Real time spectroscopic ellipsometry has been applied to develop deposition phase diagrams that can guide the fabrication of hydrogenated silicon (Si:H) thin films at low temperatures (<300 degrees C) for highest performance electronic devices such as solar cells. The simplest phase diagrams incorporate a single transition from the amorphous growth regime to the mixed-phase (amorphous+microcrystalline) growth regime versus accumulated film thickness [the a --> (a + muc) transition]. These phase diagrams have shown that optimization of amorphous silicon (a-Si:H) intrinsic layers by RF plasma-enhanced chemical vapor deposition (PECVD) at low rates is achieved using the maximum possible flow ratio of H-2 to SiH4 that can be sustained while avoiding the a --> (a + muc) transition. More recent studies have suggested that a similar strategy is appropriate for optimization of p-type Si:H thin films. The simple phase diagrams can be extended to include in addition the thickness at which a roughening transition is detected in the amorphous film growth regime. It is proposed that optimization of a-Si:H in higher rate RF PECVD processes further requires the maximum possible thickness onset for this roughening transition. (C) 2002 Elsevier Science B.V. All rights reserved.