The positive ion chemistry occurring in SiH4/GeF4 gaseous mixtures was investigated by ion trap mass spectrometry and ab initio theoretical calculations, performed at the MP2 and coupled cluster (CCSD(T)) level of theory. This system is of both fundamental and applied interest, since it is employed to deposit silicon-germanium thin films by plasma techniques. The GeF3+ cation, the only fragment obtained from ionized GeF4, resulted unreactive toward SiH4. This finding resembles our recently ascertained inability of GeF3+ to activate the C-H bonds of CH4. All the primary ions SiHn+ (n = 0-3) react instead with GeF4 so to form SiF+ or SiH2F+. The latter species reacts in turn with SiH4 and GeF4 so to form SiH3+ and SiHF2+, respectively. The open-shell cations Si+ and SiH2+ react with GeF4 and undergo the F-atom abstractions by the attack of Si+ or SiH2+ to a F atom of GeF4, with formation of the Si-F bond and concomitant cleavage of the weaker Ge-F bond. The closed-shell cations SiH+ and SiH3+ react with GeF4 and undergo the formal H/F exchange reactions. Both these processes are predicted to be exothermic, and this reflects essentially the formation of the strong Si-F bonds of SiF+ and SiH2F+. These products arise from the direct attack of SiH+ and SiH3+ to a F atom of GeF4, so to form HSi-F-GeF3+ and H3Si-F-GeF3+ intermediates, which undergo the extrusion of GeHF3, with different detailed mechanisms. In conclusion, our investigation of the positive ion chemistry occurring in mixtures of SiH4 and GeF4 improves knowledge on this system. It is remarkable that our studies did not reveal any formation of Si-Ge ionic species. This suggests that, when employed to deposit silicon-germanium thin films by plasma techniques, the species involved in the early stages of the polymerization are neutral rather than ionic. This information could contribute to optimize the running conditions during the deposition processes.
Positive ion chemistry of SiH4/GeF4 gaseous mixtures by ab initio calculations and ion trap mass spectrometry
OPERTI, Lorenza;RABEZZANA, Roberto;TURCO, Francesca;
2011-01-01
Abstract
The positive ion chemistry occurring in SiH4/GeF4 gaseous mixtures was investigated by ion trap mass spectrometry and ab initio theoretical calculations, performed at the MP2 and coupled cluster (CCSD(T)) level of theory. This system is of both fundamental and applied interest, since it is employed to deposit silicon-germanium thin films by plasma techniques. The GeF3+ cation, the only fragment obtained from ionized GeF4, resulted unreactive toward SiH4. This finding resembles our recently ascertained inability of GeF3+ to activate the C-H bonds of CH4. All the primary ions SiHn+ (n = 0-3) react instead with GeF4 so to form SiF+ or SiH2F+. The latter species reacts in turn with SiH4 and GeF4 so to form SiH3+ and SiHF2+, respectively. The open-shell cations Si+ and SiH2+ react with GeF4 and undergo the F-atom abstractions by the attack of Si+ or SiH2+ to a F atom of GeF4, with formation of the Si-F bond and concomitant cleavage of the weaker Ge-F bond. The closed-shell cations SiH+ and SiH3+ react with GeF4 and undergo the formal H/F exchange reactions. Both these processes are predicted to be exothermic, and this reflects essentially the formation of the strong Si-F bonds of SiF+ and SiH2F+. These products arise from the direct attack of SiH+ and SiH3+ to a F atom of GeF4, so to form HSi-F-GeF3+ and H3Si-F-GeF3+ intermediates, which undergo the extrusion of GeHF3, with different detailed mechanisms. In conclusion, our investigation of the positive ion chemistry occurring in mixtures of SiH4 and GeF4 improves knowledge on this system. It is remarkable that our studies did not reveal any formation of Si-Ge ionic species. This suggests that, when employed to deposit silicon-germanium thin films by plasma techniques, the species involved in the early stages of the polymerization are neutral rather than ionic. This information could contribute to optimize the running conditions during the deposition processes.
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