Letter pubs.acs.org/NanoLett
Bipolar Resistive Switching of Single Gold-in-Ga2O3 Nanowire Chia-Wei Hsu and Li-Jen Chou* Department of Materials Science and Engineering, National Tsing Hua University, No. 101, Section 2 Kuang-Fu Road, Hsinchu, 30013, Taiwan, Republic of China S Supporting Information *
ABSTRACT: We have fabricated single nanowire chips on gold-in-Ga2O3 core−shell nanowires using the electron-beam lithography techniques and realized bipolar resistive switching characteristics having invariable set and reset voltages. We attribute the unique property of invariance to the built-in conduction path of gold core. This invariance allows us to fabricate many resistive switching cells with the same operating voltage by simple depositing repetitive metal electrodes along a single nanowire. Other characteristics of these core−shell resistive switching nanowires include comparable driving electric field with other thin film and nanowire devices and a remarkable on/off ratio more than 3 orders of magnitude at a low driving voltage of 2 V. A smaller but still impressive on/off ratio of 10 can be obtained at an even lower bias of 0.2 V. These characteristics of gold-in-Ga2O3 core−shell nanowires make fabrication of future high-density resistive memory devices possible. KEYWORDS: Resistive switching, core−shell, nanowire, gold-in-Ga2O3, metal/insulator/metal
R
can reduce the operation current to nanoampere and lower the driving electric field to tens of kilovolts per centimeter.23 Moreover, heterostructure nanowires have attracted considerable attention owing to their unique properties and multifunctional characteristics.24−28 Gold-in-Ga2O3 core−shell and peapod nanowires can be synthesized in one-step process or two-step process with postannealing.29,30 This unique nanowire, metal Au embedded in dielectric material Ga2O3, has special properties in photo response with specific wavelength.31 In this study, we report for the first time a bipolar resistive switching behavior of a single gold-in-Ga2O3 core−shell nanowire. Conducting a thorough current−voltage analysis of the nanowire devices we have noticed a unique property that the set and reset voltages do not depend on the electrode distance. These results may be important to future nanodevices in resistive switching. Figure 1a shows an FESEM image of high-density gold-inGa2O3 core−shell nanowires. Highly uniform nanowires were synthesized through the vapor−liquid−solid (VLS) mechanism with gold nanoparticles as catalysts on top of nanowires. The inset shows a high-magnification FESEM image of the core− shell nanowire with a diameter around 200 nm. There is a clear gold core in the center. To measure the resistive switching of these nanowires, we transferred the core−shell nanowires from the substrate onto the electrical measurement chip by three-axis joystick micromanipulators as shown in Figure 1b. Using the electron beam lithography and e-gun system, several titanium and gold metal electrodes were defined onto a single nanowire
esistive random access memory (RRAM) with a simple sandwich structure of metal/insulator/metal (MIM) is one of the emerging nonvolatile memory technologies which has great features such as faster writing speed, smaller device feature size, lower programming voltage and multistate memory1,2 as compared to other counterparts such as phasechange RAM (PRAM),3 magnetoresistive RAM (MRAM),4 flash memory5 and ferroelectric RAM (FeRAM).6 Resistive switching phenomena are found in perovskite oxides and binary transition metal oxides such as PCMO,7 Cr-doped SrZrO3,8 NiO,9 ZnO,10 TiO2,11 and Nb2O5,12 etc. Resistive switching characteristics can be classified into bipolar and unipolar switching types based on the set and reset processes controlled by the polarity or magnitude of the applying bias. The set process defines that the resistance switches from high resistance state (HRS) to low resistance state (LRS); whereas the reset process runs inversely. 13 Several mechanisms such as filamentary model,14 charge-trap model,15 space-charge-limited current,16 and insulator−metal transition17 have been proposed to illustrate resistive switching characteristics but more in-depth investigations are still needed. Recently, riding with the revolution of nanotechnology and development of electron beam lithography, we have fabricated the electrical measurement chips on nanowires and assemble electronic or photonic devices beyond photolithographic limitation.18,19 Making single-nanowire devices provides not only a controllable method to investigate the resistive switching mechanism at sublithographic scale but also a possible means to further increasing the density of memory cells.20−22 For practical applications, reducing the operation current is important because higher current means higher power consumption and requires a larger chip size. The single-nanowire memory devices © 2012 American Chemical Society
Received: May 17, 2012 Revised: July 1, 2012 Published: July 23, 2012 4247
dx.doi.org/10.1021/nl301855u | Nano Lett. 2012, 12, 4247−4253
Nano Letters
Letter
Figure 1. Top-view FESEM and FETEM images of nanowire structures. (a) As-prepared gold-in-Ga2O3 core−shell nanowires on silicon substrate. The inset of part a is a high-magnification FESEM image of a core−shell nanowire with a diameter around 200 nm. The center bright area is the gold core and outer dark region is the Ga2O3 shell layer. (b) Chip SEM image. A core−shell nanowire is placed in the chip area with a three axis joystick micromanipulator under optical microscope. (c) As-prepared chip FESEM image. (d) Low-magnification FETEM image with a clear dark area in the center. (e) High resolution image and diffraction pattern. The zone axis is [101] direction and the d-spacings of (202̅) and (111̅) were 0.283 and 0.264 nm, respectively. Composition analysis in the inset of part e shows that the center area is gold core and the outer region is the Ga2O3 layer.
