Structural and electronic properties of ZnMgO/ZnO quantum wells

High-quality ZnMgO/ZnO quantum wells (QWs) showing no phase separation were grown on Al₂O₃(0001). Growth temperature was shown to play a major role in the stabilisation in the wurtzite phase of ZnMgO barrier layers as well as in their surface morphology. Such effects were seen in X-ray diffraction, RHEED, AFM and PL studies. Whereas the electronic properties of the narrow QWs are governed by quantum confinement effects with excitonic emission energies above that of ZnO, the electronic properties of wider QWs (>3 nm) are governed by the quantum confined Stark effect and can therefore emit several hundreds of meV below the band gap of ZnO. This is a direct consequence of the very large built-in electric field of about 1 MV/cm developed in ZnMgO/ZnO QWs grown along the c-direction.