Theory of dopant pairs in four-fold coordinated amorphous semiconductors

The position of energy levels belonging to dopant pairs was investigated in substitutionally p- or n-type doped four-fold coordinated amorphous carbon and silicon by means of Hartree-Fock ab initio and the Fragment Self-Consistent Field method. Our models contain 45 to 583 carbon or silicon atoms. Boron, phosphorus and nitrogen impurities have been incorporated into the amorphous networks. It has been found that the position of midgap states are primarily determined by the separation of the impurity atoms, and the role of the random network is only subordinate. A general relationship is proposed for the determination of midgap energy levels as a function of the distance between dopant pairs investigated for amorphous carbon and silicon. (C) 1998 Elsevier Science B.V. All rights reserved.