Generic role of the anisotropic surface free energy on the morphological evolution in a strained-heteroepitaxial solid droplet on a rigid substrate

A systematic study based on the self-consistent dynamical simulations is presented for the spontaneous evolution of an isolated thin solid droplet on a rigid substrate, which is driven by the surface drift diffusion induced by the anisotropic capillary forces (surface stiffness) and mismatch stresses. In this work, we studied the affect of surface free energy anisotropies on the development kinetics of the 'Stranski-Krastanow' island type morphology. The anisotropic surface free energy and the surface stiffness were treated with well accepted trigonometric functions. Although, various tilt angles and anisotropy constants were considered during simulations, the main emphasis was given on the effect of rotational symmetries associated with the surface Helmholtz free energy topography in 2D space. Our computer simulations revealed the formation of an extremely thin wetting layer during the development of the bell-shaped Stranski-Krastanow island through the mass accumulation at the central region of the droplet via surface drift-diffusion. In the strong (anomalous) anisotropy constant domain, we demonstrated the existence of two distinct morphological modes: i) the complete stability of the initial Cosine-shaped droplet just above a certain anisotropy constant threshold level by spontaneous slight readjustments of the base and the height of the cluster; ii) the Frank-van der Merwe mode of thin film formation for very large values of the anisotropy constant by the spreading and coalescence of the droplets over the substrate surface. During the course of the simulations, we have continuously tracked both the morphology (i.e., the peak height, the extension of the wetting layer beyond the domain boundaries, and the triple junction contact angle) and energetic (the global Helmholtz free energy changes associated with the total strain and surface energy variations) of the system.Comment: anisotropic surface free energy, Quantum Dots, elastostatic load, morphological evolution, strained-heteroepitaxial soli

Effects of anisotropic surface drift diffusion on the strained heteroepitaxial nanoislands subjected to electromigration stressing

A systematic study based on self-consistent dynamical simulations is presented for the morphological evolutionary behavior of an isolated thin Ge/Si nanoisland (quantum dot) on a rigid substrate exposed to electromigration forces. This morphological evolution is basically induced by the anisotropic surface drift diffusion, driven by the capillary forces, the lattice mismatch stresses, and the wetting potential. In this study, we have mainly focused on the size and shape development kinetics of quantum dots, known as the "Stranski-Krastanov " (SK) morphology, influenced by applied electromigration stresses. Emphasis is given to the effects of rotational symmetry associated with the anisotropic diffusivity in 2D space (i.e., quantum wires in 3D). The pointed bullet-shaped "Stranski-Krastanov " islands with high aspect ratios, xi = 0.77, are formed at the cathode edge, while the whole nanoisland slightly creeps out of the initial computational domain. The favorable configuration of the Ge-20/Si-80 alloy test module, which resulted in zeta = 0.37 enhancement in the contour surface area, has a dome shape attached to the [010] top surface of the Si substrate with a zone axis of {010}/ & lang; 001 & rang;. The anisotropic surface diffusion dyadic has a fourfold rotational symmetry axis [001] lying on the (001) plane of the Si substrate, and its major axis is tilted at about phi = 45 & DEG; from the applied electrostatic field extended along the longitudinal axis [100] of the substrate. This particular experiment resulted in a SK singlet peak with a small satellite with a very small aspect ratio of approximately equal to 0.2 that may be appropriate for the conception of quantum optoelectronic devices or inter-band structures to generate photoelectrons having large energy spectra, thereby increasing the efficiency of photovoltaics exposed to solar radiations

Morphological evolution of edge-hillocks on single-crystal films having anisotropic drift-diffusion under the capillary and electromigration forces

The morphological evolution of hillocks at the unpassivated sidewalls of single-crystal metallic thin film interconnects is investigated via computer simulations using the free-moving boundary value problem. The effect of drift-diffusion anisotropy on the development of surface topographical scenarios is fully explored under the action of electromigration and capillary forces, utilizing numerous combinations of the surface texture, the drift-diffusion anisotropy and the direction of the applied electric field. The simulation studies yield analytical relationships for the velocity of the surface solitary waves and the drift velocity of electromigration-induced internal voids as a function of the applied current densities, which contain intrinsic and structural properties of the single-crystal thin films. The threshold value of the applied current density, above which electromigration-induced internal voids can be formed and may cause the catastrophic failure of interconnects by breaching, also appears explicitly in this relationship. (c) 2006 Published by Elsevier B.V

