Step Structures and Terrace Width Ordering of Molecular Beam Epitaxially Grown GaAs Surfaces Observed by Scanning Tunneling Microscopy

We have observed the surface structures of molecular beam epitaxially (MBE) grown 30-monolayer (ML) GaAs on vicinal (001) surfaces using a multichamber ultrahigh-vacuum scanning tunneling microscope (STM). Only A-type step edges (parallel to the [11̄0] direction) were found in the STM images of vicinal surfaces cut toward [111]A (A-surface). Not only B-type step edges (parallel to the [110] direction), but also A-type step edges were observed on vicinal surfaces cut toward [111]B (B-surface); that is, wide terraces bordered by B-type step edges and narrow terraces bordered by A-type step edges co-existed on the B-surfaces. The B-surface structure is, thus, less uniform than that of the A-surface. We performed further MBE growth on the nonuniform B-surface under the step-flow condition. The surface structure evolved to be more uniform with growth.