Low coverage gallium deposition onto Si(OO1) studied by voltage-dependent STM and total energy calculations

Gallium has been deposited onto (2 X 1) reconstructed Si(OO1) by decomposition 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 filled 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 intensity of those features is approximately the same as of the Si-dimers. Imaging the empty states of the same features leads to a significant brightening up at the trench position above + 1.5 V bias voltage. We conclude in accordance with previous investigations that using either deposition method the chains which brighten up at positive voltages consist 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(OO1). We have carried out total energy calculations using the density functional theory for different Ga-configurations on the Si(OO1) (2 X 1) reconstructed surface. The lowest total energy among all likely configurations 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 compared to the perpendicular arrangement. Taking into account the voltage dependent STM results we conclude that at room temperature deposition the silicon is not mobile enough to be substituted by Gallium. The experimental images show a strong tendency for Ga-dimers to form chains; further calculations show these chains to be around 1 eV/dimer lower in energy than isolated Ga-dimers.