29 research outputs found
Influence of the surface roughness on the properties of Au films measured by surface plasmon resonance and x-ray reflectometry.
Thickness and refractive index of Au films thermally evaporated onto glass substrates and with an underlayer of Cr are determined from surface plasmon resonance. The results for the thickness are found to agree very well with those from X-ray reflectivity when a simple model of layers with flat interfaces is used. Plasmon propagation along thin films is influenced by radiative damping due to scattering from surface roughness. To study this influence the surface roughness of the glass substrate, Cr an Au layers are measured by X-ray reflectometry and the results used to introduce three intermediate layers with effective refractive indices and thicknesses corresponding to the roughness. Then Fresnel's equations are used to fit the reflectivity and to deduce the layer properties. It is found that the roughness affects to a great extent the optical parameters of the Au films even when it is smaller than 1 nm. In particular, the absolute value of real part of the dielectric constant decreases while its imaginary part increases when those effects are not taken into account. © 2011, Elsevier Ltd
Epitaxially grown III-arsenide-antimonide nanowires for optoelectronic applications
Epitaxially grown ternary III-arsenide-antimonide (III-As–Sb) nanowires (NWs) are increasingly attracting attention due to their feasibility as a platform for the integration of largely lattice-mismatched antimonide-based heterostructures while preserving the high crystal quality. This and the inherent bandgap tuning flexibility of III-As–Sb in the near- and mid-infrared wavelength regions are important and auspicious premises for a variety of optoelectronic applications. In this review, we summarize the current understanding of the nucleation, morphology-change and crystal phase evolution of GaAsSb and InAsSb NWs and their characterization, especially in relation to Sb incorporation during growth. By linking these findings to the optical properties in such ternary NWs and their heterostructures, a brief account of the ongoing development of III-As–Sb NW-based photodetectors and light emitters is also given
Determination of GaAs zinc blende/wurtzite band offsets utilizing GaAs nanowires with an axial GaAsSb insert
By applying a correlated micro-photoluminescence spectroscopy and transmission electron microscopy (TEM) approach, we have utilized molecular beam epitaxy grown self-catalysed GaAs nanowires (NWs) with an axial GaAsSb insert to determine the band offsets at the crystal phase heterojunction between zinc blende (ZB) and wurtzite (WZ) GaAs. Two distinct PL emission bands originating from the ZB GaAsSb insert were identified. The lower energy PL emission allowed an independent verification of the maximum Sb molar fraction to be ∼30%, in agreement with quantitative high-angle annular dark field scanning TEM performed on the same single NW. The higher energy PL emission revealed a low temperature ZB/WZ band offset of 120 meV at the interface between the two GaAs crystal phases occurring at the upper boundary of the insert. Separate conduction and valence band offsets develop at a higher temperature due to the different temperature dependence of the ZB and WZ GaAs band gaps, but both offset values show a relatively little variation in the range of 10–150 K
Engineering parallel and perpendicular polarized photoluminescence from a single semiconductor nanowire by crystal phase control
We report on a crystal phase-dependent photoluminescence (PL) polarization effect in individual wurtzite GaAs nanowires with a zinc blende GaAsSb insert grown by Au-assisted molecular beam epitaxy. The PL emission from the zinc blende GaAsSb insert is strongly polarized along the nanowire axis while the emission from the wurtzite GaAs nanowire is perpendicularly polarized. The results indicate that the crystal phases, through optical selection rules, are playing an important role in the alignment of the PL polarization in nanowires besides the linear polarization induced by the dielectric mismatch. The strong excitation power dependence and long recombination lifetimes (∼4 ns) from the wurtzite GaAs and zinc blende GaAsSb-related PL emission strongly indicate the existence of type II band alignments in the nanowire due to the presence of nanometer thin zinc blende segments and stacking faults in the wurtzite GaAs barrier. © 2010 American Chemical Society
GaN/AlGaN nanocolumn ultraviolet light-emitting diode using double-Layer graphene as substrate and transparent electrode
The many outstanding properties of graphene have impressed and intrigued scientists for the last few decades. Its transparency to light of all wavelengths combined with a low sheet resistance makes it a promising electrode material for novel optoelectronics. So far, no one has utilized graphene as both the substrate and transparent electrode of a functional optoelectronic device. Here, we demonstrate the use of double-layer graphene as a growth substrate and transparent conductive electrode for an ultraviolet light-emitting diode in a flip-chip configuration, where GaN/AlGaN nanocolumns are grown as the light-emitting structure using plasma-assisted molecular beam epitaxy. Although the sheet resistance is increased after nanocolumn growth compared with pristine double-layer graphene, our experiments show that the double-layer graphene functions adequately as an electrode. The GaN/AlGaN nanocolumns are found to exhibit a high crystal quality with no observable defects or stacking faults. Room-temperature electroluminescence measurements show a GaN related near bandgap emission peak at 365 nm and no defect-related yellow emission
