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F. Semond

Publications and source records attributed to F. Semond.

15 recordsLinked to original sources
arXiv2026

Enhanced Superconducting Nanowire Single Photon Detector Performances using Silicon Capping

Niobium Titanium nitride (NbTiN) based superconducting nanowire single photon detectors (SNSPDs) are known for their high performance across a wide spectral range, from the X-ray to the mid-infrared. Nonetheless, fabrication challenges and performance degradation attributable to surface oxidation and lack of uniformity in films thinner than 5 nm remain a significant barrier for achieving high-quality detectors. In this work, we study the influence of a Silicon capping layer on film properties and on the performance of SNSPDs. A Silicon capping layer effectively suppresses oxidation and increases the superconducting transition temperature. This enables superconductivity in films as thin as 3 nm at 3 K, increases critical current in patterned nanowires and significantly extends the saturation plateau from the visible to the near infrared (up to 2050 nm): These detectors maintain sub-50 ps timing jitter, even for nanowires as wide as 250 nm and with detection areas of 20x20{\mu}m2. Our results establish that thinner films protected by a capping layer allow for the fabrication of wider wires, decreasing nanofabrication challenges and extending the operating temperature range for efficient single photon detection.

quant-ph↗
arXiv2020

Strong coupling of exciton-polaritons in a bulk GaN planar waveguide: quantifiying the coupling strength

We investigate the demonstration and quantification of the strong coupling between excitons and guided photons in a GaN slab waveguide. The dispersions of waveguide polaritons are measured from T=6~K to 300~K through gratings. They are carefully analyzed within four models based on different assumptions, in order to assess the strong coupling regime. We prove that the guided photons and excitons are strongly coupled at all investigated temperatures, with a small $(11 \%)$ dependence on the temperature. However the values of the Rabi splitting strongly vary among the four models: the "coupled oscillator" model over-estimates the coupling; the analytical "Elliott-Tanguy" model precisely describes the dielectric susceptibility of GaN near the excitonic transition, leading to a Rabi splitting equal to $82 \pm 10 \ meV$ for TE0 modes; the experimental ellipsometry-based model leads to smaller values of $55 \pm 6 \ meV.$ We evidence that for waveguides including active layers with large oscillator strengths, as required for room-temperature polaritonic devices, a strong bending of the modes dispersion is not necessarily the signature of the strong-coupling, which requires for its reliable assessment a precise analysis of the material dielectric susceptibility.

cond-mat.mes-hall↗
arXiv2018

Competition between horizontal and vertical polariton lasing in planar microcavities

Planar microcavities filled with active materials containing excitonic resonances host radiative exciton-polariton (polariton) modes with in-plane wave vectors within the light cone. They also host at least one mode guided in the cavity plane by total internal reflection and which is not radiatively coupled to the vacuum modes except through defects or sample edges. We show that polariton lasing mediated by polariton stimulated scattering can occur concomitantly in both types of modes in a microcavity. By adjusting the detuning between the exciton and the radiative photon mode one can favor polariton lasing either in the radiative or in the guided modes. Our results suggest that the competition between these two types of polariton lasing modes may have played a role in many previous observations of polariton lasing and polariton Bose Einstein condensation.

cond-mat.mes-hall↗
arXiv2018

A universal description of III-V/Si epitaxial growth processes

Here, we experimentally and theoretically clarify III-V/Si crystal growth processes. Atomically-resolved microscopy shows that mono-domain 3D islands are observed at the early stages of AlSb, AlN and GaP epitaxy on Si, independently of misfit. It is also shown that complete III-V/Si wetting cannot be achieved in most III-V/Si systems. Surface/interface contributions to the free energy variations are found to be prominent over strain relief processes. We finally propose a general and unified description of III-V/Si growth processes, including the description of antiphase boundaries formation.

cond-mat.mtrl-sci↗
arXiv2015

Polariton condensation phase diagram in wide bandgap planar microcavities: GaN versus ZnO

