Etienne Puyoo

High Performance ZnO-SnO2:F Nanocomposite Transparent Electrodes for Energy Applications

Enhancing the propagation length of light without sacrificing the electro-optical properties of transparent electrodes is of particular interest to solar cells for reaching higher efficiency. This can typically be achieved by nanostructured electrodes but all too often at the expense of complexity and cost-effectiveness. In this work, we demonstrate the simple and low-cost fabrication of a new type of ZnO-SnO2:F nanocomposite thin film by combining spin-coated ZnO nanoparticles on glass with fluorine-doped SnO2 thin films deposited by atmospheric spray pyrolysis. The resulting nanocomposites exhibit a dual surface morphology featuring rough ZnO-SnO2:F nanostructures along with the original smooth SnO2:F thin film. By readily modulating the surface morphology of ZnO-SnO2:F nanocomposite thin films with the initial ZnO NP surface coverage, the scattering efficiency of the incident light can remarkably be controlled over the 400-1100 nm solar spectrum wavelength range. High quality hazy ZnO-SnO2:F thin layers are therefore formed with an averaged haze factor ranging from 0.4 to 64.2% over the 400-1100 nm solar spectrum range while the sheet resistance is kept smaller than 15 Ω/sq for an average total optical transmittance close to 80%, substrate absorption and reflection included. Eventually, optical simulations using Fourier transform techniques are performed for computing the obtained haze factors and show good agreement with experimental data in the 400-1100 nm solar spectrum wavelength range. This opens up additional opportunities for further design optimization of nanoengineered transparent electrodes.

Tunnel Junction Engineering for Optimized Metallic Single-Electron Transistor

The development of metallic single-electron transistor (SET) depends on the downscaling and the electrical properties of its tunnel junctions (TJs). These TJs should insure high-ON current, low-OFF current, and low capacitance. We propose an engineered TJ based on multidielectric stacking. A number of high-k and low-k materials were considered to optimize the TJs characteristics. The optimized TJ is proven to increase the ION current and the ION/IOFF ratio in a double-gate SET. Using TiO2 plasma oxidation and Al2O3 atomic layer deposition, an SET proof of concept, with a double layer TJ, was fabricated and characterized.

Metallic nanoparticle-based strain sensors elaborated by atomic layer deposition

Platinum nanoparticle-based strain gauges are elaborated by means of atomic layer deposition on flexible polyimide substrates. Their electro-mechanical response is tested under mechanical bending in both buckling and conformational contact configurations. A maximum gauge factor of 70 is reached at a strain level of 0.5%. Although the exponential dependence of the gauge resistance on strain is attributed to the tunneling effect, it is shown that the majority of the junctions between adjacent Pt nanoparticles are in a short circuit state. Finally, we demonstrate the feasibility of an all-plastic pressure sensor integrating Pt nanoparticle-based strain gauges in a Wheatstone bridge configuration.

Effect of nanostructuration on the thermal conductivity of thermoelectric materials

We have investigated various kinds of nanowires (Si, Bi2Te3, SiGe) in order to evaluate the influence of the nanostructuration on their thermal conductivity. The method used is a 3ω-SThM (Scanning Thermal Microscopy) technique which enables to simultaneously measure the topography and the thermal conductivity on an assembly of NWs. We detail the procedure from the measurement itself to the nanowire thermal conductivity estimation. We show that the nanostructuration leads to a thermal conductivity reduction for the 3 materials we have studied and that Si and SiGe nanowire samples seem more promising than Bi2Te3 NWs in terms of thermoelectric applications.

Development of ultrasensitive extended-gate Ion-sensitive-field-effect-transistor based on industrial UTBB FDSOI transistor

The proof of concept of a new extended-gate pH sensor, developed on an industrial ultrathin body and buried oxide (UTBB) fully-depleted silicon-on-insulator (FDSOI) transistor, is reported. The strong electrostatic coupling between the front gate and back gate of UTBB FDSOI devices provide a signal amplification opportunity for sensing applications. On the other hand, the biasing capability through a capacitive divider circuit of a floating gate ISFET offers an ample advantage for fabrication of stable and CMOS compatible solid state chemical sensors. In addition, the deep downscaling of the state-of-the-art devices enables it to be sensitive at single-charge-resolution. By integrating aluminum oxide (Al2O3) for the pH sensing purpose, we obtained an extended-gate mode ISFET having a sensitivity of 475 mV/pH, which is superior to state-of-the-art low-power ISFETs.