resistive switching behavior, i.e. the set and reset processes are controlled by the polarity of electric field. The set voltage (Vset) defined as the voltage needed to drive HRS (blue curve) to LRS (red curve) transition is at around 5 V or 5 × 104 V/cm. Changing from the LRS to HRS state, the reset voltage (Vreset) takes place at about −8.5 V or −8.5 × 104 V/cm. The operating electric field is comparable to those of thin film devices32 and single nanowire devices.33 The higher driving voltages required in nanowires are caused by the larger electrode distance as compared to that of thin film. In order to verify the type of resistive switching of the gold-in-Ga2O3 core−shell nanowire, we have performed the reverse and parallel polarity voltage sweeps after the set process. Parallel polarity means the same voltage polarity as used in the set process. Reverse polarity takes the opposite. Compared to Figure 2a, Figure 2b shows the I−V characteristics of core−shell nanowires with applied positive bias (parallel polarity) after set process. The blue, red, and green curves demonstrate the HRS, LRS, and second-time positive voltage sweep, respectively. The current state of green curve is higher than HRS but the nanowire burns off and breaks down directly at about 2 × 10−6 A due to large Joule heating. It does not show a high resistive state as a typical unipolar switching device. Data from parts a and b of Figure 2 indicate that gold-in-Ga2O3 core−shell nanowire exhibits bipolar resistive switching behavior rather than unipolar. As far as conduction mechanism is concerned, this result resonates with other published studies.34,35 During the set process, oxygen ions (O2−) migrate toward the anode to form oxygen-vacancy
as shown in Figure 1c. Figure 1d shows the low-magnification FETEM image with a clear dark area inside the light shell layer. The diameter of this nanowire was about 50 nm. Energydispersive X-ray analysis suggests that the center area is the gold core and the outer region is the Ga2O3 layer as shown in the inset of Figure 1e. Figures 1e shows the high-resolution image and the diffraction pattern of Ga2O3 shell layer. The zone axis was [101] direction and the d-spacings of (202̅) and (111̅) were 0.283 and 0.264 nm, respectively. Those two values fitted well with the theoretical values of monoclinic Ga2O3. Figure 2a shows the resistive switching characteristics, plotted in a semilogarithmic scale, of core−shell nanowire with a diameter of about 120 nm. The gold core is 40 nm in diameter and the Ga2O3 shell layer has a shell thickness of about 40 nm. The inset shows a corresponding chip FESEM image. Due to the symmetric structure of titanium/gold-inGa2O3/titanium, we can apply positive or negative biases to perform the resistive switching characteristics (see Supporting Information). The polarity of set and reset processes is defined by the polarity of every first voltage sweep. That is to say, if positive voltage is applied to execute the forming process, the parallel or reverse polarity voltage sweeps are defined as positive and negative voltages, respectively. By performing the forming process (green line), the current increased from 2.4 × 10−12 A at 1 V to the current compliance, 2 × 10−7 A at 14.3 V. This sharp increase of current indicates that gold-in-Ga2O3 core−shell nanowire may have two stable states. After the forming process, this core−shell nanowire exhibits a bipolar 4248
dx.doi.org/10.1021/nl301855u | Nano Lett. 2012, 12, 4247−4253
Nano Letters
Letter
Figure 2. (a) I−V characteristics of a single gold-in-Ga2O3 core−shell nanowire showing a bipolar switching and the inset is the chip FESEM image. The green, blue and red represent the initial state, off state and on state, respectively. (b) I−V characteristic of a single gold-in-Ga2O3 core−shell nanowire with voltage sweeping in the same polarity of bias as that in the forming process. The blue, red and green curves demonstrate the HRS, LRS and second time positive voltage sweep, respectively. It burns off and breaks down directly at about 2 × 10−6 A due to large Joule heating. The retention test curve in part c illustrates that on and off states can last more than 30000 s. The endurance test in part d shows that the on/off ratio is more than 3 orders of magnitude and the cycle number is over 100 cycles.
