| The microstructure of lateral composition in GaAs/InAs and AlAs/InAs short period superlattices was compared as a function of the growth temperature. For AlAs/InAs structures, deposited at temperatures of T = 500°C the growth occurs via layer-by-layer growth mode. No lateral composition modulation was observed. At temperatures T > 500°C some roughening occurs and strong regular and periodic lateral composition modulation is observed. The microstructure of lateral composition modulation in GaAs/InAs is significantly different. All the samples exhibit some degree of roughness and lateral composition modulation. The lateral composition modulation appears in both [110] and [11¯0] directions for almost all the Ga and Al containing structures. The lateral periodicity A is of the order of 30 nm and is not considerably temperature dependent. The differences in the microstructures between the two material systems can be explained using a kinetic model that takes In surface segregation into consideration. These data demonstrate that at some intermediate temperature, In intermixes into the individual short period superlattice layers, resulting in lower mismatch between the individual layers and more planar growth. At lower and higher temperatures, the mismatch is greater resulting in more roughening, thus more composition modulation. Too much roughening destroys the periodicity of lateral composition modulation.; Lateral composition modulation in a mixed anion material system, GaAs/GaSb was demonstrated for the first time. The microstructure of GaASn/GaSb m SPSs is investigated as a function of As species, growth temperature and short period superlattice layers thickness. Structures grown with As4 exhibit significant roughening during growth. These films are highly defective but high annular dark field images reveal the presence of regular composition modulation perpendicular to the growth direction. Surprisingly, the Ga composition varies laterally as well, suggesting that Sb interstitials or antisite defects may be present. Replacing As tetramers with As dimers improves the crystalline quality. The As sticking coefficient is calculated and found to be strongly dependent on temperature and As species, with some indication that the Sb coverage plays a role as well. |
