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Ferroelectric Random Access memories

Ferroelectric RAM (FRAM or FeRAM) is a type of non-volatile computer memory, similar to EEPROM but based on electric field orientation and with near-unlimited number of write-erase cycles. FRAM is an array of ferroelectric capacitors, a DRAM with the dielectric layer in the memory capacitors replaced with a thin ferroelectric film, typically made of lead zirconate titanate (PZT). The resulting cell is electrically similar to the capacitors used in a conventional DRAM cell, but the ferroelectric film will retain a electric field even after the charge in the capacitor quickly drains.

Depending on the direction of the current flow when the cell is charged, the film will be polarized into one of two directions. Generally the operation of FRAM similar to ferrite core memory. In comparison, FeRAM requires far less power to flip the state of the film’s polarity, and does so much faster. The requirement for a write cycle for each read cycle, together with the high but not infinite write cycle limit, poses a potential problem for some special applications. One of the main technological challenges is to find a suitable electrode material that with low electrical resistivity good thermal stability, high resistance to oxidation and good adhesion both to substrate and the ferroelectric film. The interfacial defect layers may originate from accumulation of oxygen vacancies. Under the electric field oxygen vacancies migrate towards the electrode and aggregate near the electrode interface. The pervoskite structured titanate could not afford a large population of point defects. In the case of conducting oxides electrodes or aqueous solution electrodes, oxygen vacancies in the pervoskite film in the region near the film electrode interface can be compensated by the electrodes. Thus the use of this kind of electrodes inhibits the accumulation of oxygen vacancies at the interface. As a result no interfacial defect layer forms at ferroelectric oxide or ferroelectric aqueous interfaces.

Our work is focussed on the growth of conducting pervoskite electrodes like La1-xSrxCoO3 (LSCO) and La0.5Sr0.5Co1-xNixO3 (LSCNO) by RF magnetron sputtering and ferroelectric materials like BST and PZT by PLD. The top and bottom LSCO / LSCNO electrode will act both as electrode and as template for the better growth of pervoskite ferroelectric film. Ferroelectric capacitors were fabricated using LSCO and LSCNO as the electrodes and BST and PZT as the ferroelectric material. LSCO and LSCNO electrodes were fabricated using rf magnetron sputtering at an rf power of 150 W at a target to substrate distance of 4 cm. The bottom electrodes were fabricated at substrate temperature and the top electrodes were fabricated at room temperature. Ba0.7Sr0.3TiO(BST) and PZT thin films were deposited by pulsed laser deposition using third harmonics (355 nm) of Q-switched pulsed Nd:YAG laser (10 Hz, 6-7 ns pulse width). The deposition was carried out under an oxygen partial pressure of 0.1 mbar with a substrate temperature 500 oC for BST and at 300 oC for PZT