Studio di fenomeni sulla nanoscala in film sottili ferroelettrici tipo "relaxor"

Responsabili di progetto

Franco Dinelli, Eudes Borges Araujo

Accordo

BRASILE - CNPq - Conselho Nacional Desenvolvimento Cientifico y Tecnologico

Bando

CNR/CNPq 2012-2013

Dipartimento

Materiali e Dispositivi

Area tematica

Scienze fisiche e tecnologie della materia

Stato del progetto

Nuovo

Proposta di ricerca

Subject Area: Materials Science
State of the Art
This project addresses the issue of size effects in ferroelectric and relaxor films. The properties of ferroelectrics (FE) ([i]) were exhaustively studied during the past decade and offered an exciting potential for applications since a long time. Several important technological devices, such as Random Access Memories, exploit FE([ii],[iii]) and, to keep the pace of the miniaturization in electronics, it is becoming increasingly important to investigate size effects in FE.
In such materials, a decrease in particle size causes a reduction in the lattice distortion, often leading to diffuse FE phase transition. In some cases, antiferroelectric materials become FE due to small particle size ([iv],[v]); moreover, an increase of coercive field and a decrease in the remanent polarization are observed, suggesting FE suppression in nanosized systems. However, recent results on ultrathin PZT ([vi]) and PbTiO3 films( [vii] ), showing no suppression of FE, pushes towards systematic studies, so far not appeared in the literature. Similarly, detailed investigations on size effects in relaxor materials are lacking, despite their fascinating features such as, giant piezo-electricity and electrostriction([viii],[ix]). In particular, PLZT and SBN, solid solutions with strongly composition-dependent properties[x], have received great attention for their large pyroelectric coefficient and for the excellent piezoelectric and electro-optic properties.
In FE, a different degree of ordering is expected near surfaces or interfaces, leading to an intrinsic dependence on sample size. Understanding of the involved mechanism is complicated, since the polarization interacts more strongly than other order parameters such as, strain and charge. As a result, extrinsic effects are produced if these variables are uncontrolled and consequently size dependence largely remains an unresolved issue.
The investigation of size effects on FE films is a relatively recent(vii,[xi],[xii],[xiii],[xiv]) field, where the possibilities offered by innovative synthesis and characterization methods ([xv],[xvi],[xvii]) have added strong impulse. Literature reports also a number of studies on FE powders composed of ultrafine particles([xviii]) on both classical perovskites([xix],([xx],[xxi]) and other FE, including SBTO([xxii]) and PZT([xxiii],[xxiv]).
The Curie temperature (TC) is an important parameter for ferroelectrics. The observed behavior of TC in ultrathin films appears often in disagreement with theoretical predictions(vii,xviii). The suppression of FE in thin films by a collapse of TC is typically described using Landau theory([xxv]), predicting an inverse dependence of TC on thickness. However, more complicated behaviors have been found in ultrathin films (~1.2 nm) (vii), suggesting the presence of different effects.
Besides the fundamental character of the research, there are strong applicative motivations.
Nowadays, the fabrication of FE nanostructures integrated onto Si chips is an industrial reality ([xxvi]), and understanding the phenomenology for ultrathin FE films is an important challenge in order to allow for their exploitation in nanodevices.
 
Methodology
An essential point for the research is the synthesis of high quality materials. A chemical Oxide Precursor Method (OPM)(xi) was used in the past to prepare FE thin films. The method has been recently improved through modification of the solution precursors with polyethylene glycol, and a variety of FE materials have been prepared[xxvii] ,[xxviii]. In the present research, PLZT and SBN will be produced by OPM. The general idea of this approach is to distribute the metallic ions homogeneously throughout the polymeric resin and prepare the samples according to the Pechini method[xxix] in which formation of chelate between dissolved ions and a hydroxy-carboxylic acid (citric acid) is achieved. The heating of the resin in air causes a breakdown of the polymer. Subsequently, the ions are oxidized to form the desired crystalline phases.
A large variety of techniques will be used as investigative tools. The expectation is to correlate the results of different techniques in order to create a data set that contribute to understanding the phenomenon of self-polarization in FE thin films. The main techniques to be used are X-ray diffraction (XRD), confocal Raman spectroscopy, dielectric and ferroelectric measurements, Scanning Probe Microscopy (SPM) and Scanning Electron Microscopy (SEM).
The project aims at joining high level facilities and expertise available by the Brazilian and Italian partners. First of all, thanks to the advanced fabrication capabilities of the Brazilian lab, high-quality films will be available, with stoichiometry and thickness tailored to the research goals. Moreover, the possibility to integrate the films with suitable electrodes will enable functional investigations. An added value of the research will also be the deployment of complementary diagnostic techniques. This will enable a strict comparison between different diagnostics, ranging through the more conventional methods available by the Brazilian partner (XRD, Raman, SEM, dielectric permittivity in Metal-FE-Metal structures) to the advanced nanoscale tools (Atomic Force Microscopy – AFM - with ultrasound excitation and with piezoelectric response, Polarization Modulation Scanning Near-field Optical Microscopy – PM-SNOM) developed by the Italian partner.
A large number of information on the FE films will be gathered such as, microstructure, strain gradients, electrical coefficients, mechanical properties of film and interface, morphology of the FE domains and their optical activity, local piezoelectric and ferroelectric response, which will be carefully compared each other and correlated to the fabrication parameters (material, substrate, thickness).
 
