The nature of electron states in AlN and alpha-Al2O3

Both alpha-alumina and aluminium nitride are insulators. They are widely applied as tunnel barriers. On the basis of first-principles calculations, it is shown here that the conduction band of these two compounds is of fundamentally different origin than generally assumed. The bottom of the conduction band of both compounds is primarily derived from oxygen/nitrogen 3s states with an admixture of a small amount of aluminium s character only. The presence of the anion 3s states is of importance for the size of the band gap: without them they would be significantly larger. The consequences of these differences are discussed.