Abstract
Selector devices are indispensable components of large-scale memristor array systems. The thereinto, ovonic threshold switching (OTS) selector is one of the most suitable candidates for selector devices, owing to its high selectivity and scalability. However, OTS selectors suffer from poor endurance and stability which are persistent tricky problems for application. Here, we report on a multilayer OTS selector based on simple GeSe and doped-GeSe. The experimental results show improving selector performed extraordinary endurance up to 1010 and the fluctuation of threshold voltage is 2.5%. The reason for the improvement may lie in more interface states which strengthen the interaction among individual layers. These developments pave the way towards tuning a new class of OTS materials engineering, ensuring improvement of electrical performance.Selector devices are indispensable components of large-scale memristor array systems. The thereinto, ovonic threshold switching (OTS) selector is one of the most suitable candidates for selector devices, owing to its high selectivity and scalability. However, OTS selectors suffer from poor endurance and stability which are persistent tricky problems for application. Here, we report on a multilayer OTS selector based on simple GeSe and doped-GeSe. The experimental results show improving selector performed extraordinary endurance up to 1010 and the fluctuation of threshold voltage is 2.5%. The reason for the improvement may lie in more interface states which strengthen the interaction among individual layers. These developments pave the way towards tuning a new class of OTS materials engineering, ensuring improvement of electrical performance.Introduction
Memristor study is rapidly developing and can be applied in multiple applications of storage or computing, while the array is suffering from errors caused by a sneak current flowing through the unselected cell blocks[1]. Therefore, access devices such as transistor, diode or selector are becoming indispensable for large passive crossbar arrays to ensure high resistance when the device is at low voltages[2]. In the selector domain, ovonic threshold switching (OTS) selectors, based on chalcogenide materials, exhibit high selectivity and fast response time, which makes it one of the most promising candidate selectors[3-7].
As for the ovonic threshold devices, the threshold switching phenomenon was demonstrated in 1968 for the first time[8]. A large number of reports have since been reported. A part of researchers focused on doping different chemical elements into material system in order to optimize the characteristics[9-12]. This will bring about an increased material complexity, even with more than five elements in the same alloy[13-15]. Furthermore, a common doping with arsenic element is able to improve the endurance significantly[16,17]. However, arsenic is a highly toxic element which has negative impact on environmental protection and industrialization. Therefore, other scholars have turned their attention to two-terminal OTS materials, such as GeTe, ZnTe, SiTe[18] and GeSe[9,10], which are more and more attractive due to their simple composition and competitive performance. Nevertheless, poor endurance and stability troubled these material systems, which is unable to match memristor operation. Hence, it is worthy to enhance device performance by changing the structure. It is expected that the heterostructures of different chalcogenide can be tried to improve the performance of OTS devices, referring to the phase change heterojunction of phase change devices[19-21]. Furthermore, some reported multilayer OTS selectors were recently put forward with too much layers, which make it difficult to manufacture[21,22]. Therefore, a novel structure with simple multilayer is a new idea of device optimization and is quite necessary to be proposed.
In this article, we investigated a novel multilayer OTS selector with GeSe/doped-GeSe stacked structure. The experimental results demonstrated that the device had improved characteristics such as endurance and stability.
Experiments and methods
The multilayer devices were prepared with the cell size from 2 × 2 to 50 × 50μm2 on a p-type Si wafer with 200 nm-thick thermal oxide layers. TiN electrodes and GeSe material were deposited using the DC/RF sputtering method respectively. Doped GeSe were RF co-sputtered using the Ge2Se3 and Sb2Te targets at room temperature (25 °C).
A cross-sectional TEM image of the multilayer stacked structure of GeSe/GeSeSbTe (GS/GSST sample) is given inFig. 1(a). The stack consisted of five interlaced layers, including three layers of 7.9-nm-thick GeSe and two layers of 6-nm-thick GeSeSbTe. Total thickness was 35.7 nm and the composition was GexSey/GexSeySbzTe, as determined via conventional TEM-energy dispersive X-ray spectroscopy (EDS) analysis shown inFigs. 1(b) and1(c).
