2, Low Power Nanoelectronics Research Group, Indian Institute of Technology Indore, Indore, Madhya Pradesh, India
Memristor suggested to be fourth fundamental element in 1971 has recently gained wide attention after HP scientists fabricated a real memristor. Resistive random access memory (RRAM) based on transition metal oxides has been widely investigated as a next-generation non-volatile memory to be an important application for memristor. RRAM is a two terminal device which reversibly switches between low resistance state (LRS) and high resistance state (HRS) upon applying electrical stimulus in a particular range at RESET/SET voltages respectively. In this work, we demonstrate a forming-free resistive memory with memristive characteristics using ZnO thin film. Oxygen vacancies/interstitials defects in ZnO thin film are engineered by a novel technology of Dual Ion Beam Sputtering (DIBS) to suit switching needs. In general, a forming process is necessary to activate the resistive memory devices before performing any resistive switching (RS). Abundant oxygen vacancies in film ensures forming- free behavior of device. Besides, sufficient non-lattice oxygen ions in ZnO thin film assist set/reset of device. Interfacial AlOx formation/dissolution at Al/ZnO interface ensures bipolar resistive switching with smooth transition between resistance states (HRS and LRS). To start with fabrication of device, 60 nm thick ZnO thin film is deposited over Al/SiO2/Si substrate at a substrate temperature of 100 °C, with DIBS background pressure of 1 × 10-8 mBar and Ar:O2 (2:3) (flow rate in sccm), respectively. Finally, circular Al electrodes of 500 µm is deposited on the surface of ZnO thin film. Further, I-V characteristics are measured by sweeping a DC voltage in sequence of 0-(+8 V)- 0-(-8 V)-0 in steps of 0.5 V for a compliance current of 1 mA. Set and reset voltages of device are evaluated to be at -6/6 V. Device shows excellent endurance measured at 0.1 V for 250 cycles Retention performance assessed at 0.1 V read voltage exhibits outstanding non-volatile behavior for 106s extrapolated to 10 years. Presence of oxygen vacancies (VO), interstitial oxygen ions (IO) and lattice oxygen ions are confirmed by X-ray photoelectron spectroscopy (XPS) Photoluminescence (PL) of ZnO thin film. HR-TEM image of Al/ZnO interface shows an amorphous AlOx interfacial layer (~4-5 nm) formed at interface for the device in high resistance state (HRS) state. Formation of this interfacial layer at RESET voltage transits the device from LRS to HRS and dissolution at SET voltage leads to change device state to LRS. Our memory device as fabricated by DIBS exhibits excellent performance parameters, retention and endurance to implement it for a practical RRAM. Our work could play a very significant role in realizing similar memristive devices with high performance parameters in future.