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tude faster than the erase/rewrite speeds of the best EEPROMs3 (Table I). However, fundamental questions concerning their lifetimes had delayed full commercialization. Because ferroelectrics normally have extremely large dielectric constants, their use as nonswitching capacitors in dynamic random-access memories (DRAMs) is also rapidly evolving.4 The majority of studies to date have

emphasized lead zirconate titanate (PZT)-based capacitors for nonvolatile ferroelectric random-access memories (NVFRAMs) and barium strontium titanate-based capacitor DRAMs (see Table II). Capacitors based on layered perovskites such as SrBi2Ta2O9 (SBT) and metallic electrodes such as Pt have negligible polarization fatigue (i.e., no polarization reduction upon repeated read/ erase/rewrite operations), long polarization retention, and the ability to maintain good electrical properties even when the SBT layer is plays a more complicated role since it appears in both the thermal kinetics of Equation 10 and the "avalanche-like" initiation of breakdown. The latter involves the contact potential of the electrode interface and, generally speaking, device performance is maximized via electrodes with very large work functions, such as Pt. This occurs because of the formation of an n-type layer in the ferroelectric near the cathode. In a material such as SBT, an w-type interfacial layer appears to be less significant. Hence the choice of electrode materials is much wider than with BST or PZT. Our latest study shows that there is no oxygen-depletion region near the electrode in an SBT-based capacitor.30 In fact there is a very slight oxygen excess, resulting in ca. 1% oxygen dissolved in the Pt/Ti. This explains much of the difference in device behavior between SBT and BST or PZT-based capacitors—that is, SBT has no M-type layer at the electrode interface and hence should not display p-n junction behavior.

1 2

I =3

0.1 0.2 0.5 1 2 Voltage (V)

Macroscopic Conduction There is presumably a relationship between breakdown field EB, thermal conductivity K, and specific heat Cv. In fact for bulk, homogeneous, isotropic materials within certain theoretical approximations (e.g., "impulse t h e r m a l " breakdown), the relationship is simple and analytic. However, for fine-grained ceramic thin films, it is in general not

38

5 10 20

Figure 4. Evidence for double injection in a fatigued sample of SrBi2Ta2-xNbxOg. Here the sharp transition from a quadratic to cubic I(V) dependence is a classic signature of double injection.20

1000

100

Ferroelectric Device Models Issues relevant to device physics include the following: (1) macroscopic electrostatics (i.e., the dependence of spontaneous polarization of the ferroelectric layer on variation of the coercive field and changes in the dielectric constant and capacitance), (2) macroscopic conduction (i.e., dependence of the break-down field on thermal conductivity and specific heat), and (3) switching kinetics (i.e., switching time dependence on variation of the coercive field and relation of the switching t