary to explain the RS behavior at the metal/SrTiO 3 junctions. 5,9 As the partial oxidation of Pt electrode seems to be a cause of junction inhomogeneity and of the appearance of RS behavior at Pt/SrTiO 3 , 5 we assumed that / b0 and N D were homogeneous in the entire electrode area in the sample carefully prepared with high-purity Pt electrode. Figure 3(a) shows the simulated Ti 2p HXPES profiles obtained using E c (x), as shown in Fig.…”

Interface Properties of Dielectric Oxides

“…With this techniques, it has also been possible to overcome deposition‐induced oxygen vacancy formation and obtain higher Schottky barriers . Von Wenckstern et al and Hirose et al could also prepare high Schottky barriers on n ‐type conducting In 2 O 3 and SrTiO 3 , which indicates absence of oxygen vacancy formation, by depositing Pt in an oxygen ambient . Such deposition leads, however, to the formation of Pt‐oxide, which results naturally in a different barrier compared to that of Pt due to its different work function [see also Section III ( B )].…”