The size effect of Ba0.6Sr0.4TiO3 thin films on the ferroelectric properties
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:where 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)
- et al.
Effect of grain size of Pb(Zr0.4Ti0.6)O3 sol–gel derived thin films on the ferroelectric properties
Appl. Surf. Sci.
(2001) - et al.
Structural, electrical, and optical studies on rapid thermally processed ferroelectric BaTiO3 thin films prepared by metallo-organic solution deposition technique
Thin Solid Films
(1997) - et al.
Nanomeasurement and fractal analysis of PZT ferroelectric thin films by atomic force microscopy
Microelectron. Eng.
(2003) - et al.
Structural characteristics of SrTiO3 thin films processed by rapid thermal annealing
J. Cryst. Growth
(2005) - et al.
Structural and electrical properties of Ba0.5Sr0.5TiO3 thin films on Ir and IrO2 electrodes
Jpn. J. Appl. Phys.
(1997) - et al.
Ferroelectric properties of SrRuO3/(Ba,Sr)TiO3/SrRuO3 epitaxial capacitor
Jpn. J. Appl. Phys.
(1998) - et al.
Application of ferroelectric BST thin film prepared by MOD for uncooled infrared sensor of dielectric bolometer mode
T. IEE Jpn.
(2000) - et al.
Development of Si monolithic (Ba,Sr)TiO3 thin-film ferroelectric microbolometers for uncooled chopperless infrared sensing
Proc. SPIE
(2000) - et al.
Epitaxial growth of dielectric Ba0.6Sr0.4TiO3 thin film on MgO for room temperature microwave phase shifters
Appl. Phys. Lett.
(2001) - et al.
Analysis and optimization of thin film ferroelectric phase shifters
Mater. Res. Soc. Symp. Proc.
(2000)
Large dielectric constant Ba0.4Sr0.6TiO3 thin films for high-performance microwave phase shifters
Appl. Phys. Lett.
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