Resistance Switching In Ferroelectric Materials
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0997-I05-02
Resistance Switching In Ferroelectric Materials Tingkai Li, and Sheng Teng Hsu PTL, Sharp labs. of America, Inc., 5700 NW Pacific Rim. Blvd., Camas, WA, 98607 ABSTRATCT A ferroelectric crystal with perovskite structure such as PbZr1-xTixO3 (PZT), SrBi2Ta2O9 (SBT), Bi3La1-xTixO12 (BLT) and non-perovskite structure such as Pb3Ge5O11 (PGO) have two polarization states, which can generate two resistance states: high resistance and low resistance states. These properties can be used for resistance random access memory applications (RRAM). When a ferroelectric capacitor is polarized an internal electric field opposite polarity to the external applied field is generated. As a result there is a large resistance change at a given bias voltage between the two polarization states of the capacitor. It is the purpose of this paper to show that the ferroelectric capacitor may be used as a current memory cell of non-destructive readout (NDRO) non-volatile Random Access Memory array. It will also be shown that each current sensing ferroelectric memory cell stores two bits of memory information and exhibits long memory retention and excellent endurance properties.
INTRODUCTION Materials having switchable resistance are attractive to nonvolatile random-access memories1 - 3. The perovskite metal oxides and their related structures and with transition metal doping show a reproducible switching in current with a memory effect. Research works have been made in IBM (SrZrO3 with 0.2% Cr and SrTiO3 with Cr doping)4, The University of Houston (UH) (Pr0.7Ca0.3MnO3)5, and Sharp Labs of America, Inc. (Pr0.7Ca0.3MnO3 (PCMO) by CVD, sputtering and spin on techniques)6. Compared with other memory devices, the resistance random access memory devices have a very small memory cell size, fast reading speed and nondestructive readout. Those materials are difficult to be integrated into the conventional silicon technology, however. A ferroelectric crystal with perovskite structure such as PbZr1-xTixO3 (PZT), SrBi2Ta2O9 (SBT), Bi3La1-xTixO12 (BLT) and non-perovskite structure such as Pb3Ge5O11 (PGO) have two polarization states, which generate two resistance states: high resistive low current state, and low resistive high current state7. If a positive voltage of magnitude greater than the coercive voltage is applied to the ferroelectric capacitor, the film is polarized in the positive direction. An internal field, opposite polarity to the external field is generated as is shown in Fig. 1. Once the voltage is removed from the capacitor, the polarization relaxes slightly to the Pr (called remanent polarization). We arbitrarily call this positive polarization a ìzeroî. The device current at a positive bias voltage smaller than the coercive voltage can be presented as Eq.1. IRD = (VRD - VIP)/RF
(1)
Here, IRD is the capacitor current, VRD is the applied voltage, VIP is an internal polarization voltage generated by polarization, and RF is the resistance of ferroelectric materials. Equation (1) can be rewritten as:
VIP1
VIP2
-
+ + + + + +
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