Multi-layered NiOy/NbOx/NiOy fast drift-free threshold switch with high Ion/Ioff ratio for selector application

NbO2 has the potential for a variety of electronic applications due to its electrically induced insulator-to-metal transition (IMT) characteristic. In this study, we find that the IMT behavior of NbO2 follows the field-induced nucleation by investigating the delay time dependency at various voltages and temperatures. Based on the investigation, we reveal that the origin of leakage current in NbOx is partly due to insufficient Schottky barrier height originating from interface defects between the electrodes and NbOx layer. The leakage current problem can be addressed by inserting thin NiOy barrier layers. The NiOy inserted NbOx device is drift-free and exhibits high Ion/Ioff ratio (>5400), fast switching speed (<2 ns), and high operating temperature (>453 K) characteristics which are highly suitable to selector application for x-point memory arrays. We show that NbOx device with NiOx interlayers in series with resistive random access memory (ReRAM) device demonstrates improved readout margin (>29 word lines) suitable for x-point memory array application.

performance as a selector device. We revealed that insufficient Schottky barrier height between electrode and NbO 2 formed as a result of interfacial defects, which increased the conductivity of insulating state.
Interface defects were successfully suppressed and a higher Schottky barrier can be formed by inserting a thin NiO y barrier layer between electrode and sputtered-NbO x (W/NiO y /NbO x /NiO y /W). As a result, the leakage current of W/NiO y /NbO x /NiO y /W device was significantly decreased and the device exhibited high I on /I off ratio (>5400). The W/NiO y /NbO x /NiO y /W device exhibits very fast transition speed (<2 ns) and excellent operating thermal stability (>453 K). The W/NiO y /NbO x /NiO y /W device can have very fast delay time (<30 ns) and is drift-free, which are highly suitable attributes for selector application in x-point memory array.

Result and Discussions
The cross-sectional transmission electron microscopy (TEM) image shows film structure and crystalline state of both MBE and sputter deposited films (Fig. 1). The films were analyzed by in-situ X-ray photoelectron spectroscopy (XPS) to confirm that the correct phase and composition were achieved. Details of the MBE growth and XPS phase identification are found in ref. 17 and XPS results for the sputtered films are shown in Supplementary  Fig. S1 18,19 . The MBE NbO 2 film was polycrystalline with a lattice constant of 3.4 Å. This corresponds to the d-spacing of the (400) planes of the insulating body-centered tetragonal NbO 2 phase 20 . On the other hand, the sputter-deposited NbO x film was amorphous as-grown.
In comparison with the sputter-deposited NbO x film, the electroforming process was mostly eliminated in the MBE-deposited NbO 2 film, as shown in Supplementary Fig. S2. In the case of the sputter-deposited film, the pristine state of the film was amorphous. Therefore, electroforming was needed to form crystalline tetragonal NbO 2 regions within the amorphous matrix to exhibit IMT 16 . On the other hand, electroforming was not needed in the case of MBE-deposited NbO 2 film because the pristine state of the film was already polycrystalline tetragonal NbO 2 . The difference in electroforming and IMT process between sputter-and MBE-deposited films is summarized in Supplementary Fig. S3. Because there is no longer any need for electroforming, the IMT process in MBE-deposited NbO 2 films can be precisely analyzed.
The mechanism of IMT of NbO 2 under E-field was widely interpreted by Joule-heating model, and this model suggests that electrically induced Joule-heating generate the sufficient heat over IMT temperature of NbO 2 (1080 K) 8,10 . However, the IMT temperature of NbO 2 (1080 K) is much higher than the temperature that can achieved by Joule-heating within the insulating state of NbO 2 9 . Therefore, several researches proposed that the mechanism of IMT under E-field is the result of thermal runaway model [13][14][15] . These researches simulated the conductivity of NbO 2 device as a function of temperature and E-field by fitting the I-V characteristics with Poole-Frenkel model. They showed that IMT can take place far below IMT temperature of NbO 2 (1080 K) by thermal runaway, which successfully resolved the main drawback of classical Joule-heating IMT model.
