DFT-modeling of Ammonia Molecules Protonation on a p-type Silicon Surface

In this work, DFT modeling was carried out in order to establish the conditions necessary for the protonation of ammonia molecules on the silicon surface. Simulations have shown that for an energetically favorable protonation, three conditions are necessary: the fixing of NH3 molecules on at least two OH-groups, the participation of one water molecule, and the presence of a distant boron atom. In this case, the protonation energy (decrease in the cluster energy after protonation) is Eprot  0.01 eV. Protonation occurs with the simultaneous transition of two protons: one from the surface silane group to the H2O molecule and the other from H2O to NH3. Such a transition leads to the formation of a positively charged NH4+ ion and a negatively charged pb-center (Si atom with a dangling bond). After that, the electron is transferred from the pb-center to the distant boron atom, and passivates it. Such passivation of acceptors by NH4+ ions can lead to a decrease in the concentration of free holes (and a decrease in conductivity) in p-type silicon. The proposed model of NH3 molecules protonation can also explain the processes of surface-assisted laser desorption/ionization (SALDI) of amino compounds. Free holes formed by laser radiation can recombine with electrons localized on passivated boron atoms, i.e., remove their negative charge. After this, the desorption energy of NH4+ decreases from 3.67 to 1.05 eV, which is close to the experimental values for amino ions. The simulation also showed that the absence of a distant boron atom significantly reduces the protonation efficiency, Eprot  – 0.50 eV, since the negative charge is localized not at the distant boron atom, but at the pb-center located near NH4+. Significant Coulomb attraction between the NH4+ ion and the charged pb-center leads to a substantial displacement of the silicon atom, which is energetically unfavorable. The fixation of NH3 molecules on only one OH-group or the absence of water molecules also reduces the protonation probability: the value of Eprot in these cases is – 0.37 and – 0.08 eV, respectively. This can be explained by the fact that H2O molecules and OH-groups create energetically favorable hydrogen bonds with the NH4+ ion and shield its electric field.