Resistive Switching Characteristics of Flexible TiO2 Thin Film ...

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Oct 23, 2017 - Yuanqing Chen, Lingwei Li, Xiaoru Yin, Aditya Yerramilli, Yuxia Shen, Yang Song, Weibai Bian, Na Li,. Zhao Zhao, Wenwen Qu, N. David ...

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IEEE ELECTRON DEVICE LETTERS, VOL. 38, NO. 11, NOVEMBER 2017

Resistive Switching Characteristics of Flexible TiO2 Thin Film Fabricated by Deep Ultraviolet Photochemical Solution Method Yuanqing Chen, Lingwei Li, Xiaoru Yin, Aditya Yerramilli, Yuxia Shen, Yang Song, Weibai Bian, Na Li, Zhao Zhao, Wenwen Qu, N. David Theodore, and T. L. Alford, Member, IEEE

Abstract — A novel ultraviolet photochemical method was used to prepare TiO2 resistive-switching films. Amorphous TiO2 films were formed on flexible indium-tin oxide (ITO) coated polyethylene terephthalate (PET) substrates by deep ultraviolet irradiation at 150 °C. A Pt/TiO2 /ITO/PET device was then fabricated to investigate bipolar resistive switching of the films for potential application in non-volatile memories. The ratio of on-state to off-state currents was measured, and a good value of 1000 was obtained. The retention and switch-cycling characteristics of the device were investigated for different bending radii. The resistive switching behavior of the flexible device remained stable after 600 cycles of electrical switching and 1000 cycles of bending. Index Terms — Titanium oxide, deep ultraviolet irradiation, resistive switching.

I. I NTRODUCTION ESISTIVE random access memory (RRAM) is a new kind of non-volatile memory that shows strong potential for information storage. RRAM has the advantages of fast erase speed, high storage density, and low power consumption [1], [2]. Such RRAMs are based on a resistive transition layer that experiences a reversible change between high and low resistance states under an applied electric field. Resistive switching (RS) effects have been observed in many inorganic materials. In particular, binary oxides showed excellent RS performance [3], [4]. The Binary oxides also have some advantages over other materials, including simple structure, ease of compositional control, low cost and compatibility with semiconductor processing [3], [5]. Of the binary oxides,

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Manuscript received September 2, 2017; revised September 14, 2017; accepted September 18, 2017. Date of publication September 26, 2017; date of current version October 23, 2017. This work was supported by the Foundation of Science and Technology of Shaanxi Province under Grant 2013KJXX-36. The review of this letter was arranged by Editor C. V. Mouli. (Corresponding author: T. L. Alford.) Y. Chen, L. Li, X. Yin, Y. Song, W. Bian, N. Li, and W. Qu are with the School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China. N. D. Theodore is with CHD-Fab, NXP Semiconductors, Chandler, AZ 85224 USA. Y. Chen, A. Yerramilli, Y. Shen, Z. Zhao and T. L. Alford are with the School for Engineering of Matter, Transportation and Energy, Arizona State University, AZ 85287 USA (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2017.2756444

TiO2 is one of the most attractive, due to its excellent resistive switching properties [6], [7]. Resistive switching layers can be prepared by different techniques, such as chemical solution deposition (CSD) [8], sputtering [9], plasma oxidation [10]. Of these methods, CSD has the advantages of simplicity, low cost and good compositional control. However, a high annealing temperature is required to remove organic materials. Unfortunately, this limits the application of this method for fabrication of transparent flexible electronics. Recently, a deep ultraviolet (DUV) photochemical solution method has been developed for lowtemperature deposition of oxide thin films. High-quality amorphous thin films with excellent electrical performance have been obtained for thin-film transistor applications [11]. The DUV light decomposes the gel film coated on the substrate, and promotes the formation of a metal-oxygen-metal (M-OM) network. This results in the formation of a dense inorganic thin-film structure at low temperature [11]–[13]. In this study, we utilized a deep ultraviolet photochemical solution method to fabricate flexible Pt/TiO2 /ITO/PET resistive switching devices. The results indicate that the photochemical solution processed TiO2 -based memory devices show good RS characteristics even after thousands of cycles of bending. Since current flexible RRAMs are mainly prepared by vacuum methods [14], [15], the low-cost photochemical CSD preparation of flexible RRAMs will be of potential path for the fabrication of flexible electronics. II. E XPERIMENTS Tetrabutyl titanate (Ti(OC4 H9 )4 ) and benzoylacetone (BzAcH) were dissolved in 2-methoxyethanol, and the resulting solution was stirred for 2 h at room temperature (Ti4+ :BzAcH=1:1). The concentration of this precursor solution was maintained at 0.3 mol/L, by adjusting the volume of 2-methoxyethanol. A wet TiO2 gel film was first coated on the ITO/PET substrate, by dip-coating at a rate of 2.67 mm/s. The resulting stack was then placed on a hot plate, heated to 150 °C, and exposed to DUV light for 3 h. The experimental procedure is illustrated in Fig. 1(a-d). Electrical measurements were performed with a positive voltage applied to the Pt top electrode using a Keithley 2602A sourcemeter.

