The memory effects in a memristor can be realized through the switching behavior between two distinct resistance states, low resistance state (LRS) and high resistance state (HRS) driven by low pulse voltages. ZnO-based thin films such as undoped ZnO, Mg-doped ZnO, Na-doped ZnO and Mn-doped ZnO have attracted considerable interest as promising resistive switching materials. Gallium doping electrically modulates the behavior of ZnO to suit low power switching behavior. Non-lattice oxygen ions and oxygen vacancies as detected by XPS are found to play important role in imparting forming-free resistive switching behavior.All deposition for fabrication of device has been done by dual ion beam sputtering(DIBS).
To start with fabrication of device, a 60 nm thick Ga-doped ZnO thin film as switching layer is deposited over bottom electrode (BE) Au/SiO2/Si, at a substrate temperature of 100 °C, with DIBS background pressure of 1 × 10-8mBar and Ar:O2 (1:4) (flow rate in sccm), respectively. Finally, circular Au electrodes of 300 µm is deposited on the surface of switching layer. Similarly, Al/ZnO/Al/SiO2/Si device is fabricated to compare the electrode and doping effect using Ar:O2 (2:3) and substrate temperature of 400 °C. The current-voltage (I-V) characteristics of the device are measured using Keithley 2612A sourcemeter and Everbeing probe-station. Al(BE)/ZnO interface has been observed by cross-sectional high-resolution transmission electron microscopy (HR-TEM) using HR-TEM: JEOL JEM-2010 for formation of any AlOx layer. X-ray photoelectron spectroscopy (XPS) having PHOIBOS 100 analyzer with an Al Kα radiation (1486.6 eV) as an excitation source, has been utilized to analyze the binding energy and the composition of each element in switching layer.
I-V characteristics are measured by sweeping a DC voltage in sequence of 0-10 mV-0--10 mV-0 in steps of 1 mV and 0-(+8 V)- 0-(-8 V)-0 in steps of 0.5 V for both devices respectively with a compliance current of 1 mA. I-V of Al/ZnO/Al (AZA) shows device with varying ramp rate exhibiting decreasing hysteresis with increasing ramp rate. Similarly I-V for Au/Ga-ZnO/Au (AuGZAu) conforms to zero crossing of I-V hysteresis loop and shrinking of loop area with increasing ramp rate. Device sets and resets to lower voltage as compared to AZA device. XPS of the switching layer conforms to presence of oxygen vacancies and non-lattice oxygen ions which assist in switching. An amorphous AlOx interfacial layer (~4-5 nm)  formed at Al(BE)/ZnO interface as confirmed by HRTEM for the device in high resistance state (HRS) state which assists in set/reset process.
Ga doping increases conductivity of ZnO film and hence sets and resets at lower voltages. AuGZAu device depicts unipolar memristive behavior as it shows pinched hysteresis with varying frequency, whereas AZA behaves as an ideal bipolar memristor with good endurance and retention. AuGZAu device can be utilized for low power resistive switching.