Thin film deposition of metal oxides in resistance switching devices: electrode material dependence of resistance switching in manganite films
© Nakamura et al.; licensee Springer. 2013
Received: 17 July 2012
Accepted: 29 December 2012
Published: 15 February 2013
The electric-pulse-induced resistance switching in layered structures composed of polycrystalline Pr1−xCa x MnO3 (PCMO) sandwiched between Pt bottom electrode and top electrodes of various metals (metal/PCMO/Pt) was studied by direct current current–voltage (I-V) measurements and alternating current impedance spectroscopy. The I-V characteristics showed nonlinear, asymmetric, and hysteretic behavior in PCMO-based devices with top electrode of Al, Ni, and Ag, while no hysteretic behavior was observed in Au/PCMO/Pt devices. The PCMO-based devices with hysteretic I-V curves exhibited an electric-pulse-induced resistance switching between high and low resistance states. Impedance spectroscopy was employed to study the origin of the resistance switching. From comparison of the impedance spectra between the high and low resistance states, the resistance switching in the PCMO-based devices was mainly due to the resistance change in the interface between the film and the electrode. The electronic properties of the devices showed stronger correlation with the oxidation Gibbs free energy than with the work function of the electrode metal, which suggests that the interface impedance is due to an interfacial oxide layer of the electrode metal. The interface component observed by impedance spectroscopy in the Al/PCMO/Pt device might be due to Al oxide layer formed by oxidation of Al top electrode. It is considered that the interfacial oxide layer plays a dominant role in the bipolar resistance switching in manganite film-based devices.
Recently, a large resistance change by the application of an electric pulse was observed at room temperature in metal oxides such as Pr1−xCa x MnO3 (PCMO)[1–31]. This effect provides a possibility of a next-generation nonvolatile memory, called resistance random access memory (ReRAM). ReRAM is highly expected to replace conventional flash memory due to its low power consumption, small bit cell size, and fast switching speed. The underlying mechanism of the resistance switching behavior is still poorly understood, although there have been various proposed models of the resistance switching mechanism such as formation and rupture of conductive filament paths[3, 4], field-induced electrochemical migration such as oxygen vacancy creation/diffusion[5, 6], alteration of the width and/or height of a Schottky-like barrier by trapped charge carriers in the interface states, trap-controlled space-charge-limited current[8–12], injecting electrons into and extracting electrons from the interface, and oxidation/reduction reaction at the interface[14–20]. It was also reported that the resistance switching is significantly dependent on electrode materials in the ReRAM devices[14, 18, 21–26]. The precise identity of the switching location where resistance change mainly occurs has not been revealed. The comprehensive understanding for the origin of the resistance switching is required to meet the requirement for the next-generation nonvolatile memory application.
Impedance spectroscopy is a useful technique for characterizing the resistance switching in metal oxide films, which indicates whether the overall resistance of the device is dominated by a bulk, grain boundary, or interface component[30–39]. In this work, the resistance switching mechanism in PCMO-based devices was investigated by impedance spectroscopy. In order to study the resistance switching mechanism in the PCMO-based devices, the frequency response of complex impedance was measured in the PCMO-based devices with various metal electrodes. Based on impedance spectral data, the electrode material dependence of the resistance switching in the PCMO-based devices was discussed by correlating with the standard Gibbs free energy of the formation of metal oxides and the work function of each electrode metal.
Polycrystalline PCMO films were deposited on prefabricated Pt/SiO2/Si substrates by radio-frequency (rf) magnetron sputtering with a Pr0.7Ca0.3MnO3−δ target. The base pressure was 1 × 10−6 Torr. Before the deposition, the target was presputtered for 30 min to obtain a clean target surface. A mixture of Ar and O2 gases with 25% oxygen content was used for the sputter deposition. The process pressure was controlled at 20 mTorr. The rf power was 80 W. The substrate temperature was 450°C. The film thickness was obtained by cross-sectional scanning electron microscopy. All films were about 100 nm thick.
In order to measure the electrical properties of the deposited films, we prepared layered structures composed of PCMO sandwiched between a Pt bottom electrode and top electrodes. Four kinds of metallic electrodes such as Al, Ni, Ag, and Au with a thickness of about 200 nm and a circle area with a diameter of 500 μm were deposited on top of the films by thermal evaporation. The resistance of metal/PCMO/Pt junctions was evaluated by three techniques: (1) current–voltage (I-V) characteristics, (2) resistance measurements after pulsed voltage application, and (3) Cole-Cole plots by impedance spectroscopy. The positive voltage is defined as the current flows from the top electrode to the PCMO film, and the negative bias was defined by the opposite direction. The resistance switching of the PCMO films was measured by applying a single positive electric pulse and a single negative electric pulse alternately to the top electrode. The width of the electrical pulse was 500 ns. The resistance values were read out at 0.1 V after each pulse. Impedance spectroscopy was performed in the frequency range of 100 Hz to 5 MHz. The oscillatory amplitude for the impedance measurements was 50 mV.
Results and discussion
The real part of impedance at 0 Hz measured by alternating current (ac) impedance spectroscopy corresponds to the dc resistance of the device. On the contrary, the real part values of impedance at 0 Hz shown in the impedance spectra (Figures 5,6, and7) do not show a good agreement with the resistance values shown in the electric-pulse-induced resistance switching behavior (Figures 1b,2, and3b, respectively). The same top electrode material and the same characterization technique reproducibly resulted in the similar resistance change. However, the results strongly depend on the techniques. The reason, which is not clear yet, may lie in some intrinsic difference of resistance transition processes between each technique.
