Dielectric, Piezoelectric and Nonlinear Optical Properties of Lead Titanate based Ferroelectric Thin films

Transcription

1 Dielectric, Piezoelectric and Nonlinear Optical Properties of Lead Titanate based Ferroelectric Thin films Ferroelectric oxides with perovskite structure has gained lot of interest from research as well as industrial point of view due to its multifunctionality [1]. With the discovery of BaTiO 3, due to its simple perovskite structure, there was an increased interest in basic understanding of the concept of ferroelectricity. The demand of the present day technology for the miniaturized products has pointed towards the development of thin film technology [1,2]. The increased interest for the MEMS based devices demand materials that can be processed at lower temperature as high temperature treatment may damage pre-fabricated layers [3]. Therefore in comparison with BaTiO 3 based materials, materials based on PbTiO 3 (PT) has gained importance. In addition to low temperature processing other major considerations of the industry are precise composition and low cost production. In this context, chemical solution deposition (CSD) assumes significance. Lead titanate based systems are gaining importance in device applications especially in piezoelectric and nonlinear optical (NLO) applications. Though textured thin films have been studied for dielectric and piezoelectric applications, an exact understanding of the structure -property relationship is lacking. Similarly, in such systems, the influence of defect chemistry in inducing NLO characteristics has not yet been reported. Therefore, the aims of the present study are (i) understanding structure property relationship especially with respect to dielectric and piezoelectric characteristics of textured films, (ii) to establish the role of defects in NLO characteristics. The thesis begins with a general introduction to basic concepts in ferroelectricity. It is well known that the presence of spontaneous reversible polarization on the application of electric field is the key to the onset of ferroelectricity. A brief introduction to the important theories that

2 explain ferroelectricity like (1) phenomenological theory, (2) soft mode concept and (3) first principle theory is given [4,5]. Also this chapter discusses about some of the technologically important ferroelectric systems with their applications. A brief outline of the present research work is also given in this chapter. The emphasis is given to lead titanate system based compositionally modified through isovalent substitutions of Sr 2+ and Zr 4+ at the A and B sites of the perovskite, ABO 3. Apart from the compositional modifications, the growth condition also determines the properties of thin films. Therefore Chapter 2 is dedicated to discussions on the influence of deposition techniques on the properties of thin films. Different routes usually employed for the deposition of thin films are discussed along with various substrates too. In the present work chemical solution deposition technique is adopted, in view of the ability to grow dense, crackfree films with precise stoichiometry. The as-synthesized solutions are dip/ spin -coated on the substrate and annealed at a given temperature. These films are then subjected to various characterizations depending on the requirement for particular applications. As a first step the evolution of the crystalline phase in the thin films is studied by x-ray diffraction (XRD) analysis using an x-ray diffractometer (XRD, Model D-5005, Bruker, Germany). The refractive index and film thickness are determined from the transmittance spectra of the thin films on glass substrates using a UV Visible spectrometer (Model 570V, JASCO, Japan). The dielectric and ferroelectric measurements are carried out using an impedance-gain phase analyzer (Model 4294A, Agilent Technologies, USA) and piezoelectric evaluation system (Model TF Analyser 2000, aixacct, Germany), respectively, considering a parallel plate capacitor model. The transverse piezoelectric properties of the PZT thin films are evaluated using unimorph cantilevers of PZT/Si [6]. Application of sine wave voltage between upper and bottom electrodes generates the

3 deflection by the transverse inverse piezoelectric effect, and the tip displacement is measured using a laser Doppler vibrometer (AT-3500, Graphtec, Germany) and a laser interferometer (AT- 1100, Graphtec, Germany). Microstructure of the thin film is determined using a high resolution scanning electron microscope (HR-SEM, Model FEI Quanta FEG 200, USA). For nonlinear optical studies, Z-scan technique in the open aperture configuration is employed and is discussed [7]. It is well known that the properties of PT can be tailored according to the requirements of applications through the in-corporation of different dopants in respective sites in the perovskite lattice. The advantages of A-site isovalent Sr 2+ -substituted composition, lead strontium titanate (Pb 1 x Sr x )TiO 3 (PST) system is that (i) the Curie temperature of PST can be tailored by varying the Sr content thereby enhancing their scope for applications at different temperatures and (ii) that they can be processed at lower temperatures compared with their barium-based counterparts leading to better device integration [3]. For tunable dielectric applications, the important material requirements are (i) reasonable dielectric constant, (ii) low loss tangent, (iii) high tunability, (iv) should have minimum dependence on coefficient of thermal expansion, and (v) proximity to the paraelectric phase at the operating temperatures [8, 9,10]. Ferroelectric thin films of PST have been reported to be fabricated by methods such as chemical-solution deposition including sol gel, metal organic decomposition (MOD), RF sputtering, chemical vapour deposition and pulsed laser deposition. Among these methods, chemical-solution deposition offers several advantages such as ease of control of chemical composition, precise stoichiometry and lower processing temperatures. Most of the sol gel synthetic schemes are based on the use of chelating solvents such as 2-methoxyethanol, acetic acid, alcohols and stabilizers such as β-diketones and ethanolamines. In case of refluxing with 2-methoxyethanol, deviation in the precursor solution

