B. Raju Naika,
Yadu Chandrana,
Kakunuri Rohinia,
Divya Vermaa,
Shriram Ramanathanb and
Viswanath Balakrishnan*a
aSchool of Mechanical and Materials Engineering, Indian Institute of Technology, Mandi, Himachal Pradesh 175075, India. E-mail: viswa@iitmandi.ac.in
bDepartment of Electrical and Computer Engineering, Rutgers State University, New Jersey, Piscataway, NJ 08854, USA
First published on 11th June 2024
Phase changes in oxide materials such as VO2 offer a foundational platform for designing novel solid-state devices. Tuning the V:O stoichiometry offers a vast electronic phase space with non-trivial collective properties. Here, we report the observation of discrete threshold switching voltages (Vth) with constant ΔVth between cycles in vanadium oxide crystals. The observed threshold fields over 10000 cycles are ∼100× lower than that noted for stoichiometric VO2 and show unique discrete behaviour with constant ΔVth. We correlate the observed discrete memristor behaviour with the valence change mechanism and fluctuations in the chemical composition of spatially distributed VO2–VnO2n−1 complex oxide phases that can synergistically co-operate with the insulator–metal transition resulting in sharp current jumps. The design of chemical heterogeneity in oxide crystals, therefore, offers an intriguing path to realizing low-energy neuromorphic devices.
New conceptsThis study investigates insulator–metal transition (IMT) materials in the V:O system for neuromorphic applications. Unlike prior focus on phase pure compounds, this work explores chemically heterogeneous vanadium oxide crystals. It unveils discrete threshold switching voltages (Vth) with constant ΔVth in both in-plane and out-of-plane test structures. The observed fields are ∼100× lower than for stoichiometric VO2, showcasing unique behavior over 10000 cycles. This behavior can be correlated with valence change mechanisms and chemical composition fluctuations in spatially distributed VO2–VnO2n−1 phases. The cyclic bipolar switching stability test reinforces discrete threshold voltages, confirming valence change mechanism-based memristor performance. Variations in Magnéli phases during voltage cycling guide switching events with equal ΔVth. Chemical heterogeneity and valence state fluctuations in the vanadium oxide system suggest an interesting platform for threshold switching memristor devices. |
This article demonstrates a threshold-switching device with discrete Vth voltages in chemically heterogeneous vanadium oxide samples of in-plane and out-of-plane device geometries. Discrete switching at ultra-low voltages in non-stoichiometric vanadium oxide with spatial distribution of the VO2 and VnO2n−1 heterophase is caused by electrically driven compositional fluctuations. Switching tests conducted over 10000 I–V cycles show reduced discrete switching points, and eventually, the discrete Vth voltages are stabilized to a single point. The discrete stochastic switching mechanism was explained by population dynamics of electrochemical species and fluctuations in their valence states under applied electric field stress. This implies that during continuous electric bias switching cycles, the vanadium oxide crystal experiences compositional changes dynamically. Microstructural design of heterogeneity in the V:O system is a promising direction to explore for threshold switching devices with stochastic threshold voltages that are discretized over a large number of cycles. The investigated aspects are also relevant for the fabrication of stoichiometric VO2 thin film devices to enable stable threshold switching over a large number of cycles under optimal unipolar electric fields.
