155 research outputs found
Hybrid III–V/Silicon Nanowires
International audienceSemiconducting nanowires are emerging as a route to combine heavily mismatched materials. The nanowire dimensions facilitate the defect-free integration of the two most powerful semiconductor classes, group IVs and group III-Vs. These combinations may enhance the performance of existing device concepts, and also create new applications. In this chapter we review the recent progress in heteroepitaxial growth of III-V andIVmaterials. We highlight the advantage of using the small nanowire dimensions to facilitate accommodation of the lattice strain at the surface of the structures. Another advantage of the nanowire system is that anti phase boundaries are not formed, as there is only one nucleation site per wire. In this chapter, we will discuss three different heteroepitaxial III-V/Si morphologies, III-V nanowires on group IV substrates, and axial and radial heterojunctions. Advanced analysis techniques are used tocharacterise the quality of the heterointerfaces. Finally, we address potential applications of III-V/Si nanowires
Charting the low-loss region in Electron Energy Loss Spectroscopy with machine learning
Exploiting the information provided by electron energy-loss spectroscopy
(EELS) requires reliable access to the low-loss region where the zero-loss peak
(ZLP) often overwhelms the contributions associated to inelastic scatterings
off the specimen. Here we deploy machine learning techniques developed in
particle physics to realise a model-independent, multidimensional determination
of the ZLP with a faithful uncertainty estimate. This novel method is then
applied to subtract the ZLP for EEL spectra acquired in flower-like WS
nanostructures characterised by a 2H/3R mixed polytypism. From the resulting
subtracted spectra we determine the nature and value of the bandgap of
polytypic WS, finding with a
clear preference for an indirect bandgap. Further, we demonstrate how this
method enables us to robustly identify excitonic transitions down to very small
energy losses. Our approach has been implemented and made available in an open
source Python package dubbed EELSfitter.Comment: 37 pages, 14 figures. The EELSfitter code is available from
https://github.com/LHCfitNikhef/EELSfitte
Charting the low-loss region in Electron Energy Loss Spectroscopy with machine learning
Exploiting the information provided by electron energy-loss spectroscopy (EELS) requires reliable access to the low-loss region where the zero-loss peak (ZLP) often overwhelms the contributions associated to inelastic scatterings off the specimen. Here we deploy machine learning techniques developed in particle physics to realise a model-independent, multidimensional determination of the ZLP with a faithful uncertainty estimate. This novel method is then applied to subtract the ZLP for EEL spectra acquired in flower-like WS nanostructures characterised by a 2H/3R mixed polytypism. From the resulting subtracted spectra we determine the nature and value of the bandgap of polytypic WS, finding with a clear preference for an indirect bandgap. Further, we demonstrate how this method enables us to robustly identify excitonic transitions down to very small energy losses. Our approach has been implemented and made available in an open source Python package dubbed EELSfitter
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Exciton localization mechanisms in wurtzite/zinc-blende GaAs nanowires
We investigate the emission properties of excitons in GaAs nanowires containing quantum disks formed by structural alternation between the zinc-blende and wurtzite phases, by means of temperature-dependent photoluminescence. At 10 K the emission from an ensemble of disks is distributed in a band of full width at half maximum ∼30 meV, whereas the emission linewidth for a single disk is 700 μeV. While the disk ensemble emission exhibits an S-shaped temperature dependence, the emission from single quantum disks follows the temperature dependence of the band gap over the whole temperature range. This indicates that intradisk exciton localization on impurities is negligible and that increasing the temperature induces a transfer of excitons from narrow to thick disks along the length of the wires. Our observations of the emission linewidth for single crystal-phase quantum disks show a scattering rate of excitons with acoustic phonons eight times larger than the values usually reported for (Al,Ga)As/GaAs quantum wells. This large scattering rate demonstrates that the electron effective mass in wurtzite GaAs is much heavier than in zinc-blende GaAs and is evidence of coupling between the Γ7 and Γ8 conduction bands of wurtzite GaAs.We acknowledge financial support from the Poynton
Cambridge Australia Scholarship and from the European
Union Seventh Framework Program under grant agreement
No. 265073. A.F.iM. and E.U. acknowledge funding through
the Marie Curie Excellence grant SENFED. S.C.B. thanks
S.N.F. for funding through the Marie-Heim Vögtlin scheme
Direct correlation of crystal structure and optical properties in wurtzite/zinc-blende GaAs nanowire heterostructures
A novel method for the direct correlation at the nanoscale of structural and
optical properties of single GaAs nanowires is reported. Nanowires consisting
of 100% wurtzite and nanowires presenting zinc-blende/wurtzite polytypism are
investigated by photoluminescence spectroscopy and transmission electron
microscopy. The photoluminescence of wurtzite GaAs is consistent with a band
gap of 1.5 eV. In the polytypic nanowires, it is shown that the regions that
are predominantly composed of either zinc-blende or wurtzite phase show
photoluminescence emission close to the bulk GaAs band gap, while regions
composed of a nonperiodic superlattice of wurtzite and zinc-blende phases
exhibit a redshift of the photoluminescence spectra as low as 1.455 eV. The
dimensions of the quantum heterostructures are correlated with the light
emission, allowing us to determine the band alignment between these two
crystalline phases. Our first-principles electronic structure calculations
within density functional theory, employing a hybrid-exchange functional,
predict band offsets and effective masses in good agreement with experimental
results
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