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This paper investigates the quantum confined Stark effect in AlGaN multiple quantum well structures with a high Al content grown on single-crystalline AlN substrates. The quantitative relationship between the quantum well structure parameters, photogenerated carrier density, built-in electric field and ground-level emission is discussed. It is found that the electric field strength increases from 0.5 MV cm−1 to almost 3 MV cm−1 when the Al content in the quantum well barriers is increased from 65% to 100%, which is consistent with the theory of spontaneous and piezoelectric polarization in III-nitrides. In addition, the built-in electric field increases significantly with increasing barrier thickness. Based on these results, the electric field in an Al0.55Ga0.45N single quantum well with AlN cladding is predicted to be around 5 MV cm−1. [ABSTRACT FROM AUTHOR]