Strain in epitaxial GaAs on CaF2/Si(111)

Abstract

Epitaxial layers of (111) GaAs of approximately 1 μm thickness were grown on epitaxial CaF2 buffer layers that were either 140 or 380 nm thick on Si(111) substrates. The best nucleation temperature for the GaAs on CaF2/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 transmission electron microscopy (TEM) to be ∼108 cm−2. Double-crystal x-ray diffraction measurements showed that the strain (ε⊥) was less than 2.2×10−4 in both sets of GaAs samples. However, ion channeling revealed a large tetragonal strain of 3.5×10−3 (ε⊥=1.7×10−3) in the thinner (140 nm) CaF2 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 ε⊥<8×10−5) and CaF2 (ε⊥<1.5×10−4) layers for the thicker (380 nm) CaF2 buffer layer structure. These results are in good agreement with the strain predicted from previous strain measurements of CaF2 epitaxial layers on Si.