Atomic and electronic structure of doped Si ( 111 ) ( 2 3 × 2 3 ) R 30 ∘ -Sn interfaces

The hole-doped $\mathrm{Si}(111)(2\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}2\ensuremath{\surd}3)R{30}^{\ensuremath{\circ}}$-Sn interface exhibits a symmetry-breaking insulator-insulator transition below 100 K that appears to be triggered by electron tunneling into the empty surface-state bands. No such transition is seen in electron-doped systems. To elucidate the nature and driving force of this phenomenon, the structure of the interface must be resolved. Here we report on an extensive experimental and theoretical study, including scanning tunneling microscopy and spectroscopy (STM/STS), dynamical low-energy electron diffraction (LEED) analysis, and density functional theory (DFT) calculations, to elucidate the structure of this interface. We consider six different structure models, three of which have been proposed before, and conclude that only two of them can account for the majority of experimental data. One of them is the model according to T\ornevik et al. [C. T\ornevik et al., Phys. Rev. B 44, 13144 (1991)] with a total Sn coverage of 14/12 monolayers (ML). The other is the ``revised trimer model'' with a total Sn coverage of 13/12 ML, introduced in this work. These two models are very difficult to discriminate on the basis of DFT or LEED alone, but STS data clearly point toward the T\ornevik model as the most viable candidate among the models considered here. The STS data also provide additional insights regarding the electron-injection-driven phase transformation. Similar processes may occur at other metal/semiconductor interfaces, provided they are nonmetallic and can be doped. This could open up a new pathway toward the creation of novel surface phases with potentially very interesting and desirable electronic properties.