In addition, the heterogeneity of the crystalline VO2 was verified and confirmed using various techniques, including energy dispersive spectroscopy (EDS) (Fig. S4a and b, ESI†), differential scanning calorimetry (DSC) heating and cooling curves (Fig. S5a–c, ESI†), and in situ electrical and mechanical testing (Fig. S6a–c, ESI†). All these analyses confirm that the as-grown crystal is not fully stoichiometric VO2; instead, it is non-stoichiometric, with the majority of the volume fraction dominated by stoichiometric VO2. We conducted similar experiments on polycrystalline samples to verify this unique discrete behaviour. Fig. 2a shows the schematic of the electrical measurements conducted in an out-of-plane geometry, with polycrystalline (PC) VO2 sandwiched between a bottom platinum (Pt) electrode and top Ti/Au electrodes. The obtained I–V characteristics for a repetitive cyclic voltage test over 100 cycles display similar conspicuous discreteness in Vth voltages, as shown in Fig. 2b. Thus, the device Vth voltages are highly unstable and bounce back and forth with multiple I–V cycles. Interestingly, the ΔVth between cycles remains constant for the polycrystalline VO2 device. The same is consistent with our observation on planar VO2, mentioned earlier. The scatter plot shows a ΔVth of 1.02 V over 10000 cycles (Fig. 2c). Despite the differences in device geometries and crystallinity of the VO2 devices, they exhibit the same unique discreteness in Vth with equal ΔVth behavior during I–V cycles. The PC-VO2 crystals were further verified by microscopic and spectroscopic analysis. Fig. 2d shows the SEM image and indicates the microparticle nature of the PC-VO2. Fig. 2e shows the XPS analysis with multiple oxidation states +3, +4, and +5. To elucidate the compositional variation in the vanadium oxide crystal, we have performed XPS depth profiling, as shown in Fig. S7, ESI.† It is observed that the top surface contains more +5 oxidation state along with dominant +4 oxidation state while only negligible +5 oxidation state is observed after 100 nm depth. The XRD analysis shown in Fig. 2f confirms that VO2 predominantly dominates the sample. Based on the electrical measurements, it is possible to claim that low-field switching and discrete switching over many cycles are dominated by ionic transport, supported by sweep polarity-dependent electrical I–V measurements. Fig. 3a shows I–V characteristics during a positive voltage sweep with multiple threshold switching points. Fig. 3b shows stabilized I–V characteristics within 900–1000 cycles, and the scatter plot in Fig. 3c shows Vth distribution over 1000 cycles. The off-state currents obtained in the I–V characteristics from Fig. 3a display a current of 0.296 mA before cyclic stabilization, while Fig. 3b exhibits an off-state current of 1.642 mA after stabilization. This is also true for negative polarity I–V cycles, as shown in Fig. 3d–f. This suggests that IMT is a dominant mechanism after stabilization, and valence change-based filament formation plays a dominant role before cyclic stability. This provides clear insights into the electroforming of conducting filaments with the aid of charge transfer and also reveals the effect of polarity on stochastic threshold switching during voltage sweeps. Dark and bright pattern motion dynamics further verify these compositional changes on the VO2 crystal under microscopy during heating–cooling cycles (Fig. S8a–g, ESI†). These motion dynamics are observed near room temperature and differ from the typical contrast variations from insulator–metal transition near 68 °C in the VO2 crystals (Fig. S9a–c, ESI†).
In earlier works, a systematic reduction in threshold voltage has been demonstrated by introducing oxygen vacancies in VO2 by controlled annealing.23 Oxygen vacancies have been extensively studied for their effect on electrical transport in VO2. For instance, excess oxygen vacancies increase the carrier concentration and promote metallicity, suppressing abrupt phase transition in VO2.24 Electrochemical creation and annihilation of oxygen vacancies at the nanoscale by applying an electric field through the AFM tip on VO2 is known to affect the transport properties.25 The approach of engineering the stoichiometry in VO2 by introducing either oxygen vacancies or excess oxygen has been explored for stabilizing rutile or triclinic phases.18,26 Inherent structural stochasticity with four variants of the M1 phase during insulator–metal transition in VO2 has been observed via in situ electrical triggering in TEM. However, it is unclear how the structural stochasticity is directly related to the threshold voltages, especially when a greater number of discrete threshold voltages are observed beyond four structural variants.27 Furthermore, the electric field triggered the formation of V5O9, conducting filaments in VO2, also reported to contribute to non-volatile switching at low threshold voltages.28,29 Such non-stoichiometric phases promote resistive switching at very low voltages despite large channel lengths. Earlier reports found that a stoichiometric VO2 demanded significantly high threshold electric fields in the range of 106 to 107 V m−1.30 Conversely, chemically heterogeneous VO2 shows a remarkable reduction of electric fields compared to stoichiometric VO2. The estimated threshold electric fields for switching the device vary between 4 × 104 V m−1 and 21 × 104 V m−1 within the investigated 10000 I–V cycles for VO2 planar devices with a channel length of ∼40 μm. Similarly, very low threshold electric fields, varying between 0.6 × 104 V m−1 and 2.6 × 104 V m−1, are observed over 10000 repeated I–V cycles in a heterogeneous polycrystalline VO2 film with a thickness of 250 nm in the out-of-plane geometry. Note that the estimation of threshold electric fields is carried out with the in-plane channel length of ∼40 μm, and in reality, the actual channel length might be much lower as the conducting oxygen deficient Magnéli phase is spatially distributed.
