The Effect of Nitrogen Annealing on the Resistive Switching Characteristics of the W/TiO2/FTO Memory Device

In this paper, the effect of nitrogen annealing on the resistive switching characteristics of the rutile TiO2 nanowire-based W/TiO2/FTO memory device is analyzed. The W/TiO2/FTO memory device exhibits a nonvolatile bipolar resistive switching behavior with a high resistance ratio (RHRS/RLRS) of about two orders of magnitude. The conduction behaviors of the W/TiO2/FTO memory device are attributed to the Ohmic conduction mechanism and the Schottky emission in the low resistance state and the high resistance state, respectively. Furthermore, the RHRS/RLRS of the W/TiO2/FTO memory device is obviously increased from about two orders of magnitude to three orders of magnitude after the rapid nitrogen annealing treatment. In addition, the change in the W/TiO2 Schottky barrier depletion layer thickness and barrier height modified by the oxygen vacancies at the W/TiO2 interface is suggested to be responsible for the resistive switching characteristics of the W/TiO2/FTO memory device. This work demonstrates the potential applications of the rutile TiO2 nanowire-based W/TiO2/FTO memory device for high-density data storage in nonvolatile memory devices.


Introduction
The memristor has been defined as the fourth circuit element after the capacitor, resistor, and inductor. In 1971, Chua [1] had already guessed that there should be a circuit element that could convert the magnetic flux and charge to each other according to the relationship between the four variables of the circuit, deduced that the device could memorize the characteristics of the resistance, and named it a memristor. In 2008, HP Labs [2] produced the first nanoscale memristor, which attracted the extensive attention of a large number of researchers due to the unique properties of the memristor itself. Up to now, memristors have been widely used in artificial neural networks and synapses [3][4][5], chaotic circuits [6], and secure communications [7]. Sun et al. [8] designed a neural network circuit that could relate emotion and memory based on the memristor circuit. Yang et al. [9] embedded graphene-oxide quantum dots in an HfO 2 memristor, which showed low charge loss while ensuring long-term stability, demonstrating the potential application in the nonvolatile memory devices. It is worth noting that the traditional storage technologies have encountered development bottlenecks, and the storage volume of the traditional memory devices is about to reach the size limit according to Moore's Law [10][11][12]. Thus, it is urgent to develop a new type of memory device to overcome the development bottlenecks of the traditional memory devices, and nanoscale memristors may be a potential candidate for application in the future nonvolatile memory devices.
A memristor is composed of a metal-semiconductor-metal MIM structure. The choice of the intermediate semiconductor layer for the memristor has a crucial influence on the resistive Sensors 2023, 23, 3480 2 of 10 switching characteristics of the memristor. The transition-metal-oxide-based memristors have shown many excellent advantages such as an easy fabrication process, good compatibility, and simple compositions [13]. Among them, the transition metal oxide TiO 2 is widely used in photocatalysis [14] and sensors [15] due to its low cost, easy fabrication, and corrosion resistance. Moreover, another important factor affecting the performances of the memristors is the preparation process. The transition metal oxide TiO 2 has been prepared by the hydrothermal method [16][17][18][19][20], the magnetron sputtering method [21,22], the electrochemical anodization method [23,24], the atomic layer deposition method (ALD) [25,26], and other methods [27][28][29]. In particular, the hydrothermal method with its simple experimental steps and high economic benefits is an effective approach to prepare the transition metal oxide TiO 2 . Recently, various preparation methods have been developed to prepare TiO 2 memristors. Zhang et al. [17] prepared rutile TiO 2 nanorods by the hydrothermal method, and the resistance ratio of the TiO 2 nanorod-based Pt/TiO 2 NRAs/FTO device was about 10. Wang et al. [21] carried out a study on the resistive switching property of the amorphous TiO 2 thin film deposited by the radio frequency magnetron sputtering method on a flexible copper (Cu) foil substrate, and the resistance ratio of the TiO 2 thin film-based Ni/TiO 2 /Cu device was almost one magnitude. Chen et al. [23] fabricated the Au/Cu NWs/TiO 2 NTAs/Ti device with a resistance ratio greater than 40 by the electrochemical anodization method.
The one-dimensional TiO 2 nanowires have recently received great interest for different nanoelectronics and optoelectronics applications owning to their unique physical and chemical behaviors [16,30]. Furthermore, the enhanced resistive switching properties of the TiO 2 memristors have been obtained by nitrogen (N 2 ) annealing treatment [18,31]. However, the effect of nitrogen annealing on the resistive switching behavior and mechanism of the W/TiO 2 /FTO memory device has not been reported so far. Herein, the rutile TiO 2 nanowirebased W/TiO 2 /FTO memory device was prepared, and the effect of the nitrogen annealing on the nonvolatile resistive switching behavior and mechanism of the W/TiO 2 /FTO memory device is reported.

