Electrical and Computer Engineering

The effect of compositional and positional disorder on electronic properties of (Group III)-nitride solid solutions with the wurtzite structure was studied by the method of a model empirical pseudopotential using 32-atom supercells. The calculated values of the band-gap bowing parameter are found to be equal to 0.44, 2.72, and 4.16 for AlGaN, InGaN, InAlN, respectively. It is shown that the major contribution to the band-gap bowing parameter is made by the compositional disorder, whereas the bond-length relaxation reduces the effect of compositional disorder and the effects of the volume deformation. © 2004 MAIK "Nauka/Interperiodica".

Properties of as prepared or nanoengineered III-V semiconductor surfaces provide attractive means for photonic detection of different adsorbants from surrounding gaseous or liquid environments. To be practical, this approach requires that the surface is made selectively sensitive (functionalized) to targeted species. In addition, such surface has also to stay stable over extended period of time to make it available for rapid testing. Numerous reports demonstrate attractive properties of GaAs for sensing applications. One of the most fundamental issues relevant to these applications concerns the ability to functionalize chemically, or biologically, the surface of GaAs. The most studied method of GaAs surface functionalization is based on formation of self-assembled monolayers (SAMs) of various n-alkanethiols, HS-(CH 2 ) n -T (T = CH 3 , COOH, NH 2 , etc.). In spite of multi-year research concerning this issue, it has only been recently that a comprehensive picture of SAMs formation on GaAs and an understanding of the natural limitation of the SAM-GaAs interface in some bio-chemical sensing architectures has begun to emerge.

We present the results of ab initio calculations of the effect of reconstruction and passivation of zigzag edges on the electronic and magnetic properties of triangular graphene quantum dots. We find that, similarly to nanoribbons, hydrogen-passivated ideal zigzag edges are energetically favored over the pentagon-heptagon zigzag. However, the reconstructed edge is more stable in the absence of hydrogen, thus, delayed passivation with H may lock the dot in such an unfavorable configuration. Both hydrogen-passivated edge morphologies lead to a band of states at the Fermi level. Unlike in nanoribbons, this quasidegenerate band results in net spin polarization for structures with zigzag edge of all sizes studied here. For triangular dots with pentagon-heptagon zigzag edge, a larger width of the zero-energy band is predicted, leading to the loss of net magnetization.

Post-growth selective-area bandgap tuning of quantum well (QW) microstructures has been investigated using Finite Element Method computer simulations. Laser fast scanning is proposed as a way to overcome the problem of damaging the surface with a small spot needed to obtain better spatial resolution. Influence of laser parameters, background heating, beam scanning speed and properties of QW microstructures on the ability to achieve sharp bandgap profiles in the laserwritten intermixed material are investigated. Lateral resolution of 2 µm is expected to be achievable with a 12-µm beam laser rapid thermal annealing.

On the basis of high resolution STM images and DFT modeling, we have resolved low-and high-coverage structures of methylthiolate (CH 3 S) self-assembled on the Au(111) surface. The key new finding is that the building block of all these structures has the same stoichiometry of two thiolate species joined by a gold adatom. The self-arrangement of the methylthiolate-adatom complexes on the surface depends critically on their stereochemical properties. Variations of the latter can produce local ordering of adatom complexes with either (3 × 4) or (3 × 4 3) periodicity. A possible structural connection between the (3 × 4 3) structure and commonly observed ( 3 × 3)R30°phase in methylthiolate self-assembled monolayers is developed by taking into account the reduction in the long-range order and stereochemical isomerization at high coverage. We also suggest how the observed self-arrangements of methylthiolate may be related to the c(4 × 2) phase of its longer homologues.

Infra-red (IR) laser induced selective-area quantum well intermixing (QWI) has the potential to yield multi-bandgap quantum well wafers suitable for the fabrication of monolithically integrated photonic devices. Quantitative description of the IR laser-QWI process requires knowledge of temporal and spatial temperature profiles induced by the laser. This requires solving a 3-dimensional heat diffusion equation, which takes into account laser parameters and the irradiation conditions. We report the results of modeling temperature profiles in semiconductor wafers irradiated with a stationary CW Nd:YAG laser beam (λ=1064 nm). The calculations were carried out using a finite element model and taking into account convection, background heating, as well as temperature dependent both heat conductivity and optical absorption. A reasonable agreement has been observed between calculated and measured temporal dependencies of the laser induced temperatures. Our calculations indicate that lines of the GaAs/AlGaAs QWI material could be 50% narrower than the diameter of the laser writing spot if the background temperature of the wafer is increased to about 700 °C .