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CHEN et al.: RS CHARACTERISTICS OF FLEXIBLE TiO2 THIN FILM

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Fig. 1. Preparation procedure of the flexible TiO2 RRAM. (a) Dip coating of the TiO2 gel film; (b) Deep UV irradiation of the gel film; (c) deposition of a top electrode; (d) Photograph of flexible TiO2 RRAM device.

Fig. 3. XPS of O 1s peak of different TiO2 films: (a) TiO2 gel film; (b) TiO2 film annealed at 150°C in air; (c) TiO2 amorphous film illuminated by DUV light for 1h at 150°C; (d) TiO2 amorphous film illuminated by DUV light for 3h at 150°C.

Fig. 2. (a) Dependence of switching ratio and transition voltage (inserted) on the irradiation time. (b) Dependence of switching ratio on irradiation temperature. (c) Statistics of set and reset voltages for devices irradiated by DUV light at 150°C for 3h. Electrical properties of a flexible TiO2 RRAM irradiated under DUV light at 150°C for 3h: (d) I-V curve; (e) Log I-V curve and (f) Log I-Log V curve.

III. R ESULTS AND D ISCUSSION We investigated the influence of irradiation time on the electrical properties of the TiO2 films, as shown in Fig. 2(a). We found that the ratio of HRS to LRS (RHRS /RLRS ) was very low for films irradiated for less than 2 hours. However, when the films were irradiated for more than 3 hours, the RHRS /RLRS reached a value of 103 . Moreover, the “set” and “reset” voltages decreased when the films were irradiated for more than 3 hours. After the films were irradiated for 5 h, the “set” and “reset” voltages dropped to 1-2 V. These voltages are in the desired range for low-power devices. Another factor that influences the electrical properties is the irradiation temperature. As shown in Fig. 2(b), as the temperature increases from 50 to 150°C, the RHRS /RLRS increases from 4.5×102 to 3×104. Films were then irradiated under DUV light at 150°C for 3h. The Vset and Vreset distributions of the devices, as shown in Fig. 2(c), are distinguishable without overlap. The Vset shows a broader distribution when compared with the Vreset , which is related to the variation in the ruptured filament path length [16]. Compared with the films deposited

by other vacuum methods [16], [17], the voltage and current distributions of our TiO2 films are still need to be improved. A current-voltage (I-V) curve obtained from a device is shown in Fig. 2(d). The device shows bipolar resistive switching behavior. The device exhibits a high resistance state (HRS) initially, and then switches to a low resistance state (LRS) when the voltage is ramped up to 2.4 V. When a negativevoltage scan is performed, the resistive switching device experiences a reset event at about −1.2 V, and the device changes from the LRS to the HRS (see the log I-V curve in Fig. 2(e)). Fig. 2(f) shows a linear fit of the I-V curve on a log-log scale. During the LRS, the log I-log V curve is linear, with a slope of 1.00. This indicates that the conduction mechanism follows Ohm’s Law. In contrast, during the HRS, three different slopes (1.24, 2.40 and 3.85) are observed for the TiO2 RRAM. These results are explained by a space-charge limited conduction (SCLC) mechanism. Films processed under different conditions are investigated by XPS. The O1s spectra of different films are shown in Fig. 3. For the gel film and the 150 °C annealed film in air, a dominant contribution of M–OC bonds (∼530.5eV) is found and the oxygen in M-O-M bonds (∼529.7eV) is almost negligible, indicating that the M-O-M network has not formed in the film [14]. Even after the film is irradiated by DUV light for 1h, the peak corresponding to the oxygen in M-O-M bonds is still not obvious. By prolonging the DUV irradiation to 3h, the dominant bond shifts from M–OC bond to the M–O-M bond. This indicates that after a long-time UV irradiation, the Ti-O-Ti network is basically formed in the film, leading to condensed amorphous TiO2 film with improved RS properties. For flexible applications, the memory device should exhibit good performance in both normal and bent states. We evaluated the RS properties of the flat (unbent) device, and then, after the device was bent with different bending radii (R) of 27.63 mm, 20.35 mm and 12.55 mm. During the tests, the device was slowly bent from a flat state to a semicircular state with a fixed bending radius, and then the electrical properties were tested at room temperature. Fig. 4(a) and (b) show

