6 research outputs found
Origin of Polytype Formation in VLS-Grown Ge Nanowires through Defect Generation and Nanowire Kinking
We
propose layer-by-layer growth mechanisms to account for planar
defect generation leading to kinked polytype nanowires. C<sub>s</sub>-corrected scanning transmission electron microscopy enabled identification
of stacking sequences of distinct polytype bands found in kinked nanowires,
and Raman spectroscopy was used to distinguish polytype nanowires
from twinned nanowires containing only the 3C diamond cubic phase.
The faceting and atomic-scale defect structures of twinned 3C are
compared with those of polytype nanowires to develop a common model
linking nucleation pinning to nanowire morphology and phase
Impact of Dopant Compensation on Graded <i>p</i>–<i>n</i> Junctions in Si Nanowires
The modulation between different
doping species required to produce
a diode in VLS-grown nanowires (NWs) yields a complex doping profile,
both axially and radially, and a gradual junction at the interface.
We present a detailed analysis of the dopant distribution around the
junction. By combining surface potential measurements, performed by
KPFM, with finite element simulations, we show that the highly doped
(5 × 10<sup>19</sup> cm<sup>–3</sup>) shell surrounding
the NW can screen the junction’s built in voltage at shell
thickness as low as 3 nm. By comparing NWs with high and low doping
contrast at the junction, we show that dopant compensation dramatically
decreases the electrostatic width of the junction and results in relatively
low leakage currents
Connecting Composition-Driven Faceting with Facet-Driven Composition Modulation in GaAs–AlGaAs Core–Shell Nanowires
Ternary
III–V alloys of tunable bandgap are a foundation
for engineering advanced optoelectronic devices based on quantum-confined
structures including quantum wells, nanowires, and dots. In this context,
core–shell nanowires provide useful geometric degrees of freedom
in heterostructure design, but alloy segregation is frequently observed
in epitaxial shells even in the absence of interface strain. High-resolution
scanning transmission electron microscopy and laser-assisted atom
probe tomography were used to investigate the driving forces of segregation
in nonplanar GaAs–AlGaAs core–shell nanowires. Growth-temperature-dependent
studies of Al-rich regions growing on radial {112} nanofacets suggest
that facet-dependent bonding preferences drive the enrichment, rather
than kinetically limited diffusion. Observations of the distinct interface
faceting when pure AlAs is grown on GaAs confirm the preferential
bonding of Al on {112} facets over {110} facets, explaining the decomposition
behavior. Furthermore, three-dimensional composition profiles generated
by atom probe tomography reveal the presence of Al-rich nanorings
perpendicular to the growth direction; correlated electron microscopy
shows that short zincblende insertions in a nanowire segment with
predominantly wurtzite structure are enriched in Al, demonstrating
that crystal phase engineering can be used to modulate composition.
The findings suggest strategies to limit alloy decomposition and promote
new geometries of quantum confined structures
Alloy Fluctuations Act as Quantum Dot-like Emitters in GaAs-AlGaAs Core–Shell Nanowires
GaAs-Al<sub><i>x</i></sub>Ga<sub>1–<i>x</i></sub>As (AlGaAs) core–shell nanowires show great promise for nanoscale electronic and optoelectronic devices, but the application of these nonplanar heterostructures in devices requires improved understanding and control of nanoscale alloy composition and interfaces. Multiple researchers have observed sharp emission lines of unknown origin below the AlGaAs band edge in photoluminescence (PL) spectra of core–shell nanowires; point defects, alloy composition fluctuations, and self-assembled quantum dots have been put forward as candidate structures. Here we employ laser-assisted atom probe tomography to reveal structural and compositional features that give rise to the sharp PL emission spectra. Nanoscale ellipsoidal Ga-enriched clusters resulting from random composition fluctuations are identified in the AlGaAs shell, and their compositions, size distributions, and interface characteristics are analyzed. Simulations of exciton transition energies in ellipsoidal quantum dots are used to relate the Ga nanocluster distribution with the distribution of sharp PL emission lines. We conclude that the Ga rich clusters can act as discrete emitters provided that the major diameter is ≥4 nm. Smaller clusters are under-represented in the PL spectrum, and spectral lines of larger clusters are broadened, due to quantum tunneling between clusters
He-Ion Microscopy as a High-Resolution Probe for Complex Quantum Heterostructures in Core–Shell Nanowires
Core–shell
semiconductor nanowires (NW) with internal quantum
heterostructures are amongst the most complex nanostructured materials
to be explored for assessing the ultimate capabilities of diverse
ultrahigh-resolution imaging techniques. To probe the structure and
composition of these materials in their native environment with minimal
damage and sample preparation calls for high-resolution electron or
ion microscopy methods, which have not yet been tested on such classes
of ultrasmall quantum nanostructures. Here, we demonstrate that scanning
helium ion microscopy (SHeIM) provides a powerful and straightforward
method to map quantum heterostructures embedded in complex III–V
semiconductor NWs with unique material contrast at ∼1 nm resolution.
By probing the cross sections of GaAs-AlÂ(Ga)As core–shell NWs
with coaxial GaAs quantum wells as well as short-period GaAs/AlAs
superlattice (SL) structures in the shell, the Al-rich and Ga-rich
layers are accurately discriminated by their image contrast in excellent
agreement with correlated, yet destructive, scanning transmission
electron microscopy and atom probe tomography analysis. Most interestingly,
quantitative He-ion dose-dependent SHeIM analysis of the ternary AlGaAs
shell layers and of compositionally nonuniform GaAs/AlAs SLs reveals
distinct alloy composition fluctuations in the form of Al-rich clusters
with size distributions between ∼1–10 nm. In the GaAs/AlAs
SLs the alloy clustering vanishes with increasing SL-period (>5
nm-GaAs/4
nm-AlAs), providing insights into critical size dimensions for atomic
intermixing effects in short-period SLs within a NW geometry. The
straightforward SHeIM technique therefore provides unique benefits
in imaging the tiniest nanoscale features in topography, structure
and composition of a multitude of diverse complex semiconductor nanostructures
Quantum Transport and Sub-Band Structure of Modulation-Doped GaAs/AlAs Core–Superlattice Nanowires
Modulation-doped
III–V semiconductor nanowire (NW) heterostructures
have recently emerged as promising candidates to host high-mobility
electron channels for future high-frequency, low-energy transistor
technologies. The one-dimensional geometry of NWs also makes them
attractive for studying quantum confinement effects. Here, we report
correlated investigations into the discrete electronic sub-band structure
of confined electrons in the channel of Si δ-doped GaAs−GaAs/AlAs
core−superlattice NW heterostructures and the associated signatures
in low-temperature transport. On the basis of accurate structural
and dopant analysis using scanning transmission electron microscopy
and atom probe tomography, we calculated the sub-band structure of
electrons confined in the NW core and employ a labeling system inspired
by atomic orbital notation. Electron transport measurements on top-gated
NW transistors at cryogenic temperatures revealed signatures consistent
with the depopulation of the quasi-one-dimensional sub-bands, as well
as confinement in zero-dimensional-like states due to an impurity-defined
background disorder potential. These findings are instructive toward
reaching the ballistic transport regime in GaAs−AlGaAs based
NW systems