Cathodoluminescence in a scanning electron microscope was applied to a semiconductor quantum dot in a nanowire that is able to emit single photons. We show that cathodoluminescence can be used not only for imaging and spectroscopy but also to measure the correlation function and characterize the purity of the single-photon emitter. The electron beam can be manipulated to minimize the collection of parasitic luminescence. At cryogenic temperatures, we observed that the thermal budget, as measured via the phonon sidebands, is close to that of nonresonant microphotoluminescence. This makes cathodoluminescence an efficient tool in the quest for novel single-photon sources.

The emission properties of a localized solid-state emitter are strongly influenced by its environment. The coupling to acoustic phonons impacts the coherence of the emitter and its temperature dependence, and also results in the apparition of phonon sidebands besides the sharp zero-phonon line. Here, we present a method for measuring the absolute temperature of a localized emitter by directly plotting the ratio of the Stokes and anti-Stokes components of the phonon sideband as a function of the shift from the zero-phonon line. This approach requires no calibration and knowledge of the system, making it applicable to a wide range of emitters and materials. We validate the method using a CdSe quantum dot in a ZnSe nanowire. We thus show that the quantum dot is significantly heated under nonresonant excitation when increasing the incident power at low temperature which is ascribed to the drop in thermal conductivity at these temperatures.

We present a promising solid-state system able to emit triggered single-photons in the blue-green range up to room temperature. The active element is a CdSe quantum dot (QD) embedded in a bottom-up core-shell ZnSe nanowire grown by molecular beam epitaxy. The nanowire shell acts as a waveguide and confines the fundamental optical mode HE11, channeling the photons emitted by the QD along the nanowire axis. The nanowires has a base radius of 70-90 nm and a length of 5-6 mu m with a conical ending that allows to adiabatically expands the guided mode and reduces the divergence angle. Photo-correlation measurements show anti-bunching with g((2))(0) values down to 0.3 at room temperature. Charged excitons degrade the emission properties at room temperature and we found that a neutral nanowire-quantum dot would emit single-photons with a brightness of 0.17 photon per pulse.

We propose a dual-adatom diffusion-limited model for the growth of compound semiconductor nanowires via the vapor-liquid-solid or the vapour-solid-solid mechanisms. The growth is catalyzed either by a liquid or a solid nanoparticle. We validate the model using experimental data from the growth of InAs nanowires catalyzed by a gold nanoparticle in a molecular beam epitaxy reactor. Initially, we determine the parameters (diffusion lengths, flux to the seed, Kelvin effect) that describe the growth of nanowires under an excess of one of the two beams (for instance, group III or group V atoms). The diffusion-limited model calculates the growth rate resulting from the current of atoms reaching the seed. Our dual-adatom diffusion-limited model calculates for a compound semiconductor, the instantaneous growth rate resulting from the smallest current of the two types of atoms at a given time. We apply the model to analyze the length-radius dependence of our InAs nanowires for growth conditions covering the transition from the As-limited to the In-limited regime. Finally, the model also describes the complex dependence of the transition between both regimes on the nanowire radius and length. This approach is generic and can be applied to study the growth of any compound semiconductor nanowires.

Single-photon sources are crucial for developing secure telecommunications. However, most systems operate at cryogenic temperatures. Here, we discuss a promising solid-state system emitting single photons at room temperature in the blue-green range, allowing for quantum communications in free space and underwater. The active element is a core-shell ZnSe tapered nanowire embedding a single CdSe quantum dot grown by molecular beam epitaxy. A patterned substrate enables a thorough study of the one and same nanowire by different methods. Our source exhibits anti-bunching with g(2)(0) < 0.3 near the centre of the photoluminescence line and shows high brightness. This work paves the way for developing single-photon sources operating at non-cryogenic temperatures.

The growth of ZnTe nanowires and ZnTe-CdTe nanowire heterostructures is studied by in situ transmission electron microscopy. We describe the shape and the change of shape of the solid gold nanoparticle during vapor-solid-solid growth. We show the balance between one monolayer and two monolayer steps, which characterizes the vapor-liquid-solid and vapor-solid-solid growth modes of ZnTe. We discuss the likely role of the mismatch strain and lattice coincidence between gold and ZnTe on the predominance of two monolayer steps during vapor-solid-solid growth and on the subsequent self-regulation of the step dynamics. Finally, the formation of an interface between CdTe and ZnTe is described.

