SCANNING-TUNNELING-MICROSCOPY OF STEP BUNCHING ON VICINAL GAAS(100) ANNEALED AT HIGH-TEMPERATURES

Step bunching and terrace widening are observed with scanning tunnelin g microscopy on GaAs(100), tilted 2-degrees toward [110BAR] and [110], after annealing in ultrahigh vacuum at 600-degrees-C. The resulting s urface consists of two separate phases, c(8X2) reconstructed terraces and a bunched step region that is either amorphous or (2X6) reconstruc ted. Average terrace widths increase to 2 or 4 times the nominal avera ge terrace width for a 2-degrees miscut and steps are correspondingly compressed by 40% or 75% depending on the misorientation direction. Th e observed step bunching is explained by a thermodynamic instability a gainst facetting resulting from a high step energy on c(8X2) reconstru cted surfaces.