In this paper, we report on the growth of GaN films and AlGaN/GaN multiple quantum wells (MQWs) on Si(111) substrates by molecular beam epitaxy using ammonia as nitrogen source. A smooth layer-by-layer two-dimensional growth mode allows to obtain reflection high-energy electron diffraction intensity oscillations during GaN, AlN and AlGaN growth. Low temperature photoluminescence (PL) spectra of thick nearly relaxed GaN films are dominated by an intense excitonic emission at 3.471 eV having a full width at half maximum of 5 meV. On the other hand, the PL spectra of thick strained GaN films exhibit a broad band edge emission at 3.455 eV having a full width at half maximum of 15 meV. As revealed by transmission electron microscopy, AlGaN/GaN MQW structures with sharp interfaces are achieved on Si(lll) substrates and each QW exhibits a clearly resolved PL emission peak.