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A planar dc gas discharge system with a high Ohmic semiconductor cathode is investigated with respect to temporal destabilization of the stationary homogeneous state. A subcritical Hopf bifurcation is observed, leading to a spatial homogeneous oscillation. The dependence of the oscillator's properties on control parameters is investigated. By applying spatial nonuniform optical control of the semiconductor cathode, several domains that may oscillate on different frequencies can be created. These spatially homogeneous domains can interact with each other through common boundaries. By adjusting the strength of coupling of the domains, their interaction can be controlled. In this interaction, regularities have been found that are, in some aspects, similar to those observed in externally driven nonlinear oscillators.

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In a dc-driven planar gas discharge system with a semiconductor electrode, the homogeneous stationary discharge state can be destabilized in favor of current filaments. A filament consists of a succession of spatially confined breakthroughs of the gas layer that repeatedly take place at approximately the same position. A pulsating filament is thus slowly moving over the active area of the system. At fixed parameters, processes of creation and quenching of filaments are observed, while their average spatial density depends on control parameters. Depending on the density, filaments arrange in different configurations. At an intermediate value of filament density, a pattern on a two-dimensional domain is found: it is a spatially anisotropic chain pattern that is specified by two characteristic spatial scales. It is suggested that the observed phenomena are due to a Hopf-Turing instability arising in the system.

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Doping of silicon with magnesium is investigated by a sandwich diffusion technique. Temperature dependence of the diffusion coefficient in the dislocation-free silicon in the range of 1000-1200 8C is determined. It obeys the Arrhenius behavior over the range of 600-1200 8C, when the data obtained earlier for the lower temperatures are taken into consideration. Preliminary results on Mg diffusion in the dislocated crystals are also presented. The dislocation-free Si:Mg samples are investigated with the Hall-effect measurements and the low-temperature Fourier spectroscopy. A decrease in concentration of Mg interstitials (about 15%) has been observed after 31 months of the samples storage at room temperature, when a commercially available FZ silicon was used as a starting material. The effect of the samples degradation is proposed to be due to a formation of Mg-O complexes. When using a special silicon purified from oxygen and carbon with concentrations below or equal to 1.5 Â 10 14 and 5 Â 10 14 cm À3 , respectively, a decrease in the density of interstitial magnesium has not been noticed during this period. The storage of Si:Mg samples prepared from pure silicon gives rise to the formation of an unknown center, whose ionization energy is between the corresponding values for the interstitial Mg 0 centers and (Mg-O) 0 complexes.