P-P and As-As isovalent impurity pairs in GaN:: Interaction of deep t2 levels

The electronic and atomic structure of substitutional nth neighbor (1 less than or equal to n less than or equal to 6) P-P and As-As impurity pairs in zinc blende GaN is investigated using self-consistent plane-wave pseudopotential and empirical pseudopotential methods. A single impurity introduces a deep t(2) gap level; we show that the interaction between the t(2) defect orbitals of the impurity pairs leads to pin interesting pattern of single-particle level splitting, being largest for the first (n = 1) and fourth (n = 4) neighbor pairs, both exhibiting a C-2v symmetry. The total energy of the nth order pair Delta E-(n) relative to isolated (n-->infinity) impurities indicates pairing tendency for n = 1 and n = 2 (Delta E-(1,E-2)<0) while n = 4 pairs are unstable (Delta E-(4)>0). We explain this behavior of Delta E-(n) vs n as a consequence of the interplay between two effects: an "elastic contribution" representing the interaction be tween the elastic strain fields of the two impurities and an "electronic contribution" describing the interaction of the defect orbitals of the two impurity atoms. The binding energies of the impurity-pair bound excitons are calculated for the n = 1 As-As and P-P pairs and are found to be significantly larger than for the corresponding isolated impurities. The probabilities for electronic transitions between the defect levels and conduction band are calculated. The results predict existence of a rich series of spectroscopic features distinct from single impurities. [S0163-1829(99)14415-1].