High performance white-light-controlled resistance switching memory of an Ag/α-Fe₂O₃/FTO thin film

Abstract

Non-volatile state-modulated resistive switching memory devices hold great promise for the next generation of memory chips. Herein we demonstrate the high resistance switching performance of an Ag/α-Fe₂O₃/FTO device made using a facile hydrothermal process to grow an α-Fe₂O₃ nanorod array on a fluorine-doped tin oxide substrate (FTO). The resistive switching behavior can be effectively controlled using white-light irradiation. In particular, the device possesses an OFF/ON-state resistance ratio of ∼10⁴ with exceptional stability at room temperature. Our experimental results suggest that the resistive switching effect in the Ag/α-Fe₂O₃/FTO system mainly results from the formation of conductive filaments inside the α-Fe₂O₃ nanorods. This study not only demonstrates the great potential to explore new chemistry with tailored nanostructures for high resistive switching performance, but also sheds light on its important practical applications in nonvolatile multistate memory devices.

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Introduction

The rapid development of information technology greatly relies on the high speed and large-capacity of non-volatile memories, which are key components of integrated circuits for data retention capability during power interruption.¹ In recent years, resistance random access memory (RRAM) has become the most prominent one among all new types of non-volatile memories due to its high density, simple structure, long retention time, high operation speed, and so on.² Fundamental advances are crucial for the discovery of resistive switching behavior in some new unique materials, for instance a light-controlled resistance switching material can be conveniently used for nonvolatile light-controlled memory.^2–4

To date, all solid materials exhibiting a resistance switching effect can be classified into four categories: binary transition metal oxides, oxides with a perovskite structure, solid dielectric materials with ion transport capacity and organic resistive materials. Binary transition metal oxides including ZnO,⁵ TiO_x,⁶ Cu_xO,⁷ NiO,⁸ Nb₂O₅ ⁹ and ZrO₂ ¹⁰ have been investigated, among which the NiO material shows good electrical storage ability and reliability.^8–15 The oxides with a perovskite structure such as SrTiO₃,¹⁶ PbTiO₃,¹⁷ TiPr_0.7Ca_0.3MnO₃,¹⁸ SrTi_0.99Nb_0.01O₃ ¹⁹ and Cr-doped SrZrO₃ ²⁰ have been explored. Cr-doped perovskite materials exhibit nonvolatile storage performance, low operating voltage, fast resistance change time and long state maintenance time, thus offering great application potential. Fast ion transport capacity from solid dielectric materials, for instance Cu₂S,²¹ RbAg₄I₅ ²² and Ag₂S/Ag,²³ can lead to good resistance switching behavior. Organic resistance switching memory devices possess a number of prominent advantages such as light weight, flexibility and varied composition and structure, and they are easy to cut and assemble for low cost, simple fabrication and miniaturization.^24,25 However, there are still great challenges to explore new materials and facile syntheses for mass-manufacturing, miniaturization, high performance devices and compatibility with the conventional process for practical applications. Furthermore, although some storage mechanisms for the resistance switching memory behavior have been proposed, the storage mechanism of RRAM is still not completely understood and in particular it has not been firmly confirmed experimentally.

α-Fe₂O₃ (hematite), an n-type semiconductor possessing a corundum-type structure, is the most stable iron oxide under ambient conditions.²⁶ Its band gap width is narrow, and thus it has a strong ability to absorb light in the visible region. In recent years, α-Fe₂O₃ has received increasing attention due to its extensive application in gas sensors,²⁷ catalysts,²⁸ pigments,²⁹ magnetic materials,³⁰ optical³¹ and electromagnetic devices,³² drug delivery and tissue repair engineering.³³ Therefore, various nanostructured α-Fe₂O₃ materials with different tailored morphologies have been successfully synthesized, including nanowires,³⁴ nanotubes,^35–37 nanoplates,³⁸ nanorods,³⁹ nanobelts,⁴⁰ nanocubes,⁴¹ nanoparticles,⁴² hollow spheres,⁴³ hollow nanofibers,⁴⁴ shuttle-like nanoparticles⁴⁵ and hollow nanowires.⁴⁶ Among these structures, one dimensional α-Fe₂O₃ possesses interesting physical properties such as good light-harvesting and charge transport properties. Yan et al.⁴⁷ investigated the electrically controlled resistive switching behavior of α-Fe₂O₃/FTO with only approximately two orders of magnitude for the switching ratio. Currently white-light-controlled resistance switching has received much attention^48–51 due to its long distance control and effective control without interference.^52–55 Zhao et al.⁵⁰ and Sun et al.^48,49,51 have studied the white-light-controlled resistance switching behaviors of some Fe₂O₃-based composites including BaTiO₃/γ-Fe₂O₃/ZnO, TiO₂/α-Fe₂O₃/FTO and BiFeO₃/γ-Fe₂O₃/FTO, with a resistive switching ratio of under three orders of magnitude.

