[Show abstract][Hide abstract] ABSTRACT: We report a novel graded refractive index antireflection coating for III/V
quadruple solar cells based on bottom-up grown tapered GaP nanowires. We have
calculated the photocurrent density of an InGaP-GaAs-InGaAsP-InGaAs solar cell
with a MgF2/ZnS double layer antireflection coating and with a graded
refractive index coating. The photocurrent density can be increased by 5.9 %
when the solar cell is coated with a graded refractive index layer with a
thickness of 1\mu m. We propose to realize such a graded refractive index layer
by growing tapered GaP nanowires on III/V solar cells. For a first
demonstration of the feasibility of the growth of tapered nanowires on III/V
solar cells, we have grown tapered GaP nanowires on AlInP/GaAs substrates. We
show experimentally that the reflection from the nanowire coated substrate is
reduced and that the transmission into the substrate is increased for a broad
spectral and angular range.
Solar Energy Materials and Solar Cells 10/2012; · 5.03 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The propagation of light in layers of vertically aligned nanowires is determined by their unique and extreme optical properties. Depending on the nanowire filling fraction and their diameter, layers of nanowires form strongly birefringent media. This large birefringence gives rise to sharp angle dependent peaks in polarized reflection. We demonstrate experimentally the tunability of the reflection by adding shells of SiO2 with thicknesses ranging from 10 nm to 30 nm around the nanowires. The strong modification of the reflection peaks renders nanowire layers as a promising candidate for sensing applications.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate experimentally the directional emission of light by InAsP segments embedded in InP nanowires. The nanowires are arranged in a periodic array, forming a 2D photonic crystal slab. The directionality of the emission is interpreted in terms of the preferential decay of the photoexcited nanowires and the InAsP segments into Bloch modes of the periodic structure. By simulating the emission of arrays of nanowires with the emitting segments located at different heights, we conclude that the position of this active region strongly influences the directionality and efficiency of the emission. Our results will help to improve the design of nanowire based LEDs and single photon sources.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate experimentally that arrays of base-tapered InP nanowires on top of an InP substrate form a broad band and omnidirectional absorbing medium. These characteristics are due to the specific geometry of the nanowires. Almost perfect absorption of light (higher than 97%) occurs in the system. We describe the strong optical absorption by finite-difference time-domain simulations and present the first study of the influence of the geometry of the nanowires on the enhancement of the optical absorption by arrays. Cylindrical nanowires present the highest absorption normalized to the volume fraction of the semiconductor. The absolute absorption in layers of conical nanowires is higher than that in cylindrical nanowires but requires a larger volume fraction of semiconducting material. Base-tapered nanowires, with a cylindrical top and a conical base, represent an intermediate geometry. These results set the basis for an optimized optical design of nanowire solar cells.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate experimentally that ensembles of conically shaped GaP nanorods form layers of graded refractive index due to the increased filling fraction of GaP from the top to the bottom of the layer. Graded refractive index layers reduce the reflection and increase the coupling of light into the substrate, leading to broadband and omnidirectional antireflection surfaces. This reduced reflection is the result of matching the refractive index at the interface between the substrate and air by the graded index layer. The layers can be modeled using a transfer-matrix method for isotropic layered media. We show theoretically that the light coupling efficiency into silicon can be higher than 95% over a broad wavelength range and for angles up to 60 • by employing a layer with a refractive index that increases parabolically. Broadband and omnidirectional antireflection layers are specially interesting for enhancing harvesting of light in photovoltaics.
Proceedings of SPIE - The International Society for Optical Engineering 08/2010;
[Show abstract][Hide abstract] ABSTRACT: Bio-inspired layers of semiconductor nanorods increase light coupling into a high refractive index substrate. Reflection and transmission measurements show unambiguously, that the reduced reflection is due to optical impedance matching at the interfaces.
[Show abstract][Hide abstract] ABSTRACT: We report quantitative, noninvasive and nanoscale-resolved mapping of the free-carrier distribution in InP nanowires with doping modulation along the axial and radial directions, by employing infrared near-field nanoscopy. Owing to the technique's capability of subsurface probing, we provide direct experimental evidence that dopants in interior nanowire shells effectively contribute to the local free-carrier concentration. The high sensitivity of s-SNOM also allows us to directly visualize nanoscale variations in the free-carrier concentration of wires as thin as 20 nm, which we attribute to local growth defects. Our results open interesting avenues for studying local conductivity in complex nanowire heterostructures, which could be further enhanced by near-field infrared nanotomography.
[Show abstract][Hide abstract] ABSTRACT: A generic process has been developed to grow nearly defect-free arrays of (heterostructured) InP and GaP nanowires. Soft nano-imprint lithography has been used to pattern gold particle arrays on full 2 inch substrates. After lift-off organic residues remain on the surface, which induce the growth of additional undesired nanowires. We show that cleaning of the samples before growth with piranha solution in combination with a thermal anneal at 550 degrees C for InP and 700 degrees C for GaP results in uniform nanowire arrays with 1% variation in nanowire length, and without undesired extra nanowires. Our chemical cleaning procedure is applicable to other lithographic techniques such as e-beam lithography, and therefore represents a generic process.
