Adsorption and dissociation of PH3on Si(100))2*1 and Si(111)7*7: Theoretical study

The adsorption structures and dissociation properties of PH 3 on Si(100)2*1 and Si(111)7*7 have been studied by the ASED MO method. Our calculation results show that, on the Si(100)2*1 surface, PH 3 is adsorbed undissociatively on the dangling bond, with a binding energy of 2.25 eV, Si-P bond length of 2.23 AA, an angle from the surface normal of 22 degrees , a P-H bond length of 1.52 AA and an angle of the PH 3 axis to the surface normal of 21 degrees . The dissociation energy barrier of the first H atom for PH 3 is about 1.61 eV, in agreement with the experimental results of molecular adsorption of PH 3 on Si(100) at room temperature. On S(111)7*7, the most stable site for PH 3 adsorption of S 2 DB and then the S 1 DB site; the binding energies are 2.11 eV and 1.00 eV, respectively. Using the DAS model in our calculation, our results show, that the energy barrier for PH 3 dissociation on S 1 DB and S 2 DB are 1.02 eV and 2.05 eV, respectively. If we replace the Si adatom in the DAS model by a Si 4 small cluster, the binding energies of PH 3 on S 2 DB and S 1 DB are 1.64 eV and negative, respectively. The dissociation energy barrier on S 2 DB will reduce to 0.48 eV. It seems that the dissociation of PH 3 on Si(111)7*7 is caused by some kind of defect on this surface.