SUBSTITUTIONAL GEOMETRY AND STRAIN EFFECTS IN OVERLAYERS OF PHOSPHORUS ON SI(111)

The structure and bonding topology of phosphorus adatoms on Si(lll) su rfaces have been investigated by scanning tunneling microscopy (STM) a nd scanning tunneling spectroscopy, in conjunction with low-energy ele ctron diffraction and Auger electron spectroscopy. At room temperature P adatoms substitute for the Si adatoms of the Si(111)7X7 surface at low coverages as revealed by the chemical contrast between P and Si ad atoms in the filled-state STM images. A statistical evaluation of the STM images within the framework of a simple reaction model suggests th at the corner adatoms of the faulted half of the (7X7) unit cell act a s primary reaction centers for the P-2 molecules from the gas phase, i n agreement with theoretical expectation. At elevated temperature (500 -650 degrees C) a (6 root 3x6 root 3)R30 degrees structure is the prev ailing P-induced surface reconstruction. Atomically resolved STM image s show that this structure is a domain-wall structure containing hexag onal domains that tesselate the entire surface. The 6% contracted (1X1 ) phosphorus domains are separated by straight, 4% expanded light doma in walls of irregular lengths. The structure displays a complex in-pha se-antiphase-stacking fault relationship between adjacent domains, whi ch has been modeled successfully with a P adlayer in a substitutional bonding geometry on an unreconstructed Si(lll) surface. The large tens ile surface stress introduced by the P-SI bonding is responsible for t he domain-wall formation and precludes the formation of a global (1X1) -P structure on Si(lll) surfaces.