Hydrothermally Deposited PZT Thin Films With Vertically Oriented Columnar Growth
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C11.40.1
Hydrothermally Deposited PZT Thin Films With Vertically Oriented Columnar Growth. Scott Solberg, Alexandra Rodkin, Baomin Xu, and Karl Littau Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA, USA.
ABSTRACT A novel growth mode for hydrothermal films of lead zirconate titanate (PZT) has been discovered. Previously reported hydrothermal PZT films have had a loosely-packed cubic morphology, and indeed that is the most common growth mode. However, under special growth conditions, with much more dilute reagent concentrations than previously reported, substantially vertically oriented, long rod shaped grains may be grown. Metal-organic reagents such as lead acetate trihydrate, zirconium propoxide, and titanium isopropoxide were used along with potassium hydroxide mineralizer. Metal and metal-coated substrates with appropriate perovskite seed layers, such as lead titanate (PT) and PZT were used. Unlike other film deposition methods, such as sputtering, MOCVD, or sol-gel, which typically require greater than 500°C processing temperatures, the hydrothermal processes described in this report allowed the growth of highly crystalline films at temperatures as low as 120°C. The unique vertical rod-like ferroelectric films may be useful in electrical devices such as capacitors.
INTRODUCTION Ferroelectric Lead Zirconate Titanate (PZT) films are commercially important because of their high dielectric constant and large piezoelectric effect. These films have been deposited by a variety of well known deposition methods including physical vapor deposition methods such as sputtering [1], chemical vapor deposition [2], and chemical solution methods such as sol-gel deposition [3] and hydrothermal deposition [4,5]. Forming highly crystalline PZT thin films with physical and chemical vapor deposition methods typically requires deposition or postdeposition annealing temperatures greater than about 500°C [1,2]. Hydrothermal deposition, however, allows the growth of highly crystalline PZT compounds at comparatively low temperatures of < 200° C to as low as 120°C. The term “hydrothermal”, while of geological origin, has more recently been defined as “any heterogeneous chemical reaction in the presence of a solvent (whether aqueous or non-aqueous) above room temperature and at pressure greater than 1 atmosphere in a closed system” [6]. For example, hydrothermal PZT powders have been produced from PbO powder and co-precipitated gels of hydrated titania-zirconia obtained from mixing TiOCl2 and ZrOCl2 reagents with NH3(aq) [7]. PZT films on titanium sheet substrates have been prepared using Pb(NO3)2, TiOCl2 and ZrOCl2 with KOH mineralizer [4,5]. The hydrothermal synthesis process is difficult to understand, because the nature of the closed vessel prevents easy observation of the crystallization process; however in recent years the thermodynamics of hydrothermal formation have been analyzed for PZT powders [8], and product stability and yield have been calculated for a range of solution pH for the various possible products,
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