L. Nasi

Effects of the quantum dot ripening in high-coverage InAs∕GaAs nanostructures

We report a detailed study of InAs∕GaAs quantum dot (QD) structures grown by molecular beam epitaxy with InAs coverages θ continuously graded from 1.5 to 2.9 ML. The effect of coverage on the properties of QD structures was investigated by combining atomic force microscopy, transmission electron microscopy, x-ray diffraction, photoluminescence, capacitance-voltage, and deep level transient spectroscopy. In the 1.5–2.9 ML range small-sized coherent QDs are formed with diameters and densities that increase up to 15nm and 2×1011cm−2, respectively. For θ>2.4 ML large-sized QDs with diameters of 25nm and densities ranging from 2×108to1.5×109cm−2 coexist with small-sized QDs. We explain the occurrence of large-sized QDs as the inevitable consequence of ripening, as predicted for highly lattice-mismatched systems under thermodynamic equilibrium conditions, when the coverage of the epitaxial layer exceeds a critical value. The fraction of ripened islands which plastically relax increases with θ, leading to the fo...

Self-aggregated InAs quantum dots in GaAs

We present a detailed study, both structural and optical, of GaAs/InAs/GaAs heterostructures for InAs nominal coverages (L) ranging from 0.6 to 3 ML. Planar transmission electron microscopy (TEM) provides direct evidence of the presence of InAs quantum dots (QDs) for all values of L, with an increase in their density at high values of L. Transverse TEM shows also that those QDs have mostly small base angles. Accordingly, the evolution of the optical properties of InAs/GaAs is investigated by photoluminescence (PL) and PL excitation measurements (PLE). A broad PL band is observed in all samples, which is ascribed to the recombination of heavy-hole excitons in the InAs quantum dots, observed with TEM. For thin coverages (L⩽1.6ML), a narrow PL band is also observed, which is attributed to recombination of heavy-hole excitons in a two-dimensional (2D) InAs layer. The two bands shift to lower energy for increasing L. For L⩾1.6u2002ML, the QD band has a faster shift and exhibits a complex structure, while the excit...

Metamorphic quantum dots: Quite different nanostructures

In this work, we present a study of InAs quantum dots deposited on InGaAs metamorphic buffers by molecular beam epitaxy. By comparing morphological, structural, and optical properties of such nanostructures with those of InAs/GaAs quantum dot ones, we were able to evidence characteristics that are typical of metamorphic InAs/InGaAs structures. The more relevant are: the cross-hatched InGaAs surface overgrown by dots, the change in critical coverages for island nucleation and ripening, the nucleation of new defects in the capping layers, and the redshift in the emission energy. The discussion on experimental results allowed us to conclude that metamorphic InAs/InGaAs quantum dots are rather different nanostructures, where attention must be put to some issues not present in InAs/GaAs structures, namely, buffer-related defects, surface morphology, different dislocation mobility, and stacking fault energies. On the other hand, we show that metamorphic quantum dot nanostructures can provide new possibilities of tailoring various properties, such as dot positioning and emission energy, that could be very useful for innovative dot-based devices.

High-resolution x-ray diffraction, x-ray standing-wave, and transmission electron microscopy study of Sb-based single-quantum-well structures

Ga0.6Al0.4Sb/GaSb single-quantum-well structures grown by molecular-beam epitaxy on GaSb substrates with different well thicknesses, have been studied by high-resolution x-ray diffraction and x-ray standing-wave methods. By fitting the diffraction curves, thickness, composition, and the static Debye–Waller factor were obtained for each layer of the structures. The analysis of the angular dependence of the yield of photoelectrons emitted by the x-ray standing-wave field in the range of the dynamical x-ray diffraction was used for selecting the most appropriate set of layer parameters among those which gave virtually identical fittings of the diffraction curves. Relatively broadened GaAlSb/GaSb interfaces were found in all of the samples. This result was confirmed by high-resolution transmission electron microscopy investigation of one of the samples. The effect of the surface degradation due to the chemical reaction with the atmosphere of the free surface of the upper Ga0.6Al0.4Sb layer was considered.Ga0.6Al0.4Sb/GaSb single-quantum-well structures grown by molecular-beam epitaxy on GaSb substrates with different well thicknesses, have been studied by high-resolution x-ray diffraction and x-ray standing-wave methods. By fitting the diffraction curves, thickness, composition, and the static Debye–Waller factor were obtained for each layer of the structures. The analysis of the angular dependence of the yield of photoelectrons emitted by the x-ray standing-wave field in the range of the dynamical x-ray diffraction was used for selecting the most appropriate set of layer parameters among those which gave virtually identical fittings of the diffraction curves. Relatively broadened GaAlSb/GaSb interfaces were found in all of the samples. This result was confirmed by high-resolution transmission electron microscopy investigation of one of the samples. The effect of the surface degradation due to the chemical reaction with the atmosphere of the free surface of the upper Ga0.6Al0.4Sb layer was considered.