In this work, to further improve the switching ratio of the Fe₂O₃-made device, we investigated white-light-controlled single phase α-Fe₂O₃, which has never been reported. In addition, in comparison to the composite materials, single phase α-Fe₂O₃ is acquired from a relatively abundant natural resource and its synthesis is inexpensive, thus it offers great potential for practical application in RRAM devices.

Experimental section

Preparation of an α-Fe₂O₃ nanorod array

An α-Fe₂O₃ nanorod array grown on a FTO substrate was prepared using a facile hydrothermal process (Fig. 1(a)). All chemicals were analytical grade, purchased from the commercial market, and used without further purification. We introduce the steps of the most preferred synthesis in our experiments, the detailed experimental procedure for which is outlined as follows. An aqueous solution consisting of 0.1 M FeCl₃ and 1 M NaNO₃ at pH 1.25 was continuously magnetically stirred. The clear mixed solution was transferred and sealed into a Teflon lined stainless steel autoclave (50 mL in capacity). To obtain material on fluorine-doped tin oxide (FTO)-coated glass substrates (NSG, 14 Ω per square), FTO was cleaned prior to ultrasonic treatment with acetone, deionized water and ethanol and subsequently dried in air. Then, the clean FTO substrate was put into the above mixed solution, and the conductive surface was deposited. According to Lionel Vayssieres,⁵⁶ the resulting mixture was then autoclaved in an electric oven at 95 °C for 10 h. After heating, the autoclave was cooled down to room temperature naturally. The FTO substrate was rinsed with deionized water and subsequently annealed at 550 °C for 0.5 h and 700 °C for 2 h in air.

Fig. 1 (a) The preparation process of the α-Fe₂O₃ nanorods grown on an FTO substrate. (b) The experimental test circuit.

Characterization

The crystal structure of the α-Fe₂O₃ nanorods was characterized using X-ray diffraction (XRD, MAXima-X XRD-7000, Cu Kα radiation, at a scan rate of 2° per minute with 2θ ranging from 20 to 90°) at room temperature. Scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS) analyses were obtained using a JEOL JSM-7500F scanning electron microscope equipped with an energy dispersive spectroscopy (EDS) probe. The size and morphology of the α-Fe₂O₃ nanorods were examined by transmission electron microscopy (JEM-2100) at an acceleration voltage of 200 kV. The UV-vis absorption spectrum of the α-Fe₂O₃ nanorod array was obtained using a Shimadzu UV-2550 UV-vis spectrophotometer.

Test of resistance switching

Fig. 1(b) shows a schematic representation of the studied devices. We used an ordinary filament lamp with various power densities as the light source, with white-light in the wavelength range of 400–760 nm. Ag and FTO are the top electrode and bottom electrode, respectively. The Ag electrodes with an area of about 1 mm² were prepared using silver glue. I–V characterizations were tested using the electrochemical workstation CHI-660D. The resistance switching properties of the samples were examined in the dark and under various power densities of white-light illumination.

Results and discussion

Fig. 2(a) shows the X-ray powder diffraction (XRD) pattern of the α-Fe₂O₃ nanorods at room temperature. In order to exclude the interference of FTO substrate diffraction peaks, we scraped off α-Fe₂O₃ powder from the FTO substrate for XRD characterization. The positions of the diffraction peaks are at the 2θ angles of 24.1°, 33.1°, 35.6°, 39.3°, 40.8°, 49.4°, 54.0°, 57.4°, 62.4°, 63.9°, 72.2° and 82.7°, respectively corresponding to the crystal faces (0 1 2), (1 0 4), (1 1 0), (0 0 6), (1 1 3), (0 2 4), (1 1 6), (1 2 2), (2 1 4), (3 0 0), (1 1 9) and (0 2 10) (JCPDS no. 33-0664, a = 5.035 Å and c = 13.74 Å). No characteristic peaks were observed for other impurities such as α-FeOOH, Fe₃O₄, γ-Fe₂O₃ or other inorganic ions. At the same time, the XRD peaks are very sharp, demonstrating that the crystallization of α-Fe₂O₃ is particularly perfect. In addition, the narrow width of the diffraction peaks reflects that the size of α-Fe₂O₃ is large, which is in good agreement with the results observed from the scanning electron microscopy (SEM) images discussed below. It was discovered that the relative intensity of the (1 0 4) face is stronger than the standard values in the JCPDS card no. 33-0664, indicating that (1 0 4) is the preferred growth orientation in the as-prepared product, and the material could have a single-crystalline structure. The powder of α-Fe₂O₃ was further examined using energy-dispersive X-ray spectroscopy (EDS) elemental analysis (Fig. 2(b)), confirming that it only contains O and Fe without any impurity. The inset of Fig. 2(b) shows that the atomic ratio of Fe to O is 2 : 3. Therefore, it can be concluded that the as-prepared product is pure α-Fe₂O₃.

