A LEED study of Ge(111); a high-temperature incommensurate structure

We present a detailed LEED study of the Ge(111) surface. The low-temperature structure is c(2×8). This structure undergoes a first order transition to an incommensurate reconstruction at 300°C. The diffraction from the high-temperature phase is consistent with either of two simple models. One is a 2 × 1 reconstruction modulated by antiphase walls which are oriented primarily perpendicular to the undoubled direction of the 2 × 1 unit mesh. The second is a 2 × 2 reconstruction modulated by a honeycomb arrangement of intersecting antiphase walls produced by shearing the 2 × 2 mesh by one primitive translation vector. Fluctuations in the I(2 × 2) structure preserve the total wall length and number of wall intersections; the free energy of this structure should be dominated by the entropy. The wall separation and correlation lengths for the high-temperature phase decrease continuously with increasing temperature above the transition.