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Ferroelectrics
Historical Perspective of Ferroelectrics
A subset of polar materials was first observed by Valasek in 1921 in single crystals of Rochelle salt, KNa(C4H406) • 4H20.1 The hysteretic behavior observed between polarization and applied electric field was analogous to the magnetism verses magnetic field behavior in ferromagnetic materials which were previously understood.
Therefore the phenomenon was termed « ferroelectricity. » The earliest research identified only two types of ferroelectric materials, tartrates such as Rochelle salt ( e. g. sodium ammonium tartrate tetrahydrate NaNH4(C4H40 6)•4H20 in 19282 and lithium ammonium tartrate monohydrate LiNI!t(C4H406)•HzO in 1951\ and potassium dihydrogen phosphate (e.g. KH2P04 in 19354 ) and its isomorphs (e.g. ammonium dihydrogen phosphate NH4H2P04 in 1938\ These materials’ water solubility made them impractical for use in electronic deviees and the ferroelectric phenomenon could on1y be observed at low temperatures.
In 1940, the first refractory oxide ferroelectric with room temperature ferroelectricity was discovered in the perovskite barium titanate, BaTi03, by Thumauer and Deaderick at the American Lava Co. 6 Publication of the high permittivity of BaTi03 in the open literature came from the U.S.7 concurrent independent reports and the determination that the high permittivity was due to ferroelectricity in BaTi03 was given by von Hippel of the Laboratory for Insulation Research at MIT.11 The discovery of ferroelectric barium titanate represented a breakthrough in ferroelectric research and fueled development of high permittivity deviees. Piezoelectric transducers were facilitated by the discovery of the ferroelectric perovskite solid solution lead zirconate titanate, Pb(Zr,Ti)03 or PZT,12 and the existence of a temperature independent polymorphie phase boundary at Zr:Ti mole ratios of 52:48.13 To date, perovskite based ferroelectrics are the mostly widely utilized class of solid-state materials for high capacitance density capacitors and high strain/sensitivity transducers. Despite the large number of publications each year, their potential for commercial electronics is yet to be exhausted.
Phase Transitions in Ferroelectrics
A necessary criterion for classification for ferroelectricity is the existence of a phase transition from a low temperature low symmetry ferroelectric phase to a higher symmetry non-polar paraelectric phase above a transition temperature called the Curie Temperature (Tc). The temperature dependence of permittivity for a ferroelectric is given by Fig. 2.1.
where C is constant, T is temperature, 8 is the Curie-Weiss temperature, and e reaches maximum at Tc, the transition temperature. B=Tc andE: declines steeply forT< Tc. For 1 st order transition B<T c and e discontinuously decreases at this temperature. For barium titanate, no exact indisputable value for C has been determined yet, ali measurements are on the order of 105 and the Currie-Weiss temperature Bis typically 10 oc lower than the transition temperature.14 Curie-Weiss behavior is valid for many other non-linear dielectrics, for example, strontium titanate (SrTi03), which is considered an incipient ferroelectric since the predicted transition temperature is less than 0 K. The Curie-Weiss constants have been measured for SrTi03 to be C slightly less than 105 and e ~ 20 oc. 15 Near and at the transition temperature, Tc, anomalies in dielectric, elastic, optical, and thermal properties are observed. The transitions can be of first or second order.
These classifications constitute a transition that occurs discontinuously in first order cases, or smoothly over a range of temperatures in the second order scenario. Both scenarios were modeled by Devonshire in 1954. This phenomenological theory is based on the thennodynamic function for a ferroelectric system. The equations of state for ferroelectric crystals relate the system free energy to polarization based on the influences of temperature, stress and extemal electric fields. The results of these calculations show that polarization bas equivalent minima at P !- 0 at temperatures below Tc supporting the observation of spontaneous polarization. Results of the calculations show for a second order transition the minima slowly collapse toward P = 0 yet for a first order transition free energy minima move very little yet a localized reduction of the free energy appears at P = 0 and becomes a global minima at T ?:_ Tc. 17 First order ferroelectric transitions are associated with discontinuous changes in polarization, latent beat, and specifie beat. Most perovskite oxide ferroelectric show first order transition behavior. Devonshire’s description adequately describes the behavior of measured parameters though the phase transition yet a more complete understanding of the physical phenomenon can be proposed using the dynamics of lattice vibrations. The transverse optical vibration mode in perovskites showing displacive transitions is associated with the vibration of the B-site cation and is defined as the « soft mode ». As the temperature is lowered from a temperature above the transition temperature this mode decreases in frequency (increases in wavelength) at the Brillouin Zone center at zero wave number.
Here wm is the angular frequency of the transverse optical mode or soft mode, To is the temperature where the frequency of the mode falls to zero, and K is a constant that determines how fast the frequency falls with T. The non-zero frequency of the soft mode above the transition shows that the cations are free to vibrate around their respective unit cell centers and therefore the time and spatially averaged charge density is located at the unit cell center, thus polarization cannot be supported. As the soft mode frequency falls to zero the mode is frozen and all cations vibrate in phase over a longer distance within the crystal. This can interpreted as an overall spontaneous polarization since at a low enough temperature (where frequency goes to zero, or wavelength goes to infinity) all cations are displaced away from the center of the unit cell at any point intime. The permittivity is a result of the linear combination of ali displacements which can be described by all the optic modes at k = 0 yet the soft mode contributes the most significantly to the permittivity in ferroelectrics as seen in Eq. (2.3).
