On the Role of AlOx Thickness in AlOx/SiNy: H Layer Stacks Regarding Light- and Elevated Temperature-Induced Degradation and Hydrogen Diffusion in c-Si

Light- and elevated temperature-induced degradation (LeTID) is assumed to be triggered by the hydrogen content in the crystalline silicon bulk. This article investigates differently thick atomic layer-deposited aluminum oxide (AlO_x) layers acting as diffusion barrier for hydrogen originating from a hydrogen-rich silicon nitride (SiN_y:H) layer. We demonstrate that the extent of LeTID can be significantly reduced by adjusting the AlO_x layer thickness up to 25 nm. To directly measure the diffusing species, a deuterium-rich SiN_y:D layer is deposited and the deuterium content is measured in an amorphous Si layer at the back side of the wafer via secondary ion mass spectrometry. Thus, a diffusion length of deuterium in the AlO_x layer of (3.8 ±1.6) nm is determined at a firing temperature of (743 ±2)^° C. These results are not only a contribution to determine the LeTID formation dynamics, but also can be used to control LeTID in silicon wafers and solar cells.

I. Introduction

Light- and elevated temperature-induced degradation (LeTID [1]) is known to negatively impact the high-efficiency passivated emitter and rear cell concept, as first observed in multicrystalline silicon (mc-Si) [2]. It has been shown that LeTID can cause an efficiency loss of up to 10%_rel on cell level already during the first several hundred hours of illumination (depending on temperature and injection conditions) [1] but also impacts effective minority charge carrier lifetime (τ_eff) samples [3], [4] and may be followed by a phase of regeneration. The observed effect cannot be described by the boron-oxygen (BO) related degradation or FeB pair dissociation [2], [5]. It has been proven that LeTID occurs in both B- and Ga-doped mc-Si [2], [6], and further that Czochralski-grown (Cz) and float zone (FZ) Si materials are also affected by this kind of defect [7]–[10]. As in Cz-Si the BO defect formation also leads to a severe reduction of τ_eff, both degradation phenomena are usually measured in parallel, but recent studies already showed a way to identify the dominant defect [11].