7 research outputs found
Abrupt GaP/GaAs Interfaces in Self-Catalyzed Nanowires
We
achieve the self-catalyzed growth of pure GaP nanowires and GaAs<sub>1â<i>x</i></sub>P<sub><i>x</i></sub>/GaAs<sub>1â<i>y</i></sub>P<sub><i>y</i></sub> nanowire
heterostructures by solid-source molecular beam epitaxy. Consecutive
segments of nearly pure GaAs and GaP are fabricated by commuting the
group V fluxes. We test different flux switching procedures and measure
the corresponding interfacial composition profiles with atomic resolution
using high-angle annular dark field scanning transmission electron
microscopy. Interface abruptness is drastically improved by switching
off all the molecular beam fluxes for a short time at the group V
commutation. Finally, we demonstrate that the morphology of the growth
front can be either flat or truncated, depending on the growth conditions.
The method presented here allows for the facile synthesis of high
quality GaP/GaAs axial heterostructures directly on Si (111) wafers
Sharpening the Interfaces of Axial Heterostructures in Self-Catalyzed AlGaAs Nanowires: Experiment and Theory
The
growth of IIIâIIIâV axial heterostructures in nanowires
via the vaporâliquidâsolid method is deemed to be unfavorable
because of the high solubility of group III elements in the catalyst
droplet. In this work, we study the formation by molecular beam epitaxy
of self-catalyzed GaAs nanowires with Al<sub><i>x</i></sub>Ga<sub>1â<i>x</i></sub>As insertions. The composition
profiles are extracted and analyzed with monolayer resolution using
high-angle annular dark-field scanning transmission electron microscopy.
We test successfully several growth procedures to sharpen the heterointerfaces.
For a given nanowire geometry, prefilling the droplet with Al atoms
is shown to be the most efficient way to reduce the width of the GaAs/Al<sub><i>x</i></sub>Ga<sub>1â<i>x</i></sub>As
interface. Using the thermodynamic data available in the literature,
we develop numerical and analytical models of the composition profiles,
showing very good agreement with experiments. These models suggest
that atomically sharp interfaces are attainable for catalyst droplets
of small volumes
Measuring and Modeling the Growth Dynamics of Self-Catalyzed GaP Nanowire Arrays
The bottom-up fabrication of regular
nanowire (NW) arrays on a
masked substrate is technologically relevant, but the growth dynamic
is rather complex due to the superposition of severe shadowing effects
that vary with array pitch, NW diameter, NW height, and growth duration.
By inserting GaAsP marker layers at a regular time interval during
the growth of a self-catalyzed GaP NW array, we are able to retrieve
precisely the time evolution of the diameter and height of a single
NW. We then propose a simple numerical scheme which fully computes
shadowing effects at play in infinite arrays of NWs. By confronting
the simulated and experimental results, we infer that re-emission
of Ga from the mask is necessary to sustain the NW growth while Ga
migration on the mask must be negligible. When compared to random
cosine or random uniform re-emission from the mask, the simple case
of specular reflection on the mask gives the most accurate account
of the Ga balance during the growth
Growth of Vertical GaAs Nanowires on an Amorphous Substrate via a Fiber-Textured Si Platform
We demonstrate the vertical self-catalyzed
molecular beam epitaxy
(MBE) growth of GaAs nanowires on an amorphous SiO<sub>2</sub> substrate
by using a smooth [111] fiber-textured silicon thin film with very
large grains, fabricated by aluminum-induced crystallization. This
generic platform paves the way to the use of inexpensive substrates
for the fabrication of dense ensembles of vertically standing nanowires
(NWs) with promising perspectives for the integration of NWs in devices
Photon Cascade from a Single Crystal Phase Nanowire Quantum Dot
We report the first comprehensive experimental and theoretical study
of the optical properties of single crystal phase quantum dots in
InP nanowires. Crystal phase quantum dots are defined by a transition
in the crystallographic lattice between zinc blende and wurtzite segments
and therefore offer unprecedented potential to be controlled with
atomic layer accuracy without random alloying. We show for the first
time that crystal phase quantum dots are a source of pure single-photons
and cascaded photon-pairs from type II transitions with excellent
optical properties in terms of intensity and line width. We notice
that the emission spectra consist often of two peaks close in energy,
which we explain with a comprehensive theory showing that the symmetry
of the system plays a crucial role for the hole levels forming hybridized
orbitals. Our results state that crystal phase quantum dots have promising
quantum optical properties for single photon application and quantum
optics
Determination of nâType Doping Level in Single GaAs Nanowires by Cathodoluminescence
We
present an effective method of determining the doping level
in n-type IIIâV semiconductors at the nanoscale. Low-temperature
and room-temperature cathodoluminescence (CL) measurements are carried
out on single Si-doped GaAs nanowires. The spectral shift to higher
energy (BursteinâMoss shift) and the broadening of luminescence
spectra are signatures of increased electron densities. They are compared
to the CL spectra of calibrated Si-doped GaAs layers, whose doping
levels are determined by Hall measurements. We apply the generalized
Planckâs law to fit the whole spectra, taking into account
the electron occupation in the conduction band, the bandgap narrowing,
and band tails. The electron Fermi levels are used to determine the
free electron concentrations, and we infer nanowire doping of 6 Ă
10<sup>17</sup> to 1 Ă 10<sup>18</sup>âcm<sup>â3</sup>. These results show that cathodoluminescence provides a robust way
to probe carrier concentrations in semiconductors with the possibility
of mapping spatial inhomogeneities at the nanoscale
Morphology Tailoring and Growth Mechanism of Indium-Rich InGaN/GaN Axial Nanowire Heterostructures by Plasma-Assisted Molecular Beam Epitaxy
We
investigate the growth mechanism of axially heterostructured
InGaN/GaN nanowires (NWs) as a function of the flux conditions. The
InGaN heterostructure morphology critically depends on the In/Ga flux
ratio affecting the local V/III ratio at the NW growth front. Locally
N-rich conditions are associated with tapered island-like morphologies,
while metal-rich conditions, leading to the formation of a stable
Indium adsorbed layer at the NW growth front, promote the growth of
heterostructures with a disk-like shape. Based on experimental results
and theoretical predictions, we demonstrate that this indium ad-layer
acts as a surfactant inducing a modification of the InGaN heterostructure
growth mode. The impact of flux conditions and strain relaxation on
the Indium incorporation are also addressed. The resulting insertions
present abrupt interfaces and a homogeneous In distribution for In
contents up to 40%