The near-band-edge optical properties of a series of 15 crystals including the two binary compounds GaSe and GaTe have been investigated in great detail at 300, 77, and 1.6°K. The absorption spectra at 77 and 1.6° K show well-resolved excitonic structures and can be classified in two different series. The first one, GaSe-like crystals, corresponds with compositions ranging between $x=0.7\mathrm{}$ and $x=1\mathrm{}$. It is associated with a small interband matrix element for polarization $\overrightarrow{\mathrm{E}}\perp \overrightarrow{\mathrm{C}}$ and a large experimental shift of the fundamental edge. The corresponding transition has a predominant anionlike character in good agreement with previous experimental and theoretical finding for the direct gap in the $\mathrm{Ga}{\mathrm{S}}_x{\mathrm{Se}}_{1-x}$ alloy system. The second series of crystals corresponds with compositions ranging between $x=0\mathrm{}$ and $x=0.35\mathrm{}$ (GaFe-like crystals). It is characterized by a much larger interband matrix element and a smaller energy shift. It corresponds with the fundamental absorption edge in GaTe and can be associated with a mixed anion-cation character. A simple analytical model is found which accounts satisfactorily for the change of interband matrix element between GaSe and GaTe. No alloy composition exists in the range: $0.35<x<\mathrm{}0.7\mathrm{}$.

• Received 23 December 1977

DOI:https://doi.org/10.1103/PhysRevB.19.1060