Carbon precipitation and diffusion in SiGeC alloys under silicon self-interstitial injection

In this work we investigate the diffusion and precipitation of supersaturated substitutional carbon in 200-nm-thick SiGeC layers buried under a silicon cap layer of 40 nm. The samples were annealed in either inert (N₂) or oxidizing (O₂) ambient at 850 °C for times ranging from 2 to 10 h. The silicon self-interstitial (I) flux coming from the surface under oxidation enhances the C diffusion with respect to the N₂-annealed samples. In the early stages of the oxidation process, the loss of C from the SiGeC layer by diffusion across the layer/cap interface dominates. This phenomenon saturates after an initial period (2–4 h), which depends on the C concentration. This saturation is due to the formation and growth of C-containing precipitates that are promoted by the I injection and act as a sink for mobile C atoms. The influence of carbon concentration on the competition between precipitation and diffusion is discussed. Received: 19 October 2001 / Accepted: 19 December 2001 / Published online: 20 March 2002 / Published online: 20 March 2002     

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.     

comments on the paper “contribution of quantum effects to the mechanism of positronium reactions”

Some results recently published on the o-Ps quenching reactions promoted by FeCl₂ and K₂Cr₂O₇ in glycerol aqueous solutions were interpreted by supposing that a significant fraction of o-Ps atoms diffuses by a quantum-mechanical effect or in quasi-free states even at room temperature. It is shown that the results can be interpreted in a different manner, so they cannot be invoked for supporting the hypotheses mentioned above about the mechanism of o-Ps diffusion at room temperature.     

Flat Complex Vector Bundles,the Beltrami Differential and W-Algebras

Since the appearance of the paper by Bilal et al. in 1991, it has been widely assumed that W-algebras originating from the Hamiltonian reduction of an SL(n,C)-bundle over a Riemann surface give rise to a flat connection, in which the Beltrami differential may be identified. In this Letter, it is shown that the use of the Beltrami parametrization of complex structures on a compact Riemann surface over which flat complex vector bundles are considered, allows the construction of the above mentioned flat connection. It is stressed that the modulus of the Beltrami differential is of necessity less than one, and that solutions of the so-called Beltrami equation give rise to an orientation-preserving smooth change of local complex coordinates. In particular, the latter yields a smooth equivalence between flat complex vector bundles. The role of smooth diffeomorphisms which induce equivalent complex structures is specially emphasized. Furthermore, it is shown that, while the construction given here applies to the special case of the Virasoro algebra, the extension to flat complex vector bundles of arbitrary rank does not provide generalizations of the Beltrami differential usually considered as central objects for such non-linear symmetries.     

Low-temperature spectrally resolved cathodoluminescence study of degradation in opto-electronic and microelectronic devices

This study reports on the microcharacterization of devices for optoelectronic and for microelectronic applications using low temperature (T = 5 and 77 K) spectrally resolved cathodoluminescence (SCL). The mechanisms leading to compositional inhomogeneities in the regrowth regions of InP-based butt-coupled laser-waveguide devices for semiconducting optical amplifiers (SOAs) and for defect generation in the active and cladding layers of GaAs based pump lasers for erbium-doped optical fibre amplifiers (EDFAs) were studied. Beryllium outdiffusion in the base regions of GaAs-based heterojunction bipolar transistors (HBTs) after bias ageing was also studied. By comparing the CL results with TEM, SIMS and HRXRD studies and with the existing literature, the observed growth and operation induced defects were attributed, respectively, to the following mechanisms: recombination-enhanced defect glide (REDG) in the pump lasers, recombination enhanced impurity diffusion (REID) in the HBTs and electrostatically induced growth flux instabilities in the butt-coupled laser-waveguide devices.     

On the postronium spin exchange reactions promoted by 3d aqua ions

In order to ascertain possible relationships between the rate constants, K_SE, of positronium, Ps, spin exchange /SE/ reactions promoted by 3d aqua ions and the electron configuration of the ion, the K_SE's of Mn _aq ²⁺ and Co _aq ²⁺ aqua ions were measured together with that of [Cr(NH₃)₆]³⁺. The K_SE's values obtained are discussed together with those of the Cr _aq ³⁺ , Fe _aq ²⁺ , Ni _aq ²⁺ and [Ni(NH₃)₆]²⁺ ions previously measured. It was found that Cr _aq ³⁺ and Mn _aq ²⁺ K_SE's are 1 M^–1ns^–1, while those of Fe _aq ²⁺ , Co _aq ²⁺ and Ni _aq ²⁺ ions are 2.5 M^–1ns^–1. Thus the K_SE values of 3d aqua ions do not depend solely on the number of their unpaired electrons as it was found for the 4f aqua ions. The trend observed parallels that of the electron delocalization from the metal atom; the delocalization occurs only with Fe _aq ²⁺ , Co _aq ²⁺ and Ni _aq ²⁺ ions, but not with the Cr _aq ³⁺ and Mn _aq ²⁺ aqua ions. The trend of the K_SE's of aqua, ammine and perhaps of chloro complexes parallels that of ligand capabilities to cause d-electron cloud expansion, and thus d-electron delocalization from metal atoms.     

Positronium Spin-Conversion Reactions Promoted by 3d Complexes in Highand Low-Spin States

Continuing in our work on the correlations between the rate constants, k _CR, of the ortho into para-positronium conversion reactions, CR, promoted by complexes of various 3d ions and the metal electron delocalization caused by the ligands, the relationship between the parameters of the correlation lines pertinent to highand low-spin complexes of Cr^II, Mn^II, and Co^II ions was ascertained. Moreover, it was experimentally verified, for the first time, that the statistical probability of the CR promoted by paramagnetic compounds with S = 1/2 is three times larger than that of the CR caused by compounds with S > 1/2.     

Photoreflectance characterization of InAs/GaAs self-assembled quantum dots grown by ALMBE

We report on a photoreflectance investigation in the 0.8-1.5 eV photon energy range and at temperatures from 80 to 300 K on stacked layers of InAs/GaAs self-assembled quantum dots (QDs) grown by Atomic-Layer Molecular Beam Epitaxy. We observed clear and well-resolved structures, which we attribute to the optical response of different QD families. The dependence of the ground state transition energy on the number of stacked QD layers is investigated and discussed considering vertical coupling between dots of the same column. It is shown that Coulomb interaction can account for the observed optical response of QD families with different morphology coexisting in the same sample. Received 17 November 1999     

Existence of a somewhere injective pseudo-holomorphic disc

Given a smooth totally real submanifold L {\cal L} in an almost complex manifold (M,J) and a J-holomorphic disc with boundary in L {\cal L} , by restriction of the initial disc and factorization, one gets a smooth simple J-holomorphic curve still with boundary in L {\cal L} . As a consequence one gets a proof of the Arnold-Givental conjecture for a class of Lagrangian submanifolds in a symplectic manifold.     

Relative frames on J-holomorphic curves

The aim of this work is to prove that any non-constant J-holomorphic disc with its boundary in a given Lagrangian submanifold can be decomposed in homology into a sum of finitely many J-holomorphic simple discs with the same Lagrangian boundary condition. As a consequence, in dimension higher than 6, any generic J-holomorphic disc is multicovered.