Experiments
All the chemicals, including titanium butoxide (97%) and concentrated hydrochloric acid (36%-38% by mass), were of analytical grade and used without further purifying; they were purchased from Sigma-Aldrich. Fluorine-doped tin oxide (FTO, 15 Ω/square) was used as the substrate for the epitaxial growth of the TiO 2 nanowire arrays. The preparation process was as follows: First, 15 mL of concentrated hydrochloric acid was mixed with 15 mL of deionized water and stirred continuously for 15 min at room temperature. Then, 0.5 mL of titanium butoxide was slowly dripped into the above mixed solution containing 15 mL of deionized water and 15 mL of concentrated hydrochloric acid. After sufficient stirring for another 15 min, a transparent solution was generated, which acted as the TiO 2 precursor solution. Subsequently, the precursor solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave, in which a piece of FTO substrate with the conducting surface facing down was kept at an angle against the inner wall of the Teflon liner. After that, the autoclave was sealed and continuously heated at 140 • C for 4 h. Finally, the Teflonlined stainless-steel autoclave was cooled down to room temperature and the sample was taken out, washed with ethanol and deionized water, and then allowed to dry in ambient air. After the synthesis, the TiO 2 sample was annealed at 450 • C in a tube furnace for 1 h under a N 2 atmosphere. The tungsten electrodes were deposited on the TiO 2 sample by the DC magnetron sputtering process.
The crystal structure and surface topography of the prepared samples were detected by X-ray diffraction (XRD) and a field emission scanning electron microscope (FESEM), respectively. X-ray Photoelectron Spectroscopy (XPS) was performed to survey the chemical composition and surface states of the TiO 2 nanowire arrays. The I-V characteristics of the TiO 2 nanowire-based W/TiO 2 /FTO memory device were measured by using an Agilent B2901A analyzer at room temperature.  Figure 1a shows the XRD pattern of the TiO 2 nanowire arrays. It is clear that there were only two sharp diffraction peaks, such as (101) and (002), observed at 36.20 • and 62.84 • , respectively, which can be assigned to the tetragonal rutile phase (JCPDS No. 88-1175) [17]. Moreover, the (002) diffraction peak was clearly enhanced compared with the (101) diffraction peak, while some diffraction peaks such as the (111), (211), and (110) crystal planes were absent, which suggests that the highly oriented TiO 2 nanowire grew preferentially along the [001] orientation with the growth axis perpendicular to the FTO substrate. Figure 1b,c indicate the top-view and tilt-view FESEM images of the TiO 2 nanowire arrays, respectively. It is observed that the FTO substrate was densely and uniformly covered by the vertically aligned TiO 2 nanowire arrays with smooth edges and rough top surfaces. Figure 1d displays the cross-sectional FESEM image of the TiO 2 nanowire arrays. It is clearly shown that the average height and diameter of the rutile TiO 2 nanowire arrays were about 1.5 µm and 180 nm, respectively.
respectively. X-ray Photoelectron Spectroscopy (XPS) was performed to survey the chemical composition and surface states of the TiO2 nanowire arrays. Thecharacteristics of the TiO2 nanowire-based W/TiO2/FTO memory device were measured by using an Agilent B2901A analyzer at room temperature. Figure 1a shows the XRD pattern of the TiO2 nanowire arrays. It is clear that there were only two sharp diffraction peaks, such as (101) and (002), observed at 36.20° and 62.84°, respectively, which can be assigned to the tetragonal rutile phase (JCPDS No. 88-1175) [17]. Moreover, the (002) diffraction peak was clearly enhanced compared with the (101) diffraction peak, while some diffraction peaks such as the (111), (211), and (110) crystal planes were absent, which suggests that the highly oriented TiO2 nanowire grew preferentially along the [001] orientation with the growth axis perpendicular to the FTO substrate. Figure 1b,c indicate the top-view and tilt-view FESEM images of the TiO2 nanowire arrays, respectively. It is observed that the FTO substrate was densely and uniformly covered by the vertically aligned TiO2 nanowire arrays with smooth edges and rough top surfaces. Figure 1d displays the cross-sectional FESEM image of the TiO2 nanowire arrays. It is clearly shown that the average height and diameter of the rutile TiO2 nanowire arrays were about 1.5µm and 180 nm, respectively.  