In this work the relationship between the characteristic energy of the Urbach edge $E_0$ and the parameter $B^{\frac{1}{2}}$ of the Tauc's representation of the absorption coefficient of $a$-SiN- and $a$-GeN-based alloys is presented and discussed. No correspondence has been experimentally found between $B^{\frac{1}{2}}$ and the topological disorder induced by small impurity concentrations in the network (less than a few at.%), which provokes a broadening of the Urbach tail. In the alloy regime, nevertheless, $E_0$ and $B^{\frac{1}{2}}$ present a linear correspondence. This fact is discussed in terms of the structural changes induced by atoms of different atomic coordination, i.e., on the base of the dominant bonding character (which changes from purely covalent to partially ionic) and the electronic states at the top of the valence band, as the nitrogen content is increased. The effects of hydrogen, carbon, and silicon in the $a$-Si and $a$-Ge networks are also discussed in terms of the Tauc slope $B^{\frac{1}{2}}$ parameter.

• Received 6 July 1995

DOI:https://doi.org/10.1103/PhysRevB.53.3833