Received 5 September 2022DOI:https://doi.org/10.1103/PhysRevB.106.119901©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasSpin diffusionSpin relaxationTechniquesDiffusion & random walksLight scatteringSpin noise spectroscopyCondensed Matter, Materials & Applied Physics

Quantum dots acting as single photon emitters in the blue-green range are fabricated and characterized at cryogenic temperature. They consist in CdSe dots inserted in (Zn,Mg)Se nanowires with a thick shell. Photoluminescence spectra, decay curves and autocorrelation functions were measured under nonresonant continuous-wave and pulsed excitation. An analytical approach is applied simultaneously to the decay curves and correlation functions. It allows a quantitative description of how these two quantities are affected by the exciton rise due to biexciton feeding, the bright exciton decay, the effect of the dark exciton, and the re-excitation between two laser pulses. Linewidths at our limit of resolution (200 $\mu$eV) are recorded. The reported correlation counts vary from a full control by re-excitation from traps, to a small contribution of re-excitation by mobile carriers or other QDs, as low as 5%.

( )Starting from the numerical solution of the 6-band k . p description of a lattice-mismatched ellipsoidal quantum dot situated inside a nanowire, including a spin Zeeman effect with values appropriate to a dilute magnetic semiconductor, we propose and test phenomenological models of the effect of the built-in strain on the heavy hole, light hole, and exciton states. We test the validity and the limits of a description restricted to a (Gamma(8)) quadruplet of ground states and we demonstrate the role of the interactions of the light hole state with light hole excited states. We show that the built-in axial strain not only defines the character, heavy hole or light hole, of the ground state, but also mixes significantly the light hole state with the split-off band's states: Even for a spin-orbit energy as large as 1 eV, that mixing induces first-order modifications of properties such as the spin value and anisotropy, the oscillator strength, and the electron-hole exchange, for which we extend the description to the light hole exciton. CdTe/ZnTe quantum dots are mainly used as a test case but the concepts we discuss apply to many heterostructures, from mismatched II-VI and III-V quantum dots and nanowires, to III-V nanostructures submitted to an applied stress, and to silicon nanodevices with even smaller residual strains.

Spatiotemporal spin noise spectroscopy is combined with dynamic light scattering in order to reach spatial resolutions down to $\ensuremath{\sim}\ensuremath{\lambda}/10$. Applied to a system of localized electron spins, an insulating n-doped CdTe layer, this allows us to reveal long spin jump distances $\ensuremath{\ell}\ensuremath{\sim}2.7\phantom{\rule{4.pt}{0ex}}\ensuremath{\mu}\mathrm{m}$. Spin noise spectra at large wave vectors $q$ ($q\ensuremath{\ell}\ensuremath{\gg}1$) provide a snapshot of the spin dynamics before jump (therefore not affected by spin motion), while at smaller $q$, spin motion sets in. This allows us to unravel the contributions of spin-orbit and hyperfine fields in the electron spin relaxation and to determine self-consistently all parameters relevant to the spin dynamics. We propose a phenomenological equation inspired by studies of atomic jump diffusion by neutron scattering, which includes the relevant spin relaxation mechanisms and the effect of time of flight of the spin fluctuation across the laser spot. This modeling reproduces all experimental results.

The propagation of sidewall steps during the growth of nanowires is calculated in the frame of the Burton-Cabrera-Frank model. The stable shape of the nanowire comprises a cylinder section on top of a cone section: their characteristics are obtained as a function of the radius of the catalyst-nanowire area, the desorption-limited diffusion length of adatoms on the terraces, and the sticking of adatoms at step edges. The comparison with experimental data allows us to evaluate these last two parameters for InP and ZnTe nanowires; it reveals a different behavior for the two materials, related to a difference by an order of magnitude of the desorption-limited diffusion length.

A whole series of complementary studies have been performed on the same, single nanowire containing a quantum dot: cathodoluminescence spectroscopy and imaging, micro-photoluminescence spectroscopy under magnetic field and as a function of temperature, and energy-dispersive X-ray spectrometry and imaging. The ZnTe nanowire was deposited on a Si 3 N 4 membrane with Ti/Al patterns. The complete set of data shows that the CdTe quantum dot features the heavy-hole state as a ground state, although the compressive mismatch strain promotes a light-hole ground state as soon as the aspect ratio is larger than unity (elongated dot). A numerical calculation of the whole structure shows that the transition from the heavy-hole to the light-hole configuration is pushed toward values of the aspect ratio much larger than unity by the presence of a (Zn,Mg)Te shell, and that the effect is further enhanced by a small valence band offset between the semiconductors in the dot and around it.

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 bulblike (pendant-droplet) shape attributed to the Kirkendall effect.

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.

