| As the development of economy, the requirement for storage became increasingly higher, such as, high density, fast write and read access, non-volatile, low energy consumption and so on. Today, Si-based flash memory device represent the most prominent NVM (non-volatile memory), however, flash suffers from low write and read speed. Moreover, as the dimension of the devices scale down, the thickness of oxide layer reduce, which will lead to poor cooling effect and current leakage problem, so the outlook is not optimistic. Now, people turned to the new-generation random memory, which has both fast speed and non-volatile property, mainly including magnetoresistive random access memory (MRAM), ferroelectric random access memory (FeRAM), resistive random access memory (ReRAM), and phase change random access memory (PPAM) and so on. ReRAM is the latest memory, it has fast speed, low consumption, low cost, simple technology and easy to combine with traditional CMOS technology, so it has a great potential to become the new generation of memory. In1962, H. W. Hick Mott fabricated the metal/oxide/metal sandwiched heterojunction, Al/Al2O3/Au and Zr/ZrO2/Au, studied their â… -â…¤ characteristic curve and firstly reported the resistance switching phenomenon. Recently, ReRAM attracts more and more attention in the RAM field.Ta2O5has outstanding physical and chemical property, which makes it attract lots of attention and be applied in many fields. For example, Ta2O5is a kind of coating materials with high index of refraction2.1. Due to the high-quality interface and low leakage current between Si and Ta2O5, Ta2O5is used as insulator in high-end capacitor. In the ReRAM field, Ta2O5has stable resistance switching characteristics and become the most studied material.In this paper, we fabricated Ag/Co/Ta2O5/Ag heterojunction by magnetron sputtering device. Large bipolar resistance switching was found in our sample, the Roff/Ron ratio is high up to104, the optical performances give it great application prospect. By changing the thickness and area of Ta2O5, we found that the resistance in HRS (high resistance state) increase with the increase of thickness and the decrease of area of Ta2O5, but there is not obvious difference in the LRS (low resistance switching), which indicates the mechanism of resistance switching is bulk dominant. In the next experiment, we replace the Ag electrode with Au electrode and fabricated the Au/Co/Ta2O5/Au heterostructure. A similar bipolar resistance switching was observed in this structure which can exclude the formation of Ag filament and we speculate that the resistance switching is due to the migration of oxygen ion. We also fabricated the Ag/Ta2O5/Ag and measured their IV curve, which show a very unstable complementary RS. Moreover, the electroforming process is not necessary for our device and the existence of CoO is demonstrated by the magnetic measurement of the Co/Ta2O5bilayer. We think that the inert of Co will break the symmetrical structure of the device, Co is an active metal, when the Ta2O5was spurted, a thin layer of CoO is synthesized simultaneously, an oxygen vacancy space gradient distribution will formation in the Ta2O5. The RS process is as follows:when a positive bias voltage is applied to the junction,(oxygen vacancy) will accumulate in the cathode, because of this increased conductivity, the electrical field decreases significantly in this region, we call it virtual cathode. As times goes on, the virtual cathode is moving to the anode, the junction finally converts from HRS to LRS. On the contrary, a negative bias voltage will lead to the rupture of the filament composed of and reset the junction to HRS. In conclusion, our work reported a stable, giant bipolar RS, interpreted the RS mechanism and the importance of Co. |
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