conducting filaments inside the Ga2O3 layer. After the set process, the oxygen reservoir forms at the anode side because of the accumulation of O2‑ ions.34 For reset, it can occur either under the reverse polarity bias (bipolar behavior) to terminate the conducting filaments by migrating O2‑ ions from the anode to Ga2O3 layer because drift and diffusion forces perform at the same direction or under the same polarity bias (unipolar behavior) without the interfacial barrier since the diffusion force can overcome the drift force.35 Noble metals such as platinum, gold and rubidium used as electrodes on both ends typically show unipolar behavior because noble metal is resistant to oxidation and no interfacial layer forms. However, most electrodes made of oxidizable materials such as titanium and aluminum may show bipolar characteristics due to the formation of an interfacial layer to act as the barrier for O2− ions migration.35 Our nanowire devices having titanium electrodes show they are bipolar. That is consistent with what reported in literature about bipolar behavior with titanium as the electrode in which an interface layer forms. Moreover, in comparison with the I−V curve of pure Ga2O3 nanowire as shown in Figure S2 (see Supporting Information), only core− shell nanowire can exhibit bipolar resistive switching characteristics. Parts c and d of Figure 2 show the retention behavior and the switching endurance of gold-in-Ga2O3 core−shell nanowire, respectively. The LRS and HRS were demonstrated to remain at least 3 × 104 s at a bias of 2 V and the on/off (LRS to HRS) ratio was over 103. The high on/off ratio is desired in commercial memory devices. A smaller but still impressive on/ off ratio of 10 can be obtained at an even lower bias of 0.2 V. In addition to the high on/off ratios, the switching endurance of
these nanowire devices lasts at least 100 cycles of set and reset processes, and the operation current can reduce to tens of nanoampere for low power consumption. As reported in numerous published papers, there are many great features in resistive switching memory devices such as faster writing speed, smaller device feature size, lower programming voltage and multistate memory in RRAM. Nevertheless, the Vset and Vreset are found to depend on the distance between two electrodes. As the distance increases, it needs a larger driving voltage to set and reset the devices.36−38 This distance-dependent voltage variation makes the precise control of the thickness of insulation layer critical to the fabrication of resistive switching devices. In contrast, gold-inGa 2 O 3 core−shell nanowire can provide the distance independent Vset and Vreset. Figure 3 shows the resistive switching characteristics with different distance between electrodes, and the inset indicates an FESEM image of a device with six metal electrodes. This core−shell nanowire is composed of a gold core of 50 nm in diameter and Ga2O3 shell layer of 40 nm in thickness. The blue, red and green curves represent the resistive switch characteristics corresponding to different electrode distances at 900, 1630, and 2440 nm, respectively. These three curves demonstrate that the bipolar switch characteristic and the driving voltages are similar with Vset and Vreset around 5.4 V and −8.5 V, respectively. The almost invariable Vset and Vreset is a unique feature compared with other reports.36−38 We attribute the invariance to the presence of a highly conductive gold metal core. As the gold core can serve as a conductor channel in the center of core− shell nanowire, the conducting path between two electrodes is 4249
dx.doi.org/10.1021/nl301855u | Nano Lett. 2012, 12, 4247−4253
Nano Letters
Letter