We correlate the mechanism of low Vth voltages that show discreteness with constant ΔVth with dynamic fluctuation in composition and related shuttling of oxygen ion vacancies. Fig. 4 shows a schematic of the mechanism for discrete threshold voltages during reversible I–V sweeps. Fig. 4a and b show a representative I–V hysteresis curve with six segments of operation, and crystalline VO2 with dark and bright patterns, respectively. Segment I–III is for a positive voltage sweep and IA–IIIA for a negative voltage sweep. During part I, the reduction process occurs with the creation of oxygen vacancies. As the voltage increases, a conducting filament by VnO2n−1 starts forming by migration of oxygen vacancies across VO2, as shown in Fig. 4c. The VnO2n−1 (bright region) filament formation is due to small volume fractions of Magnéli phases between VO2 (dark region) as shown in Fig. 4b marked region. This filament formation increases the current rapidly, and Joule heating becomes significant to drive the subsequent growth of the conducting filament.31 These filaments act like a guiding path and their formations are sensitive to these dynamic fluctuations in oxygen ion vacancy concentration. In segment II, the insulator–metal transition is triggered once the required threshold voltage is reached, leading to an abrupt increase in current. During segment III, oxidation occurs, and these filaments start rupturing by annihilation of oxygen vacancies by migration of oxygen ions, which results in an abrupt decrease in current (Fig. 4d). The above processes are also valid in the negative voltage direction. Since the reduction process and filament formation require more supply of oxygen vacancies over repeated electric cycles, the filament formation path becomes random, generating several Vth. On the contrary, for rupturing, even a minute annihilation of oxygen vacancies can break the conducting filament. During repeated electrical cycles, the conducting filament thickness increases, which results in a decrease in Vth, which is evident from the results of both planar and out of plane VO2 samples.
Nevertheless, the origin of discreteness in Vth with an equal voltage difference between each consecutive current–voltage characteristic of VO2 needs further validation. It is possible to correlate the electrochemical redox reaction during the electrical triggering and changes in threshold voltage. We use the Nernst equation to understand the mechanism described below.32
(1) |
(2) |
Assuming the constant E° value of 0.34 V for all reactions involving reduction of V+4 to V+3 with fractional charge transfer, formation of V2O3, V3O5, V4O7, V5O9, etc., values of E, and free energy changes (ΔG = −nFE) are calculated and shown in Table S1 (ESI†). It is possible to evolve several phases derived from a parent structure by crystallographic shear (c.s). Such systematic composition variations have been demonstrated in TiO2 by incorporating c.s structures. For example, Magnéli phases of TinO2n−1 wherein n is varied from 4 to 9, resulting in derived structures from TiO2, a rutile type with regular crystallographic shear on the (121) planes.33 Moreover, it is known that slight changes in composition can be accommodated by varying only the orientation without any changes in the number of c.s. planes. When the VO2 undergoes slight reduction, the oxygen deficiency may be accommodated by disordered c.s. planes. As the oxygen deficiency increases, they form parallel ordered groups, forming lamellae structures.33 In the present case, we have observed lamellae-like structures with parallel microdomains of various sizes that are seen in optical and electron microscope images. TEM bright field images and selected area diffraction patterns with superlattice reflections of such micro domain structures in the VO2 crystals are shown in Fig. S10, ESI† in agreement with earlier work on TinO2n−1.34 The revealed lamellar and micro domain structures in the TEM bright field images, Fig. S10(a) and (b) (ESI†) along with the superlattice reflections in the electron diffraction patterns provide clear evidence for the oxygen deficient heterogeneous structure of vanadium oxide. Interestingly such heterogeneity can be achieved in both oxygen deficient and oxygenated VO2 with excess oxygen. During the reduction and oxidation, creation of excess cations and/or oxygen may flow up or flow down the c.s. planes, respectively, during voltage sweeps instead of limited only to thermal processing at high temperatures. Note that such minute compositional variations, introduced by annealing at high temperatures with controlled partial pressure of oxygen, are known to induce discretized free energy changes in TinO2n−1.33 In the present case, theoretical calculation shows a similar feature of discrete free energy changes against their compositions with different n values for VnO2n−1, as shown in Fig. 4e. It is known that the stability range of each of these Magnéli phases will be small in the order of a few kJ moles−1. Since the variations in the compositions are minimal, the step heights are also very similar, 0.65 kJ mole−1 when n varies from 9 to 11. At the same time, for lower values of n, the composition variations and the related step heights are large, evident from Fig. 4e. The presented free energy change against the composition for varied ratios of O/V is calculated with the fractional charge transfer, while the mentioned number of cycles is roughly approximated to correlate with the experimental observation of cycle-dependent Vth.
