Interaction of nitrogen dioxide molecules with the surface of silicon nanocrystals in porous silicon layers

Abstract

The methods of infrared absorption spectroscopy and electron paramagnetic resonance are used for studying the effect of adsorption of NO2 molecules, which are strong acceptors of electrons, on the electronic and optical properties of silicon nanocrystals in mesoporous silicon layers. It is found that the concentration of free charge carriers (holes) in silicon nanocrystals, which exhibits a nonmonotonic dependence on the NO2 pressure, sharply increases in the presence of these molecules. At the same time, a monotonic increase in the concentration of dangling silicon bonds (Pb1 centers) is observed. A microscopic model proposed for explaining this effect presumes the formation of donor-acceptor pairs P+b1-(NO2)− on the surface of nanocrystals, which ensure an increase in the hole concentration in nanocrystals, as well as Pb1 centers, which are hole-trapping centers. The proposed model successfully explains a substantial increase in photoconductivity (by two or three orders of magnitude) in the layers of porous silicon in the presence of NO2 molecules; the increment in the concentration of free charge carriers is detected within an order of magnitude of this quantity. The results can be used in designing electronic and luminescence devices based on silicon nanocrystals.