We report on a combined spectroscopic/structural study of MOVPE-grown GaAs-AlGaAs core-multishell nanowires, containing thin GaAs quantum well tubes (QWTs) wrapped around the central GaAs core. Low temperature (7K) cathodoluminescence (CL) spectroscopic imaging combined with Z-contrast scanning transmission electron microscopy (STEM) tomography performed on single core-multishell nanowires allowed robust correlation between QWT emission and the nanowire inner structure down to the nano-scale. Besides the core luminescence and minor defects-related contributions, each nanowire showed one or more QWT peaks in the 1.53-1.65 eV spectral region, which correlated with sections of the nanowire trunk having different diameters. Average values of QWT thickness (in the 3-7 nm range) were thus extracted from measured nanowire diameter through the application of a multishell growth model, the latter validated against experimental data (core/nanowire diameter, shell thicknesses) obtained from 3-dimensional (3D) reconstructed STEM tomograms of single QWT nanowires. Our data evidenced that the QWT emissions appear redshifted (by about 40-120 meV) with respect to values expected for uniform QWTs of the same thickness. CL mapping evidenced nanoscale localization of QWT exciton emissions along the nanowire, demonstrating that their emission is affected by carrier localization at confinement-potential inhomogeneity. The latter have been ascribed to azimuthal asymmetries as well as (azimuthal and axial) random fluctuations of the GaAs QWT thickness within each nanowire, as evidenced by detailed statistical analysis of the 3D tomograms.
We 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.
We report on the effects of changing the surface densities of MOVPE-grown free-standing GaAs-AlGaAs core-shell
nanowires on the resulting nanostructure size and their photoluminescence (PL) properties. It is demonstrated that
decreasing the local density of GaAs nanowires within the array leads to an increase of the overgrown AlGaAs shell
thickness and to a substantial redshift of the nanostructure excitonic emission. Application of a vapor mass-transport
limited growth model of the AlGaAs shell allows explaining the dependence of shell growth rate on nanowire density.
The observed redshift of the nanowire PL emission is then experimentally correlated with these density-induced changes
of the nanostructure size, namely with the nanowire shell-thickness to core-radius ratio hs/Rc. To account for a possible
contribution of the nanostructure built-in elastic strain to the energy shift of the peak excitonic emission, the strain field
in present core-shell nanowires was calculated as function of the nanostructure relevant geometrical parameters, based on
a uniaxial elastic energy equilibrium model, and its effect on valence and conduction band shifts of the GaAs core
evaluated by means of the Pikus-Bir Hamiltonian. Good agreement is obtained for hs/Rc<1, the strain-free excitonic
emission being identified at 1.510 eV and ascribed to bound heavy-hole excitons. For hs/Rc>1 increasingly larger
redshifts (up to ~9 meV in excess of values calculated based on the elastic strain model) are observed, and tentatively
ascribed to shell-dependent exciton localization effects.
We report on the self-assembly by Au-catalyzed metalorganic vapor phase epitaxy (MOVPE) of GaAs-based nanowires
(NWs) and their applications to novel and efficient nano-devices. The growth of GaAs and GaAs-AlGaAs core-shell
NWs is presented as case study, focusing on the dependence of their structural, optical and electrical properties on
MOVPE conditions. MSM diodes fabricated using as-grown core-shell NWs are reported, along with their photoelectric
performances. These devices show potentials for applications as fast photo-detectors and efficient solar cells.
We report on the growth of thick CdTe layers on ZnTe/(100) GaAs hybrid substrates by the novel H2 transport vapor phase epitaxy (H2T-VPE) method. High crystalline quality (100)-oriented CdTe single crystal epilayers can be fabricated under atmospheric pressure and at growth temperatures (TD) in the 600 - 800 degree Celsius interval. Double crystal X-ray diffraction measurements performed on epilayers thicker than 30 micrometer show CdTe (400) peaks with FWHM < 59 arcsec. Samples grown under optimized conditions exhibit mirror-like surfaces. Nominally undoped epilayers grown < 650 degrees Celsius are p-type and low resistive, but they turn n-type above 650 degrees Celsius, as a result of donor (likely Ga) diffusion from the substrate. RT resistivities ((rho) ) approximately 106 (Omega) (DOT)cm are obtained for 675 degrees Celsius < TD < 700 degrees Celsius, but (rho) decreases for higher temperatures and thinner samples. Layers grown under these conditions show RT electron concentrations in the 1014 - 1011 cm-3 range. The detection capability of H2T-VPE grown CdTe is demonstrated by time- of-flight measurements performed at RT on Au/n-CdTe/n+- GaAs diode structures under reverse bias conditions. The present results show the potentials of H2T-VPE for the growth of detector-grade CdTe.
The nucleation of CdTe onto basal plane sapphire and the subsequent growth of a CdTe buffer layer has been studied using in-situ laser reflectance (probe wavelength 633 nm, HeNe laser). The production of midwave infrared focal plane arrays requires the growth of typically 10 micrometer of CdTe (111)B buffer layer in order to grow out problems due to stacking faults, dislocation clusters and twinning. A-face and B-face growth of CdTe is seen to produce different reflectance 'signatures' within the first 6000 angstroms of growth, so enabling the early identification of problems with the growth process. Laser reflectance was also successfully demonstrated to predict the thickness of the buffer layer. Oscillations in the laser reflectance are attenuated due to absorption by the film at the probe wavelength used after approximately 6000 angstrom. However by the on-line calculation of the growth rate at every half wavelength oscillation, it is possible to extrapolate a film thickness for the total growth time. This extrapolated value is seen to be in good agreement with the thickness calculated ex-situ by beta-back scattering. The dependence on the buffer layer growth on the nucleation conditions was also investigated. The determination of whether the buffer layer grows A-face or B-face is seen to be more influenced by the II:VI ratio than the temperature during nucleation. For a nucleation temperature of 400 degrees Celsius, with a II:VI ratio of 6:1 the growth of the buffer layer is seen to be 100% A-face. As the II:VI ratio is increased the degree of A-face growth is seen to decline and the material becomes dominated by B-face growth. At a II:VI ratio of 60:1 the material is entirely B-face and predominantly untwinned. The difference in the two growth modes is manifested in the laser reflectance. Greater scattering of the laser light occurs during B-face growth due to the increased roughening compared to A-face growth. Consequently the reflectance signal in the B-face signature is seen to fall away more rapidly than is the case with A-face growth.
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