space-change-limited mechanism.39 These mechanisms can be identified by the isothermal current density to electric field (J− E) curve. Figure 4a shows the J−E curve in the HRS of Figure 2 and parts b−d of Figure 4 indicate the blue, red and green lines in Figure 3, respectively, and the insets show corresponding curve fitting results. The J−E curve of a gold-in-Ga2O3 core− shell nanowire in the HRS shows a linear characteristic (red fitting curve) in the low electric field region. However, in the high electric field, the current density is exponentially proportional to the electric field (blue fitting curve). Between two distinct J−E regions, there exists a critical electric field Ec. As the applied field increases above Ec, the exponential relation is consistent with the thermionic emission model. Typically, the thermionic emission mechanism describes that the heatinduced charge carriers transport owing to a high potential barrier, which is much larger than kT at room temperature. According to the thermionic emission theory, the current density is defined as39
Figure 3. Bipolar switching characteristic of a single gold-in-Ga2O3 core−shell nanowire with different distances between electrodes and the inset is the FESEM chip image. The blue, red and green curves are for electrode distance at 900, 1630, and 2440 nm, respectively. These set and reset voltages are almost the same with different distances.
ln J = ln(A**T 2) −
q q ϕB + V kBT nkBT
where A** is the effective Richardson constant, ϕB is the barrier height, q is the electron charge, and kB is the Boltzmann constant. We have deduced the barrier height qϕB ≈ 0.52 eV at A** = 33.6 A/cm2 K2 40 from the intercepts of the natural logarithm J to E curves. Since the work function of titanium is 4.33 eV and the electron affinity of Ga2O3 is 3.13 eV,41,42 we can deduce that the Schottky barrier height to be 1.2 eV. A difference between the calculated value of qϕB and theoretically expected values has been often noticed in the Schottky junction systems. This departure may result from charge tunneling, interfacial states and spatial barrier inhomogeneity.43 Besides,
dominated by the conductivity path in the Ga2O3 layer between the metal electrode and gold core. Therefore, the values of Vset and Vreset depend mainly on the thickness of the Ga2O3 shell layer but not on the length of nanowires. This unique property, i.e. invariant biases of Vset and Vreset, can greatly enhance the density of cells in a single nanowire device and takes the commercial nanowire memory industry a big step forward. There exist several models to explain the electrical transport mechanisms of metal-to-semiconductor contact such as the Fowler−Nordheim tunneling, thermionic emission, Frenkel− Poole emission, Ohmic conduction, ionic conduction and
Figure 4. J−E curves in the HRS with curve fitting. (a) J−E curve of the device shown in Figure 2a. (b−d) J−E curves of the blue, red and green lines shown in Figure 3, respectively. These curves show linear characteristic in the low operated electric field region; however in the high electric field, the current density is exponentially proportional to the electric field. There exists a critical electric field Ec, between linear region and thermionic emission region. The Ec values are similar for different distances between electrodes implied that the barrier is the same. 4250
dx.doi.org/10.1021/nl301855u | Nano Lett. 2012, 12, 4247−4253
Nano Letters
Letter
Figure 5. J−E curves in the LRS and the insets show the curve fitting results. (a) J−E curve of the devices shown in Figure 2a. (b−d) J−E curves of the blue, red and green lines shown in Figure 3, respectively. Those curves with a quadratic current density to electric field behavior obey the spacecharge-limited mechanism.
