Resistive switching behavior in Lu2O3 thin film for advanced flexible memory applications
© Mondal et al.; licensee Springer. 2014
Received: 26 November 2013
Accepted: 17 December 2013
Published: 3 January 2014
In this article, the resistive switching (RS) behaviors in Lu2O3 thin film for advanced flexible nonvolatile memory applications are investigated. Amorphous Lu2O3 thin films with a thickness of 20 nm were deposited at room temperature by radio-frequency magnetron sputtering on flexible polyethylene terephthalate substrate. The structural and morphological changes of the Lu2O3 thin film were characterized by x-ray diffraction, atomic force microscopy, and x-ray photoelectron spectroscopy analyses. The Ru/Lu2O3/ITO flexible memory device shows promising RS behavior with low-voltage operation and small distribution of switching parameters. The dominant switching current conduction mechanism in the Lu2O3 thin film was determined as bulk-controlled space-charge-limited-current with activation energy of traps of 0.33 eV. The oxygen vacancies assisted filament conduction model was described for RS behavior in Lu2O3 thin film. The memory reliability characteristics of switching endurance, data retention, good flexibility, and mechanical endurance show promising applications in future advanced memory.
Resistive switching (RS) behavior, which utilizes the resistance change effect of oxide material, has attracted considerable attention and been widely investigated due to its potential application in future nonvolatile memory (NVM) devices . Several metal oxide materials including NiO , TiO2, Cu x O , and Al2O3 have been studied for resistive random access memory (ReRAM) applications. On the other hand, the flexible electronics are an emerging class of devices in an intriguing technological paradigm. The demand for flexible electronics is revived because of their inherit merits of low cost, light weight, excellent portability, and user-friendly interfaces over conventional rigid silicon technology . Despite these advantages, there is very little in the works about the flexible and NVM devices because of the difficulty to satisfy the dual requirements of memory element. A major challenge for flexible electronics is the lack of good performance NVM devices fabricated at low temperature [7, 8]. The ReRAM shows promising memory performance on plastic flexible substrate when processed at low temperature, but the degradation behavior due to excessive mechanical and electrical stress, large switching power, and distribution is the basic limitation for high-density electronic applications [9–12]. It is expected that an achievement of such flexible- and nonvolatile-type memory device will be the next step toward the realization of flexible electronic systems. Recently, flexible resistive memories have been reported in various oxides including graphene oxide (GO) , HfO2, NiO , and single-component polymer  thin films. However, the huge dispersion in switching parameters, deprived reliability, and poor understanding of the RS behavior are some of the fundamental issues which hinder its application for high-density flexible electronics.
It is well articulate that the amorphous high-κ gate dielectrics, which have already been established to be promising for semiconductor transistor technologies, can be good alternative for ReRAM applications as long as such these materials can perform good RS behaviors. Rare earth metal oxides as high-κ dielectrics are considered as the replacement of hafnium-based technology [17–19], among which Lu2O3 is the promising one as it shows well-insulating property, large bandgap (5.5 eV), better hygroscopic immunity, good thermal stability, and adequate dielectric constant of approximately 11 . Gao et al. reported promising unipolar RS behavior in amorphous Lu2O3 oxide . In contrast, we previously demonstrated the bipolar RS in various high-κ rare earth metal oxides, such as Tm2O3, Yb2O3, and Lu2O3, on silicon substrate . The different RS behavior may be originated from their distinguished morphological changes. However, no flexible memory device has been demonstrated and detail switching dynamics is still unclear in this material. The superior experimental switching characteristics in Lu2O3 and room temperature deposition process allow it to be a possible functional material for flexible electronics. Therefore, in this study we investigate the RS behaviors of the sputter deposited lutetium sesquioxide (Lu2O3) thin film on flexible substrate for nonvolatile flexible memory application. In addition, we demonstrate that the memory performance of ReRAM on a flexible substrate has excellent electrical and mechanical reliabilities due to the high ductility of amorphous Lu2O3 thin film and the merit of the low-temperature process. Unlike other typical flexible resistive memory, better RS characteristics were achieved for advanced flexible memory applications.
Flexible Ru/Lu2O3/ITO RS memory devices were fabricated on flexible polyethylene terephthalate (PET) substrates. The sputtered ITO-coated PET substrate was glued on a Si dummy wafer with polyimide tape to mechanically support the flexible substrate during fabrication process. The Lu2O3 thin films with a thickness of 20 nm were deposited by reactive radio frequency magnetron sputtering system in argon-oxygen (3:1) medium at room temperature from a Lu metal target. The chamber working pressure was maintained at 10 mTorr with the rf power of 130 W during deposition. The sputtering rate and time of the film were about 0.17 Å/s and 20 min, respectively. Finally, a 50-nm-thick square shape (100 × 100 μm2) Ru metal top electrode was deposited on the oxide films through shadow mask by DC sputtering technique operated at 10 mTorr in Ar environment.
The crystalline structure and the chemical compositions of the films were examined by x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS), respectively. The crystal structure of the Lu2O3/ITO film was determined in a Bruker-AXS D5005 diffractometer (Bruker Biosciences Inc., Billerica, MA, USA) using Cu Kα (λ = 1.542 Å) radiation. The composition and chemical bonding in the Lu2O3 film were analyzed using a Thermo Scientific Microlab 350 VG system (Thermo Fisher Scientific, Inc., Waltham, MA, USA) with a monochromatic Al Kα (1,486.7 eV) source. The surface of the Lu2O3 film was pre-sputtered using an Ar ion source. The chemical shifts in the spectra were corrected with reference to the C 1 s peak (from adventitious carbon) at a binding energy of 285 eV. Curve fitting was performed after Shirley background subtraction using a Lorentzian-Gaussian fitting. The roughness of the film was measured using an NT-MDT Solver P47 (NT-MDT Co., Zelenograd, Moscow, Russia). The atomic force microscope (AFM) was operated in the tapping mode for imaging. The electrical properties of the Ru/Lu2O3/ITO memory devices were measured by a semi-automated cascade measurement system equipped with Agilent E5260 high-speed semiconductor parameter analyzer (Agilent Technologies, Sta. Clara, CA, USA).
Results and discussion
In this work, the RS behavior in the Lu2O3 thin films on flexible PET substrate is explored for advanced flexible nonvolatile random access memory applications. The current conduction mechanism is dominated by the bulk-limited SCLC conduction in HRS and the ohmic-like conduction in LRS. A shallow trap level at 0.33 eV below the conduction band was evaluated in Lu2O3 thin films. The filament conduction via oxide defects was described for the RS behavior in the Lu2O3 thin films on ITO/PET substrate. Low-voltage RS and good device uniformity were obtained in the Ru/Lu2O3/ITO flexible ReRAM cell. Good memory reliability characteristics of switching endurance, data retention, flexibility, and mechanical endurance were promising for future memory applications. The superior switching behaviors in Ru/Lu2O3/ITO flexible ReRAM device have great potential for future advanced nonvolatile flexible memory applications.
This work was supported by the National Science Council (NSC) of Republic of China under contract no. NSC-102-2221-E-182-072-MY3.
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