Applied Physics Express 4 (2011) 015204 DOI: 10.1143/APEX.4.015204
Volatile/Nonvolatile Dual-Functional Atom Transistor Tsuyoshi Hasegawa1;2 , Yaomi Itoh1;2 , Hirofumi Tanaka3 , Takami Hino1 , Tohru Tsuruoka1;2 , Kazuya Terabe1 , Hisao Miyazaki1 , Kazuhito Tsukagoshi1 , Takuji Ogawa3 , Shu Yamaguchi2;4 , and Masakazu Aono1 1
WPI Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan Japan Science and Technology Agency, CREST, Chiyoda, Tokyo, 102-0075, Japan 3 Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan 4 Department of Materials Engineering, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan 2
Received November 19, 2010; accepted December 5, 2010; published online December 24, 2010 We demonstrate a conceptually new atom transistor operation by electric-field control of the nanoionic state. The new atom transistor possesses novel characteristics, such as dual functionality of selective volatile and nonvolatile operations, very small power consumption (pW), and a high ON/OFF ratio [106 (volatile operation) to 108 (nonvolatile operation)], in addition to complementary metal oxide semiconductor (CMOS) process compatibility enabling the development of future computing systems that fully utilize highly-integrated CMOS technology. Cyclic endurance of 104 times switching was achieved with the prototype. # 2011 The Japan Society of Applied Physics
he international technology roadmap for semiconductors (ITRS) 2009 has predicted that in the year 2019, by introducing new materials and structures, present-day complementary metal oxide semiconductor (CMOS) transistors will be scaled to a technology node of 16 nm.1) However, ITRS 2009 also predicted that there are many difficult challenges in downscaling CMOS further than the 16 nm technology node, although the 8 nm technology node is expected to be achieved by 2024. Nanoionics devices, such as resistance random access memories (ReRAMs)2–4) and atomic switches,5–10) are also listed in ITRS 2009 as some of the most promising memory devices. Instead of the electronic charges controlled in most conventional memory devices, these two-terminal devices achieve electrical switching by controlling ionic diffusion and their reduction/oxidation processes. Since ionic control can be achieved in a small area to achieve electrical switching,3) nanoionics devices are expected to have the potential to achieve scaling smaller than 16 nm. A fast switching time of 5 ns was also reported for these nanoionic devices.4) Although two-terminal devices can configure completely new logic circuits, such as crossbar-circuits,11) threeterminal devices have potential as logic operation devices that can fully use semiconductor circuit technologies. Thus, three-terminal nanoionics devices have also been developed12–14) based on the above-mentioned two-terminal operation, in which a metal atomic filament is formed to connect source and drain electrodes. However, the state variable in these devices is the number of reduced/oxidized atoms/ions to form/dissolve the metal filament. Consequently, gate voltages (VG ) for turning-on/off vary depending on the ramp speed of VG and the previous states being similar to memristors,15) resulting in the difficulty of their being used as conventional logic devices. A large gate leak current of about 1 A, cyclic endurance and a low ON/OFF ratio (104 )14) have been issues that needed to be solved. We solved the subject issues by using nanoionic conductive states formed in an insulator, which can be controlled by a gate electric field. Figure 1(a) shows the operating concept of an atom transistor, where metal cations brought from the gate electrode form a conductive channel between the source and drain electrodes. When the concentration of metal cations, which is a function of the gate electric field, reaches a certain
T
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(a) Atom transistors in turning-off and turning-on operations. Metal cations/atoms form a conductive path between the source and the drain. (b) Scanning electron microscopy images of the fabricated atom transistor. (c) Cross-sectional schematics. Full stack layers of the atom transistor (left) and A–A cross-section of the stacked layers viewed from the left (right).
Fig. 1.
threshold value, a drastic change in conductivity, i.e., an insulator to conductive-state transition, takes place. When the gate electric field becomes smaller, metal cations move towards the gate electrode to turn off the atom transistor. Thus, the atom transistor works as a circuit element in a similar way to a silicon transistor, i.e., the state variable is VG . Figures 1(b) and 1(c) show electron micrographs of the developed atom transistor and cross-sectional schematics of the device. The source and drain electrodes of Pt (30 nm)/ Ti (10 nm) are separated by a 10-nm-thick layer of SiO2 . A 20- or 30-nm-thick layer of Ta2 O5 was formed on the sidewall of the stacked layers as an ionic conductive layer. Here, the sidewall was fabricated using a focused ion beam (FIB). A gate electrode of Pt/Cu or that of Pt/Ag was formed on the Ta2 O5 layer with the variation of their film thickness. All the layers were deposited by DC/RF sputtering with electron beam (EB)-lithographic patterning. Figure 2 shows the operating results of the atom transistor using Cu (30 nm) as the gate electrode material. Figure 2(a)
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(a) Change in drain (red) and gate (blue) currents during gate voltage sweeping from 0 to 1.5 V, and vice versa, is shown. 5 mV of VD was applied between the source and drain electrodes for measuring the currents. The green dashed line indicates the subthreshold slope of the volatile operation taking the hysteresis into account. (b) Change in drain (red) and gate (blue) currents during gate voltage sweeping from 0 to 3 V and vice versa is shown. VD ¼ VG 0:2 was applied for measuring the currents. (c) Temperature dependence of the on-resistance in the nonvolatile operation. (d) The distribution of turning on VG in the volatile operations. Atom transistors with 30-nm-thick Ta2 O5 did not show volatile operation in this study. (e) The distribution of turning-on VG in the nonvolatile operations. VG changes depending on the Ta2 O5 thickness.
