The quality of a Si crystal determines the cost and efficiency of a solar cell. It was found that Ge doping into Si is beneficial as it improves the quality of Si by controlling the void defects which improves the photovoltaic characteristics of B-doped CZ-Si [1]. However, Ge is obviously larger than Si and thus the addition of Ge should increase the lattice strain as the lattice mismatch between Si and Ge is about 4 %. Therefore, to explore the maximum range of Ge concentration which is beneficial for preparing high quality of Si, B and Ge codoped CZ-Si crystals with high Ge concentrations were grown and their defect and photovoltaic characteristics were studied. The experimental results show that the minority carrier lifetime (MCL) increased when the initial Ge concentration in Si melt (C_l^Ge) increases up to 3×10²⁰ cm^-3 and decreased drastically when the C_l^Ge increased beyond 3×10²⁰ cm^-3. The etching analysis revealed that the Secco etch pit defect (SEPD) density and flow pattern defect (FPD) density decreased up to C_l^Ge of 3×10²⁰ cm^-3 and increased when the C_l^Ge increased beyond 3×10²⁰ cm^-3, which resulted low MCL in heavily Ge codoped CZ-Si. The intentional variation of growth rates (v) of heavily Ge codoped CZ-Si (to avoid the cellular growth) may be influenced on the change in type of GMDs from void defects (reveled as FPDs for the crystals grown with relatively high growth rate, 0.7 mm/min) to A/B type self interstitial defects (revealed as SEPDs in the crystals grown with relatively low growth rates, 0.4 to 0.15 mm/min). Furthermore, in the heavily Ge codoped CZ-Si, the influence of Ge concentration is relatively smaller than v of crystal on the formation of GMDs.

Reference:

1.      M.Arivanandhan et al., J. Appl. Phys. 111, 043707 (2012).

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