A stable undoped low-voltage memristor cell based on Titania (TiOx)

An asymmetric memristive device fabricated with a titania (TiOx)-based switching layer deposited through atomic layer deposition with a thickness of ∼37 nm was investigated. X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy coupled with energy-dispersive x-ray spectroscopy were employed for device structural characterization. A unipolar resistive switching behavior (both at positive and negative voltages) was observed through the memristor’s current–voltage characteristics. A remarkably smaller forming voltage (from the top Pt electrode to the grounded Au electrode) of 0.46 V was achieved, while it approached (positive bias from the Au electrode and holding Pt electrode as grounded) 0.25 V, which is a much smaller forming voltage than has ever been reported for titanium-based oxides without doping. The retention and endurance characterization over 2000 switching cycles were satisfactory without degradation.


Introduction
Memristive devices with resistive switching features have gathered tremendous consideration owing to their attractive competency in emerging applications such as evolving semiconducting materials with memories, logic circuits, and neuromorphic applications [1][2][3][4][5].In addition, memristive devices hold simple device structures, are compatible with complementary metal oxide semiconductor (CMOS) technologies, and exhibit an exceptional capacity for next-generation volatile and nonvolatile memories owing to their high storage densities, fast read/write operations, reasonable durability, and retention time [6][7][8][9].Various engineered materials have recently been reported as the switching layer in memristive devices.Transition metal oxides (TMO) have been extensively investigated due to their thermally stable structures, excellent stochiometric control, sample preparation methods, and scalability [10][11][12][13][14]. Several studies have focused on TiO 2 -based memristive devices, including symmetric devices with an identical top electrode (TE) and bottom electrode (BE) and asymmetric devices with a different TE and BE, and both convincingly realized TiO 2 as a potential contender to be employed as a switching layer in memristive architectures [15][16][17].
Resistance switching (RS) in TMO-based memristive devices (including TiO 2 ) is commonly understood to occur through the formation and dissolution of conductive filaments in the presence of oxygen vacancies when an appropriate voltage is applied [18,19].The current-voltage (I-V) curves obtained from device characterization offer two distinct mechanisms in RS devices; a uniformity in I-V switching supports homogeneous switching (linking the smoothly migrated oxygen vacancies as carriers), while an abrupt shift during I-V characteristics is considered filamentary switching [20].Such a functional mechanism operating across the switching layer changes the electrical conductivity of the memristive device between the highresistance state (HRS) and low-resistance state (LRS).Considering I-V characteristics, resistive switching behavior can be distinguished between bipolar switching (dependent on the polarity of the applied bias voltage and its amplitude) and unipolar switching (characterized by the amplitude of the applied bias voltage) [19].
The literature has witnessed that memristive devices require several volts during electroforming (EF)/ switching operations (except diffusive memristors, which involve silver or copper) [21].However, considering artificial synaptic sensitivity and power consumption, depletion at higher voltages is particularly crucial [22,23].The operational voltages can be correlated to the ion flow (copper (Cu) or silver (Ag) ions and oxygen anions), which shows proportionality to the outcome of both the concentration and diffusion coefficient of drifting entities in electrochemical metallization (ECM) and valence change memory (VCM) devices [24,25].However, considering an ECM, an electrochemically active metallic electrode, such as Ag, is required for the memory effect.The highly mobile cationic Ag ions drift through the dielectric towards the counter electrode, forming dendrites of a highly conductive nature, while VCM takes place in TMO and is provoked by the anion migration (oxygen anions), which can also be described as the shift in corresponding oxygen vacancies.As reported, the lower operating voltage of 0.2 V was achieved for the ZrO 2 -based device attributed to the higher diffusion coefficient of active electrode silver (Ag) [26].Moreover, low switching voltages are also observed for sulfidebased electrolytes ascribed to the porosity of the sulfide layer, which facilitates ion migration in ECM [27].Similarly, lower operational voltages of VCM are supported by the migration of mobile defects offered by their lower activation energies, i.e., halide (iodide) perovskites and GaSe nanosheets [11,28].Furthermore, various strategies are implemented for tunning the switching voltages, including impurity doping [29,30], resulting in narrow memory windows, vacancy-induced approaches for maximizing memories [31,32], employing metallic nanoisland arrays [33], especially ZnS with the lightly oxidized state, which even surpasses the synaptic sensitivity level of biological species [34].
Although TiO 2 -based memristive architectures are extensively investigated, both in single and multilayer resistive films (a brief survey and comparison to this work are given in table 1), low-forming and switching voltage TiO 2 -based memristive cells (in a non-diffusive system) is rarely reported.Besides, devices operating at higher voltages (especially elevated electroforming steps) and currents are hindering the application of TiO 2 -based films in low-power memory operations.Thus, motivated by this disadvantage, we report a highly stable, low-switching, unipolar, TiO x -based memristor.The I-V characteristics displayed astonishingly low forming and operating voltages.Such low-forming characteristics of the memristive cell delivered appealing characteristics (2000 switching cycles) with excellent stability and could be attributed to higher oxygen vacancies in the titania film.Electrical characterization was conducted after a month (300 switching cycles), followed by a six-month (3 cycles before sending out for further analysis) interval, revealing no significant change in the device functionality.The results were supported by an X-ray photoelectron spectroscopic (XPS) technique and highresolution transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (TEM-EDS).

