Low-temperature silicon surface passivation for bulk lifetime studies based on corona-charged Al2O3

Abstract

Bulk lifetime studies of crystalline silicon wafers, e.g. with the aim of studying the light-induced degradation and regeneration behavior, require low-temperature surface passivation schemes that do not alter the silicon bulk properties, e.g. through hydrogenation. Aluminum oxide (Al2O3) can provide an excellent and stable surface passivation, however, in order to achieve the best possible surface passivation, an annealing step at ~400°C is typically required, which has been found to alter the bulk properties of some silicon materials. Hence, in this contribution we examine the possibility of passivating the silicon surface using Al2O3 layers that have seen a much lower thermal budget. We demonstrate that we are able to achieve an excellent silicon surface passivation using atomic-layer-deposited Al2O3 with measured effective surface recombination velocities Seff as low as 1.3 cm/s without exceeding a temperature of 250°C. We are able to achieve such excellent low-temperature passivation by applying a post-deposition annealing step at 250°C in combination with the deposition of negative Corona charges on the Al2O3 surface. For samples annealed at only 220°C, we still reach very low Seff values of 2 cm/s after deposition of negative Corona charges. We demonstrate that the Corona-charged low-temperature Al2O3 passivation shows only a slight degradation from an Seff of 1.6 cm/s to an Seff of 5 cm/s after 218 days of storage. Even without any post-deposition anneal and only negative Corona charges deposited, we achieve stable Seff values of 15 cm/s. As an alternative to Corona charging, a short exposure to intense UV light (O = 395 nm) also significantly improves the surface passivation quality of low-temperature-annealed Al2O3-passivated silicon samples. However, the best surface passivation for the latter method is limited to an Seff value of 6.6 cm/s, which is still quite reasonable for bulk lifetime studies.