Resistive switching of Au/ZnO/Au resistive memory: an in situ observation of conductive bridge formation
© Peng et al.; licensee Springer. 2012
Received: 9 August 2012
Accepted: 6 September 2012
Published: 8 October 2012
A special chip for direct and real-time observation of resistive changes, including set and reset processes based on Au/ZnO/Au system inside a transmission electron microscope (TEM), was designed. A clear conducting bridge associated with the migration of Au nanoparticles (NPs) inside a defective ZnO film from anode to cathode could be clearly observed by taking a series of TEM images, enabling a dynamic observation of switching behaviors. A discontinuous region (broken region) nearby the cathode after reset process was observed, which limits the flow of current, thus a high resistance state, while it will be reconnected to switch the device from high to low resistance states through the migration of Au NPs after set process. Interestingly, the formed morphology of the conducting bridge, which is different from the typical formation of a conducting bridge, was observed. The difference can be attributed to the different diffusivities of cations transported inside the dielectric layer, thereby significantly influencing the morphology of the conducting path. The current TEM technique is quite unique and informative, which can be used to elucidate the dynamic processes in other devices in the future.
Resistive random access memory (ReRAM) is one of the most significantly nonvolatile memories because of its fast switching speed, low power consumption, excellent endurance, and easy integration with current device processes [1, 2]. Among the materials for ReRAM application, metal oxides have attracted an increasing interest in material choice because of controllable compositions. Therefore, making a high-quality metal oxide layer with a bistable resistance state is a key issue in achieving a high-performance ReRAM device. A typical configuration of the ReRAM device usually consists of metal/insulator/metal structures, and the operation is based on the switching of high resistance state (HRS) and low resistance state (LRS) (off and on states) after a larger bias was applied due to the formation of a conductive bridge/path . In general, the switching behaviors can be classified into two types in terms of current–voltage (I V) behaviors, namely bipolar and unipolar switching [4, 5]. The bipolar resistive switching shows a directional resistive switching depending on the polarity of the applied voltage, while the unipolar resistive switching depends on the amplitude of the applied voltage without any polarity.
Zinc oxide (ZnO) has many superb characteristics in optics and electronics, such as a direct bandgap of approximately 3.37 eV, adjustable electrical properties by doping with different dopants, and low synthesis temperature [6, 7]. ZnO-based thin film ReRAM devices were found to have promising resistive switching characteristics with either unipolar or bipolar resistive switching behaviors, while the detailed switching behavior is still not clear. Therefore, unveiling the switching behavior of ReRAM based on ZnO system is imperative to shed light on the fundamental understanding of device operation, enabling the further exploration of ReRAM devices.
Recently, the existence of conductive bridges/paths has been confirmed and observed by different methods. Pan et al. used transmission electron microscopy (TEM) to observe the formation of the conductive bridge/path after a couple of set/rest processes [8–12]. Takimoto et al. used conducting atomic force microscopy to locate the formation of the conducting filament [13, 14]. However, these methods for the observation of the conducting filament formation can only be considered as ex situ observations. Some controversies regarding resistive switching mechanisms remain, and the underlying physical mechanism of the resistive switching is still not fully understood.
Consequently, a direct observation of conductive bridge/path formation in real time is imperative. In this regard, we demonstrate a real-time observation of a conductive bridge/path formation in the Au/ZnO/Au system inside the TEM to clarify how the conducting bridge/path formed between two electrodes, including set and reset processes when the resistance is changed from high resistance to low resistance states in the Au/ZnO/Au system. To realize this goal, it is important to directly observe in situ images in real time [15–17]. The switching mechanisms of the Au/ZnO/Au device in ultrahigh vacuum and air conditions were investigated for comparison.
Fabrication of TEM chip for in situ observation
Si substrates were cleaned by standard processes. Si3N4/SiO2 thin film layers with thicknesses of 80/20 nm were deposited on both sides of a Si substrate with a low-pressure chemical vapor deposition (LPCVD) system at 780 °C in a vacuum of 350 mTorr. Position of the membrane area was defined by photolithography on the back side of the chips, and the Si3N4 and SiO2 layers were etched away by reactive ion etching process to expose the Si substrate, followed by KOH solution etching from the back side to fabricate a freestanding Si3N4 membrane which is transparent to electron beam in TEM. The etching time and temperature are kept at 1 hr and 80 °C, respectively. After etching, we cleaned the sample with D.I. water. A 25-nm-thick ZnO layer was then deposited on the top side of the chips by radio frequency magnetron sputtering at room temperature with pressure kept at 10 to 6 Torr and with an Ar/O ratio of 0.1. Subsequently, Au contact electrodes with a gap approaching 100 nm were fabricated by electron beam lithography with poly(methyl methacrylate) (PMMA) as photoresist, followed by metal deposition, and lift-off process. Finally, the TEM chip was loaded into the specially designed TEM holder, with which a bias can be directly applied through the Au electrodes inside the TEM.
