A. Balzarotti

EXAFS measurements on FeB metallic glasses: Asymmetry of the radial distribution function

Abstract The EXAFS structure measured above the K-edge of Fe in the disordered Fe80B20 alloy is reported. Complete agreement is obtained with X-ray diffraction results in the position of the first neighbours coordination shell when the asymmetry of the Radial Distribution Function (R.D.F.) into the EXAFS formula is included. A Finney-like R.D.F. is used to describe the pair distribution of these metallic glasses. The weak temperature dependence of the spectra is discussed in terms of the structural properties of these amorphous alloys.

Self-ordering of Ge islands on step-bunched Si(111) surfaces

By using step-bunched Si(111) surfaces as templates, we demonstrate the self-assembly of an ordered distribution of Ge islands without lithographic patterning. Initially, islands nucleate and evolve at step edges, up to complete ripening, forming long ribbons. Subsequently, island nucleation takes place at the center of flat terraces. Ge islands appear to be regularly spaced in scanning tunneling microscope images. The exploitation of this effect provides a possible route to grow ordered arrays of semiconducting nanostructures.

Apparent critical thickness versus temperature for InAs quantum dot growth on GaAs(001)

We studied the temperature dependence of the two-dimensional to three-dimensional growth transition in InAs∕GaAs(001) heteroepitaxy by means of reflection high energy electron diffraction and atomic force microscopy. The observed shift of the transition to higher InAs deposition times, at temperatures above 500°C, is not a change of critical thickness for islanding, which instead, is constant in the 450–560°C range. Consequently, In-Ga intermixing and surface and interface strain have a negligible dependence on temperature in this range.

Step erosion during nucleation of InAs∕GaAs(001) quantum dots

We have investigated, by means of atomic force microscopy, the complete evolution of InAs∕GaAs(001) quantum dots as a function of deposited InAs. Direct evidence is found for step erosion by quantum dots nucleated onto the step edge and an estimate of the eroded volume is provided. By studying the quantum dots volume as a function of InAs coverage, we show that the wetting layer contribution is confined within a narrow range of coverage around the two- and three-dimensional transition.

Kinetic aspects of the morphology of self-assembled InAs quantum dots on GaAs(001)

We analyzed by atomic force microscopy self-assembled quantum dots of InAs on GaAs(001) in a series of samples prepared by molecular beam epitaxy (MBE). Two different growth procedures have been applied, namely, the usual continuous growth and the migration-enhanced growth. At equal depositions of InAs, larger than the critical thickness for the two- to three-dimensional transition, marked differences are found in the evolution of the nanoparticle density and volume, despite of the same set of growth parameters were used. Above 2 ML, a small fraction of ripened islands is also present, which is responsible for the nonlinear increase of the total volume of the dots with InAs coverage caused by an anomalous participation of the underlying layers. The different morphologies obtained substantiate the overwhelming role of kinetics on thermodynamics in the nonequilibrium MBE growth.

InAs/GaAs(001) epitaxy: kinetic effects in the two-dimensional to three-dimensional transition

Step instability and surface mass transport strongly influence the kinetics of the two- to three-dimensional (2D-3D) transition in InAs/GaAs self-assembly epitaxy. In this paper we report evidence of the step erosion of quantum dots (QDs) nucleated on step edges for samples having different surface morphologies and the explosive nucleation of 3D QDs triggered by it. Related issues such as the temperature dependence of the critical thickness, the volume dependence of the surface mass transport, and the scaling behaviour are illustrated by means of RHEED and AFM measurements and data analysis for small increments of InAs coverage throughout the 2D-3D transition.

EXAFS investigation of amorphous-to-crystal transition in Ge

Abstract The structure of a-Ge as a function of the deposition temperature Ts has been studied by the EXAFS technique for the first time. The results demonstrate the sensitivity of EXAFS to structural differences in the medium range. The data provide strong evidence for a continuous transition from the amorphous to the crystalline phase over a temperature interval form 130°C to 300°C. The analysis of the data is in line with the microcrystal model of amorphous Ge films and allows an estimate of the average grain size for the lower range Ts used.

Self-assembly of InAs and Si/Ge quantum dots on structured surfaces

We discuss the self-aggregation process of InAs and Si–Ge quantum dots (QDs) on natural and patterned GaAs(001) and Si(001) and Si(111) surfaces, with reference to our recent studies with scanning tunnelling and atomic force microscopy and current experimental and theoretical works. Various methods for obtaining naturally structured surfaces are briefly surveyed, as the patterning formed by the surface instability and by the strain in mismatched heteroepitaxy, and the latest methods of pre-patterning and growth at selected sites are discussed. Basic topics are also addressed that determine the final morphology of QDs, such as the wetting layer formation, the elastic strain field and the two-dimensional to three-dimensional phase transition.

How kinetics drives the two- to three-dimensional transition in semiconductor strained heterostructures: The case of InAs/GaAs(001)

The two- to three-dimensional growth mode transition in the InAs∕GaAs(001) heterostructure has been investigated by means of atomic force microscopy. The kinetics of the density of three-dimensional islands indicates two transition onsets at 1.45 and 1.59 ML of InAs coverage, corresponding to two separate families, small and large dots. According to the scaling analysis and volume measurements, the transition between the two families of quantum dots and the explosive nucleation of the large ones is triggered by the erosion of the step edges.

Reflection high energy electron diffraction observation of surface mass transport at the two- to three-dimensional growth transition of InAs on GaAs(001)

We have followed by reflection high-energy electron diffraction the nucleation of InAs quantum dots on GaAs(001), grown by molecular-beam epitaxy with growth interruptions. Surface mass transport gives rise, at the critical InAs thickness, to a huge nucleation of three-dimensional islands within 0.2 monolayers (ML). Such surface mass diffusion has been evidenced by observing the transition of the reflection high-energy electron diffraction pattern from two- to three-dimensional during the growth interruption after the deposition of 1.59 ML of InAs. It is suggested that the process is driven by the As2 adsorption-desorption process and by the lowering of the In binding energy due to compressive strain. The last condition is met first in the region surrounding dots at step edges where nucleation predominantly occurs.