Improvement of seed layer smoothness for epitaxial growth on porous silicon

Abstract

In the last decades many techniques have been proposed to manufacture thin (<50µm) silicon solar cells. The main issues in manufacturing thin solar cells are the unavailability of a reliable method to produce thin silicon foils with contained material losses (kerf-losses) and the difficulties in handling and processing such fragile foils. A way to solve both issues is to grow an epitaxial foil on top of a weak sintered porous silicon layer. The porous silicon layer is formed by electrochemical etching on a thick silicon substrate and then annealed to close the top surface. This surface is employed as seed layer for the epitaxial growth of a silicon layer which can be partially processed while attached on the substrate that provides mechanical support. Afterward, the foil can be bonded on glass, detached and further processed at module level. The efficiency of the final solar cell will depend on the quality of the epitaxial layer which, in turn, depends on the seed layer smoothness.

Several parameters can be adjusted to change the morphology and, hence, the properties of the porous layer, both in the porous silicon formation and the succeeding thermal treatment. This work focuses on the effect of the parameters that control the porous silicon formation on the structure of the porous silicon layer after annealing and, more specifically, on the roughness of the top surface. The reported analysis shows how the roughness of the seed layer can be reduced to improve the quality of the epitaxial growth.