78 research outputs found
Near-field properties of plasmonic nanostructures with high aspect ratio
Using the Green's dyad technique based on cuboidal meshing, we compute the
electromagnetic field scattered by metal nanorods with high aspect ratio. We
investigate the effect of the meshing shape on the numerical simulations. We
observe that discretizing the object with cells with aspect ratios similar to
the object's aspect ratio improves the computations, without degrading the
convergency. We also compare our numerical simulations to finite element method
and discuss further possible improvements
Effect of the pn junction engineering on Si microwire-array solar cells
We report on the impact of the doping concentration design on the performance of silicon microwire arrays as photovoltaic devices. We have fabricated arrays with different p- and n-doping profiles and thicknesses, obtaining mean efficiencies as high as 9.7% under AM 1.5G solar illumination. The results reveal the importance of scaling the microwire diameter with the depletion width resulting from doping concentrations. The doping of the core should be kept low in order to reduce bulk recombination. Furthermore, the thickness of the n-shell should be kept as thin as possible to limit the emitter losses
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
III-V nanowire arrays: growth and light interaction
Semiconductor nanowire arrays are reproducible and rational platforms for the realization of high performing designs of light emitting diodes and photovoltaic devices. In this paper we present an overview of the growth challenges of III-V nanowire arrays obtained by molecular beam epitaxy and the design of III-V nanowire arrays on silicon for solar cells. While InAs tends to grow in a relatively straightforward manner on patterned (111) Si substrates, GaAs nanowires remain more challenging; success depends on the cleaning steps, annealing procedure, pattern design and mask thickness. Nanowire arrays might also be used for next generation solar cells. We discuss the photonic effects derived from the vertical configuration of nanowires standing on a substrate and how these are beneficial for photovoltaics. Finally, due to the special interaction of light with standing nanowires we also show that the Raman scattering properties of standing nanowires are modified. This result is important for fundamental studies on the structural and functional properties of nanowires
Phase-locking of a 2.7-THz quantum cascade laser to a mode-locked erbium-doped fibre laser
We demonstrate phase-locking of a 2.7-THz metalmetal waveguide quantum cascade laser (QCL) to an external microwave signal. The reference is the 15th harmonic, generated by a semiconductor superlattice nonlinear device, of a signal at 182 GHz, which itself is generated by a multiplier-chain (x2x3x2) from a microwave synthesizer at 15 GHz. Both laser and reference radiations are coupled into a hot electron bolometer mixer, resulting in a beat signal, which is fed into a phase-lock loop. Spectral analysis of the beat signal (see fig. 1) confirms that the QCL is phase locked. This result opens the possibility to extend heterodyne interferometers into the far-infrared range
Dramatic reduction of surface recombination by in-situ surface passivation of silicon nanowires
Nanowires have unique optical properties [1-4] and are considered as
important building blocks for energy harvesting applications such as solar
cells. [2, 5-8] However, due to their large surface-to-volume ratios, the
recombination of charge carriers through surface states reduces the carrier
diffusion lengths in nanowires a few orders of magnitude,[9] often resulting in
the low efficiency (a few percent or less) of nanowire-based solar cells. [7,
8, 10, 11] Reducing the recombination by surface passivation is crucial for the
realization of high performance nanosized optoelectronic devices, but remains
largely unexplored. [7, 12-14] Here we show that a thin layer of amorphous
silicon (a-Si) coated on a single-crystalline silicon nanowire (sc-SiNW),
forming a core-shell structure in-situ in the vapor-liquid-solid (VLS) process,
reduces the surface recombination nearly two orders of magnitude. Under
illumination of modulated light, we measure a greater than 90-fold improvement
in the photosensitivity of individual core-shell nanowires, compared to regular
nanowires without shell. Simulations of the optical absorption of the nanowires
indicate that the strong absorption of the a-Si shell contributes to this
effect, but we conclude that the effect is mainly due to the enhanced carrier
lifetime by surface passivation
Study of the temperature distribution in Si nanowires under microscopic laser beam excitation
The use of laser beams as excitation sources for the characterization of semiconductor nanowires (NWs) is largely extended. Raman spectroscopy and photoluminescence (PL) are currently applied to the study of NWs. However, NWs are systems with poor thermal conductivity and poor heat dissipation, which result in unintentional heating under the excitation with a focused laser beam with microscopic size, as those usually used in microRaman and microPL experiments. On the other hand, the NWs have subwavelength diameter, which changes the optical absorption with respect to the absorption in bulk materials. Furthermore, the NW diameter is smaller than the laser beam spot, which means that the optical power absorbed by the NW depends on its position inside the laser beam spot. A detailed analysis of the interaction between a microscopic focused laser beam and semiconductor NWs is necessary for the understanding of the experiments involving laser beam excitation of NWs. We present in this work a numerical analysis of the thermal transport in Si NWs, where the heat source is the laser energy locally absorbed by the NW. This analysis takes account of the optical absorption, the thermal conductivity, the dimensions, diameter and length of the NWs, and the immersion medium. Both free standing and heat-sunk NWs are considered. Also, the temperature distribution in ensembles of NWs is discussed. This analysis intends to constitute a tool for the understanding of the thermal phenomena induced by laser beams in semiconductor NWs
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