Fabrication of Sb2S3/Sb2Se3 heterostructure for potential resistive switching applications

The exponential growth of large data and the widespread adoption of the Internet of Things (IoT) have created significant challenges for traditional Von Neumann computers. These challenges include complex hardware, high energy consumption, and slow memory access time. Researchers are investigating novel materials and device architectures to address these issues by reducing energy consumption, improving performance, and enabling compact designs. A new study has successfully engineered a heterostructure that integrates Sb2Se3 and Sb2S3, resulting in improved electrical properties. This has generated significant interest in its potential applications in resistive switching. In this study, we have demonstrated the fabrication of a device based on Sb2S3/Sb2Se3 heterostructure that exhibits resistive switching behavior. The device has different resistance states that can be switched between high and low resistance levels when exposed to an external bias (−1 V to 0 V to 1 V). It also has good non-volatile memory characteristics, including low power consumption, high resistance ratio (∼102), and reliable endurance (∼103). The device enables faster data processing, reduces energy consumption, and streamlines hardware designs, contributing to computing advancements amidst modern challenges. This approach can revolutionize resistive switching devices, leading to more efficient computing solutions for big data processing and IoT technologies.


Introduction
Modern society has entered a new era of large data, high speed, and the Internet of Things (IoT), which creates significant challenges for Von Neumann architecture computers and places high expectations on their ability to process data [1][2][3][4][5][6].In order to improve the processing capacity of computers and reduce bottlenecks in the von Neumann architecture, the concept of neuromorphic computing systems has been explored.Therefore, a new memristor system, such as resistive random-access memory (RRAM) based on Resistive switching, has been developed recently [7][8][9].Resistive switching is a phenomenon where the resistance of the material may be switched between high and low states by applying voltage pulses [10].It has two sandwiched terminal configurations, considered good prospects for logic systems, neuromorphic computing, and next-generation non-volatile memory.RRAM still has limitations in achieving high computing power, low energy consumption, and ultrahigh storage density [10].Researchers are actively exploring new materials to improve performance, reduce energy consumption, and create compact designs [11,12].
Several materials, like metal oxide, metal chalcogenides, and metal-organic frameworks (MoF), have been explored for their potential use in resistive switching applications [13].Among them, antimony chalcogenides such as antimony selenide (Sb 2 Se 3 ) and sulfide (Sb 2 S 3 ) have shown promising properties for resistive switching applications.Significant efforts have been made to understand the resistive-switching behavior of Sb 2 Se 3 and Sb 2 S 3 thin films individually [14][15][16][17].However, combining the favorable properties of both materials, a heterostructure composed of Sb 2 Se 3 and Sb 2 S 3 might be promising for resistive switching applications.
In this report, we have synthesized a Sb 2 S 3 /Sb 2 Se 3 heterostructure using the thermal evaporation method.Various characterization techniques have been employed to determine the structural, electrical, and morphological properties of the heterostructure.To analyze the resistive switching behaviour of Sb 2 S 3 /Sb 2 Se 3 heterostructure, a device in vertical architecture was fabricated, which demonstrates various resistance states that may be switched between high and low resistance levels upon exposure to external bias (−1 V to 0 V to 1 V).Additionally, the device exhibits excellent non-volatile memory properties with a notable resistance ratio and reliable endurance (10 3 ) at low power, underscoring its potential for creating advanced resistive switching devices.

