POSSIBLE MECHANISM FOR THE ROOM-TEMPERATURE STABILIZATION OF THE GE(111) T-GREATER-THAN-300-DEGREES-C PHASE BY GA

At low coverages, Ga on Ge(111) induces a hexagonal, domain wall modulated (2 x 2) adatom phase, stable at room temperature, that is characterized in low energy electron diffraction (LEED) by split 1/2-order reflections. This pattern closely resembles the one observed for a phase of clean Ge(111) appearing at temperatures above 300 degrees C (T > 300 degrees C phase). We report scanning tunneling microscopy, LEED, as well. as surface x-ray diffraction measurements on the Ga-induced room-temperature (RT) phase and compare it with a model for the T > 300 OC phase of clean Ge(111). RT deposition of Ga yields a metastable c(2 x 8) structure which upon annealing transforms to the hexagonal (2 x 2) one. The transition occurs at considerably lower temperatures compared to clean Ge(111) and is irreversible due to pinning of adatom domains at Ga-induced defects, preventing the reordering of the adatoms and the correct stacking of the c(2 x 8) structure when cooling to RT. For the lowest Ga coverages investigated, a stabilized phase is obtained that resembles a striped (2 x 2) rather than a hexagonal (2 x 2) structure. We discuss the possible existence of a striped (2 x 2) phase as an intermediate state in the transition from the c(2 x 8) of dean Ge(111) to the T > 300 degrees C phase. Driven by entropy - and in the presence of Ga by defects - this intermediate phase transforms to a quasihexagonal (2 x 2) structure above 300 degrees C.