Evolution of the optical properties of III-V nitride alloys: Direct band-to-band transitions in GaN y P 1 − y ( 0 y 0 . 0 2 9 )

Incorporation of nitrogen into III-V semiconductor compound materials dramatically alters their electronic properties. We report on the experimental observation of the evolution of the direct transition energies at the $\ensuremath{\Gamma}$ point ${(E}_{0}$ and ${E}_{0}^{\ensuremath{'}}),$ and along the $\ensuremath{\Lambda}$ direction ${(E}_{1}$ and ${E}_{1}^{\ensuremath{'}})$ in ${\mathrm{GaN}}_{y}{\mathrm{P}}_{1\ensuremath{-}y}$ $(0l~yl~0.029)$ using spectroscopic ellipsometry. We detect two additional transitions, labeled here ${E}_{2}^{1}$ and ${E}_{2}^{2},$ which cannot be assigned unambiguously. We observe a strong blueshift of the direct transition energies ${E}_{0}$ and ${E}_{1}$ with increasing nitrogen incorporation, in contrast to the well-known downshift of the conduction-band edge, which we also obtain for our samples using transmission and photoluminescence measurements. The critical-point transition energies ${E}_{0}^{\ensuremath{'}},$ ${E}_{2}^{1},$ ${E}_{2}^{2},$ and ${E}_{1}^{\ensuremath{'}}$ do not significantly shift with increasing nitrogen concentration. Our observations can be well understood within the model of III-nitride alloy formation proposed recently by Kent and Zunger.