Negative-U behavior of the Si donor in Al0.77Ga0.23N

Electron paramagnetic resonance (EPR) spectrum of a shallow donor is observed at low temperatures in darkness in Si-doped Al0.77Ga0.23N epitaxial layers grown on 4H-SiC substrates. It is shown from the temperature dependence of the donor concentration on the neutral donor state measured by EPR that Si is a DX (or negative-U) center but behaves as a shallow donor due to a small separation of only ∼3 meV between the neutral state Ed and the lower-lying negative state EDX. The neutral state is found to follow the effective mass theory with Ed ∼ 52–59 meV.