Geometric and Local Electronic Structure of Si(111)-As

The Si(111)-As surface is imaged by scanning tunneling microscopy under UHV conditions. A threefold-symmetric 1 × 1 structure is observed, consistent with models suggesting substitution of As for Si in the outer half of the top double layer. No evidence for residual stacking faults on the scale of the 7 × 7 mesh is detected, but atomic debris, interpreted as displaced Si atoms, is seen. $I-V$ characteristics of the junction show features separated by 1.9-2.3 eV consistent with an empty surface state in the projected gap and a filled state resonant with the bulk as predicted by self-energy calculations for this surface.