ZnO/(ZnMg)O single quantum wells with high Mg content graded barriers

ZnO/Zn 1− x Mg x O single quantum wells (SQWs) were grown by plasma-assisted molecular beam epitaxy on c -plane sapphire substrates. Compositional grading allows the application of optimized growth conditions for the fabrication of Zn 1− x Mg x O barriers with high crystalline quality and a maximum Mg content of x =0.23. High resolution x-ray diffraction reveals partial relaxation of the graded barriers. Due to exciton localization, the SQW emission is found to consist of contributions from donor-bound and free excitons. While for narrow SQWs with well width d W ≤ 2 . 5 nm , the observed increase of the exciton binding energy is caused by quantum confinement, the drop of the photoluminescence emission below the ZnO bulk value found for wide SQWs is attributed to the quantum-confined Stark effect. For a Mg content of x =0.23, a built-in electric field of 630kV/cm is extracted, giving rise to a decrease of the exciton binding energy and rapid thermal quenching of the SQW emission characterized by an activation energy of (24±4) meV for d W =8.3nm.