Investigation of the in-plane and out-of-plane electrical properties of metallic nanoparticles in dielectric matrix thin films elaborated by atomic layer deposition

Pt nanoparticles in a Al2O3 dielectric matrix thin films are elaborated by means of atomic layer deposition. These nanostructured thin films are integrated in vertical and planar test structures in order to assess both their in-plane and out-of-plane electrical properties. A shadow edge evaporation process is used to develop planar devices with electrode separation distances in the range of 30nm. Both vertical and planar test structures show a Poole-Frenkel conduction mechanism. Low trap energy levels (<0.1eV) are identified for the two test structures which indicates that the Pt islands themselves are not acting as traps in the PF mechanism. Furthermore, a more than three order of magnitude current density difference is observed between the two geometries. This electrical anisotropy is attributed to a large electron mobility difference in the in-plane and out-of-plane directions which can be related to different trap distributions in both directions.Pt nanoparticles in a Al2O3 dielectric matrix thin films are elaborated by means of atomic layer deposition. These nanostructured thin films are integrated in vertical and planar test structures in order to assess both their in-plane and out-of-plane electrical properties. A shadow edge evaporation process is used to develop planar devices with electrode separation distances in the range of 30 nm. Both vertical and planar test structures show a Poole-Frenkel conduction mechanism. Low trap energy levels (<0.1 eV) are identified for the two test structures which indicates that the Pt islands themselves are not acting as traps in the PF mechanism. Furthermore, a more than three order of magnitude current density difference is observed between the two geometries. This electrical anisotropy is attributed to a large electron mobility difference in the in-plane and out-of-plane directions which can be related to different trap distributions in both directions.

Piezo-tunnel effect in Al/Al2O3/Al junctions elaborated by atomic layer deposition

In this work, the electrical transport in Al/Al2O3/Al junctions under mechanical stress is investigated in the perspective to use them as strain sensors. The metal/insulator/metal junctions are elaborated with a low temperature process (≤200 °C) fully compatible with CMOS back-end-of-line. The conduction mechanism in the structure is found to be Fowler-Nordheim tunneling, and efforts are made to extract the relevant physical parameters. Gauge factors up to −32.5 were found in the fabricated devices under tensile stress. Finally, theoretical mechanical considerations give strong evidence that strain sensitivity in Al/Al2O3/Al structures originates not only from geometrical deformations but also from the variation of interface barrier height and/or effective electronic mass in the tunneling oxide layer.

Study of piezo-tunnel effect in Metal/A 2 O 3 /metal junctions

In this work, the electrical transport in Al/A₂O₃/Pt and Al/A₂O₃/Al junctions under mechanical stress was investigated. The junctions were fabricated by evaporation for the metals (with shadow mask lithography) and Atomic Layer Deposition (ALD) for the A₂O₃. First, IV characteristics were extracted from the unstressed sample and these curves were fitted with the Fowler-Nordheim current expression to identify the conduction mechanism. Then, gauges were tested under mechanical strain using the beam deflection method. Gauge factors of -34 for Al/A₂O₃/Al and -70 for Al/A₂O₃/Pt junctions were measured. Finally, the geometrical gauge factor is discussed and a possible effect of the variation of the Metal/A₂O₃ interfaces barrier height under stress is suggested as a plausible cause for the difference in gauge factor observed between the two junction types.

Study and characterization of the irreversible transformation of electrically stressed planar Ti/TiOx/Ti junctions

We investigate the properties and characteristics of planar Ti/TiOx/Ti junctions, which consist of transverse TiOx lines drawn on Ti test patterns. Junctions are elaborated by means of local anodic oxidation using atomic force microscopy. An irreversible morphological transformation occurring in a reproducible manner is observed when these planar junctions are electrically stressed under ambient atmosphere. Structural and chemical analyses based on transmission electron microscopy techniques reveal the extension of the initial amorphous TiOx into a crystalline rutile phase. This irreversible transformation is proven to vanish completely if the electrical stress occurs under vacuum atmosphere. Finally, we carry out temperature dependent electrical measurements in order to elucidate their conduction mechanism: Schottky emission above an ultra-low potential barrier is assumed to dominate under vacuum atmosphere whereas ionic conduction seems to prevail in air.