Resistive switching studies in VO2 thin films

The hysteretic insulator-to-metal transition of VO2 is studied in detail for pulsed laser deposition grown thin films on TiO2 substrates, under variation of temperature and applied bias currents. This system is of interest for novel electronics based on memristive concepts, in particular as the resistive transition in these films occurs close to room temperature. Multiple, stable resistance states can be set controllably in the temperature range of the hysteretic phase transition by tailored temperature sweeps or by Joule heating induced by current pulses.

. In such Mott insulators, large resistance changes can be achieved e.g., through Joule heating near the MIT temperature or by carrier doping using electric field gating 5,21,22 . In particular vanadium-oxide (VO 2 ) is an alluring resistive switching material, due to its very sharp, hysteretic MIT close to room temperature. Employing tailored temperature sweeps, multiple history-dependent resistance values can be defined 11,21,[23][24][25][26][27] , as we will show below. Recently, Del Valle et al. have shown that the resistance values of vanadium-oxide films grown by RF magnetron sputtering on sapphire substrates can be tuned by electric field pulses, attributed to Joule heating effects when the pulses are applied at a temperature just below the MIT and by filamentary changes in the crystallographic configurations when the pulses are applied at considerably lower temperatures 25 . It is of interest to further explore this and to study the current-and temperature-induced changes in the resistance for the entire temperature range spanning the hysteretic metal-to-insulator transition and to explore in which ways multiple stable states can be induced. Both aspects are addressed here. In doing this, we also extend the range of deposition techniques and substrate materials.

Methods
For our studies, epitaxial VO 2 thin films showing a metal-to-insulator transition near room temperature were deposited on single crystal TiO 2 (001) substrates using pulsed laser deposition, using a KrF excimer laser (λ = 248 nm, 20 ns pulse duration) and employing reflection high energy electron diffraction (RHEED) monitoring. The films were deposited at a temperature of 420 °C and in an oxygen background pressure of 1.2·10 −2 mbar, by ablation from a V 2 O 5 target with an energy density of ~1.3 J/cm 2 and a pulse repetition rate of 2 Hz. After deposition, the samples were cooled at 10 °C/min at the same oxygen pressure. The high crystalline quality was verified using X-ray diffraction (XRD) (in both θ-2θ and reciprocal space mapping) and atomic force microscopy. A typical film thickness of ~10 nm was used, confirmed by X-ray reflectivity measurements. The electronic characteristics were measured in a four probe geometry, employing RF sputtered Ti-Au contacts defined   www.nature.com/scientificreports www.nature.com/scientificreports/ this manuscript, we will refer to these transition temperatures as T 1 and T 2 , respectively. Further, we indicate with T INS & T MET temperature ranges at which the material is in a completely insulating or metallic state, respectively. Figure 2(a,b) shows the stable intermediate resistance states in the R(T) curves that were obtained when the temperature-sweep was reversed before completing the full transition, corroborating results obtained for sol-gel deposited films on sapphire 27 . After an initial relaxation, characterized by a long (i.e. several minutes) time scale (Fig. 2c), the intermediate resistance stabilized as shown in Fig. 2d. It is known that the overall metal-to-insulator transition coincides with a structural transition from a conducting rutile phase at high temperatures to an insulating monoclinic phase at low temperatures, concomitant with the opening of a band-gap in the band structure due to vanadium-vanadium dimerization 28,29 . In the hysteretic region there is a coexistence of metallic and insulating domains, as was seen in infrared microscopy 30,31 . If at a given intermediate resistance value within the MIT, the temperature is reversed and maintained between the limiting temperatures of the hysteresis loop for that  www.nature.com/scientificreports www.nature.com/scientificreports/ resistance, the metallic puddle structure is stabilized 27 . This explains why in Fig. 2(a,b) the resistance remained roughly constant at the intermediate set-values when the temperature sweep was reversed at the corresponding point in the transition.

Results and Discussion
The voltage vs. current V(I) characteristics show a qualitative development as a function of temperature as displayed in Fig. 3. At high temperatures, in the T MET regime, Ohmic behavior is seen. When reducing temperature, in addition to the appearance of non-Ohmic behavior, hysteresis starts to occur systematically, which gets stronger as temperature is decreased. While on the linear scale of Fig. 3 it appears that the resistance around zero current/voltage has only a single value independent of the history of the current scan, a plot on a logarithmic scale (Fig. 4) reveals that the high and low resistive states actually stay distinct 23 . The V(I) characteristics entail a region with negative differential resistance (indicated in Fig. 3). It should be noted though that when reversing the current direction within this region, the V(I)-characteristics do not trace back along the same line, but close the hysteresis loop.
The resistive switching effects are symmetric for positive and negative current bias directions, as can be expected for a thermally driven process. When completing a sweep for one current bias polarity and repeating this without going first to the opposite, nearly the same trace was obtained, i.e. first a high resistance state was seen before the critical current was reached at which the resistance decreases. This implies that under these biasing conditions the system restores to a large extend to the higher resistance state near low bias. When measuring the V(I) characteristics starting at low temperatures and warming up, similar behavior was found as compared to measurements during cool down, as follows also from Fig. 3.
The V(I) characteristics of Figs. 3 and 4 were taken at temperatures close to the MIT. It is to be noted that the characteristics within this temperature range are different from the resistive switching observed at lower temperatures, further away from the MIT [32][33][34][35][36] . In the switching displayed in Figs. 3 and 4, electroforming and ion/defect migration are not expected to play a role. In line with this, the switching does not present a typical forming-set-reset loop and there is no need for opposite polarity to reset the switching. Also, switching is highly reversible with biasing and temperature, excluding all possibilities of permanent changes in the material or at interfaces. A more likely mechanism for the observed resistive switching is Joule heating, which can also lead to filamentary conductance paths 21,37 . www.nature.com/scientificreports www.nature.com/scientificreports/ To study the bias-current dependence in more detail, the voltage was recorded for subsequent sweeps with varying return currents, as shown in Fig. 5. Figure 5(b) shows the behavior at 307 K, which is still within the MIT. The resistance changed each time when a higher current was applied. This memristive behavior is similar to that reported in 27 . Figure 5c shows the V(I) characteristics at a lower temperature (297 K). Clearly, as the current range is gradually increased, the heating effects lead to large and somewhat irregular hysteresis loops, suggesting more a avalanche-like formation and partial reconstruction of conducting filaments.
To explore further, we have also obtained current vs. voltage I(V) characteristics at the comparatively low temperature of 297 K, shown in Fig. 6. Interestingly, a sharp jump in the first sweep was observed and all subsequent curves show similar hysteric behavior as was observed in Fig. 5. The first sweep, going down to −10 V, gives rise to a typical resistive forming step 21,35 . This sharp resistive switching could be exactly reproduced each time after fully cycling the temperature to above the MIT hysteresis region. Therefore, this switching is also due to heating effects, in accordance with 21 , and not due to defect migration.

Conclusion
In conclusion, our studies underline that thermally induced phase transitions, either invoked by temperature sweeps or by current-induced Joule heating, provide useful pathways to create tunable resistors based on VO 2 . It should be noted though that the speed if this process is relatively slow, due to the inherent long time-scale of thermal processes and possible slow reconstruction processes within the sample after setting a certain resistance value. An advantage of the VO 2 system over ionic migration-based devices is that by sweeping the temperature to above the metal-to-insulator transition, the written configuration can be completely erased. A prospect for future developments is the local writing of functional electronic circuits, including neuromorphic circuitry, by e.g. laser beams or heated AFM tips.