Electron Microscopy and X‐Ray Diffraction Determinations of Strain Release in InGaAs / GaAs Superlattices Grown by Molecular Beam Epitaxy

Strain-induced dislocations have been studied in low and medium mismatched (001) oriented In x Ga 1-x As/GaAs superlattice heterostructures. Their optical quality, composition, residual strain, dislocation nature, and location have been studied by x-ray diffraction and electron microscopy techniques. Different predictions of the residual strain have been discussed briefly. All the samples presented perpendicular networks of misfit dislocations with different densities along the [110] and [110] directions. The dislocations were confined inside the buffer layer or at the buffer-superlattice interface without threading the superlattice

Interfaces in AlGaSb∕GaSb multiquantum well structures

The Al0.4Ga0.6Sb∕GaSb∕Al0.4Ga0.6Sb quantum well (QW) structures were grown by a molecular beam epitaxy on the GaSb substrates at different growth temperatures (Tg) and with different Sb4∕Ga beam equivalent pressure ratios (BEPR) and were studied using high-resolution x-ray diffraction, photoluminescence (PL), and transmission electron microscopy. The x-ray diffraction analysis showed that the AlGaSb∕GaSb interfaces have smooth Al composition profiles with graded region thicknesses in the range of 0.83nm–2.17nm, depending upon the growth conditions; the normal interfaces are generally sharper than the inverse ones. The low-temperature PL spectra from QWs show relatively broad peaks and energies of the peak emission that can be related to the growth parameters. Good agreement was obtained between the PL peak energies and the transition energies calculated using a QWs model that takes into account the one-dimensionally graded confining potential for describing the experimentally determined Al composition pro...

Triple crystal diffractometry, x-ray standing wave, and transmission electron microscopy investigation of shallow BF2 implantation in Si

Si wafers implanted at 5, 15, and 50 keV with different BF2+ doses and next annealed at 945u200a°C for 45 s, were studied by means of x-ray triple crystal diffraction, x-ray standing wave, and transmission electron microscopy methods. Due to the implantation energies used, very narrow subsurface regions with a depth ranging from a few tens of nanometers to a few nanometers were damaged. By fitting the diffraction curves and using the information obtained from the photoelectrons emitted by the x-ray standing wave field, it was possible to determine the most appropriate strain and damage profiles versus depth within the disturbed region of the crystal. The above results made it possible to find: (i) the distribution of interstitial ions produced during the implant processes; (ii) the depth of amorphization of the implanted regions at the highest doses; and (iii) the appearance of extended defects (dislocation loops band) at the amorphous/crystal interface during the restoration of the lattice by the annealing p...

Damage profiles determination in ultra-shallow B+ implanted Si by triple crystal X-ray diffraction and transmission electron microscopy

B + ions were implanted in Si at ultra-low energies: 0.25, 0.5 and 1 keV, respectively, and at different doses: 1 x 10 14 , 1 x 10 15 cm -2 . Lattice distortion and disorder due to the implantation process were investigated by means of a high resolution X-ray diffraction method. Due to the very low implantation depth (a few nm), the X-ray diffraction measurements were carried out by triple-crystal diffractometry. With this experimental configuration it was possible to separate coherent from diffuse scattering, considerably improving the signal-to-noise ratio. For the analysis of the experimental curves, the subsurface region was divided in several thin layers. The layer thickness, the static Debye-Waller factor, which is related to the lattice damage, and the lattice spacing modification (strain) were the parameters of the fitting procedure. Despite the small thickness of the subsurface-damaged area, it was possible to obtain the main parameters describing the depth distribution of the lattice distortions in the analyzed crystals. Transmission electron microscopy investigations were made on two samples implanted at the lowest energies and the results obtained by the X-ray diffraction were confirmed.

Structural and electrical investigation of high temperature annealed As-implanted Si crystalsa)

Si wafers implanted at 80keV with different As doses, and next annealed at different temperatures for different times, were studied by means of x-ray triple crystal diffraction, x-ray standing wave, transmission electron microscopy, spreading resistance profile, and electrochemical C-V profiling methods. The implantation processes produced heavily damaged subsurface regions hundreds of nanometers deep. By fitting both the x-ray diffraction curves and the x-ray standing wave photoelectron emission profiles, it was possible to determine the most appropriate strain and atomic static displacement behavior versus depth within the disturbed region of the crystal. The results obtained by x-ray diffraction measurements were confirmed by transmission electron microscopy investigations. Therefore, making use of different structural and electrical characterization techniques it was possible to find: (i) the depth of amorphization of the implanted regions, (ii) the appearance of extended defects (dislocation loops ba...