RAMAN-SPECTROSCOPY OF LOCALIZED VIBRATIONAL-MODES FROM CARBON AND CARBON-HYDROGEN PAIRS IN HEAVILY CARBON-DOPED GAAS EPITAXIAL LAYERS

Heavily carbon-doped GaAs layers have been studied by Raman spectroscopy of localized vibrational modes. Films grown by three different epitaxial techniques-namely molecular-beam epitaxy, metalorganic vapor-phase epitaxy (MOVPE), and metalorganic molecular-beam epitaxy-have been examined. Samples grown by MOVPE show-besides scattering by the C-12(As) local vibrational mode near 583 cm-1, which is observed for all three growth techniques-two additional lines at 452 and 2640 cm-1 for carbon concentrations > 5 X 10(19) cm-3. These lines are assigned to the stretch mode of C-12(As)-H pairs (2640 cm-1) and to a carbon mode of these pairs (452 cm-1). The analysis of polarization selection rules indicates that the 452-cm-1 mode is longitudinal (A1 symmetry). It shows a pronounced resonance enhancement in scattering strength for incident-photon energies in resonance with the GaAs E1 band-gap energy.