Investigation of Hydrogenation in n-type Wafers with Ring- and Disc-shaped Defect Zones

Abstract

This paper investigates hydrogenation processes for improvement of the bulk recombination lifetime in n-type Czochralski (n-Cz) silicon wafers. Since in the near future an increase in the market share of n-type monocrystalline silicon solar cells is expected, research into the specific issues of using n-Cz Si wafers is rising. One of the widely recognized issues is the occurrence of ring- and disc-shaped defect regions, visible in recombination lifetime maps of the wafers or cells, that are responsible for significant efficiency reduction in solar cells. In this paper, it is demonstrated that these defect regions can be effectively passivated by an NH3 plasma treatment. Also after deposition of silicon nitride, used for passivation and antireflection coating on both sides of n-PERT solar cells, the recombination activity in the defect regions is reduced.The paper also reports on the evolution of the defect regions, after the passivation, in subsequent thermal processes. If wafers with passivated defect regions are subsequently exposed to 500-600°C, the patterns reappear in lifetime maps. After exposure to temperatures above 650°C for only seconds their recombination also increases. However, after these losses of passivation, the passivation can to some extent be restored by re-exposure to an NH3 plasma or by annealing at lower temperatures up to 450°C in the presence of a hydrogen source.We correlate these phenomena to hydrogen sources and diffusion. Thus, the restoration of passivation depends on the presence of a hydrogen source in the wafer coating layers. For very strong ring shaped defect patterns, complete restoration of the passivation cannot be achieved in a stable fashion.