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High resolution X-ray diffraction and Raman spectroscopy have been used to study GaAs epilayers grown on GaAs substrates by conventional molecular beam epitaxy and by atomic layer epitaxy, at growth temperatures ranging between 600 and 200°C. No scattering was observed by TO phonons, indicating high-quality crystallinity. Epilayers grown at 200°C are tetragonally strained with a relaxed lattice constant greater than that of GaAs. The level of residual strains depends on the type of growth. The LO phonon frequencies were downshifted compared to GaAs, due to volume expansion by As excess, misfit strains, and changes in the effective charge and reduced mass of the unit cell. An estimate for the As excess has been obtained for the epilayers grown at 200°C. © 1993. |
