Solution Growth of Microcrystalline Silicon on Amorphous Substrates

This work deals with low–temperature solution growth of micro–crystalline silicon on glass. The task is motivated by the application in low–cost solar cells. As glass is an amorphous material, conventional epitaxy is not applicable. Therefore, growth is conducted in a two–step process. The first step aims at the spatial arrangement of silicon seed crystals on conductive coated glass substrates, which is realized by means of vapor–liquid–solid processing using indium as the solvent. Seed crystals are afterwards enlarged by applying a specially developed steady–state solution growth apparatus. This laboratory prototype mainly consists of a vertical stack of a silicon feeding source and the solvent (indium). The growth substrate can be dipped into the solution from the top. The system can be heated to a temperature below the softening point of the utilized glass substrate. A temperature gradient between feeding source and growth substrate promotes both, supersaturation and material transport by solvent convection. This setup offers advanages over conventional liquid phase epitaxy at low temperatures in terms of achievable layer thickness and required growth times. The need for convective solute transport to gain the desired thickness of at least 50μm is emphasized by equilibrium calculations in the binary system indium–silicon. Material transport and supersaturation conditions inside the utilized solution growth crucible are analyzed. It results that the solute can be transported from the lower feeding source to the growth substrate by applying an appropriate heating regime. These findings are interpreted by means of a hydrodynamic analysis of fluid flow and supporting FEM simulation. To ensure thermodynamic stability of all materials involved during steady–state solution growth, the ternary phase equilibrium between molybdenum, indium and silicon at 600°C was considered. Based on the obtained results, the use of molybdenum disilicide as conductive coating material is proposed. MoSi2 thin films on glass, produced by annealing near–stoichiometric Mo–Si multilayers, are shown to resist solution contact. Subsequent investigation of feasibility of the vapor–liquid–solid mechanism revealed the success of indium microdroplet formation to be determined by both, the multilayer deposition parameters and the substrate temperature during indium deposition. Steady–state solution growth at 610°C was utilized to enlarge silicon seed crystals to diameters of up to 200μm. The grown material has been subject of characterization regarding the crystallinity, orientation and purity. Additionally, morphological anomalies are considered. The outgrown material was found to be bound by {111} facets. Many of these microcrystallites contain at least two twin domains. Twin–assisted growth at formed re-entrant edges promotes