References
[i] F. Jona and G. Shirane, Ferroelectric crystals, Dover Publications, New York, 1993.
[ii] J.F. Scott and C.A. Araujo, Science 246, 1400 (1989).
[iii] J.F. Scott, Ferroelectric Memories (Springer, Heidelberg, Germany, 2000).
[iv] S. Chattopadhyay et al., J. Phys.: Cond. Matter 9, 8135 (1997).
[v] P. Ayyub, S. Chattopadhyay, R. Pinto and M.S. Multani, Phys. Rev. B 57, R5559 (1998).
[vi] T. Tybell, C.H. Ahn and J.-M. Triscone, Appl. Phys. Lett. 75, 856 (1999).
[vii] D.D. Fong et al., Science 304, 1650 (2004).
[viii] R. Blinc, V.V. Laguta, B. Zalar, J. Banys, J. Mat. Sci. 41,  27 (2006)
[ix] A. A. Bokov, Z.-G. Ye, J. Mat. Sci. 41,  31 (2006)
[x]. R.R. Neurgaonkar, W.K. Cory, W.W. Ho and W.F. Hall, Ferroelectrics 38, 857 (1981).
[xi] T. M. Shaw, S. Trolier-McKinstry, P. C. McIntyre, Annu. Rev. Mater. Sci. 30, 263 (2000).
[xii] C.H. Ahn, K.M. Rabe, J.-M. Triscone, Science 303, 488 (2004).
[xiii] N.A. Spaldin, Science 304, 1606 (2004).
[xiv] R. Waser, A. Rudiger, Nature Mater. 3, 81 (2004).
[xv] D.D. Fong and C. Thompson, Annu. Rev. Mater. Res. 36, 431 (2006).
[xvi] L. Despont, et al., Eur. Phys. J. B 49, 141 (2006).
[xvii] S. Wicks, V. Anbusathiah and V. Nagarajan, Nanotechnology 18, 465502 (2007).
[xviii] B. Jiang, J.L. Peng, L.A. Bursill, W.L. Zhong, J. Appl. Phys. 87, 3462 (2000).
[xix] Y.S. Kim et al., Appl. Phys. Lett. 18, 102907 (2005).
[xx] C. Lichtensteiger and J.-Marc Triscone, Phys. Rev. Lett. 94, 047603 (2005).
[xxi] D.D. Fong et al., Phys. Rev. Lett. 96, 127601 (2006).
[xxii] A. Gonzalez, R. Jimenez, J. Mendiola, C. Alemany and M.L. Calzada, Appl. Phys. Lett. 81, 2599 (2002).
[xxiii] P. Paruch, T. Giamarchi and J.-M. Triscone, Phys. Rev. Lett. 94, 197601 (2005).
[xxiv] X.H. Zhu et al., Appl. Phys. Lett. 89, 122913 (2006).
[xxv] R. Kretschmer and K. Binder, Phys. Rev. B 20, 1065 (1979).
[xxvi] J.F. Scott, Science 315, 954 (2007).
[xxvii] D. Valim et al., Journal of Physics D: Appl. Phys. 37, 744 (2004).
[xxviii] K. Yao, S. Yu and F.E.H. Tay, Appl. Phys. Letters 88, 052904 (2006).
[xxix] P.A. Lessing, Ceramic Bulletin 68, 1002 (1989).

Obiettivi della ricerca

The focus of the present research project will be centered on both scientific and technological aspects for materials such as PLZT and SBN. The analysis of the extrinsic and size effects on the structure of relaxor ferroelectric films is our main aim. The absence of systematic studies involving size effects on the dielectric properties at the interface thin/ultrathin ferroelectric films represents another important motivation. The objectives of this Brazil/Italy collaboration are:
1) Synthesis of PLZT and SBN relaxor thin films with different thicknesses at compositions La/Zr/Ti = 9/65/35 and Sr/Ba = 75/25, respectively;
2) Understand the size effects and residual stress on the structure, nanostructure and dielectric properties at the ferroelectric thin/ultrathin film interface;
3) Thorough investigation of nanostructured PLZT and SBN samples from the macro- and microscale down to the nanoscale;
4) Quantitative nanoscale characterization of ferroelectric thin films by Atomic Force Microscopy;
5) Understand the size effects on the mechanism of polarization reversal in thin films, nucleation processes and domain wall motion;
6) Investigation of size effects on the dielectric properties as a function of frequency, temperature and electric field (Eac and Edc);
7) Establish a scientific cooperation in the nanoscience of ferroelectric materials between Brazil and Italy;
8) Formation of students (M.Sc. and Ph.D.) and training of young researchers.
 
The expected results are:
1) Comprehension and control of the size effects on the studied films from the macro- and microscale to the nanoscale;
2) Understanding the size effects on the relaxor properties of the PLZT and SBN films;
3) Joint publication of scientific articles in indexed journals;
4) Joint contributions to international workshops and conferences.
During this collaboration, the two partners will constantly exchange ideas and cooperate by sharing their complementary techniques and resources to realize the common aims. In addition to exchange visits, informal correspondence by phone, Skype or e-mail, discussions, formation of young students, decisions on the future directions will be developed.
 
 

Ultimo aggiornamento: 27/04/2024