Disproportionation Phenomena on Free and Strained Sn/Ge(111) and Sn/Si(111) Surfaces
Abstract
Distortions of the 3×3 Sn/Ge(111) and Sn/Si(111) surfaces are shown to reflect a disproportionation of an integer pseudocharge, Q, related to the surface band occupancy. A novel understanding of the (3×3)-1U (``1 up, 2 down'') and 2U (``2 up, 1 down'') distortions of Sn/Ge(111) is obtained by a theoretical study of the phase diagram under strain. Positive strain keeps the unstrained value Q=3 but removes distorsions. Negative strain attracts pseudocharge from the valence band causing first a (3×3)-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a (3×3)-3U (``all up'') state with Q=6. The possibility of a fluctuating phase in unstrained Sn/Si(111) is discussed.
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