Laurence Méchin

Low-noise La0.7Sr0.3MnO3 thermometers for uncooled bolometric applications

We report measurements of the temperature coefficient of the resistance (TCR) and the low-frequency noise of epitaxial La0.7Sr0.3MnO3 (LSMO) thin films deposited on SrTiO3 (STO) and (LaAlO3)0.3(Sr2AlTaO6)0.7 substrates. An x-ray-diffraction study showed that the films were (001) oriented. A normalized Hooge parameter of 9×10−31m3 was measured at 300K in the case of a 10‐μm-wide, 575‐μm-long line patterned in the 200‐nm-thick film grown on STO substrate. This value is among the lowest reported values for manganites and close to values measured in standard metals and semiconductors. The corresponding noise equivalent temperature (NET) was constant in the 300–340K range and equal to 6×10−7KHz−1∕2 at 10Hz and 150μA for a 10‐μm-wide, 575‐μm-long line patterned in a 200‐nm-thick LSMO film. This very low NET value is comparable to the best published results for manganites and was even found to be lower than the NET of other uncooled thermometers such as amorphous semiconductors, vanadium oxides, or semiconductin...

Correlation between structure and properties in multiferroic La0.7Ca0.3MnO3∕BaTiO3 superlattices

Superlattices composed of ferromagnetics, namely La0.7Ca0.3MnO3 (LCMO), and ferroelectrics, namely, BaTiO3 (BTO), were grown on SrTiO3 at 720 °C by a pulsed laser deposition process. While the out-of-plane lattice parameters of the superlattices, as extracted from the x-ray diffraction studies, were found to be dependent on the BTO layer thickness, the in-plane lattice parameter is almost constant. The evolution of the strains, their nature, and their distribution in the samples were examined by the conventional sin2ψ method. The effects of structural variation on the physical properties, as well as the possible role of the strain on inducing the multiferroism in the superlattices, have also been discussed.

Investigation of laser-ablated ZnO thin films grown with Zn metal target: A structural study

High quality ZnO thin films were grown using the pulsed laser deposition technique on (0001) Al2O3 substrates in an oxidizing atmosphere, using a Zn metallic target. We varied the growth conditions such as the deposition temperature and the oxygen pressure. First, using a battery of techniques such as x-ray diffraction, Rutherford-backscattering spectroscopy, and atomic force microscopy, we evaluated the structural quality, the stress, and the degree of epitaxy of the films. Second, the relations between the deposition conditions and the structural properties that are directly related to the nature of the thin films are discussed qualitatively. Finally, a number of issues on how to get good-quality ZnO films are addressed.

Influence of pulsed laser deposition growth conditions on the thermoelectric properties of Ca3Co4O9 thin films

Thin films of the misfit cobaltite Ca3Co4O9 were grown on (0001)-oriented (c cut) sapphire substrates, using pulsed-laser deposition techniques. The dependence of the thermoelectric/transport properties on the film growth conditions was investigated. The optimal conditions (for low resistivities) were found to be 600°C, 0.1–0.2mbar of oxygen pressure, and 1.7J∕cm2. These films exhibited slightly metallic behavior, consistent with in-plane resistivity curves of single crystals and c-axis magnetically aligned samples. Hall effect measurements showed the density of the holelike carriers was 6.8×1020∕cm3. The in-plane epitaxial relationship between the thin film and the sapphire substrate is investigated.

Suspended epitaxial YBaCuO microbolometers fabricated by silicon micromachining: Modeling and measurements

Suspended epitaxial YBaCuO microbolometers were successfully fabricated by two silicon micromachining techniques. The first one used the reactive ion etching (RIE) of Si substrates and the second one the etching of the SiO2 layer in separated by implanted oxygen (SIMOX) substrates. This work aims at the modeling and the measurement of the bolometric performances of IR pixels (100×100 μm2 detection area) constituted by suspended bridges in series. The influence of both the dimensions and the thermophysical properties of the materials constituting the membrane is discussed. Thermal conductances and time constants were measured as functions of the length and the width of different suspended bridges fabricated by RIE. Comparison of a “RIE type” bridge with a bridge of same dimensions fabricated from a SIMOX substrate shows that the sensitivity-bandwidth product of the SIMOX bridge is improved by one order of magnitude. All measurements on suspended bridges are consistent with calculations from thermal model. ...

