Heteroepitaxial GaAs layers were grown on Si (001) substrates by metalorganic chemical vapor deposition. The tetragonal distortion induced by the lattice and the thermal expansion coefficient mismatches gives substantial effects on the acceptor energy level as well as the valence band structure. The biaxial tensile strain in GaAs layers is investigated using low-temperature photoluminescence. The origins of intrinsic exciton lines and carbon-related extrinsic lines observed in the photoluminescence spectra are identified by the two-band model. It is also found that the binding energy of the carbon acceptor is reduced as biaxial tensile strain increases. ©1997 American Institute of Physics.