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shows the changes in the drain current (ID ) and the gate current (IG ) in the atom transistor (Ta2 O5 : 20 nm) operation when subjected to VG sweeping. When VG was swept from 0 to 1.5 V, the atom transistor was turned on with a VG of 1.25 V. While sweeping the gate voltage to 0 V, ID decreased to the order of pA at VG ¼ 0:65 V. Thus, in this volatile operation, the ID changed by 6 orders of magnitude in the VG range of 0.65 to 1.25 V, corresponding to a subthreshold slope of 100 mV/decade. As shown in Fig. 2(b), when the VG was swept to 3 V, two types of switching were observed. First, switching occurred at VG ¼ 1:4 V, where ID increased to the order of 10 A, which is similar to the result shown in Fig. 2(a). Second, switching occurred at VG ¼ 2:65 V, increasing the ID by another two orders of magnitude. Once the second switching occurred, the on-state was kept at VG ¼ 0 V, requiring negative VG application to turn off the atom transistor. The ON/OFF ratio in the nonvolatile operation is 8 orders of magnitude larger. The on-resistance of the nonvolatile operations linearly decreased with decreasing ambient temperature, as shown in Fig. 2(c), suggesting that the conductive path consists of metal atoms. In both volatile and nonvolatile operations, the IG remained small. The range for turning on VG for the volatile operation and that for the nonvolatile operation are shown in Figs. 2(d) and 2(e), respectively. Although the VG ranges of the volatile and nonvolatile operations overlap because of the nonuniformity in the thickness of the Ta2 O5 layer of the fabricated devices in this study, VG for the volatile switching was about half of that for the nonvolatile switching in each device. Consequently, selective volatile and nonvolatile operation is available by controlling the range of the gate voltage. An additional point here is that the VG range for nonvolatile operations depends on the thickness of the Ta2 O5 layer, as
(a) Changes in the source (IS ), drain (ID ), and gate (IG ) currents while VG was swept between 0 and 2 V. 5 mV of VD was applied between the source and drain electrodes for measuring the currents. The red arrowhead indicates the VG achieved the turning-on process, and the blue arrowhead indicates the VG completed the turning-off process. (b) The distribution of VG for turning on (red) and for turning off (blue) in one hundred sequential switchings. (c) Distribution of RON and ROFF in the cyclic endurance test. (d) The ranges of gate voltage for turning on (red bars) and for turning off (blue bars). The ranges for volatile and nonvolatile operations are indicated. Fig. 3.
shown in Fig. 2(e), indicating that an electrical field of about 0.1 V/nm is required for the nonvolatile switching. Figure 3 shows the operating results of the atom transistor using an Ag (3 nm)-gate on a 20-nm-thick Ta2 O5 layer. The atom transistor showed volatile operation when the VG was swept in the range from 0 to 2 V, as shown in Fig. 3(a). The source current (IS ) and the drain current (ID ) abruptly increased at VG ¼ 1:35 V while VG was swept from 0 to 2 V, i.e., the atom transistor was turned on. The IS and ID decreased to the order of pA at 0.5 V while VG was swept from 2 to 0 V. During the switching operation, IG remained on the order of pA, as shown in Fig. 3(a). Consequently, the power consumption of the atom transistor is on the order of pW. The atom transistor with the Ag-gate achieved 104 times switching in the cyclic endurance measurement. Figure 3(b) shows the distribution of turning on VG and turning off VG in one hundred switching operations in series, which is part of the cyclic endurance measurement. Even though both the turning on VG and the turning off VG have ranges, they were clearly separated by VG ¼ 1 V. Although the VG s have ranges, both on-resistance and off-resistance were relatively uniform, as shown in Fig. 3(c). This uniformity, both in the on- and off-resistances, suggests that resistive switching is achieved by something like phase transition caused by a certain nanoionic state determined by the atomic/ionic arrangement. Atom transistors using an Ag-gate also show nonvolatile operation by sweeping VG to a value larger than 2 V. Figure 3(d) shows the ranges of turning on VG and turning off VG for the nonvolatile operations with those for the volatile operations. The results suggest that electrical fields of 75 and 190 mV/nm are required for volatile and nonvolatile operations, respectively. Thus, Ag-gate atom transistors also enable selective volatile and nonvolatile operations by controlling the range of the VG . Although further experiments will be required to reveal the whole mechanism of the selective volatile and nonvolatile atom transistor operations, the operations are understood as
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Schematic diagrams of the device structure (top) and atomistic structure at the interface along the source and drain electrodes (bottom) are illustrated. (a) VG application introduces metal cations (Cuþ ) into the Ta2 O5 layer, migrating towards the counter source and drain electrodes. (b) When the concentration reaches a certain value, CV , which puts a single Cuþ into each Ta2 O5 lattice, a conductive channel is formed by the Ta2 O5 lattices containing a Cuþ cation. (c) Application of larger VG achieves supersaturation of metal cations at the interface, resulting in Cu cluster nucleation. (d) Once the Cu cluster is formed, a negative VG application is required to dissolve the Cu clusters. Fig. 4.
