The Eect of Film Thickness on the Ferroelectric Properties of Sol±Gel Prepared Lanthanum Modi®ed Lead Titanate Thin Films M. AlgueroÂ, M. L. Calzada and L. Pardo* Instituto de Ciencia de Materiales, CSIC, Cantoblanco 28049, Madrid, Spain Abstract Lanthanum modi®ed lead titanate (PTL) thin ®lms with a thickness ranging between 100 and 650 nm have been prepared by a sol±gel method. Hysteresis loops have been measured in two sets of ®lms: ther- mally crystallized with 10  C min 1 and more than 500  C min 1 . A thickness dependence of the ferro- electric parameters has been found in both sets. This dependence is related to the presence of a modi®ed layer near the bottom electrode, with a dierent ori- gin in the two sets of ®lms. In those treated with 10  C min 1 , the layer is a Ti de®cient one, most probably with the perovskite structure but La de®- cient. In those treated with more than 500  C min 1 , it is a layer that bears the ferroelectric-substrate interfacial stress. # 1999 Elsevier Science Limited. All rights reserved Keywords: (Pb,La)TiO 3 , ®lms, sol±gel processes, ferroelectric properties. 1 Introduction Among the ferroelectric compositions, lanthanum modi®ed lead titanate (PTL) with small amounts of La has been found to be very suitable for thin ®lm in infrared sensors, 1,2 and it is also under focus for electromechanical applications. In a previous work, 3 we have shown that PTL ®lms with a 8% of La can be prepared on Si based substrates by a diol based sol±gel method showing good ferroelectric properties, provided that a 20 mol% excess of PbO were added to the pre- cursor solution. It was found that the structure and microstructure of the ®lms can be tailored using dierent heating rates for the crystallization ther- mal treatment. In this work, the dependence of the switchable polarization on thickness is studied for the two types of ®lms. A thickness dependence is expected in ®lms when a low dielectric permit- tivity layer is developed near the electrodes. 4,5 Previous rutherford backscattering spectrometry (RBS) results 6 obtained for the PTL ®lms of this work, suggest that Ti diusion occurs from the ®lm to the Pt bottom electrode during the thermal treatment depending on the heating rate of crys- tallization. 2 Experimental Methods PTL ®lms were prepared as in Ref. 3 from a pre- cursor solution with a 20 mole% excess of PbO with respect to the nominal composition of Pb 088 La 008 TiO 3 . Films with successive increasing thickness were obtained by repeating solution deposition up to ®ve times. The average thickness of the PTL ®lms was measured by pro®lometry. Changes of the struc- ture and microstructure with thickness were moni- tored by grazing incidence X-ray diraction (GIXRD) and scanning electron microscopy (SEM), respectively. Electrical characterization was accomplished after 500 m diameter Pt elec- trodes were deposited by sputtering on the ®lms. The capacitance and losses at 1 kHz were measured with a HP 4284A precision LCR meter. Ferro- electric hysteresis loops were measured by using a modi®ed Sawyer±Tower circuit and an oscilloscope. A sine wave voltage of variable frequency (between 2 and 200 Hz) was used. To improve the switchable polarization, an electrical treatment consisting in trains of alternating square pulses with a 1 min length, a 200 Hz frequency, and a 250±400 kV cm 1 amplitude, 3,7 was applied before measurements. Journal of the European Ceramic Society 19 (1999) 1481±1484 # 1999 Elsevier Science Limited Printed in Great Britain. All rights reserved PII:S0955-2219(98)00459-2 0955-2219/99/$ - see front matter 1481 *To whom correspondence should be addressed. Fax: +34- 1327-0623; e-mail: lpardo@icmm.csic.es