1D Nanostructure-Based Piezo-Generators

International audienceWith the amount of connected objects constantly on the rise, both in our daily life and in high-technology applications, it becomes critical to deal with their associated increase in energy consumption. Their energetic autonomy is currently a key worldwide challenge with strong economic and environmental benefits. The recent miniaturization of electronic micro-devices have resulted in the reduction of energy consumption to mW and even µW, combined with the progress in micro-nano-fabrication, and have opened, in these last years, new perspectives to develop autonomous power systems based on the renewable energy harvesting

GaN/Ga2O3 Core/Shell Nanowires Growth: Towards High Response Gas Sensors

International audienceThe development of sensors working in a large range of temperature is of crucial importance in areas such as monitoring of industrial processes or personal tracking using smart objects. Devices integrating GaN/Ga2O3 core/shell nanowires (NWs) are a promising solution for monitoring carbon monoxide (CO). Because the performances of sensors primarily depend on the material properties composing the active layer of the device, it is essential to control them and achieve material synthesis in the first time. In this work, we investigate the synthesis of GaN/Ga2O3 core-shell NWs with a special focus on the formation of the shell. The GaN NWs grown by plasma-assisted molecular beam epitaxy, are post-treated following thermal oxidation to form a Ga2O3-shell surrounding the GaN-core. We establish that the shell thickness can be modulated from 1 to 14 nm by changing the oxidation conditions and follows classical oxidation process: A first rapid oxide-shell growth, followed by a reduced but continuous oxide growth. We also discuss the impact of the atmosphere on the oxidation growth rate. By combining XRD-STEM and EDX analyses, we demonstrate that the oxide-shell is crystalline, presents the β-Ga2O3 phase, and is synthesized in an epitaxial relationship with the GaN-core

On the mechanisms of spontaneous growth of III-nitride nanocolumns by plasma-assisted molecular beam epitaxy

A study of the GaN nanocolumns nucleation and growth by molecular beam epitaxy on Si(1 1 1) is presented. Ga droplets with different diameters (340–90 nm) were deposited on the substrate, prior to growth, to determine any effect on the nanocolumns size and distribution. Results indicate that there is no difference in nanocolumnar size and density whether Ga droplets are used or not, meaning that Ga droplets do not act as catalysts for the nanocolumns nucleation. In addition, Ga droplets were never observed on the nanocolumn tips upon growth termination. These findings rule out the vapor–liquid–solid mechanism. Instead, driven by a strong lattice mismatch nanocolumnar nucleation occurs spontaneously by Volmer–Weber growth mechanism, whereas nitrogen excess prevents the nucleation sites coalescence. Further nanocolumnar growth proceeds by direct Ga incorporation on the nanocolumns top and by Ga diffusion along the nanocolumns sidewalls up to their apex. Related to this diffusion mechanism, we found that Ga droplets, when used, may act as reservoirs to feed Ga atoms to the neighboring nanocolumns. Nanocolumns preserve a constant diameter if growth conditions are not modified because of a strong metal ad-atom diffusion length along their sidewalls. The effect of using AlN buffer layers on the nanocolumnar growth and morphology is also addressed

Nonlinear optics and saturation behavior of quantum dot samples under continuous wave driving

The nonlinear optical response of self-assembled quantum dots is relevant to the application of quantum dot based devices in nonlinear optics, all-optical switching, slow light and self-organization. Theoretical investigations are based on numerical simulations of a spatially and spectrally resolved rate equation model, which takes into account the strong coupling of the quantum dots to the carrier reservoir created by the wetting layer states. The complex dielectric susceptibility of the ground state is obtained. The saturation is shown to follow a behavior in between the one for a dominantly homogeneously and inhomogeneously broadened medium. Approaches to extract the nonlinear refractive index change by fringe shifts in a cavity or self-lensing are discussed. Experimental work on saturation characteristic of InGa/GaAs quantum dots close to the telecommunication O-band (1.24-1.28 mm) and of InAlAs/GaAlAs quantum dots at 780 nm is described and the first demonstration of the cw saturation of absorption in room temperature quantum dot samples is discussed in detail

1D Nanostructure-Based Piezo-Generators

With the amount of connected objects constantly on the rise, both in our daily life and in high-technology applications, it becomes critical to deal with their associated increase in energy consumption [...

Influence of thermal annealing on the structural and optical properties of GaN/AlN quantum dots

The influence of the post-growth thermal annealing on the structural and optical properties of GaN/AlN quantum dots (QDs) is reported. X-ray techniques suggest smooth and high quality interfaces of the stacked multilayer structures for the as-grown and annealed samples without any period thickness change. High-angle annular dark field images by scanning transmission electron microscopy show an intermixing between the GaN QDs and AlN spacers after annealing. The QDs recombination shifts to lower energies (red shift) for big dots and to higher energies (blue shift) for small dots, reflecting two competitive processes taking place during the thermal annealingFCT-PTDC/FIS/66262/2006FCT-PTDC/CTM/ 100756/2008FCT-SFRH/BD/45774/2008FCT-SFRH/BD/44635/2008Ciência 200