follows. When a positive VG is applied, metal cations (Cuþ ) are supplied into the Ta2 O5 layer from the gate electrode, as shown in Fig. 4(a). Since the current flowing through the Ta2 O5 layer is on the order of pA, we can assume that the Ta2 O5 layer is almost an insulator and the electrical potential () linearly changes in the Ta2 O5 layer. In the equilibrium condition with a certain VG application, the electrochemical potential of the metal cations, ¼ 0 þ F þ RT ln C (0 is the chemical potential at C ¼ 1, F, R, T , and C are the Faraday constant, molar gas constant, temperature, activity coefficient and concentration of the metal cation, respectively), is constant everywhere in the Ta2 O5 layer, making an exponential distribution of metal cations with respect to the position in the Ta2 O5 layer.16) Namely, the electrical field determines the concentration of metal cations at the interface between the Ta2 O5 layer and source/drain electrodes. Theoretical study has predicted that a chain of Ta2 O5 lattices containing a single Cuþ cation in each forms a conductive channel.17) Thus, we can understand that the atom transistor is turned on when the concentration of metal cations reaches a certain value of CV , enabling all Ta2 O5 lattices at the interface between the source and drain electrodes to have a single Cuþ cation in each, as shown in Fig. 4(b). Decreasing the VG causes redistribution of the metal cations, i.e., moving of some cations toward the gate electrode, resulting in the disappearance of the conductive channel. On the other hand, nonvolatile operation is achieved by super-saturation-induced Cu cluster nucleation at the interface between the source and drain electrodes. In Cu cluster nucleation, Cuþ cations are reduced to Cu atoms. Consequently, the conductive channel in the nonvolatile operation is Cu clusters, which is consistent with the observed onresistance range from 180 to 1 k and their temperature dependence. The Cu clusters remain stable without VG application, resulting in nonvolatile operation. Negative VG application is required to re-ionize the Cu atoms for dissolving the Cu clusters, i.e., turning off the atom transistor which is in the nonvolatile-on state, as shown in Fig. 4(d). The Cu clusters make the atom transistors into two-terminal devices by electrically connecting the drain and source electrodes, being similar to the two-terminal atomic
switches.5–10) Although the two-terminal atomic switches form a metal filament between their top and bottom electrodes, the filament formation between the gate (top) electrode and the source/drain (bottom) electrodes rarely occurs in the atom transistor operations. This is because VG for the nonvolatile-on state of the atom transistors is smaller than that for the forming of the two-terminal atomic switches.18) In addition, limiting the amount of metal cations introduced into the Ta2 O5 layer using a thin Cu/Ag film (2– 3 nm) as the gate electrode can avoid the filament formation. Here, we demonstrated conceptually new three-terminal operations. The atom transistor shows volatile/nonvolatile selective operations with a high ON/OFF ratio (106 –108 ) and very small power consumption (pW). The volatile operation has a potential to achieve low VG , such as 0.5 V, operation by using a thin Ta2 O5 layer, since the electrical field required for volatile operation is 75 mV/nm. The nonvolatile operation is also suitable for achieving nonvolatile logic,19) where a very small write/erase current of pA is advantageous to the other devices, such as MRAMs, requiring a large current on the order of sub-mA.1,20) In addition, the selective volatile and nonvolatile operations might enable new types of logic operations/architectures. Acknowledgments Part of this work was conducted under the KeyTechnology Research Project, ‘‘Atomic Switch Programmed Device’’, supported by the Ministry of Education, Culture, Sports, science and Technology of Japan (MEXT), and the Japan Science and Technology Agency (JST)/CREST ‘‘Atom Transistor Project’’.
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