Mixing inhibition and crystalline defects in heavily Si-doped AlAs/GaAs superlattices

Superlattice mixing in heavily silicon-doped AlAs/GaAs superlattices has been examined by secondary-ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM). Samples were grown by molecular beam epitaxy with Si concentrations of 1018 to 1020 cm-3 introduced during the growth process. Interdiffusion of Al was inhibited at a Si concentration of 1020 cm -3. Defect clusters and prismatic dislocation loops were found to be associated with Si concentrations of 1020 and 1019 cm-3, respectively. Si was observed by SIMS to segregate preferentially into the GaAs layers and TEM observation revealed defect formation in these same layers during the diffusion process, suggesting a strong correlation between Si segregation and defect formation. In the range of Si concentrations employed, the Al diffusion coefficient is found to vary as the third power of the estimated electron concentration, consistent with our previous results at lower concentrations. These results suggest that the diffusion inhibition at high Si concentrations may arise from the trapping of mobile Si species by defects with a consequent reduction of the carrier concentration.