Wurtzite AlGaN/GaN quantum well (QW) structures were grown by molecular beam epitaxy on c-plane sapphire substrates and the QW transition energies were measured by low temperature photoluminescence. Both the well widths and the Al mole fraction in the AlxGa1-xN(0<x<0.3) 100-Angstrom-thick barriers have been varied in order to assess the built-in electric field present in the quantum heterostructures. It is found that the electric field increases linearly with the Al composition. The magnitude of this electric field is as high as 1 MV/cm for an Al mole fraction of 0.27. The main consequence is that whatever the investigated Al composition range, the well thickness must be lower than 30 Angstrom in order to get a transition energy greater than the band gap of GaN. (C) 1999 American Institute of Physics. [S0003-6951(99)03316-1].