The High Density Information Storage Characteristic In Epitaxial BaTiO₃（111） Thin Films

With the rapid development of the society, especially in information technology, memory is becoming a necessity. Memory is divided into volatile memory and non-volatile memory. The stored information will be lost in case of power failure in volatile memory. On the contrary, non-volatile memory has been able to save the information we need in case of power failure. The flash memory(Flash) we usually use at present is a kind of non-volatile memory. The feature size of MOS pipe will reach a certain value. In addition, the power consumption of the flash memory(Flash) is relatively high and the data retention will be affected by the small size. The above disadvantages about flash memory(Flash) limit its development and cannot satisfy the requirements for storage. Resistance random access memory(RRAM) is a new type of memory and can meet the needs of people. So we need to work hard on it and make it widely used.We need to make efforts in the following aspects to make the RRAM device into use:(1) try to use simple structure to explore the switching mechanism as the mechanism is not clear;(2)high density storage;(3) without the forming process;(4) long retention time and so on. High density storage is one direction we all need to work on in resistance random access memory(RRAM).We carry on some research on the widely investigated perovskite material barium titanate(BaTiO3).Firstly, barium titanate(BaTiO3) thin films were fabricated on the(111) niobium doped strontium titanate(Nb:SrTiO3(NSTO)) substrate by pulsed laser deposition(PLD) technique. We confirmed the(111) orientation growth of prepared BaTi O3(BTO) from the X-ray diffraction patterns of θ-2θ scans. The X ray diffraction patterns of the ф-scans identify the epitaxial growth of the(111) oriented BTO film on NSTO. The X-ray diffraction patterns of Omega scanning indicate the good quality of the epitaxial film we prepared.The top electrodes Pt were deposited on BTO films by DC sputtering through a patterned mask. Then a sandwich structure of Pt/BTO/NSTO was prepared. The In bottom electrode was pressed on the back of NSTO with ohmic contact, which is confirmed by the current-voltage measurement.After preparing the device, we performed the current-voltage(I-V) measurements using Keithley 2400 source meter. No eletroforming process is necessary before the observation of the hysteretic I-V curves. Negative differential resistance(NDR) behaviors appear in the reverse voltage region at about-5V when the voltage is larger than 7V, which is useful for high density storage, and a bipolar resistive switching(RS) is also observed. When a small voltage(<4V) is applied to the device, a typical rectifying behavior is observed.Do studies on NDR behaviors. The NDR behaviors are observed in the negative voltage region only after applying a sufficiently large positive voltage by recording the current-voltage curves by applying different positive voltages and different negative voltages. The NDR behaviors are also observed when large positive pulses are applied on the device and only negative voltage region is scanned. The relation between compliance current and the NDR behaviors is also measured, indicating that the compliance current has impact on the NDR behaviors.Do studies on bipolar resistive switching. We measured the effect of the maximum positive voltage, the maximum negative voltage and compliance current et al on high resistance states(HRS) and low resistance states(LRS) of the device because we are studying the the resistive switching characteristics of our device. The resistance of LRS decreases with increasing the maximum of positive voltage and finally reaches a minimum, while that of HRS remains constant. The resistance of LRS decreases with increasing the compliance current and finally reaches a minimum, while that of HRS remains constant. The resistance of HRS increases with increasing the absolute of maximum of negative voltage and finally reaches a maximum, while that of LRS remains almost constant. There are tunable bipolar resistive switching characteristics in our device. To summarize, the above results imply the promising application for nonvolatile multistate memory devices, which has the potential application for high density information storage.The retention test and fatigue test are also performed. The initial decay in the LRS is observed in the retention test, which could be related to the diffusion of oxygen vacancies. As we all know, NSTO is a typical n-type and BTO is also n-type. Moreover, the barrier height at the interface of BTO/NSTO is very low for the epitaxial BTO film on the NSTO substrate and the contact on BTO/NSTO can be regarded as an ohmic. The electrical measurements show that the contact on NSTO/In is ohmic, while the contact on Pt/BTO interface is Schottky for different work function. The resistive switching characteristics of the device are dominated by the Pt/BTO interface. We explain the observed negative differential resistance behavior and tunable bipolar resistive switching characteristic by considering the oxygen vacancies which can trap or release electrons as a trapping layer at the Pt/BTO interface and the trapping/detrapping process will change the Schottky barrier width.We also do studies on the influence of white-light on the Schottky behavior. The Schottky behavior becomes worse with white-light.In a word, both the NDR behavior and promising application for nonvolatile multistate memory in our prepared device suggest that the device has the potential application for high density information storage, which is useful for the studies and applications of resistive switching in the future.