Bidirectional-nonlinear threshold switching behaviors and thermally robust stability of ZnTe selectors by nitrogen annealing

Three-dimensional stackable memory frames involving the integration of two-terminal scalable crossbar arrays are expected to meet the demand for high-density memory storage, fast switching speed, and ultra-low power operation. However, two-terminal crossbar arrays introduce an unintended sneak path, which inevitably requires bidirectional nonlinear selectors. In this study, the advanced threshold switching (TS) features of ZnTe chalcogenide material-based selectors provide bidirectional threshold switching behavior, nonlinearity of 104, switching speed of less than 100 ns, and switching endurance of more than 107. In addition, thermally robust ZnTe selectors (up to 400 ℃) can be obtained through the use of nitrogen-annealing treatment. This process can prevent possible phase separation phenomena observed in generic chalcogenide materials during thermal annealing which occurs even at a low temperature of 250 ℃. The possible characteristics of the electrically and thermally advanced TS nature are described by diverse structural and electrical analyses through the Poole–Frankel conduction model.


Scientific RepoRtS
| (2020) 10:16286 | https://doi.org/10.1038/s41598-020-73407-3 www.nature.com/scientificreports/ electrical/thermal degradation that occurs during thermal annealing at temperatures above 250 ℃. We expect that this issue is likely associated with a rapid decomposition or phase separation of OTS material by thermal annealing. Thus, because thermal annealing is a generic approach requiring a high temperature of approximately 400 ℃ at the back end of the line, the manipulation of thermal stability along with achieving the desired electric characteristics is also a key challenge for future 3D stacked crossbar array semiconductor processing 19 .
In this paper, we report the TS behaviors and electrical responses of as-grown and annealed ZnTe selectors in vacuum and nitrogen ambient states. Systematic structural and electrical analyses of ZnTe thin films are done by X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and DC/pulse electrical characteristics to aid interpretation of the experimental findings using the Poole-Frankel conduction model. Particularly, the incorporation of a suitable nitrogen post annealing approach verifies the thermally robust electrical responses of ZnTe selectors even at a high annealing temperature of 400 ℃, which suggests the efficient protection of annealing-dependent phase separation phenomena of ZnTe active layer.

Result and discussion
Structural and electrical characteristics of ZnTe selector. Figure 1a shows the schematic cross-sectional transmission electron microscope (TEM) images and representative I-V response of W/ZnTe/W selectors. The ZnTe film has a thickness of 40 nm and is sandwiched by both top and bottom W electrodes, as illustrated in Fig. 1a. The TEM image of Fig. 1b indicates the uniformly distributed growth of the ZnTe film. The representative I-V curve for as-grown ZnTe selector (Fig. 1c) reveals typical bidirectional TS behavior (black lines) after the forming process (red line) under voltage sweep modes: an initial state exhibits a high resistance, but then the resistance decreases abruptly at a forming voltage (V f ); this process is nonreversible. After forming, the resistance decreases gradually with bias voltage sweeping until reaching the threshold voltage (V th ) followed immediately by a sharp decrease in resistance above V th , clearly verifying the representative TS behavior. A similar trend also occurs in the negative region. This volatile low-resistance state is maintained as long as the applied voltage is higher than the holding voltage (V h ). If not, it then switches to a high resistance state. Comparisons of TS curves under voltage and current sweep modes demonstrate the characteristic sharp current increases and decreases at V th and V h voltages, respectively, whereas the current sweep reveals the representative S-shaped curve with a negative differential resistance event (Fig. S1 in the Supplementary Information). In this study, an additional resistor in series is also employed to avoid possible overshoot problems during electrical analyses. Figure S2, in the Supplementary Information, plots the representative I-V responses of ZnTe selectors under a serially connected external resistance ranging from 1 to 10 kΩ. As shown on Fig. S2, there is no significant vari- cross-sectional TEM image of the W/ZnTe/W selector integrated with a serially connected resistor to avoid the overshoot during the measurements. (c) Representative I-V plot of the ZnTe selector, clearly revealing the forming voltage (V f ) and bidirectional TS behavior. Over a certain voltage, namely, threshold voltage, the resistance of the TS decreases sharply. The low resistance state is maintained as long as a given voltage is above the holding voltage. (d) Comparison of TS curves under voltage (red line) and current (blue line) sweep modes. The voltage sweep shows the abrupt current increase/decrease at V th /V h , respectively, while the current sweep reveals the S-shape curve with a negative differential resistance event. www.nature.com/scientificreports/ ation in operation voltage of V th and V h , except for a slight variation in off-and on-current levels, regardless of the magnitudes of additional resistors.
