Gettering of transition metal impurities during phosphorus emitter diffusion in multicrystalline silicon solar cell processing

Abstract

We have investigated the gettering of transition metals in multicrystalline silicon wafers during a phosphorus emitter diffusion for solar cell processing. The results show that mainly regions of high initial recombination lifetime exhibit a significant lifetime enhancement upon phosphorus diffusion gettering. Nevertheless, transition metal profiles extracted by secondary ion mass spectrometry in a region of low initial lifetime reveal significant gradients in Cr, Fe, and Cu concentrations towards the surface after the emitter diffusion, without exhibiting a significant enhancement in the lifetime. In a region of higher initial lifetime, however, diminutive concentration gradients of the transition metal impurities are revealed, indicating a significantly lower initial concentration in these regions. From spatial maps of the dislocation density in the wafers, we find that lifetime enhancements mainly occur in regions of low dislocation density. Thus, it is believed that a generally higher concentration of transition metals combined with an impurity decoration of dislocations in regions of high dislocation density limit the initial lifetime and the lifetime after the phosphorus diffusion, in spite of the notable gettering of transition metal impurities towards the surface in these regions. Furthermore, after a hydrogen release from overlying silicon nitride layers, we observe that only regions of low dislocation density experience a significant lifetime enhancement. This is attributed to impurity decoration of the dislocations in the regions of both high dislocation density and high transition metal impurity concentration, reducing the ability of hydrogen to passivate dislocations in these regions.

Understanding phosphorus diffusion into silicon in a MOVPE environment for III–V on silicon solar cells

“…1, the saturation condition was reached for all samples #1 to #3 (i.e. for all temperatures in the range studied) measuring a surface P concentration in the range of 3-5 x 10 20 cm -3 , in agreement with values reported in the literature for the solid solubility of P in Si in this range of temperature [21,22].…”

Building intuition of iron evolution during solar cell processing through analysis of different process models

“…A strong correlation is seen between precipitation site density and low [Fe,] because Fe,-internally getters to and precipitates at the structural defects as the silicon cools during crystallization. In all Models, regions of higher structural defect density and thus nucleation site density contain more precipitated iron after crystallization, as observed experimentally [15,68,69]. During subsequent processing steps, precipitated iron can reduce device performance because precipitates can limit the lifetime [20] and dissolving precipitates release Fe, into the bulk [69][70][71].…”

Synchrotron-based analysis of chromium distributions in multicrystalline silicon for solar cells

“…14 Sch€ on et al found that minority carrier lifetime increased and [Cr i ] decreased after phosphorous diffusion gettering. 13 Other quantitative studies of the effect of phosphorous diffusion gettering have measured high chromium concentrations at near-surface regions, suggesting external gettering, [15][16][17] as well as a reduction of the total bulk chromium concentration. 18 For this study, two adjacent (sister) wafers were selected from a 12 kg laboratory-scale intentionally chromiumcontaminated mc-Si ingot.…”

“…13,[15][16][17][18] Lifetimes (Cr i , Dn ¼ 10 15 cm À3 ) and interstitial concentrations as measured by quasi-steady-state photoconductance (QSSPC) before and after gettering are shown in Fig. 3.…”

“…The remaining chromium (%10 13 cm À3 assuming no change in precipitate size or density) is likely externally gettered, diffusing to the emitter and PSG layer as has been observed by SIMS. [15][16][17] With one-half of the total Cr concentration in precipitated form, a diffusion temperature of %990 C (Cr solubility equal to 4 Â 10 13 cm À3 ) would be required to fully dissolve precipitates.…”