Breakdown of Corner States and Carrier Localization by Monolayer Fluctuations in Radial Nanowire Quantum Wells

Scanning Acousto-Optoelectric Spectroscopy on a Transition Metal Dichalcogenide Monolayer

The charge carrier dynamics are investigated by surface acoustic waves (SAWs) inside a WSe2 monolayer on LiNbO3 by scanning acousto-optoelectric spectroscopy. A strong enhancement of the PL emission intensity is observed almost over the entire area of the flake. This enhancement increases with increasing amplitude of the wave and is especially strong at or in the vicinity to defects. The latter is attributed to the SAW-driven Poole-Frenkel activation of trapped charge carriers bound to trapping sites at these defects. In addition, the PL intensity exhibit clear periodic modulations at the SAW's frequency fSAW and at 2 fSAW. These modulations are clear and unambiguous fingerprints of spatio-temporal carrier dynamics driven by the SAW. These occur on sub-nanosecond timescales which are found in good agreement with calculated exciton dissociation times. Mapping and analyzing both effects, this study shows that scanning acousto-electric spectroscopy provides a highly sensitive and local contact-free probe which uncovers distinct local features not resolved by conventional quasi-static photoluminescence techniques. The method is ideally suited to study carrier transport in 2D and other types of nanoscale materials and to reveal dynamic exciton modulation, and carrier localization and activation dynamics in the technologically important megahertz to gigahertz frequency range.

Direct bandgap quantum wells in hexagonal Silicon Germanium

Silicon is indisputably the most advanced material for scalable electronics, but it is a poor choice as a light source for photonic applications, due to its indirect band gap. The recently developed hexagonal Si1-xGex semiconductor features a direct bandgap at least for x > 0.65, and the realization of quantum heterostructures would unlock new opportunities for advanced optoelectronic devices based on the SiGe system. Here, we demonstrate the synthesis and characterization of direct bandgap quantum wells realized in the hexagonal Si1-xGex system. Photoluminescence experiments on hex-Ge/Si0.2Ge0.8 quantum wells demonstrate quantum confinement in the hex-Ge segment with type-I band alignment, showing light emission up to room temperature. Moreover, the tuning range of the quantum well emission energy can be extended using hexagonal Si1-xGex/Si1-yGey quantum wells with additional Si in the well. These experimental findings are supported with ab initio bandstructure calculations. A direct bandgap with type-I band alignment is pivotal for the development of novel low-dimensional light emitting devices based on hexagonal Si1-xGex alloys, which have been out of reach for this material system until now.

Sub-nanosecond acousto-electric carrier redistribution dynamics and transport in polytypic GaAs nanowires

The authors report on a combined structural, optical and acousto-electric study of polytypic GaAs nanowires. Two types of nanowires with different zincblende and wurtzite crystal phase mixing are identified by transmission electron microscopy and photoluminescence spectroscopy. The nanowires exhibit characteristic recombination channels which are assigned to different types of spatially direct recombination (electron and hole within the same crystal phase segment) and spatially indirect recombination (electron and holes localized in different segments). Contact-free acousto-optoelectric spectroscopy is employed to resolve spatiotemporal charge carrier dynamics between different recombination channels induced by a piezoelectric surface acoustic wave. The observed suppression of the emission and its dynamic temporal modulation shows unambiguous fingerprints of the local bandedge variations induced by the crystal phase mixing. A nanowire, which exhibits a variation from a near-pristine zinc blende crystal structure to a highly mixed crystal phase, shows a clear dependence on the propagation direction of the acoustic wave. In contrast, no pronounced directionality is found for a nanowire with an extended near-pristine zincblende segment. The experimental findings are corroborated by solving the drift and diffusion equations of electrons and holes induced by the surface acoustic wave. The key characteristics observed in our experimental data are well reproduced in the numerical simulations by assuming two general bandedge modulations and realistic parameters for the bandedge discontinuities and transport mobilities of electrons and holes. This evidences that even all relevant physical processes are accounted for in the model.

Ultrafast electron cycloids driven by the transverse spin of a surface acoustic wave

Spin-momentum locking is a universal wave phenomenon promising for applications in electronics and photonics. In acoustics, Lord Rayleigh showed that surface acoustic waves exhibit a characteristic elliptical particle motion strikingly similar to spin-momentum locking. Although these waves have become one of the few phononic technologies of industrial relevance, the observation of their transverse spin remained an open challenge. Here, we observe the full spin dynamics by detecting ultrafast electron cycloids driven by the gyrating electric field produced by a surface acoustic wave propagating on a slab of lithium niobate. A tubular quantum well wrapped around a nanowire serves as an ultrafast sensor tracking the full cyclic motion of electrons. Our acousto-optoelectrical approach opens previously unknown directions in the merged fields of nanoacoustics, nanophotonics, and nanoelectronics for future exploration.

Measuring, controlling and exploiting heterogeneity in optoelectronic nanowires

Fabricated from ZnO, III-N, chalcogenide-based, III-V, hybrid perovskite or other materials, semiconductor nanowires offer single-element and array functionality as photovoltaic, non-linear, electroluminescent and lasing components. In many applications their advantageous properties emerge from their geometry; a high surface-to-volume ratio for facile access to carriers, wavelength-scale dimensions for waveguiding or a small nanowire-substrate footprint enabling heterogeneous growth. However, inhomogeneity during bottom-up growth is ubiquitous and can impact morphology, geometry, crystal structure, defect density, heterostructure dimensions and ultimately functional performance. In this topical review, we discuss the origin and impact of heterogeneity within and between optoelectronic nanowires, and introduce methods to assess, optimise and ultimately exploit wire-to-wire disorder.

