The gettering effect of dielectric films for silicon solar cells

We present our recent findings on the strong gettering effect of dielectric films for removing impurities from the silicon bulk. Iron was used as a model impurity in silicon to study the gettering effects. The iron concentration in silicon is found to be significantly reduced after annealing silicon wafers coated with either plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SiNx) films, or atomic layer deposited (ALD) aluminium oxide (Al2O3) films. Both PECVD SiNx coated and ALD Al2O3 coated silicon wafers demonstrate largely diffusion-limited iron reduction rates at around 400 degrees C, removing half of the bulk iron concentration within 30 min for a 160-mu m thick silicon wafer with dielectric films on both sides. PECVD SiNx films are shown to achieve diffusion-limited gettering of iron at temperatures below 700 degrees C, meaning that 90% of iron can be gettered by the surface SiNx films within 5 min for a 260-mu m silicon wafer, or within 2 min for a 160-mu m wafer at 700 degrees C. Secondary ion mass spectrometry reveals that the gettering effect of PECVD SiNx films is caused by impurity segregation into the bulk of the SiNx films, and the gettering effect of ALD Al2O3 films comes from impurity accumulation at the Al2O3/Si interfaces.