6 research outputs found
Nanowires Bending over Backward from Strain Partitioning in Asymmetric CoreâShell Heterostructures
The flexibility and
quasi-one-dimensional nature of nanowires offer
wide-ranging possibilities for novel heterostructure design and strain
engineering. In this work, we realize arrays of extremely and controllably
bent nanowires comprising lattice-mismatched and highly asymmetric
coreâshell heterostructures. Strain sharing across the nanowire
heterostructures is sufficient to bend vertical nanowires over backward
to contact either neighboring nanowires or the substrate itself, presenting
new possibilities for designing nanowire networks and interconnects.
Photoluminescence spectroscopy on bent-nanowire heterostructures reveals
that spatially varying strain fields induce charge carrier drift toward
the tensile-strained outside of the nanowires, and that the polarization
response of absorbed and emitted light is controlled by the bending
direction. This unconventional strain field is employed for light
emission by placing an active region of quantum dots at the outer
side of a bent nanowire to exploit the carrier drift and tensile strain.
These results demonstrate how bending in nanoheterostructures opens
up new degrees of freedom for strain and device engineering
Three-Dimensional Magneto-Photoluminescence as a Probe of the Electronic Properties of Crystal-Phase Quantum Disks in GaAs Nanowires
Crystal-phase
engineering has emerged as a novel method of bandgap
engineering, made feasible by the high surface-to-volume ratio of
nanowires. There remains intense debate about the exact characteristics
of the band structure of the novel crystal phases, such as wurtzite
GaAs, obtained by this approach. We attack this problem via a low-temperature
angle-dependent magneto-photoluminescence study of wurtzite/zinc-blende
quantum disks in single GaAs nanowires. The exciton diamagnetic coefficient
is proportional to the electronâhole correlation length, enabling
a determination of the spatial extent of the exciton wave function
in the plane and along the confinement axis of the crystal-phase quantum
disks. Depending on the disk nature, the diamagnetic coefficient measured
in Faraday geometry ranges between 25 and 75 ÎŒeV/T<sup>2</sup>. For a given disk, the diamagnetic coefficient remains constant
upon rotation of the magnetic field. Along with our envelope function
calculation accounting for excitonic effects, we demonstrate that
the electron effective mass in wurtzite GaAs quantum disks is heavy,
mostly isotropic and results from mixing of the two lower-energy conduction
bands with Î<sub>7</sub> and Î<sub>8</sub> symmetries.
Finally, we discuss the implications of the results of the angle dependent
magneto-luminescence for the likely symmetry of the exciton states.
This work provides important insight in the band structure of wurtzite
GaAs for future nanowire-based polytypic bandgap engineering
Radial Stark Effect in (In,Ga)N Nanowires
We study the luminescence of unintentionally
doped and Si-doped
In<sub><i>x</i></sub>Ga<sub>1â<i>x</i></sub>N nanowires with a low In content (<i>x</i> < 0.2) grown
by molecular beam epitaxy on Si substrates. The emission band observed
at 300 K from the unintentionally doped samples is centered at much
lower energies (800 meV) than expected from the In content measured
by X-ray diffractometry and energy dispersive X-ray spectroscopy.
This discrepancy arises from the pinning of the Fermi level at the
sidewalls of the nanowires, which gives rise to strong radial built-in
electric fields. The combination of the built-in electric fields with
the compositional fluctuations inherent to (In,Ga)N alloys induces
a competition between spatially direct and indirect recombination
channels. At elevated temperatures, electrons at the core of the nanowire
recombine with holes close to the surface, and the emission from unintentionally
doped nanowires exhibits a Stark shift of several hundreds of meV.
The competition between spatially direct and indirect transitions
is analyzed as a function of temperature for samples with various
Si concentrations. We propose that the radial Stark effect is responsible
for the broadband absorption of (In,Ga)N nanowires across the entire
visible range, which makes these nanostructures a promising platform
for solar energy applications
Observation of Dielectrically Confined Excitons in Ultrathin GaN Nanowires up to Room Temperature
The realization of semiconductor
structures with stable excitons
at room temperature is crucial for the development of excitonics and
polaritonics. Quantum confinement has commonly been employed for enhancing
excitonic effects in semiconductor heterostructures. Dielectric confinement,
which gives rises to much stronger enhancement, has proven to be more
difficult to achieve because of the rapid nonradiative surface/interface
recombination in hybrid dielectric-semiconductor structures. Here,
we demonstrate intense excitonic emission from bare GaN nanowires
with diameters down to 6 nm. The large dielectric mismatch between
the nanowires and vacuum greatly enhances the Coulomb interaction,
with the thinnest nanowires showing the strongest dielectric confinement
and the highest radiative efficiency at room temperature. In situ
monitoring of the fabrication of these structures allows one to accurately
control the degree of dielectric enhancement. These ultrathin nanowires
may constitute the basis for the fabrication of advanced low-dimensional
structures with an unprecedented degree of confinement
Crystal-Phase Quantum Wires: One-Dimensional Heterostructures with Atomically Flat Interfaces
In
semiconductor quantum-wire heterostructures, interface roughness
leads to exciton localization and to a radiative decay rate much smaller
than that expected for structures with flat interfaces. Here, we uncover
the electronic and optical properties of the one-dimensional extended
defects that form at the intersection between stacking faults and
inversion domain boundaries in GaN nanowires. We show that they act
as crystal-phase quantum wires, a novel one-dimensional quantum system
with atomically flat interfaces. These quantum wires efficiently capture
excitons whose radiative decay gives rise to an optical doublet at
3.36 eV at 4.2 K. The binding energy of excitons confined in crystal-phase
quantum wires is measured to be more than twice larger than that of
the bulk. As a result of their unprecedented interface quality, these
crystal-phase quantum wires constitute a model system for the study
of one-dimensional excitons
Molecular Beam Epitaxy of GaN Nanowires on Epitaxial Graphene
We demonstrate an
all-epitaxial and scalable growth approach to
fabricate single-crystalline GaN nanowires on graphene by plasma-assisted
molecular beam epitaxy. As substrate, we explore several types of
epitaxial graphene layer structures synthesized on SiC. The different
structures differ mainly in their total number of graphene layers.
Because graphene is found to be etched under active N exposure, the
direct growth of GaN nanowires on graphene is only achieved on multilayer
graphene structures. The analysis of the nanowire ensembles prepared
on multilayer graphene by Raman spectroscopy and transmission electron
microscopy reveals the presence of graphene underneath as well as
in between nanowires, as desired for the use of this material as contact
layer in nanowire-based devices. The nanowires nucleate preferentially
at step edges, are vertical, well aligned, epitaxial, and of comparable
structural quality as similar structures fabricated on conventional
substrates