Experiment
Highly crystalline Sb 2 S 3 and Sb 2 Se 3 materials were synthesized using the solid-state reaction method.A specific stoichiometric ratio of high purity (99.999%) antimony (Sb), Sulfur (S), and selenide (Se) powder (Sigma Aldrich) were used.Sb-S and Sb-Se were mixed separately and vacuum sealed in a quartz tube at 10 −6 torr, then heated at 800 °C, and 700 °C respectively, for 48 h, resulting in ingots of Sb 2 S 3 and Sb 2 Se 3 , which was confirmed by the Raman spectroscopy.Figure 1(a) further, the synthesized Sb 2 S 3 and Sb 2 Se 3 were crushed into a powdery form, which was used as a precursor to deposit heterostructure of Sb 2 S 3 /Sb 2 Se 3 on Fluorine-doped Tin Oxide (FTO) substrate using thermal evaporation technique at a base pressure of 2.4 × 10 −6 torr (Sb 2 Se 3 and Sb 2 S 3 with the thickness of ∼130 nm and ∼125 nm, respectively).Figure 1(b) prior to deposition, the thermogravimetric investigation was performed on Sb 2 S 3 and Sb 2 Se 3 powders at 250 °C with a ramp rate of 10 °C min −1 to obtain an understanding of the thermal evaporation parameters.
The phase structure, morphology, vibrational characteristics, and optical behavior of the heterostructure were assessed using Al Kα radiation (λ = 1.5406Å) via a Bruker AXS D8 Advance x-ray diffraction (XRD) instrument, Raman spectrometer (Jobin Yvon T64000) employing a 100 mW argon-ion laser with an excitation source at 532 nm, absorption spectra by Avantes ultraviolet-visible (UV-vis) spectrometer, respectively.Further, the surface morphology was examined through field emission scanning electron microscopy (FESEM) using the TESCAN Magna GMH Model.The x-ray photoelectron spectroscopic (XPS) measurements were performed in a Multiprobe Surface Analysis System (Scienta Omicron, Germany) operating at a base pressure of 5 × 10 −11 Torr.High-resolution XPS spectra were recorded using MgKα (1253.6 eV) radiation source.The pass energy of the scans was kept at 20 eV with an ultimate spectral resolution of ∼0.02 eV.The calibration of the spectrometer was performed via a standard gold sample.
To investigate resistive switching beahviour, a vertical device consisting of an Sb 2 S 3 /Sb 2 Se 3 heterostructure sandwiched between FTO and Au electrodes was fabricated.A schematic diagram of the constructed device is shown in figure 1(c).All the measurements were executed using top and bottom electrode Au and FTO, respectively.The current-voltage (I-V) measurements were performed using a probe station equipped with an S10 Triax probe station and a Keithley-2450 Source meter.

