Nanoscale spectroscopic imaging of GaAs-AlGaAs quantum well tube nanowires: correlating luminescence with nanowire size and inner multishell structure

The luminescence and inner structure of GaAs-AlGaAs quantum well tube (QWT) nanowires were studied using low-temperature cathodoluminescence (CL) spectroscopic imaging, in combination with scanning transmission electron microscopy (STEM) tomography, allowing for the first time a robust correlation between the luminescence properties of these nanowires and their size and inner 3D structure down to the nanoscale. Besides the core luminescence and minor defects-related contributions, each nanowire showed one or more QWT peaks associated with nanowire regions of different diameters. The values of the GaAs shell thickness corresponding to each QWT peak were then determined from the nanowire diameters by employing a multishell growth model upon validation against experimental data (core diameter and GaAs and AlGaAs shell thickness) obtained from the analysis of the 3D reconstructed STEM tomogram of a GaAs-AlGaAs QWT nanowire. We found that QWT peak energies as a function of thus-estimated (3-7 nm) GaAs shell thickness are 40-120 meV below the theoretical values of exciton recombination for uniform QWTs symmetrically wrapped around a central core. However, the analysis of the 3D tomogram further evidenced azimuthal asymmetries as well as (azimuthal and axial) random fluctuations of the GaAs shell thickness, suggesting that the red-shift of QWT emissions is prominently due to carrier localization. The CL mapping of QWT emission intensities along the nanowire axis allowed to directly image the nanoscale localization of the emission, supporting the above picture. Our findings contribute to a deeper understanding of the luminescence-structure relationship in QWT nanowires and will foster their applications as efficient nanolaser sources for future monolithic integration onto silicon

Three-Dimensional Composition and Electric Potential Mapping of III–V Core–Multishell Nanowires by Correlative STEM and Holographic Tomography

The nondestructive characterization of nanoscale devices, such as those based on semiconductor nanowires, in terms of functional potentials is crucial for correlating device properties with their morphological/materials features, as well as for precisely tuning and optimizing their growth process. Electron holographic tomography (EHT) has been used in the past to reconstruct the total potential distribution in three-dimension but hitherto lacked a quantitative approach to separate potential variations due to chemical composition changes (mean inner potential, MIP) and space charges. In this Letter, we combine and correlate EHT and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) tomography on an individual ⟨111⟩ oriented GaAs–AlGaAs core–multishell nanowire (NW). We obtain excellent agreement between both methods in terms of the determined Al concentration within the AlGaAs shell, as well as thickness variations of the few nanometer thin GaAs shell acting as quantum well tube. Subtracting the MIP determined from the STEM tomogram, enables us to observe functional potentials at the NW surfaces and at the Au–NW interface, both ascribed to surface/interface pinning of the semiconductor Fermi level

Determination of surface lattice strain in ZnTe epilayers on {100}GaAs by ion channeling and reflectance spectroscopy

We report on the direct measurements of surface lattice strain in ZnTe epitaxial layers on {100}GaAs substrates by ion channeling Rutherford backscattering spectrometry and low‐temperature (10 K) reflectance spectroscopy measurements. The measured ZnTe strain is the superposition of the expected thermal (tensile) strain and a thickness‐dependent residual compressive strain. Our data indicate that the removal of this residual strain is slower than the rate predicted by the equilibrium theory, following an apparent h−1/2 power‐law dependence on the epilayer thickness h, above ∼100 nm

Lattice Strain Relaxation and Compositional Control in As-Rich GaAsP/(100)GaAs Heterostructures Grown by MOVPE

The fabrication of high-efficiency GaAsP-based solar cells on GaAs wafers requires addressing structural issues arising from the materials lattice mismatch. We report on tensile strain relaxation and composition control of MOVPE-grown As-rich GaAs1−xPx/(100)GaAs heterostructures studied by double-crystal X-ray diffraction and field emission scanning electron microscopy. Thin (80–150 nm) GaAs1−xPx epilayers appear partially relaxed (within 1−12% of the initial misfit) through a network of misfit dislocations along the sample [011] and [011−] in plane directions. Values of the residual lattice strain as a function of epilayer thickness were compared with predictions from the equilibrium (Matthews–Blakeslee) and energy balance models. It is shown that the epilayers relax at a slower rate than expected based on the equilibrium model, an effect ascribed to the existence of an energy barrier to the nucleation of new dislocations. The study of GaAs1−xPx composition as a function of the V-group precursors ratio in the vapor during growth allowed for the determination of the As/P anion segregation coefficient. The latter agrees with values reported in the literature for P-rich alloys grown using the same precursor combination. P-incorporation into nearly pseudomorphic heterostructures turns out to be kinetically activated, with an activation energy EA = 1.41 ± 0.04 eV over the entire alloy compositional range

