Dimer length variation for different reconstructions of Si, Ge, and mixed Si-Ge dimers on Si(001) and Ge(001) substrates

We report on ab initio pseudopotential calculations for geometrictiucture o f Ge, Si, and mixed Si-Ge dimers on the Si(001)-(1 x 2), Si(001)-(2 x 2), G e(001)-(1);(2), and Ge(001)-(2 x 2) surfaces. Our studies show that there i s a clear energy gain in going from the (1 x 2) reconstruction to a semiant iphase (2 x 2) reconstruction for all of these systems. However, for the ab ove reconstruction change, significant elongation is found to take place fo r the pure Ge-Ge and mixed Si-Ge dimers, but not for the pure Si-Si dimers despite similar beneficial change in substrate relaxation. [S0163-1829(99)0 2427-3].