ADSORPTION AND DISSOCIATION OF PH3 ON SI(100) 2X1 AND SI(111) 7X7 - THEORETICAL-STUDY

The adsorption structures and dissociation properties of PH3 on Si(100 ) 2 x 1 and Si(111) 7 x 7 have been studied by the ASED MO Method. Our calculation results show that, on the Si(100) 2 x 1 surface. PH3 is a dsorbed undissociatively on the dangling bond, with a binding energy o f 2.25 eV, Si-P bond length of 2.23 angstrom, an angle from the surfac e normal of 22-degrees, a P-H bond length of 1.52 angstrom and an angl e of the PH3 axis to the surface normal of 21-degrees. The dissociatio n energy barrier of the first H atom for PH3 is about 1.61 eV, in agre ement with the experimental results of molecular adsorption of PH3 on Si(100) at room temperature. On S(111) 7 x 7, the most stable site for PH3 adsorption of S2DB, and then the S1DB site; the binding energies are 2.11 eV and 1.00 eV, respectively. Using the DAS model in our calc ulation, our results show that the energy barrier for PH3 dissociation on S1DB and S2DB are 1.02 eV and 2.05 eV, respectively. If we replace the Si adatom in the DAS model by a Si4 small cluster, the binding en ergies of PH3 On S2DB and S1DB are 1.64 eV and negative, respectively. The dissociation energy barrier on S2DB will reduce to 0.48 eV. It se ems that the dissociation of PH3 on Si(111) 7 x 7 is caused by some ki nd of defect on this surface.