Phase transitions on the Ge(111) and Si(111) surfaces from core-level studies

Detailed studies of clean Ge(111) and Si(111) surfaces in a large interval of temperatures around the c(2×{}8){}(1×{}1) and (7×{}7){}(1×{}1) transition points have been made by means of core-level photoemission spectroscopy. Through the decomposition of the Ge 3d and Si 2p core levels we bring the evidence that with increasing temperature a part of the surface reconstruction of both semiconductors progressively changes to a nonadatom structure. This new atom configuration seems to be the (2×{}1) reconstruction and it appears far below the transition-point temperature.