INVESTIGATION OF OXIDE-BASED PMC MEMORY CELLS INVESTIGATION OF OXIDE-BASED PMC MEMORY CELLS In the past several years, the Programmable Metallization Cell (PMC) research program at ASU has yielded highly significant results. One of the most notable recent achievements was the fabrication and characterization of Cu-SiO2 based devices, with and without rectifying characteristics. The rectifying devices were developed for use in high-density passive memory arrays. The material study involved an investigation of Cu diffusion at various temperatures in thin SiO2 films and the influence of diffusion conditions on the switching of PMC devices formed from such Cu-doped films. Film composition and diffusion products were analyzed using secondary ion mass spectroscopy, Rutherford backscattering spectrometry, X-ray diffraction and Raman spectroscopy methods. We found a strong dependence of the diffused Cu concentration, which varied between 0.8 at.% and 10-3 at. %, on the annealing temperature. X-ray diffraction and Raman studies revealed that Cu does not react with the SiO2 network and remains in elemental form after diffusion for the annealing conditions used. PMC resistive memory cells were fabricated with such Cu-diffused SiO2 films and device performance, including the stability of the switching voltage, was examined in the context of the material characteristics. Cu-SiO2 based devices were modified to have inherent rectifying characteristics for passive array applications via the use of an n-type silicon bottom electrode with doping densities of 1018 cm-3 and 5x1014 cm-3. The low resistance state of the resistive switching element produced by Cu ion migration and electrodeposition results in the formation of a Cu/n-Si Schottky interface, the behavior of which has been shown to be comparable to a Cu-Si Schottky diode in forward and reverse bias. The leakage current in the off-state was in the order of 5/50 nA at +/-1 V for a 6 m diameter device but since this current is carried by the Cu-SiO2 material, it is expected to scale with device area, e.g., 50/500 fA at 20 nm, resulting in acceptable levels for cell-to-cell isolation even without a diode element. The reverse bias leakage current in the on-state diode was dependent on the current employed to program the device as Iprog influences the area of the electrodeposit and hence the area of the Cu/n-Si junction. Our analysis revealed that Isat is<1 nA at 1 V for Iprog <10 A, providing GO-level cell-to-cell isolation in a passive array. As in the case of all PMC-like memory elements, the on-state resistance of the Cu/Cu-SiO2/n-Si device could be controlled via the magnitude of Iprog and this allows multi-level cell (MLC) operation, in which discrete resistance levels are used to represent multiple logical bits in each physical cell. Our Cu/Cu-SiO2/n-Si elements with heavily-doped silicon electrodes were readily erasable at reasonable voltage (less than -5 V) which allows them to be re-programmed. The lightly-doped silicon electrode devices were not able to be erased due to their very high reverse breakdown voltage but since their leakage current levels were around three orders of magnitude lower than in the case of the highly doped silicon electrode devices, they are actually ideal one-time programmable (OTP) devices as they can be programmed at low voltage and current and are extremely well isolated by their extremely high Roff in the off-state and ultra-low Isat in the on-state.
|Effective start/end date||7/16/12 → 5/13/13|
- Arizona Department of Economic Security: $16,149.00
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