the critical electric field Ec, is almost the same regardless of the different electrode distances. This fact implies that the titanium/Ga2O3 barrier height is invariable along the whole nanowire. Figure 5 shows the J−E curves in the LRS state and the insets indicate the curve fitting results. Those curves with quadratic current density dependence to the electric field follow the space-charge-limited mechanism. The space-change-limited behavior is a mobility-dominated transport model due to the low effective carrier concentration caused by low intrinsic doping and charge traps.44,45 In this mechanism, once all the traps are filled with carriers, further injected carriers from the electrodes can then transport through the condition band and the current will suddenly rise significantly. Applying the spacecharge-limited mechanism to nanowires we can express the current density as follows46
potential barrier between Ga2O3 and titanium electrode, which is much larger than kT at room temperature. Simultaneously, during the set process, the applied forward bias drives O2‑ ions from lattice toward the anode side and the oxygen vacancies (VO) move to the cathode side to form the oxygen-vacancy conducting filaments in the Ga2O3 layer.34 This process is called soft breakdown. The conduction in the dielectric layer after set process is mainly due to electron transport among these VO.47 As the oxygen-vacancy conducting filaments connect with titanium electrode and gold core, the electrons can transport along the path of titanium/Ga2O3/gold/Ga2O3/ titanium which is like connecting two resistive switching cells in series. That is to say, electrodes can conduct electrons via VO easily due to the soft breakdown occurred in the Ga2O3 layer under large driving electric field as compared to normal operation. In the LRS, under the reverse bias, the I−V behavior may follow the space-charge-limited mechanism owing to excessive electrons being injected from the electrode.44 Ga2O3, a wide bandgap material with low carrier concentration and a high charge trap density during the fabrication process should enhance the space-charge-limited behavior. Because of the Ohmic transport behavior of metal gold core, the current flow is limited by the carrier transport within Ga2O3 layer between titanium electrode and gold core following the space-chargelimited mechanism. Therefore, the transport behavior in the LRS is governed by the space-charge-limited mechanism. Simultaneously, during the reset process, the applied reverse voltage drives the O2‑ ions from the anode side to the Ga2O3 layer to break the oxygen-vacancy conducting filaments. After reset process, the condition is similar to the original Ga2O3 since some O2‑ ions reoccupy VO. However, the Ga2O3 layer may not return to its original status. The microstructure of Ga2O3 contains few VO, and it will change the transport mechanism from the space-charge-limited mechanism back to
⎛R⎞ E2 J = ξ⎜ ⎟kε0μ ⎝L⎠ L where R and L are the radius and the length of core−shell nanowire, respectively, ξ(R/L) is an aspect ratio function, k is the dielectric constant of Ga2O3, ε0 is the permittivity of free space and μ is the carrier mobility. In this space-charge-limited model for nanowires, the current density depends on the square of the applied electric field. Ga2O3 is a wide bandgap material (∼5 eV) with low carrier concentration and the nanostructure with a high charge trap density during synthesis should enhance the effect of the space-charge-limit current. In short, two transport mechanisms, i.e., the thermionic emission mechanism in HRS and space-charge-limited mechanism in LRS, account for different electron transport and conducting paths upon biasing. In the HRS of gold-in-Ga2O3 core−shell nanowire, the I−V behavior follows the thermionic emission mechanism due to a 4251
dx.doi.org/10.1021/nl301855u | Nano Lett. 2012, 12, 4247−4253
Nano Letters
Letter
Figure 6. Schematic drawings depicting conducting paths and the transport mechanism that explain the properties of invariant Vset and Vreset against different distances between electrodes. (a) Demonstration that the oxygen vacancy filament connects to the gold core in the center of nanowire because the distance between two electrodes, 900 nm, is much larger than the thickness of Ga2O3 shell layer, 40 nm. (b) Indication that the oxygen vacancy channel can link to the gold core due to the smaller diameter compared to the distance. (c) Illustration of a possible resistive switching device array picture formed gold-in-Ga2O3 core−shell nanowires.
