Investigation of Dead Layer Thickness in SrRuO 3 /Ba 0.5 Sr 0.5 TiO 3 /Au Thin Film Capacitors
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Investigation of Dead Layer Thickness in SrRuO3/Ba0.5Sr0.5TiO3/Au Thin Film Capacitors L. J. Sinnamon, R. M. Bowman and J. M. Gregg Department of Pure and Applied Physics Queen’s University Belfast Belfast BT7 1NN ABSTRACT Thin film capacitors with barium strontium titanate (BST) dielectric layers of 7.5 to 950 nm were fabricated by Pulsed Laser Deposition. XRD and EDX analyses confirmed a strongly oriented BST cubic perovskite phase with the desired cation stoichiometry. Room temperature frequency dispersion (ε100 kHz /ε100 Hz) for all capacitors was greater than 0.75. Absolute values for the dielectric constant were slightly lower than expected. This was attributed to the use of Au top electrodes since the same sample showed up to a threefold increase in dielectric constant when Pt was used in place of Au. Dielectric constant as a function of thicknesses greater than 70 nm, was fitted using the series capacitor model. The large interfacial parameter ratio di /εi of 0.40 ± 0.05 nm implied a significant dead-layer component within the capacitor structure. Modelled consideration of the dielectric behaviour for BST films, whose total thickness was below that of the dead layer, predicted anomalies in the plots of d/ε against d at the dead layer thickness. For the SRO/BST/Au system studied, no anomaly was observed. Therefore, either (i) 7.5 nm is an upper limit for the total dead layer thickness in this system, or (ii) dielectric collapse is not associated with a distinct interfacial dead layer, and is instead due to a through-film effect.
INTRODUCTION The decrease in dielectric constant with film thickness is a well-established observation in thin film ferroelectric capacitors. This decrease can be effectively modelled by assuming the existence of very low dielectric constant ‘dead layers’ at the electrode-ferroelectric interfaces.1 These interfacial dead layers act as parasitic capacitors in series with the bulk-like ferroelectric and hence the decrease in dielectric constant is said to follow the ‘series capacitor model.’ In this model, the effective capacitance of a thin film dielectric layer is given by: 1 1 1 = + C eff Cb Ci
(1)
where subscripts b, and i refer to bulk and interface respectively. If the interfacial dead layer thickness di is independent of the total thickness d, then: 1 1 d d d d = b + i = + d i − ε eff ε b ε i ε b εi εb
(2)
It is usually assumed that either εb >> εi1 or db >> di2 giving a linear relationship between d/εeff and d with a gradient of 1/εb and y-axis intercept of di/εi. CC3.2.1
Despite the wide acceptance of the dead layer concept, an experimentally consistent explanation for the nature of such layers is not yet evident. Various models have been proposed: low dielectric constant space charge layers;3 oxygen depletion zones adjacent to metals with a high oxygen affinity;4 formation of surface states;4 local diffusion of electrode material into the ferroelectric;5,6 lattice-mismatch-induced ion vacancy formation;7 chemically distinct surface phase;8 intrinsic surface
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