The Si (100) surface: Symmetric or asymmetric dimers?

We present new results obtained with medium energy ion beam crystallography on the clean Si (100)?(2 × 1) surface. This study, meant to discriminate between symmetric and asymmetric dimer models for the reconstruction, shows that symmetric dimers are not consistent with our results. We conclude that the Si (100)?(2 × 1) surface is reconstructed in asymmetric dimers, accompanied by extensive subsurface distortions.