As the temperature is lowered below the transition temperature the soft mode decreases in wavelength leading to vibrations including longer-range coordination of polarizing displacements and the permittivity increases. At the transition temperature the permittivity peaks and spontaneous polarization is supported. If hysteretic behavior is to be avoided then a deviee can be operated just above the transition temperature while still taking advantage ofhigh permittivity.
Theory ofDielectric Response ofFerroelectrics
Ferroelectrics are known for their high permittivity and high non-linear dielectric response dependent electric field. The typical 2-E curve is shown in Fig. 2.2. The feature of this 2-E curve is called dielectric tunability, which has been widely ielectric permittivity. The ongm of the high dielectric permittivity of ferroelectrics in the paraelectric phase is a delicate compensation of various kinds of microscopie forces that maintain the material in a non-poled state in the absence of a microscopie electric field. Because of this compensation, the restoring force opposing the po ling action of the applied field is relatively weak. This result in a high dielectric permittvity of the material. 18 In the ferroelectric phase, the newly appeared spontaneous polarization is not very stable either. This makes the dielectric permittivity high in the ferroelectric phase. In addition to this, the permittivity may be further increased by contributions stemming from ferroelectric domains.
Table of contents :
Acknowleqgements
Abstract
Résumé
CHAPTER 1 Introduction
CHAPTER 2 Backgrounds and Literature Review
2.1 F erroelectrics
2.1.1 Historical Perspective ofFerroelectrics
2.1.2 Phase Transitions in Ferroelectrics
2.1.3 Theory ofDielectric Response ofFerroelectrics
2.1.4 Ferroelectric Thin Films
2.1.5 Applications ofFerroelectric Thin Films in Tunable Deviees
2.2 Suitable Materials for Tunable Applications
2.2.1 Fundamentals ofBaxSr1.xTi03 Material Properties
2.2.1.1 Bulk BaxSrl-xTÏÜJ
2.2.1.2 BaxSr1-xTi03 Thin Films
2.2.2 Fundamentals of Bh03-ZnO-Nb20s Material Properties
2.2.2.1 Crystal Structure ofBb03-ZnO-Nb205 System
2.2.2.2 Bh.sZn1.0Nb1.507 Thin Films
2.3 Motivations for the Present Study
2.4 Reference
CHAPTER 3 Experimental Procedures
3.1 Preparation ofSputtering Ceramics target
3.1.1 Preparation ofBaxSr1.xTi03 Ceramic Target
3.1.2 Preparation ofBh.s+xZn1.oNb1.507 Ceramic Target
3.1.3 Ceramics Powders Used in This Thesis
3.2 RF Magnetron Sputtering System
3.3 Film Preparations
3.3.1 Pt Metal Electrodes
3.3.2 BST Thin Film Deposition
3.3.3 BZN Thin Film Deposition
3.4 Characterization Techniques
3.4.1 X-ray Diffraction (XRD) Analysis
3.4.2 Scanning Electron Microscopy (SEM)
3.4.3 Atomic Force Microscopy (AFM)
3.4.4 Electrical Properties Measurements
3.4.4.1 Metal-Insulator-Metal Structure
3.4.4.2 Coplanar Waveguide Transmission Lines
CHAPTER 4 Growth of(001)-and (111)-0riented (Ba,Sr)Ti03 Thin Films
4.1 Perfectly (00 1 )-and (111 )-Oriented (Ba,Sr)Ti03 Thin Films on Pt/TiOx/Si02/Si Without Buffer Layers
4.1.1 Introduction
4 .1.2 Experimental Procedure
4.1.3 Results and Discussions
4.1.4 Summary
4.2 Effects ofUltrathin TiOx Seeding Layer on Crystalline Orientation and Electrical Properties of (Ba,Sr)Ti03 Thin films
4.2.1 Introduction
4.2.2 Experimental Procedure
4.2.3 Results and Discussions
4.2.4 Summary
4.3 Reference
CHAPTER 5 Studies BZN/BST composites films for Microwave Applications
5.1 Effects ofSubstrate Temperature on the Crystalline ofBZN films
5.1.1 Introduction
5 .1.2 Experimental Procedure
5.1.3 Results and Discussions
5.1.4 Summary
5.2 Improved Dielectric Properties of BZN/(111)-0riented BST Bilayered Films* for Tunable Microwave Applications
5.2.1 Introduction
5.2.2 Experimental Procedure
5.2.3 Results and Discussions
5.2.4 Summary
5.3 Reference
CHAPTER 6 Thickness and Interfacial Properties for BST(111) Thin Film
6.1 Introduction
6.2 Experimental Procecure
6.3 Results and Discussions
6.4 Summary
6.5 Reference
CHAPTER 7 Microwave Properties of BST up to 50 GHz
7.1 Microwave Properties of Epitaxial BST(lll) Thin Films on Ah03 (0001) up to 40 GHz
7 .1.1 Introduction
7 .1.2 Experimental Procedure
7 .1.3 Results and Discussions
7.1.4 Summary
7.2 Microwave Properties of BZN/BSTbilayered Thin Films Directly Deposited on High Resistivity Si Toward 50 GHz
7.2.1 Introduction
7.2.2 Experimental Procedure
7.2.3 Results and Discussions
7.2.4 Summary
7.3 Reference
CHAPTER 8 Conclusions and Future Work
8.1 Conclusions
8.2 Future Work
List of Publications .