On the MOVPE growth and luminescence properties of GaAs-AlGaAs core-multishell nanowire quantum structures

none5noWe report on the growth of GaAs-AlGaAs core-multishell nanowire quantum heterostructures by metalorganic vapor phase epitaxy, and their photoluminescence (PL) properties. Dense arrays of vertically-aligned GaAs nanowires were fabricated onto (111)B-GaAs wafers by Au-catalyzed self-assembly, and radially overgrown by two AlGaAs shells between which a few-nm thin GaAs shell was introduced to form a quantum well tube (QWT). Besides the GaAs nanowire core emission band peaked at around 1.503 eV, 7K PL spectra showed an additional broad peak in the 1.556-1.583 eV energy interval, ascribed to the transition between electron and hole confined states within the QWT. The emission blue-shifts with the shrinkage of as-grown GaAs well tubes, as the nanowire local (on the substrate) density and height change.Prete, P.; Rosato, R.; Stevanato, E.; Marzo, F.; Lovergine, N.Prete, Paola; Rosato, R.; Stevanato, Elena; Marzo, Fabio; Lovergine, Nicol

Shape, Size Evolution, and Nucleation Mechanisms of GaAs Nanoislands Grown on (111)Si by Low-Temperature Metal–Organic Vapor-Phase Epitaxy

The shape, size evolution, and nucleation mechanisms of GaAs nanoislands grown at 400 degrees C on As-stabilized (111)Si by metal-organic vapor phase epitaxy are reported for the first time. GaAs crystallizes in the zincblende phase in the very early nucleation stages until a continuous epilayer is formed. GaAs nanoislands grow (111)-oriented on Si as truncated hexagonal pyramids, bound by six equivalent {120} side facets and a (111) facet at the top. Their diameter and height appear to increase linearly with the deposition time, yielding a constant aspect ratio of similar to 1/4. The nanoisland density (before coalescence) stays constant with time at similar to 2 x 10^(10) cm(-2), suggesting that the nucleation occurs at specific Si surface sites (defects) during the very early growth stages, rather than being due to the continuous formation of new nuclei. To understand the molecular-level mechanisms driving the low-temperature MOVPE growth of GaAs on Si, we applied a deposition-diffusion-aggregation (DDA) nucleation model, which predicts a linear evolution of the overall GaAs growth rate with surface coverage, in good agreement with experimental observations, under the assumption that direct impingement of trimethylgallium (Me3Ga) molecules onto the nanoisland surface dominates the material nucleation and growth rate; the contribution of Me3Ga adsorbed onto the As-stabilized (111)Si is negligible, pointing out the reduced reactivity of the Si surface (As passivation). Our DDA model allows estimation of the effective reactive sticking coefficient of Me3Ga onto GaAs, which is equal to 2.82 X 10^(-5) : the small value is compatible with the Me3Ga large steric hindrance and the competitive role of methyl radicals in surface adsorption at low temperature

Observation and Impact of a “Surface Skin Effect” on Lateral Growth of Nanocrystals

We investigate the impact of a quasi-crystalline two-dimensional (2D) surface on the lateral epitaxy of one-dimensional (1D) nanocrystals. The quasi-2D surface was formed by locally conditioning a GaN crystalline lattice at and below surface using a low-dose focused Ga ion beam ranging from 9 to 354 ions/pulse. Short ion pulses/site are used to create the 2D arrays of sub-10-nm circular disks modulating the GaN lattice to a depth of about 30 nm. Impact of this localized lattice modulation was investigated on lateral epitaxy of 1D ZnO nanocrystal and electronic structure of formed heterojunctions. High resolution transmission electron microscopy (HRTEM) below the GaN surface reveals direct evidence of a “skin effect” that influences the surface epitaxy of low-dimensional nanocrystals. We define this effect as the crystallinity of the top 10 nm of the substrate that is found to be a key factor in occurrence of lateral epitaxy. HRTEM shows that lateral epitaxy stops, if the thin skin layer is disrupted. However, if the lattice structure of this layer rebounds, the lateral epitaxy occurs. Results indicate that, beyond the skin depth of about 10 nm, the disorder of the subsurface lattice does not impact the structure of the overgrown nanocrystals. These findings suggest the possibility of surface engineering for enabling spatially controlled modulation of the electronic structure of low-dimensional nanocrystals on a scalable fashion