Nature of the fundamental band gap in GaNxP1−x alloys

The optical properties of ${\mathrm{GaN}}_x{\mathrm{P}}_{\text{1−x}}$ alloys $\text{(0.007⩽x⩽0.031)}$ grown by gas-source molecular-beam epitaxy have been studied. An absorption edge appears in ${\mathrm{GaN}}_x{\mathrm{P}}_{\text{1−x}}$ at energy below the indirect ${\Gamma }_V{\mathrm{–X}}_C$ transition in GaP, and the absorption edge shifts to lower energy with increasing N concentration. Strong photomodulation signals associated with the absorption edges in ${\mathrm{GaN}}_x{\mathrm{P}}_{\text{1−x}}$ indicate that a direct fundamental optical transition is taking place, revealing that the fundamental band gap has changed from indirect to direct. This N-induced transformation from indirect to direct band gap is explained in terms of an interaction between the highly localized nitrogen states and the extended states at the Γ conduction-band minimum.