Elsevier

Applied Surface Science

Volume 252, Issue 12, 15 April 2006, Pages 4171-4177
Applied Surface Science

The size effect of Ba0.6Sr0.4TiO3 thin films on the ferroelectric properties

https://doi.org/10.1016/j.apsusc.2005.06.027Get rights and content

Abstract

Barium strontium titanate (BST) thin films were prepared by RF magnetron sputtering. The dielectric constant–voltage curves and the hysteresis loops of BST thin films with different grain sizes and film thicknesses were investigated. When the grain size increases from 12 nm to 35 nm, remarkable increases in dielectric constant and tunability were observed. Above 12 nm, the BST films exhibited size effects, i.e. a decrease in maximal polarization (Pm) and an increase in coercive electric field (Ec) with reduction in grain size. In our investigation, the dielectric constant, tunability and maximal polarization increased as the film thickness increased. Furthermore, the size dependence of the dielectric constant and tunability of Ba0.6Sr0.4TiO3 thin films is determined by that of the maximal polarization and the coercive electric field.

Introduction

Ferroelectric barium strontium titanate (BST) thin films have been widely investigated as potential materials for the applications of microelectronic devices such as dynamic random access memories, infrared detectors, microwave devices, and hydrogen gas sensors owing to their high dielectric constant, relatively low dielectric loss tangent, large electric-field tunability, long lifetime, and good temperature stability [1], [2], [3], [4], [5], [6], [7], [8], [9]. In particular, dielectric non-linearity is one of the most important factors for tunable microwave applications. It is found that the tunability depend on the microstructures of the thin films, such as the phase, grain size, composition, defect, strain, etc. [10], [11].

It is well known that ferroelectric bulk materials, such as BaTiO3 and PbTiO3, show “size effect”. The size effect means that properties of ferroelectrics are dependent on the size of materials and show different behavior from a single crystal. The size effect is also observed in ferroelectric thin films [10], [12], [13], [14], [15], [16]. For example, the phase transitions in PbTiO3 films were investigated by high-temperature XRD measurements [16]. It is found that the phase transition from tetragonal to cubic in the film of grain size ∼210 nm occurs at about 480 °C, whereas it occurs at around 460 °C in the film of grain size ∼130 nm. Our previous results indicated that in Ba0.6Sr0.4TiO3 thin films there are three critical grain sizes. The first critical grain size is about 10 nm from amorphous to crystalline. The second one refers to the disappearance of ferroelectric behavior, which is about 20 nm. The third one is about 31 nm from multi-domain to single-domain. The domain in ferroelectrics is denoted as a region with uniform polarization direction. The configuration of ferroelectric domains, controlled by composition, grain size, etc., is the most important factor that determines the macroscopic ferroelectric properties.

The grain sizes of thin films are usually controlled by annealing temperature and time [17], [18], [19], [20]. It is well known that the grain size increases with an increase in annealing temperature [17]. However, Jiang et al. reported that grain sizes of SrTiO3 thin films decreased at low temperatures while increased slightly at high temperatures with increasing temperature [18]. On the other hand, many experimental results show that the average grain size increase linearly with annealing time [19]. However, Enriquez and Mathew found that beyond 30 min the grain sizes of CdTe films remain more or less constant with a tendency to slightly increase in size [20]. Moreover, the relation between the grain size and annealing time can be described by the law:(D2D02)1/2=Atnwhere D0 and D are the average grain sizes before and after the annealing, t the annealing time, A the constant and n is the grain growth exponent (the ideal value of n above the half-melting temperature is 0.5).

In this paper, Ba0.6Sr0.4TiO3 thin films were prepared by RF-magnetron sputtering system. The ferroelectric properties of Ba0.6Sr0.4TiO3 thin films with different grain sizes and film thicknesses were investigated.

Section snippets

Film preparation

The SiO2/Si substrates were cleaned by ultrasonic stirring in acetone (2 min) and ethanol (2 min). To measure the dielectric properties of BST thin films, the metal–insulator–metal (MIM) structures were fabricated. The buffer layer titanate (∼30 nm) and platinum electrode layer (∼100 nm) were successively deposited at 300 °C on the substrates by dc magnetron sputtering. A hard mask was placed over the substrate and the Ba0.6Sr0.4TiO3 thin films were deposited using a RF-magnetron sputtering system

Grain size dependence of ferroelectric properties

Average grain sizes of the Ba0.6Sr0.4TiO3 thin films were estimated using a contact mode atomic force microscopy (AFM). Fig. 2 shows the typical AFM micrographs of films annealed at different temperature for 10 min in O2 atmosphere. The films were very dense, smooth and crack-free. The average grain size of the thin films in Fig. 2(b–f) was 12 nm, 18 nm, 25 nm, 35 nm and 58 nm, respectively. It is found that the average grain size increased as the annealing temperature increased and the annealing

Conclusions

In this paper, the ferroelectric properties of Ba0.6Sr0.4TiO3 thin films with different grain sizes and film thicknesses were investigated. When the grain size increases from 12 nm to 35 nm, remarkable increases in dielectric constant and tunability were observed. Above 12 nm, the maximal polarization (Pm) increases and the coercive electric field (Ec) decreases with the increasing grain size. It is found that the maximum dielectric constant increases as the grain size and film thickness of BST

References (22)

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