122 research outputs found
Dirac delta nucleation in the framework of Avrami's model: The case of diamond growth on deformed Si(100)
No full textThe nucleation and growth kinetics of diamond deposited by hot-filament chemical vapour deposition (HFCVD) on Si(100) substrates, previously deformed by uniaxial compression dong the [100] direction, have been investigated Although the nucleation density at saturation (0.04-0.06 mu m(-2)) was similar to those measured on virgin, as-received Si(100) wafers, the kinetics of stable nucleus formation resembled the fast kinetics observed for substrates which were mechanically abraded prior to CVD in order to enhance diamond nucleation. The results definitely indicate that diamond nucleation occurs randomly with a rate that is a Dirac delta function. The time dependence of the substrate fraction which is covered by islands was measured, and a good agreement with Avrami's kinetics for 2D phase transitions was found. The total island perimeter has also been measured as a function of the covered surface, and is well described by the analytical model recently developed [M. Tomellini and M. Fanfoni, Surf Sci. 349 (1996) L191]. The observed fast nucleation has been attributed to stress-induced defects pre-existing at the surface and which provides suitable sites for diamond growth. (C) 1997 Elsevier Science B.V
Kinetics study of the GaP(110)/Cu interface via P L2,3VV Auger line shape and x-ray-photoemission spectroscopies
No full textWe have studied the evolution of the GaP(110)/Cu interface as prepared at room-temperature (RT) versus low-temperature (LT100 K) conditions. Electron-excited P L2,3VV and Cu M1VV Auger line shapes and x-ray-excited (h=1253.6 eV) Ga 2p3/2, Ga 3d, P 2p, Cu 2p3/2, and Cu 3p core-level emissions were followed as a function of the Cu coverage (2 up to 80). Regardless of the substrate temperature, substrate disruption is observed at 1 monolayer Cu, with P atoms participating in chemical bonds with deposited Cu. In parallel, the topmost Ga atoms are dislodged from their pristine locations and segregate in a metalliclike environment. These processes persist in the intermediate-coverage range (212 Cu) at both temperatures, being only relatively attenuated at LT. At very high Cu thicknesses, strong chemical intermixing and out-diffusion of substrate-derived species are still observed at RT, while these processes are nearly completely inhibited at LT. © 1990 The American Physical Society
Apparent critical thickness versus temperature for InAs quantum dot growth on GaAs(001)
No full textWe 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 degrees C, is not a change of critical thickness for islanding, which instead, is constant in the 450-560 degrees C range. Consequently, In-Ga intermixing and surface and interface strain have a negligible dependence on temperature in this range
I fenomeni fisici e chimici irreversibili: La diffusione
No full textL’ articolo è dedicato ad uno dei processi più affascinanti, interessanti ed importanti della chimica-fisica: la diffusione. E’ rivolto ad un pubblico “matematicamente” adulto, poiché è necessario conoscere la serie e la trasformata di Fourier. C’è comunque un’ampia appendice, che sebbene sia ben lontana dall’essere esaustiva , introduce in modo semplice alle serie e trasformate di Fourier. Il docente, una volta approfondito l’argomento, potrà sviluppare una sua personale serie di lezioni. Nella parte conclusiva viene descritta la simulazione al calcolatore della passeggiata casuale o dell’ubriaco. I risultati presentati sono stati ottenuti tramite un codice scritto con il linguaggio Mathematica, ma possono essere ottenuti, evidentemente, attraverso un qualsiasi linguaggio di programmazione
Modeling adatom life time in thin film growth in case of spatially correlated nuclei
No full textWe model the adatom life time for island growth in the general case of spatially correlated nuclei. The model is based on a stochastic approach, which relies on the use of the m-dots spatial correlation functions. The adatom life time is evaluated in the entire range of surface coverage. The impact of the spatial correlation among nuclei on the life time is analysed and discussed also with reference to the case of random nucleation. © 2004 Elsevier B.V. All rights reserved
Observation of interface states by high-resolution electron-energy-loss spectroscopy in metal-GaAs(110) junctions
No full textAg, Au, and In/GaAs (110) interfaces have been investigated by high-resolution electron-energy-loss spectroscopy, ultraviolet photoemission spectroscopy, and x-ray photoemission spectroscopy. Energy-loss peaks at 0.72 and 1.2 eV-that is, in the GaAs energy gap region-have been detected at the early stage of the junction formation. The intensity behavior of the 1.2-eV feature, as a function of metal coverage, strictly follows the evolution of the total perimeter of the metal islands. Similar features have been also found on cleaved stepped surfaces. These facts suggest that the loss structures are due to transitions among defect states induced by the metal overlayer and localized at the interface
A Design Pattern for translating UML software models into IEC 61131-3 Programming Languages
No full textThe paper deal with a deep analysis and application of object-oriented methodologies for the design process of industrial machine controllers. The process pass thought the phases of (1) conceptual model development of the software artifacts, and (2) model code deployment for the automation system target, which is in general a Programmable Logic Controller (PLC) compliant with the international standard IEC 61131-3 for programming languages. The paper describes a design pattern for the software conceptual model deployment, with a particular emphasis on practical requirements enforced by PLC of different brands. In fact, IEC 61131-3 compliance provides a common ``look-and-feel'' for programming languages, but does not guarantee straightforward code portability between different PLC vendors. The paper has a great focus on applications and provides an example, based on a generic flow-pack machine, to better explain the proposed methodology
Mean field approach for describing thin film morphology: 2. Adatom life time
No full textBy means of the analytical expression of the decay of the number of rigid discs as a function of the fraction of covered surface, we are able to evaluate averages of stochastic variables related to the distribution of impenetrable discs. In particular, we consider the edge-edge mean distance between discs, lambda p, and the distance from the edge disc of a point chosen at random, delta. The second moment of d and its connection to the adatom life time in thin film growth is highlighted and discussed on the basis of the comparison with Monte Carlo results. We also demonstrate numerically that the adatom life time in mean field coalescence is the same as that calculated in genuine coalescence
Electronic anisotropy of the GaAs(001) surface studied by energy loss spectroscopy
No full textHigh-Resolution Electron-Energy-Loss Spectroscopy (HREELS) has been applied to investigate the anisotropy of the GaAs(001)-c(4 x 4) and beta2(2 x 4) reconstructions. Measurements have been performed on high-quality samples grown in situ by Molecular Beam Epitaxy. The loss intensity is different in the directions parallel and perpendicular to dimers, particularly close to the fundamental gap. We construct relative difference intensity spectra which can be directly compared with the differential reflectivity spectra of the RAS spectroscopy. A one-to-one correspondence is found between experimental and calculated electronic transitions up to about 3 eV. The surface anisotropy given by EELS is about two orders of magnitude higher than that measured optically. The contributions to the anisotropy originate entirely from a few atomic layers beneath the surface. In the beta2 phase we find direct evidence of transitions involving the dimers of the top atomic layer which are well separated by those involving bulk states modified by the surface. (C) 2003 Elsevier Science Ltd. All rights reserved
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