Microbolomètres supraconducteurs YBCO suspendus<br />réalisés par micro-usinage du substrat de silicium

We demonstrated the feasibility of both sensitive and relatively fast YBaCuOsuspended bolometers by silicon micromachining. This work includes a technological partwhere three fabrication sequences are described and a characterisation part. The YBaCuOfilms on CeO2 / YSZ-buffered silicon are c axis textured and show only one epitaxial in-planeorientation. Their critical temperature Tc (zero resistance) is 88 K and their critical currentdensity is above 106 A / cm2 at 77 K. The reactive ionic etching (RIE) of silicon substratesenabled to fabricate a large number of samples and to confirm our calculations by comparisonwith the measurements. The technique of fabrication based on SIMOX substrates did notdegrade the YBaCuO superconducting properties and was found to be very promising fordetection. At last, we defined a meander line, constituted by seventeen 4 µm wide tracks andfabricated by RIE. The performances of this detector of 100 ´ 100 µm2 area, measured at 85 Kin the wavelength range 3-5 µm, are among the best reported results:Sensitivity (3-5 µm) = 11950 V / W.Thermal response time t = 564 µs.Optical NEP = 4,0 10-12 W Hz-1/2Detectivity D* = 2,5 109 cm Hz-1/2 / WNous avons démontré la faisabilité de bolomètres suspendus YBaCuO sensibles etrelativement rapides par micro-usinage du substrat de silicium. Ce travail comprend une partietechnologique décrivant trois techniques de fabrication de structures suspendues YBaCuO etune partie de caractérisation. Les films d'YBaCuO obtenus sur silicium avec une doublecouche tampon CeO2 / YSZ sont texturés dans la direction c et ne possèdent qu'une seuleorientation dans le plan. Leur température critique mesurée à résistance nulle vaut 88 K, etleur densité de courant critique dépasse 106 A / cm2 à 77 K. La gravure ionique réactive (GIR)du substrat de silicium nous a permis de réaliser un grand nombre d'échantillons et de validerainsi nos calculs par confrontation avec les mesures. La technique de fabrication utilisant unsubstrat SIMOX s'est révélée très peu dégradante pour l'YBaCuO et très prometteuse pour ladétection. Enfin, nous avons défini un méandre constitué de 17 brins de largeur de piste 4 µm,et fabriqué par GIR. Les performances de ce détecteur de surface 100 * 100 µm2, mesurées à85 K dans la gamme de longueur d'onde 3-5 µm, sont au niveau des meilleures publiées:Sensibilité (3-5 µm) = 11950 V / WTemps de réponse t = 564 µsNEP optique = 4,0 10-12 W Hz-1/2Détectivité D* = 2,5 109 cm Hz-1/2 /

Uncooled bolometer response of a low noise La2/3Sr1/3MnO3 thin film

We report measurements of the optical responses of a La2/3Sr1/3MnO3 (LSMO) sample at a wavelength of 533 nm in the 300-400 K range. The 200 nm thick film was grown by pulsed laser deposition on (100) SrTiO3 substrate and showed remarkably low noise. At 335 K the temperature coefficient of the resistance of a 100 micrometers wide 300 micrometers long LSMO line was 0.017 K-1 and the normalized Hooge parameter was 9 e-30 m3, which is among the lowest reported values. We then measured an optical sensitivity at I = 5 mA of 10.4 V.W-1 and corresponding noise equivalent power (NEP) values of 8.1 e-10 W.Hz-1/2 and 3.3 e-10 W. Hz-1/2 at 30 Hz and above 1kHz, respectively. Simple considerations on bias current conditions and thermal conductance G are finally given for further sensitivity improvements using LSMO films. The performances were indeed demonstrated on bulk substrates with G of 10-3 W.K-1. One could expect a NEP reduction by three orders of magnitude if a membrane-type geometry was used, which makes this LSMO device competitive against commercially available uncooled bolometers.Comment: 15 pages. Accepted for publication in Appl. Phys. Let

