Y. Genuist

Ultrafast Room Temperature Single-Photon Source from Nanowire-Quantum Dots

Epitaxial semiconductor quantum dots are particularly promising as realistic single-photon sources for their compatibility with manufacturing techniques and possibility to be implemented in compact devices. Here, we demonstrate for the first time single-photon emission up to room temperature from an epitaxial quantum dot inserted in a nanowire, namely a CdSe slice in a ZnSe nanowire. The exciton and biexciton lines can still be resolved at room temperature and the biexciton turns out to be the most appropriate transition for single-photon emission due to a large nonradiative decay of the bright exciton to dark exciton states. With an intrinsically short radiative decay time (≈300 ps) this system is the fastest room temperature single-photon emitter, allowing potentially gigahertz repetition rates.

Optical properties of single ZnTe nanowires grown at low temperature

Optically active gold-catalyzed ZnTe nanowires have been grown by molecular beam epitaxy, on a ZnTe(111) buffer layer, at low temperature 350°C under Te rich conditions, and at ultra-low density (from 1 to 5 nanowires per micrometer²). The crystalline structure is zinc blende as identified by transmission electron microscopy. All nanowires are tapered and the majority of them are oriented. Low temperature micro-photoluminescence and cathodoluminescence experiments have been performed on single nanowires. We observe a narrow emission line with a blue-shift of 2 or 3 meV with respect to the exciton energy in bulk ZnTe. This shift is attributed to the strain induced by a 5 nm-thick oxide layer covering the nanowires, and this assumption is supported by a quantitative estimation of the strain in the nanowires.

Quantitative Reconstructions of 3D Chemical Nanostructures in Nanowires

Energy dispersive X-ray spectrometry is used to extract a quantitative 3D composition profile of heterostructured nanowires. The analysis of hypermaps recorded along a limited number of projections, with a preliminary calibration of the signal associated with each element, is compared to the intensity profiles calculated for a model structure with successive shells of circular, elliptic, or faceted cross sections. This discrete tomographic technique is applied to II-VI nanowires grown by molecular beam epitaxy, incorporating ZnTe and CdTe and their alloys with Mn and Mg, with typical size down to a few nanometers and Mn or Mg content as low as 10%.

Diffusion-driven growth of nanowires by low-temperature molecular beam epitaxy

With ZnTe as an example, we use two different methods to unravel the characteristics of the growth of nanowires (NWs) by gold-catalyzed molecular beam epitaxy at low temperature. In the first approach, CdTe insertions have been used as markers, and the nanowires have been characterized by scanning transmission electron microscopy, including geometrical phase analysis and energy dispersive electron spectrometry; the second approach uses scanning electron microscopy and the statistics of the relationship between the length of the tapered nanowires and their base diameter. Axial and radial growth are quantified using a diffusion-limited model adapted to the growth conditions; analytical expressions describe well the relationship between the NW length and the total molecular flux (taking into account the orientation of the effusion cells), and the catalyst-nanowire contact area. A long incubation time is observed. This analysis allows us to assess the evolution of the diffusion lengths on the substrate and along the nanowire sidewalls, as a function of temperature and deviation from stoichiometric flux.

Nanowire growth and sublimation: CdTe quantum dots in ZnTe nanowires

The role of the sublimation of the compound and of the evaporation of the constituents from the gold nanoparticle during the growth of semiconductor nanowires is exemplified with CdTe-ZnTe heterostructures. Operating close to the upper temperature limit strongly reduces the amount of Cd present in the gold nanoparticle and the density of adatoms on the nanowire sidewalls. As a result, the growth rate is small and strongly temperature dependent, but a good control of the growth conditions allows the incorporation of quantum dots in nanowires with sharp interfaces and adjustable shape, and it minimizes the radial growth and the subsequent formation of additional CdTe clusters on the nanowire sidewalls, as confirmed by photoluminescence. Uncapped CdTe segments dissolve into the gold nanoparticle when interrupting the flux, giving rise to a bulb-like (pendant-droplet) shape attributed to the Kirkendall effect.

Control of the incubation time in the vapor-solid-solid growth of semiconductor nanowires

Nanowires grown in the vapor-solid-solid mode using solid gold nanoparticles as a catalyst may exhibit a strong fluctuation of their length mostly due to the presence of an incubation time with a large distribution. We show that this is efficiently cured by an appropriate preparation of the catalyst nanoparticle—in the case of ZnTe nanowires by adding a Zn flux during the dewetting process. While nanowires start at any time after dewetting in vacuum (resulting in a broad length distribution, up to a factor of 10), the incubation time is quite uniform after dewetting under Zn exposure. Residual fluctuations (reduced to below a factor of 2) are due to fluctuations of the nanoparticle size and to a change of the nanoparticle morphology during the growth.

Self-catalyzed growth of highly vertical GaAs core-shell nanowires on chemically-treated Si(111) surfaces

The reproducibility of self-catalyzed molecular beam epitaxy growth of GaAs nanowires (NWs) on oxide-covered Si(111) substrates depends on the consistent quality of the oxide layer. We have developed an effective chemical treatment method to create oxide surfaces that are conducive to the nucleation of vertical GaAs NWs on Si(111). Here we demonstrate a high yield, exceeding 90%, of vertical GaAs NWs with density in the order of 109 cm-2. Using a combination of chemical oxide treatment and in situ growth conditions, we investigate how to achieve a good control of the morphology and density of GaAs and GaAs/III-As core-shell NWs grown on Si.