4 chemistry due to the formation of ester and water has been reported to affect the thin film homogeneity and uniformity, whereas the use of glycols lead to more shrinkage stress in the films. Previously a hybrid MOD process have been reported that uses alkaline-earth metal carboxylates in conjunction with transition-metal alkoxides, to yield good quality thin films of (Ba 1 x Sr x )TiO 3. The objective of this work, therefore, is to adapt this methodology for the synthesis of highly stable precursor solutions for dip-coating (Pb 1 x Sr x )TiO 3 ; x = 0.5 thin films and study their tunable dielectric characteristics. Details are discussed in Chapter 3. The composition (Pb 0.5 Sr 0.5 )TiO 3 acquires significance as it has T c near the room temperature thereby enhancing their suitability for tunable dielectric applications. In PT, the B-site Zr 4+ -substituted composition, Pb(Zr 1-x Ti x )O 3 [PZT], is a well known piezoelectric. It has been recognized that, in PZT the morphotropic phase boundary composition with Zr/Ti ~ 53/47 is the most suited composition for various applications due to the high value of piezocoefficients. Again the crystallographic orientation control of ferroelectric materials has attracted much attention. High orientation control in ferroelectric thin film is indispensable for realization of highly reliable ferroelectric random access memories (FeRAM) and micro-electromechanical systems (MEMS). In microactuators, fabrication of piezoelectric thin films with thickness in the range of 0.5-2µm are necessary to realize maximum displacement and force [11]. For this application, the geometry of the thin film structure makes their effective in-plane transverse piezoelectric coefficient e 31*, the most important parameter to be considered. The important factors which influence e 31* are the thickness, density, film texture and the compositional gradient across the film [11, 12]. PZT films of thickness in this range have been reported to be fabricated by techniques such as aerosol deposition, Chemical solution deposition (CSD) and sputtering. Chemical solution deposition (CSD) methods like Sol-gel and Metal

5 organic decomposition (MOD), not only are economical but also are promising for producing dense microstructures. However during CSD the large shrinkage of the films during annealing and the thermal mismatch with the silicon substrate often lead to the formation of cracks and/ or high porosities thereby limiting their applications in microactuators. One of the approaches for minimizing the shrinkage stresses during thin film crystallization is the increase in the inorganic content in the precursor solutions. Also SrTiO 3 (ST) has been reported to act as an effective seed layer for the low-temperature crystallization of PZT thin films. Thus, in conjunction with the use of SrTiO 3 (ST) template layers, a careful control of the solution chemistry also assumes significance in the context of fabrication of dense, crack-free PZT thin films. Also these seed layers can effectively control the orientation of these films. Therefore, Chapter-4 discusses about the fabrication of oriented PZT thin films ({001}, {110}, {111}) of high-quality by spin-coating technique using various seed layers and heat treatment conditions and the evaluation of their transverse piezoelectric coefficient e 31 by measuring the piezoelectric vibration of PZT/Si. * unimorphs. A detailed discussion on the factors influencing e 31 in textured PZT thin films is also given in this chapter. The theory governing the heterogeneous bimorphs is also discussed as the studies were carried out by considering PZT/Si as a hetrogeneous bimorph [13]. Though PT- based thin films such as PLZT have been reported to exhibit third order NLO characteristics, the mechanism responsible for this effect has not yet been studied in detail. In this study, cubic third order optical nonlinearity of PST and PZT thin films were studied employing the z-scan technique in the open aperture configuration. Initially the series, (Pb 1- xsr x )TiO 3 ; x = 0, 0.5, 1.0 was chosen, as their application in photonic industry have been limited so far due to lack of information on their nonlinear optical characteristics and also due to the fact that, in this class, the Pb content is systematically varied. The defects associated with the

6 volatility of lead, [ V ] can aid in the two photon absorption (TPA) process. The TPA Pb originates from genuine two photon absorption or two step (excited state) absorption [14]. The defects states lies in the forbidden gap of the material there by satisfying the conditions for TPA 1 E g < ω < E g. Since [ V Pb ] can be systematically altered through A-site La 3+ -doping, the 2 study was extended to establish a correlation between [ V Pb ] and β, the nonlinear absorption coefficient. For this PZT system was selected because (i) it is well known that PZT is inherently p-type [15] and (ii) the low levels of lanthanum doping could vary the electrical properties of PZT. La 3+ with 0, 0.001, 0.004, and 0.01 was chosen for this study. The results were explained on the basis of defect chemistry and were further supported through leakage current and microstructure analysis. In short, thesis describes the correlation of thin film texture on dielectric and piezoelectric characterizatics as well as that of defect chemistry on NLO characteristics. References: 1. Gene H Haertling, Ferroelectric Ceramics: History and Technology, J. Am. Ceram. Soc., 82 (1999) N Setter et.al, Ferroelectric thin films: Review of materials, properties, and application, J. Appl. Phys., 100 (2006) Jyh-Liang Wang, Yi-Sheng Lai, Der-Chi Shye, Chen-Chia Chou, Bi-Shiou Chiou, Chuan-Ping Juan, and Huang-Chung Cheng, Dependence of Ferroelectric Characteristics on the Deposition Temperature of (Pb,Sr)TiO3 Films Jpn. J. Appl. Phys., 46 (2007) 6727.