Materials and methods
The fabricated asymmetric RS device employed titanium oxide (TiO x ) as a switching layer, with a device area of 5×5 μm 2 , and the device architecture contained Pt/Cr/TiO x /Au/SiO 2 /Si.The physical vapor deposition (PVD) method was used to deposit the Au, ∼30 nm thick, as the BE while keeping chromium (Cr ∼5 nm thick) as the adhesive layer.Then, the TiO x switching layer (∼37 nm thick) was deposited on the top of the BE via plasma-enhanced ALD (PE-ALD).Finally, Cr (5 nm) was deposited through PVD as an adhesive, followed by the Pt as the TE (∼60 nm thick).The deposition patterns were accomplished through shadow masks, standard photolithography, and liftoff procedures.The XPS (Thermo Fisher ESCALAB Xi+, UK) was applied for a surface elemental analysis of the memristor device.In addition, TEM-EDS (Thermo Fisher Talos F200XG2, Czech Republic) was employed for the memristive device's cross-sectional identification and elemental analysis.The electrical characterization of the Pt/Cr/TiO x /Au/SiO 2 /Si device was evaluated through a probe station (Cascade Summit 11k, Germany) and potentiostat (Bio-Logic VMP-300, France).The device was investigated by applying a bias voltage to the Pt considered as the TE while Au was grounded, and then bias voltage was applied to the Au electrode while the Pt electrode was grounded.

