Ab Initio Chemical Kinetics for SiH_x Reactions with Si₂H_y (x = 1,2,3,4; y = 6,5,4,3; x + y = 7) under a-Si:H CVD Condition

Gas-phase reactions of SiH_x with Si₂H_y (x = 1,2,3,4; y = 6,5,4,3) species, respectively, which may coexist under chemical vapor deposition (CVD) conditions, have been investigated by means of ab initio molecular orbital and statistical theory calculations. Potential energy surface (PES) predicted at the CCSD(T)/CBS//B3LYP/6–311++G(3df,2p) level shows that these reactions take place primarily via trisilany radicals, n-Si₃H₇ and i-Si₃H₇. For example, SiH₂ can associate with Si₂H₅ producing n-H₂SiSiH₂SiH₃ exothermically by 55.8 kcal/mol; SiH₃ can undergo addition to H₂SiSiH₂ to produce n-Si₃H₇ or associate with H₃SiSiH barrierlessly forming i-Si₃H₇; whereas SiH can insert into one of the Si─H bonds of Si₂H₆ to give excited n-Si₃H₇. Similarly, H₂SiSiH and SiSiH₃ can undergo insertion reactions with SiH₄ producing n/i-Si₃H₇ intermediates, respectively, to be followed by fragmentation to various smaller species. These processes are fully depicted in the complete PES. The predicted heats of formation of various species agree well with available thermochemical data. The rate constants and product branching ratios for the low-energy channel products have been calculated for the temperature range 300–1000 K by variational RRKM (Rice–Ramsperger–Kassel–Macus) theory with Eckart tunneling corrections. The results may be employed for realistic kinetic modeling of the plasma-enhanced chemical vapor deposition growth of a-Si:H thin films under practical conditions.