The Relation Between Precursor Gas Flows, Thickness Dependent Material Phases, and Opto-Electrical Properties of Doped a/nc-SiO$_{X≥0}$:H Films

Doped layers are a determining factor for the performance of photovoltaic devices such as silicon heterojunction and thin film silicon solar cells. The material properties of doped hydrogenated amorphous/nanocrystalline silicon-oxide (a/nc-SiOX≥0:H) films processed by plasma-enhanced chemical vapor deposition generally exhibit a tradeoff between optical and electrical performance. The optoelectrical properties are the result of different material phases in these heterogeneous films, such as hydrogenated amorphous silicon and silicon-oxide tissue, nanocrystalline silicon grains, their corresponding fractions and extent of doping. In this article, all the precursor gas flows are varied to achieve a wide range of doped a/nc-SiOX≥0:H phases. A material phase diagram is introduced to clarify the complex interplay between processing conditions, dominant growth mechanisms, a/nc-SiOX≥0:H phases, and the resulting optoelectrical properties. In addition, it is discussed that material properties are strongly dependent on the thickness of the films, as the mix of different material phases is not uniform along the growth direction.