Interaction of dopants with a host GaAs lattice: The case of low-temperature grown molecular beam epitaxial GaAs(Si)

Superlattice structures of alternately undoped and [Si]=1×10¹⁹ cm^-3 doped GaAs have been grown by molecular beam epitaxy at a substrate temperature of 250°C. X-ray diffraction profiles give an average value of lattice constant in the structures in between the values measured in single, nominally undoped and [Si]=1×10¹⁹ cm^-3 doped epilayers grown under identical conditions. Transmission electron microscopy dark-field (004) beam images of the as-grown structures contain bands of strain contrast and hybrid diffraction/imaging reveals periodic variations in the position of the higher-order Laue zone lines in the superlattice structures. We believe the results signify a modulation of the lattice constant along the growth direction: the lattice constant being smaller in the doped regions than in the undoped regions due to a reduction of excess As concentration at this high doping level. On annealing, dense bands of large As precipitates appear in the undoped regions with bands of fewer, smaller precipitates in the doped regions. Hall-effect measurements on the as-grown structures yield electron concentrations comparable with those measured in heavily Si-doped, low-temperature grown single epilayers, and the mobilities are indicative of band conduction in the doped regions of the superlattices.