NEAR-BAND-EDGE OPTICAL-PROPERTIES OF GASEXTE1-X MIXED-CRYSTALS

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 and x=1. It is associated with a small interband matrix element for polarization E 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 GaSxSe1-x alloy system. The second series of crystals corresponds with compositions ranging between x=0 and x=0.35 (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<0.7. © 1979 The American Physical Society.