Thomas Maroutian

Wavelength dependence of Pockels effect in strained silicon waveguides

We investigate the influence of the wavelength, within the 1.3μm-1.63μm range, on the second-order optical nonlinearity in silicon waveguides strained by a silicon nitride (Si₃N ₄) overlayer. The effective second-order optical susceptibility χxxy(2)¯ evolutions have been determined for 3 different waveguide widths 385 nm, 435 nm and 465 nm and it showed higher values for longer wavelengths and narrower waveguides. For wWG = 385 nm and λ = 1630 nm, we demonstrated χxxy(2)¯ as high as 336 ± 30 pm/V. An explanation based on the strain distribution within the waveguide and its overlap with optical mode is then given to justify the obtained results.

Low electrical resistivity in thin and ultrathin copper layers grown by high power impulse magnetron sputtering

Ultrathin copper (Cu) layers are in continuous demand in several areas, such as within microelectronics and space, as well as in instrumentation technology requiring an electrical resistivity as low as possible. However, the performance of modern copper connections is limited by the size-dependent value of the film resistivity, which is known to increase when the layer thickness is reduced to a few tens of nanometer. In this work, the authors have successfully deposited Cu thin films from 20 to 800 nm exhibiting reduced electrical resistivity by using a high power impulse magnetron sputtering (HiPIMS) process. The electrical and microstructural properties of such films were compared to samples deposited by conventional direct current magnetron sputtering (DCMS) within the same thickness range. For films as thin as 30 nm, the electrical resistivity was reduced by ∼30% when deposited by HiPIMS compared to DCMS, being only three times larger than the copper bulk value. The HiPIMS Cu films exhibit larger grai...

Memristive and neuromorphic behavior in a Li x CoO 2 nanobattery

The phenomenon of resistive switching (RS), which was initially linked to non-volatile resistive memory applications, has recently also been associated with the concept of memristors, whose adjustable multilevel resistance characteristics open up unforeseen perspectives in cognitive computing. Herein, we demonstrate that the resistance states of LixCoO2 thin film-based metal-insulator-metal (MIM) solid-state cells can be tuned by sequential programming voltage pulses, and that these resistance states are dramatically dependent on the pulses input rate, hence emulating biological synapse plasticity. In addition, we identify the underlying electrochemical processes of RS in our MIM cells, which also reveal a nanobattery-like behavior, leading to the generation of electrical signals that bring an unprecedented new dimension to the connection between memristors and neuromorphic systems. Therefore, these LixCoO2-based MIM devices allow for a combination of possibilities, offering new perspectives of usage in nanoelectronics and bio-inspired neuromorphic circuits.

Spin electronic magnetic sensor based on functional oxides for medical imaging

To detect magnetic signals coming from the body, in particular those produced by the electrical activity of the heart or of the brain, the development of ultrasensitive sensors is required. In this regard, magnetoresistive sensors, stemming from spin electronics, are very promising devices. For example, tunnel magnetoresistance (TMR) junctions based on MgO tunnel barrier have a high sensitivity. Nevertheless, TMR also often have high level of noise. Full spin polarized materials like manganite La0.67Sr0.33MnO3 (LSMO) are attractive alternative candidates to develop such sensors because LSMO exhibits a very low 1/f noise when grown on single crystals, and a TMR response has been observed with values up to 2000%. This kind of tunnel junctions, when combined with a high Tc superconductor loop, opens up possibilities to develop full oxide structures working at liquid nitrogen temperature and suitable for medical imaging. In this work, we investigated on LSMO-based tunnel junctions the parameters controlling the overall system performances, including not only the TMR ratio, but also the pinning of the reference layer and the noise floor. We especially focused on studying the effects of the quality of the barrier, the interface and the electrode, by playing with materials and growth conditions.

Development of a microwave capacitive method for the spectroscopy of the complex permittivity

We describe a vector network analyzer-based method to study the electromagnetic properties of nanoscale dielectrics at microwave frequencies (1 MHz–40 GHz). The complex permittivity spectrum of a given dielectric can be determined by placing it in a capacitor accessed on its both electrodes by coplanar waveguides. However, inherent propagation delays along the signal paths together with frequency-dependent effective surface of the capacitor at microwave frequencies can lead to significant distortion in the measured permittivity, which in turn can give rise to artificial frequency variations of the complex permittivity. We detail a fully analytical rigorous correction sequence with neither recourse to extrinsic loss mechanisms nor to arbitrary parasitic signal paths. We illustrate our method on 3 emblematic dielectrics: ferroelectric morphotropic lead zirconate titanate, its paraelectric pyrochlore counterpart, and strontium titanate. Permittivity spectra taken at various points along the hysteresis loop help shedding light onto the nature of the different dielectric energy loss mechanisms. Thanks to the analytical character of our method, we can discuss routes to extend it to higher frequencies and we can identify unambiguously the sources of potential artifacts.

Strain induced by functional oxides for silicon photonics applications

The purpose of this work is to explore an alternative approach for high speed and low power consumption optical modulation based on the use of the Pockels effect in silicon. Unfortunately, silicon is a centro-symmetric crystal leading to a vanishing of the second order nonlinear coefficient, i.e. no Pockels effect. To overcome this limitation, on possibility would be to break the crystal symmetry by straining the silicon lattice with the epitaxial growth of crystalline functional oxides. Indeed, the induced strain due to lattice parameter mismatch and the difference in the thermal expansion coefficients between oxides and silicon are strong and may induce strong strain into silicon. Furthermore, functional oxides can exhibit very interesting multiferroicity and piezoelectricity properties that pave the way to a new route to implement silicon photonic circuits with unprecedented functionalities.

Low noise all-oxide magnetic tunnel junctions based on a La0.7Sr0.3MnO3/Nb:SrTiO3 interface

All-oxide magnetic tunnel junctions with a semiconducting barrier, formed by the half-metallic ferromagnet La0.7Sr0.3MnO3 and n-type semiconductor SrTi0.8Nb0.2O3, were designed, fabricated, and investigated in terms of their magneto-transport properties as a function of applied bias and temperature. We found that the use of the heavily Nb-doped SrTiO3 as a barrier results in significant improvement in the reproducibility of results, i.e., of large tunnel magnetoresistance (TMR) ratios, and a spectral noise density reduced by three orders of magnitude at low temperature. We attribute this finding to a considerably decreased amount of point defects in SrTi0.8Nb0.2O3, especially oxygen vacancies, compared with the conventional insulating SrTiO3 barrier.

Temperature Study of the Antiferromagnetic Coupling at the Interface of La

We have studied the antiferromagnetic coupling at the interface of the La0.7 Sr 0.3MnO3/SrRuO3 bilayer as function of temperature. We measured the La0.7 Sr 0.3MnO3 magnetization reversal while the SrRuO 3 magnetization is blocked due to a field cooled procedure and because of its high coercive field. Even if in this system, the La 0.7Sr0.3MnO3 magnetization cycle may be explained by two exchange couplings which coexist up to 120 K, the exchange bias and the coercive field from 5 K to 300 K have a similar behavior from some ferromagnetic/antiferromagnetic systems.