|Budget Amount *help
¥1,800,000 (Direct Cost : ¥1,800,000)
Fiscal Year 1996 : ¥700,000 (Direct Cost : ¥700,000)
Fiscal Year 1995 : ¥1,100,000 (Direct Cost : ¥1,100,000)
Ab initio electronic state calculations have been performed for five model structures of covalent adatoms on the Si (111) surface. Norm-conserving pseudopotentials have been used in the calculation with cutoff wavenumber 2.5-3.4alpha_B^<-1> for plane wave basis functions. Calculated results for the total energy are utilized in the investigation of relative stability of these adsorption structures.
Calculations have been done for Ga as group-III,Ge as group-IV,and As and Sb as group-V adatoms. The following findings are obtained as common features among them :
(1) Relaxation of substrate atoms is large for the two models with single adatoms at the hollow sites of the top layr. Especially for the model with As and Sb adatoms just above the second layr substrate atoms, the energy gain due to the relaxation amounts to as large as leV.
(2) Relaxation effects are small for the trimer and substitutional adsorption models.
(3) Cutoff value 3.1alpha_B^<-1> for wavenumber of plane waves, although too small to give convergent values for the adsorption energy, is sufficient for reliable results for relative stability among these adsorption models.
A marked individuality of adsorbed atoms is found in some cases with relative stability among the adsorption models. Adatoms Ga, Ge and Sb give nearly the same order of stability for the five models investigated. , but different order is obtained for As. The surface with As adatoms is most stable for the substitutional model with the 1*1 periodicity, in which all the top-layr Si atoms of the truncated surface are replaced with the adatoms. On the other hand for adatoms Ga, Ge and Sb, the most stable structure is found in the "1T model" with the ROO<3> * ROO<3> periodicity, in which single adatoms are located at the hollow sites of the top layr just above the second layr atoms. These results are consistent with the reported experimental observations.