The standard Gibbs free energy of the formation of metal oxides is also shown together with the work function of the electrode metals in Figure 9. The difference in the oxidation Gibbs free energy between Al and Ag shows a good correspondence with the difference in the resistance switching behavior between the Al/PCMO/Pt and Ag/PCMO/Pt devices. An applied electric field may enhance the oxidation at the interface with the electrode metals with lower oxidation Gibbs free energy. The oxidation near the interface plays a role in the electrical hysteresis and resistance switching. The opposite switching polarity of the Ag/PCMO/Pt device to the Al/PCMO/Pt device is due to the difference in the oxidation Gibbs free energy.
As stated above, the resistance switching behavior was significantly different between the Ni/PCMO/Pt and Au/PCMO/Pt devices, although Au has a similar work function to Ni. This difference in the resistance switching also can be explained well by the difference in the oxidation Gibbs free energy between Ni and Au. Whether resistance switching can be observed or not seems to be dependent on the oxidation Gibbs free energy.
Recently, an amorphous Al oxide layer with the thickness of several nanometers was found at the Al/PCMO interface by high-resolution transmission electron microscopy (HRTEM). It was also reported that the oxidation of Al metal in PCMO films at the Al/PCMO interface was observed by X-ray photoemission spectroscopy (XPS)[19, 20]. In order to evaluate the capacitance due to the Al oxide layer at the Al/PCMO interface, the observed impedance spectra shown in Figure 5 were analyzed by comparing with the simulated spectra constructed on the basis of an equivalent circuit composed of parallel connection of resistance and capacitance (RC). Three sets of parallel RC components in series were required as an equivalent circuit to reproduce the observed spectra by theoretical simulation, although the experimental impedance spectra seemed to be composed of two semicircular arcs. These three components can be assigned to grain bulk, grain boundary, and film-electrode interface. By fitting the experimental impedance spectra with the simulated ones, the interface resistance values of high and low resistance states were evaluated to be 915 and 15 kΩ, respectively. Simultaneously, the interface capacitance values of high and low resistance states were determined to be 2.5 × 10−9 and 7 × 10−9 F as the best-fit parameters, respectively. Knowing the interface capacitance C, the thickness of the Al oxide interfacial layer, d = ε0εS / C, can be estimated, where ε0, ε, and S are the vacuum permittivity, the dielectric constant of aluminum oxide, and the electrode area, respectively. With ε0 = 8.85 × 10−14 F/cm, ε = 10, and S = 2 × 10−3 cm2, d is obtained to be 7 and 2.5 nm in the high and low resistance states, respectively. The thickness of the Al oxide interfacial layer obtained by impedance spectroscopy in this work was in good agreement with that estimated by HRTEM and XPS[18–20]. The oxidation of the Al electrode plays a dominant role in the bipolar resistance switching in the PCMO-based devices. On the contrary, the resistance change at the interface might not give a dominant contribution to the overall resistance change of Ni/PCMO/Pt and Ag/PCMO/Pt devices because with Ni and Ag, it is difficult to form the oxide interface layer as compared with Al. As a result, the resistance change ratio of Ni/PCMO/Pt and Ag/PCMO/Pt devices is smaller than that of the Al/PCMO/Pt device. It is rather difficult to categorize Ni and Ag into the group of top electrode materials that cause the ReRAM effect.
The electric-pulse-induced resistance switching in manganite film-based devices with various metal electrodes of Al, Ni, Ag, and Au was studied by dc current–voltage measurements and ac impedance spectroscopy. The hysteretic I-V characteristics and resistance switching were observed in the PCMO-based devices with top electrode of Al, Ni, and Ag. The Al/PCMO/Pt device showed larger resistance switching than other PCMO-based devices with top electrode of Ni and Ag. The electrode material dependence of the resistance switching in polycrystalline manganite films was investigated in more detail by impedance spectroscopy. Two semicircular arcs were observed in the impedance spectra of the Al/PCMO/Pt device, while the Cole-Cole plots in the devices with Ni, Ag, and Au showed only one semicircular arc. These two distinctive features of the Al/PCMO/Pt device could be assigned to the PCMO bulk and to the interface between the PCMO film and the Al electrode, respectively. By comparing the impedance spectra between the high and low resistance states in the Al/PCMO/Pt device, we suggested that the resistance switching in the PCMO-based devices was mainly due to the resistance change in the interface between the film and the electrode. According to the theoretical simulation of impedance spectra, the interface component observed by impedance spectroscopy in the Al/PCMO/Pt device might be due to Al oxide layer formed by oxidation of Al top electrode. The interfacial transition layer of Al oxides is possibly responsible for the large resistance change in the Al/PCMO/Pt device. The oxidation Gibbs free energy rather than work function gives dominant influence on the resistance switching behavior of the PCMO-based devices. Large resistance switching ratio is expected by choosing a metal with lower oxidation Gibbs free energy as an electrode material and using the interface resistance component due to metal oxide layer in the PCMO-based devices.
This work was supported in part by a Grant-in-Aid for Challenging Exploratory Research (no. 23656215) from the Japan Society for the Promotion of Science (JSPS).
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