Ab initio calculations of the structure and dynamical properties of boron double acceptors in GaAs

Ab initio local density functional cluster calculations are used to investigate the structure and dynamic properties of B As in its various charge states in GaAs. The authors find B - - is an on-site defect with a triplet mode at 604 cm -1 ( 11 B). This is to be compared with an observed mode at 601 cm -1 . B - moves slightly off-site along (111) away from one of its Ga neighbours, whereas the neutral defect, B 0 , suffers a large distortion along this direction. E-vibratory modes of the neutral defect are higher than the modes of B - - but their effective charge is only about half that of B -- . - . The singly charged defect, B - , may tunnel between its four equivalent locations. Explanations are advanced for the loss of the infrared absorption due to B As when the Fermi level drops below about E v +0.2 eV.