Stranski-Krastanow islanding initiated on the stochastic rough surfaces of the epitaxially strained thin films

Quantum dots (QD) have discrete energy spectrum, which can be adjusted over a wide range by tuning composition, density, size, lattice strain, and morphology. These features make quantum dots attractive for the design and fabrication of novel electronic, magnetic and photonic devices and other functional materials used in cutting-edge applications. The formation of QD on epitaxially strained thin film surfaces, known as Stranski-Krastanow (SK) islands, has attracted great attention due to their unique electronic properties. Here, we present a systematic dynamical simulation study for the spontaneous evolution of the SK islands on the stochastically rough surfaces (nucleationless growth). During the development of SK islands through the mass accumulation at randomly selected regions of the film via surface drift-diffusion (induced by the capillary and mismatch stresses) with and/or without growth, one also observes the formation of an extremely thin wetting layer having a thickness of a few Angstroms. Above a certain threshold level of the mismatch strain and/or the size of the patch, the formation of multiple islands separated by shallow wetting layers is also observed as metastable states such as doublets even multiplets. These islands are converted into a distinct SK islands after long annealing times by coalescence through the long range surface diffusion. Extensive computer simulation studies demonstrated that after an initial transient regime, there is a strong quadratic relationship between the height of the SK singlet and the intensity of the lattice mismatch strain (in a wide range of stresses up to 8.5GPa for germanium thin crystalline films), with the exception at those critical points where the morphological (shape change with necking) transition takes place. (C) 2014 AIP Publishing LLC

Brain-Computer Interface for Control of Wheelchair Using Fuzzy Neural Networks

The design of brain-computer interface for the wheelchair for physically disabled people is presented. The design of the proposed system is based on receiving, processing, and classification of the electroencephalographic (EEG) signals and then performing the control of the wheelchair. The number of experimental measurements of brain activity has been done using human control commands of the wheelchair. Based on the mental activity of the user and the control commands of the wheelchair, the design of classification system based on fuzzy neural networks (FNN) is considered. The design of FNN based algorithm is used for brain-actuated control. The training data is used to design the system and then test data is applied to measure the performance of the control system. The control of the wheelchair is performed under real conditions using direction and speed control commands of the wheelchair. The approach used in the paper allows reducing the probability of misclassification and improving the control accuracy of the wheelchair

Does aspirin use prevent acute coronary syndrome in patients with pneumonia: multicenter prospective randomized trial

Objectives The aim of this study was to test the hypothesis that aspirin would reduce the risk for acute coronary syndromes (ACSs) in patients with pneumonia

Prognosis of Second Primary Malignancies in Pediatric Acute Lymphoblastic Leukemia Survivors: A Multicenter Study by the Turkish Pediatric Hematology Society

The improved survival rates of childhood cancers raise the long-term risk of second primary malignancy (SPM) in childhood and adolescent cancer survivors. The intensity of the treatment protocol used, the use of some groups of chemotherapeutics, and radiotherapy were found to be risk factors for the development of second primary malignancies (SPMs). Forty-one patients who developed acute myelocytic leukemia or any solid organ cancer within 25 years of follow-up, after completion of pediatric acute lymphoblastic leukemia (ALL) treatment, were included in the study. The mean duration of initial ALL diagnosis to SPM was 9.3 ± 6.1 years. The 3 most common SPMs were acute myelocytic leukemia, glial tumors, and thyroid cancer. Thirteen (81%) of 16 patients exposed to cranial irradiation had cancer related to the radiation field. In total 13/41 (32%) patients died, and the 5-year overall survival rate was 70 ± 8%. Patients older than 5 years old at ALL diagnosis had significantly worse overall survival than cases younger than 5 years old. In conclusion, children and adolescents who survive ALL have an increased risk of developing SPM compared with healthy populations, and physicians following these patients should screen for SPMs at regular intervals