Substrate-induced magnetic anisotropy in La0.7Sr 0.3MnO3 epitaxial thin films grown onto (110) and (118) SrTiO3 substrates

We show a detailed magneto-optical Kerr study at room temperature of well characterized epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto (110) and (118) SrTiO3 substrates. The films present a well-defined uniaxial (two-fold) magnetic anisotropy ascribed to substrate-induced anisotropy. In particular, the in-plane uniaxial anisotropy in the(110)-oriented LSMO films originates from the existence of elongated in-plane [001]-oriented structures. Similar elongated structures, parallel to the [110] crystallographic direction, are found for LSMO films grown on (118) STO surfaces. In all films, such a uniaxial magnetic anisotropy is characterized by an easy axis lying along the elongated structures. Furthermore, the vectorial-resolved hysteresis loops as a function of the in-plane applied field direction are interpreted in terms of rotation and propagation and nucleation of magnetic domains processes. Our results demonstrate the tailoring of magnetic anisotropy by exploiting the substrate-induced anisotropy in epitaxial thin filmsThis work was supported in part by the Spanish MICINN through Project No. CSD2007-00010 and by the Comunidad de Madrid through Project No. S2009/MAT-1726. P.P. thanks the European Science Foundation (ESF) through the activity entitled ’Thin Films for Novel Oxide Devices’ (http://www.ims.tnw.utwente.nl/thiox/) for partial financial support through exchange grant

Modulated optical reflectance measurements on La2/3Sr1/3MnO3 thin films

The modulated optical reflectance (MOR) measurement technique was applied to colossal magnetoresistive materials, in particular, La2/3Sr1/3MnO3 (LSMO) thin films. The contactless measurement scheme is prospective for many applications spanning from materials characterization to new devices like reading heads for magnetically recorded media. A contrasted room temperature surface scan of a 100 microns wide 400 microns long bridge patterned into LSMO film provided preliminary information about the film homogeneity. Then the temperature was varied between 240 and 400 K, i.e. through the ferromagnetic to paramagnetic transition. A clear relation between the MOR signal measured as function of the temperature and the relative derivative of the resistivity up to the Curie temperature was observed. This relationship is fundamental for the MOR technique and its mechanism was explored in the particular case of LSMO. Analysis in the framework of the Drude model showed that, within certain conditions, the measured MOR signal changes are correlated to changes in the charge carrier concentration.Comment: 29 pages, accepted for publication in J. Appl. Phy

La0.7Sr0.3MnO3 thin films on SrTiO3 and CaTiO3 buffered Si substrates: structural, static, and dynamic magnetic properties

International audienceNearly 50-nm thick La0.7Sr0.3MnO3 (LSMO) films were grown on Si substrates using molecular beam epitaxy on (001) Si substrates overlayered by a 20 nm thick SrTiO3 (STO) or by a 20 nm thick CaTiO3 (CTO) film. In addition, a reference LSMO film was directly deposited on a (001) STO substrate by pulsed laser deposition. For all the samples, X-ray diffraction revealed an excellent epitaxy of the LSMO film and small mosaicity around (001), with in-plane [100] and [010] cubic axes. The LSMO/CTO films are in-plane compressed while the LSMO/STO ones are in-plane extended. The temperature dependence of their static magnetic properties was studied using a SQUID, showing a Curie temperature overpassing 315 K for all the samples. Hysteresis loops performed at room temperature (294 K) with the help of a vibrating sample magnetometer (VSM) are also discussed. At 294 K Micro-strip ferromagnetic resonance (MS-FMR) was used to investigate the dynamic magnetic properties. It allows concluding to a strong anisotropy perpendicular to the films and to a weak fourfold in-plane anisotropy with easy axes along the [110] and [1 10] directions. Their values strongly depend on the studied sample and are presumably related to the strains suffered by the films

Evidence of charge carrier number fluctuations in InN thin films?

