InGaN/GaN quantum wells (QWs) were grown by molecular-beam epitaxy on c-plane sapphire substrates. The growth of InGaN is carried out at 550 degrees C with a large V/III ratio to counteract the low efficiency of NH3 at that temperature and to promote the two-dimensional mode of growth. An In composition of 16%+/- 2% was determined by high-resolution x-ray diffraction experiments. Room-temperature photoluminescence of InGaN/GaN single QWs can be obtained over the whole visible spectrum (from 0.4 to 0.66 mu m) by varying the well thickness from 1 to 5 nm. These heterostructures exhibit very large Stokes shifts between the emission and the absorption edge energies. (C) 2000 American Institute of Physics. [S0003- 6951(00)01435-2].