Preparation and ferroelectric properties of mixed composition layered lead zirconate titanate thin films for nonvolatile
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Preparation and ferroelectric properties of mixed composition layered lead zirconate titanate thin films for nonvolatile memory applications Seung-Hyun Kim, Dong-Joo Kim, S. K. Streiffer, and A. I. Kingon Dapartment of Materials Science and Engineering, North Carolina University, Raleigh, North Carolina 27695 (Received 2 July 1998; accepted 9 February 1999)
Mixed composition layered lead zirconate titanate (PZT) films sZryTi ratio 30y70 1 65y35d with stoichiometric lead containing PZT thin layer at the film/electrode interface were successfully fabricated by a modified chemical solution deposition method. These modified PZT thin films are highly (111) textured, and have square-shaped P-E hysteresis loops with large remanent polarization and low coercive field, as well as low saturation voltage. In addition, these films show good fatigue and imprint behavior with Pt electrodes; the retained polarization of the modified film was above 50% after fatigue testing to 109 cycles, and the thermally induced voltage shifts sDVd were 0.51 V after heating at 150 ±C for 4410 s, two times lower than for films without a stoichiometric thin layer.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
The basic demands on a ferroelectric memory are that it can be switched at low voltage (5 V or less), that the switched polarization is sufficiently large, and that the number of read/write cycles is large (ideally 1015 cycles, but 1010 cycles also offers device possibilities).1–3 The voltage needed for operation depends on the film composition, the coercive field sEc d, and the shape of the hysteresis loop (squareness). The preparation and optimization of ferroelectric thin films for low voltage operation can be considered as one of the most vital tasks for the realization of devices. Generally, while Ti-rich tetragonal PZT thin films sZryTi 30y70d have square P-E hysteresis loops and large remanent polarizations, they have large coercive fields and saturate at high voltages. On the other hand, Zr-rich films have good loops with low coercive fields, but usually have low remanent polarization and slanted hysteresis loops.4,5 To compensate for these drawbacks, we have prepared mixed composition layered PZT thin films by modifiedchemical solution deposition. These films consist of a bottom thin layer (50 nm) that is Ti-rich sZryTi 30y70d without lead excess in the precursor solution, followed by a main layer (200 nm) that is Ti-rich (30y70) with 12% lead excess in the solution, and then by a top thin layer (50 nm) that is Zr-rich (65y35). Using this method, we have produced films that are highly (111) oriented with square-shaped P-E hysteresis loops, large remanent polarization, and low coercive field, as well as low saturation voltage. In addition, these films show good fatigue and imprint behavior. Our studies on new preparation methods and the ferroelectric properties of mixed composition layered PZT thin films will be presented here.
The choice of starting materials and solvent is very im
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