Different growth modes have been identified in thick (≥ 500 nm) AlGaN and InGaN layers grown by plasma-assisted molecular beam epitaxy as a function of parameters such as the growth temperature or the III-V ratio. Al mole fractions between 0% and 60% have been explored in AlGaN alloys, finding that a two-dimensional growth only occurs above a critical temperature under a metal-stable regime. This critical temperature increases with the Al content and depends strongly on the strain induced by the substrate and previous layers. A thermodynamical model is proposed to explain this behaviour. InGaN layers were also grown with In compositions in the 0-10% range. In this case, a metal stable regime may induce In accumulation at the surface preventing the InGaN growth. Thus, slightly N-rich conditions are needed to reach an efficient In incorporation. To broaden the photodetection range of III-nitrides, photoconductors have been fabricated on both ternary alloys aiming to obtain high responsivity, solar-blind UV detectors (AlGaN) and narrow-band visible detectors (InGaN). © 2005 Elsevier B.V. All rights reserved.