Trimer adatom structure of phosphorus on Ge(111)

The surface structures of phosphorus on a $\mathrm{Ge}\mathrm{}\left(111\right)c(2\mathrm{}\times 8)$ surface have been investigated by scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. Phosphorus adatoms are stable on the Ge(111) surface up to ∼350 °C, where desorption and/or diffusion into the bulk sets in. The P adatoms cluster into small islands and form a local $(\sqrt{3}\times \sqrt{3})R30\mathrm{}°$ structure. The stable bonding configuration of phosphorus on Ge(111) between room temperature and ∼260 °C is the so-called $T_4$ trimer geometry, where the P adatoms form triangular trimer units above second-layer Ge atoms, thus saturating all the Ge surface dangling bonds in the local $(\sqrt{3}\times \sqrt{3})R30\mathrm{}°$ array. However, only poor long-range order is obtained. The ${\mathrm{P}}_3$ trimers replace the Ge adatoms of the $c(2\mathrm{}\times 8)$ reconstruction as revealed in atomically resolved STM images. The P trimer structure on Ge(111) is discussed in relation to the substitutional adsorption geometry of P on Si(111), and it is argued that the $\mathrm{Ge}\mathrm{}\left(111\right)(\sqrt{3}\times \sqrt{3})R30\mathrm{}°-{\mathrm{P}}_3$ structure is the result of the larger tensile stress on the Ge surface and the weaker Ge-P bond strength as compared to the Si case.