Result and discussion
Figure 2(a) illustrates the XRD pattern of the heterostructure deposited by the thermal evaporation method.The graph indicates sharp peaks that correspond to diffraction planes, revealing that the diffraction peaks of Sb 2 Se 3 can be indexed to (112), (123), and (131) with 2θ values of approximately 27.66°, 32.9°, and 35°, respectively.These peaks correspond to the crystallographic planes of the face-centered cubic (FCC) structure of Sb 2 Se 3 (JCPDS card no.072-1184) [30].Similarly, the diffraction peaks of Sb 2 S 3 corresponding indices are (020), (142), (154), and (170), which are from the orthorhombic phases of Sb 2 S 3 (JCPDS card no.42-1393), with 2θ values around 45.94°, 54.6°, 56.3°, and 59.12° [31].The observed peaks align with known crystallographic data for Sb 2 S 3 and Sb 2 Se 3 , proving successful heterostructure formation.Moreover, the vibrational modes of heterostructure were investigated using Raman spectroscopy figure 2(b).The Raman peaks observed at 83.79 cm −1 , 118.1 cm −1 , and 189.6 cm −1 correspond to specific phonon modes of Sb 2 Se 3 [32].These modes are often associated with lattice vibrations, such as Se-Se stretching and Sb-Se bending, reflecting the structural and vibrational properties unique to Sb 2 Se 3 .The characteristic phonon modes of Sb 2 S 3 are evidenced by peaks observed at 253.7 cm −1 , 372 cm −1 , and 449 cm −1 [33].These modes are typically linked to various vibrational patterns within the Sb 2 S 3 lattice, including Sb-S stretching and Sb-Sb stretching, indicative of the specific bonding configurations and structural characteristics of Sb 2 S 3 .The shift in peak position and intensities of the individual materials showed potential interactions and lattice distortions at the heterointerface [30].
Further, to analyze the morphology of the thin film, FESEM characteristic was used figure 2(c).It was observed that the heterostructure exhibits nonuniform morphology (island-like structure), which plays a crucial role in enhancing the performance of resistive switching devices in terms of the formation and dynamics of conductive filaments.This non-uniformity leads to varied local electric field distributions and charge accumulation, facilitating conductive filaments-controlled generation and dissolution during resistive switching.This non-uniformity also offers multilevel resistance states, which are valuable for encoding data in non-volatile memory devices [34].Moreover, the isolated islands act as insulating barriers, reducing sneak path currents and enhancing selectivity in memory arrays [35].Further, the absorption spectra Sb 2 S 3 /Sb 2 S 3 heterostructure investigation reveals a distinct peak of maximum absorption within the visible region of the electromagnetic spectrum, as shown in figure 2(d).This phenomenon shows that the heterostructure effectively absorbs and interacts with the visible and NIR region spectrum [36].
X-ray Photoelectron Spectroscopy technique was used to analyze the surface chemical composition and electronic states of the Sb 2 S 3 /Sb 2 Se 3 heterostructure.Figure 3(a) shows the survey spectrum of the heterostructure in which we can observe that the elements of S, Sb, and Se coexist in the Sb 2 S 3 /Sb 2 Se 3 heterostructure.The elements that were present and their chemical states were determined by examining the binding energies of the core-level peaks.The core level spectra of the Sb element are shown in figure 3(b).Two peaks at 539.3 eV and 530.04 eV binding energies are associated with Sb 3d 3/2 and Sb 3d 5/2 , respectively.Additionally, due to the accidental surface oxidation of Sb 2 S 3 /Sb 2 Se 3 , two minor peaks of Sb 3d 3/2 and Sb 3d 5/2 at 528.9 eV and 538.3 eV are attributed to Sb 2 O 3 [36].The core level spectrum of Se is shown in figure 3(c), and the Se signal exhibits three distinct peaks at energies of 53.8 eV, 54.89 eV, and 52.0 eV, which correspond to Se 3d 3/2 , Se 3d 5/2 , and Se, respectively.The deconvoluted core level spectra of the S 2p (figure 3(d)) reveal the evolution of two binding energy peaks of S 2p at 162.6 eV and 163.8 eV, which belong to S 2p 3/2 and S 2p 1/2 orbitals of S, respectively.
Comparing these binding energy shifts to their bulk values reveals insights into the chemical environment and electronic interactions in the heterostructure.Shifts or broadening of peaks reveal oxidation states and bonding configuration changes, likely due to charge transfer or interfacial processes.
The unique combination of Sb 2 S 3 and Sb 2 Se 3 materials within the heterostructure likely plays a pivotal role in the resistive switching behavior.The Sb 2 S 3 /Sb 2 Se 3 heterostructure was subjected to resistive switching tests to investigate its possible use in resistive switching gadgets.The device was put through voltage sweeps with variable compliance currents to record the I-V characteristics.The resistive switching behavior's ON/OFF ratios, switching voltage, retention, and endurance were examined.
The I-V characteristics of the device showed separate ON/OFF states as a result of the resistive switching experiments shown in figure 4(a).Under external voltage biasing, the devices displayed reversible transitions between high-resistance OFF and low-resistance ON states.The ON/OFF resistance ratios were found to be several orders of magnitude, indicating a stable and dependable switching behaviour.The current of the device was increased by the relatively low switching voltages required to initiate the change between the two states.
The I-V characteristics are replotted in a log(I)-log (V) scale to better understand the conduction process of the device, as illustrated in figure 4(b).The positive sweeps show two separate linearly fitting slope domains, and it observed from the I-V relation which is nearlyOhmic (I α Vi.e. the conductive law of low resistance state (LRS)).This result is consistent with the conductive filament mechanism, which states that the Au and FTO are connected via the formation of conductive filaments [37].A similar trend was followed in high resistance state (HRS) states.On the application of high voltage, the thermally generated carriers increase.Due to the high density of these carriers, the slope of fitting increases by more than one, corresponding to the Child's square law (I µ V 2 ) [37].Additionally, figure 4(c) illustrates that the device repeats cycles I-V characteristics, and the stable bipolar RS behaviour throughout the 500 cycles was demonstrated.Further, the endurance characteristics of the devices were evaluated using the consistency of the ON/OFF states over a number of 500 Cycles.Figure 4(d) displays the change in resistance of states for 500 cycles, which is nearly constant.The device exhibited reasonable endurance behaviour by maintaining their resistance states (order of resistance in HRS 10 5 and for LRS 10 4 ) [38].The device also showed durability competencies by withstanding many switching cycles without visibly deteriorating performance.Additionally, the retention of the device was evaluated (shown in the inset figure 4(c)).The resistance of the device remained unchanged for 10 3 sec which illustrates the nonvolatility of the memristor by allowing the device to be consistently maintained in either the HRS or the LRS [39].These findings underscore the practicality of the Sb 2 S 3 /Sb 2 Se 3 heterostructure-based device in-memory applications, emphasizing its capacity for enduring multiple read/write cycles, maintaining data integrity without power, and exhibiting long-term retention of stored information.Such characteristics are critical for developing nextgeneration non-volatile memory devices, offering high-density storage, low-power operation, and superior reliability for many memory-centric applications.
The postulated fundamental mechanism governing the resistive switching phenomenon within the asprepared device is elucidated herein.This mechanism is conjectured to entail the migration of ions and vacancies within the Au/Sb 2 S 3 /Sb 2 Se 3 /FTO device.In the absence of an external power supply, as depicted in figure 5(a), a discernible absence of ion and vacancy movement is observed.When the negative voltage is applied to the top electrode (Au) while connecting the lower electrode (FTO) to a positive terminal, a noticeable transformation ensues, as portrayed in figure 5(b).During this process, vacancies of S/Se species are generated and traverse toward the upper electrode, while S 2− /Se 2− ions commence migration toward the lower electrode in the heterostructure, as visually represented in Figure (b).These migrating vacancies of S/Se entities serve as the foundation for developing slender, electrically conductive filaments [40].These filaments facilitate the transit of electrons from the Au electrode to the FTO electrode, thereby inducing a transformation of the device's electrical state from HRS to LRS, as illustrated in figure 5(b).Conversely, when the polarity of the upper and lower electrodes is reversed, the conductive filaments undergo deformation due to the recombination of vacancies and ions as demonstratively portrayed in figure 5(c).This event reverses the electrical state of the device from LRS to HRS.Thus, it is distinguished that the principal mechanisms underpinning the operation of the Au/Sb 2 S 3 /Sb 2 Se 3 /FTO synaptic devices are intricately linked to the ionic movement, the generation and dissolution vacancies species, as well as the formation and subsequent destruction of conductive filaments [41,42].

