Atomic step organization in homoepitaxial growth on GaAs(111)B substrates

When homoepitaxial growth is performed on exactly oriented (singular) (1̄1̄1̄) GaAs substrates, while maintaining the √19 x √19 surface reconstruction, the originally flat surface spontaneously evolves vicinal (1̄1̄1̄) facets that are tilted approximately 2.5° toward the <211> azimuthal directions. These facets form pyramid-like structures where the distance between adjacent peaks can be varied from as little as 1 μm to tens of μm. When these surfaces are observed with atomic force microscopy (AFM), we find that they are extremely smooth with the observed tilt resulting from atomic steps which are spaced at approximately 7.5 nm. We have also studied growth on vicinal GaAs(1̄1̄1̄) substrates. Our results are interpreted as indicating that the 2.5° vicinal (1̄1̄1̄) surface has a minimum free energy for the √19 x √19 reconstruction (i.e., that 10 nm spacing of <011> steps is thermodynamically preferred). Exactly oriented (1̄1̄1̄) facets are only observed when their facet width is less than a couple of micrometers implying a minimum nucleation size. This is a surprising result since conventional wisdom argues the surfaces with low Miller indexes are preferred. A possible explanation is an anisotropy in the surface in the two degenerate phases of √19 x √19 reconstruction which are rotated ±23° from the unreconstructed surface.