Excitonic Instabilities and Bond Theory of III-VI Sandwich Semiconductors

Bond lengths and fundamental optical spectra of the layer semiconductors GaS, GaSe, GaTe, and InSe are discussed. It is shown that the role played by ionicity in these crystals is quite different from that in $A^N{B}^{8-N}$ tetrahedrally coordinated semiconductors. The importance of the covalent energy gap in stabilizing the structure against alternative metallic or ionic structures is discussed. The conclusion is that only by an improbable accident could the formation of virtual excitons make a significant contribution toward determining the relative stabilities of any covalent material.