GaN/AlxGa1-xN quantum wells (QWs) are grown by molecular beam epitaxy (MBE) on c-plane sapphire substrates. Both the Al composition and the well thickness are determined in situ from reflection high-energy electron diffraction intensity oscillations. It is demonstrated that MBE growth allows controlling the QW width at the monolayer (ML) scale from sample to sample. 9 K photoluminescence (PL) spectra exhibit well-resolved QW-related emission peaks (linewidths of 20-30 meV), The variation of the GaN/Al0.11Ga0.89N QW energy versus the well thickness (from 3 to 15 MLs) shows the presence of a strong built-in electric field in the quantum structure. Room-temperature PL of QWs is also presented. (C) 1998 American Institute of Physics. [S0003-6951(98)00835-3].