Theory of the structure and dynamics of the C impurity and C-H complex in GaAs

Carbon is an important impurity in metalorganic molecular-beam-epitaxy-grown GaAs. ${\mathrm{C}}_{\mathrm{As}}$ is a single acceptor that can be passivated by H. We describe local-density-functional cluster calculations on the structure and dynamics of the impurities and passivated complexes. The lowest-energy structure of the passivated acceptor is a H atom located 1.1 Å from C in a bond-centered orientation. The H-Ga distance is 2.1 Å. The other three C-Ga lengths are 2.18 Å. The H stretch frequency is found to be 2605 ${\mathrm{cm}}^{\mathrm{-}1}$ and is observed at 2635 ${\mathrm{cm}}^{\mathrm{-}1}$. We have also calculated C-H bend modes that should be visible in Raman but not in infrared experiments. The activation energy for the reorientation of the complex is 0.67 eV. Also described are the local modes of the two substitutional C defects in addition to a C-C pair. Modes of the latter are found around 553 and 425 ${\mathrm{cm}}^{\mathrm{-}1}$, respectively, and have effective charges of about 0.5.