[Show abstract][Hide abstract] ABSTRACT: Birefringent layers of GaP nanowires are grown by metal-organic vapor phase epitaxy on top of a GaP substrate. We modified the reflection of the as-grown layer by adding a shell of 12 nm of SiO 2 around the nanowires. The effect of the shell on the effective refractive indices of the birefringent nanowire layer was calculated using Maxwell–Garnett effective medium theory for coated cylinders. The large change of the reflection due to the shell renders nanowire layers a promising material for sensing applications.
Semiconductor Science and Technology 02/2010; 25:24008-5. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that highly tunable nanowire arrays with optimized diameters, volume fractions, and alignment form one of the strongest optical scattering materials to date. Using a new broad-band technique, we explore the scattering strength of the nanowires by varying systematically their diameter and alignment on the substrate. We identify strong Mie-type internal resonances of the nanowires which can be tuned over the entire visible spectrum. The tunability of nanowire materials opens up exciting new prospects for fundamental and applied research ranging from random lasers to solar cells, exploiting the extreme scattering strength, internal resonances, and preferential alignment of the nanowires. Although we have focused our investigation on gallium phosphide nanowires, the results can be universally applied to other types of group III-V, II-VI, or IV nanowires.
[Show abstract][Hide abstract] ABSTRACT: A novel class of optically anisotropic materials is presented. Layers of semiconductor nanowires fabricated in a bottom-up process exhibit a large in-plane birefringence and show quarter-wavelength retardation for a wavelength of 690 nm. These nanowire metamaterials are promising materials for optical gas- and biosensing.
[Show abstract][Hide abstract] ABSTRACT: Semiconductor nanowires are novel nanostructures full of promise for optical applications. Nanowires have subwavelength diameters
and large aspect ratios, which combined with the high permittivity of semiconductors lead to a strong optical anisotropy.
We review in this chapter this optical anisotropy, focusing on the polarization anisotropy of the photoluminescence of individual
nanowires and the propagation of light through birefringent ensembles of aligned nanowires.
Recent developments in bottom-up nanofabrication techniques allow the growth of free-standing semiconductor nanowires with
controlled composition, lateral dimensions of typically 10–100 nm, and lengths of several micrometers (see Fig. 6.1). The
small lateral dimensions of nanowires enables to grow them heteroepitaxially onto different substrates [1–3] or even to design
heterostructures with segments, shells, and/or quantum dots of different semiconductors in a single nanowire [4–8]. Nanowires
are full of promise for monolithic integration of high-performance semiconductors with new functionality [8–11] into existing
silicon technology [2, 3, 12]. These nanostructures will offer new possibilities as next generation of optical and optoelectronical
components. Junctions in semiconductor nanowires and light emitting devices have been demonstrated [4, 13–17]. Although the
quantum efficiency of these nano-LEDs is still low, fast progress is being made on the passivation of the nanowire surface
and the increase of their efficiency [18, 19]. Also, optically and electrically driven nanowire lasing have been reported
[9, 20, 21]. Nanowires have been proposed as polarization sensitive photodetectors [22, 23] and as a source for single photons
The encouraging perspectives for novel applications has lead to improved control over nanowire synthesis and materials composition
[4, 5, 25–27]. However, little is known about how light is emitted by individual nanowires or how light is scattered by ensembles
of these nanostructures. The large geometrical anisotropy of nanowires and the high refractive index of semiconductors give
rise to a huge optical anisotropy, which has been reported as a strongly polarized photoluminescence of individual nanowires
along their long axis [22, 28]. In this chapter we review the polarization anisotropy in the photoluminescence of individual
nanowires. We also describe the propagation of light through ensembles of nanowires oriented perpendicularly to the surface
of a substrate. The controlled growth and alignment of the nanowires leads to a medium with giant birefringence , i.e.,
a medium with a large difference in refractive indexes for different polarizations. The giant birefringence in ensembles of
nanowires can be easily tuned by changing the semiconductor filling fraction and is not restricted to narrow frequency bands
as in periodic structures . Broadband and giant birefringence constitutes an elegant example of the extreme optical anisotropy
of nanowires, which may lead to nanoscale polarization controlling media , the efficient generation of nonlinear signals
, and the observation of novel surface electromagnetic modes on birefringent materials .
[Show abstract][Hide abstract] ABSTRACT: Here, we present the first broadband birefringence measurements on samples of GaP nanowires. The samples are grown vertically in the metal-organic vapor phase epitaxy (MOVPE) using the vapor-liquid-solid (VLS) growth mode.