10 research outputs found
Thermoelectric properties of In-rich InGaN and InN/InGaN superlattices
The thermoelectric properties of n-type InGaN alloys with high In-content and InN/InGaN thin film superlattices (SL) grown by molecular beam epitaxy are investigated. Room-temperature measurements of the thermoelectric properties reveal that an increasing Ga-content in ternary InGaN alloys (0 < x(Ga) < 0.2) yields a more than 10-fold reduction in thermal conductivity (κ) without deteriorating electrical conductivity (σ), while the Seebeck coefficient (S) increases slightly due to a widening band gap compared to binary InN. Employing InN/InGaN SLs (x(Ga) = 0.1) with different periods, we demonstrate that confinement effects strongly enhance electron mobility with values as high as ∼820 cm2/V s at an electron density ne of ∼5×1019 cm−3, leading to an exceptionally high σ of ∼5400 (Ωcm)−1. Simultaneously, in very short-period SL structures S becomes decoupled from ne, κ is further reduced below the alloy limit (κ < 9 W/m-K), and the power factor increases to 2.5×10−4 W/m-K2 by more than a factor of 5 as compared to In-rich InGaN alloys. These findings demonstrate that quantum confinement in group-III nitride-based superlattices facilitates improvements of thermoelectric properties over bulk-like ternary nitride alloys
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
Crystal Phase Quantum Dots in the Ultrathin Core of GaAs–AlGaAs Core–Shell Nanowires
Semiconductor quantum dots embedded
in nanowires (NW-QDs) can be used as efficient sources of nonclassical
light with ultrahigh brightness and indistinguishability, needed for
photonic quantum information technologies. Although most NW-QDs studied
so far focus on heterostructure-type QDs that provide an effective
electronic confinement potential using chemically distinct regions
with dissimilar electronic structure, homostructure NWs can localize
excitons at crystal phase defects in leading to NW-QDs. Here, we optically
investigate QD emitters embedded in GaAs–AlGaAs core–shell
NWs, where the excitons are confined in an ultrathin-diameter NW core
and localized along the axis of the NW core at wurtzite (WZ)/zincblende
(ZB) crystal phase defects. Photoluminescence (PL)-excitation measurements
performed on the QD-emission reveal sharp resonances arising from
excited electronic states of the axial confinement potential. The
QD-like nature of the emissive centers are suggested by the observation
of a narrow PL line width, as low as ∼300 μeV, and confirmed
by the observation of clear photon antibunching in autocorrelation
measurements. Most interestingly, time-resolved PL measurements reveal
a very short radiative lifetime <1 ns, indicative of a transition
from a type-II to type-I band alignment of the WZ/ZB crystal interface
in GaAs due to the strong quantum confinement in the ultrathin NW
core
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