AlGaN/GaN quantum well (QW) structures were grown entirely by molecular-beam epitaxy on c-plane sapphire substrates. Reflection high-energy electron diffraction intensity oscillations, which testify a smooth growth front surface at the molecular monolayer scale, were used to precisely measure in situ both the growth rate and the Al content in AlGaN alloys. High-qualily AlGaN/GaN QWs with thicknesses varying from 10 to 80 Angstrom were achieved. Their photoluminescence (PL) linewidth is 10 to 30 meV, and the Stokes shift, determined by temperature-dependent FL, also ranges between 10 to 30 meV. The sample geometry and the Al composition in the barriers were varied in order to assess the built-in electric Grid present in wurtzite nitride-based structures. The magnitude of the deduced electric field points out the role of spontaneous polarization.