In conclusion, we have successfully demonstrated invariable set and reset voltages in gold-in-Ga2O3 core−shell nanowires, which are synthesized through the VLS growth mechanism. This invariant result because electrons transport through the conducting paths formed at the Ga2O3 shell layer and Au metal core. The invariant set and reset voltages against the distance between electrodes makes the fabrication of high-density RRAM devices possible by depositing repetitive electrodes along the nanowires. These gold-in-Ga2O3 core−shell nanowires exhibit comparable driving electric field (5 × 104 V/cm for set process and −8.5 × 104 V/cm for reset process) compared to other thin film and single nanowire devices. The on/off ratio more than 3 orders of magnitude at 2 V is large enough to make effective resistive switching devices. A smaller but still impressive on/off ratio of 10 can be obtained at an even lower bias of 0.2 V. In addition, the retention time and the endurance cycles are more than 3 × 104 s and 100 cycles, respectively. Different mechanisms account for carriers transport in the HRS (thermionic emission mechanism) and LRS (space-charge-limited mechanism) states. Experimental Section. Gold-in-Ga2O3 core−shell nanowires were synthesized on an amorphous SiO2 substrate with commercial Ga powders (purity 99.9999%) source and sizecontrolled gold nanoparticles catalyst through the vapor− liquid−solid (VLS) growth in the three-zone heating horizontal furnace at a pressure of 1 × 10−2 Torr. Annealing was carried out at 800 °C with a ramping rate of 20 °C/min for 30 min. The details of structure and growth mechanism can be seen in our previous reports.18,29,30 The morphologies and crystal structures were analyzed by field-emission scanning (FESEM, JSM-6500F) and transmission electron microscopes (FETEM, JEM-3000F). In order to investigate the electron transport behaviors of gold-in-Ga2O3 core−shell nanowires, we have fabricated the single nanowire devices on the heavily doped ntype silicon wafer covered with 500 nm thick thermally grown SiO2 layer as the insulating layer. On the top of it, we have employed photolithography to define platinum electrodes.
the thermionic emission mechanism. On the other hand, after set process, the charge traps increase due to the formation of VO by forward bias to move the O2‑ to the anode and it then changes the transport mechanism from the thermionic emission to the space-charge-limited. Parts a and b of Figure 6 illustrate the possible transport mechanism attributed to the observed invariance Vset and Vreset for different distances between electrodes. As we apply the forward bias between two electrodes, oxygen vacancy filament forms in the Ga2O3 shell layer along the direction of electric field. Since the distance between two electrodes, 900 nm, is much larger than the thickness of Ga2O3 shell layer, 40 nm, the oxygen vacancy filament tree can form, touch and connect easily to the gold core in the center of nanowire as shown in Figure 6a. As the oxygen vacancy channels form, electrons can transport through the gold core. When we increase the distance to 1630 or 2440 nm in our experiments, the oxygen vacancy filament trees can still link to the gold core due to the smaller Ga2O3 shell thickness as compared to the distance between electrodes as shown in Figure 6b. We attribute the finite Ga2O3 layer thickness and the conducting gold core to the invariable set and reset voltage in the gold core-Ga2O3 shell nanowire structures. Taking advantage of this unique property of invariant biases of Vset and Vreset, we can realize numerous resistive switching cells in a single core−shell nanowire by depositing repetitive metal electrodes. A possible design of resistive switching device array is proposed in Figure 6c. The electrodes parallel and vertical to the gold-in-Ga2O3 core−shell nanowires would serve as word lines and bit lines, respectively, to write, read and erase data. Because of the invariable Vset and Vreset, we can apply the same operation voltage between a word line and any one of the bit lines to store information into the resistive switching cells which form below the bit lines. By narrowing the spacing between bit lines, we can achieve high density resistive switching cells in a single gold-in-Ga2O3 core− shell nanowire. 4252
