  SiAlON is a prominent example of systems suitable as hosts for creating
materials for light-emitting diodes (LEDs). In this work, the electronic
structure of a series of semiordered and disordered SiAlON systems is
investigated by means of ab initio calculations, using the FLAPW and Green
function KKR methods. Finite temperature effects are included by averaging over
thermodynamic configurations within the alloy analogy model. We found that the
dependence of the electronic structure on the (Al,O) concentration $z$ is
similar for semiordered and disordered structures. The electronic band gap
decreases with increasing $z$ by about 1.5 eV when going from $z$=0 to $z$=2.
States at the top of the valence band are mostly associated with N atoms
whereas the states at the bottom of the conduction band are mostly derived from
O~atoms. Increasing the temperature leads to a shift of the bottom of the
conduction band to lower energies. The amount of this shift increases with
increasing $z$.