GaN and ZnO microcavities have been grown on patterned silicon substrate. Thanks to a common platform these microcavities share similar photonic properties with large quality factors and low photonic disorder which gives the possibility to determine the optimal spot diameter and to realize a complete comparative phase diagram study. Both systems have been investigated under the same experimental condition. Experimental results are well reproduced by simulation using Boltzmann equations. Lower polariton lasing threshold has been measured at low temperature in the ZnO microcavity as expected due to a larger Rabi splitting. However the threshold is strongly impacted by LO phonons through phonon-assisted polariton relaxation. We observe and discuss this effect as a function of temperature and detuning. Finally the polariton lasing threshold at room temperature is quite similar in both microcavities. This study highlights polariton relaxation mechanism and their importance for threshold optimization.

cond-mat.mtrl-sci↗
arXiv2014

Origins of GaN(0 0 0 1) surface reconstructions

The reconstructions of the Ga polarity GaN(0 0 0 1) surface with and without trace amounts of arsenic and prepared by molecular beam epitaxy (MBE) have been studied with in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). Various reconstructions are observed with RHEED by analyzing patterns while the substrate is exposed to a fixed NH3 flux or after depositing known amounts of Ga as a function of substrate temperature. In situ STM images reveal that only a few of these reconstructions yield long-range periodicity in real space. The controversial role of arsenic on Ga induced reconstructions was also investigated using two independent MBE chambers and X-ray photoelectron spectroscopy.

cond-mat.mes-hall↗
arXiv2014

Patterned silicon substrates: a common platform for room temperature GaN and ZnO polariton lasers

A new platform for fabricating polariton lasers operating at room temperature is introduced: nitride-based distributed Bragg reflectors epitaxially grown on patterned silicon substrates. The patterning allows for an enhanced strain relaxation thereby enabling to stack a large number of crack-free AlN/AlGaN pairs and achieve cavity quality factors of several thousands with a large spatial homogeneity. GaN and ZnO active regions are epitaxially grown thereon and the cavities are completed with top dielectric Bragg reflectors. The two structures display strong-coupling and polariton lasing at room temperature and constitute an intermediate step in the way towards integrated polariton devices.

cond-mat.mes-hall↗
arXiv2011

LO-phonon assisted polariton lasing in a ZnO based microcavity

Polariton relaxation mechanisms are analysed experimentally and theoretically in a ZnO-based polariton laser. A minimum lasing threshold is obtained when the energy difference between the exciton reservoir and the bottom of the lower polariton branch is resonant with the LO phonon energy. Tuning off this resonance increases the threshold, and exciton-exciton scattering processes become involved in the polariton relaxation. These observations are qualitatively reproduced by simulations based on the numerical solution of the semi-classical Boltzmann equations.

cond-mat.mtrl-sci↗
arXiv2011

Laser emission with excitonic gain in a ZnO planar microcavity

The lasing operation of a ZnO planar microcavity under optical pumping is demonstrated from T=80 K to 300 K. At the laser threshold, the cavity switches from the strong coupling to the weak coupling regime. A gain-related transition, which appears while still observing polariton branches and, thus, with stable excitons, is observed below 240K. This shows that exciton scattering processes, typical of II-VI semiconductors, are involved in the gain process.

physics.optics↗
arXiv2011

Fabrication and Optical Properties of a Fully Hybrid Epitaxial ZnO-Based Microcavity in the Strong Coupling Regime

In order to achieve polariton lasing at room temperature, a new fabrication methodology for planar microcavities is proposed: a ZnO-based microcavity in which the active region is epitaxially grown on an AlGaN/AlN/Si substrate and in which two dielectric mirrors are used. This approach allows as to simultaneously obtain a high-quality active layer together with a high photonic confinement as demonstrated through macro-, and micro-photoluminescence ({\mu}-PL) and reflectivity experiments. A quality factor of 675 and a maximum PL emission at k=0 are evidenced thanks to {\mu}-PL, revealing an efficient polaritonic relaxation even at low excitation power.

