Highly carbon doped (001) oriented GaAs layers were grown by metalorganic vapor phase epitaxy using the precursor trimethylgallium with either arsine or trimethylarsenic. Postgrowth infrared measurements using polarized light demonstrate that proposed grown-in planar carbon–hydrogen[H–(CAs)2]complexes, giving vibrational absorption at2688 cm−1,have the twoCAsatoms aligned along the [110] direction. A previously unreported absorption peak at576 cm−1exhibits polarization dependent absorption orthogonal to that of the2688 cm−1line. Investigation of a sample containing the[D–(CAs)2]complex indicates that the576 cm−1line would have to be a transverse mode of the unpairedCAsatom. Polarization difference spectra show the presence of other aligned carbon related complexes giving absorption close to the line(582 cm−1)due to isolatedCAsatoms. ©1997 American Institute of Physics.