dx.doi.org/10.1021/nl301855u | Nano Lett. 2012, 12, 4247−4253
Nano Letters
Letter
(18) Hsieh, C. H.; Chang, M. T.; Chien, Y. J.; Chou, L. J.; Chen, L. J.; Chen, C. D. Nano Lett. 2008, 8, 3288−3292. (19) Lu, W.; Xie, P.; Lieber, C. M. IEEE Trans. Electron Devices 2008, 55, 2859−2876. (20) Nagashima, K.; Yanagida, T.; Oka, K.; Taniguchi, M.; Kawai, T.; Kim, J. S.; Park, B. H. Nano Lett. 2010, 10, 1359−1363. (21) Nagashima, K.; Yanagida, T.; Oka, K.; Kanai, M.; Klamchuen, A.; Kim, J. S.; Park, B. H.; Kawai, T. Nano Lett. 2011, 11, 2114−2118. (22) Kim, K. H.; Gaba, S.; Wheeler, D.; Cruz-Albrecht, J. M.; Hussain, T.; Srinivasa, N.; Lu, W. Nano Lett. 2012, 12, 389−395. (23) Oka, K.; Yanagida, T.; Nagashima, K.; Tanaka, H.; Kawai, T. J. Am. Chem. Soc. 2009, 131, 3434−3435. (24) Lauhon, L. J.; Gudiksen, M. S.; Wang, D.; Lieber, C. M. Nature 2002, 420, 57−61. (25) Xiang, J.; Lu, W.; Hu, Y.; Wu, Y.; Yan, H.; Lieber, C. M. Nature 2006, 441, 489−493. (26) Qian, F.; Gradecak, S.; Li, Y.; Wen, C. Y.; Lieber, C. M. Nano Lett. 2005, 5, 2287−2291. (27) Chen, C. Y.; Lin, Y. K.; Hsu, C. W.; Wang, C. Y.; Chueh, Y. L.; Chen, L. J.; Lo, S. C.; Chou, L. J. Nano Lett. 2012, 12, 2254−2259. (28) Chang, M. T.; Chen, C. Y.; Chou, L. J.; Chen, L. J. ACS Nano 2009, 3, 3776−3780. (29) Hsieh, C. H.; Chou, L. J.; Lin, G. R.; Bando, Y.; Golberg, D. Nano Lett. 2008, 8, 3081−3085. (30) Chen, P. H.; Hsieh, C. H.; Chen, S. Y.; Wu, C. H.; Wu, Y. J.; Chou, L. J.; Chen, L. J. Nano Lett. 2010, 10, 3267−3271. (31) Wu, Y. J.; Hsieh, C. H.; Chen, P. H.; Li, J. Y.; Chou, L. J.; Chen, L. J. ACS Nano 2010, 4, 1393−1398. (32) Sawa, A. Materialstoday 2008, 11, 28−36. (33) Oka, K.; Yanagida, T.; Nagashima, K.; Kawai, T.; Kim, J. S.; Park, B. H. J. Am. Chem. Soc. 2010, 132, 6634−6635. (34) Gao, B.; Sun, B.; Zhang, H.; Liu, L.; Liu, X.; Han, R.; Kang, J.; Yu, B. IEEE Electron Device Lett. 2009, 30, 1326−1328. (35) Yu, S.; Philip Wong, H. −S. IEEE Electron Device Lett. 2010, 31, 1455−1457. (36) You, Y. H.; So, B. S.; Hwang, J. H.; Cho, W.; Lee, S. S.; Chung, T. M.; Kim, C. G.; An, K. S. Appl. Phys. Lett. 2006, 89, 222105. (37) Kim, S. I.; Lee, J. H.; Chang, Y. W.; Hwang, S. S.; Yoo, K. H. Appl. Phys. Lett. 2008, 93, 033503. (38) Kang, Y. H.; Choi, J. H.; Lee, T. I.; Lee, W.; Myoung, J. M. Solid State Commun. 2011, 151, 1739−1742. (39) Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices; WileyInterscience: New York, 2007. (40) Irmscher, K.; Galazka, Z.; Pietsch, M.; Uecker, R.; Fornari, R. J. Appl. Phys. 2011, 110, 063720. (41) Huang, Y.; Yue, S.; Wang, Z.; Wang, Q.; Shi, C.; Xu, Z.; Bai, X. D.; Tang, C.; Gu, C. J. Phys. Chem. B 2006, 110, 796−800. (42) Gowtham, S.; Deshpande, M.; Costales, A.; Pandey, R. J. Phys. Chem. B 2005, 109, 14836−14844. (43) Shang, D. S.; Sun, J. R.; Shi, L.; Shen, B. G. Appl. Phys. Lett. 2008, 93, 102106. (44) Gu, Y.; Lauhon, L. J. Appl. Phys. Lett. 2006, 89, 143102. (45) Rose, A. Phys. Rev. 1955, 97, 1538−1544. (46) Chiang, Y. D.; Chang, W. Y.; Ho, C. Y.; Chen, C. Y.; Ho, C. H.; Lin, S. J.; Wu, T. B.; He, J. H. IEEE Trans. Electron Devices 2011, 58, 1735−1740. (47) Xu, N.; Liu, L.; Sun, X.; Liu, X.; Han, D.; Wang, Y.; Han, R.; Kang, J.; Yu, B. Appl. Phys. Lett. 2008, 92, 232112.
