Changes in functional behavior of 93 % Pb(Mg1/3Nb2/3)O-3-7 % PbTiO3 thin films induced by ac electric fields

The dielectric properties of Au/[93%Pb(Mg1/3Nb2/3)O-3-7%PbTiO3] (PMN-PT)/(La0.5Sr0.5)CoO3/MgO thin-film capacitor heterostructures, made using pulsed laser deposition, have been investigated, with particular emphasis on the changes in response associated with increasing the magnitude of the ac measuring field. It was found that increasing the ac field caused a change in the frequency spectrum of relaxators, increasing the speed of response of "slow" relaxators, with an associated decrease in the freezing temperature (T-f) of the relaxor system; in addition, other characteristic parameters relating to polar relaxation (activation energy E-a and attempt frequency 1/tau(0)), described by fitting of the dielectric response to a Vogel-Fulcher expression, were found to change continuously as ac field levels were increased.

Investigation of dead-layer thickness in SrRuO3/Ba0.5Sr0.5TiO3/Au thin-film capacitors

Thin-film capacitors, with barium strontium titanate (BST) dielectric layers between 7.5 and 950 nm in thickness, were fabricated by pulsed-laser deposition. Both crystallography and cation chemistry were consistent with successful growth of the BST perovskite. At room temperature, all capacitors displayed frequency dispersion such that epsilon (100 kHz)/epsilon (100 Hz) was greater than 0.75. The dielectric constant as a function of thickness was fitted, using the series capacitor model, for BST thicknesses greater than 70 nm. This yielded a large interfacial d(i)/epsilon (i) ratio of 0.40 +/-0.05 nm, implying a highly visible parasitic dead layer within the capacitor structure. Modeled consideration of the dielectric behavior for BST films, whose total thickness was below that of the dead layer, predicted anomalies in the plots of d/epsilon against d at the dead-layer thickness. In the capacitors studied here, no anomaly was observed. Hence, either (i) 7.5 nm is an upper limit for the total dead-layer thickness in the SRO/BST/Au system, or (ii) dielectric collapse is not associated with a distinct interfacial dead layer, and is instead due to a through-film effect. (C) 2001 American Institute of Physics.

Exploring grain size as a cause for dead-layer effects in thin film capacitors

Pulsed laser deposition was used to make a series of Au/Ba0.5Sr0.5TiO3 (BST)/SrRuO3/MgO thin film capacitors with dielectric thickness ranging from similar to15 nm to similar to1 mum. Surface grain size of the dielectric was monitored as a function of thickness using both atomic force microscopy and transmission electron microscopy. Grain size data were considered in conjunction with low field dielectric constant measurements. It was observed that the grain size decreased with decreasing thickness in a manner similar to the dielectric constant. Simple models were developed in which a functionally inferior layer at the grain boundary was considered as responsible for the observed dielectric behavior. If a purely columnar microstructure was assumed, then constant thickness grain-boundary dead layers could indeed reproduce the series capacitor dielectric response observed, even though such layers would contribute electrically in parallel with unaffected bulk- like BST. Best fits indicated that the dead layers would have a relative dielectric constant similar to40, and thickness of the order of tens of nanometers. For microstructures that were not purely columnar, models did not reproduce the observed dielectric behavior well. However, cross-sectional transmission electron microscopy indicated columnar microstructure, suggesting that grain boundary dead layers should be considered seriously in the overall dead-layer debate. (C) 2002 American Institute of Physics.

Scaling of domain periodicity with thickness measured in BaTiO3 single crystal lamellae and comparison with other ferroics

The focused ion beam microscope has been used to cut parallel-sided {100}-oriented thin lamellae of single crystal barium titanate with controlled thicknesses, ranging from 530 nm to 70 nm. Scanning transmission electron microscopy has been used to examine domain configurations. In all cases, stripe domains were observed with {011}-type domain walls in perovskite unit-cell axes, suggesting 90 degrees domains with polarization in the plane of the lamellae. The domain widths were found to vary as the square root of the lamellar thickness, consistent with Kittel's law, and its later development by Mitsui and Furuichi and by Roytburd. An investigation into the manner in which domain period adapts to thickness gradient was undertaken on both wedge-shaped lamellae and lamellae with discrete terraces. It was found that when the thickness gradient was perpendicular to the domain walls, a continuous change in domain periodicity occurred, but if the thickness gradient was parallel to the domain walls, periodicity changes were accommodated through discrete domain bifurcation. Data were then compared with other work in literature, on both ferroelectric and ferromagnetic systems, from which conclusions on the widespread applicability of Kittel's law in ferroics were made.

