41 research outputs found
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The impact of ultraviolet laser excitation during Raman spectroscopy of hexagonal boron nitride thin films
We utilized excitation in the ultraviolet (UV) spectral range for the study of hexagonal boron nitride (h-BN) thin films on different substrates by Raman spectroscopy. Whereas UV excitation offers fundamental advantages for the investigation of h-BN and heterostructures with graphene, the actual Raman spectra recorded under ambient conditions reveal a temporal decay of the signal intensity. The disappearance of the Raman signal is found to be induced by thermally activated chemical reactions with ambient molecules at the h-BN surface. The chemical reactions could be strongly suppressed under vacuum conditions which, however, favor the formation of a carbonaceous surface contamination layer. For the improvement of the signal-to-noise ratio under ambient conditions, we propose a line-scan method for the acquisition of UV Raman spectra in atomically thin h-BN, a material which is expected to play a key role in future technologies based on 2D van der Waals heterostructures. © 2020 The Authors. Journal of Raman Spectroscopy published by John Wiley & Sons Lt
Density control of GaN nanowires at the wafer scale using self-assembled SiN patches on sputtered TiN(111)
The self-assembly of heteroepitaxial GaN nanowires using either molecular
beam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE) mostly results
in wafer-scale ensembles with ultrahigh ( m) or ultralow (
m) densities, respectively. A simple means to tune the density of
well-developed nanowire ensembles between these two extremes is generally
lacking. Here, we examine the self-assembly of SiN patches on TiN(111)
substrates which are eventually acting as seeds for the growth of GaN
nanowires. We first found that if prepared by reactive sputtering, the TiN
surface is characterized by \{100\} facets for which the GaN incubation time is
extremely long. Fast GaN nucleation is only obtained after deposition of a
sub-monolayer of SiN atoms prior to the GaN growth. By varying the amount
of pre-deposited SiN, the GaN nanowire density could be tuned by three
orders of magnitude with excellent uniformity over the entire wafer, bridging
the density regimes conventionally attainable by direct self-assembly with MBE
or MOVPE. The analysis of the nanowire morphology agrees with a nucleation of
the GaN nanowires on nanometric SiN patches. The photoluminescence analysis
of single freestanding GaN nanowires reveals a band edge luminescence dominated
by excitonic transitions that are broad and blue shifted compared to bulk GaN,
an effect that is related to the small nanowire diameter and to the presence of
a thick native oxide. The approach developed here can be principally used for
tuning the density of most III-V semiconductors nucleus grown on inert surfaces
like 2D materials
a resonant Raman study
We report resonant Raman scattering (RRS) by the TO, LO, and 2 LO modes of
single wurtzite and zinc-blende GaAs nanowires. The optical band gap of
wurtzite GaAs is 1.460eV ± 3meV at room temperature, and 35 ± 3meV larger than
the GaAs zinc-blende band gap. Raman measurements using incoming light
polarized parallel and perpendicular to the wire c axis allowed us to
investigate the splitting of heavy Γ9 and light-hole Γ7 band at the Γ point of
65 ± 6meV
a combined photoluminescence and resonant Raman scattering study
We used spatially resolved photoluminescence (PL) and resonant Raman
spectroscopy to study the electronic structure of single GaAs nanowires (NWs)
consisting of zinc-blende (ZB) and wurtzite (WZ) segments. For narrow ZB
segments and stacking faults the energy range of the observed PL peak
positions is found to deviate from that of the maxima in resonance Raman
profiles. These different energy ranges reflect the fact that the PL
recombination is dominated by spatially indirect transitions whereas the
resonance enhancement of Raman scattering is caused by direct transitions. Our
results provide evidence for the type II band alignment between ZB and WZ GaAs
and a coherent picture of all near-band-gap transition energies in GaAs NWs
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Electronic properties of wurtzite GaAs: A correlated structural, optical, and theoretical analysis of the same polytypic GaAs nanowire
III-V compound semiconductor nanowires are generally characterized by the
coexistence of zincblende and wurtzite structures. So far, this polytypism has
impeded the determination of the electronic properties of the metastable
wurtzite phase of GaAs, which thus remain highly controversial. In an effort to
obtain new insights into this topic, we cross-correlate nanoscale spectral imaging
by near-field scanning optical microscopy with a transmission electron microscopy
analysis of the very same polytypic GaAs nanowire dispersed onto a Si wafer.
Thus, spatially resolved photoluminescence spectra could be unambiguously
assigned to nanowire segments whose structure is known with lattice-resolved
accuracy. An emission energy of 1.528 eV was observed from extended zincblende
segments, revealing that the dispersed nanowire was under uniaxial strain
presumably due to interaction with its supporting substrate. These crucial
information and the emission energy obtained for extended pure wurtzite
segments were used to perform envelope function calculations of zincblende
quantum disks in a wurtzite matrix as well as the inverse structure. In these
calculations, we varied the fundamental bandgap, the electron mass, and the
band offset between zincblende and wurtzite GaAs. From this multi-parameter
comparison with the experimental data, we deduced that the bandgap between
the Γ8 conduction and A valence band ranges from 1.532 to 1.539 eV in strain-free
wurtzite GaAs, and estimated values of 1.507 to 1.514 eV for the Γ7–A bandgap.
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