Fig. 2 (a) The X-ray diffraction (XRD) patterns of the α-Fe₂O₃ nanorods and PDF#33-0664. (b) The energy-dispersive X-ray spectrum (EDS) of the α-Fe₂O₃ nanorods. The inset shows the atomic percentages in the α-Fe₂O₃ nanorods.

Fig. 3(a) and (b) show SEM images of the α-Fe₂O₃ nanorod array grown on the FTO substrate. It can be seen from Fig. 3(a) that the sample comprises uniform nanorods. A cross-sectional SEM image of the α-Fe₂O₃ nanorod array synthesized on the FTO substrate is shown in Fig. 3(b), revealing that the average length of the α-Fe₂O₃ nanorods is about 5 μm. Fig. 3(c) displays a high resolution transmission electron microscope (HRTEM) image of an α-Fe₂O₃ nanorod. The fringe with a spacing of 0.28 nm corresponds to the (1 0 4) plane of Fe₂O₃. The TEM image of an individual α-Fe₂O₃ nanorod in Fig. 3(d) shows a diameter of ∼200 nm, while the inset displays the corresponding selected area electron diffraction (SAED) pattern. The HRTEM image and the corresponding SAED pattern indicate that the α-Fe₂O₃ nanorods possess a single-crystalline structure, which agrees well with the XRD characterization results.

Fig. 3 (a) and (b) Scanning electron microscopy (SEM) images of the α-Fe₂O₃ nanorod array grown on the FTO substrate. (c) High resolution TEM (HRTEM) image of an α-Fe₂O₃ nanorod. (d) TEM image of an individual α-Fe₂O₃ nanorod, the inset shows the corresponding SAED pattern of the α-Fe₂O₃ nanorod.

The absorption spectrum of the as-prepared Fe₂O₃ sample shown in Fig. 4 was transformed from the diffuse reflection spectra according to Kubelka–Munk (K–M) theory.⁵⁷ The absorption spectrum indicates that the as-prepared α-Fe₂O₃ nanorod array has good white-light absorption properties in the visible-light region. Many previous works^58–62 have reported that the bandgap width values of α-Fe₂O₃ are in a range of 1.9 to 2.2 eV, depending on the crystalline status and preparation methods. According to the Tauc relation, αhν = A_o(hν − E_g), the E_g value of α-Fe₂O₃ prepared in our work calculated from the measured UV-vis is 2.06 eV, which is in good agreement with the previously reported E_g range. Thus, the prepared Fe₂O₃ nanorods we prepared are pure.

Fig. 4 The UV-vis absorption spectrum of the α-Fe₂O₃ nanorod array.

Fig. 5(a) exhibits the current–voltage (I–V) curves of Ag/α-Fe₂O₃/FTO in the dark and under white-light illumination using various power densities and at room temperature. The arrows in the figure denote the sweeping direction of the voltage. The dc voltage was swept from 0 → +V_max → 0 → −V_max, and then back to 0 V at the end. An obvious bipolar resistive switching behavior is clearly observed, indicating rapid conversion and good reproducibility. The switch behavior can be explained as follows. An applied electrical field can result in the polarization of Fe₂O₃. When applying white-light illumination, Fe₂O₃ absorbs the light energy and a transition occurs, which enhances the absolute values of V_Set and V_Reset. Fig. 5(a) shows that the greater the light power density, the larger the change is. The reported TiO₂/α-Fe₂O₃ nanorod composite only has a resistive switching ratio of up to ∼3 orders of magnitude at a light power density of 50 mW cm⁻².⁴⁹ In contrast, the resistive switching ratio in our work is up to ∼4 orders of magnitude at an even lower power density (10 mW cm⁻²). Moreover, I–V curves show that Ag/α-Fe₂O₃/FTO possesses a rectifying effect.

Fig. 5 (a) Current–voltage curves in the dark and under white-light illumination using various power densities at room temperature. (b) The corresponding resistance switching effects in a logarithmic scale. (c) The evolution of switching voltages including V_Set and V_Reset during the 100 resistive switching cycles. (d) The resistance–cycle curve with a positive bias voltage of 0.5 V.