Effect of ferroelectric layers on the magnetocapacitance properties of superlattices-based oxide multiferroics

A series of superlattices composed of ferromagnetic La0.7Ca0.3MnO3 (LCMO) and ferroelectric/paraelectric Ba1−xSrxTiO3 (0⩽x⩽1) were deposited on SrTiO3 substrates using pulsed laser deposition. Magnetotransport properties of the films reveal a ferromagnetic Curie temperature in the range of 145–158K, and negative magnetoresistance as high as 30%, depending on the type of ferroelectric layers employed for their growth (i.e., “x” value). Ferroelectricity at temperatures ranging from 55Kto105K is also observed, depending on the barium content. More importantly, the multiferroic nature of the film is determined by the appearance of negative magnetocapacitance, which is maximum around the ferroelectric transition temperature (3% per tesla). These results are understood based on the role of the ferroelectric/paraelectric layers and strains in inducing the multiferroism.

Epitaxial Growth of Sputtered Ultra-Thin NbN Layers and Junctions on Sapphire

High crystalline quality of ultra-thin NbN layers and of NbN-MgO-NbN tri-layers, epitaxially grown by DC-magnetron sputtering in the superconducting B1-cubic phase has been achieved in a reproducible way on three different orientations of sapphire substrates i.e. R-, A- and M-planes. Significant improvements such as higher Tc, higher Jc and lower resistivity have been obtained by growing un-twined (110) oriented NbN layers on M-plane orientation of sapphire. Uniform, low roughness, 3-5 nm thick films with Tc above 12 K and Jc above 5 MA/cm2 at 4.2 K were obtained. Characterizations by TEM, AFM and X-Ray diffraction evidence that growth of un-twined NbN on M-plane lead to a better epitaxy in comparison with twinned films observed on other sapphire orientations. We observe that the reduction of the substrate temperature from 600degC to 300degC during the deposition of NbN or NbN-MgO-NbN layers thicker than 20 nm prevents the nucleation of the competing HCP NbN phase. Moreover, 1.5 nm thick AlN or MgO over-layers sputtered in-situ prevent ultra-thin NbN films degradation through aging. The formation of Nb2NyO5-x ( ~ 2.2 nm) at the unprotected NbN surface and of interfacial NbO ( ~ 0.7 nm) native oxides has been observed by XPS. It is forecasted that such improvements in ultra-thin NbN films deposited uniformly on 3 and 4 inch sapphire wafers is a key in the future development of superconducting single photon detectors, THz HEB mixers and also in low noise quantum analogical and digital Josephson devices.

Resistive hystersis effects in perovskite oxide-based heterostructure junctions

The electrical and structural properties of the oxide-based metal/ferroelectric/metal (MFM) junctions are reported. The heterostructures are composed of ultrathin layers of La0.7Ca0.3MnO3 (LCMO) as a metallic layer and BaTiO3 (BTO) as a ferroelectric layer. Junction based devices, having the dimensions of 400×200μm2, have been fabricated upon LCMO/BTO/LCMO heterostructures by photolithography and Ar-ion milling technique. The dc current-voltage (I-V) characteristics of the MFM junctions were carried out. At 300K, the devices showed the linear (I-V) characteristics, whereas at 77K, (I-V) curves exhibited some reproducible switching behaviors with well-defined remnant currents. The resulting resistance modulation is very different from what was already reported in ultrathin ferroelectric layers displaying resistive switching. A model is presented to explain the data.

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. At 335 K the temperature coefficient of the resistance of a 100-μm-wide, 300-μm-long LSMO line was 0.017K−1 and the normalized Hooge parameter was 9×10−30m3, which is among the lowest reported values. We then measured an optical sensitivity at I=5mA of 10.4VW−1 and corresponding noise equivalent power (NEP) values of 8.1×10−10WHz−1∕2 and 3.3×10−10WHz−1∕2 at 30 Hz and above 1 kHz, respectively. Simple considerations on bias current conditions and thermal conductance G are finally given for further sensitivity improvements using LSMO films.

Tailoring magnetic anisotropy in epitaxial half metallic La0.7Sr0.3MnO3 thin films

We present a detailed study on the magnetic properties, including anisotropy, reversal fields, and magnetization reversal processes, of well characterized half-metallic epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto SrTiO3 (STO) substrates with three different surface orientations, i.e., (001), (110), and (11−8). The latter shows step edges oriented parallel to the [110] (in-plane) crystallographic direction. Room temperature high resolution vectorial Kerr magnetometry measurements have been performed at different applied magnetic field directions in the whole angular range. In general, the magnetic properties of the LSMO films can be interpreted with just the uniaxial term, with the anisotropy axis given by the film morphology, whereas the strength of this anisotropy depends on both structure and film thickness. In particular, LSMO films grown on nominally flat (110)-oriented STO substrates presents a well defined uniaxial anisotropy originated from the existence of elongated in-plane [001]-orient...