M. Gervais

Synthesis of Ca0.25Cu0.75TiO3 and infrared characterization of role played by copper

Abstract Ca0.25Cu0.75TiO3 compounds with a perovskite-type structure have been reported to display the double advantage for capacitance applications to show high dielectric constant and little dependence on temperature near room temperature. Powders have been prepared by using an organic gel-assisted citrate process. The infrared reflectivity spectra obtained on sintered ceramic discs are very different from those of simple cubic perovskites. Copper indeed is expected to form hybridized bonds with oxygen, contrary to calcium. As a result, instead of remaining equidistant from calcium or copper as in simple cubic perovskite, oxygen forms shorter bonds with copper inducing a strong distortion of oxygen octahedra. Spectra have been Kramers–Kronig transformed and fitted with the factorized form of the dielectric function. Results show that the origin of high dielectric constant is little phononic but rather due to a relaxational process with the characteristic frequency in the gigahertz range. Thin films grown by PLD on (100) SrTiO3 substrate show preferential (100) orientation.

Characterization of pulsed laser deposited Ba0.6Sr0.4TiO3 on Pt-coated silicon substrates

Abstract The high dielectric constant (Ba,Sr)TiO 3 (BST) films have been widely used to realize capacitors integrated on silicon with a high value of capacitance. The multilayer Pt/Ti/SiO 2 /Si is one of the most currently used bottom electrodes for the integration of BST on silicon. However, the crystal orientation and the dielectric properties of ferroelectric thin films are greatly influenced by the underlying Pt/Ti metallization, and particularly by the out-diffusion of titanium towards the platinum surface during thermal treatments. In this study, we show that the heating stage of the Pt/Ti/SiO 2 /Si substrate before the BST pulsed laser deposition is of primary importance in both favoring the (1xa01xa01) growth of the BST material within a wide range of oxygen deposition pressure and in reducing drastically the loss tangent values of Al/BST/Pt capacitors because of the oxygen saturation of platinum. Electrical measurements indicate the existence of an interfacial layer degrading the capacitance. They support the presence of an interfacial depleted layer. The dramatic increase of the loss tangent under positive polarities applied on the platinum electrode is attributed to the ohmic contact of the BST/Al interface. Except this increase, all of the electrical properties are very promising in view to realize capacitors with high capacitance value and low dispersion.

Pulsed laser deposition of ferroelectric BST thin films on perovskite substrates: an infrared characterization ☆

Abstract The interest in Ba1−xSrxTiO3 (BST) thin films is motivated by capacitive applications because of its high dielectric constant, low leakage current and high dielectric breakdown strength. Pulsed laser deposition (PLD) has emerged as an excellent technique for the preparation of high quality oriented films of perovskite materials with high compositional consistency between thin film and target. In a first step, (100) preferentially oriented Ba0.6Sr0.4TiO3 thin films were grown by pulsed laser ablation of sintered ceramic targets on (100) SrTiO3 single crystal substrates at 700°C and under oxygen pressures between 5×10−3 and 10−1 mbar. The dielectric constants of the films in the terahertz range were deduced from the analysis of the far-infrared reflectivity.

Upshift of ferromagnetic-paramagnetic phase transition temperature of La0.8Sr0.2Mn1-xRuxO3 probed by electron spin resonance

The effect of ruthenium doping upon the temperature of the ferromagnetic-to-paramagnetic (FM–PM) transition in a series of La0.8Sr0.2Mn1−xRuxO3 (0<x<0.2) has been studied by electron spin resonance (ESR). The onset of the FM–PM transition has been determined from the variation of the effective g factor with temperature. TC was shown to increase from 310 K for x=0 up to a maximum value at 352 K for x=0.15.

Processing and physical properties of La0.8−zYzSr0.2MnO3 bulk, thick films and single crystal

Abstract Bulk and thick film manganese perovskite La 0.8− z Y z Sr 0.2 MnO 3 (LYSMO) with 0⩽ z ⩽0.8 have been processed by standard ceramic process. A single crystal of La 0.78 Y 0.02 Sr 0.2 MnO 3 made by floating zone method had been prepared and thick films were deposited on (001) MgO substrates using an air-brushing method. The structure and microstructure were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electron paramagnetic resonance (EPR) and the magneto-transport properties have been performed to determine the paramagnetism–ferromagnetism transition temperature ( T C ). The insulator-to-metal transition (I–M) occurs at about room temperature ( T IM ) and T C is in range of 311–355 K. The magnetoresistance of 70% at 300 K has been observed. Effects of Y-doping are also discussed.

Optical and electrical conductivity of La0.8Sr0.2MnO3 thin films deposited by laser ablation

Abstract La0.8Sr0.2MnO3 thin films have been grown by pulsed laser deposition on a SrTiO3 (1xa00xa00) single crystal substrate. The ‘metal’ (low-temperature)–‘insulator’ (high-temperature) phase transition manifests itself by a jump of resistivity near room temperature. A possible dependence of these properties upon the film thickness is investigated. The optical conductivity is deduced from the best fit to reflectivity spectra of a ‘double-damping Drude’ model, itself derived from the factorized form of the dielectric function. The model allows in particular to discriminate the contributions to the optical conductivity of trapped charges (polarons) and mobile charge carriers.