Majorana zero modes in nanowires with combined triangular and hexagonal geometry

The effects of geometry on the hosting of Majorana zero modes are explored in core-shell nanowires with a hexagonal core and a triangular shell, and vice versa. The energy interval separating electronic states localized in the corners from states localized on the sides of the shell is shown to be larger for a triangular nanowire with a hexagonal core, than a triangular one. We build the topological phase diagram for both cases and compare them to earlier work on prismatic nanowires with matching core and shell geometry. We suggest that a dual core nanowire is needed to allow for braiding operations of Majorana zero modes at the nanowire end plane.

Cathodoluminescence visualisation of local thickness variations of GaAs/AlGaAs quantum-well tubes on nanowires

We present spatially and spectrally resolved emission from nanowires with a thin radial layer of GaAs embedded in AlGaAs barriers, grown radially around taper-free GaAs cores. The GaAs layers are thin enough to show quantization, and are quantum wells. Due to their shape, they are referred to as quantum well tubes (QWTs). We have investigated three different nominal QWT thicknesses: 1.5, 2.0, and 6.0 nm. They all show average emission spectra from the QWT with an energy spread corresponding to a thickness variation of ±30%. We observe no thickness gradient along the length of the nanowires. Individual NWs show a number of peaks, corresponding to different QW thicknesses. Apart from the thinnest QWT, the integrated emission from the QWTs shows homogeneous emission intensity along the NW. The thinnest QWTs show patchy emission patterns due to the incomplete coverage of the QWT. We observe a few NWs with larger diameters. The QWTs in these NWs show spatially resolved variations across the NW. An increase in the local thickness of the QWT at the corners blocks the diffusion of carriers from facet to facet, thereby enabling us to visualise the thickness variations of the radial quantum wells.

Quantum-Confinement-Enhanced Thermoelectric Properties in Modulation-Doped GaAs-AlGaAs Core-Shell Nanowires

Nanowires (NWs) hold great potential in advanced thermoelectrics due to their reduced dimensions and low-dimensional electronic character. However, unfavorable links between electrical and thermal conductivity in state-of-the-art unpassivated NWs have, so far, prevented the full exploitation of their distinct advantages. A promising model system for a surface-passivated one-dimensional (1D)-quantum confined NW thermoelectric is developed that enables simultaneously the observation of enhanced thermopower via quantum oscillations in the thermoelectric transport and a strong reduction in thermal conductivity induced by the core-shell heterostructure. High-mobility modulation-doped GaAs/AlGaAs core-shell NWs with thin (sub-40 nm) GaAs NW core channel are employed, where the electrical and thermoelectric transport is characterized on the same exact 1D-channel. 1D-sub-band transport at low temperature is verified by a discrete stepwise increase in the conductance, which coincided with strong oscillations in the corresponding Seebeck voltage that decay with increasing sub-band number. Peak Seebeck coefficients as high as ≈65-85 µV K^-1 are observed for the lowest sub-bands, resulting in equivalent thermopower of S² σ ≈ 60 µW m^-1 K^-2 and S² G ≈ 0.06 pW K^-2 within a single sub-band. Remarkably, these core-shell NW heterostructures also exhibit thermal conductivities as low as ≈3 W m^-1 K^-1 , about one order of magnitude lower than state-of-the-art unpassivated GaAs NWs.

Corner and side localization of electrons in irregular hexagonal semiconductor shells

We discuss the low energy electronic states in hexagonal rings. These states correspond to the transverse modes in core-shell nanowires built of III-V semiconductors which typically have a hexagonal cross section. In the case of symmetric structures the 12 lowest states (including the spin) are localized in the corners, while the next following 12 states are localized mostly on the sides. Depending on the material parameters, in particular the effective mass, the ring diameter and width, the corner and side states may be separated by a considerable energy gap, ranging from few to tens of meV. In a realistic fabrication process geometric asymmetries are unavoidable, and therefore the particles are not symmetrically distributed between all corner and side areas. Possibly, even small deformations may shift the localization of the ground state to one of the sides. The transverse states or the transitions between them may be important in transport or optical experiments. Still, up to date, there are only very few experimental investigations of the localization-dependent properties of core-shell nanowires.

Anisotropic light scattering by prismatic semiconductor nanowires

Anisotropic transverse light scattering by prismatic nanowires is a natural outcome of their geometry. In this work, we perform numerical calculations of the light scattering characteristics for nanowires in the optical and near-infrared range and explore the possibility of tuning the directivity by changing the angle of light incidence. The scattering cross section and the directivity of the scattered light when it is incident perpendicular to a facet or to an edge of the prism are investigated both with transverse electric and with transverse magnetic polarizations. The phenomenology includes Mie resonances and guided modes yielding together rich and complex spectra. We consider nanowires with hexagonal, square and triangular cross sections. The modes that are most sensitive to the incidence angle are the hexapole for the hexagonal case and the quadrupole for the square case. Higher order modes are also sensitive, but mostly for the square geometry. Our results indicate the possibility of a flexible in-situ tunability of the directivity simply by rotating the nanowire profile relatively to the direction of the incident light which could offer potential advantages in applications such as switching or sensing.