Single nanowire is picked up onto the measurement chip with a three-axis joystick micromanipulator having glass tips via static electricity under optical microscope. The titanium contact metal and gold antioxidant metal electrodes are deposited to connect to the platinum electrodes by e-gun system and defined by electron beam lithography. The electrical characteristics are measured with a Keithley 4200 in atmospheric air at room temperature.
■
ASSOCIATED CONTENT
S Supporting Information *
Figures showing positive and negative biases to perform the resistive switching characteristics and the I-V characteristics of pure Ga2O3 nanowires. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Authors gratefully acknowledge Chii Dong Chen, Kuang Chien Hsieh, Chih Chien Chiang, and Chih Yen Chen for helpful equipment and discussions. We also acknowledge National Science Council and the Foundation Grants 101F2289A8 and 99-2923-E-007-002-MY3 for financial assistance.
■
REFERENCES
(1) Strukov, D. B.; Snider, G. S.; Stewart, D. R.; Williams, R. S. Nature 2008, 453, 80−83. (2) Waser, R.; Dittmann, R.; Staikov, G.; Szot, K. Adv. Mater. 2009, 21, 2632−2663. (3) Kolobov, A. V.; Fons, P.; Frenkel, A. I.; Ankudinov, A. L.; Tominaga, J.; Uruga, T. Nature Mater. 2004, 3, 703−708. (4) Moritomo, Y.; Asamitsu, A.; Kuwahara, H.; Tokura, Y. Nature 1996, 380, 141−144. (5) Pavan, P.; Bez, R.; Olivo, P.; Zanoni, E. Proc. IEEE 1997, 85, 1248−1271. (6) Scott, J. F.; Araujo, C. A. P. D. Science 1989, 246, 1400−1405. (7) Li, S. L.; Shang, D. S.; Li, J.; Gang, J. L.; Zheng, D. N. J. Appl. Phys. 2009, 105, 033710. (8) Liu, C. Y.; Wu, P. H.; Wang, A.; Jang, W. Y.; Young, J. C.; Chiu, K. Y.; Tseng, T. Y. IEEE Electron Device Lett. 2005, 26, 351−353. (9) Park, G. S.; Li, X. S.; Kim, D. C.; Jung, R. J.; Lee, M. J.; Seo, S. Appl. Phys. Lett. 2007, 91, 222103. (10) Chang, W. Y.; Lin, C. A.; He, J. H.; Wu, T. B. Appl. Phys. Lett. 2010, 96, 242109. (11) Fujimoto, M.; Koyama, H.; Konagai, M.; Hosoi, Y.; Ishihara, K.; Ohnishi, S.; Awaya, N. Appl. Phys. Lett. 2006, 89, 223509. (12) Sim, H.; Choi, D.; Lee, D.; Seo, S.; Lee, M. J.; Yoo, I. K.; Hwang, H. IEEE Electron Device Lett. 2005, 26, 292−294. (13) Waser, R.; Aono, M. Nat. Mater. 2007, 6, 833−840. (14) Kim, D. C.; Seo, S.; Ahn, S. E.; Suh, D.-S.; Lee, M. J.; Park, B.H.; Yoo, I. K.; Baek, I. G.; Kim, H. −J.; Yim, E. K.; Lee, J. E.; Park, S. O.; Kim, H. S.; Chung, U-In; Moon, J. T.; Ryu, B. I. Appl. Phys. Lett. 2006, 88, 202102. (15) Chen, A.; Haddad, S.; Wu, Y. C.; Lan, Z.; Fang, T. N.; Kaza, S. Appl. Phys. Lett. 2007, 91, 123517. (16) Xia, Y.; He, W.; Chen, L.; Meng, X.; Liu, Z. Appl. Phys. Lett. 2007, 90, 022907. (17) Kim, D. S.; Kim, Y. H.; Lee, C. E.; Kim, Y. T. Phys. Rev. B 2006, 74, 174430. 4253
dx.doi.org/10.1021/nl301855u | Nano Lett. 2012, 12, 4247−4253
