Phase change materials, which show different electrical characteristics across the phase transitions, have attracted considerable research attention for their potential electronic device applications. Materials with metal‐to‐insulator or charge density wave (CDW) transitions such as VO2and 1
The metal-to-insulator transition is a hallmark phenomenon predicted by Peierls' theory for explaining charge density wave (CDW) in ideal one-dimensional metals 1,2 . However, this transition is not a universal feature in real CDW materials, with some systems failing to exhibit transport anomalies at the transition temperatures 3,4 . Despite over half a century of study into CDW phases and phase transitions, much of the research has centered on the physical aspects such as the driving force behind CDW [5][6][7] and its relation to superconductivity [8][9][10] , rather than exploring potential electronic device applications. Nonetheless, CDW systems with hysteric resistive phase transitions have the potential to offer unique opportunities for novel electronic device development 11,12 . One such system that has attracted significant attention is the quasi-twodimensional Mott insulator 1T-TaS2, which exhibits several CDW phase transitions with resistivity changes and hysteresis and has been utilized to construct electronic devices such as phase change oscillators [13][14][15] and memristors 16,17 , both of which are key device elements to achieve energyefficient neuromorphic computing 18 .
BaTiS3 is a quasi-one-dimensional small bandgap semiconductor 19 that has recently shown unique resistive phase transitions 20 , including one CDW transition near 250 K and another structural transition emerging at even lower temperatures (from 120 K to 150 K during cooling cycle). Upon cooling from room temperature, the system switches from a semiconducting state to a CDW state, resulting in an increase in electrical resistivity, bandgap opening, and a periodic lattice distortion 20 . This change in electrical resistivity presents opportunities to create devices that can be modulated by external stimuli such as electrical and optical fields. In this study, we demonstrate threshold resistive switching behavior with negative differential resistance (NDR) utilizing the CDW phase transition in bulk BaTiS3 single crystal. The electrical switching mechanism was extensively investigated through temperature-dependent current-voltage (I-V) characteristics and pulsed I-V measurements. Furthermore, voltage oscillations with a frequency close to 1 kHz were observed from a two-terminal BaTiS3 device. Potential strategies for optimizing device performance, such as reducing channel sizes and optimizing thermal management, were also explored. Our findings shed light on electronic device applications in CDW systems.
The phase transition of BaTiS3 near 250 K, referred to as the CDW transition, results in an abrupt increase in resistance and a thermal hysteresis of over 10 K, both of which are crucial for its potential applications as an electronic device. Figure 1a To demonstrate the resistive switching behavior in BaTiS3, the device was initially set to the high-resistive CDW state near the completion of the CDW transition (230 K). A DC currentvoltage characterization was performed on a two-terminal BaTiS3 device by sweeping voltage (Vmode), as illustrated in Figure 1c. The system exhibited a transition to a more conductive state above a certain threshold voltage VF during forward scan, and it returned to its original highresistive state below another critical voltage VR (VR < VF) during reverse scan, forming a characteristic hysteresis window. Additionally, 'S-type' negative differential resistance regions (dV / dI < 0) were observed during transitions when testing in current mode (I-mode) by sourcing current, as shown in Figure 1d. The onset of NDR in BaTiS3 is directly associated with the phase transition and the critical voltages VF and VR extracted from I-mode are consistent with those obtained via voltage sweeps. This threshold resistive switching behavior with NDR has been previously observed in other crystalline phase-change systems such as VO2 [21][22][23] and 1T-TaS2 13,24,25 , and is utilized to construct electronic devices such as oscillators 13,14,22,23 . The 'S-type' NDR behavior in those systems can be either attributed to the formation of a current path caused by the local inhomogeneities 26,27 , related to defects that pin CDW condensates 28 , or due to the coupling between CDW domains 29 , as well documented in the literature. In the CDW compound BaTiS3, one cannot ignore the contributions from defects on interpreting NDR. Further experimental and theoretical studies are needed to pin down its mechanism.
The mechanism behind such electrical voltage / current resistive switching behavior in those systems is primarily attributed to the local temperature rise beyond the transition point due to Joule heating 25,30,31 , although there are ongoing debates regarding the potential role of electrical field effect 24 and Mott transition 22 . The effect of Joule heating is highly dependent on the specific material, device structure, and bias mode (DC or pulse). In this experiment scheme, where a BaTiS3 crystal is embedded in a polymer medium with low thermal conductivity and the switching is triggered by DC sweeps, the local Joule heating can be substantial.
To evaluate the contribution of Joule heating to the resistive switching behavior observed in BaTiS3, we conducted four-probe I-V sweeps at various temperatures across the CDW transition (Figure 2a). The results show that the critical voltage required to switch the resistance state increases as temperature decreases, and there is no threshold voltage switching observed at a temperature of 260 K. The thermal power generated by Joule heating at threshold fields were calculated ( 𝑃 !" = 𝑉 !" × 𝐼 !" ) and found to exhibit a linear relationship with temperature, as illustrated in Figure 2b and 2c. Two characteristic temperatures of 245 K and 258 K were identified at which the threshold thermal power approaches zero, which aligns with the transition temperatures observed in the temperature-dependent resistance measurements (Figure 1a). This analysis suggests that the resistive switching in BaTiS3 is primarily driven by Joule heating.