Results and discussions
The fabricated RS device implicated TiO x (∼37 nm) as a switching layer deposited via PEALD; a detailed fabrication process is given in the supplementary information (figure S1), and the device architecture is illustrated in figure 1(a).For electrical characterization, a positive voltage (for EF) was applied to the TE (Pt) while grounding the BE (Au).An abrupt increase in current was observed after the forming voltage, and the device approached the LRS from the HRS.To accomplish extensive, I-V characteristics of the memristive device, no compliance current (I CC ) was employed to restrain the operational current.The observed I-V characteristic expressed unipolar switching behavior at positive and negative voltages.A remarkably small forming voltage of 0.46 V is realized, attributed to higher oxygen vacancies in the titania film, which is associated with the Gibbs free energy of formation (oxide formation) [35].Additionally, the electroforming voltage may become lower than the memristive device's switching voltage if the formation's Gibbs free energy is sufficiently low [36].Such low electroforming insignificantly protects the device from damage, and electroforming could contribute to the first switching phase of the device.Thus, the necessity of high forming voltages and a separate electroforming process can be avoided.Moreover, rich oxygen vacancies of TiO x reveal a low dielectric constant, generating a comparatively higher electric field, concentrating in the TiO x layer, eventually directing the conductive filament formation [37].
To characterize the electrical behavior of the memristive device, a test bench was prepared that included a probe station and potentiostat.A DC voltage sweep was performed within the range of ±0.6 V, and information regarding current and voltage was recorded.An I-V curve is shown in figure 1(b) for the first test that involved grounding BE (Au) and applying a voltage to TE (Pt).The device was initially in HRS mode, but at the Set voltage (approximately 0.51 V), it switches to LRS mode, allowing quite a few microamps to flow.As the voltage was decreased, the current abruptly decreased at the Reset voltage (about 8 mV), resetting the device to HRS (Reset).Negative bias regions exhibited similar characteristics but opposite polarities of Set and Reset voltages.Such RS establishes indistinguishable potential and is an eminent feature of unipolar resistive switching [38].
We conducted over 2000 cycle tests (the extended I-V values are given in figure S2) to determine the stability of our device.Figure 1(c) illustrates HRS and LRS results, while figure 1(d) illustrates the Set and Reset voltage results.Based on the plot of HRS and LRS, it is apparent that the memristive device remained exceptionally stable (very small standard deviation) and showed excellent repeatability in terms of cycles.Additionally, both the Set and Reset voltages show good stability.The durability cycle presents the HRS at approximately 46.47 kΩ, while the LRS was approximately 9.15 kΩ, where a clear memory window more significant than 37 kΩ can be realized to distinguish between the two states.Slight variations in HRS and LRS, which are strongly dependent on the movement of oxygen ions and the reduction of oxygen ions on the electrodes during the switching operations, may cause variations in HRS/LRS values [17].The identified current at the EF stage was 1.1 μA, while it jumped to 10 μA at HRS, and the highest current at LRS was 55.7 μA.Since the device architecture is asymmetric, we grounded the TE (Pt) and applied the voltage to the BE (Au) in the second test.For 500 cycles, a DC sweep test was performed, and its I-V curve, resistance values, and switching voltages are shown in figure 2 (the extended I-V values are given in figure S3).In comparison with the first experiment, a similar behavior was observed, although with different resistance values and switching voltages.In particular, the HRS, LRS, and Set voltage were reduced to 19kΩ, 1.9kΩ, and 210 mV, respectively, while the Reset voltage increased to 8.5 mV.Based on the statistical analysis results, the device exhibits good stability in electrical behavior in this configuration as well.Similar behavior of the device is observed at negative voltages with different polarities of switching voltages, establishing a unipolar resistive switching feature.Surprisingly, a reduced forming voltage (0.25 V) was attained on this occasion.The established current during EF was 1.1 μA, while it increased to 10 μA at HRS, and the highest current at LRS was 41.6 μA.
Additionally, the device was tested after one month of storage at room temperature without proper environmental protection (the I-V characteristics are given in figure S4).This test was conducted on a similar test bench, but the number of cycles decreased to 300.Compared to the tests conducted after initial fabrication, no noticeable changes were observed in electrical characteristics.Similar measurements were examined for the device (6 months later for three cycles only) before sending it for cross-sectional TEM and EDS characterization (requiring FIB technology to cut the device), and the outcomes disclosed that the device performed the same as on the first day (the I-V characteristics are presented in figure S5).Considering all the respective data, it can be concluded that the achieved memristor is highly stable under normal conditions, and the oxygen vacancies can alter via oxygen pulses during ALD deposition.
For comparison, we also inserted thicker Cr-layers (the I-V characteristics are given in figure S6) as an adhesive Pt/Cr (7.5 nm)/TiO x /Au (the I-V characteristics are given in figure S6a) and Pt/Cr (10 nm)/TiO x /Au (the I-V characteristics is shown in figure S6b), respectively.The results demonstrated switching at the negative bias, and after switching once, they did not switch back at both positive and negative voltages, instead following a straight resistor-like path (shown by red arrows in the I-V curve in figure S6).It can also be seen that as the Cr thickness increases from 7.5 nm to 10 nm, the switching voltage also increases.We then fabricated devices with Cr functioning as the TE (the I-V characteristics are given in figure S7).The I-V characteristics showed switching at −2.05 V, allowing 695 microamps of current to flow.Identical switching was observed (like Cr 7.5 nm and Cr10 nm); once the device is switched on, it follows the write-once read many times (WORM) structure and does not switch on or off.In addition, we fabricated a device architecture (Pt/Cr (2.5 nm)/TiO x /Au) (given in figure S8) with Cr having a thickness of 2.5 nm.The electrical characterization of the memristive cell revealed that the devices did not switch on the positive voltage but switched at the negative voltage (identical switching to the above-mentioned devices except the switching voltage), and after switching once, it did not switch back at both positive and negative voltage, instead following a straight resistor-like path (shown by red arrows).
To elucidate the chemical dynamics during RS behavior in a TiO x -based unipolar volatile RS memristor, the double logarithmic I-V characteristics were plotted to describe the nature of conduction.Figure 3 illustrates the RS between HRS and LRS, figure 3(a) shows the RS switching mechanism from the Pt electrode to Au as the grounded electrode, and figure 3(b) demonstrates the RS switching mechanism from the Au electrode to the Pt (grounded) electrode.The RS mechanistic approach behind the RS switching has not been clarified completely; however, the hypothesis regarding TiO x devices reveals that switching at room temperature is mainly dominated by oxygen vacancies (V ox ) in the switching layer [39].Generally, the intrinsic V ox in the nonconductive TiO x layer assists the growth of the conductive filament.Initially, the memristor occupied the HRS owing to the immobilized charge carriers under no external bias; however, the external voltage supply first accomplishes the EF, then dissociates the oxygen ions (O 2− ) from TiO x , followed by the O 2− drifts from the cathode to the anode.Once the conductive filament is established, the device shifts from HRS to LRS owing to the flow of high current through it and to the electronic drift from the anode to the cathode [40].The log-log fitting derived from the I-V sweeps exhibited an identical slope value equalizing unity (1.0) for the HRS and LRS (similar slope values were attained for the negative region because of the unipolarity of the device), irrespective of the sweep direction, demonstrating the ohmic conduction.In addition, the abrupt transition appeared at threshold switching (HRS switched to LRS), generating no data points to access analysis during this switching phase.The literature has described the probability of generating linear and metal-like conductive channels extended throughout the insulating layer [41].
As the oxygen vacancies regulate the conductive filaments formation (Set) and their dissolution (Reset) is brought on by current controlled Joule heating, thermally enabling the conductive filaments to disintegrate at an accelerated speed [42].So, as soon as the LRS is achieved, the maximum current through the device approaches 55.7 μA (0.51 V), which seems to be the threshold value, and the current falls gradually (as soon as the voltage decreases) but at 8 mV, the current fall abruptly which bring the device to HRS.At the maximum current value, the local temperature associated with the conductive filament increases, and owing to this high temperature, the oxygen vacancies start diffusing from the conductive filament (thermally initiation of the conductive filament) to the neighboring region, initializing the reduction of the conductive filament diameter (from the hottest region).Such localized reduction of the conductive filament upsurges local current density (elevating the localized temperature), resulting in the faster diffusion of the oxygen vacancies (speeding the diminution of the conductive filament).So, heat-mediated conductive filament diminution raises the resistance, generating additional heat, and ultimately, the conductive filament is cracked, ensuring the Reset.Further studies are needed to thoroughly understand the conductive channel through unipolar volatile RS devices.
In general, heavy doping governs Ohmic in metal/semiconductor contacts, while low doping results in rectifying (Schottky-like) contacts [43].Concluding from our results, especially HRTEM, we can observe that many oxygen vacancies are created in the TiO x film, which could potentially contribute to highly doped behavior.Literature guidance supported the conduction mechanism, and the O/Ti ratio via XPS is ∼1.5348, revealing a large V ox concentration, which would transform the TiO x into a heavily n-doped electrically conductive state and thus facilitate the facile flow of current through it.We believe more research is needed to understand the Cr/TiO x interface.
Owing to the huge device area, the analysis, such as XPS, STEM, and EDS, was performed via randomly selected zones.The XPS measurement was employed to analyze the device to ascertain the surface pattern and chemical formulation of the ALD-deposited TiO x switching layer.The stoichiometric surface chemical analysis was evaluated via integral intensity.The study revealed that the ALD-deposited TiO x film comprised O and Ti elements, as shown in figures 4(a) and (b).For Ti, the observed peaks for Ti 2p 3/2 and Ti 2p 1/2 states were identified at the binding energies of 458.5 and 464.3 eV, respectively, which relates to the previous findings [44,45].The corresponding spin-orbit splitting between the Ti 2p 3/2 and Ti 2p 1/2 peak positions was approximately 5.8 eV [46] which signifies the Ti 4+ oxidation state, associating oxygen vacancies (V ox ) in the TiO x switching layer [45].Similarly, the oxygen peak O 1 s spectrum was observed at 530.1 eV, which relates to the O−Ti bond (oxygen ions) [45].Figure 5(a) displays the cross-sectional TEM image (HRTEM image is given in figure S8) of the memristor architecture of Pt/Cr/TiO x /Au/SiO 2 /Si, validating the thickness of the TiO x (∼37 nm) layer.In addition, 60 nm thick Pt as the TE and 30 nm Au functioning as the BE, each fabricated on the top of an adhesive (Cr ∼5 nm) layer, can also be distinguished.Figure 5(b) illustrates the EDS deposits in a scanning TEM (STEM) image for the Pt/Cr/TiO x /Au/SiO 2 /Si stack (a detailed image is provided in figure S9).Each corresponding layer indicates Si, Ti, O, Pt, Au, Cr, etc Generally, the material employed regulates its figureof-merit, which can be varied and strongly associated with its application, such as nonvolatile memories, advanced computing, etc.
In contrast to bipolar memristors, the unipolar memristor displays symmetry in voltage polarity; both transitional (HRS and LRS) operations are accomplished simply between voltages of similar polarity.Such symmetric voltage changes (unipolar memristor) may make it easier to integrate memristors into arrays of circuit components assembled to carry out sophisticated logic operations.Herein, a brief survey of the TiO 2 /TiO x -based resistive random-access memory (RRAM) devices (earlier reports) is presented in table 1. Various preparation techniques were employed for TiO 2 dielectric deposition, which includes radio frequency (RF) sputtering, direct current (DC) sputtering, RF magnetron sputtering, reactive sputtering, ALD, focused electron beam induced deposition (FEBID), and dip coating.Both bipolar and unipolar switching behavior was evaluated, and the comparative analysis proves the low forming and operating voltages of our fabricated unipolar resistive switching memristor.The devices presented individually by [16,49,51,54], respectively, were EF-free, which is better for integration into arrays, but the switching voltage was much higher than for our device.References [42, 47, 57] reported unipolar memristive architectures; the first two had elevated EF voltages  along with higher operating voltages, while the latter [57] had a close resemblance in switching voltages to this study, but the operating current and switching layer thickness (three layers of TiO 2 deposition) are higher.Similarly, [46,48] fabricated devices with thin switching layers (the former had low switching voltages) but higher EF voltages, while the latter operated at higher voltages as well as had elevated EF voltages.Again, [50,51] presented very thin bipolar devices (three layers and two layers, respectively); the former had a high EF voltage, while the latter revealed a higher operating voltage.Reference [52] also fabricated a double-switching layer device with moderate EF voltage, but the operating voltage and current were higher.Moreover, [55,58] reported very thin devices (7.5 and 10 nm, respectively) where 7.5 nm devices showed very close switching voltages to our device, but EF voltage was higher while the 10 nm device operated at higher current and voltage, respectively.To conclude, this study demonstrates a simple memristive architecture fabricated with conventional materials deprived of additional doping or annealing, ultimately operating at a low voltage, which could pave the approach towards multifunctional memristive devices operating at low voltages.

