203 research outputs found
Growth study of indium-catalyzed silicon nanowires by plasma enhanced chemical vapor deposition
Indium was used as a catalyst for the synthesis of silicon nanowires in a plasma enhanced chemical vapor deposition reactor. In order to foster the catalytic activity of indium, the indium droplets had to be exposed to a hydrogen plasma prior to nanowire growth in a silane plasma. The structure of the nanowires was investigated as a function of the growth conditions by electron microscopy and Raman spectroscopy. The nanowires were found to crystallize along the , or growth direction. When growing on the and directions, they revealed a similar crystal quality and the presence of a high density of twins along the {111} planes. The high density and periodicity of these twins lead to the formation of hexagonal domains inside the cubic structure. The corresponding Raman signature was found to be a peak at 495 cm−1, in agreement with previous studies. Finally, electron energy loss spectroscopy indicates an occasional migration of indium during growt
Local modification of GaAs nanowires induced by laser heating
GaAs nanowires were heated locally under ambient air conditions by a focused laser beam which led to oxidation and formation of crystalline arsenic on the nanowire surface. Atomic force microscopy, photoluminescence and Raman spectroscopy experiments were performed on the same single GaAs nanowires in order to correlate their structural and optical properties. We show that the local changes of the nanowires act as a barrier for thermal transport which is of interest for thermoelectric applications
Raman spectroscopy of wurtzite and zinc-blende GaAs nanowires: polarization dependence, selection rules and strain effects
Polarization dependent Raman scattering experiments realized on single GaAs
nanowires with different percentages of zinc-blende and wurtzite structure are
presented. The selection rules for the special case of nanowires are found and
discussed. In the case of zinc-blende, the transversal optical mode E1(TO) at
267 cm-1 exhibits the highest intensity when the incident and analyzed
polarization are parallel to the nanowire axis. This is a consequence of the
nanowire geometry and dielectric mismatch with the environment, and in quite
good agreement with the Raman selection rules. We also find a consistent
splitting of 1 cm-1 of the E1(TO). The transversal optical mode related to the
wurtzite structure, E2H, is measured between 254 and 256 cm-1, depending on the
wurtzite content. The azymutal dependence of E2H indicates that the mode is
excited with the highest efficiency when the incident and analyzed polarization
are perpendicular to the nanowire axis, in agreement with the selection rules.
The presence of strain between wurtzite and zinc-blende is analyzed by the
relative shift of the E1(TO) and E2H modes. Finally, the influence of the
surface roughness in the intensity of the longitudinal optical mode on {110}
facets is presented.Comment: 28 pages, 12 figures. to be published in Phys. Rev.
Optical study of the band structure of wurtzite GaP nanowires
We investigated the optical properties of wurtzite (WZ) GaP nanowires by performing photoluminescence (PL) and time-resolved PL measurements in the temperature range from 4 K to 300 K, together with atom probe tomography to identify residual impurities in the nanowires. At low temperature, the WZ GaP luminescence shows donor-acceptor pair emission at 2.115 eV and 2.088 eV, and Burstein-Moss band-filling continuum between 2.180 and 2.253 eV, resulting in a direct band gap above 2.170 eV. Sharp exciton α-β-γ lines are observed at 2.140-2.164-2.252 eV, respectively, showing clear differences in lifetime, presence of phonon replicas, and temperature- dependence. The excitonic nature of those peaks is critically discussed, leading to a direct band gap o
Thermal conductivity of GaAs nanowires studied by micro-Raman spectroscopy combined with laser heating
The thermal properties of freely suspended GaAs nanowires are investigated by applying a method which relies on laser heating and the determination of the local temperature by Raman spectroscopy. In order to determine the values for the thermal conductivity kappa, the fraction of the laser power absorbed inside the GaAs nanowire is estimated by numerical simulations. The thermal conductivity of nanowires with homogeneous diameter is found to lie in the range of 8-36 W m(-1) K-1. The change of the temperature profile in the presence of a tapering was investigated. Furthermore, we discuss the influence of laser heating in ambient conditions on the value of kappa. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3532848
Effect of the GaAsP shell on optical properties of self-catalyzed GaAs nanowires grown on silicon
We realize growth of self-catalyzed core-shell GaAs/GaAsP nanowires (NWs) on
Si substrates using molecular-beam epitaxy. Transmission electron microscopy
(TEM) of single GaAs/GaAsP NWs confirms their high crystal quality and shows
domination of the zinc-blende phase. This is further confirmed in optics of
single NWs, studied using cw and time-resolved photoluminescence (PL). A
detailed comparison with uncapped GaAs NWs emphasizes the effect of the GaAsP
capping in suppressing the non-radiative surface states: significant PL
enhancement in the core-shell structures exceeding 2000 times at 10K is
observed; in uncapped NWs PL is quenched at 60K whereas single core-shell
GaAs/GaAsP NWs exhibit bright emission even at room temperature. From analysis
of the PL temperature dependence in both types of NW we are able to determine
the main carrier escape mechanisms leading to the PL quench
Spatially resolved Raman spectroscopy on indium-catalyzed core-shell germanium nanowires: size effects
The structure of indium-catalyzed germanium nanowires is investigated by atomic force microscopy, scanning confocal Raman spectroscopy and transmission electron microscopy. The nanowires are formed by a crystalline core and an amorphous shell. We find that the diameter of the crystalline core varies along the nanowire, down to few nanometers. Phonon confinement effects are observed in the regions where the crystalline region is the thinnest. The results are consistent with the thermally insulating behavior of the core-shell nanowires
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