STRUCTURE AND DYNAMICS OF THE DX CENTER IN GAAS-SI

We have carried out local-density-functional cluster calculations on the Si(Ga-) defect in GaAs. We find that the distortion proposed by Chadi and Chang involving a large Si movement along <111> breaking an Si-As bond has the same energy as a simple breathing distortion. The calculated local vibratory modes of the Chadi-Chang structure do not agree with those recently assigned to DX from an infrared-absorption experiment. On the other hand, the calculated triplet mode due to the other structure above is in reasonable agreement with these observations. The effective charge of the second type of defect is about three times that of the first. It is proposed that both defects coexist with the Chadi-Chang one being almost infrared inactive.