BONDING OF H-C-AS PAIRS IN ALXGA1-XAS ALLOYS

The local structure of C-As accepters in AlxGa1-xAs has been investiga ted by studying the nondegenerate localized vibrational modes of H-C-A s pairs with A(1) symmetry, rather than those of isolated C-As impurit ies. infrared absorption and Raman scattering measurements have been m ade on AlxGa1-xAs: C-12 epilayers that (a) had been exposed to a radio -frequency hydrogen (deuterium) plasma or (b) contained hydrogen incor porated during growth. Arguments are advanced that indicate that the H (D) atom should occupy a bond-centered site between C-As and Ga atoms rather than between C-As and Al atoms at low temperatures. An ab initi o local-density-functional calculation indicates that the energy is th en lowered by 0.24 eV. This analysis has led to the assignment of five antisymmetric stretch modes and five symmetric (X) modes to H-C-As pa irs at sites where the carbon atom that was originally unpaired had ze ro, one, two, three, or four Al nearest neighbors.