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 M-v,M-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 weakly perturbed alloy. In the composition ranges where these alloys have a direct band gap, we find that (i) in GaP1-xAsx, M-v,M-c is large (like the virtual-crystal value) and nearly composition independent; (ii) in GaAs1-xNx, M-v,M-c is strongly composition dependent: large for small x and small for large x; and (iii) in GaP1-xNx, M-v,M-c is only slightly composition dependent and is significantly reduced relative 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-->1 or x-->0 limits of GaP1-xAsx, (b) an isolated As impurity in GaN (Ga (N) under bar:As) has a deep band gap impurity level but no deep impurity state is found for N in GaAs, and (c) Ga (N) under bar:P exhibits a P-localized deep band-gap impurity state and Ga (P) under bar: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 M-v,M-c. These impurity levels are facilitated by atomic relaxations, as evident by the fact that unrelaxed Ga (N) under bar:As and Ga (N) under bar:P, show no deep levels, have extended wave functions, and have large interband transition elements. [S0163-1829(97)05339-3].