Numerical and Comparative Study of the Agility of Planar Transmission Lines printed on a Ferroelectric Thin Film

International audienceIn this paper, we report on simulation results at 10 GHz of planar transmission lines (TL) printed on a ferroelectric thin film deposited on sapphire and lanthanum aluminate substrates. The ferroelectric film permittivity is supposed to vary from 700 to 500. First, the main properties of microstrip (MS), coplanar waveguide (CPW) and coplanar strips (CS) namely their effective permittivity, characteristic impedance and insertion loss are computed as a function of the physical and electrical parameters. Then their tunability and figure of merit are defined and compared. CPW and CS lines present a tunability (eff/eff_max) of about 16 % for a gap value g = 30 μm. The MS lines show a much less tenability of 2%. Moreover, the figure of merit of CPW and CS configurations are of 6.3 and 12.2 %/dB for a gap value of 30 μm, respectively. By increasing the gap, these figures of merit can be improved up to the limiting values of 8.7 and 15.4 %/dB, respectively

KTa0.6Nb0.4O3 Ferroelectric Thin Film Behavior at Microwave Frequencies for Tunable Applications

"©20xx IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE."International audienceIn this study about the relationships between structural and microwave electrical properties of KTa1-xNbxO3 (KTN) ferroelectric materials, a KTN thin film was deposited on different substrates to investigate how KTN growth affects the microwave behavior. Interdigital capacitors and stubs were made on these films through a simple engraving process. Microwave measurements under a static electric field showed the importance of the substrate on the circuit behavior and, notably, on the tuning factor

Randomly organized and self-assembled Na0.5Bi0.5TiO3 nanodots elaborated by sol-gel and pulsed laser deposition routes

International audienceThis work reports for the first time on the elaboration, by both chemical (sol-gel) and physical (pulsed laser deposition) routes, of lead-free ferroelectric Na0.5Bi0.5TiO3 nanodots deposited on bare c-sapphire single crystal substrates presenting a 5° miscut angle along the [110] direction. Prior to any deposition, the sapphire substrates were treated at 1350 °C, during 24 h in air, in order to increase the height of the surface steps, reaching by this way ~8 nm. The experimental parameters adjusted for the growth of Na0.5Bi0.5TiO3 dots were the concentration of sols and the number of laser pulses (50 and 100) for the sol-gel and pulsed laser deposition routes, respectively. Whereas the sol-gel route leads to randomly organized Na0.5Bi0.5TiO3 nanodots in respect to the surface steps, the pulsed laser deposition route provokes the self-assembly for some important proportion of these dots along the same surface steps. Despite the lack of organization for the sol-gel dots, the latter present a much more regular distribution in size (~100 and ~10-20 nm as an average lateral dimensions and height, respectively) compared to dots deposited by laser ablation, where three different populations of grains can be observed. In each case, the dots do not seem to be epitaxially grown

Caractérisation large bande en ligne coplanaire de couches minces ferroélectriques

National audienceNous avons présenté dans cet article une méthode de caractérisation " large-bande " de films minces ferroélectriques à partir de la mesure sous pointes d'une ligne coplanaire incorporant le film. Nous avons caractérisé un film mince de KTN de 0.5μm d'épaisseur jusqu'à 50 GHz. Nous avons quantifié les erreurs liées aux hypothèses simplificatrices de l'analyse électromagnétique rendue nécessaire par le caractère destructif de la méthode et dont le domaine de validité est limité. Enfin, la complexité de la correction des erreurs systématiques dans le cas de la caractérisation des couches minces ferroélectriques a été mise en évidenc

SrSnO3:N – nitridation and evaluation of photocatalytic activity

International audienceThe SrSnO3 was prepared in powders form by a conventional method of solid state reaction and submitted to thermal treatment under NH3 in order to obtain the oxynitride and then promote photocatalytic activity at the visible region. Three different nitridation conditions were tested. The results showed that when the SrSnO3 is subjected to nitridation at moderate conditions (600 °C and 650 °C) oxynitide is formed preserving the perovskite structure. On the other hand, when the nitridation occurs at more severe conditions (750 °C), it is possible to observe the decomposition of the structure instead of the incorporation of N. Samples in which oxynitride is formed, SrSnO3:N, showed high potential photocatalysis under visible light, showing to be about 4 times more active than SrSnO3 or TiO2 at the same condition

Pulsed laser deposited KNbO3_3 thin films for applications in high frequency range

