LOW-COVERAGE GALLIUM DEPOSITION ONTO SI(001) STUDIED BY VOLTAGE-DEPENDENT STM AND TOTAL-ENERGY CALCULATIONS

Gallium has been deposited onto (2 x 1) reconstructed Si(001) by decom position of trimethylgallium (TMGa) and MBE, respectively. The surface was investigated by scanning tunneling microscopy (STM) over a range of bias voltages at room temperature. Both deposition methods lead to similar STM pictures. Imaging the filed states at a bias voltage of -2 V elongated features with sidelobes on either side appear in the STM image perpendicular to the Si-dimer rows. At this bias voltage the int ensity of those features is approximately the same as of the Si-dimers . Imaging the empty stairs of the same features leads to a significant brightening up at trench position above + 1.5 V bias voltage. We conc lude in accordance with previous investigations that using either depo sition method the chains which brighten up at positive voltages consis t of Ga. From the experimental results, it is obvious that Ga forms ad -dimers located in the trenches of the (2 x 1) reconstructed Si(001). We have carried out total energy calculations using the density functi onal theory for different Ga-configurations on the Si(001) (2 x 1) rec onstructed surface. The lowest total energy among all likely configura tions was found for the substitutional Ga-dimer. This position has at least a 1.15 eV/Ga-dimer lower energy than the dimer adsorption at any site. It is shown that the geometry with Ga-dimers parallel to the Si -dimers is energetically favourable by around 1.35 eV/Ga-dimer compare d to the perpendicular arrangement. Taking into account the voltage de pendent STM results we conclude that at room temperature deposition th e silicon is not mobile enough to be substituted by Gallium. The exper imental images show a strong tendency for Ga-dimers to form chains; fu rther calculations show these chains to be around 1 eV/dimer lower in energy than isolated Ga-dimers.