Figure 1.(Color online) (a) TEM image of the multilayer selector device. (b) EDS line scanning spectrum along the direction marked by the line in (a). (c) EDS mapping of multilayer selector device.
To compare the performance of the device, two types of other devices were prepared, namely a GeSe (GS sample) and a GeSe/GeSeSb (GS/GSS sample). The electrical characteristics of the devices with 10 × 10μm2 area were measured using a standard semiconductor parameter analyzer (Keithley 4200 SCS).
Results and discussion
The electrical characterization was mainly tested by quasi-static analysis, dynamic AC response and endurance analysis. The characteristics were tested under pulsed mode including triangular and trapezoidal voltage pulses. For chalcogenide glass, the electrical characteristics of the sample are mainly characterized by pulse testing to avoid enormous thermal and electrical stresses resulting from time lag in the DC voltage test[5,23].
Fig. 2 presents cycle curves of triangle pulse test under the same triangular pulse with rising/falling edge of 500 ns/1μs and a plateau length of 50 ns. As illustrated, all three samples show obvious threshold switching behavior.Figs. 2(a)–2(c) are the test results of monolayer GS sample, multilayer GSS sample and multilayer GSST sample, respectively. InFig. 2(a), the ON current of GS sample degrades greatly and the OTS characteristic becomes inconspicuous with the increasing number of cycles. This is probably due to the unstable chemical bonds within GeSe material without doping other elements. The threshold voltage (Vth) of GSS sample is gradually degenerated and the OTS characteristic becomes inconspicuous, which is the same as the GS sample shown inFig. 2(b). Owing to the increase of layers, a higherVth with rapid decline exists. InFig. 2(c), we report stable OTS characteristic of GS/GSST sample retaining with merely performance degradation after at least 20 cycles. By way of improving the structure and composition of the device, GS/GSST sample exhibits lowVth at 3.3 V and large selectivity at 104. The cumulative probability ofVth andVh (holding voltage) is shown inFig. 2(d). As demonstrated, theVth coefficient of variation (Cv) of GSST sample is 2.5% calculated byCv =σ/μ, which means stableVth andVh. According to our analysis, Sb and Te doping contribute to the final result. Sb and Te are adopted to counteract theVth rise and contribute to larger selectivity at the same time[11,24]. Te concentration reduces the band gap and concentrates defects[24], while Sb is able to bring about lower threshold voltage[11]. The multilayer structure also leads to stronger interaction among individual layers[19], which helps to improve device stability. The increasing of layers acting as effective diffusion barriers against electron transport during extensive cycling.
Figure 2.(Color online) Cycle curves of triangle pulse test. (a) Monolayer GS sample. (b) Multilayer GSS sample. (c) Multilayer GSST sample. (d) Cumulative probability ofVth andVh. The insets are the DC tests of each type of selector.
Table Infomation Is Not Enable
Fig. 3 demonstrates the results of multiple trapezoidal pulse test with the rising and falling edge of trapezoidal pulse of 100 ns/100 ns and a plateau length of 200 ns.Figs. 3(a)–3(c) are the test results of monolayer GS sample, multilayer GSS sample and multilayer GSST sample, respectively. InFig. 3(a), the GS sample loses OTS characteristic after several cycles of trapezoidal pulse. The GSS sample is able to present performance repeatability after at least 20 cycles, illustrated inFig. 3(b). However, it is noted that the continuous curves demonstrate the randomness of delay time, indicating poor stability. We cycle GS/GSST sample for over 50 cycles as observed inFig. 3(c). The curves of cycles pulse test are nearly superposed and the cumulative probability of delay time is presented inFig. 3(d), which exhibits the lowest delay time and variance compared with GS sample and GS/GSS sample, indicating high stability and merely performance degradation.
Figure 3.(Color online) Cycle curves of trapezoidal pulses test. (a) Monolayer GS sample. (b) Multilayer GSS sample. (c) Multilayer GSST sample. (d) Cumulative probability of delay time.
