Elsevier

Applied Surface Science

Volume 254, Issue 17, 30 June 2008, Pages 5492-5496
Applied Surface Science

Mechanical properties of sol–gel derived lead zirconate titanate thin films by nanoindentation

https://doi.org/10.1016/j.apsusc.2008.02.094Get rights and content

Abstract

Lead zirconate titanate (PZT) thin films are deposited on platinized silicon substrate by sol–gel process. The crystal structure and surface morphology of PZT thin films are characterized by X-ray diffraction and atomic force microscopy. Depth-sensing nanoindentation system is used to measure mechanical characteristics of PZT thin films. X-ray diffraction analyses confirm the single-phase perovskite structures of all PZT thin films. Nanoindentation measurements reveal that the indentation modulus and hardness of PZT thin films are related with the grain size and crystalline orientation. The increases of the indentation modulus and hardness with grain size are observed, indicating the reverse Hall–Petch effect. Furthermore, the indentation modulus of (1 1 1)-oriented PZT thin film is higher than those of (1 0 0)- and random-oriented films. The consistency between experimental data and numerical results of the effective indentation moduli for fiber-textured PZT thin films using Voigt–Reuss–Hill model is obtained.

Introduction

Recently, large attention has been given to lead zirconate titanate (PZT) thin films with respect to their potential applications in non-volatile ferroelectric random access memory (FRAM) and micro-electromechanical systems (MEMS) [1], [2], [3]. It is well known that piezoelectric coefficients depend on mechanical coefficients which are related to the mechanical states of materials [4]. Moreover, the mechanical behavior plays a crucial role in the delamination, cracking or fracture, and polarization fatigue of multilayer thin film structures [5], [6], [7]. Consequently, the investigation on the mechanical properties of PZT thin films is of importance for the design of piezoelectric devices and practical applications in various environments.

Nanoindentation technology is frequently used to evaluate the key mechanical properties, such as hardness and Young's modulus of isotropic materials, due to its high sensitivity, good resolution and easy operation. Many researchers have employed this technique to investigate the mechanical properties of various kinds of thin films including some ferroelectric thin films [8], [9], [10]. It has been suggested that PZT thin films prepared by different methods usually have different textures or crystal preferred orientations [10], [11], [12] which affect the ferroelectric and mechanical performances of the films. Considering the anisotropy of PZT thin films, the indentation modulus is of more scientific and practical importance than Young's modulus in many cases [13]. Some groups have examined the effects of grain size and crystal preferred orientation on the mechanical properties of PZT thin films by nanoindentation technology [14], [15], [16], [17], [18]. However, these factors are usually combined together, which makes it difficult to obtain a coincident conclusion. Moreover, several theoretical methods suited for bulk monocrystal materials have been used to evaluate the indentation modulus [10], [18], elastic constants [19] and biaxial modulus [20], [21] of fiber-textured films. To our knowledge, however, there have been no reports on the calculation of indentation modulus of PZT thin films using Voigt–Reuss–Hill (VRH) model considered an excellent theoretical estimation [21].

In this paper, crystalline structure, surface morphology and mechanical behavior of PZT thin films fabricated by sol–gel method are evaluated using X-ray diffraction (XRD), atomic force microscopy (AFM) and nanoindentation technique. The effects of grain size and crystalline orientation on the mechanical properties of the thin films, including indentation modulus and hardness, are discussed in detail, respectively. Furthermore, theoretical estimations on the indentation moduli of PZT thin films with different textures by VRH model are provided for comparison with the experimental data.

Section snippets

Preparation of PZT thin films

Pb(Zr0.52Ti0.48)O3 thin films were prepared on Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrates by sol–gel technique. Lead acetate trihydrate [Pb(CH3COO)2·3H2O], tetra-n-butyl titanate [Ti(OC4H9)4] and zirconium n-propoxide [Zr(OC3H7)4] which were dissolved in 1-propanol were chosen as the source materials with 2-methoxyethanal (MOE) as the solvent. After Pb(CH3COO)2·3H2O had been completely dissolved in MOE solution, the mixed solution was distilled at 123 °C to remove the water associated with the Pb salts,

Microstructure and crystallographic orientation

Fig. 1 shows the XRD patterns of PZT films prepared under different prefired and annealing temperatures. All films exhibit a complete perovskite phase, i.e. there is no observable pyrochlore phase in the range of prefired and annealing temperatures used. The strong diffuse scatterings at 39.9° are considered from the Pt substrate. After uniform 375 °C prefired and different annealing processes, samples A and B represent (1 1 1) preferred orientations, and all other reflections including (1 0 0), (1 1 

Conclusions

Lead zirconate titanate (PZT) thin films are fabricated by sol–gel method and their crystal structures and mechanical properties are investigated using X-ray diffraction and nanoindentation technology. All films exhibit a pure perovskite phase and different preferred orientations by controlling prefired and annealing temperatures. The average grain size develops from 32 to 59 nm with the increasing annealing temperature from 600 to 700 °C. The enhancement of annealing temperature leads to an

Acknowledgements

This work is supported by the National Science Foundation with Excellent Young Investigators of China (10325208).

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