Focused ion beam assisted chemically etched mesas on GaAs(001) and the nature of subsequent molecular beam epitaxial growth

Focused ion beam assisted chemical etching is examined as an ultrahigh vacuum compatible in situ direct-write approach to patterning substrates to create mesas with arbitrary shapes (i.e., sidewall angles) independent of crystallographic orientation. Ga+ ion beam assisted Cl2 etching of GaAs(001) is employed as a vehicle. A phenomenological model is used to predict mesa profiles as a function of ion beam conditions. Mesas created under various conditions are characterized via atomic force microscopy and good agreement is found between the measured and predicted profiles. Examination of the growth profile evolution on such mesas with as-patterned sidewall angles between ∼10° and ∼60° reveal that mesa top size reduction suitable for nanostructure fabrication on stripes oriented along the [11̄0] and [100] directions occurs only for as-patterned mesa sidewall angles greater than ∼19° and ∼45°, respectively, which are the angles subtended by the {114} and {101} facets that emerge during growth and cause mesa top pinch-off for the two mesa stripe orientations, respectively. An as-patterned surface roughness in the sidewall and valley regions of ∼20 nm is found to heal with typically ∼50 nm of GaAs buffer growth via molecular beam epitaxy. Cross-section transmission electron microscope studies evidence the high quality of growth on the mesas and the lack of extended defects in the as-patterned substrate as well as in the overlayers, thus making such in situ prepared mesas suitable for creation of nanostructures via the technique of size-reducing epitaxy.