Quantum dots inserted inside semiconductor nanowires are extremely promising candidates as building blocks for solid-state based quantum computation and communication. They provide very high crystalline and optical properties and offer a convenient geometry for electrical contacting. Having a complete determination and full control of their emission properties is one of the key goals of nanoscience researchers. Here we use strain as a tool to create in a single magnetic nanowire quantum dot a light-hole exciton, an optically active quasiparticle formed from a single electron bound to a single light hole. In this frame, we provide a general description of the mixing within the hole quadruplet induced by strain or confinement. A multi-instrumental combination of cathodo-luminescence, polarisation-resolved Fourier imaging and magneto-optical spectroscopy, allow us to fully characterize the hole ground state, including its valence band mixing with heavy hole states.

We report here on the quantitative 3D reconstruction of core‐shell nanostructures by STEM/EDX using two X‐ray maps acquired at two different tilt angles perpendicular to each other (Rueda et al., 2016; fig. 1). The method is based on the modelling of the NW cross‐section using a series of imbricated ellipses whose dimensions are defined by their major and minor diameters (fig. 2). The number of ellipse depends on the number of chemical phases which are identified from the concentration profiles. The position and orientation of each ellipse are determined by the coordinates of their respective centers and the overall tilt of the nanowire, respectively. More sophisticated models, using hexagons or rectangles instead of ellipses, have been developed in order to take into account the crystal structure of nanowires exhibiting facetted sidewalls. These models are based on the elliptical model, by constructing the tangents to an ellipse, and hence, are defined by the same parameters, which is useful when comparing models. Considering a system of a number of K ellipses with ξ k,j the local concentration of element j for the k th ellipse (k=1 for the largest ellipse), then the average concentration C i,j of element j for the i th pixel along the x‐axis must satisfy the following equations (equations 1): With t 1,i and t k,i , the local thickness at pixel i of the first and k th ellipse, respectively. The local thickness of the first ellipse (= the total thickness of the cross‐section) and the average concentration C i,j of element j present along the beam axis is determined using the zeta‐factor method (Watanabe and Williams, 2006): With: m the total number of element, I b the beam current; ρ the sample density; ζ j the zeta‐factor of element j determined using reference samples of known composition and thickness (Lopez‐Haro et al., 2014); I i,j and A i,j the net X‐ray intensity and the absorption correction term for element j at pixel i, respectively. The absorption correction term is estimated from a simple model that takes into account the direction of the X‐ray emission relative to the position of the detectors, knowing the thickness, density, and mass absorption coefficient of the material through which the radiation travels (Rueda et al., 2016). The method for reconstructing the cross‐section can be divided into three steps: 1) the appropriate cross‐sectional model is selected by comparing the thickness profile calculated from equation [2] with the thickness profile simulated for elliptical, hexagonal, and rectangular cross‐sections (figure 3); 2) the number of ellipses is determined, and their dimensions are evaluated, from the concentration profiles; 3) the local concentrations ξ k,j are determined and the dimensions of the ellipses are adjusted by minimizing the compositional differences between profiles calculated from equation [3] and simulated by equation [1]. This method was applied for reconstructing core‐shell nanostructures on (Mg, Mn, Cd, Zn)(Te,Se) and (Al, Cu)Ge nanowires and (Pt, Co) nanoparticles. Advantages and limitations of the method will be presented and discussed at the conference.

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%.

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.

Spin noise spectroscopy is an optical technique which can probe spin resonances non-perturbatively. First applied to atomic vapours, it revealed detailed information about nuclear magnetism and the hyperfine interaction. In solids, this approach has been limited to carriers in semiconductor heterostructures. Here we show that atomic-like spin fluctuations of Mn ions diluted in CdTe (bulk and quantum wells) can be detected through the Kerr rotation associated to excitonic transitions. Zeeman transitions within and between hyperfine multiplets are clearly observed in zero and small magnetic fields and reveal the local symmetry because of crystal field and strain. The linewidths of these resonances are close to the dipolar limit. The sensitivity is high enough to open the way towards the detection of a few spins in systems where the decoherence due to nuclear spins can be suppressed by isotopic enrichment, and towards spin resonance microscopy with important applications in biology and materials science.

We propose a comprehensive description of the strain configuration induced by the lattice mismatch in a core-shell nanowire with circular cross-section, taking into account the crystal anisotropy and the difference in stiffness constants of the two materials. We use an analytical approach which fully exploits the symmetry properties of the system. Explicit formulae are given for nanowires with the wurtzite structure or the zinc-blende structure with the hexagonal/trigonal axis along the nanowire, and the results are compared to available numerical calculations and experimental data on nanowires made of different III–V and II–VI semiconductors. The method is also applied to multishell nanowires, and to core-shell nanowires grown along the 〈0 0 1〉 axis of cubic semiconductors. It can be extended to other orientations and other crystal structures.