Blandine Alloing

Single-Photon Detection System for Quantum Optics Applications

We describe the design and characterization of a fiber-coupled double-channel single-photon detection system based on superconducting single-photon detectors (SSPD), and its application for quantum optics experiments on semiconductor nanostructures. When operated at 2-K temperature, the system shows 10% quantum efficiency at 1.3-¿m wavelength with dark count rate below 10 counts per second and timing resolution <100 ps. The short recovery time and absence of afterpulsing leads to counting frequencies as high as 40 MHz. Moreover, the low dark count rate allows operation in continuous mode (without gating). These characteristics are very attractive-as compared to InGaAs avalanche photodiodes-for quantum optics experiments at telecommunication wavelengths. We demonstrate the use of the system in time-correlated fluorescence spectroscopy of quantum wells and in the measurement of the intensity correlation function of light emitted by semiconductor quantum dots at 1300 nm.

On the polarity of GaN micro- and nanowires epitaxially grown on sapphire (0001) and Si(111) substrates by metal organic vapor phase epitaxy and ammonia-molecular beam epitaxy

The polarity of GaN micro- and nanowires grown epitaxially by metal organic vapor phase epitaxy on sapphire substrates and by molecular-beam epitaxy, using ammonia as a nitrogen source, on sapphire and silicon substrates has been investigated. On Al2O3(0001), whatever the growth technique employed, the GaN wires show a mixture of Ga and N polarities. On Si(111), the wires grown by ammonia-molecular beam epitaxy are almost entirely Ga-polar (around 90%) and do not show inversion domains. These results can be understood in terms of the growth conditions employed during the nucleation stage.

GaN microwires as optical microcavities: whispering gallery modes Vs Fabry-Perot modes

GaN microwires grown by metalorganic vapour phase epitaxy and with radii typically on the order of 1-5 micrometers exhibit a number of resonances in their photoluminescence spectra. These resonances include whispering gallery modes and transverse Fabry-Perot modes. A detailed spectroscopic study by polarization-resolved microphotoluminescence, in combination with electron microscopy images, has enabled to differentiate both kinds of modes and determined their main spectral properties. Finally, the dispersion of the ordinary and extraordinary refractive indices of strain-free GaN in the visible-UV range has been obtained thanks to the numerical simulation of the observed modes.

Dislocation filtering and polarity in the selective area growth of GaN nanowires by continuous-flow metal organic vapor phase epitaxy

The early growth stages of GaN nanowires on GaN-on-sapphire templates with a patterned dielectric mask have been characterized by using transmission electron microscopy. The dielectric mask aperture (200–800 nm) determines the presence or absence of threading dislocations arising from the underlying template, which results in dislocation-free nanowires for small apertures and dislocation bending for larger apertures, owing to three-dimensional (3D) growth. The Ga polarity of the underlying template is conserved in all nanowires irrespective of the aperture size, even in regions grown laterally above the mask. The pure Ga polarity assures spatially homogeneous optical properties as evidenced by cathodoluminescence.

Selective Area Sublimation: A Simple Top-down Route for GaN-Based Nanowire Fabrication

Post-growth in situ partial SiNx masking of GaN-based epitaxial layers grown in a molecular beam epitaxy reactor is used to get GaN selective area sublimation (SAS) by high temperature annealing. Using this top-down approach, nanowires (NWs) with nanometer scale diameter are obtained from GaN and InxGa1-xN/GaN quantum well epitaxial structures. After GaN regrowth on InxGa1-xN/GaN NWs resulting from SAS, InxGa1-xN quantum disks (QDisks) with nanometer sizes in the three dimensions are formed. Low temperature microphotoluminescence experiments demonstrate QDisk multilines photon emission around 3 eV with individual line widths of 1-2 meV.

Fabrication and growth of GaN-based micro and nanostructures

GaN is considered as the material of choice for electromechanical, electronic and optoelectronic applications in the visible/UV regions of the electromagnetic spectrum. However, the lack of an adapted substrate results in a very large density of defects, in particular dislocations, stacking faults and cracks. Under these circumstances, the growth of nanostructures appears as an appealing means of circumventing the problem. In this paper, we describe the fabrication and characterisation of GaN–based nanostructures – cantilevers, microdiscs, photonic crystals, micro and nanowires – grown by metallorganic vapour phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) and fabricated either by a bottom–up or a top–down approach. The applications envisaged at CRHEA will be examined.

Selective area sublimation of GaN for top-down fabrication of nanostructures (Conference Presentation)

A fraction of a SiNx mono-layer is formed on a GaN layer by exposing the surface to a Si flux. When the sample is heated under vacuum at high temperature (900°C), we observe the sublimation of GaN in the regions uncovered by the thermally resistant SiNx mask. This selective area sublimation (SAS) process can be used for the formation of nanopyramids and nanowires with a diameter down to 4 nm. Also, if InGaN quantum wells are included in the structures before sublimation, InGaN quantum disks with quasi identical sizes in the 3 dimensions of space can be formed using SAS.