NH3 is used as a nitrogen precursor for growing III-V nitride materials by molecular beam epitaxy on c-plane sapphire substrates. The sapphire nitridation step is followed in situ by reflection high-energy electron diffraction. Subsequently, it is demonstrated that the buffer layer growth temperature has a drastic effect on the structural and optical properties of GaN epilayers. The influence of the V/III ratio on the GaN growth is also studied. It is found that N-rich conditions lead to the best material properties. P-type doping is achieved allowing the realization of both GaN and InGaN/GaN based light emitting diodes. The influence of the growth conditions on the properties of InGaN alloys is discussed. It is shown that the window for optimum growth parameters is very sharp. Finally, AlGaN/GaN quantum wells with a thickness control at the monolayer scale are realized. The smoothness of the interfaces are demonstrated by the linewidth of the QW photoluminescence peak which can be as low as 15 meV. A strong electric field in the quantum structure is evidenced. (C) 1999 Elsevier Science S.A. All rights reserved.