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IEEE ELECTRON DEVICE LETTERS, VOL. 38, NO. 11, NOVEMBER 2017

Fig. 5. (a) and (b) The retention characteristics and switch-cycling characteristics of a Pt/TiO2 /ITO/PET device after being bent and straightened 100 times. (c) and (d) The retention characteristics and switchcycling characteristics of Pt/TiO2 /ITO/PET device after being bent and straightened 1000 times. The samples were cycled between the flat state and a bent state (with a bending radius of 20.35mm).

Fig. 4. (a) and (b) are the retention characteristics of a Pt/TiO2 /ITO/PET device with bending radius of R = ∞(flat) and R=12.55 mm, respectively; (c) The dependence of the average LRS and HRS resistance on the curvature; (d) The schematic diagram of the bending device. (e) and (f) are the cycle characteristics of Pt/TiO2 /ITO/PET device with bending radius of R = ∞(flat) and R=12.55 mm, respectively.

the retention characteristics of the Pt/TiO2 /ITO/PET device with R= ∞(flat) and R=12.55 mm, respectively. The LRS and HRS resistances were found to be stable, over a period of 104 seconds, in both flat and bent states of the memory device. This indicates that the device has good flexibility and good retention characteristics. There is not much difference between the LRS resistances for different bending radii. However, the average HRS resistance decreases slowly as the curvature K (K=1/R) increases from 0 to 79.7 m−1 , as shown in Fig. 4(c). The switch-cycling characteristics of the flexible Pt/TiO2 /ITO/PET device were tested using a pulse voltage of 3 V and a pulse width of 100 ms. Fig. 4(e) and (f) show the switchcycling characteristics of the Pt/TiO2 /ITO/PET device with bending radii of R = ∞(flat) and R=12.55 mm. The results show that the electrical switching between HRS and LRS is repeatable. Over the course of 600 resistance-switching cycles, the switching ratio of the device remained constant. For use in flexible electronics, the device should also continue to perform well after multiple cycles of bending. So, we studied the electrical performance of the device after multiple cycles of bending. The device was cycled between the flat state and a bent state with a bending radius of 20.35 mm. Fig.5 shows the retention characteristics and the switch-cycling characteristics of a Pt/TiO2 /ITO/PET device which was bent and then straightened 100 times and 1000 times at room temperature. The device shows stable resistive behavior even after having being bent and straightened 100 times and 1000 times. In addition, even after 600 cycles of electrical resistance switching, the switching

ratio of the flexible device remains the same (with a value of about 1000). There is almost no change in the switching ratio between the bent device and the planar device. Since our photochemical-solution-processed TiO2 film is amorphous, it is believed that the cracks do not easily form when the bending radius is larger than 10mm, and thus the conducting filament are not destroyed. This may account for the Pt/TiO2 /ITO/PET device maintaining good RS properties even after being bent for thousands of times. However, results indicated that when the bending radius is less than 10 mm, our flexible TiO2 -RRAM devices show failure. Our device is not as flexible as typical polymer memories and nanofiber based RRAMs, which were reported to be able to keep good RS properties even after being bent with bending radius of 1-5mm [16], [18]. However, considering the moderate flexibility of our devices, the low-cost photochemical production technique, and good retention and switch-cycling characteristics (for different bending radii), our devices are still of potential in many flexible RRAM applications. IV. C ONCLUSION An amorphous TiO2 thin film was prepared by DUV photochemical method. The DUV irradiation promoted the formation of the formation of Ti-O-Ti network. A Pt/TiO2 /ITO/PET RRAM device was then fabricated using this TiO2 film. Analysis of I-V characteristics indicated that the conduction was ohmic in nature during the low-resistance state of the device, and space-charge limited during the high-resistance state of the device. A good ratio (of 103 ) of on-state to off-state currents was obtained. The retention characteristics and the switch-cycling characteristics were investigated for devices bent with different bending radii. The resistive switching behavior of the flexible device remained stable after being bent and straightened 1000 times.

CHEN et al.: RS CHARACTERISTICS OF FLEXIBLE TiO2 THIN FILM

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