Measurements and characterizations
Crystal structures of the ZnO films was characterized with a Shimadzu X-ray diffractometer (XRD; Shimadzu, Kyoto, Japan), and grazing incident angle XRD with Cu Ka (λ = 0.154 nm) as the radiation source. In situ operation of the ReRAM devices, including forming, set, and reset processes, was investigated in an ultrahigh vacuum TEM (JEM-2000 V, JEOL Ltd., Tokyo, Japan) with a specially designed holder which is capable of applying electric current directly on the electrodes. Morphologies of the device and microstructures were investigated by field-emission scanning electron microscopy (FE-SEM; JSM-6500 F, JEOL) and transmission electron microscopy (JEM-3000 F, JEOL). The I-V characteristics of the MIM structure were measured with a Keithley 4200 semiconductor parameter analyzer (Keithley Instruments Inc., Cleveland, OH, USA) at room temperature in air.
Results and discussion
The formation of the conducting bridge from our in situ observation, which is opposite the conventional formation of the conducting bridge via the ECM mechanism, can be explained as due to the different diffusivity of cations transported inside the dielectric layer, thereby significantly influencing the morphology of the conducting bridge. For the conventional ECM mechanism, the matrix is typically composed of a solid electrolyte with a low melting point, for which cations, after an ionization process from the active electrode, can easily diffuse through the electrolyte to be reduced at the cathode with the wide base and then terminated at the anode with the narrow neck, resulting in the formation of the conducting bridge with the conical shape. In the typical ECM mechanism, the rate limit is determined by the reduction process at the interface of the cathode rather than the ion transport inside the electrolyte film. For the Au/ZnO/Au case as an example in our study, the rate limit is determined by the speed of diffusion for Au cations transported inside the ZnO film because the ZnO layer can be considered as a dense film, thereby limiting the diffusion of Au cations. Once Au cations ionized from Au electrode transport into the ZnO film, they will be reduced into Au NPs immediately, leading to the wide base of the conducting bridge near the anode and the formation of the narrow neck near the cathode, which is consistent with the results observed by other groups in Ag/SiO2/Pt  and Ag/ZrO2/Pt systems . Furthermore, an issue regarding the formation of a conducting bridge on the surface of the ZnO or inside the ZnO film and the role of oxygen vacancies inside the ZnO for the formation and dissolution of the Au-conducting bridge have to be investigated systematically.
In summary, we have designed a specific TEM chip for the in situ observation of ReRAM operations, including set and reset processes based on Au/ZnO/Au system inside a TEM. An obvious bipolar resistive switching behavior with a nanogap distance of 190 nm could be achieved in air with forming, set, and reset voltages of approximately 8.5, 4.6, and ~4.8 V, respectively. A clear conducting bridge associated with the migration of Au ions inside a defective ZnO film from cathode to anode via the conducting bridge during forward/reverse biases could be clearly observed by taking a series of consecutive TEM images, while the formed positions of the conducting bridge with a wide base and narrow neck is different from a typical formation of conducting bridge via the ECM mechanism. The difference can be ascribed to the different diffusivities of cations transported inside the dielectric layer, thereby significantly influencing the morphology of the conducting path. In addition, a broken region near the cathode after reset process was observed, while it will be reconnected after the set process, which provides a direct proof of the formation of the conducting bridge. The current TEM technique is quite unique and informative, which can be used to elucidate the dynamic processes in other devices in the future.
The research was supported by the National Science Council through grants no. NSC 101-2112-M-007-015-MY3 and NSC 101-2120-M-007-003, and National Tsing Hua University through grant no. 100N2024E1. YL Chueh greatly appreciates the use of the facility at CNMM, National Tsing Hua University, through grant no. 101N2744E1.
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