Application of Deposition Phase Diagrams for the Optimization of a-Si:H-Based Materials and Solar Cells

Real time spectroscopic ellipsometry (RTSE) has been applied to develop deposition phase diagrams that can guide the fabrication of hydrogenated silicon (Si:H) thin films at low temperatures (<300°C) for highest performance electronic devices such as solar cells. The simplest phase diagrams incorporate a single transition from the amorphous (a) growth regime to the mixed-phase amorphous + microcrystalline (a+μc) growth regime versus accumulated film thickness [the a→(a+μc) transition]. These phase diagrams have shown that optimization of amorphous silicon (a-Si:H) intrinsic (i) layers by rf plasma-enhanced chemical vapor deposition (PECVD) at low rates is achieved using so-called protocrystalline Si:H. This material is deposited with the maximum possible flow ratio of H₂ to SiH₄ while stopping short of the a→(a+μc) transition that inevitably occurs for film thicknesses greater than the desired value in the optimized device. The simplest 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.