Strain in Epitaxial GaAs on CaF 2 /Si(111)

Epitaxial layers of (111) GaAs of approximately 1 µ m thickness were grown on epitaxial CaF 2 buffer layers which were either 140 or 380 nm thick on Si(111) substrates. The best nucleation temperature for the GaAs on CaF 2 /Si(111) we have observed was 620 °C. This resulted in high quality GaAs films which exhibited channeling minimum yields of 4%. The density of threading dislocations in the GaAs layers was observed by TEM to be ~10 8 cm -2 . Double-crystal x-ray diffraction measurements showed that the strain (e ┴ .) was less than 2.2×10 -4 in both sets of GaAs samples. Ion channeling, however, revealed a large tetragonal strain of 3.5×10 -3 (e ┴ = 1.7×10 -3 ) in the thinner (140 nm) CaF 2 buffer layers. By doing ion channeling with high energy (2.5 MeV) protons, it was possible to determine strain more accurately. Using this technique, we were able to set an upper limit for the tetragonal strain of 2.5×10 -4 in both the GaAs (which implies e ┴ -5 and CaF 2 (e ┴ -4 ) layers for the thicker (380 nm) CaF 2 buffer layer structure. These results are in good agreement with the strain predicted from previous strain measurements of CaF 2 epitaxial layers on Si.