Dissociative Adsorption of PH3 on the Si(111)-7 × 7 Surface: A Theoretical Investigation

Abstract

Density functional theory at the level of (U)B3LYP has been used to explore the dissociation of PH3 on the adatom site (Sia) and rest atom site (Sir) of the Si(111)-7 × 7 surface. A detailed comparison between PH3 and NH3 adsorption on Si(111)-7 × 7 is performed. Our results show that PH3 initial dissociation to adsorbed species, PH2(a) and H(a), is facile and preferentially occurs on the Sir site. The same trend was found for NH3, but PH3 shows a site selectivity higher than NH3. XH2(a) is thermally stable, and an elevated temperature is required for further X−H (X = N or P) bond decomposition. The general mechanism for further X−H bond decomposition is XHn (n = 2 or 1) insertion into Si−Si backbond, followed by H2 liberation, with the former usually being the rate-determining step. Full XH3 decomposition may lead to the formation of Si═X or Si3X unit with the preference on the Sir site for N and that on the Sia site for P. Such a difference should be attributed to X−H bond energy difference, the atomic radius difference between P and N, and the release of the strain energy of the reconstructed surface. We anticipate that the detailed energetics obtained from this study can be used as the quantum-mechanical input for a chemical-kinetics model of chemical vapor deposition.