Adsorption and reaction of N₂H₄ on Si(1 0 0)-2 × 1: A computational study with single- and double-dimer cluster models

We have studied the adsorption and decomposition of N₂H ₄ on Si(1 0 0)-2 × 1 surface using the hybrid density functional B3LYP method with Si₉H₁₂ and Si ₁₅H₁₆ as single and double surface dimer models for cluster calculations, respectively. We also compared the energetic results with slab calculations using density functional theory with the generalized gradient approximation. The result of our single-dimer surface model calculation shows that the activation energy for the dissociative adsorption of N ₂H₄ producing 2NH₂(a), 29.6 kcal/mol, is higher than that for the dissociative adsorption giving N₂H₃(a) + H(a), 5.3 kcal/mol although the overall exothermicity of the former process, 97 kcal/mol, is considerably higher than that of the latter, 56 kcal/mol. Both processes occur via the stable N₂H₄(a) intermediate formed with 23.7 kcal/mol adsorption energy. The result of our calculation with the double-dimer surface model reveals that the activation energies for the aforementioned processes are somewhat lower than the single-dimer surface for either one or two N₂H₄ molecules, but with a similar trend. The energies of stable species predicted by the slab model calculation are consistent with the double-dimer results to within 10%. The predicted stabilities of various surface species and their vibrational frequencies are also consistent with the results of our previous thermal annealing studies with HREELS, XPS and UPS measurements. With the double-dimer surface model, we have also examined the effect of adsorbate interactions.