Scanning tunneling microscopy of step bunching on vicinal GaAs(100) annealed at high temperatures

Step bunching and terrace widening are observed with scanning tunneling microscopy on GaAs(100), tilted 2°toward [1̄10] and [110], after annealing in ultrahigh vacuum at 600°C. The resulting surface consists of two separate phases, c(8×2) reconstructed terraces and a bunched step region that is either amorphous or (2×6) reconstructed. Average terrace widths increase to 2 or 4 times the nominal average terrace width for a 2°miscut and steps are correspondingly compressed by 40% or 75% depending on the misorientation direction. The observed step bunching is explained by a thermodynamic instability against facetting resulting from a high step energy on c(8×2) reconstructed surfaces.