Olenych I. Non-equilibrium electronic processes in silicon-based nanosystems

Dissertation addresses scientific and applied problem related to the study of regularities of light, temperature and adsorption stimulated electronic processes in silicon-based nanosystems. Main regularities of the formation of nc-Si – dielectric, nc-Si – semiconductor, nc-Si – conductor and also hybrid organic-inorganic porous silicon-based nanosystems with predictable structural, optical and electrical properties were studied to establish the mechanisms of environmental influence on the electronic processes in silicon nanostructures. Using AFM and SEM methods structural and fractal properties of obtained nanosystems were studied. Molecular structure and mechanisms of interaction between the components of porous silicon based inorganic and organic-inorganic systems were investigated by means of Fourier IR spectroscopy. The mechanisms of influence of irradiation from 226Ra isotope on the photoluminescent and electrical properties of porous silicon and silicon-based film nanocomposites have been established. The influence of silicon nanocrystals orientation on optical-luminescent properties of silicon nanostructures in epoxy matrix was studied. It is demonstrated that nanocrystals shape anisotropy leads to anisotropy in photoluminescence polarization and effective refraction index of the nanosystem as a whole. Optical properties of the nanosystems are analyzed in the frame of effective medium model with cylindrical particles shape. The possibility of controlling the spectrum of multi-band photoemission of the porous silicon – zinc oxide nanosystems by adjusting the excitation energy is demonstrated. Peculiarities of the field effect in porous semiconductors with cylindrical pore shape were studied. Dependencies of electrostatic potential on the coordinate were analyzed for different pore radius, interpore distance and surface charge density. It is established that surface potential and Debye screening length decrease with increasing surface curvature. It is demonstrated that redistribution of charge carriers not only changes the electric conductivity of the porous layer but can also cause the inversion of the conductivity type. The inversion depth depends on the surface charge density and pore radius and can extend even through the entire layer. The possibility of controlling electronic parameters of silicon nanostructures by means of adsorption of chemically active or polar molecules from vapor or liquid phases was demonstrated. The formation of photosensitive electrical barriers in porous silicon-based structures due to adsorption doping of porous layer has been found. The effect of modification of porous layer by metallic nanoparticles having different inner work function and the influence of ZnO and graphene nanosheets deposition on electrical and photoelectrical properties of porous silicon structures was studied. It was established that incorporation of metals and graphene into porous silicon leads to the increase in conductivity and photoinduced voltage of sandwich structures due to the passivation of silicon nanocrystals and the formation of additional conductive channels in the porous layer. The expansion of spectral photosensitivity of porous silicon based structures due to ZnO deposition is revealed. Charge transport and relaxation processes in porous silicon based nanostructures were studied. Electrical conductivity mechanisms were established for the wide range of temperatures. Fractal structures based on porous silicon were shown to have several mechanisms of conductivity. Hopping charge transport via the system of localized states near the Fermi level in the temperature range of 12–120 K was revealed. At higher temperatures, activation mechanism of electric conductivity is dominating which is characterized by different activation energies in the temperature ranges of 120–200 K and 220–290 K. Activation energy of the electric conductivity for porous silicon in the low temperature region was found to be around 0.05 eV. The increase in activation energy due to oxide or polymer passivation of silicon nanostructures and graphene nanoparticle deposition was detected. Energy distribution of the localized states population density in porous silicon nanostructures was determined by means of thermally stimulated depolarization method. The nature of electrically active defects was identified and it was established that the change of silicon nanocrystals surface passivation causes a redistribution of the density of states in different energy ranges. The sensory properties of nanosystems based on porous silicon have been studied. Dependencies of adsorption sensitivity on the concentration were analyzed and time characteristics of sensor elements were determined. The possibility of gas identification based on overall impedance response of the multisensory system was demonstrated.