Fabrication of continuous ultrathin ferroelectric films by chemical solution deposition methods

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S. Holgado Escuela Politécnica Superior, Universidad Autónoma de Madrid, Cantoblanco, E-28049 Madrid, Spain

Z. Huang Department of Materials, School of Applied Sciences, Cranfield University, Bedfordshire MK43 0AL, United Kingdom

P. Ramos Departamento de Electrónica, Universidad de Alcalá, E-28871 Alcalá de Henares, Spain

R. Fernández and M.L. Calzada Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid, Spain (Received 13 March 2008; accepted 11 July 2008)

The integration of ferroelectrics in nanodevices requires firstly the preparation of high-quality ultrathin films. Chemical solution deposition is considered a rapid and cost-effective technique for preparing high-quality oxide films, but one that has traditionally been regarded as unsuitable, or at least challenging, for fabricating films with good properties and thickness below 100 nm. In the present work we explore the deposition of highly diluted solutions of pure and Ca-modified lead titanates to prepare ultrathin ferroelectric films, the thickness of which is controlled by the concentration of the precursor solution. The results show that we are able to obtain single crystalline phase continuous films down to 18 nm thickness, one of the lowest reported using these methods. Below that thickness, the films start to be discontinuous, which is attributed to a microstructural instability that can be controlled by an adequate tailoring of the processing conditions. The effect of the reduction of thickness on the piezoelectric behavior is studied by piezoresponse force microscopy. The results indicate that films retain a significant piezoelectric activity regardless of their low thickness, which is promising for their eventual integration in nanodevices, for example, as transducer elements in nanoelectromechanical systems.

I. INTRODUCTION

Ferroelectric lead titanate based thin films have extensively been studied for their applications as active elements in nonvolatile ferroelectric memories (NVFeRAM) 1,2 and microelectromechanical systems (MEMS).3 However, the modern electronics industry demands an ever-larger degree of integration, shrinking the device dimensions down to the nanoscale. The reduction of the lateral memory cell size in FeRAMs means higher storage densities, which has recently reached 10 Tbit/ in.2.4 Regarding the nanoelectromechanical systems (NEMS), one of the major challenges they face now is

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0345 J. Mater. Res., Vol. 23, No. 10, Oct 2008

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related to the search of alternative transduction elements,5 as most of the techniques used in MEMS reach their limits in the nanoscale. Several attempts have been carried out, such as the use of piezoresistors for sensing6 or piezoelectric actuation of semiconductor GaAs,7 but the use of piezoelectric transduction with ferroelectric films has not been explored yet. Further development in the fabrication methods of ferroelect

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