Figure 2 shows the XPS spectra of the Ti 2p and O 1 s in the rutile TiO2 nanowire arrays before and after the rapid nitrogen annealing treatment. As shown in Figure 2a, the Ti 2p3/2 and Ti 2p1/2 peaks around 458.48 eV and 464.08 eV can be observed. The spin-orbit   suggests that the as-prepared rutile TiO 2 nanowire arrays contained a considerable amount of oxygen vacancies, after the rapid nitrogen annealing treatment at 450 • C for 1 h, as shown in Figure 2c,d. It is obvious that the Ti 2p 3/2 and Ti 2p 1/2 peaks were found at 458.39 eV and 464.09 eV, respectively. In addition, the peak area of the 531.58 eV binding energy corresponding to the oxygen vacancies in Figure 2c was about 12.4% larger than that of the 530.78 eV binding energy in Figure 2a, which means that the nitrogen atoms entered the rutile TiO 2 nanowire arrays and replaced the lattice oxygen atoms [32], resulting in an increase in the oxygen vacancies in the rutile TiO 2 nanowire arrays. splitting binding energy between Ti 2p3/2 and Ti 2p1/2 was approximately 5.6 eV, in the existence of Ti-O bonds in the rutile TiO2 nanowire arrays. Figure 2b reveals spectrum of O 1 s. It was found that the peak at the binding energy of 529.68 sh attributed to the lattice oxygen in the rutile TiO2 nanowire arrays, while the pea binding energy of 530.78 eV may be assigned to the oxygen vacancies, which sugg the as-prepared rutile TiO2 nanowire arrays contained a considerable amount o vacancies, after the rapid nitrogen annealing treatment at 450 °C for 1 h, as show ure 2c,d. It is obvious that the Ti 2p3/2 and Ti 2p1/2 peaks were found at 458.39 eV an eV, respectively. In addition, the peak area of the 531.58 eV binding energy corres to the oxygen vacancies in Figure 2c was about 12.4% larger than that of the 5 binding energy in Figure 2a, which means that the nitrogen atoms entered the t TiO2 nanowire arrays and replaced the lattice oxygen atoms [32], resulting in an in the oxygen vacancies in the rutile TiO2 nanowire arrays.

The Electrical Characteristics of the W/TiO2/FTO Memory Device
The typical current-voltage (I-V) measurements of the W/TiO2/FTO memor were carried out to explain the resistive switching characteristics of the W/T memory device before and after the rapid nitrogen annealing treatment, as show ure 3a. The I-V curve plotted in semi-logarithmic scale was obtained by setting the voltage in a sequence of 0 Figure 3b displays the sc diagram of the W/TiO2/FTO memory device, which is composed of the top W e the TiO2 nanowire arrays, and the bottom FTO electrode. During the measurem voltages of the W/TiO2/FTO memory device were applied to the W electrode with electrode grounded. It is clear that the W/TiO2/FTO memory device showed no bipolar resistive switching characteristics. The pristine resistance state of the W/T memory device was the high resistance state (HRS). When the applied voltage in

The Electrical Characteristics of the W/TiO 2 /FTO Memory Device
The typical current-voltage (I-V) measurements of the W/TiO 2 /FTO memory device were carried out to explain the resistive switching characteristics of the W/TiO 2 /FTO memory device before and after the rapid nitrogen annealing treatment, as shown in Figure 3a. The I-V curve plotted in semi-logarithmic scale was obtained by setting the applied voltage in a sequence of 0 Figure 3b displays the schematic diagram of the W/TiO 2 /FTO memory device, which is composed of the top W electrode, the TiO 2 nanowire arrays, and the bottom FTO electrode. During the measurements, the voltages of the W/TiO 2 /FTO memory device were applied to the W electrode with the FTO electrode grounded. It is clear that the W/TiO 2 /FTO memory device showed nonvolatile bipolar resistive switching characteristics. The pristine resistance state of the W/TiO 2 /FTO memory device was the high resistance state (HRS). When the applied voltage increased from 0 V to +6 V, the W/TiO 2 /FTO memory device switched from the HRS to the LRS with a steep jump of current at +1.17 V (V set ), which indicated that the set process occurred. After that, the W/TiO 2 /FTO memory device maintained the LRS before the applied voltage reduced to −5.36 V (V reset ). Subsequently, the reset process occurred at V reset , which induced a switch to the pristine HRS with a dramatic decrease in the current in the device, indicating the nonvolatile bipolar resistive switching behavior of the Sensors 2023, 23, 3480 5 of 10 W/TiO 2 /FTO memory device. After the rapid nitrogen annealing treatment, the V set and V reset reduced to 1.09 V and −4.87 V, respectively, and the operation current in the LRS for the device was higher than that of the device before the rapid nitrogen annealing treatment. Figure 3c,d show the I-V curves plotted in In(I) ∼ V 1/2 scale before and after the rapid nitrogen annealing treatment, respectively. It is observed that the W/TiO 2 Schottky barrier heights of the W/TiO 2 /FTO memory device were about 0.38 eV and 0.37 eV before and after the nitrogen annealing, respectively. Thus, the change in the W/TiO 2 Schottky barrier modified by the oxygen vacancies is suggested to be responsible for the resistive switching characteristics of the W/TiO 2 /FTO memory device.   The current-voltage characteristics of the Schottky emission are described as [29]: where J is the current density, T is the absolute temperature, V is the electric field, A is the Richardson constant, k is the Boltzmann's constant, ε is the dielectric constant, q is the electric charge, and ∅ B is the Schottky barrier height. According to the above fitting results, the conduction behaviors of the W/TiO 2 /FTO memory device are attributed to the Schottky emission in the high resistance state, and the change in the W/TiO 2 Schottky barrier depletion layer thickness and barrier height modified by the oxygen vacancies at the W/TiO 2 interface is suggested to be responsible for the resistive switching characteristics of the W/TiO 2 /FTO memory device. For the W/TiO 2 /FTO memory device, the work function of tungsten was 4.6 eV, and the work function of the intrinsic rutile TiO 2 was about 4.2 eV. Therefore, when the W/TiO 2 Schottky junction was formed, electrons from the Fermi level of the rutile TiO 2 migrated toward the W until the Fermi levels on both sides equalized, and the height of the W/TiO 2 Schottky junction depletion barrier was about 0.4 eV. As shown in Figure 3, the Schottky barrier heights of the W/TiO 2 Schottky barrier depletion layer were about 0.38 eV and 0.37 eV before and after the rapid nitrogen annealing treatment, respectively, which were smaller than that of the intrinsic rutile TiO 2 because of the existence of oxygen vacancies in the rutile TiO 2 . In order to evaluate the effect of the nitrogen annealing on the resistive switching characteristics of the W/TiO 2 /FTO memory device, Figure 4a,b exhibit the retention tests of the device at the reading voltage of 0.1 V before and after the rapid nitrogen annealing treatment, respectively. As shown in Figure 4a, the W/TiO 2 /FTO memory device displayed the nonvolatile resistive switching characteristics with a high resistance ratio (R HRS /R LRS ) of about two orders of magnitude, which could be stably preserved for over 10 3 s without obvious degradation. Figure 4b indicates the retention tests of the device at the reading voltage of 0.1 V after the rapid nitrogen annealing treatment, it is appreciable that the R HRS /R LRS of the W/TiO 2 /FTO memory device increased from about two orders of magnitude to three orders of magnitude after the rapid nitrogen annealing treatment. In comparison with the previous reports about TiO 2 memory devices as summarized in Table 1 [16][17][18][19][21][22][23][24][25][26][27][28][29]31], the W/TiO 2 /FTO memory device in this work has a relatively lower V set and the highest resistance ratio of about three orders of magnitude, which demonstrates the outstanding potential of the W/TiO 2 /FTO memory device for the future nonvolatile memory applications. equalized, and the height of the W/TiO2 Schottky junction depletion barrier was a eV. As shown in Figure 3, the Schottky barrier heights of the W/TiO2 Schottky ba pletion layer were about 0.38 eV and 0.37 eV before and after the rapid nitrogen an treatment, respectively, which were smaller than that of the intrinsic rutile TiO2 of the existence of oxygen vacancies in the rutile TiO2.
