Charge‐Density‐Wave Resistive Switching and Voltage Oscillations in Ternary Chalcogenide BaTiS3

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 VO2 and 1T‐TaS2 have demonstrated voltage oscillations due to their robust bi‐state resistive switching behavior with some basic neuronal characteristics. BaTiS3 is a small bandgap ternary chalcogenide that has recently reported the emergence of CDW order below 245 K. Here, the discovery of DC voltage / current‐induced reversible threshold switching in BaTiS3 devices between a CDW phase and a room temperature semiconducting phase is reported. The resistive switching behavior is consistent with a Joule heating scheme and sustained voltage oscillations with a frequency of up to 1 kHz are demonstrated by leveraging the CDW phase transition and the associated negative differential resistance. Strategies of reducing channel sizes and improving thermal management may further improve the device's performance. The findings establish BaTiS3 as a promising CDW material for future electronic device applications, especially for energy‐efficient neuromorphic computing.


Abstract
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 VO2 and 1T-TaS2 have demonstrated voltage oscillations due to their robust bi-state resistive switching behavior with some basic neuronal characteristics. BaTiS3 is a small bandgap ternary chalcogenide that has recently reported the emergence CDW order below 245 K. Here, we report on the discovery of DC voltage / current-induced reversible threshold switching in BaTiS3 devices between a CDW phase and a room temperature semiconducting phase. The resistive switching behavior is consistent with a Joule heating scheme and sustained voltage oscillations with a frequency of up to 1 kHz has been demonstrated by leveraging the CDW phase transition and the associated negative differential resistance. Strategies of reducing channel sizes and improving thermal management may further improve the device performance. Our findings establish BaTiS3 as a promising CDW material for future energy-efficient electronics, especially for neuromorphic computing.

Main Content
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 5-7 and its relation to superconductivity 8-10 , 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 .
BaTiS3 is a quasi-one-dimensional small bandgap semiconductor 18 that has recently shown unique resistive phase transitions 19 , 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 19 . 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 4 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.

Reversible resistive switching
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 plots the temperature-dependent resistance of BaTiS3 along the c-axis from 220 to 280 K, with an inset showing an optical image of a typical multi-terminal BaTiS3 device with varying channel sizes. Structurally, the unit cell doubles along the a-and b-axis across the transition (a = 2a0, b = 2b0, c = c0), as depicted in Figure 1b.
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. This threshold resistive switching behavior with NDR has been previously observed in other phase change systems such as VO2 20-22 and 1T-TaS2 13,23,24 , and is utilized to construct electronic devices such as oscillators 13,14,21,22 .

Mechanism of electrical switching
The mechanism behind such electrical voltage / current resistive switching behavior in those systems is believed to be primarily due to local Joule heating [24][25][26] , although there are ongoing debates regarding the potential role of electrical field effect 23 and Mott transition 21 . 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 ( !" = !" × !" ) 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.

CDW voltage oscillations
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 25,27 and experimentally demonstrated at MHz frequencies 13,21 . 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 (RL = 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 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. (d) Effect of device channel size. The oscillation frequency increases more than three times when reducing the channel size from 10 µm to 5 µm.

Effect of thermal management and channel sizes
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 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 28 . In early days, the voltage oscillation frequency in millimeter-scale VO2 bulk single crystals was only around 5 kHz 29 , while nowadays, MHz-level oscillation frequencies have been achieved in VO2 thin film devices with sub-micron channels 30 . 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.

Conclusion
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 11 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.

Device fabrication
Single crystals of BaTiS3 were grown by a vapor transport method as reported elsewhere 18,19 .

Electrical characterization
Electrical transport measurements were performed in a JANIS 10 K closed-cycle cryostat.
Temperature dependent resistances of BaTiS3 as shown in Figure 2a were

Conflict of interest
The authors declare no competing financial interests.