Conclusion
We fabricated an undoped and asymmetric memristive device carrying ALD-deposited TiO x as the switching layer with a thickness of ∼37 nm.The memristor fabrication was accomplished through PVD and ALD deposition techniques, where all the thin film patterns were finalized using a shadow mask, followed by standard photolithography and liftoff procedures.The stoichiometric balance between Ti and O ensured low forming and operating voltages for a unipolar resistive switching memristor.Based on the measurements, the Set and Reset voltage values were ± 0.51 V and ± 8 mV (2000 cycles), respectively, presenting a memory window of 37 kΩ for distinguishing between On and Off states.In addition, when the voltage was applied to Au, and the Pt electrode was kept grounded, the Set and Reset voltages changed to ±0.31 V and ±16.4 mV, respectively (500 switching cycles).In addition, the device was tested after one month (stored at room temperature without proper protection from humidity, etc).No significant deviation in the switching cycles was observed for both directions of operating voltages (300 cycles).To the best of our knowledge, it's the first example of such a low-forming voltage than the switching voltage for TiO x -based memristive cells.Besides, it's the lowest reported forming and switching voltage for TiO x -based non-diffusive memristive architectures.The outcomes confirm that such lowoperating-threshold switching volatile devices have the potential to be implemented in low-power neuromorphic chips as artificial neurons.

