Gold Induced SILICON(111) Surface Reconstructions Studied by Ultrahigh Vacuum Transmission Electron Microscopy

Due to its growing scientific and technical importance in semiconductor metallization, the Au on Si(111) system has recently drawn the attention of many researchers. Therefore, the atomic structures of two gold induced Si(111) surface reconstructions were determined using ultrahigh vacuum transmission electron microscopy. The Si(111)-(5 x 2) Au atomic structure was determined using a combination of off-zone HREM imaging, "heavy-atom holography", and chi^2 electron diffraction refinement. It contains two rows of gold atoms between expanded surface arrangements of silicon atoms. Si(111) -(5 x 2) Au passivates the surface to oxygen attack and room temperature gold deposition onto it indicates surface diffusion is important in its disordering. Transmission electron diffraction data of the Si(111)-(surd 3 x surd3)R30 ^circAu surface support the missing top layer twisted trimer model of this surface in which gold and silicon atoms in the top two layers form rotated like-atom trimers. This gold-silicon structure is quite similar to part of the Si(111)-(5 x 2) Au structure. The degree of trimer rotation and the interatomic spacings of the Si(111)-(surd3 x surd3)R30 ^circAu structure vary significantly with the sharpness of the structure's diffraction spots. These variations and large fitted Debye Waller term values indicate substantial static disorder is present. Evidence of subsurface displacements and charge transfer was also found. The presence of gold trimers is confirmed by the local symmetry seen in high resolution micrographs, which also show surface domain morphology differences between diffuse and sharp diffraction spot regions. Based on these images, models for the Si(111)-(surd3 x surd3)R30 ^circAu domain walls are proposed. Room temperature gold deposition onto the Si(111) -(surd3timessurd3)R30 ^circAu surface shows that higher order surface diffracted beams decay more quickly with coverage than lower order beams. Direct phasing analysis of this result indicates deposited gold occupies unstable T _4 like honeycomb sites within the structure. The site could play a key role in the disordering of the Si(111) -(surd3times surd3)R30 ^circAu surface. Finally a partial surface phase diagram and driving forces are proposed for the Au on Si(111) system based on these and other literature results.