ABSTRACT OF THE THESIS
Write-Once Read-Many-Times (WORM) Memory based of Zinc Oxide on Silicon
Master of Science, Graduate Program in Electrical Engineering
University of California, Riverside, August 2011
Dr. Jianlin Liu, Chairperson
Write-once read-many-times (WORM) memory feature is found in ZnO films deposited on silicon substrates. The resistance ratio (R ratio) between the high resistance state (HRS or ON state) and low resistance state (LRS or OFF state) is mostly around 104 for all tested devices. The programming power required to switch the memory devices from HRS to LRS can be as low as 10-3 watts when current compliance is set at 50µA. Lowering the current compliance will result in lower R ratio, because the maximum LRS current level will also be limited by the current compliance.
To test the devices for long term storage purpose, endurance to reading pulse with width of 2μs and amplitude of 1V were tested for several randomly selected devices with various top contact sizes. Results shown all devices sustained their R ratio with hardly noticeable resistance change throughout 108 reading cycles. Retention was also tested on several devices. The results showed that only R ratio of HRS showed some drop in resistance after 105 seconds (27.78 hours) testing period, while LRS current level was almost constant throughout. R ratio still retained around 103 after being extrapolated to 100 years. Temperature stability of the devices was also tested up to 120ºC. There was current drop in both HRS and LRS under high temperature over time, but the overall R ratio did not change much, still above 103 for all tested devices. Endurance and retention test results under high temperature showed similar trend as room temperature.
Au metal contact devices showed almost undetectable HRS current by the equipment because the probe station used has current noise level which is almost the same as the current level of the HRS devices. Devices with Au and Ti metal have almost the same conductivity at LRS, and Au top contact has the potential to provide higher R ratio for WORM devices. Higher memory performance can be clearly observed in devices fabricated on p-Si substrates than that on n-Si substrates. Devices on p-Si substrate showed low rectifying ratio in HRS, and high rectifying ratio in LRS, thus the R ratio of the devices is higher. The high rectifying ratio in both HRS and LRS in devices with n-Si substrate resulted in lower R ratio of around 102.
The switching mechanism of all devices is explained by the filament model, in which conducting filaments consisting of oxygen vacancies are formed after applying external electric field to switch the device to ON state. The filament model explained why all devices' LRS conductivity is independent of contact size. As write-once memory, the LRS devices have to sustain their resistivity through any external electric field. Test results showed that all devices do not reset to HRS after repeatedly attempting to reset devices by looping the -5V ~ 5V sweep on them. Although all devices kept their LRS status, a slight reset can be observed in the negative bias region, positive bias region showed no sign of reset.