ATOMIC STEP ORGANIZATION IN HOMOEPITAXIAL GROWTH ON GAAS(111)B SUBSTRATES

When homoepitaxial growth is performed on exactly oriented (singular) (111BAR) GaAs substrates, while maintaining the square-root 19 x square-root 19 surface reconstruction, the originally flat surface spontaneously evolves vicinal (111BAR) facets that are tilted approximately 2.5-degrees toward the [211BAR] azimuthal directions. These facets form pyramidlike structures where the distance between adjacent peaks can be varied from as little as 1 mum to 10's of mum. 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(111BAR) substrates. If the substrate surfaces are tilted 3-degrees toward the [211BAR] direction, then a very smooth surface is obtained with a rather uniform spacing of atomic steps running along the [011BAR] direction. For vicinal substrates that are tilted 2-degrees or less toward the [211BAR] azimuth, the surface evolves a grating structure consisting of approximately singular (111BAR) facets and vicinal (111BAR)-facets that are tilted approximately 2.5-degrees toward the [211BAR] azimuth. As the sizes of the singular (111BAR) facets are made larger (by growing on vicinal substrates with smaller misorientations), occasional inclined three-sided pyramids are observed with the three symmetric, vicinal (111BAR) facets. Vicinal substrates that are tilted along other directions are observed to form zigzag terraces that result from appropriate combinations of oriented and vicinal (111BAR) facets. Our results are interpreted as indicating that the 2.5-degrees vicinal (111BAR) surface has a minimum free energy for the square-root 19 x square-root 19 reconstruction (i.e., that 10 nm spacing of [011] steps is thermodynamically preferred). Exactly oriented (111BAR) facets are only observed when their facet width is less than a couple of micrometers implying a minimum nucleation size. This is a very surprising result since conventional wisdom argues the surfaces with low Miller indexes are preferred. This tendency to form step bunches could be utilized to better control the growth of quantum wires and quantum dots.