Variation in the composition and valence state during repeated cycling influences the value of E and results in discrete free energy changes for forming several non-stoichiometric phases. Because the memristive switching effect in the transition metal oxide memories is often associated with the electrochemical valence change mechanism under electrical triggering, the threshold voltage, VT, and E are correlated.35 The electrochemical valence state fluctuations may occur during the repeated electrical switching in VO2 in the solid state. Such dynamic electrochemical changes have been reported in the literature for transition metal oxide-based memristors.36,37 The observed discreteness in the Vth voltages that are seemingly random in their occurrence over 10000 repeated cycles could be explained by the variation in species undergoing valence change and the formation of several non-stoichiometric Magnéli phases during positive and negative voltage sweeps. In this scenario, the actual triggering voltage will be a random variable R(VT), which may change due to the valence state reaction's highly repeated threshold voltage, Vx and formal potential (E). Note that any slight variation in the population dynamics of electrochemical species and their valence states between the positive and negative voltage sweeps would affect the VT as per the following expression with definite voltage difference (ΔVth) across multiple switching events.
R(VT) = Vx ± nΔVth | (3) |
Here, the n is an integer (n = 1, 2, 3, 4, etc.) related to the compositional variations in Magnéli phase (VnO2n−1) that forms the conducting channel during the switching cycles. The population dynamics of V+4 species undergoing valence state are stochastic but result in switching at discrete threshold voltages dictated by the driving force for electroforming of Magnéli phase channels. Furthermore, the ΔVth is related to the step heights in the free energy change plot (Fig. 4e). The discreteness in threshold voltages with constant ΔVth arises due to the dynamic changes in composition of the Magnéli phase, wherein ‘n’ takes larger values above 5. In other words, the mechanism of resistive switching at the threshold voltage is governed by IMT and valence change-induced ionic species transport in VO2; the discrete variation in threshold voltages is arising due to fluctuations in composition, induced by electrical cycling. To understand the individual contributions of IMT and valence change in restive switching we carried out temperature dependent electrical measurements across the IMT. Transport measurement was performed at elevated temperatures, and with increasing temperature, the Eth values decrease, and beyond 68 °C, VO2 transitions into a metallic state, as shown in Fig. S11a and b, ESI.† We performed temperature-dependent I–V characteristics measurements over 100 cycles at temperatures approximately 7–8 °C below and above the VO2 phase transition temperature. These results suggest that the discrete resistive switching behaviour over repeated cycles in the temperature range of 25–60 °C is dominated by the valence change mechanism. The electrical transport behaviour at ∼70 °C is dominated by IMT behavior characterized by linear I–V curves (Fig. S12, ESI†). Nevertheless, the actual resistive switching in chemically heterogeneous vanadium oxide may involve both valency changes as well as phase change contributions.
Due to the presence of chemical heterogeneity in the crystals, the apparent channel length between nominally insulating regions is reduced in spite of the large electrode gap in the fabricated device. Furthermore, during cycling, the V/O ratio changes with the help of c.s structures, and certain Magnéli phases are being stabilized. The variation in compositional changes and associated free energy changes of the Magnéli phases at specific O/V ratios during voltage cycling guide the switching events with equal ΔVth. It is difficult to capture the compositional fluctuations happening in real-time during electrical measurements. Nevertheless, we have carried out post annealing measurements in an Ar + H2 reducing environment to establish correlation between the compositional fluctuation and discrete threshold voltages. For post growth annealing treatment, Ar + H2 gas was used and the annealing temperature was maintained at 850 °C. The XPS and electrical measurements of the annealed samples are shown in Fig. S13a–f and S14a–f, ESI.† In the annealed samples, the +3 oxidation state appears along with +4 and +5 oxidation states and its atomic percentage increases steadily from 24.5% to 35.2% with increasing annealing time from 15 minutes to 60 minutes. From XPS analysis, it is inferred that with increasing annealing time, there is an increase in the +3 oxidation state, indicating the formation of the oxygen-deficient phase of vanadium oxide. In addition, the I–V plots show stochastic switching in both as-grown and all annealed samples with discrete stochastic switching, as shown in S14b–f, ESI.† The current–voltage characteristics of the annealed samples result in a reduced number of transition points (Vth) compared to the multiple discrete Vth observed in the as-grown samples. The number of discrete electrical switching points is quite high in as-grown and post-annealed samples for a short duration, and those switching points decrease as the annealing time increases. This suggests that reducing multiple discrete stochastic switching points in annealed samples could be due to increased compositional variations. From the XPS and electrical measurements, chemical heterogeneity and the observed fluctuations in valence states result in the distribution of discrete Vth voltages.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4mh00034j |
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