Conference Programme

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Overview
Session
O-03: Ferroelectric materials and structures
Time:
Tuesday, 20/Jun/2017:
10:30am - 12:00pm

Session Chair: Andrew John Bell, University of Leeds
Location: Rm 322

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Presentations
10:30am - 11:00am
Invited

Engineering New Crystal Structures, Multifunctional and Photovoltaic Behavior of Epitaxial BFO and Pb-free Thin Films

John WANG1, Zhen FAN2, Ping YANG3

1National University of Singapore, Singapore; 2South China Normal University, China; 3Singapore Synchrotron Light Source, National University of Singapore, Singapore

There has been considerable effort in manipulating the crystal structure and coupling between ferroelectric order and magnetic order in bismuth ferrite (BFO)-based thin films. On the other hand, BFO-based compositions are a class of important Pb-free electroceramics. We have investigated different crystal structures and ferroelectric behavior in epitaxial BFO-based thin films manipulated by two general approaches, namely substrate strain and chemical doping. For BFO thin films deposited on oxide-buffered single crystal substrates, it can be fully strained to tetragonal phase, where unique twinning rotation structure with monoclinic symmetry is present. The super-tetragonal BFO phase is another interesting phase for BFO. Our first approach was to develop BFO films on STO substrates by controlling the growth rate, where the parasiticβ-Bi2O3 (BO) phase appears to be the origin for the formation of super-tetragonal BFO phase. By employing BO as a buffer layer, the super-tetragonal phase BFO could be grown. The second approach is to distort BFO lattice by substrates with large compressive strain above ~4.5%. For the ultrathin BFO film below 10 nm, a monoclinic MA structure with a giant c/a is developed. The large compressive strain of ~7% constrains BFO into a strictly super-tetragonal phase for the film thickness of ~30 nm. In our third approach, the super-tetragonal phase can also be developed by chemical doping, such as Ga for Fe in BFO. Their ferroelectric behavior supports the microscopic origin of ferroelectric polarization from the ion displacement induced dipole moments. In addition to multiferroic behavior, an emerging function of BFO-based thin films is the photovoltaic behavior, which can well be tuned and much enhanced by both chemical doping and formation of proper heterolayers.


11:00am - 11:15am
Oral

Declamping in Lead Magnesium Niobate – Lead Titanate Films

R. KEECH1, L. YE2, J.L. BOSSE2, G. ESTEVES3, J. GUERRIER3, J. L. JONES3, M. A. KURODA4, B.D. HUEY2, S. TROLIER-MCKINSTRY1

1Penn State University, United States; 2University of Connecticut, United States; 3North Carolina State University, United States; 4Auburn University, United States

Piezoelectronic Transistors provide a potential CMOS replacement, but necessitate finely patterned, thin piezoelectric films with excellent piezoelectric response. This paper will discuss growth and characterization of both epitaxial and highly {001}-oriented lead magnesium niobate – lead titanate (PMN-PT) thin films for this application. It was found that both film types exhibited similar, thickness-independent high-field εr of ~300 with highly crystalline electrode/dielectric interfaces. Irreversible domain wall motion is the major contributor to the overall dielectric response and its thickness dependence. In clamped epitaxial films the irreversible Rayleigh coefficients reduced 85% upon decreasing thickness from 350 to 100 nm. The effective interfacial layers are found to contribute to the measured thickness dependence in d33,f measured by X-ray diffraction. High field piezoelectric characterization revealed a field-induced rhombohedral to tetragonal phase transition in epitaxial films. Using optical and electron beam lithography combined with reactive ion etching, the PMN-PT films were systematically patterned down to lateral feature sizes of 200 nm in spatial scale with nearly vertical sidewalls. Upon lateral scaling, there was an increase in both small and large signal dielectric properties, including a doubling of the relative permittivity in structures with width-to-thickness aspect ratios of 0.7. The longitudinal piezoelectric coefficient, d33,f, was interrogated as a function of position across the patterned structures by finite element modeling, piezoresponse force microscopy, and nanoprobe synchrotron X-ray diffraction. It was found that d33,f increased from the clamped value of 40-50 pm/V to ~160 pm/V at the free sidewall under 200 kV/cm excitation. The sidewalls partially declamped the piezoelectric response 500-600 nm into the patterned structure, raising the piezoelectric response at the center of features with lateral dimensions less than 1 μm (3:1 width to thickness aspect ratio). The normalized data from all three characterization techniques are in excellent agreement.


11:15am - 11:30am
Oral

Crack free and Device Worthy 0.65PMN-0.35PT One Micron Thick Film Grown using PLD

Pius AUGUSTINE1,2, Martando RATH1, MS Ramachandra RAO1

1Department of Physics, Nano Functional Material Technology Centre and Material Science Research Centre, Indian Institute of Technology Madras, India; 2Department of Physics, Sacred Heart College (Autonomous), India

Superior functional response of the bulk PMN-PT ceramics compared to many other similar systems makes it a smart material to explore. Synthesis of PMN-PT relaxor thick films invites special attention, because of its suitability in various applications like actuators, piezo sensors, MEMS etc. However, precise stoichiometry and phase purity are highly warranted for enhanced response of the system.

Ability to make stoichiometric transfer of material from mutlicomponent target to growing film made PLD an excellent thin film technique for lead based systems like PMN-PT. However, the growth of secondary paraelectric pyrochlore phase is a challenge, as it disturbs the ferroelectric response of the system and make it unsuitable for applications. Lead evaporation during thin film growth is one of the major reasons for the formation of pyrochlore phase. It will also make the film conducting and leaky and unsuitable for applications. Various deposition parameters like, quality of the target, nature of the substrate (Pt/TiO2/SiO2/Si), template layer (La0.5Sr0.5CoO3), vacuum conditions, throw distance, energy fluence, substrate temperature during deposition and annealing etc. play significant role in the formation of phase pure, crack free and insulating films, which will be discussed. Results on the structure, microstructure, topography and electrical behavior exhibited by the PLD grown films will be presented.

References:

[1] S. Yokoyama et.al; J.Appl.Phys.98(2005)p.094106(1-8).

[2] D. B. Chrisey et.al; Wile-Interscience, New York (1994).



 
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