Valérie Bouquet

Epitaxial growth of LiNbO3 thin films in a microwave oven

Abstract Oriented LiNbO 3 thin films were prepared using a polymeric precursor solution deposited on (0001) sapphire substrate by spin coating and crystallized in a microwave oven. Crystallization of the films was carried out in a domestic microwave oven. The influence of this type of heat treatment on the film orientation was analyzed by X-ray diffraction and electron channeling patterns, which revealed epitaxial growth of films crystallized at 550 and 650 °C for 10 min. A microstructural study indicated that the films treated at temperatures below 600 °C were homogeneous and dense, and the optical properties confirmed the good quality of these films.

INFLUENCE OF HEAT TREATMENT ON LINBO3 THIN FILMS PREPARED ON SI(111) BY THE POLYMERIC PRECURSOR METHOD

Departamento de Quimica LIEC Univ. Federal de Sao Carlos, P.O. Box 676, 13565-905 Sao Carlos, SP

Ferroelectric Thin Films for Applications in High Frequency Range

Abstract SrBi 2 Nb 2 O 9 and KTa 1− x Nb x O 3 thin films were grown by pulsed laser deposition on various substrates. Structural investigations evidenced that single phase polycrystalline randomly oriented films were grown on sintered alumina whereas epitaxial films were grown on (100)MgO. The microstructure was strongly controlled by the structural characteristics. Interdigited capacitors patterned on SrBi 2 Nb 2 O 9 films on both substrates as well as permittivity and losses measurements performed at 12.5 GHz showed a strong effect of the structural characteristics on the dielectric behavior.

KTN Dielectric Properties at Microwave Frequencies: Substrate Influence

To determine the dielectric properties of ferroelectric thin-films over a wide range of frequencies we developed a simple and efficient method based on the measurement of coplanar waveguide realized on ferroelectric/dielectric heterostructures by using a Spectral Domain Approach. We applied this method to KTa 1−x Nb x O 3 (KTN) thin-films deposited on different substrates. The strong differences observed in the dielectric behavior of KTNs according to the growth substrate suggest that it is affected not only via the resulting microstructure, but also by the induced strains. Accuracy of the method was estimated to 15 and 10% on permittivity and loss tangent, respectively.

EFFECT OF THIN KNbO3 SEED LAYERS ON PULSED LASER DEPOSITED FERROELECTRIC KTa0.65Nb0.35O3 FILMS FOR MICROWAVE TUNABLE APPLICATION

ABSTRACT Ferroelectric KTa0.65Nb0.35O3 thin films were grown by pulsed laser deposition on R-plane sapphire substrates. The potassium enriched ceramic targets were used for compensating potassium losses. In order to improve the KTa0.65Nb0.35O3 film growth on sapphire, a thin KNbO3 layer was introduced as seed layer. The experimental results show that the crystalline quality is lowered when increasing the K-excess. Simultaneously, phase purity, orientation and crystalline quality of perovskite (h00)-oriented KTa0.65Nb0.35O3 are greatly improved by the use of the KNbO3 seed layer. In particular, the presence of the latter effectively suppresses any pyrochlore secondary phase formation.

Lead-Free Oxide Thin Films for Gas Detection

In view to develop gas sensors, a first generation of lead-free thin films was deposited by different techniques on commercial Si and Al2O3 substrates. During our research project, the correlation between the micro structure of films, the structure of the embedded sensors and the applied temperature range, is being studied. In the first year, doped and undoped BaTiO3, KNbO3 and ZnO thin films have been deposited by sol-gel and PLD techniques. BT and BST films have shown a polycrystalline structure with very fine and regular grains, while disoriented grains with an average size ranging from 50 to 200 nm were observed on the KNbO3 film surface, and ZnO films exhibited a columnar growth. All films were characterized and finally embedded to make semiconductor gas sensors which have been tested under different gases. In this first generation of sensors, ZnO sensors have shown encouraging preliminary results under CO and H2S gases. Keywords : thin films, lead-free oxide, sensor, gas.