Conclusion
In summary, the Sb 2 S 3 /Sb 2 Se 3 heterostructure shows promise for resistance-switching devices that can toggle between resistance states in response to an external bias (−1 V to 0 V to 1 V).Notably, the device boasts advantageous characteristics, including low power consumption, a substantial resistance ratio (∼10 2 ), and robust endurance (∼10 3 ).The device holds potential applications in various sectors, including logic circuits, neuromorphic computing, and non-volatile memory, offering the tantalizing prospect of high-density, lowenergy memory solutions.This successful demonstration not only validates the efficacy of the Sb 2 S 3 /Sb 2 Se 3 heterostructure but also points towards its potential integration into practical applications, laying the foundation for the development of cutting-edge electronic devices with enhanced performance and reliability.The resistive switching behavior central to these devices hinges on the growth and decay of conductive filaments within the material, leading to alterations in its resistance properties.The generated knowledge advances the fundamental understanding and opens doors to practical applications in electronics and nanotechnology.

Figure 1 .
Figure 1.(a) Synthesis of materials via solid-state reaction in a vacuum-sealed tube; (b) Thin film deposition process utilizing thermal evaporation method; (c) Schematic representation of the fabricated device for resistive switching applications.

Figure 3 .
Figure 3. (a) Survey scan (b)-(d) Core level spectra of present elements such as Sb, S, Se.

Figure 4 .
Figure 4. (a) I-V characteristics of the Sb 2 S 3 /Sb 2 Se 3 heterostructure-based devices (b) re-plotted I-V in terms of log(I)-log(V) (c) I-V characteristic of the device with the specific number of sweep cycles (inset Retention of the device) (d) Endurance of the device.