Gettering of transition metals by porous silicon in epitaxial silicon solar cells



Epitaxial silicon solar cells (“epicells”) are based on an epitaxial active layer (“epilayer”) grown on top of a low-cost, inactive p+ silicon substrate. A key challenge is to mitigate transition metal out-diffusion from the low-purity substrate into the active layer. An embedded porous silicon (PSi) layer can be used to getter metals within the substrate. This was studied theoretically using density functional theory where large binding energies (∼1.9–2.2 eV) for metal segregation to PSi void surface were calculated for Fe and Cu. Incorporating this in a diffusion model yielded large gettering coefficients of ∼104 even at 1000 °C. To verify this experimentally, a test structure consisting of a 2-µm thick epilayer grown on top of an 8.5 × 8.5 cm2 area of re-organized PSi etched into the middle of an 8″ Cz, p+ wafer was used. These wafers were surface-contaminated with metals (Fe, Ni, Cu) to ∼1014–1015 cm−2 and annealed at high temperatures (950–1000 °C) for up to 15 min. This allowed the metals to distribute throughout the wafer and getter to preferential sites. Direct total reflection X-ray fluorescence mapping of Cu on the front side showed that the embedded PSi reduced the amount of Cu reaching the top surface by ∼103 times, compared to the areas without PSi. Moreover, SIMS depth profiling revealed large metal concentrations (1018–1019 cm−3) in the depth associated with PSi, while the metal concentrations were below detection limits in the surrounding area, suggesting a gettering coefficient of ∼103–104. A slow cooling rate and smaller pore radii were also found to be beneficial for gettering.