7 research outputs found
Atomic Scale Strain Relaxation in Axial Semiconductor IIIāV Nanowire Heterostructures
Combination of mismatched materials
in semiconductor nanowire heterostructures
offers a freedom of bandstructure engineering that is impossible in
standard planar epitaxy. Nevertheless, the presence of strain and
structural defects directly control the optoelectronic properties
of these nanomaterials. Understanding with atomic accuracy how mismatched
heterostructures
release or accommodate strain, therefore, is highly desirable. By
using atomic resolution high angle annular dark field scanning transmission
electron microscopy combined with geometrical phase analyses and computer
simulations, we are able to establish the relaxation mechanisms (including
both elastic and plastic deformations) to release the mismatch strain
in axial nanowire heterostructures. Formation of misfit dislocations,
diffusion of atomic species, polarity transfer, and induced structural
transformations are studied with atomic resolution at the intermediate
ternary interfaces. Two nanowire heterostructure systems with promising
applications (InAs/InSb and GaAs/GaSb) have been selected as key examples
Nanowires Grown on InP (100): Growth Directions, Facets, Crystal Structures, and Relative Yield Control
Growth of IIIāV nanowires on the [100]-oriented industry standard substrates is critical for future integrated nanowire device development. Here we present an in-depth analysis of the seemingly complex ensembles of epitaxial nanowires grown on InP (100) substrates. The nanowires are categorized into three types as vertical, nonvertical, and planar, and the growth directions, facets, and crystal structure of each type are investigated. The nonvertical growth directions are mathematically modeled using a three-dimensional multiple-order twinning concept. The nonvertical nanowires can be further classified into two different types, with one type growing in the āØ111ā© directions and the other in the āØ100ā© directions after initial multiple three-dimensional twinning. We find that 99% of the total nanowires are grown either along āØ100ā©, āØ111ā©, or āØ110ā© growth directions by {100} or {111} growth facets. We also demonstrate relative control of yield of these different types of nanowires, by tuning pregrowth annealing conditions and growth parameters. Together, the knowledge and controllability of the types of nanowires provide an ideal foundation to explore novel geometries that combine different crystal structures, with potential for both fundamental science research and device applications
Strong Amplified Spontaneous Emission from High Quality GaAs<sub>1ā<i>x</i></sub>Sb<sub><i>x</i></sub> Single Quantum Well Nanowires
Quantum
confinement in semiconductor nanowires is of contemporary
interest. Enhancing the quantum efficiency of quantum wells in nanowires
and minimizing intrinsic absorption are necessary for reducing the
threshold of nanowire lasers and are promising for wavelength tunable
emitters and detectors. Here, we report on growth and optimization
of GaAs<sub>1ā<i>x</i></sub>Sb<sub><i>x</i></sub>/Al<sub>1ā<i>y</i></sub>Ga<sub><i>y</i></sub>As quantum well heterostructures formed radially around pure
zinc blende GaAs core nanowires. The emitted photon energy from GaAs<sub>0.89</sub>Sb<sub>0.11</sub> quantum well (1.371 eV) is smaller than
the GaAs core, thus showing advantages over GaAs/Al<sub>1ā<i>y</i></sub>Ga<sub><i>y</i></sub>As quantum well nanowires
in photon emission. The high optical quality quantum well (internal
quantum efficiency reaches as high as 90%) is carefully positioned
so that the quantum well coincides with the maximum of the transverse
electric (TE01) mode intensity profile. The obtained superior optical
performance combined with the supported FabryāPerot (FāP)
cavity in the nanowire leads to the strong amplified spontaneous emission
(ASE). Detailed studies of the amplified cavity mode are carried out
by spatialāspectral photoluminescence (PL) imaging, where emission
from nanowire is resolved both spatially and spectrally. Resonant
emission is generated at nanowire ends and is polarized perpendicular
to the nanowire, in agreement with the simulated polarization characteristics
of the TE01 mode in the nanowire. The observation of strong ASE for
single QW nanowire at room temperature shows the potential application
of GaAs<sub>1ā<i>x</i></sub>Sb<sub><i>x</i></sub> QW nanowires as low threshold infrared nanowire lasers
Zn<sub>3</sub>As<sub>2</sub> Nanowires and Nanoplatelets: Highly Efficient Infrared Emission and Photodetection by an Earth Abundant Material
The development of earth abundant
materials for optoelectronics and photovoltaics promises improvements
in sustainability and scalability. Recent studies have further demonstrated
enhanced material efficiency through the superior light management
of novel nanoscale geometries such as the nanowire. Here we show that
an industry standard epitaxy technique can be used to fabricate high
quality IIāV nanowires (1D) and nanoplatelets (2D) of the earth
abundant semiconductor Zn<sub>3</sub>As<sub>2</sub>. We go on to establish
the optoelectronic potential of this material by demonstrating efficient
photoemission and detection at 1.0 eV, an energy which is significant
to the fields of both photovoltaics and optical telecommunications.
Through dynamical spectroscopy this superior performance is found
to arise from a low rate of surface recombination combined with a
high rate of radiative recombination. These results introduce nanostructured
Zn<sub>3</sub>As<sub>2</sub> as a high quality optoelectronic material
ready for device exploration
Simultaneous Selective-Area and VaporāLiquidāSolid Growth of InP Nanowire Arrays
Selective-area
epitaxy is highly successful in producing application-ready size-homogeneous
arrays of IIIāV nanowires without the need to use metal catalysts.
Previous works have demonstrated excellent control of nanowire properties
but the growth mechanisms remain rather unclear. Herein, we report
a detailed growth study revealing that fundamental growth mechanisms
of pure wurtzite InP āØ111ā©A nanowires can indeed differ
significantly from the simple picture of a facet-limited selective-area
growth process. A dual growth regime with and without metallic droplet
is found to coexist under the same growth conditions for different
diameter nanowires. Incubation times and highly nonmonotonous growth
rate behaviors are revealed and explained within a dedicated kinetic
model
Selective-Area Epitaxy of Pure Wurtzite InP Nanowires: High Quantum Efficiency and Room-Temperature Lasing
We report the growth of stacking-fault-free
and taper-free wurtzite
InP nanowires with diameters ranging from 80 to 600 nm using selective-area
metalāorganic vapor-phase epitaxy and experimentally determine
a quantum efficiency of ā¼50%, which is on par with InP epilayers.
We also demonstrate room-temperature, photonic mode lasing from these
nanowires. Their excellent structural and optical quality opens up
new possibilities for both fundamental quantum optics and optoelectronic
devices
Unit Cell Structure of Crystal Polytypes in InAs and InSb Nanowires
The atomic distances in hexagonal polytypes of IIIāV compound semiconductors differ from the values expected from simply a change of the stacking sequence of (111) lattice planes. While these changes were difficult to quantify so far, we accurately determine the lattice parameters of zinc blende, wurtzite, and 4H polytypes for InAs and InSb nanowires, using X-ray diffraction and transmission electron microscopy. The results are compared to density functional theory calculations. Experiment and theory show that the occurrence of hexagonal bilayers tends to stretch the distances of atomic layers parallel to the <i>c</i> axis and to reduce the in-plane distances compared to those in zinc blende. The change of the lattice parameters scales linearly with the hexagonality of the polytype, defined as the fraction of bilayers with hexagonal character within one unit cell