We take a different perspective by using field-induced nucleation theory to explain IMT mechanism in this research. Devices that abruptly change their resistance at a certain electric field, such as phase change random access memory (PRAM) or VO 2 -based IMT devices, energetically favor metallic nuclei with a cylindrical shape upon nucleation via the applied electric field [21][22][23][24][25] . Similarly, the field-induced IMT of NbO 2 is expected to result of a Peierls transition of conductive NbO 2 (metallic, i.e. rutile NbO 2 ) regions formed as cylindrical shape nuclei within an insulating NbO 2 matrix (tetragonal, distorted rutile NbO 2 ) under the influence of an electric field 21,22 . The formation of nuclei is favorable and forms a conductive path through the insulating host material. The free energy the system ∆G consist of: Here, σ and μ are the surface tension and the chemical potential difference between the two NbO 2 phases, respectively. The transition energy barrier is lowered by an external electric field where ε is the dielectric constant of the host and n is the depolarizing factor ( = n 1 3 for a sphere)) as shown in Fig. 2(a). If we assume that spherical nuclei exist in zero-field (W E = 0), then the surface area and volume of the nuclei can be defined as A = 4πR 2 and Ω = 4πR 3 /3, respectively. By using the differential form of the free energy at zero-field (∆ = π σ − π µ ), we can define the energy barrier at zero-field ( π = σ µ W 16 /3 0 3 2 ) and the equivalent radius of the nuclei (R 0 = 2σ/μ). However, because the nuclei with cylindrical shape are energetically more favorable than spherical ones when an E-field is applied 22 , it is preferable to modify Equation (1) as follows: with R being the cylinder radius and h its height. Following from Eq. (2), the reduced barrier energy W(E) under E-field is given by: Here, E 0 is the voltage acceleration factor of the first order, independent of the external voltage or temperature, and its conventional value is 1 MV/cm 22 , d is the thickness of film; and α is geometric factor of cylinder radius compared with equivalent radius of the nuclei at zero-field (R = αR 0 ) where 0.1 ≤ α < 0.5. We assume α = 0.5 because this value corresponds to the maximum barrier 23 . The theory predicts the delay time between application of the field and the switching event expressed as 24 : The value of τ d for the film was measured by using rising ramp pulses that can minimize RC delay effect and can reveal the delay time with various voltages (V A ) and temperatures 26 . τ d is defined as the point where I D (V D /50 Ω) suddenly increases, as shown in Fig. 2(b). Here, τ d decreases exponentially with V A and temperature. Figure 2(c) shows that the relation between temperature/V A and τ d can be described by an Arrhenius plot, which follows Equation (4). We found that the experimentally determined value of the zero field barrier W 0 , which is 47-63 meV, agrees well with the calculated minimum energy pathway (MEP) found between rutile and tetragonal NbO 2 during the Peierls transition, which is 43 meV 27 . The alternative mechanism of diffusion or electromigration of oxygen (vacancies) has also been discussed in terms of the IMT in niobium oxides 11,28,29 . The values of the diffusion barrier height of roughly 290-550 meV deduced from the diffusion studies in Nb 2 O 5 reported in ref. 30. Also, the observed oxygen diffusion coefficient in NbO 2 is lower than that of the pentoxide (indicating a higher diffusion barrier for NbO 2 than for Nb 2 O 5 ). Therefore, we can conclude that oxygen diffusion is energetically less favorable than Peierls phase transition due to the high barrier for oxygen diffusion. Furthermore, the diffusion barrier at zero field is estimated to be reduced by only ~10 meV under electric field application considering the electric potential drop along a typical diffusion length of ~1 Å. Therefore, IMT of NbO 2 is likely due to a Peierls phase transition through field induced nucleation rather than oxygen electromigration. The events that occur during IMT are illustrated in Fig. 2(d).
