We present analytical and computer modeling along with an experiment on the formation of sharp conical tips on monocrystalline silicon thin films, silicon-on-insulator, subjected to irradiation by single 25 ns pulses from a KrF excimer laser focused into a spot several micrometers in diameter. These fabricated structures have heights of about 1 m and apical radii of curvature of several tens of nanometers. We offer a simplified analytical model for the formation of these structures. The computer simulation includes two-dimensional time-dependant heat transfer and phase transformations in Si films on SiO 2 substrates that result from the laser irradiation ͑the Stefan problem͒. It is shown that upon irradiation and initial melting, the liquid/solid interface remains mainly parallel to the surface of the film. After the laser pulse, the molten material self-cools and resolidifies. The solid/liquid interface moves predominately laterally toward the center of the irradiated spot, forming an almost vertical front. We discuss the relation between the dynamics of the melting/freezing front movement and the displacement of material in the irradiated spot.

We present computer modeling along with experimental data on the formation of sharp conical tips on silicon-based three-layer structures that consist of a single-crystal Si layer on a 1 m layer of silica on a bulk Si substrate. The upper Si layers with thicknesses in the range of 0.8− 4.1 m were irradiated by single pulses from a KrF excimer laser focused onto a spot several micrometers in diameter. The computer simulation includes two-dimensional time-dependent heat transfer and phase transformations in Si films that result from the laser irradiation ͑the Stefan problem͒. After the laser pulse, the molten material self-cools and resolidifies, forming a sharp conical structure, the height of which can exceed 1 m depending on the irradiation conditions. We also performed computer simulations for experiments involving single-pulse irradiation of bulk silicon, reported by other groups. We discuss conditions under which different types of structures ͑cones versus hollows͒ emerge. We confirm a correlation between the presence of the lateral resolidification condition after the laser pulse and the presence of conical structures on a solidified surface.

Negative-effective-mass ballistic field-effect transistor: Theory and modelingWe consider the stationary characteristics and high-frequency conductance of dual-gate field-effect transistors with a substantially inhomogeneous capacitive connection between gates and a current-conducting channel for an electron enhancement control mode. In most cases, such transistors are quasihomogeneous because their stationary JV-characteristics are not different from characteristics of perfectly homogeneous devices, which can be described by a certain effective channel length. But a nonstationary small-signal conductance of such transistors with the same stationary characteristics can be noticeably different since it is dependent on detailed space distributions of the capacitive inhomogeneity. In particular, a high-frequency gate current can depend on the stationary current direction in channels with symmetrical stationary characteristics. We discuss in detail the cases when a specific channel-gate capacitance increases or decreases monotonically along the channel current direction. Then high-frequency gate currents depend substantially on the stationary channel current direction ͑in spite of the same stationary JV-characteristics for both directions͒.

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The behavior of short-wave-length acoustic phonons in the vicinity of a quantum well in a GaAs/AlGaAs heterostructure has been investigated. Hot two-dimensional electrons in the well produce longitudinal optical phonons, which decay into almost monoenergetic short-wave-length longitudinal acoustic (LA) phonons. The latter undergo elastic scattering and down-conversion into transverse acoustic (TA) phonons. The distribution of the LA and TA phonons over frequency and distance to the well have been found by solving semi-analytically a system of two kinetic equations with nontrivial boundary conditions and nonlinear dispersion. The distribution functions have essentially non-temperature form even at substantial distance from the well.