Ptashchenko F. Influence of long-range interaction between impurity atoms and surface defects on the process of formation, conductivity and sensor sensitivity to NO2 and NH3 molecules of porous silicon and other silicon structures

The dissertation is devoted to the study of the mechanism and manifestations of the remote interaction between impurity atoms and surface defects of the pb-center type and the influence of this interaction on the process of electrochemical etching, conductivity and sensor sensitivity to active NO2 and NH3 molecules of porous silicon and other silicon structures. It has been shown that impurity phosphorus and boron atoms in silicon structures can remotely interact with pb-centers if the distance between them does not exceed ~25 Å. During this interaction, the pb-centers and impurity atoms become charged, which is accompanied by passivation of the impurity and the appearance of a region of increased or decreased potential around the pb-centers. It is shown that the formation of PS during electrochemical etching in aqueous HF solutions occurs by direct fluorination of surface silicon atoms. Direct fluorination is easiest on the (110), (100) faces and at their intersection, under the action of "heavy" fluoride complexes and with the participation of free holes. Near impurity boron atoms, the process of electrochemical etching of silicon slows down or stops, which explains the formation of a characteristic PS structure in the form of a skeleton of nanocrystals with boron atoms inside. It has been demonstrated that the activation character of PS conductivity is explained by the presence of barriers for free carriers that exist around charged pb centers in the thinnest sections of nanowires in the PS network. This also explains the scatter in the values of the thermal activation energy of conductivity for different PS samples, the presence of two linear segments in the temperature dependence of PS conductivity in Arrhenius coordinates, and the existence of two types of dependence of PS conductivity on the external field. It has been shown that the adsorption of NO2 molecules on a hydroxylated silicon or silicon oxide surface can lead to the formation of free states in the band gap of silicon. The appearance of shallow acceptor states in p-PS is also observed when NO2 molecules are adsorbed on OH-groups near positively charged pb-centers, which passivate subsurface boron atoms. This explains the increase in the concentration of free holes in p-type PS in an NO2 atmosphere. It is shown that protonation of ammonia molecules can occur on hydrogenated and oxidized silicon surfaces. The protonation of NH3 molecules is facilitated by the presence of surface OH groups, adsorbed water molecules, and subsurface boron atoms. The process of protonation of ammonia molecules is accompanied by remote passivation of the boron impurity by NH4+ ions and the appearance of donor states and explains the increase in the concentration of free electrons (or the decrease in the concentration of free holes) in silicon. The protonation of amino compounds on the surface of PS or amorphous silicon determines the experimentally established patterns of SALDI processes.