Resistive Switching Characteristics in TiO2/LaAlO3 Heterostructures Sandwiched in Pt Electrodes
TiO2/LaAlO3 (TiO2/LAO) heterostructures have been deposited on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition. Resistive switching characteristics of Pt/TiO2/LAO/Pt have been studied and discussed in comparison with those of Pt/TiO2/Pt. It is observed that the switching uniformity and the ON/OFF resistance ratio can be greatly improved by introducing the LAO layer. The observed resistive switching characteristics are discussed as a function of LAO thickness and explained by the preferential formation and rupture of conductive filaments, composed of oxygen vacancies, in the LAO layer.
Resistive random access memories (RRAMs), in which binary logical states are represented by a low resistance and a high resistance state , have attracted considerable attention, due to their distinctive features and advantages meeting the demands of next generation high-speed [2, 3], low-power consumption , and high-density  memory devices. A variety of oxides, including binary metal oxides [5–8], perovskite dielectrics [9–12], and complex electron correlated oxides [13–16], have been demonstrated to show resistance switching behaviors, among which TiO2 [17, 18] is one of the most extensively studied, owing to its unique advantages such as wide band gap, high temperature stability, and high dielectric constant with flexibility to exhibit both unipolar and bipolar switching characteristics. However, several issues, including device stability, endurance, and the ON/OFF resistance ratio , still need to be improved. Although a large ON/OFF resistance ratio about 104 has been reported in Pt/TiO2/Pt devices , most unipolar TiO2 devices show an ON/OFF resistance ratio about 10~100, as in the Pt/TiO2/TiN devices reported by Choi and colleagues . Some methods have been investigated to improve the resistance switching performance, such as doping impurities [21, 22], optimizing electrode materials , and controlling the oxygen content of films .
Various models have been proposed to explain the resistive switching phenomenon. Liu et al. reported that the resistive switching performances of Zr+-implanted-ZrO2 films can be explained by charge trapping and detrapping . Electrochemical migration of oxygen vacancies may result in resistive switching in TiO2−δ/La2/3Sr1/3MnO3 stacks . Modification of Schottky barriers with interface states in Pt/Nb-doped SrTiO3 Schottky junction  represents another important resistive switching mechanism. Specifically, the conductive filament model  has been widely accepted to explain the resistive switching behavior in binary oxides. In this model, resistive switching is achieved by formation and rupture of conductive filaments, which have been observed by in situ transmission electron microscopy [29, 30] and local conductivity measurement, due to the migration of charged species on the nanoscale .
In this paper, we report the improvement of resistive switching properties in Pt/TiO2/Pt heterostructures by inserting a LaAlO3 (LAO) layer between TiO2 and Pt bottom electrode. By comparing switching characteristics as a function of LAO thickness, it is proposed that the switching uniformity and ON/OFF resistance ratio can be improved by the formation and rupture of conductive filaments in the LAO layer.
2. Experimental Section
TiO2 thin films and TiO2/LAO bilayers were deposited on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition using a coherent CompexPro 205F KrF excimer laser with a 248 nm output at a repetition rate of 2 Hz. The oxygen pressure and substrate temperature during the deposition were maintained at 0.2 Pa and 350°C for TiO2 and 0.1 Pa and 750°C for LAO, respectively. The thickness of TiO2 was about 50 nm, while that of LAO varied from 10 to 30 nm. Pt top electrodes, 100 μm in diameter, were sputter-deposited at room temperature, using a shadow mask. The surface morphology, chemical composition, and cross-sections of the heterostructures were characterized by an Asylum Research Cypher-ES atomic force microscopy (AFM), a Zeiss ULTRA 55 field emission scanning electron microscope (SEM), and a Thermo Fisher K-Alpha X-ray photoelectron spectroscope (XPS), respectively. The resistive switching behavior was measured using a Keithley 2400 source-measure unit. The bias voltage was applied on the top electrodes while the bottom electrode was always grounded.
3. Results and Discussion
Figure 1(a) shows the surface morphology of TiO2/LAO deposited on Pt/TiO2/SiO2/Si substrates. A smooth surface with a root mean square roughness of about 1.3 nm is obtained over an area of 3 3 μm2. Both TiO2 and LAO layers are deposited below their crystallization temperature [32, 33] and are amorphous as indicated by X-ray diffraction. Figure 1(b) shows the cross-sectional SEM image of a TiO2/LAO sample in which the LAO layer is deposited for 20 min. The total thickness of the TiO2/LAO bilayer is about 80 nm. The deposition rate of LAO is calibrated separately as about 1.5 nm/min. Therefore, the thickness of TiO2 and LAO in this sample is about 50 and 30 nm, respectively. We keep the thickness of TiO2 at 50 nm throughout this paper. Figure 1(c) displays the XPS spectra of the Ti 2p core-level electron in TiO2/LAO. The Ti 2p peak can be decomposed into four components, centered at 457.0, 458.3, 463.1, and 464.0 eV. Two strong peaks correspond to the spin-orbit split of Ti4+ 2p electrons [34, 35]. The other two peaks can be attributed to Ti3+ ions. As estimated from the integrated intensity of Ti4+ and Ti3+ components, there are about 4.5% Ti3+, indicating the existence of oxygen vacancies in TiO2 .
