Evolution of the optical properties of III-V nitride alloys: Direct band-to-band transitions in GaN_yP_1-y (0≤_y≤0.029)

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 Γ point (E₀ and E′₀), and along the Λ direction (E₁ and E′₁) in GaN_yP_1-y (0≤_y≤0.029) using spectroscopic ellipsometry. We detect two additional transitions, labeled here E₂¹ and E₂², which cannot be assigned unambiguously. We observe a strong blueshift of the direct transition energies E₀ and E₁ 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, E₂¹, E₂², and E′₁ 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.