Due to its small band-gap and its high mobility, InN is a promising material for a large number of key applications like band-gap engineering for high efficiency solar cells, light emitting diodes, and high speed devices. Unfortunately, it has been reported that this material exhibits strong surface charge accumulation which may depend on the type of surface. Current investigations are conducted in order to explain the mechanisms which govern such a behavior and to look for ways of avoiding it and/or finding applications that may use such an effect. In this framework, low frequency noise measurements have been performed at different temperatures on patterned MBE grown InN layers. The evolution of the 1/f noise level with temperature in the 77 K-300 K range is consistent with carrier number fluctuations thus indicating surface mechanisms: the surface charge accumulation is confirmed by the noise measurements

Low-frequency noise considerations for sensors based on manganites

International audienceLow frequency noise considerations for sensors based on La0.33Sr0.67MnO3 (LSMO) thin films are discussed in this paper. Thanks to special attention on the film quality, onthe electrical readout electronics and on the patterned geometries, epitaxially grown LSMO thin films can show a very low level of low-frequency noise and can thus be used to fabricate high signal-to-noise ratio sensors such as uncooled bolometers and uncooled low-field magnetoresistances

High Curie temperature for La0.7Sr0.3MnO3 thin films deposited on CeO2/YSZ-based buffered silicon substrates

International audienceTwo kinds of epitaxial structures were grown by standard pulsed laser deposition on (001) Si, namely La0.7Sr0.3MnO3/Bi4Ti3O12/CeO2/YSZ/Si (BTO-based), and La0.7Sr0.3MnO3/SrTiO3/ CeO2 /YSZ/Si (STO-based) multilayers. The samples were investigated by means of x-ray diffraction, transmission electron microscopy, magnetic and transport measurements. The Curie temperature TC of the BTO-based samples was found to be higher (360 K) than for the typical reference epitaxial LSMO film grown on (001) SrTiO3 single crystal (345 K), due to high compressive in-plane strain. The STO-based samples show high structural quality, low roughness and high TC (350 K), making them interesting candidates for use in innovative LSMO-based bolometers or spintronic devices operating at room temperature. (Some figures in this article are in colour only in the electronic version

Influence of fabrication steps on optical and electrical properties of InN thin films

This paper reports on a case study of the impact of fabrication steps on InN material properties. We discuss the influence of annealing time and sequence of device processing steps. Photoluminescence (PL), surface morphology and electrical transport (electrical resistivity and low frequency noise) properties have been studied as responses to the adopted fabrication steps. Surface morphology has a strong correlation with annealing times, while sequences of fabrication steps do not appear to be influential. In contrast, the optical and electrical properties demonstrate correlation with both etching and thermal annealing. For all the studied samples PL peaks were in the vicinity of 0.7 eV, but the intensity and full width at half maximum (FWHM) demonstrate a dependence on the technological steps followed. Sheet resistance and electrical resistivity seem to be lower in the case of high defect introduction due to both etching and thermal treatments. The same effect is revealed through 1/f noise level measurements. A reduction of electrical resistivity is connected to an increase in 1/f noise level

La0.7Sr0.3MnO3 suspended microbridges for uncooled bolometers made using reactive ion etching of the silicon substrates

International audienceSuspended La0.7Sr0.3MnO3 (LSMO) microbridges were fabricated using standard silicon micromachining techniques. First epitaxial LSMO thin films were deposited on SrTiO3 (STO) buffered Si (001) substrates by molecular-beam epitaxy. A simple two photolithography step process using the reactive ion etching of the silicon substrate to release the suspended microbridges was developed. The electrical resistivity as a function of temperature of 4 lm wide 50-200 lm long and 75 nm thick LSMO/STO fully processed suspended microbridges was very close to the characteristics of the initial LSMO thin films, demonstrating that the fabrication process did not degrade the quality of the LSMO. The thermal conductance of the processed bolometers was very low (of the order of 10 7WK 1) at 300 K. These structures are promising for uncooled bolometer applications and other micro-electromechanical systems based on LSMO or other epitaxial functional oxides