Methods of two- and three-crystal X-ray diffraction and measurements of pho
toluminescence (PL) spectra are used to study structural perfection and var
iations in the PL spectra shape of GaAs epitaxial layers (ELs) grown by the
molecular-beam epitaxy (MBE) technology on substrates with (100), (111)A,
and (111)B orientation. A strong influence of arsenic-to-gallium molecular
flow-rate ratio gamma on the structural property and shape of the PL spectr
a of ELs is revealed. The optimum values of parameter gamma for each of the
se substrate orientations are found.