To gain an insight into the difference in current characteristics before and after forming, which were derived from the previous work 17, 18 , the cell size-dependent current density-voltage (J-V) and I-V responses for the ZnTe selector were recorded as shown in Fig. 2a,b. The cell size is defined by the size of the bottom W electrode, which is in the range of 420-1414 nm. As plotted in Fig. 2a, the initial current densities in the off-current regime before forming clearly reveal non-dependency on cell size features under identical compliance currents. Such a result in an off-current regime before forming suggests uniform current flow across the whole ZnTe layer. After forming, both off and on currents are also independent to the cell size, as seen in Fig. 2b. This may suggest the presence of the local conductive filaments (CFs) inside the ZnTe layer generated by forming 20 : that is, after forming, the local CFs represent the dominant conductance in the ZnTe active layer, whereas current flow in the other region is negligible. Figure 2c,d show the possible models for the ZnTe layer matrix before and after forming, respectively. It can be seen that the initially uniform ZnTe layer has zinc-blended crystal structures and maintains a high resistance state before forming. However, after forming, the application of a high electric field serves to break the pristine crystal state, which has a more significant impact on the generation of localized charge defects (filaments) inside the ZnTe layer, thereby contributing to the TS characteristics observed in previous work 17,18 .
To further clarify the above conduction nature before and after forming, the Poole-Frankel conduction model frequently used in previous reports is adopted for simulation, based on the observed I-V responses. This model well-describes the off-current conduction of OTS materials through thermally assisted hopping between the dense and localized charge traps 21,22 . An external electric field permits electrons to transfer by lowering the energy barrier between adjacent traps: that is, carriers can be trapped and de-trapped at defect sites under electrical stress. Below is the quantitative amount of current that flows in a subthreshold regime.
where q is element charge, A is the region causing switching, N T , tot is the concentration of trap, τ 0 is the timeto-escape of traps, Δz is the trap-to-trap (or defect-to-defect) distance, kT is the thermal energy, and u a is the thickness of the ZnTe layer. Under bias, electrons can hop between local charge traps with a distance of Δz and a barrier height of E C -E F within τ 0 . As expressed in Eq. (1), it can be seen that the off current depends on the area (A) of the trap-rich region and N T,tot , whereas the hyperbolic sine term contributes to the electron transfer probability. The slope in the log (I)-V curve of the subthreshold regime determines Δz, where a longer Δz is associated with a lower electron hopping probability. This model also gives the relationship between V th and the thickness of the chalcogenide layer. Figure S3 of the Supplementary Information show a clear linear thickness dependency of I-V responses and V th in the ZnTe-based selector, thus reflecting the appreciable tunability of V th for practical selection window margins. Figure 3 presents the representative I-V plots of the off current under diverse parameters necessary for analytical simulations. As shown in Fig. 3a, all devices give two different current slopes at a certain voltage boundary during voltage sweeps: linear (red line) and exponential (blue line) behaviors in currents for extremely small and subthreshold voltage regimes, respectively. The linear I-V response is explained by linear approximation of Eq. (1) with the same physical origin caused by thermal hopping. Figure 3b,c refer to the analytical simulation results for the Δz and E C -E F levels, where the Δz of 6.5 nm is Current-voltage (I-V) curves for the ZnTe selector before and after forming, respectively. Both characteristics clearly confirm cell-size-independent features, where the cell size is defined by the size of the bottom W electrode. Possible models of (c) resistive and (d) conductive state behaviors before and after forming, respectively. The resistive state features before forming may represent the uniform flow of initial off currents through the whole ZnTe layer between electrodes, whereas the conductive behavior after forming may reflect the presence of dominant conduction through the locally created filament paths by the forming process, which implies indirect evidence of cell-size independence. www.nature.com/scientificreports/ derived by extracting the slope in log (I)-V curve of the subthreshold regime. The corresponding E C -E F level is given by adjusting the current levels from the measured off-current responses with an assistance of Δz value. The resultant value is found to be approximately 0.5 eV. To further verify the derived values of Δz and E C -E F taken from the above analytical simulations, the temperature dependent off-current