Figure 1 .
Figure 1.(a) Optical image of the crossbar structure of Pt/Cr/TiO x /Au/SiO 2 /Si device with the effective line width of 5 μm and the crossbar memristive cell is 5×5 μm 2 , and the inset displayed schematic illustration of the crossbar memristive cell, (b) Plot of the I-V curve of the device recorded for 2000-voltage sweeps when grounding BE (Au) and applying a voltage to TE (Pt) (A red arrow indicates electroforming, while a blue arrow indicates the path followed by each cycle after EF is completed), (c) HRS and LRS state values as well as their statistical information, and (d) Set and Reset voltages recorded in each cycle and their histograms.

Figure 2 .
Figure 2. (a) The I-V characteristics of the device recorded for 500-voltage sweeps when grounding TE (Pt) and applying a voltage to BE (Au) ( A red arrow indicates electroforming, while a blue arrow indicates the path followed by each cycle after EF is completed), (b) HRS and LRS state values as well as their statistical information, and (c) Set and Reset voltages recorded in each cycle and their histograms.

Figure 3 .
Figure 3. Log-log plot of the I-V characteristics in the positive voltage region displaying unity slope at both HRS and LRS switching: (a) at 0.51 V (voltage applied to the Pt electrode while Au was grounded), and (b) at 0.3 V (voltage applied to the Au electrode while Pt was grounded).

Table 1 .
Comparison of TiO x -based RRAM, active layer prepared/deposited by different methods.