In order to evaluate the effect of the nitrogen annealing on the resistive sw characteristics of the W/TiO2/FTO memory device, Figure 4a,b exhibit the retenti of the device at the reading voltage of 0.1 V before and after the rapid nitrogen an treatment, respectively. As shown in Figure 4a, the W/TiO2/FTO memory device di the nonvolatile resistive switching characteristics with a high resistance ratio (R of about two orders of magnitude, which could be stably preserved for over 10 3 s obvious degradation. Figure 4b indicates the retention tests of the device at the voltage of 0.1 V after the rapid nitrogen annealing treatment, it is appreciable RHRS/RLRS of the W/TiO2/FTO memory device increased from about two orders of tude to three orders of magnitude after the rapid nitrogen annealing treatment. parison with the previous reports about TiO2 memory devices as summarized in [16][17][18][19][21][22][23][24][25][26][27][28][29]31], the W/TiO2/FTO memory device in this work has a relatively lo and the highest resistance ratio of about three orders of magnitude, which demo the outstanding potential of the W/TiO2/FTO memory device for the future non memory applications. (a)

The Resistive Switching Mechanism of the W/TiO 2 /FTO Memory Device
To further illustrate the resistive switching mechanism of the W/TiO 2 /FTO memory device, Figure 5 shows the schematic diagram of the relative band positions of the W, TiO 2 , and FTO before and after the formation of the W/TiO 2 Schottky interface. It was found that the work function of W was about 4.6 eV, which was higher than that of TiO 2 (4.2 eV). Therefore, the electrons migrated from the Fermi level of the TiO 2 to the W electrode until the Fermi levels on both sides equalized when the W/TiO 2 Schottky interface was formed, which pushed the conduction band of the W electrode to a relatively higher energy level with respect to the conduction band position of the TiO 2 . Thus, the change in the W/TiO 2 Schottky barrier depletion layer thickness and barrier height modified by the oxygen vacancies at the W/TiO 2 interface is suggested to be responsible for the resistive switching characteristics of the W/TiO 2 /FTO memory device. During the set process as displayed in Figure 6, when the positive voltage was applied to the W/TiO 2 /FTO memory device, the electrons were injected from the bottom FTO electrode and captured by the oxygen vacancies in the W/TiO 2 Schottky barrier depletion layer, resulting in lowering the W/TiO 2 Schottky barrier height and barrier thickness. Once the W/TiO 2 Schottky barrier became low and thin enough, large amounts of electrons from the TiO 2 side crossed the W/TiO 2 Schottky barrier, thus switching the W/TiO 2 /FTO memory device from the HRS to the LRS with an abrupt increase in the current at V set . Subsequently, the device maintained the LRS until a large enough negative voltage V reset was applied, indicating the nonvolatile resistive switching behavior of the W/TiO 2 /FTO memory device. During the reset process, when the negative voltage was applied to the W/TiO 2 /FTO memory device, the electrons injected from the top W electrode were retarded by the W/TiO 2 Schottky barrier, which induced a large electric field to cross the W/TiO 2 Schottky barrier depletion layer. Thus, the electrons captured by the oxygen vacancies were activated and emitted from the W/TiO 2 Schottky barrier depletion layer to the bottom FTO electrode, which induced a recovery to the initial state of the W/TiO 2 Schottky barrier, and the device switched back from the LRS to the HRS with a drastic drop in the current at V reset . This work suggests that the W/TiO 2 /FTO memory device may be a potential candidate for future nonvolatile memory applications.   After the rapid nitrogen annealing treatment, N atoms entered the surface of the TiO 2 nanowire arrays and replaced the O atoms, which resulted in the increase in oxygen vacancies at the W/TiO 2 Schottky barrier. It is worth noting that the increase in the oxygen vacancies at the W/TiO 2 Schottky interface led to reducing the depletion layer thickness and lowering the barrier height of the W/TiO 2 Schottky barrier. Thus, a lower Schottky barrier height of 0.37 eV and smaller set voltage of 1.09 V (V set ), as well as a higher resistance ratio of about three orders of magnitude, were observed after the rapid nitrogen annealing treatment, as shown in Figure 3.

Conclusions
In this paper, the rutile TiO 2 nanowire-based W/TiO 2 /FTO memory device with a high resistance ratio of about three orders of magnitude was successfully obtained by the rapid N 2 annealing treatment. The as-prepared W/TiO 2 /FTO memory device exhibits nonvolatile bipolar resistive switching behavior. Furthermore, the R HRS /R LRS of the W/TiO 2 /FTO memory device was obviously increased from about two orders of magnitude to three orders of magnitude after the rapid N 2 annealing treatment. The conduction behaviors of the W/TiO 2 /FTO memory device are attributed to the Ohmic conduction mechanism and the Schottky emission in the low resistance state and the high resistance state, respectively. In addition, the change in the W/TiO 2 Schottky barrier depletion layer thickness and barrier height modified by the oxygen vacancies at the W/TiO 2 interface has been suggested to be responsible for the nonvolatile resistive switching phenomena of the W/TiO 2 /FTO memory device. This work demonstrates that the rutile TiO 2 nanowire-based W/TiO 2 /FTO memory device may be an outstanding candidate for application in the future nonvolatile memory devices.
Author Contributions: Z.Y. guided the experimental method and the paper writing. X.H. completed the experiment and wrote the paper. J.X., C.C. and X.Q. verified the repeatability of the experiment. B.L., Z.S. and T.S. defined the review scope, context, and purpose of the study. All authors have read and agreed to the published version of the manuscript.