GaN and InGaN layers are grown by molecular beam epitaxy using ammonia as nitrogen precursor. The lattice mismatch between InN and GaN is very large and a Stranski-Krastanov (SK) growth mode transition can occur above a critical In composition. However, changing the growth conditions, namely increasing the NH3 flux, allows one to promote the 2D growth. This phenomenon can be ascribed to a surfactant effect of hydrogen atoms (or NHx radicals) present at the growth surface. The optical properties of InGaN/GaN quantum dots, made by SK growth mode, and InGaN/GaN quantum wells are compared.