Moreover, pulsed I-V measurements with varying pulse widths were performed to gain insights into the switching mechanism by deconvoluting the contributions from Joule heating and electrical field effects. Figure 2d shows the results of pulsed I-V measurements conducted on a two-terminal BaTiS3 device at 210 K. Voltage pulses with a width of 8 ms and a pulse period of 10 ms were swept between 0.8 V to 1.8 V. In such pre-defined voltage pulses, less than 10% of the total time was used for cooling. Hence, similar to that observed in DC sweeps, the hysteretic switching behavior persisted, as evidenced by the asymmetric measured current profile. To reduce the contribution of Joule heating, the pulse width was decreased from 8 ms to 1 ms while maintaining the voltage sweep ranges and pulse period. Figure 2e illustrates the reconstructed I-V curves from pulsed measurements for different pulse widths. As the pulse width was decreased, the width of the hysteresis window reduced while the switching voltage increased. No hysteresis was revealed in the I-V curves when the pulse width was 1 ms. These observations are consistent with the hypothesis of a thermally driven transition, as the Joule heating power decreased with decreasing pulse width, while the electric field applied to the BaTiS3 device remained the same.
The capability of inducing sustained phase change-based voltage oscillations is crucial for constructing electronic devices such as oscillators. Systems such as 1T-TaS2 and VO2 have shown promising results, with the potential for GHz-level switching speeds theoretically predicted 30,32 and experimentally demonstrated at MHz frequencies 13,22 . In the case of BaTiS3, we observe stable voltage oscillations at frequencies from 16 Hz to 18 Hz when a two-terminal BaTiS3 device is connected in series with a load resistor (RS = 11.25 kOhm) and a parallel capacitor (CP = 10 µF) and subjected to a DC bias between 16 V and 23 V. The voltage oscillation is illustrated in Figure 3b, with the I-V characteristics at 220 K and circuit diagram of the oscillation measurements shown in Figure 3a, together with the load line of the resistor. This oscillation behavior is achieved by leveraging the threshold resistive switching behavior as discussed above, and it can be understood as the periodic changes in electrical resistance as a result of the repetitive cycles of local heating and cooling processes across the phase transition. When the applied constant DC voltage reaches certain value and hence the voltage drop across the BaTiS3 channel exceeds the critical voltage VF, a transition to the low-resistance state is triggered due to Joule heating (P =
), resulting in a sudden increase in current and a subsequent increase in voltage across the load resistor. The Joule heating power P is not sufficient to sustain the temperature after the transition, because P decreases as R is reduced for R < RS. This drives the BaTiS3 device back into the CDW state, and the cycle repeats, leading to sustained voltage oscillations. The oscillation is not sustained when the DC voltage exceeds 23 V or falls below 16 V.
On the other hand, although stable voltage oscillations were obtained for the first time from the CDW transition in BaTiS3, its frequency remains orders of magnitude lower than the MHz level achieved in VO2 and 1T-TaS2 systems. This low switching speed is primarily attributed to the poor heat dissipation of the bulk BaTiS3-polyimide system, which has a low thermal conductivity and results in an inefficient cooling process that limits its overall performance, despite the effective heating procedure. This is evidenced by the asymmetric heating and cooling cycles of voltage oscillations with a much longer cooling time, as revealed in Figure 3b.
One approach to enhance the switching speed is by improving the cooling efficiency of the system, for example, by decreasing the operating temperature. Figure 4a to 4c plot the voltage oscillations of the same BaTiS3 device measured at 200 K, 170 K and 130 K, respectively, with an observed increase in frequency from 67 Hz to 910 Hz. However, it is difficult to maintain this stable CDW oscillation when the temperature is further reduced, as the low-temperature structural transition in BaTiS3 begins to interfere and complicate the results. Thus, it is challenging to significantly improve the oscillator performance solely by tuning the measurement temperatures.
Another strategy to further enhance the oscillation frequency is to decrease the size of the device channel, as has been proven effective in other oscillating systems, such as VO2 33 . In early days, the voltage oscillation frequency in millimeter-scale VO2 bulk single crystals was only around 5 kHz 34 , while nowadays, MHz-level oscillation frequencies have been achieved in VO2 thin film devices with sub-micron channels 35 . Figure 4d plots the oscillation waveforms from two different BaTiS3 devices with channel sizes of 10 µm and 5 µm, respectively, both of which were measured at 170 K for direct comparison. With reduced channel size down to 5 µm, the oscillation frequency increases more than three times compared to the 10 µm BaTiS3 device. This effect can be understood as the improved efficiency for both cooling and heating processes as the channel size decreases. Further reduction of sample sizes both laterally and vertically is expected to result in even higher oscillation frequencies in BaTiS3.
In conclusion, we have shown reversible threshold switching in the recently discovered CDW system BaTiS3, driven by DC voltage or current, whose mechanism is consistent with a Joule heating scheme. Moreover, sustained voltage oscillations were achieved in BaTiS3 based on bistate resistive switching between the semiconducting phase and CDW phase. The oscillation frequencies were improved through appropriate thermal managements and reduced channel sizes.
Our work on BaTiS3 opens new opportunities in electronic device applications of CDW phase change materials beyond 1T-TaS2.