The expected transition speed of NbO 2 film is quite fast because only short-range atomic arrangement is needed for the transition (Peierls transition). Therefore, NbO 2 based IMT device is well suited for selector device in x-point memory array. However, sufficiently high resistivity in the insulating state of NbO x film has not yet been obtained. Moreover, the off-current of NbO 2 film can be suppressed under 1 nA at 1 V (Area = 50 × 50 nm 2 , Thickness = 25 nm) because insulating state of NbO 2 conductivity is about 10 −4 S/cm 31 . Likely, the relatively high conductivity of the insulating state of NbO x originates from interface defects between electrode and NbO x layer. In fact, many defects (Grain boundary, dislocation, and point defects) are observed between electrode and MBE-deposited NbO 2 film by TEM image (Supplementary Fig. S4). Additionally, these interface defects were observed in sputter-deposited NbO x device from our previous research 32 .
These defects can pin the Fermi level between electrode and NbO x layer. As a result, the device does not have a sufficiently high Schottky barrier. These defects can be eliminated and a high Schottky barrier can be obtained by inserting a NiO y layer, which consists of NiO and Ni 2 O 3 phases ( Supplementary Fig. S1), between electrode and NbO x layer (W/NiO y /NbO x /NiO y /W) as shown in Supplementary Fig. S5 33 . Based on DC I-V characteristics at various temperatures for both devices, current-temperature dependencies at low field (V = 0.1 V, saturation region) for both devices follow the Richardson relation (Eq. (5)) and the effective Schottky barrier can be obtained using the slope of the Richardson plot ( Supplementary Fig. S5). W/NiO y /NbO x /NiO y /W devices have higher Schottky barrier height (ϕ B ~ 0.25 eV) than W/NbO x /W devices (ϕ B ~ 0.15 eV).
Bp 0 2 (J 0 = Current density at saturation region, A* = Richardson constant, T = temperature, k = Boltzmann constant, q = electronic charge, ϕ Bp = effective schottky barrier energy). Before comparing the device performance of W/NiO y /NbO x /NiO y /W device with W/NbO x /W device, we analyzed the delay time of the W/NiO y /NbO x /NiO y /W. Interestingly, the zero field barrier W 0 of W/NiO y /NbO x / NiO y /W device (42-70 meV), which was extracted from delay time Arrhenius plot, also corresponds well to the calculated minimum energy pathway (MEP) found between rutile and tetragonal NbO 2 during the Peierls transition, which is 43 meV. This value is the same as that obtained from MBE film analysis (Fig. 2(c)). These results inferred that the barrier layers were not affected by transition mechanism of NbO x . Meanwhile, interface structure of NbO x device can control the conductivity of insulating state of the device. Therefore, we can suppress the high conductivity of NbO x film by simply inserting NiO y barrier layer without compromising fast transition characteristics of NbO x IMT layer. Figure 3(a) shows IMT characteristics after electroforming both W/NbO x /W and W/NiO y /NbO x /NiO y /W devices. Compared with the W/NbO x /W device, the W/NiO y /NbO x /NiO y /W device exhibited decreased conductivity in the insulating state. The I on /I off ratio of W/NiO y /NbO x /NiO y /W device improved to >5400 from >480, which is the I on /I off ratio of the W/NbO x /W device. Both devices have superior endurance that persisted over 10 8 AC cycles as shown in Fig. 3(b). The W/NiO y /NbO x /NiO y /W device has very uniform device-to-device and cycle-to-cycle stability during several DC I-V sweeps (Supplementary Fig. S6). Moreover, we measured the transition time and delay time of W/NiO y /NbO x /NiO y /W device to investigate the temporal characteristics of the device. The device has a transition time under 2 ns and a delay time down to 30 ns for variable voltage ramps (Supplementary Fig. S7). We expect that the delay time of W/NiO y /NbO x /NiO y /W can be even shorter for square pulses.
Since IMT mechanism of NbO x is a second-order structural transition of the Peierls type and involves only very short range atomic displacements, the drift-free characteristic is available in NbO x based device. As a matter of fact, Fig. 4(a) shows that W/NiO y /NbO x /NiO y /W device can recover its insulating state under less than 10 ns. Figure 4(b) illustrates the drift-free operation of the W/NiO y /NbO x /NiO y /W device when V th does not change at different time intervals 34 . These results indicate that the W/NiO y /NbO x /NiO y /W device can be used for fast operating applications.