Intrinsic dielectric response in ferroelectric nano-capacitors

Measurements on 'free-standing' single-crystal barium titanate capacitors with thickness down to 75 nm show a dielectric response typical of large single crystals, rather than conventional thin films. There is a notable absence of any broadening or temperature shift of the dielectric peak or loss tangent. Peak dielectric constants of similar to25 000 are observed, and Curie-Weiss analysis demonstrates first order transformation behaviour. This is in dramatic contrast to results on conventionally deposited thin film capacitor heterostructures, which show large dielectric peak broadening and temperature shifts (e.g. Parker et al 2002 Appl. Phys. Lett. 81 340), as well as an apparent change in the nature-of the paraelectric-ferroelectric transition from first to second order. Our data are compatible with a recent model by Bratkovsky and Levanyuk (2004 Preprint cond-mat/0402100), which attributes dielectric peak broadening to gradient terms that will exist in any thin film capacitor heterostructure. The observed recovery of first order transformation behaviour is consistent with the absence of significant substrate clamping in our experiment, as modelled by Pertsev et al (1998,Phys. Rev. Lett. 80 1988), and illustrates that the second order behaviour seen in conventionally deposited thin films cannot be attributed to the effects of reduced dimensionality in the system, nor to the influence of an intrinsic universal interfacial capacitance associated with the electrode- ferroelectric interface.

Perovskite lead zirconium titanate nanorings: Towards nanoscale ferroelectric

Rings of perovskite lead zirconium titanate (PZT) with internal diameters down to similar to 5 nm and ring thicknesses of similar to 5-10 nm have been fabricated and structurally, crystallographically, and chemically characterized using an analytical transmission electron microscope. Ring fabrication involved conformal solution deposition of a thin layer of PZT on the inside of a thin film of anodized aluminum oxide nanopores, and subsequent sectioning of the coated pores perpendicular to their cylinder axes. Although the starting solution used for the solution deposition was made from morphotropic phase boundary PZT, the nanorings were found to be on the zirconium-rich side of the PZT phase diagram. Nevertheless, coatings were found to be of perovskite crystallography. The dimensions of these nanorings are such that they have the potential to demonstrate polarization vortices, as modeled by Naumov [Nature (London) 432, 737 (2004)], and moreover represent the perfect morphology to allow vortex alignment and the creation of the ferroelectric "solenoid" as modeled by Gorbatsevich and Kopaev [Ferroelectrics 161, 321 (1994)].

Toward Self-Assembled Ferroelectric Random Access Memories: Hard-Wired Switching Capacitor Arrays with Almost Tb/in.2 Densities

We report on the successful fabrication of arrays of switchable nanocapacitors made by harnessing the self-assembly of materials. The structures are composed of arrays of 20-40 nm diameter Pt nanowires, spaced 50-100 nm apart, electrodeposited through nanoporous alumina onto a thin film lower electrode on a silicon wafer. A thin film ferroelectric (both barium titanate (BTO) and lead zirconium titanate (PZT)) has been deposited on top of the nanowire array, followed by the deposition of thin film upper electrodes. The PZT nanocapacitors exhibit hysteresis loops with substantial remnant polarizations, while although the switching performance was inferior, the low-field characteristics of the BTO nanocapacitors show dielectric behavior comparable to conventional thin film heterostructures. While registration is not sufficient for commercial RAM production, this is nevertheless an embryonic form of the highest density hard-wired FRAM capacitor array reported to date and compares favorably with atomic force microscopy read-write densities.