Figure 2(a) shows 50 consecutive I-V curves of Pt/TiO2/Pt devices in a semilogarithmic plot. The devices are initially in the low resistance state. A typical unipolar resistive switching can be observed. However, the critical voltages, at which the resistance toggles between a high and a low level, are quite dispersive. The reset voltage, at which the devices are switched from a low resistance state to a high resistance state, is from 1.3 to 1.7 V, while a voltage between 2.0 and 2.7 V can set the devices into the low resistance state.
We then insert a layer of LAO, 30 nm in thickness, between TiO2 and the bottom electrode. Figure 2(b) shows 50 consecutive I-V cycles of Pt/TiO2/LAO/Pt devices. Contrary to the Pt/TiO2/Pt devices, these devices are initially in the high resistance state. A +8.0 V electroforming voltage with 1 mA current compliance was applied and the resistive switching was triggered at +7.0 V. After the forming process, TiO2/LAO bilayers sandwiched in Pt electrodes exhibit reproducible and uniform unipolar resistive switching characteristics. By sweeping the voltage from zero to about 1.0 V, the current drops abruptly and the devices are switched to the high resistance state. While sweeping the voltage again, a sudden jump of current appears at about 4.7 V, and the devices are set again into the low resistance state. A 10 mA current compliance was applied in the set process. The statistics of the set and reset voltages from 50 switching cycles are shown in Table 1. For the Pt/TiO2/Pt devices, the standard deviations of set and reset voltages are 0.21 V and 0.11 V, respectively. However, after inserting the LAO layer, the standard deviations of set and reset voltages for Pt/TiO2/LAO/Pt devices decrease to about 0.01 V. Although the Pt/TiO2/Pt devices exhibit an ON/OFF resistance ratio about only 102, the Pt/TiO2/LAO/Pt devices could maintain a greatly improved ON/OFF ratio about 103 after 400 times consecutive switching, as shown in Figure 3.
In previous studies, the resistive switching behavior in TiO2 has been reported to be governed by the formation and rupture of conducting filaments, which are formed by percolation of certain kind of charged defects such as oxygen vacancies . Due to the random nature of filaments, uniform switching parameters are difficult to achieve. To improve the switching uniformity, Hirose et al. doped Co into TiO2 thin films  and found that the conductive filaments preferentially formed or ruptured in the vicinity of the impurities. Chang et al.  embedded Pt nanocrystals into TiO2 thin films sandwiched in Pt electrodes and improved the resistive switching uniformity by local enhancement of electric field adjacent to the Pt nanocrystals. Liu et al.  reported that compared with the TiN/TiO2/Pt devices, by inserting a ZnO layer between the TiO2 and Pt bottom electrode, the uniformity can be greatly improved because the distribution and migration of oxygen vacancies across the multilayers can be adjusted by the embedded ZnO layer. This can be used to explain the role that LAO plays in the improvement of resistive switching uniformity. In Pt/TiO2/LAO/Pt devices, the resistivity of LAO is much higher than that of TiO2 due to the wide band gap of LAO (about 5.5 eV). The high resistance LAO layer connected in series with semiconductive TiO2 leads to the initial high resistance state in the Pt/TiO2/LAO/Pt devices. However, positive charged oxygen vacancies in TiO2, as evidenced by the XPS results shown in Figure 1(c), can be driven into the LAO layer to form the conductive paths during the electroforming process. The observation that both Pt/TiO2/Pt and Pt/TiO2/LAO/Pt devices exhibit identical resistance values in the low resistance state supports that conductive filaments are formed in LAO. In the high resistance state, the resistance in Pt/TiO2/LAO/Pt is two orders higher than that of Pt/TiO2/Pt. This indicates the preferential rupture of the conductive filaments in LAO, which sets the LAO layer back into insulator. This also makes the ON/OFF resistance ratio of Pt/TiO2/LAO/Pt 10 times greater than that of Pt/TiO2/Pt devices.
To check the scenario proposed, resistive switching characteristics of Pt/TiO2/LAO/Pt devices with various LAO thicknesses are compared, as shown in Figure 4(a). The resistance of the ON and the OFF states is summarized in Figure 4(b). With increasing the thickness of LAO layer, all the devices show approximately the same resistance values in low resistance ON state, while the resistance value in high resistance OFF state increases. The set voltage shows a strong dependence on the thickness of LAO layer, increasing from 2.2 to 4.7 V with the increase of LAO thickness from 0 to 30 nm. This can be ascribed to the increase of LAO resistance. However, the reset voltage decreases from 1.4 to 1.0 V by inserting the LAO layer and does not change with the thickness of LAO. This indicates that the LAO layer is conductive in the low resistance state. These observations further confirm that the conductive filament is formed and ruptured preferentially in LAO layer.
By introducing a LAO layer into TiO2 and the Pt bottom electrode, we improved the resistive switching characteristics in TiO2 based heterostructures. The dispersion of switching voltage in Pt/TiO2/Pt is greatly suppressed and the ON/OFF resistance ratio has been increased from 102 to 103 by inserting a LAO layer of 30 nm in thickness. XPS measurements reveal an amount of oxygen vacancies in the TiO2 layer. These oxygen vacancies may be driven into the LAO layer, which shows an initial high resistance. The resistance of the high resistance state increases with the LAO thickness, while that of the low resistance state is insensitive of the LAO thickness. These indicate the preferential formation and rupture of conductive filaments in the LAO layer.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was jointly sponsored by the State Key Program for Basic Research of China (2015CB921203), the Natural Science Foundation of China (51222206 and 11374139), and the Jiangsu Provincial Natural Science Foundation (BK2012016).
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