Annealing prior to contact firing: A potential new approach to suppress LeTID

Abstract

In this work, we introduce a new approach to suppress light and elevated temperature-induced degradation (LeTID) by applying a pre-fire annealing step using rapid thermal processing (RTP) and discuss the impact of this process on the evolution of bulk and surface lifetime components. We demonstrate that pre-fire annealing at low temperatures and/or shorter holding times allows a significant amount of hydrogen to migrate into the bulk to passivate bulk defects as including grain boundaries and dislocation clusters, without causing surface deterioration. As such, the addition of the pre-fire annealing step results in larger improvements in bulk and surface lifetime than that of the control samples. These conditions also significantly suppress LeTID. Increasing pre-fire annealing temperature and duration is shown to completely mitigate LeTID. However, this process may cause surface deterioration, possibly due to the excessive effusion of hydrogen out of the dielectric layer. Injection-dependent lifetime analysis shows that at the most degraded state, the bulk lifetime of the pre-fire annealed samples (650 °C–1 min and 3 min) remains relatively higher (∼110 μs–∼120 μs) than that of the control sample (∼40 μs). Applying pre-fire annealing process at 700 °C on Cz-Si samples and testing the boron-oxygen (B-O) generation behavior suggest that these processes cause a reduction in the hydrogen concentration in the bulk, resulting in slower B-O regeneration rate and reduction of regeneration extent. This result also implies that the suppression of LeTID in mc-Si by applying a pre-fire thermal treatment is likely due to a reduction of hydrogen in the bulk, and this highlights that the proposed method of pre-fire annealing may be unsuitable for material such as p-type Cz silicon subjected to B-O related degradation.