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

Abstract

Scientific breakthroughs in silicon surface passivation have enabled commercial high-efficiency photovoltaic devices making use of the black silicon nanostructure. In this study, we report on factors that influence the passivation stability of black silicon realized with industrially viable Spatial Atomic Layer Deposited (SALD) aluminum oxide (AlOx) 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 2 passivation over a period of 1000 h under both testing conditions. Both surface textures degrade at similar rates with 5 nm 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 AlOx 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 SiOx layer at the interface between the AlOx 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 on surface passivation degradation mechanisms on nanostructured silicon surfaces, and pave the way for the industrial production of highly stable black silicon devices.

Coupled Investigation of Contact Potential and Microstructure Evolution of Ultra-Thin AlOx for Crystalline Si Passivation

“…By taking advantage of the negative fixed charge density of AlO x itself, a passivation with surface recombination velocity (SRV) close to 10 cm/s has been achieved for p-type silicon [10]. Two main approaches have been developed to improve the passivation quality provided by AlO x including thermal annealing [11][12][13][14] and light-soaking [15][16][17].…”

Nanofabrication via Maskless Localized Atomic Layer Deposition of Patterned Nanoscale Metal Oxide Films

“…These results confirm the possibility to homogenously coat nonplanar surfaces using the LOCALD approach, in agreement with previous SALD works on comparable structured substrates. 31,37,66,67 In particular, Munõz-Rojas' group reported conformal deposition of ZnO and Al doped-ZnO, respectively, on textured Si 66 and ZnO nanowires deposited on a Si wafer 68,69 using an open-air atmospheric pressure SALD reactor. Nevertheless, LOCALD on higher aspect ratio substrates such as black silicon reveals less homogeneity in depth with poor step coverage.…”

Field-Effect Passivation of Undiffused Black Silicon Surfaces

“…Surface passivation by the industry-standard plasma-enhanced chemical vapor deposited (PECVD) hydrogenated amorphous silicon (a-Si:H) and silicon nitride (SiN x ) could be problematic for b-Si because of the nonconformal deposition resulting from the PECVD process for high-aspect-ratio surface structures [4]. Atomic layer deposition (ALD) is the most promising deposition technique for the surface passivation of the extreme b-Si surfaces since it can achieve a conformal deposition on such structures [5]- [8], and especially ALD Al 2 O 3 has been widely studied [2], [5], [6], [9]- [12]. Experimental studies have noted that the passivation provided by ALD Al 2 O 3 layers on b-Si surfaces is often better than what would be expected when taking into account the increased surface area.…”