C. Bocchi

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...

Properties of wetting layer states in low density InAs quantum dot nanostructures emitting at 1.3 μm: Effects of InGaAs capping

In this work we study the properties of energy levels of the two-dimensional quantum system composed by wetting layers and thin capping layers in low density InAs/InGaAs quantum dot structures, that can be used as single photon sources at the fiber-optic wavelength of 1.3 μm. We show how, thanks to the low density of quantum dots, x-ray characterization of structures allows to extract thicknesses and compositions of the InAs wetting layer and the quantum well formed by the InGaAs capping layer, resulting in substantial deviations from the simplified picture of a wetting layer consisting of a 1.6 monolayer thick InAs square well. The agreement between model calculations of quantum confined energy levels based on x-ray data and photoluminescence peak energies substantiates the validity of this calculation, that also allows to investigate on carrier localization. The increase in In composition in the InGaAs capping layer results in reduced localization of heavy holes in the wetting layer, that are pushed int...

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.

Investigation of lattice distortions in InP crystals implanted with Fe+ ions by means of high‐resolution x‐ray diffraction and x‐ray standing‐wave methods

Lattice distortions due the implantation of Fe+ ions in InP semi‐insulating crystals have been investigated by means of high‐resolution x‐ray‐diffraction and x‐ray standing‐wave methods. The effects of both the implantation dose and the annealing time were studied. It is shown that the x‐ray standing‐wave method provides valuable complementary information on strain and damage in the subsurface layer and permits one to distinguish between different distortion profiles that give practically the same kinematical diffraction curve.

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.

Study of structural transition from metastable zinc blende to rocksalt crystal in molecular beam epitaxy MgS∕ZnSe∕GaAs multilayer system

The stable crystal structure of magnesium sulfide (MgS) is rocksalt. However, the metastable zinc-blende structure is obtained when MgS is deposited by molecular beam epitaxy (MBE) on (001) zinc-blende substrates with a relatively small misfit. In the present work, the zinc blende to rocksalt phase transition is analyzed in MgS∕ZnSe∕GaAs multilayer samples grown by MBE with different MgS layer thicknesses. By x-ray diffraction and transmission electron microscopy methods, a partial nucleation of MgS rocksalt is evidenced and correlated to the presence of stacking faults at the bottom interface. The unexpected coexistence of both rocksalt and zinc-blende MgS structural phases is discussed.

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 945 °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...

Structural properties of Zn-diffused InP layers

A sealed tube method has been adopted to prepare Zn-diffused InP layers. Both Zn3P2 and Zn+InP have been used as sources. The samples were prepared at 500 °C. The diffusion time ranged from 5 up to 120 min. Both S- and Zn-doped InP crystals have been used as substrates. The Zn depth profile has been measured by secondary ion mass spectroscopy, while the lattice strain produced by diffusion has been carefully investigated by x-ray double crystal diffraction and the standing-waves method of recording photoelectrons. The results show that in the S-doped crystal the diffused/virgin interface is very sharp and the diffused layers are lattice contracted. The concentration of Zn, as well as the lattice strain, do not depend on the diffusion time, whereas the thickness of the diffused layer increases with time. The plot of diffused layer thickness versus the square root of the diffusion time showed different slopes depending on the diffusion sources. Both lattice strain and diffusion depth depend on the diffusion...

Structural characterization of Zn‐diffused InP layers by x‐ray diffraction and standing‐waves method

Zn‐diffused InP layers have been prepared in a sealed quartz ampoule at 500 °C using a Zn3P2 source. The diffusion time ranged from 7 up to 90 min. As substrates, nearly dislocation‐free S‐doped InP crystals have been used. The lattice strain produced by diffusion has been carefully investigated by x‐ray double crystal diffraction and the standing‐waves method recording photoelectrons. The results show that the diffused layers exhibit a lattice contraction probably related to the Zn atoms substituting In. The diffuse/virgin interface has been found to be very sharp and the secondary ion mass spectrometry measurement of the Zn concentration profile has been confirmed. Finally, the maximum strain seems not to depend on the diffusion time, whereas the thickness of the diffused layer increases by increasing the time as expected for a diffusion process.