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Molecular beam epitaxy (MBE) is an important technique for the creation of new, non-equilibrium semiconductor materials and structures exhibiting novel physical phenomena. Surface diffusion plays an important role in the growth of these structures, influencing such fundamental growth processes and constants as islanding, critical thickness and epitaxial temperatures. Two approaches to the general problem of surface diffusion and islanding, using the SiGe system as a prototypical semiconductor heterostructure, are discussed: The time evolution of patterned deposits, and kinetic studies of nucleation and growth. While disordered laminar growth occurs for deposition at 300 K, elevated temperatures lead to Stranski-Krastanow (SK) growth (uniform coverage theta SK with excess Ge in islands). Diffusion coefficients for Ge on Si(100) have been determined for coverages below theta SK and show a significant coverage dependence. They are extremely sensitive to contamination with carbon on the order of approximately 0.05 ML, as well as to e-beam irradiation. In situ annealing experiments were performed to study the islanding process in real time. Provided the initial coverage exceeds the thickness of the SK layer, theta SK approximately 3 ML on Si(100)2 x 1, the initially uniform but disordered layer begins to collapse into a SK-type morphology at about 250 degrees C. At a ramping rate of 0.1 degrees C/s this process is completed at approximately 400 degrees C. A temperature dependence of the SK-layer thickness has been discovered for the first time. It is in excellent agreement with theoretical predictions.
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PMID:Surface diffusion and islanding in semiconductor heterostructures. 208 Apr 23

We consider the problem of diffusion-controlled evolution of the A -particle-island- B -particle-island system at propagation of the sharp annihilation front A+B-->0 . We show that this general problem, which includes as particular cases the sea-sea and island-sea problems, demonstrates rich dynamical behavior from self-accelerating collapse of one of the islands to synchronous exponential relaxation of both islands. We find a universal asymptotic regime of the sharp-front propagation and reveal the limits of its applicability for the cases of mean-field and fluctuation fronts.
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PMID:Diffusion-controlled death of A -particle and B -particle islands at propagation of the sharp annihilation front A+B-->0. 1851 10

The feasibility of applying multiphoton excitation fluorescence microscopy-related techniques in planar membrane systems, such as lipid monolayers at the air-water interface (named Langmuir films), is presented and discussed in this paper. The non-linear fluorescence microscopy approach, allows obtaining spatially and temporally resolved information by exploiting the fluorescent properties of particular fluorescence probes. For instance, the use of environmental sensitive probes, such as LAURDAN, allows performing measurements using the LAURDAN generalized polarization function that in turn is sensitive to the local lipid packing in the membrane. The fact that LAURDAN exhibit homogeneous distribution in monolayers, particularly in systems displaying domain coexistence, overcomes a general problem observed when "classical" fluorescence probes are used to label Langmuir films, i.e. the inability to obtain simultaneous information from the two coexisting membrane regions. Also, the well described photoselection effect caused by excitation light on LAURDAN allows: (i) to qualitative infer tilting information of the monolayer when liquid condensed phases are present and (ii) to provide high contrast to visualize 3D membranous structures at the film's collapse pressure. In the last case, computation of the LAURDAN GP function provides information about lipid packing in these 3D structures. Additionally, LAURDAN GP values upon compression in monolayers were compared with those obtained in compositionally similar planar bilayer systems. At similar GP values we found, for both DOPC and DPPC, a correspondence between the molecular areas reported in monolayers and bilayers. This correspondence occurs when the lateral pressure of the monolayer is 26+/-2 mN/m and 28+/-3 mN/m for DOPC and DPPC, respectively.
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PMID:Multiphoton excitation fluorescence microscopy in planar membrane systems. 2022 61