New Model for the Structure of the Ge “Hut” Facet

The STM images of Ge nanocrystals that grow on Si(001), the so-called “huts” discovered in our lab in 1990, have become poster art in many books and articles and in fact form the permanent cover of Progress in Surface Science. The atomic structure of the very distinct (105) facets of these nanocrystals was postulated by us in 1990 on the basis of the STM im-ages and has been accepted since then. It turns out this structure is incorrect. We have made new and vastly improved STM measurements, now on of the Si(105) surface with Ge deposited. With the help of density functional theory (DFT) calculations provided by the Terakura group at JRCAT, Tsukuba, Japan, we have proposed a new structure that is consis-tent with all data. The paper is accepted for publication at PRL. The morphology evolution with increasing amount of Ge deposited documents the existence of a tensile surface strain in Si(105) and its relaxation with increasing coverage of Ge. The new model for the Ge(105)-1x2 structure formed on Si(105) includes the existence of the surface strain. The new structural model includes rebonded SB steps rather than the non-bonded SB steps that are the central feature of the currently accepted model. Structure optimization shows large relaxation of dimers at the rebonded SB steps and causes significant charge redistribution among dangling bonds.

Figure 1. Experimental ((a) and (b)) and theoretical ((c)-(f)) STM images. (a), (c) and (e) are filled state images, while (b), (d) and (f) are empty state images. Si(105) covered with 3ML of Ge was observed with sample bias of (a) -1:0 V and (b) +1:0 V. STM images of the old model are shown in (c) and (d), and those of the new rebounded-steps model are shown in (e) and (f). (From Terakura et al., submitted).

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