Mechanisms of Changing the Conductivity of Porous Silicon in an Ammonia Atmosphere – DFT Modeling

Based on quantum-chemical calculations by the density functional theory (DFT) method, four possible mechanisms of the influence of ammonia vapors on the conductivity of silicon nanostructures, in particular, porous silicon (PS), were examined. The first mechanism involves the emergence of donor states in the interaction of NH3 molecules with pb-centers (surface Si atoms with dangling bonds). The change in conductivity by the second and third mechanisms can occur in p-type silicon structures. The second mechanism involves the protonation of an ammonia molecule with the subsequent passivation of subsurface impurity boron atoms by NH4 ions. The third mechanism combines the first two. At the first stage, it involves the interaction of NH3 molecules with passivated B-pb-center pairs. After protonation of the NH3 molecule, the boron impurity atom is already passivated by the NH4 ion, and the paramagnetic state of the pb-center is restored. At the second stage, the formation of donor states occurs during the interaction of NH3 molecules with already paramagnetic pb-centers. The processes according to the fourth mechanism can occur in n-type silicon structures. It provides for the restoration of donor properties of surface phosphorus atoms passivated by two hydrogen atoms. Such a restoration occurs after protonation of the NH3 molecule, when the proton (the ion of the surface hydrogen atom) is separated from the phosphorus atom. The last three models involve the protonation of NH3 molecules with the necessary participation of water molecules and surface OHgroups, the important role of which has been demonstrated in most experimental studies.