AlO_x surface passivation of black silicon by spatial ALD: Stability under light soaking and damp heat exposure

Scientific breakthroughs in silicon surface passivation have enabled commercial high-efficiency photovoltaic devices making use of the black silicon nanostructure. In this study, the authors report on factors that influence the passivation stability of black silicon realized with industrially viable spatial atomic layer deposited (SALD) aluminum oxide (AlO_x) under damp heat exposure and light soaking. Damp heat exposure conditions are 85 °C and 85% relative humidity, and light soaking is performed with 0.6 sun illumination at 75 °C. It is demonstrated that reasonably thick (20 nm) passivation films are required for both black and planar surfaces in order to provide stable surface passivation over a period of 1000 h under both testing conditions. Both surface textures degrade at similar rates with 5 and 2 nm thick films. The degradation mechanism under damp heat exposure is found to be different from that in light soaking. During damp heat exposure, the fixed charge density of AlO_x is reduced, which decreases the amount of field-effect passivation. Degradation under light soaking, on the other hand, is likely to be related to interface defects between silicon and the passivating film. Finally, a thin chemically grown SiO_x layer at the interface between the AlO_x film and the silicon surface is shown to significantly increase the passivation stability under both light soaking and damp heat exposure. The results of this study provide valuable insights into surface passivation degradation mechanisms on nanostructured silicon surfaces and pave the way for the industrial production of highly stable black silicon devices.