The endurances of GS and GS/GSS sample are shown inFigs. 4(a) and4(b), respectively. GS sample performs poor endurance with a degradation of high resistance after 104 pulses. Meanwhile, it is found that GS/GSS device presents an improvement of device endurance, after which it still presents a degradation of high resistance, indicating that multilayer structure has benefit effects on endurance improvement. However, it is still not up to expectations. In this case,Fig. 4(c) presents the endurance testing result of GS/GSST sample that is acquired using 7 V/200 ns programming pulses and 3.5 V/100μs reading pulses with rising/falling edge is 100 ns/100 ns. The endurance result of the sample does not show any degradation up to more than 1010 switching cycles as illustrated. It is worthy that the phenomenon, multilayer sample electronically switches between off and on states repeatedly, which also indicate the material's stability. However, owing to the dependence of pulse test accuracy on pulse width and range, high current can be measured within a shorter time while low current can only be tested accurately in much longer time. Therefore, although it is limited by the test conditions, we suppose that the sample has potential to achieve a higher endurance over than 1010. In addition,Fig. 4(d) illustrates the cumulative probability of on-state and off-state current. Compared with GS sample and GS/GSS sample, it performs the most stable current, which confirmed reliable switching on and off.
Figure 4.(Color online) Results of endurance test. (a) Monolayer GS sample. (b) Multilayer GSS sample. (c) Multilayer GSST sample. (d) Cumulative probability of ON and OFF resistance.
As-deposited materials and multilayer structure probably contribute to the improved endurance and stability. The increase of the layers indicates more interfaces, which bring strong interaction among individual layers. Take Ge–Se bonds as an example. The presence of Ge–Se bonds even in multilayers contributes to the strong inter-layers interaction and the high probability of Ge–Se formation[25]. Ge-Se bonds are important to avoid the OTS mechanism degradation. Besides, the doping of Te and Sb plays the same role. Sb–Se bonds and Sb–Sb bonds make the same contributions[11]. The increased concentration of Te and Sb is reported to decrease band-gap and concentrate traps[21,26]. Therefore, GS/GSST sample performs a superior endurance and stability.
The characteristics of the devices proposed in this paper are compared with those of various reported bidirectional threshold switching devices as shown inTable 1. It can be seen that the multilayer structure can bring up improvement of endurance, but at the same time bring about a decline in stability. Therefore, by doping other elements to optimize the device, we can finally get a device with improved endurance and stability. As illustrated, the device we proposed demonstrates the endurance at over 1010 with simple multilayer structure, indicating greater potential for integration with memristors. The coefficient of variation (Cv) ofVth is 2.5 % and theCv of delay time is 3.9 %, which indicate the improved stability. Furthermore, it can be seen that off-state current (Ioff) and selectivity are able to achieve a pretty good level. Other multilayer device with the best reported performance is shown inTable 1[21]. In comparison, the multilayer device performs a better endurance and stability with a simpler structure. Further optimization can be achieved in selectivity and on state current. Therefore, it can be seen that the OTS selector can be optimized in the future for memristor integration and other applications.
Conclusion
Endurance and stability are crucial characteristics for OTS devices in a variety of applications. In this article, a reliability optimization method for OTS devices is investigated. We propose a symmetric multilayer OTS selector with improved endurance and stability. Compared with the characteristics of two other devices in the same material system, we provided a five interlaced multilayer structure based on GeSe/doped-GeSe stacked structure. It is observed that multilayer structure performs endurance more than 1010 cycles and 2.5% fluctuation ofVth. Therefore, an optimization of selector device is given by improving structure and doping. The device structure can provide a direction of device optimization based on improved structure of selector cells to manufacture high-performance selector and to be utilized to memristor integration with promising outlook.
Acknowledgements
This work was supported by National Natural Science Foundation of China (Grant Nos. 61974164, 62074166, 61804181, 62004219, and 6200422).
References
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