Ferroelectric Properties of Lead Titanate


Ferroelectric thin films of lead titanate (PbTiO3) have been prepared on SiO2/Si(100) and LaNiO3(110)/SiO2/Si(100) substrates by a sol-gel spin coating method, using lead acetate trihydrate, and titanium isopropoxide as precursors. X-ray diffraction patterns showed the formation of the perovskites structure of PbTiO3 thin films deposited on SiO2/Si substrate at 650 °C and its ferroelectric hysteresis measurements gave values of remanent polarization of Pr=18.37 μC/cm2 and a coercive field of Ec=121.53 kV/Cm. The current-voltage characteristic was found to be space charge conduction in the high voltage region and its piezoelectric coefficient d33 gave a value of 69 pm/V.

Keywords: Sol-gel coatings, Lead titanate thin film, Structural characterization, Ferroelectric properties

1. Introduction

Investigation on the ferroelectric materials has drawn much intention for numerous applications, including non-volatile memories, dynamic random access memories, electro-optic switches, pyroelectric detectors, etc. [1-4]. Considerable attention has recently focused on the development of the technology for their growth in thin films finding that the film deposition method used in the production of multilayered structure devices (substrate, ferroelectric film, and electrodes) is a decisive step to obtain high-performance films [5,6].

Lead titanate PbTiO3 is well known to have good ferroelectric and pyroelectric properties [7], such as the high Curie temperature of 490 °C, large pyroelectric coefficient, low relative permittivity, the largest lattice anisotropy (c/a=1.064 in the tetragonal phase) in the simple perovskites family and an easy spontaneous polarization [8,9]. It is also considered one of the most promising materials because of its significant applications in various domains such as ultrasonic sensors [10], non-volatile memory devices [11], etc.

Numerous techniques have been applied to prepare lead-based titanate ferroelectric materials such as solid-state reaction [12], coprecipitation [13], hydrothermal processes [14], chemical vapour deposition [15], pulsed laser ablation [16], etc, however, the sol-gel chemical technique is being increasingly used because it offers excellent control over the stoichiometry, better homogeneity, low processing temperature, and low-cost equipment required has been extensively used for the fabrication of ferroelectric perovskites [17,18].

In this paper, we are interested to study the structural, electrical and ferroelectric properties of sol-gel derived lead titanate (PbTiO3) thin films prepared by polymeric precursor method and deposited on two substrates, SiO2/Si(100) and LaNO3(110)/SiO2/Si(100).

2. Experimental tools

Thin films of lead titanate (PbTiO3) have been prepared using the precursors of lead acetate trihydrate Pb(CH3COO)2,3H2O and titanium isopropoxide Ti(OC3H7)4. During this preparation, the 2-methoxy-ethanol CH3OCH2CH2OH was used as a solvent and acetic acid CH3COOH as the chelating agent for the alkoxides. Lead acetate trihydrate

was first dissolved in heated 2-methoxy-ethanol (70 oC) (with a molar ratio of 1:26). Excess of 10 mol% lead acetate was added to compensate the losses during subsequent thermal treatment. The obtained solution was heated at 120 oC under stirring to eliminate water, the dehydrated solution was cooled down to 70 oC, and then titanium isopropoxide was added with constant stirring to prepare PbTiO3 sol, followed by refluxing for 3h. Filtered sol is dispensed using a 0.2 µm syringe filter and spin-coated at a speed of 3000 rpm for the 30s on LaNiO3/SiO2/Si (100) and SiO2/Si (100) substrates. The films were pyrolyzed at 350 °C for 30 min after each coating to remove residual volatile organics. This step was repeated until obtaining thicker films. The films were finally heated at 650 °C in the furnace for 1 hour with a heating rate of 2 oC/min for crystallization.

The PbTiO3 film structure characterization was performed using an X-ray diffractometer system (XPERT-PRO, Cu Kα radiation λ=1.5405Å). While its ferroelectric properties and piezoresponse were measured by aix ACCT system and Laser Scanning Doppler Vibrometer (SLDV), respectively.