Metal Removal and Defect Passivation Performed on Si Wafers for Solar Cell Use by HCN Treatments

Abstract

Cleaning of Si wafers for solar cell use with HCN solutions can remove metal contaminants almost completely. Fe and Ni atoms with ∼2 × 1011 and ∼3 × 1010 atoms/cm2 concentrations reside on Si even after formation of pyramidal textured Si surfaces produced by anisotropic alkaline etching. The Fe and Ni concentrations decrease to less than ∼6 × 109 atoms/cm2 (i.e., detection limit of a total-reflection X-ray fluorescence (TXRF) spectrometer for Fe) and ∼1 × 1010 atoms/cm2, respectively, after cleaning with pH 11.5 HCN solutions containing 1.5 M isopropanol (IPA). Minority carrier lifetime for the pyramidal textured p-type single crystalline Si specimens increases from 58 to 220 μs by the HCN treatment with IPA. The micro-roughness on Si(111) surfaces treated with HCN solutions containing IPA is much less (i.e., 0.27 nm) than that without IPA (i.e., 0.52 nm). Adsorption of IPA on Si retards the etching rate from 32.7 to 8.6 nm/min, leading to enhancement of anisotropic etching, thus resulting in the formation of more complete pyramidal structure. Cleaning with pH 11.5 HCN solutions without IPA increases reflectivity due to isotropic etching, while that with IPA slightly decreases it due to the formation of more complete pyramidal textured surfaces. The increase in the minority carrier lifetime caused by HCN cleaning with IPA is attributable to i) removal of metal contaminants, ii) passivation of Si dangling bond defect states by CN− ions, and iii) a decrease in the defect density due to a decrease in the micro-roughness.