Stability scheme of ZnO-thin film resistive switching memory: influence of defects by controllable oxygen pressure ratio
© Huang et al.; licensee Springer. 2013
Received: 11 September 2013
Accepted: 3 November 2013
Published: 16 November 2013
We report a stability scheme of resistive switching devices based on ZnO films deposited by radio frequency (RF) sputtering process at different oxygen pressure ratios. I-V measurements and statistical results indicate that the operating stability of ZnO resistive random access memory (ReRAM) devices is highly dependent on oxygen conditions. Data indicates that the ZnO film ReRAM device fabricated at 10% O2 pressure ratio exhibits the best performance. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) of ZnO at different O2 pressure ratios were investigated to reflect influence of structure to the stable switching behaviors. In addition, PL and XPS results were measured to investigate the different charge states triggered in ZnO by oxygen vacancies, which affect the stability of the switching behavior.
KeywordsZnO O2 partial pressure Oxygen defects Resistive change memory
Recently, resistive random access memory (ReRAM) has intensively attracted much attention, which will become one of the potential candidates in next-generation memory, owing to its advantages, including nonvolatility, high speed, high density, and low power consumption[1, 2]. From the materials science point of view, many metal oxide materials, such as perovskite-type oxides, ferroelectric oxides, and binary transition metal oxides, have exhibited differently resistive switching characteristics[3–5]. Up to date, the best switching behaviors of ReRAM devices were observed on the binary transition metal oxides, such as NiO and TiO2[6–9].
ZnO is one of binary transition metal oxides with several applications as optoelectronics because of a wide optical direct bandgap of approximately 3.37 eV, a high exciton binding energy of around 60 meV, and has been exhibited excellent resistive behavior[10–12]. However, optimized conduction in ReRAM applications for the ZnO-based ReRAM is not well investigated yet, whose defects resulted from pristine conditions or doping in the ZnO film are not easily controlled. Most of the studies have indicated that migration of oxygen ionic atoms plays an important role in the resistive switching process[13, 14]. The conductivity of metal oxide is highly sensitive regardless whether the oxygen atom existed at a lattice site or not.
In this regard, by varying partial pressures of oxygen gases (O2) during sputtering process, native defects related to resistive behavior in the ZnO layer, including oxygen vacancies, Zn vacancies, oxygen interstitials, and Zn interstitials, were investigated in detail, respectively[15–17]. The amount of these defects would significantly affect the resistive switching behaviors of the ZnO layers as well as the stability. Here, photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) were used to identify the native defects.
ZnO films of 100-nm thick were deposited on Pt/Ti/SiO2/Si substrates at room temperature (RT) by RF sputtering of the ZnO target at different O2 pressure ratios from 0%, 10%, 33% to 50%. Pt electrode with a diameter of around 200 μm was used to fabricate a symmetrical metal-insulator-metal (MIM) sandwich structure by shadow mask. The I-V behaviors of these devices under different temperatures were measured by a Keithley 4200 semiconductor parameter analyzer (Keithley Instruments Inc., Cleveland, OH, USA). The crystalline structures of the ZnO films were examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The PL measurements were performed using a He-Cd laser with an excitation wavelength of 325 nm at RT to unveil the defects in the ZnO layer. XPS was used to observe the chemical bonding energy with different oxygen states.
Results and discussion
The intensity ratio from the PL spectra
Activation energy obtained from fitting of Arrhenius relations
Activation energy (eV)
In conclusion, the ZnO films prepared by sputtering processes for ReRAM device at different oxygen pressure ratios were measured and investigated. The statistical results from the electrical properties indicate that the ZnO ReRAM devices fabricated at the 10% O2 pressure ratio exhibit a very stable I-V behavior with a high operation yield of approximately 75% compared to the other conditions. The XRD and TEM images showed a large grain aligned at (001) direction in the 10% O2 pressure ratio, which limits the electrons migrating along these grain boundaries, thereby stabilizing the switching behavior. From the PL results, a near-band edge emission was observed in the lower O2 pressure ratio, owing to the defects that existed in the ZnO film. Two charge states of oxygen vacancies were found according to the calculation using Arrhenius equation, which affect the stability of the switching behavior. Finally, the region near the Zn ions containing a relatively higher conductive characteristic with a higher O2 pressure ratio is the reason of unstable I-V behavior, owing to the creation of more leakage paths, while the electrons trapped by oxygen vacancies at the positive valence state in the lower O2 pressure ratio can be an explanation for enhancing the stability of ReRAM devices.
Full width at half maximum
Resistive random access memory
Transmission electron microscopy
X-ray photoelectron spectroscopy.
The authors greatly acknowledge Professor Tai-Bor Wu at the National Tsing Hua University. The research was supported by the National Science Council through grant nos. 101-2112-M-007-015-MY3, NSC 101-2120-M-007-003, and 101-2218-E-007-009-MY3, and the National Tsing Hua University through grant no. 102N2022E1. YL Chueh greatly appreciates the use of facility at CNMM, National Tsing Hua University through grant no. 102N2744E1.
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