Low-temperature photoluminescence and reflectance measurements were employed to study the optical transitions present in two-dimensional electron systems confined at MgxZn1–xO/ZnO heterojunctions. Transitions involving A- and B-holes and electrons from the two lowest subbands formed within the confinement potential are detected. In the studied density range of 2.0–6.5 × 1011 cm−2, the inter-subband splitting is measured and the first excited electron subband is shown to be empty of electrons.

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