Bi-stable resistive switching characteristics in Ti-doped ZnO thin films
© Younis et al.; licensee Springer. 2013
Received: 20 February 2013
Accepted: 22 March 2013
Published: 4 April 2013
Ti-doped ZnO (ZnO/Ti) thin films were grown on indium tin oxide substrates by a facile electrodeposition route. The morphology, crystal structure and resistive switching properties were examined, respectively. The morphology reveals that grains are composed of small crystals. The (002) preferential growth along c-axis of ZnO/Ti could be observed from structural analysis. The XPS study shows the presence of oxygen vacancies in the prepared films. Typical bipolar and reversible resistance switching effects were observed. High ROFF/RON ratios (approximately 14) and low operation voltages within 100 switching cycles are obtained. The filament theory and the interface effect are suggested to be responsible for the resistive switching phenomenon.
Resistance switching in metal oxide structures has attracted considerable attention because of its potential application to non-volatile memories [1–5]. Resistive random access memories (RRAMs) have many advantages over other technologies of data storage, such as much faster reading and writing rate, smaller bit cell size and lower operating voltages and very high retention time up to 10 years [2, 6–8].
In general, the metal oxide thin films are prepared by physical methods, such as radio frequency magnetron sputtering and pulsed laser deposition, etc. It not only involves high fabrication cost but also limit the size and massive production. On the other hand, chemical methodologies, such as chemical bath deposition and hydrothermal, suffer from the problems of low crystallinity, disconnection of substrate and film or high-temperature calcinations. Compared with the aforementioned techniques, electrodeposition provides an effective way to fabricate high-quality metal oxide thin films at low temperature and ambient atmosphere. Moreover, in this process, the deposition of metal oxide layers on the substrate is driven by the external electric field. Therefore, it is facile to precisely control the layer microstructure by this method and further design heterostructures with novel functionalities.
To date, various methods including doping , interface engineering  and nanoparticle incorporation [11, 12] were used to improve the performance of RRAM devices. The effects of Au and Pt nanoparticles embedded in ZrO2 and TiO2 oxide films have also been studied [12, 13].
Among the resistance switching materials, ZnO is especially attractive for its several unique advantages, such as the coexistence of unipolar and bipolar switching behaviour [14, 15], the larger high resistance state to low resistance state (HRS/LRS) window  and the transparent and flexible application aspects [6, 17]. The doping method has already been adopted to optimize the switching performance of ZnO, including Mn, Co, Cu and Ga [15, 16, 18–20], but the switching properties were not as optimized as for practical applications. Very few studies of the electric conduction mechanism for Ti-doped ZnO films have been reported [21–23]. Since the ionic radius of titanium is smaller than that of the zinc, when titanium atoms doped into a ZnO lattice, they act as scattering centres/donors by providing two free electrons. However, only a small amount of doped Ti4+ could induce more electrons and avoid acting scattering centres . Also, Ti-doped ZnO films have more than one charge valence state in comparison to that of the ZnO films doped with other Group III elements.
The Ti precursor in aqueous solution controls the hydrolysis process of Ti ions, and this reaction is very fast in conventional precursors, such as TiCl4. The coordination number of Ti is six; therefore, ammonium hexafluorotitanate is more stable, and thus, it is suitable to use as a dopant. In this present work, we find that ammonium hexafluorotitanate is the most suitable compound for Ti doping and for controlled structural morphology.
In this paper, a study has been carried out on resistance switching properties of Ti-doped ZnO, where the films were prepared by a simple electrochemical deposition method at low temperature. Ti dopants were introduced into ZnO in order to enlarge the memory window via increasing the resistivity of the high-resistance state.
Electrodeposition was carried out using an Autolab 302 N electrochemical workstation (Metrohm, Utrecht, The Netherlands). A standard three-electrode setup in an undivided cell was used. ITO (indium tin oxide) (9.3 to 9.7 Ω, 1.1 mm × 26 mm × 30 mm, Asashi Glass Corporation, Japan) was used as the working electrode while platinum foil (0.2 mm × 10 mm × 20 mm) as the counter electrode. The distance between the two electrodes was 30 mm. The reference electrode was an Ag/AgCl electrode in a 4-M KCl solution, against which all the potentials reported herein were measured.
