Marylline Guilloux-Viry

(Text in French) Contribution of crystallochemistry to the optimization of lead-free oxide thin films by pulsed laser deposition. Through interdisciplinary research, this article presents several aspects of the crystallochemistry of thin films grown by pulsed laser deposition, which contributes to i) optimizing lead-free piezoelectric and ferroelectric materials on single-crystal substrates, ii) control the crystalline characteristics of thin films on less expensive substrates, iii) the introduction of thin films of interest into energy conversion technology, and iv) explore phase-change materials for new, high-performance devices for information processing that consume less energy.

K-enriched KNN target effectively prevents alkali cation deficiency, ensuring typical piezoelectric and ferroelectric behavior. In particular, it avoids the secondary phase K ₄ Nb ₆ O ₁₇ in KNN thin films obtained by PLD using a stoichiometric target.

We present a study of the effect of electrically induced cracks on both the ferroelectric and piezoelectric properties of Pt/PbZr_0.52Ti_0.48O₃ (PZT)/Pt capacitors and correlations with domain structures of PZT films. Above a threshold bipolar electric field, cracks appear inside the PZT layer thickness leading to an increase in the ferroelectric polarization (+50% for the remnant polarization, from 16 to 25 μC/cm²) and the longitudinal piezoelectric coefficient d_33,f (from ∼150 to ∼220 pm/V). The use of x-ray diffraction during in situ biasing provides direct evidence for a modification of the PZT crystalline structure as well as the a/c domain configuration. After cracking, the fraction of c-domains is strongly increased, thus contributing to higher polarization and larger strain in the out-of-plane direction.

In 2008, the modified European Restriction of Hazardous Substances (RoHS) directive prohibited the use of hazardous substances such as lead, cadmium, and mercury. As such, an urgent need for lead-free components emerged in Europe. In this frame, we have decided to study the microstructure influence of zinc oxide thin films on the detection of hydrogen sulfide (H₂S). Zinc oxide thin films were deposited by PLD on silicon substrates under different conditions to modify the microstructure. In order to compare our demonstrators to current commercial semiconductor gas sensors, measurements under H₂S were also performed with sensors from Figaro and Winsen corporations. Gas sensors were therefore implemented by using commercial cases in view to test them with Simtronics gas detector DG477. The good sensitivity values measured at T = 400 °C under 100 ppm H₂S, and response times as low as 30 s, definitely confirm that ZnO thin films could be developed for commercial sensors.

A miniature frequency-tunable slot-loop antenna (0.128λ ₀ × 0.077λ ₀ ) based on ferroelectric K _0.5 Na _0.5 NbO ₃ (KNN) interdigitated varactors is investigated at Ku-band. KNN material has been selected among the ferroelectric oxide family because of its competitive properties for microwave applications. Its high permittivity value (ϵ _r ≈ 360 at 10 GHz) and its wide frequency tunability under an external static electric field E _bias make it a promising candidate for frequency-agile antennas and circuits at microwaves. The antenna prototype has been designed and elaborated on MgO substrate (ϵ _r = 9.8 and tanδ ≈ 10 ^-4 at 10 GHz). Fabrication details including laser microetching process of the ferroelectric layer are also described. The center frequency of the slot-loop antenna can therefore be tuned monotonously from 15.22 to 15.97 GHz under E _bias = 88 kV/cm, leading to a frequency tunability rate equal to 4.6%. These results demonstrate for the first time the ability of the KNN material to be used for miniature tunable antennas, in comparison with Ba _x Sr _1-x TiO ₃ , currently considered as the most relevant material for such applications.

Ferroelectric oxide films are attractive to design and fabricate reconfigurable and miniaturized planar devices operating at microwaves due to the large electric field dependence of their dielectric permittivity. In particular, KTa1-xNbxO3 (KTN) ferroelectric material presents a high tunability under moderate DC bias electric field. However, its intrinsic dielectric loss strongly contributes to the global loss of the related devices and limits their application areas at microwaves. In this work, a twofold approach is investigated to reduce the device loss. The intrinsic loss of KTN is firstly reduced by doping the ferroelectric material with a low loss dielectric material, namely MgO. Second, the doped ferroelectric films are confined using an original laser micro-etching process. Both routes have been implemented here to provide a synergic effect on the total insertion loss of the microwave test device, namely a coplanar waveguide stub resonator. The experimental data demonstrate a decrease of the intrinsic loss by a factor ~2 and a decrease of the global loss by a factor ~4 with a frequency tunability close to 10% at ~10 GHz under a moderate biasing (80 kV/cm).