behaviors are also recorded in the range of 20-80 ℃, as shown in Fig. 3d. A high temperature leads to an increase in the off current owing to the enhanced hopping probability caused by a higher thermal energy. The resulting curves correlate well with the measurements over the entire temperature range. Such consistency implies that the conduction nature of ZnTe selectors is also similar to that observed form the previously reported OTS materials. Figure 4 reveals the time-resolved transient pulse response and endurance tests for the ZnTe selector, where the programmed input (purple line)/output (black line) voltages and the corresponding currents (blue line) are shown. As evident in Fig. 4a, the selector remains in an off state with an extremely low current when applied voltage (V a ) is less than V th . After exceeding V th , the current starts to increase with a delay time (t delay ) of 60 ns and reaches the compliance current level within 40 ns. Therefore, the current rises from I off to the compliance current of 25 mA within less than 100 ns, suggesting the corresponding switching speed of less than 100 ns for our device, which is closely comparable to other works. An endurance test is also conducted to test the repeatability and nonlinearity of the ZnTe selector, as shown in Fig. 4b, where a pulse measurement was done to avoid   Figure S4b shows the representative I-V responses of each 1S (black line) and 1R (red line) elements, where the 1S is also serially connected with a 1 kΩ resistor (black line). The 1R device gives typical bipolar switching characteristics with an on/off ratio of about 10 2 . In an integrated 1S1R device, the nonlinearity of 1R device is greatly enhanced by the 1S, as shown in Fig. S4c. The current maintains an initially low level in the 1S device until V th is reached. After exceeding V th , the 1S1R device switches to the on state with a lower resistance state than device 1R. This implies that the overall I-V response of 1S1R devices employs the unique advantages of each 1S and 1R device, such as high (low) on (off) current levels and an enhanced nonlinearity of 10 4 in 1S and nonvolatile on/off ratio of 1R. Thus, we anticipate that these experimental findings can be generalized and extended to engineer future crossbar array configurations, even though more experiments are necessary in diverse multi-stacked array levels.
Thermal stability and method for method for improvement. In an effort to address the structural decomposition events observed in previously reported OTS materials upon thermal post annealing, the thermal stability features of the ZnTe selector were also tested at various temperatures. Due to the requirements of high temperature of approximately 400 ℃ at the back end of the line in 3D stacked crossbar array semiconductor processing, the temperatures ranges from 200 to 400 ℃ in vacuum and nitrogen (N 2 ) atmospheres 23 . The representative I-V curves for as-grown and annealed ZnTe selectors are displayed in Fig. 5; all devices exhibit typical TS behavior, regardless of annealing conditions. As seen in the Fig. 5a, the selector annealed at 200 ℃ always maintains a relatively stable off current, whereas the selectors annealed at 250 ℃ and 300 ℃ experiences the rapid electrical degradation even at those low annealing temperatures. This deterioration effect is likely linked with the inevitable phase separation or stoichiometric decomposition of ZnTe materials upon thermal annealing. However, as seen in Fig. 5b, the selectors annealed in N 2 atmosphere provide thermally stable features up to 400 ℃, which permits N 2 annealing to be employed as a reliable option for achieving a thermally robust ZnTe selector. In addition, the time-resolved transient pulse response of nitrogen-annealed ZnTe exhibited similar a trend during fast transition from off state to on state without exhibiting any degradation, as evident in Fig. S5 in the Supplementary Information.
To exploit indirect evidence for the advanced thermal stability upon N 2 annealing, closer microstructural investigations of the ZnTe thin film were conducted. Figure 6 reveals the representative XRD patterns of ZnTe films annealed at the same temperature in vacuum and N 2 atmospheres. As seen in Fig. 6a, the as-grown ZnTe www.nature.com/scientificreports/ film gives produces the expected zinc-blended FCC crystal structures. Vacuum annealing at 200 ℃ allows for the presence of slightly improved (220) and (311) peaks of ZnTe materials, while the phase separated Te peaks (red circle) are clearly shown at more than 250 ℃. This implies a possible phase-separation event in ZnTe film during vacuum annealing, which negatively affects electric performance. On the other hand, the N 2 -annealed ZnTe layer does not follow such a trend: no phase separation took place up to 400 ℃. This would explain why the N 2 -annealed device maintains the same TS behavior as as-grown selectors without having any degradation, as shown in Fig. 6b. These approximate findings only imply the eligibility of N 2 annealing to the OTS-based selectors.
