Minimizing lifetime degradation associated with thermal oxidation of upright randomly textured silicon surfaces

Abstract

Thermally grown silicon dioxide is commonly used in high-efficiency mono-crystalline silicon solar cell designs as a diffusion mask, electroless plating mask, passivation layer or as a rudimentary anti-reflection coating. Some of these device designs also use upright random texturing, to minimize front surface reflection, in combination with thermally grown silicon dioxide layers. The volume expansion associated with oxidation, generates large stresses on the silicon substrate when oxidizing sharp surface features. If this stress exceeds the fracture stress of silicon, defects are generated within the bulk region. This paper reviews oxidation-induced stress, and highlights two techniques for reducing lifetime degradation during the growth of silicon dioxide on textured wafers: (i) reducing the generation of stress, and (ii) decrease the relaxation time of the silicon dioxide layer. This paper experimentally investigates, and evaluates, several practical methods for implementing these techniques on upright random textured p-type float zoned silicon using photoconductance lifetime.