Composition Dependence of Interband Transition Intensities in GaPN, GaAsN, and GaPAs Alloys

Using large (512-atom) pseudopotential supercell calculations, we have investigated the composition dependence of the momentum matrix element Mv,c for transitions between the valence-band maximum and the conduction-band minimum of three semiconductor alloys: GaP1-xNx and GaAs1-xNx, exhibiting large chemical and size differences between their alloyed elements, and GaP1-xAsx, which is a weaklyperturbed alloy. In the composition ranges where these alloys have a direct band gap, we find that (i) in GaP1-xAsx, Mv,c is large (like the virtual-crystal value) and nearly composition independent; (ii) in GaP1-xNx, Mv,c is strongly composition dependent: large for small x and small for large x; and (iii) in GaP1-xNx, Mv,c is only slightly composition dependent and is significantly reducedrelative to the virtual-crystal value. The different behavior of GaP1-xAsx, GaP1-xNx, and GaAs1-xNx is traced to the existence/absence of impurity levels at the dilute alloy limits: (a) there are no gap-level impurity states at the x..fwdarw..1 or x..fwdarw..0 limits of GaP1-xAsx, (b) an isolated As impurity in GaN (Ga..lovin..N:As) has a deep band gap impurity level but no deep impurity stateis found for N in GaAs, and (c) Ga..lovin..N:P exhibits a P-localized deep band-gap impurity state and Ga..lovin..P:N has an N-localized resonant state. The existence of deep levels leads to wave-function localization in real space, thus to a spectral spread in momentum space and to a reduction of Mv,c. These impurity levels are facilitated by atomic relaxations, as evident by the fact thatunrelaxed Ga..lovin..N:As and Ga..lovin..N:P, show no deep levels, have extended wave functions, and have large interband transition elements.