P-P and As-As isovalent impurity pairs in GaN: Interaction of deep t(2) 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 pseudopot ential 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) impuriti es 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 Delt a E-(n) vs n as a consequence of the interplay between two effects: an "ela stic contribution" representing the interaction be tween the elastic strain fields of the two impurities and an "electronic contribution" describing t he interaction of the defect orbitals of the two impurity atoms. The bindin g 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 transit ions between the defect levels and conduction band are calculated. The resu lts predict existence of a rich series of spectroscopic features distinct f rom single impurities. [S0163-1829(99)14415-1].