cond-mat.mtrl-sci↗
arXiv2010

Epitaxial integration of the intrinsic ferromagnetic semiconductor GdN with silicon technology

A major challenge for the next generation of spintronics devices is the implementation of ferromagnetic-semiconductor thin films as spin injectors and detectors. Spin-polarised carrier injection cannot be accomplished efficiently from metals, and coupled with the rarity of intrinsic ferromagnetic semiconductors this has driven intensive study of diluted magnetic semiconductors. Chief among these is the doped III-V compound (Ga,Mn)As. These materials suffer from a number of drawbacks; they (i) require magnetic-ion doping well above the solubility limit, and (ii) must be hole doped to above the degenerate limit, preventing independent control of the carrier concentration and charge sign. Here we demonstrate the first epitaxial growth of a recently-characterised intrinsic ferromagnetic semiconductor, GdN, on silicon substrates, providing an essential step on the way to integrate new spintronics functionalities into Si-based technology. The films have been characterised as regards their growth toward fully relaxed GdN, the density and mobility of their carriers, and their magnetic behaviour.

cond-mat.mtrl-sci↗
arXiv2008

Polarized emission of GaN/AlN quantum dots : single dot spectroscopy and symmetry-based theory

We report micro-photoluminescence studies of single GaN/AlN quantum dots grown along the (0001) crystal axis by molecular beam epitaxy on Si(111) substrates. The emission lines exhibit a linear polarization along the growth plane, but with varying magnitudes of the polarization degree and with principal polarization axes that do not necessarily correspond to crystallographic directions. Moreover, we could not observe any splitting of polarized emission lines, at least within the spectral resolution of our setup (1 meV). We propose a model based on the joint effects of electron-hole exchange interaction and in-plane anisotropy of strain and/or quantum dot shape, in order to explain the quantitative differences between our observations and those previously reported on, e.g. CdTe- or InAs-based quantum dots.

cond-mat.other↗
arXiv2008

Influence of the mirrors on the strong coupling regime in planar GaN microcavities

The optical properties of bulk $λ/2$ GaN microcavities working in the strong light-matter coupling regime are investigated using angle-dependent reflectivity and photoluminescence at 5 K and 300 K. The structures have an Al$_{0.2}$Ga$_{0.8}$N/AlN distributed Bragg reflector as the bottom mirror and either an aluminium mirror or a dielectric Bragg mirror as the top one. First, the influence of the number of pairs of the bottom mirror on the Rabi splitting is studied. The increase of the mirror penetration depth is correlated with a reduction of the Rabi splitting. Second, the emission of the lower polariton branch is observed at low temperature in a microcavity containing two Bragg mirrors and exibiting a quality factor of 190. Our simulations using the transfer-matrix formalism, taking into account the real structure of the samples investigated are in good agreement with experimental results.

cond-mat.mtrl-sci↗
arXiv2003

Electronic structure of wurtzite and zinc-blende AlN

The electronic structure of AlN in wurtzite and zinc-blende phases is studied experimentally and theoretically. By using x-ray emission spectroscopy, the Al 3p, Al 3s and N 2p spectral densities are obtained. The corresponding local and partial theoretical densities of states (DOS), as well as the total DOS and the band structure, are calculated by using the full potential linearized augmented plane wave method, within the framework of the density functional theory. There is a relatively good agreement between the experimental spectra and the theoretical DOS, showing a large hybridization of the valence states all along the valence band. The discrepancies between the experimental and theoretical DOS, appearing towards the high binding energies, are ascribed to an underestimation of the valence band width in the calculations. Differences between the wurtzite and zinc-blende phases are small and reflect the slight variations between the atomic arrangements of both phases.

cond-mat.mtrl-sci↗