Structural, Optical, and Dielectric Properties of Bi1.5–xZn0.92–yNb1.5O6.92−δ Thin Films Grown by PLD on R-plane Sapphire and LaAlO3 Substrates

Bi(1.5-x)Zn(0.92-y)Nb(1.5)O(6.92-δ) thin films have the potential to be implemented in microwave devices. This work aims to establish the effect of the substrate and of the grain size on the optical and dielectric properties. Bi(1.5-x)Zn(0.92-y)Nb(1.5)O(6.92-δ) thin films were grown at 700 °C via pulsed-laser deposition on R-plane sapphire and (100)(pc) LaAlO(3) substrates at various oxygen pressures (30, 50, and 70 Pa). The structure, morphology, dielectric and optical properties were investigated. Despite bismuth and zinc deficiencies, with respect to the Bi(1.5)Zn(0.92)Nb(1.5)O(6.92) stoichiometry, the films show the expected cubic pyrochlore structure with a (100) epitaxial-like growth. Different morphologies and related optical and dielectric properties were achieved, depending on the substrate and the oxygen pressure. In contrast to thin films grown on (100)(pc) LaAlO(3), the films deposited on R-plane sapphire are characterized by a graded refractive index along the layer thickness. The refractive index (n) at 630 nm and the relative permittivity (ε(r)) measured at 10 GHz increase with the grain size: on sapphire, n varies from 2.29 to 2.39 and ε(r) varies from 85 to 135, when the grain size increases from 37 nm to 77 nm. On the basis of this trend, visible ellipsometry can be used to probe the characteristics in the microwave range quickly, nondestructively, and at a low cost.

KTa0.5Nb0.5O3 ferroelectric thin films: processing, characterization and application to microwave agile devices

This paper is aimed at demonstrating the high capabilities of KTa0.5Nb0.5O3 thin films for microwave agility applications. Tunable capacitors, resonators and phase shifters were realized with KTN materials. Their high frequency measurements showed, in particular, the very high tuning factors of these films under quite low electric field.

Sinterização de filmes finos de LiNbO3 em forno microondas: estudo da influência da direção do fluxo de calor

LiNbO3 thin films were prepared using a polymeric precursor solution deposited by spin coating on (0001) sapphire substrate. Heat treatment of the films was carried out in a microwave oven at 400 oC for 15 and 20 min. A SiC susceptor (material with high dielectric loss) was used to absorb microwave energy and transfer the heat to the film in order to promote crystallization. The susceptor was placed above the film or below the substrate. Thus, the influence of the heat flux direction on the sample crystallization was verified. The films were characterized by X-ray diffraction, atomic force microscopy and spectrophotometry (transmittance) in the UV-Visible region and the refractive index was determined with an ellipsometer. The epitaxial growth was observed for the film with the susceptor placed below the substrate. Random growth was verified when the susceptor was placed above the film. The films were relatively dense, homogeneous and smooth with good optical properties.

Electric Pulse Induced Resistive Switching in the Narrow Gap Mott Insulator GaMo4S8

We report here on resistive switching measurements on GaMo4S8 a lacunar spinel compound with tetrahedral Mo4 clusters filled with 11 electrons. Alike other clustered lacunar spinel compounds with 7 or 8 electrons per cluster, this narrow gap Mott Insulator exhibits both a volatile and a non-volatile unipolar resistive switching. We found that the volatile resistive switching appears above a threshold electric field in the 7 kV/cm range. For electric field much larger than this threshold, the resistive switching becomes non-volatile. Successive electric pulses allow switching back and forth between high and low resistance states. All these results demonstrate that the narrow gap Mott insulator compound GaMo4S8 could be a relevant candidate for a new type of non-volatile memory based on an electric field induced breakdown of the Mott insulating state.