Fig. 5(b) presents the corresponding I–V curve with resistance switching effects in a logarithmic scale, which exhibits asymmetric behavior with significant hysteresis. It is obvious that a sudden current increase occurs at about 2.0 V (V_Set), indicating a resistive switching from the high resistance state (HRS or ‘OFF’) to the low resistance state (LRS or ‘ON’), which is called the “Set” process. When the applied voltage swept from zero to a certain negative voltage of about −2.0 V (V_Reset), the device can return to the HRS, which is called the “Reset” process. During the subsequent “Set” and “Reset” cycle on the same device, an I–V curve identical to the first one was obtained. Actually, V_Reset and V_Set remain almost unchanged in the subsequent cycles.

Fig. 5(c) displays the evolutions of V_Set and V_Reset over 100 successive resistive switching cycles on the same device. It is very clear that there is no conspicuous decay in switching voltages. V_Set and V_Reset are 2.0 ± 0.2 V and −2.0 ± 0.2 V, respectively, indicating a very low variation in switching voltages for the Ag/α-Fe₂O₃/FTO structure, and excellent switching stability to a certain extent.

The evolutions of the resistance in the HRS and LRS with a positive bias of 0.5 V, over 100 successive resistive switching cycles was tested, as shown in Fig. 5(d). The device possesses an OFF/ON-state resistance ratio of about 10⁴ at room temperature, while the resistances at both the LRS (ON state) and HRS (OFF state) remain stable. These results show excellent repeatability and reliability in the white-light controlled resistive switching of the Ag/α-Fe₂O₃/FTO device. In addition, the low operating voltages (≤1.5 V) and low operating current densities (<10⁻⁷ mA cm⁻²) in our system are intriguing traits for possible memory applications with low power consumption. Furthermore, in contrast to the darkness condition, the OFF/ON-state resistance ratio of the device was reduced under white-light illumination. This further proves the high white-light-controlled resistance switching performance of the Ag/α-Fe₂O₃/FTO device.

To analyse the mechanisms for the resistive switching of the Ag/α-Fe₂O₃/FTO structure in this work, the device after 100 cycles of HRS (OFF) and LRS (ON) was etched to remove the Ag film electrode to expose the α-Fe₂O₃ layer (Fig. 6(a)). The EDS data for this layer (Fig. 6(b)) clearly shows the presence of Ag, strongly evidencing that Ag ions from the Ag film electrode diffuse into the α-Fe₂O₃ structure under a voltage bias. In addition, the I–V characteristics of the ON state were plotted to fit in a double logarithmic scale (ESI Fig. S2†), clearly showing an ohmic conduction behaviour with a slope of m < 1.0, to evidence the formation of conductive paths in the device during the SET process. Further I–V^m fitting confirms the presence of oxygen vacancies following the stress induced by a high electric field, in agreement with previously reported studies^63–65 (ESI Fig. S2†). The mechanisms for metal/metal oxide/FTO structured resistive switching have been extensively investigated but are still controversial.^66–69 In terms of our results, we propose a hybrid filament mechanism for our Ag/α-Fe₂O₃/FTO structure as shown in Fig. 7, in which formation/rupture of the metal filaments occur when Ag atoms ionize into Ag ions, and then diffuse from the metal electrode under a bias voltage, while the oxygen ions move in the opposite direction under the same electric field, resulting in oxygen vacancies. This is strongly supported by Wang et al.’s work, which also reported a hybrid filament mechanism and is in good agreement with our device structure.⁷⁰

Fig. 6 (a) The etching process. (b) The energy-dispersive X-ray spectra (EDS) when the device achieved the LRS or ‘ON’ state after 100 cycles.

Fig. 7 The schematic mechanism of conductive filaments in the device.

Conclusions

In brief, an α-Fe₂O₃ nanorod array was successfully grown on an FTO substrate using a facile hydrothermal process, and bipolar resistive switching characteristics of the vertically aligned α-Fe₂O₃ nanorod array were observed. The results suggest that the resistive switching effect in the Ag/α-Fe₂O₃/FTO system largely results from the formation of conductive filaments inside the α-Fe₂O₃ nanorods. The high performance light-controlled resistive switching of the α-Fe₂O₃ nanorods holds great promise for their application in practical next-generation nonvolatile memory devices.

Acknowledgements

We gratefully appreciate the financial support from the 973 program of China (No. 2013CB127804), National Natural Science Foundation of China, (No. 21205098, and 21273173), Fundamental Research Funds for the Central Universities (XDJK2012C049), Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, start-up Grant under SWU111071 from the Southwest University, Institute for Clean Energy & Advanced Materials, Southwest University, Chongqing, P.R. China. This work has been also supported by the National Key Project for Basic Research (No. 2014CB921002) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra24057c

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