Adsorption of alkanethiols on GaAs (001) surface under low coverage conditions was studied using density functional calculations in a periodic supercell approach. The study of physisorbed precursor and transition to chemisorption revealed that hydrogen atoms stay on the surface upon S-H bond cleavage and significantly affect desorption products and energies, in agreement with available experimental data. Binding of thiols to GaAs is found to be comparable or stronger than that of thiols to noble metals surfaces. Calculated thiolate-surface binding energies are found to be higher for Ga-rich than for As-rich surfaces, and are strongly dependent on surface reconstruction, adsorption site and coverage. This dependence is explained by the violation or fulfillment of electron counting rule, rehybridization of surface atom orbitals and strain relaxation upon addition of extra electrons brought by adsorption.

Using alkanethiol dense packing as a starting point, we have found six prototypical packing structures commensurate with the (3 Â 2 √ 3) supercell of the Au (111) surface. Five of the six structures are not compatible with the flat surface conditions but can be fitted to a reconstructed surface. Combined with density functional theory calculations and simulations of grazing incidence X-ray diffraction maps and of scanning tunneling microscopy images, this allowed us to refine and assess the recently proposed models of the c(4 Â 2) self-assembled monolayers involving thiolate-adatom and thiolate-adatom-thiolate species and to propose a new model with four gold adatoms per unit cell. Langmuir 2009, 25(13), 7353-7358 Voznyy and Dubowski Article (52) Torrelles, X.; Vericat, C.; Vela, M. E.; Fonticelli, M. H.; Millone, M. A. D.; Felici, R.; Lee, T. L.; Zegenhagen, J.; Munoz, G.; Martin-Gago, J. A.; Salvarezza, R. C.

Abstract. Chemical bonding and electronic properties of III-nitrides solid solutions are studied using a model empirical pseudopotential method in modified virtual crystal approximation taking into account positional and compositional disorder. The changes in chemical bonding are analyzed in different ionicity models. The bowing is mainly caused by compositional disorder, volume deformation effects and valence charge redistribution in the alloys.

Abstract: Theory of electronic and optical properties of exciton and biexciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic sp3d5s* tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactions, and model surface passivation. The optically excited states are expanded in electron-hole configurations and the many-body spectrum is computed in the configuration-interaction approach.

Abstract Adsorption of alkanethiols on GaAs (001) surface under low coverage and high-coverage conditions was studied using density functional calculations in a periodic supercell approach. The thiolate adsorption site and tilt angle are dictated by the high directionality and covalent character of S-As bond. Calculated sulfur-surface binding energies are found to be significantly different for Ga-rich and As-rich surfaces and stronger than that of thiols on gold and copper surfaces.

Abstract From physiology we learn that the auditory system extracts simultaneous features from the underlying signal, giving birth to simultaneous representations of audible signals. The use of the Rank Order Coding has also been recently hypothesized in the mammalian auditory system. In a first application we compare a very simplistic speech recognition prototype that uses the Rank Order Coding with a conventional Hidden Markov Model speech recognizer.

We incorporate auditory-based features into an unconventional pattern classification system, consisting of a network of spiking neurones with dynamical and multiplicative synapses. Although the network does not need any training and is autonomous, the analysis is dynamic and capable of extracting multiple features and maps. The neural network allows computing a binary mask that acts as a dynamic switch on a speech vocoder made of an FIR gammatone analysis/synthesis bank of 256 filters.

ABSTRACT We propose a new biologically-inspired paradigm for universal audio coding based on neural spikes. Our proposed approach is based on the generation of sparse 2-D representations of audio signals, dubbed as spikegrams. The spikegrams are generated by projecting the signal onto a set of overcomplete adaptive gammachirp (gammatones with additional tuning parameters) kernels. A masking model is applied to the spikegrams to remove inaudible spikes and to increase the coding efficiency.

Abstract We present an example of an anthropomorphic approach, in which auditory-based cues are combined with temporal correlation to implement a source separation system. The auditory features are based on spectral amplitude modulation and energy information obtained through 256 cochlear filters. Segmentation and binding of auditory objects are performed with a two-layered spiking neural network.