The ITO substrates were first cleaned by detergent, then rinsed well with ethanol and DI water and then electrodeposited in a solution of 0.1 M Zn (NO3)2·6H2O with 2% (NH4)2TiF6 at 1 mA for 30 min, at 75°C. The phase composition of the samples was characterized by X-ray powder diffraction (Philips X’pert Multipurpose X-ray Diffraction System with Cu Kα; Philips, Amsterdam, The Netherlands). The morphologies of the samples were observed by scanning electron microscopy (Nova Nano SEM 230, FEI, Hillsboro, OR, USA). To measure the electrical property of the films, Au top electrodes were patterned and deposited by sputtering using a metal shadow mask. Voltage–current curves of the films were measured using an Autolab 302 N electrochemical workstation controlled with Nova software (with a possible error in current and voltage values as ±5%; Nova Software, Chongqing, China). All measurements were repeated at least twice to confirm the results. During measurement, the working electrode and sensor electrode were connected to the top Au electrode, and the reference and counter electrode were connected to the ITO substrate.
X-ray photoelectron spectroscopy (XPS) was performed with an ESCALAB250Xi spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) using a monochromatized Al K alpha X-ray source (hV) 1486.6 eV with 20 eV pass energy. Hall effect measurements were carried out by the Accent HL5500PC (Nanometrics, Milpitas, CA, USA). All measurements were performed at room temperature.
Results and discussion
For more understanding of the conduction and switching mechanisms of the memory device, the I-V characteristics are replotted in a log-log scale. Figure 3b shows the logarithmic plot of the previous I-V curve for the positive voltage sweep region, while it is similar for the negative branch. The I-V curve in LRS clearly shows an ohmic behaviour, which might be due to the formation of conductive filaments in the device during the set process. However, the conduction mechanism in off state is much more complicated. The charge transportation in this region is in agreement to the classical trap-controlled space-charge-limited conduction (SCLC), which consists of three regions: the ohmic region (I ∝ V), the Child's law region (I ∝ V2) and the steep current increase region . The totally different conduction behaviours in these two states (LRS and HRS) also suggest that the high conductivity in on-state device should be a confined, filamentary effect rather than a homogenously distributed one. This indicates the fact that the active medium is much smaller than the device size, providing a potential of scaling.
The endurance characteristics of the Au/ZnO: Ti/ITO memory cell are shown in Figure 4a. The memory window defined by the two resistance states, i.e., (ROFF − RON) / RON ≈ ROFF/RON, is more than 14. This is a high memory margin, making the device circuit very easy to distinguish the storage information between ‘1’ and ‘0’. The resistance of the HRS scatter in a certain extent during cycling. However, due to high ROFF/RON ratio of the present device, this kind of scattering may be tolerated. It can be seen that the memory margin keeps beyond 14 times during cycling, and the cell shows little degradation after 100 repeated sweep cycles. The endurance measurements ensured that the switching between on and off states is highly controllable, reversible and reproducible. After the device was switched on or off, no electrical power was needed to maintain the resistance within the given state.
To further demonstrate the stability of the resistive switching properties, data retention was gauged by examining the current level of the device in the on state over a long period of time (>2000 s) in air ambient. In this case, no appreciable change in resistance ratio (HRS/LRS) was observed in these devices, as shown in Figure 4b, while the information storage in these devices is likely to persist for an even longer time judging from the present trend of data.
The current–voltage measurements of pure ZnO sample were also performed and presented in the supporting information in Additional file 1: Figure S2. The memory margin of the device with 2% Ti@-ZnO was much better than the standard device (pure ZnO) as shown in Additional file 1: Figure S3. We also did perform the same measurements for both devices (pure and 2% Ti@-ZnO) without gold top electrode to see the possible effect of top electrode (results not shown here). Interestingly, both devices exhibited almost the same results as with the gold top electrode suggesting that gold top electrode is not playing critical/dominating role in resistive switching characteristics of these devices.
In our recent study , the resistive switching characteristics of pure ZnO were improved (on/off, approximately 7) with Co doping in ZnO. In the present report, with the addition of Ti in ZnO, the resistive switching characteristics were further improved with on/off ratio (>14) and data retention time of 2,000 seconds was achieved.
Ti-doped ZnO thin films were prepared by a facile electrochemical deposition method. The SEM, XPS and EDS mapping indicates that Ti is homogenously doped in ZnO films. The Ti-doped ZnO film had a similar structure to that of the pure ZnO film and had a preferential orientation in the (002) direction. The prepared film exhibits excellent resistance switching behaviour with a HRS/LRS ratio of about 14 during endurance test, much better than pure ZnO. In addition, the dominant conduction mechanism of LRS and HRS were explained by trap-controlled space-charge-limited conduction. The present work demonstrates that Ti doping can further enhance switching characteristics of pure ZnO films and thus have the potential for next-generation non-volatile memory applications.
The authors would like to acknowledge the financial support from the Australian Research Council Projects of DP110102391, DP1096769, FT100100956 and DP0988687 in this work.
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