Structural characterization techniques for the analysis of semiconductor strained heterostructures

A combined method for structural characterization of strained epitaxial heterostructures involving different techniques such as Rutherford backscattering spectrometry (RBS), multiple crystal X-ray diffractometry (MCD) and transmission electron microscopy (TEM) is presented. In order to obtain a complete characterization of the analysed structure, three different quantities are measured independently: the epilayer thickness, the density of misfit dislocations which may appear at the interface, and the significant components of the strain tensor, mainly the tetragonal distortion, affecting the epilayer lattice. In this way the thermodynamic state and the mechanisms of plastic deformation of the structures can be fully investigated. In this contribution we present and discuss the experimental results concerning a set of InP/GaAs samples having different layer thicknesses ranging from 5 to 500 nm. The thickness of the samples has been determined by RBS. Measurements of in-plane strain and tetragonal distortion have been performed by MCD and RBS-channelling respectively, finally TEM has been used for determining the defects densities and distribution.     

X-ray double-crystal rocking curves in GaAlAs/GaAs heterostructures

Summary X-ray double-crystal rocking curves of Ga_1−x Al _x As/GaAs heterostructures have been calculated using a dynamical diffraction model for the general case of Bragg reflection geometry. Different experimental configurations have been considered and the possibility of studying both slightly mismatched and relatively thin layers has been investigated. Experimental rocking curves have been measured using the CuKα ₁ radiation, the 004 symmetric reflection and a perfect crystal as the monochromator. An excellent agreement between calculated and experimental rocking curves has been found and this demonstrates the reliability of both the experimental procedure and the theoretical approach.     

Localization of Bose-Einstein condensates in optical lattices

The dynamics of repulsive bosons condensed in an optical lattice is effectively described by the Bose-Hubbard model. The classical limit of this model, reproduces the dynamics of Bose-Einstein condensates, in a periodic potential, and in the superfluid regime. Such dynamics is governed by a discrete nonlinear Schrödinger equation. Several papers, addressing the study of the discrete nonlinear Schrödinger dynamics, have predicted the spontaneous generation of (classical) breathers in coupled condensates. In the present contribute, we shall focus on localized solutions (quantum breathers) of the full Bose-Hubbard model. We will show that solutions exponentially localized in space and periodic in time exist also in absence of randomness. Thus, this kind of states, reproduce a novel quantum localization phenomenon due to the interplay between bounded energy spectrum and non-linearity.     

Dynamical instability in a trimeric chain of interacting bose-einstein condensates

We analyze thoroughly the mean-field dynamics of a linear chain of three coupled Bose-Einstein condensates, where both the tunneling and the central-well relative depth are adjustable parameters. Owing to its nonintegrability, entailing a complex dynamics with chaos occurrence, this system is a paradigm for longer arrays whose simplicity still allows a thorough analytical study. We identify the set of dynamics fixed points, along with the associated proper modes, and establish their stability character depending on the significant parameters. As an example of the remarkable operational value of our analysis, we point out some macroscopic effects that seem viable to experiments.     

Weak and strong chaos in Fermi-Pasta-Ulam models and beyond

We briefly review some of the most relevant results that our group obtained in the past, while investigating the dynamics of the Fermi-Pasta-Ulam (FPU) models. The first result is the numerical evidence of the existence of two different kinds of transitions in the dynamics of the FPU models: (i) A stochasticity threshold (ST), characterized by a value of the energy per degree of freedom below which the overwhelming majority of the phase space trajectories are regular (vanishing Lyapunov exponents). It tends to vanish as the number N of degrees of freedom is increased. (ii) A strong stochasticity threshold (SST), characterized by a value of the energy per degree of freedom at which a crossover appears between two different power laws of the energy dependence of the largest Lyapunov exponent, which phenomenologically corresponds to the transition between weak and strong chaotic regimes. It is stable with N. The second result is the development of a Riemannian geometric theory to explain the origin of Hamiltonian chaos. Starting this theory has been motivated by the inadequacy of the approach based on homoclinic intersections to explain the origin of chaos in systems of arbitrarily large N, or arbitrarily far from quasi-integrability, or displaying a transition between weak and strong chaos. Finally, the third result stems from the search for the transition between weak and strong chaos in systems other than FPU. Actually, we found that a very sharp SST appears as the dynamical counterpart of a thermodynamic phase transition, which in turn has led, in the light of the Riemannian theory of chaos, to the development of a topological theory of phase transitions.     

Electrical and photovoltaic properties of Cd1? x Zn x S/ p -GaAs heterojunctions

Cd_1−x Zn _x S/p-GaAs heterojunctions for solar cell applications have been prepared by growing single crystal Cd_1−x Zn _x S epitaxial layers on (111)GaAs substrates through a vapour phase chemical transport method using the close-spaced geometry and H₂ as a transport agent. Electrical and photovoltaic properties of the heterojunctions have been investigated and discussed in connection with the main features of the growth technique. AM1 power conversion efficiencies up to 6.2% have been measured and possible improvements have been examined.     

X-ray Topographic Assessment of Defects in Pure and Substituted Hexaferrite Crystals

Results of X-ray diffraction topography, in reflection and transmission scanning geometry, of flux grown single crystals of substituted and unsubstituted hexaferrites bearing composition SrGa_xIn_yFe_12-(x+y)O₁₉ (where x = 0, 5, 7, 9; y = 0, 0.8, 1.3, 1.0) are presented. Diffraction topographs reveal defects like misoriented grains, dislocations, cavities, inclusions, and the strain patterns in these crystals. The unsubstituted hexaferrites exhibit better perfection when compared to the substituted ones. The study is reported to support the results obtained by chemical etching and fractography, besides yielding additional information covering defects.     

Microcanonical Entropy and Dynamical Measure of Temperature for Systems with Two First Integrals

We consider a generic classical many particle system described by an autonomous Hamiltonian H(x ¹,…,x ^N+2) which, in addition, has a conserved quantity V(x ¹,…,x ^N+2)=v, so that the Poisson bracket {H,V} vanishes. We derive in detail the microcanonical expressions for entropy and temperature. We show that both of these quantities depend on multidimensional integrals over sub-manifolds given by the intersection of the constant energy hyper-surfaces with those defined by V(x ¹,…,x ^N+2)=v. We show that temperature and higher order derivatives of entropy are microcanonical observable that, under the hypothesis of ergodicity, can be calculated as time averages of suitable functions. We derive the explicit expression of the function that gives the temperature.