Enhanced stability of passivation quality on diffused silicon surfaces under light-induced degradation conditions

Abstract

Significant surface related degradation (SRD) is observed in samples passivated with either SiN x :H or AlO x :H/ SiN x :H during treatment at 150°C and 1 sun equivalent illumination intensity. Degradation of SiN x :H passivation is caused by a decrease of chemical passivation quality whereas degradation of AlO x :H/SiN x :H is caused by a decrease of fixed charge density. SRD is, however, strongly suppressed on highly doped silicon surfaces resulting from a diffusion step. Device simulations indicate that this cannot only be explained by reduced sensitivity to changes at the silicon surface due to the diffused region, and implications for defect formation are discussed.

Bulk and Surface‐Related Degradation in Lifetime Samples Made of Czochralski Silicon Passivated by Plasma‐Enhanced Chemical Vapor Deposited Layer Stacks

“…It was observed before that degradation of AlO x :H/SiN x :H stacks at 150 °C is mainly caused by loss of fixed charge density Q , and quantitative determination of J 0s in that kind of samples has proven unreliable at elevated temperatures . For the treatment at 150 °C shown in Figure , J 0s analysis fails, too, during the decline toward minimum III, and τ eff at higher Δ n surpasses τ eff at lower Δ n after ≈15 h of treatment (data not shown) which is suspected to result from loss of Q . Owing to the complex sample structure and difficult J 0s extraction, further SRD analysis has not been carried out for AlO x :H/SiO x N y :H/SiN x :H samples.…”

“…This increase of J 0s indicates (as discussed in more detail elsewhere) that the long‐term degradation of τ eff results from deterioration of surface passivation quality and supports the conclusion that the PECVD‐deposited AlO x :H/SiO x N y :H/SiN x :H passivation stack is affected by significant SRD. It should be noted that earlier studies which investigated fired lab‐type stacks comprising AlO x :H made by atomic layer deposition (ALD) and capped with PECVD SiN x :H observed much higher stability of passivation quality during similar treatments …”

“…Additionally, it can now be seen that the sample shows a recovery after reaching a minimum III of τ eff after ≈100 h of treatment. It was observed before that degradation of AlO x :H/SiN x :H stacks at 150 °C is mainly caused by loss of fixed charge density Q , and quantitative determination of J 0s in that kind of samples has proven unreliable at elevated temperatures . For the treatment at 150 °C shown in Figure , J 0s analysis fails, too, during the decline toward minimum III, and τ eff at higher Δ n surpasses τ eff at lower Δ n after ≈15 h of treatment (data not shown) which is suspected to result from loss of Q .…”

“…Unfortunately, this implies that neither determined k app nor E app are related to a single process but rather the result of a complex superposition of changes in the number of defects and density of fixed charges at the sample surface. Simultaneous changes in both parameters have been observed in AlO x :H/SiN x :H passivated samples before, whereas in SiN x :H passivated samples fixed charges remain constant and SRD appears to be caused by changes in defect density only …”

“…While in these bulk‐related degradation (BRD) phenomena the bulk excess charge carrier lifetime τ b of a sample is susceptible to light‐induced changes, recent research results showed that the passivation quality of layers/stacks comprising hydrogen‐rich silicon nitride (SiN x :H) can degrade and recover under illumination and temperature treatment as well as after a fast firing step as used for contact formation in solar cell production . This surface‐related degradation (SRD) appears to be a carrier‐induced effect and is not related to UV photons which may cause degradation of surface passivation, too . Samples with a highly doped region at the surface (forming an emitter or a high‐low junction) have been found to be less affected by SRD .…”