Gas-phase positive and negative ion/molecule reactions in diborane and silane/diborane mixtures

The gas phase ion chemistry of diborane and of silane/diborane mixtures was studied by ion trap mass spectrometry (ITMS), in both positive and negative ionization mode. Positive ion/molecule reactions in diborane mainly proceed through H2 or BH3 loss and lead to formation of B hydride clusters containing up to six B atoms, while in negative ionization clustering reactions proceed to a higher degree with formation of B8Hn− ions. In the silane/diborane system, the main ion/molecule reactions leading to ions containing both silicon and boron atoms were identified and rate constants of the main processes were determined for positive ions. In positive ionization, Si/B ions are mainly formed in reactions of few diborane ions with silane, while reactions of silane ions with B2H6 yield B2H5+ as the main product. Negative ionization of the same mixture produces a much larger amount of Si/B ionic species, due to several reactions of silane anions with diborane, resulting in BH3 or H2 neutral losses. These results indicate that negative ions may play an important role in formation of Si/B ion clusters in plasma chemical vapour deposition processes for production of semiconductor and photovoltaic materials.