Substrate temperature dependence of arsenic precipitate formation in AlGaAs and GaAs

GaAs epilayers which are grown by molecular-beam epitaxy under "normal" group III-V fluxes but at very low substrate temperatures contain as much as 1% excess arsenic. Upon annealing these epilayers at a temperature of 600°C, the excess arsenic forms precipitates. We have undertaken a systematic study of the substrate growth temperature dependence of this incorporation of excess arsenic in both GaAs and AlGaAs epilayers. The substrate growth temperature was varied in increments of 25°C from 225 to 375°C after every 0.25 μm of film growth for a GaAs and an AlGaAs epilayer. Both epilayers were grown using a dimer arsenic source and a group V to total group III beam equivalent pressure ∼20. After growth the films were annealed for 1 h in the As flux at a temperature of 600°C. Cross-sectional samples were than prepared by the ion thinning technique and examined by transmission electron microscopy (TEM). Both epilayers contained arsenic precipitates; this is the first observation of arsenic precipitates in an AlGaAs epilayer. The density of the arsenic precipitates in the two epilayers had a strong dependence on substrate growth temperature. Details of the film growth and of the TEM observations are reported.