Single-Mode Near-Infrared Lasing in a GaAsSb-Based Nanowire Superlattice at Room Temperature
Semiconductor nanowire lasers can produce guided coherent light emission with miniaturized geometry, bringing about new possibilities for a variety of applications including nanophotonic circuits, optical sensing, and on-chip and chip-to-chip optical communications. Here, we report on the realization of single-mode and room-temperature lasing from 890 to 990 nm, utilizing a novel design of single nanowires with GaAsSb-based multiple axial superlattices as a gain medium under optical pumping. The control of lasing wavelength via compositional tuning with excellent room-temperature lasing performance is shown to result from the unique nanowire structure with efficient gain material, which delivers a low lasing threshold of ∼6 kW/cm2 (75 μJ/cm2 per pulse), a lasing quality factor as high as 1250, and a high characteristic temperature of ∼129 K. These results present a major advancement for the design and synthesis of nanowire laser structures, which can pave the way toward future nanoscale integrated optoelectronic systems with superior performance
Graphene-Based Transparent Conducting Substrates for GaN/AlGaN Nanocolumn Flip-Chip Ultraviolet Light-Emitting Diodes
Flip-chip ultraviolet light-emitting diodes based on self-assembled GaN/AlGaN nanocolumns have been fabricated, exploiting single-layer graphene not only as a growth substrate but also as a transparent conducting electrode. High crystalline quality of the nanocolumns is confirmed by detailed electron microscopy characterization, also showing the intrinsic GaN quantum disk in the active region of the nanocolumns. These features are further confirmed in the optical emission, where the absence of defect-related yellow emission and the presence of blue-shifted (from the usual 365 nm band gap emission of bulk wurtzite GaN) emission at ∼350 nm, ascribed to quantum confinement and strain effects, are observed. Despite a noticeable graphene damage after the nanocolumn growth that causes high sheet resistance of graphene and high turn-on voltage, the proof of concept of single-layer graphene used as the transparent conducting substrate for a nanocolumn device is demonstrated. This study offers an alternative platform for the fabrication of next-generation nano-optoelectronic and electronic devices
Photoluminescence of the single wurtzite GaAs nanowire with different powers and temperatures
Single-Mode Near-Infrared Lasing in a GaAsSb-Based Nanowire Superlattice at Room Temperature
Semiconductor nanowire
lasers can produce guided coherent light
emission with miniaturized geometry, bringing about new possibilities
for a variety of applications including nanophotonic circuits, optical
sensing, and on-chip and chip-to-chip optical communications. Here,
we report on the realization of single-mode and room-temperature lasing
from 890 to 990 nm, utilizing a novel design of single nanowires with
GaAsSb-based multiple axial superlattices as a gain medium under optical
pumping. The control of lasing wavelength via compositional tuning
with excellent room-temperature lasing performance is shown to result
from the unique nanowire structure with efficient gain material, which
delivers a low lasing threshold of ∼6 kW/cm<sup>2</sup> (75
μJ/cm<sup>2</sup> per pulse), a lasing quality factor as high
as 1250, and a high characteristic temperature of ∼129 K. These
results present a major advancement for the design and synthesis of
nanowire laser structures, which can pave the way toward future nanoscale
integrated optoelectronic systems with superior performance
A Story Told by a Single Nanowire: Optical Properties of Wurtzite GaAs
The optical properties of the wurtzite (WZ) GaAs crystal
phase
found in nanowires (NWs) are a highly controversial topic. Here, we
study high-quality pure WZ GaAs/AlGaAs core–shell NWs grown
by Au-assisted molecular beam epitaxy (MBE) with microphotoluminescence
spectroscopy (μ-PL) and (scanning) transmission electron microscopy
on the very same single wire. We determine the room temperature (294
K) WZ GaAs bandgap to be 1.444 eV, which is ∼20 meV larger
than in zinc blende (ZB) GaAs, and show that the free exciton emission
at 15 K is at 1.516 eV. On the basis of time- and temperature-resolved
μ-PL results, we propose a Γ<sub>8</sub> conduction band
symmetry in WZ GaAs. We suggest a method for quantifying the optical
quality of NWs, taking into consideration the difference between the
room and low temperature integrated PL intensity, and demonstrate
that Au-assisted GaAs/AlGaAs core–shell NWs can have high PL
brightness up to room temperature