Thickness independence of true phase transition temperatures in barium strontium titanate films

The functional properties of two types of barium strontium titanate (BST) thin film capacitor structures were studied: one set of structures was made using pulsed-laser deposition (PLD) and the other using chemical solution deposition. While initial observations on PLD films looking at the behavior of T-m (the temperature at which the maximum dielectric constant was observed) and T-c(*) (from Curie-Weiss analysis) suggested that the paraelectric-ferroelectric phase transition was progressively depressed in temperature as BST film thickness was reduced, further work suggested that this was not the case. Rather, it appears that the temperatures at which phase transitions occur in the thin films are independent of film thickness. Further, the fact that in many cases three transitions are observable, suggests that the sequence of symmetry transitions that occur in the thin films are the same as in bulk single crystals. This new observation could have implications for the validity of the theoretically produced thin film phase diagrams derived by Pertsev [Phys. Rev. Lett. 80, 1988 (1998)] and extended by Ban and Alpay [J. Appl. Phys. 91, 9288 (2002)]. In addition, the fact that T-m measured for virgin films does not correlate well with the inherent phase transition behavior, suggests that the use of T-m alone to infer information about the thermodynamics of thin film capacitor behavior, may not be sufficient. (C) 2004 American Institute of Physics.

Electrode field penetration: A new interpretation of tunneling currents in barium strontium titanate (BST) thin films

This paper shows that penetration of the applied electric field into the electrodes of a ferroelectric thin film capacitor produces both an interfacial capacitance and an effective mechanism for electron tunneling. The model predictions are compared with experimental results on Au-BST-SrRuO3 capacitors of varying thicknesses, and the agreement is excellent.

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.

Studies of switching kinetics in ferroelectric thin films

Experimental studies are reported concerning the importance of interfacial capacitance (including electrode screening, space-charge layers, and/or chemically discrete dead layers). on domain switching behaviour in thin films of ferroelectric lead zirconate-titanate (PZT), strontium bismuth tantalate (SBT), and barium strontium titanate (BST). Emphasis is placed upon studies at applied field values very near the coercive field E, asymmetry in fatigue for positive and negative polarity coercive fields, and in the case of BST, of the coexistence of ferroelectric and paraelectric phases Studies of dielectric loss show important correlations between tan 6 and fatigue (polarization decrease) as a function of bipolar switching cycles N. This is a priori not obvious, since the former is a linear response and the latter, a nonlinear response. Modelling of enlarged interfacial,space-charge layers in PZT films and chemically distinct dead (paraelectric) layers in BST films shows contradictory tendencies of coercive-voltage changes with the growth of passive layers.

Switching dynamics in ferroelectric thin films: An experimental survey

We have conducted a broad survey of switching behavior in thin films of a range of ferroelectric materials, including some materials that are not typically considered for FeRAM applications, and are hence less studied. The materials studied include: strontium bismuth tantalate (SBT), barium strontium titanate (BST), lead zicronate titanate (PZT), and potassium nitrate (KNO3). Switching in ferroelectric thin films is typically considered to occur by domain nucleation and growth. We discuss two models of frequency dependence of coercive field, the Ishisbashi-Orihara theory where the limiting step is domain growth and the model of Du and Chen where the limiting step is nucleation. While both models fit the data fairly well the temperature dependence of our results on PZT and BST suggest that the nucleation model of Du and Chen is more appropriate for the experimental results that we have obtained.