We also evaluated the feasibility of x-point memory array using a novel W/NiO y /NbO x /NiO y /W device. The W/NiO y /NbO x /NiO y /W device was connected in series with a TiN/Ti/HfO x /TiN ReRAM device (ReRAM, 1 R) which has DC I-V characteristic shown in Fig. 5(a). Set voltage (V set ) and reset voltage (V reset ) of ReRAM is about 0.6 V and −1.2 V, respectively. To prevent the hard breakdown of 1 R, we set the compliance current to 500 μA during operation. Figure 5(b) shows DC I-V characteristics of 1S-1R device with superior DC endurance (>300 cycles). The V set and V reset of 1S-1R was about 1.8 V and −2 V, respectively. The state of the device is determined by applying a read voltage (V read ) of 1.4 V.
We simulated the x-point memory using novel W/NiO y /NbO x /NiO y /W selector based on measurement in Fig. 5(b). Since the leakage current of unselected cell at ½V read is suppressed to about 300 nA in both LRS and HRS state by adopting the W/NiO y /NbO x /NiO y /W selector, we demonstrate that the readout margin (Eq. (6)) can improve up to 2 9 vs. 2 1 word lines (W.L.) as shown in Fig. 5(d) 35,36 .

Conclusion
We have successfully fabricated NbO 2 poly-crystalline film using MBE, which does not require an electroforming process. We find that the IMT in NbO 2 undergoes the Peierls phase transition through the field-induced nucleation with the formation of a conductive filament of rutile NbO 2 in an insulating host matrix of tetragonal NbO 2 .
We also showed that the leakage current of NbO 2 IMT device originates from the insufficient Schottky barrier height between electrode and NbO x layer as a result of interfacial defects. Sufficiently high Schottky barrier and improved IMT characteristics can be obtained by introducing a NiO y layer between electrode and NbO x layer. A novel W/NiO y /NbO x /NiO y /W device has high I on /I off ratio (>5400), high operating temperature (>453 K), fast transition speed (<2 ns) and drift-free operation. We employed the W/NiO y /NbO x /NiO y /W device as a selector device on ReRAM memory cell. Due to the excellent selector characteristics of W/NiO y /NbO x /NiO y /W device, we show a significantly improved readout margin (up to 2 9 word lines) is possible in a large x-point memory array.

Methods
First, to analyze the IMT mechanism under E-field, we fabricated NbO x films using both MBE and RF-sputtering. About 25 nm-thick NbO x film was deposited by MBE and RF-sputtering on a 50 × 50 nm 2 TiN bottom electrode (B.E). The MBE-deposited NbO 2 film was deposited at 700 °C. Nb metal was evaporated from an electron beam source and molecular oxygen at a pressure of 5 × 10 −6 Torr were used. The sputter-deposited NbO x film was deposited at room temperature by RF reactive sputtering with a process gas of Ar/O 2 (30 sccm/1.3 sccm), at a working pressure of 5 × 10 −3 Torr and forward power of 100 W using a 2-inch Nb metal target. After both of NbO x were deposited, positive photoresist was spincoated at 3000 rpm for 35 s and baked at 100 °C for 90 s. The photoresist were exposed under the lithography mask which has 50 × 50 um 2 pattern and removed with developer to deposit contactable top electrode. Afterwards, W top electrode was deposited by RF reactive sputtering at room temperature with a process gas of Ar (30 sccm), at a working pressure of 5 × 10 −3 Torr and forward power of 100 W using a 2-inch W metal target. Secondly, to reduce the leakage current of NbO x film, NiO y barrier layer inserted NbO x structure deposited on a 50 × 50 nm 2 W bottom electrode (B.E). About 2-3 nm thick NiO y layer was sputter deposited additionally by RF reactive sputtering with a process gas of Ar/O 2 (30 sccm/2.0 sccm), at a working pressure of 5 × 10 −3 Torr and forward power of 30 W using a 2-inch Ni metal target as a barrier layer between W electrodes and sputtered NbO x layer. The condition for sputtering NbO x is same with above. As a result, W/NiO y /NbO x /NiO y /W device was fabricated and its electrical characteristics compared to a W/NbO x /W control sample.