Evolution of the optical properties of III-V nitride alloys:: Direct band-to-band transitions in GaNyP1-y (0≤y≤0.029) -: art. no. 245207

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 Gamma point (E-0 and E-0(')), and along the Lambda direction (E-1 and E-1(')) in GaNyP1-y (0less than or equal toyless than or equal to0.029) 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('), E-2(1), E-2(2), 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.