Defects in GaAs on Si studied with the positron-beam technique

Positron-beam experiments have been carried out to characterize defects in metal-organic vapor-phase-epitaxially grown GaAs layers on (100)GaAs and (100)Si substrates. The Te-doped GaAs/GaAs layer (n=2×{}10¹⁵ cm^-3) appears to be free of vacancies which are present in similarly doped bulk n-type GaAs crystals. The heteroepitaxial GaAs/Si layer with the low (10⁷ cm^-2) dislocation density is also free of positron trapping. At a higher dislocation density of 10⁷--10⁸ cm^-2 positrons detect Si clusters. In the epilayer with the dislocation density 10⁸ cm^-2 vacancies and small vacancy clusters at the concentration 10¹⁷ cm^-3 are found. In addition, in this epilayer a 0.3-{μ}m layer next to the interface has a high concentration of Si clusters which trap all positrons stopped there. The findings are in agreement with the earlier observations of Si diffusion in GaAs/Si epilayers and vacancy-type point defects in deformed bulk GaAs.