To provide further indirect evidence for the above observations regarding the effect of N 2 annealing, XPS analyses of the as-grown (Sample A) and ZnTe layers annealed at 300 ℃ in N 2 (Sample B) and a vacuum (Sample C) were carried out. Figure 7 shows the XPS spectra of Te 3d 5/2 and Zn 2p 3/2 for each sample (A-C) revealing the variation in chemical states, where each layer has the same 40 nm thickness. The Te 3d 5/2 spectra of each sample is de-convoluted to obtain two dominant peaks at 572.11 eV and 575.54 eV, suggesting the presence of metal Te 0 atom and Te 2+ , respectively, whereas the Zn 2p 3/2 spectra reveal two main peaks of metal Zn 0 atom and Zn 2+ at 1021.31 eV and 1021.49 eV, respectively. It is widely believed that a Te chalcogenide component (Te 2+ in this study) combines with other components. Te 2+ initially creates the well-known lone-pair electrons, providing the localized charge traps 11,24 . Thus, suitable Te-Zn chemical binding may resemble the behavior of OTS-based selectors. However, the absence or reduction of the Te 2+ peak from the Zn-Te binding may cause leaky I-V characteristics without exhibiting TS behavior due to a low amount of unlinked Te chains, as plotted in the blue I-V curve of Fig. 6a. Figure 7a,b shows the representative chemical binding states of Te 3d 5/2 for Samples A and B, respectively. As seen, Sample B reveals no clear change in the Te 2+ peak, which means that N 2 annealing does not affect the chemical states of Te atoms in the ZnTe layer. However, vacuum annealing above 250 ℃ (Sample C) causes a variation in the binding states of Te atoms, possibly induced by the phase-separation event upon annealing [25][26][27] . This causes the disappearance of the Te 2+ peak and the generation of unclear Te* elements, as evident in Fig. 7c. It is well-known that the Te atoms have various oxidation states of unknown Te* peak, which is not assigned even in other previous works 17 . However, the absence of preexisting defect-related Te 2+ peaks observed in the as-grown sample may suggest the corresponding electrical degradation of ZnTe selectors coming from the inevitable phase-separation event upon vacuum annealing. Figure 7d-f reveals Zn peaks with no clear peak variation in each sample, regardless of annealing conditions. This suggests that the Zn component has no significant impact on the electrical TS performance. Our observations from the XRD and XPS measurements suggest that a suitable amount of Te 2+ in ZnTe active layer may be essential to the TS observation, even though more results and comparisons are needed to establish a clearer explanation for the origin of the electrical degradation upon annealing. The presence of phase-separated Te peaks may be highly associated with the electrical degradation observed from the higher annealing temperature over 250 ℃ while the nitrogen-annealed ZnTe layer refers to thermally robust and stable patterns associated with having no phase separation characteristics.

conclusion
In this study, we address the performance and thermal stability of simple binary ZnTe selectors. Diverse electrical parameters in off current regimes were mainly determined by applying the Poole-Frankel conduction model to experimental observations, yielding information on the generation of local CFs and OTS behaviors. Simple structural analyses revealed that the representative electrical degradation upon vacuum annealing may be ascribed to a phase-separation event of ZnTe layer, while a simple N 2 -annealing approach contributes to the presence of thermally robust stability of the ZnTe selector without having a phase-separation event, even at higher annealing temperatures. We anticipate that the above experimental findings will lead to future practical applications for 3D stackable/scalable crossbar array frames.

Method
Sample fabrication. The micro-and nano-sized W bottom electrodes are, at first, prepared on the Si 3 N 4 substrates to define the device cell areas during electrical analyses and then followed by the deposition of 40-nmthick Zn 0.5 Te 0.5 (ZnTe) films through a radio frequency (RF) magnetron sputtering using a single ZnTe target of the 1:1 composition ratio. The base and working pressures for the growth of ZnTe thin films are 2 × 10 -7 Torr and 3 × 10 -3 Torr under Ar only ambient, respectively. The top W electrode dimension is a 50 μm square pattern defined by a typical photolithography approach. Two post annealing processes are also conducted in vacuum and nitrogen ambient of 10 -2 Torr and 2 Torr for comparison, respectively.
Chemical and electrical characterization. Crystal structure and chemical bonding analyses of ZnTe films verse annealing temperatures are taken by the high-resolution X-ray diffractometer (XRD) and the X-ray photoemission spectroscopy (XPS) using a K-Alpha+. DC current-voltage (I-V) and pulse characteristics are determined using a Keithley 4200 semiconductor parameter analyzer (Keithley 4200 SPA, Keithley Instruments, Inc.), where the compliance current of 1 mA is set to prevent the problem of possible breakdown of ZnTe selectors.
Received: 3 August 2020; Accepted: 16 September 2020 www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.