Potassium niobate thin films were grown by pulsed laser deposition on various substrates. Influence of deposition conditions on film characteristics was studied. Structural investigation evidenced that single phase polycrystalline randomly oriented films were grown on sintered alumina whereas epitaxial films were grown on (100)SrTiO$_3$ and (100)MgO substrates. The microstructure was highly controlled by the structural characteristics. Interdigited capacitors built from KNbO3 films on two different substrates (alumina and MgO) showed the strong influence of the structural characteristics on the dielectric behavior. The variation of the equivalent capacitance measured on the interdigital capacitor on MgO was 6.4% at 2.5 GHz while it was 1.5% on alumina, in both cases for a moderate applied field of $\sim$15 kV cm$^{-1}$. The results show the potentiality of these ferroelectric materials for use in frequency agile microwave electronics

Epitaxial growth and properties of lead-free ferroelectric Na0.5Bi0.5TiO3 thin films grown by pulsed laser deposition on various single crystal substrates

International audienceThe epitaxial growth of lead-free ferroelectric Na0.5Bi0.5TiO3 (NBT) thin films on various single crystal substrates was successfully achieved, using the pulsed laser deposition technique (PLD). The present work is divided in two parts, focused on: (i) the growth of NBT layers on c- and r-sapphire (Al2O3) substrates, with and without introducing a CeO2 buffer layer, and (ii) the growth of NBT layers on bare (001)SrTiO3 substrates, with and without introducing a LaNiO3 layer, that could be used as a bottom electrode. In the first part, it was shown that the introduction of a CeO2 buffer layer completely modifies the out-of-plane growth orientation of the NBT films, as well as their microstructure. Indeed, (001)NBT films epitaxially grow only on r-Al2O3 substrates buffered with epitaxial (001)CeO2 layers, while, growing simply NBT on top of bare c or r-Al2O3 substrates, or on top of CeO2/c-Al2O3 heterostructures leads to polycrystalline or textured films. In the second part, we demonstrate that (001)-oriented NBT layers deposited on either bare (001)SrTiO3 or (001)SrTiO3 substrates (STO) covered with (001)LaNiO3 (LNO) are systematically epitaxially grown. Furthermore, the microstructure of the samples is strongly affected by the introduction of the LaNiO3 layer

Study of ferroelectric/dielectric multilayers for tunable stub resonator applications at microwaves

International audienceTunable coplanar waveguide stub resonators deposited on various ferroelectric/dielectric heterostructures are studied in the 10-GHz band. A frequency tunability of up to ~ 45% is achieved under a moderate biasing field (Ebias < 100 kV/cm) when the resonator is printed on KTa0.5Nb0.5O3 (KTN) ferroelectric thin film alone: this comes from the large permittivity agility of the KTN material (εr(KTN) varies from ~ 700 to ~ 200). Nevertheless this also leads to significant insertion loss due to the dielectric loss of the ferroelectric material itself (tanδr(KTN) ≈ 0.15-0.30 at 10 GHz). In this paper, an original route has been considered to reduce the device loss while keeping up a high frequency tunability. It consists in associating the KTN film with a dielectric film to elaborate ferroelectric/dielectric multilayers. The Bi1.5Zn0.9Nb1.5O7−δ (BZN) oxide material is selected here for two main reasons, namely its low dielectric loss (tanδr(BZN) ≈ 0.005-0.0075) and its moderate relative permittivity (εr(BZN) ≈ 95-125) at 12.5 GHz. The relevance of this approach is studied numerically and experimentally. We compare numerically two different heterostructures for which the ferroelectric film is grown on the dielectric film (KTN/BZN), or vice versa (BZN/KTN). A stub resonator printed on the most relevant heterostructure has been fabricated, and experimental data are discussed and compared to the numerical results

Loss reduction technique in ferroelectric tunable devices by laser micro-etching. Application to a CPW stub resonator in X-band

International audienceFerroelectric materials are known to be lossy at microwaves. A local microetching technique based on laser ablation is implemented here to reduce the insertion loss of highly tunable devices fabricated on KTa1-xNbxO3 (KTN) ferroelectric thin films. The relevance of this approach is studied in X-band by comparing numerically and experimentally the performance of a frequency-tunable coplanar waveguide stub resonator before and after KTN microetching. The experimental data demonstrate a large loss reduction (by a factor 3.3), while keeping a high-frequency tunability (47%) under a moderate biasing static electric field (80 kV/cm). This approach paves the way for the design of ferroelectric reconfigurable devices with attractive performance in X-band and even beyond