Thermal activation and deactivation of grown‐in defects limiting the lifetime of float‐zone silicon

Abstract

By studying the minority carrier lifetime in recently manufactured commercially available n‐ and p‐type float‐zone (FZ) silicon from five leading suppliers, we observe a very large reduction in the bulk lifetime when FZ silicon is heat‐treated in the range 450–700 °C. Photoluminescence imaging of these samples at the wafer scale revealed concentric circular patterns, with higher recombination occurring in the centre, and far less around the periphery. Deep level transient spectroscopy measurements indicate the presence of recombination active defects, including a dominant center with an energy level at ∼Ev + 0.5 eV. Upon annealing FZ silicon at temperatures >1000 °C in oxygen, the lifetime is completely recovered, whereby the defects vanish and do not reappear upon subsequent annealing at 500 °C. We conclude that the heat‐treatments at >1000 °C result in total annihilation of the recombination active defects. Without such high temperature treatments, the minority carrier lifetime in FZ silicon is unstable and will affect the development of high efficiency (>24%) solar cells and surface passivation studies.

Light-induced activation and deactivation of bulk defects in boron-doped float-zone silicon

“…The lower lifetime level and the injection dependent lifetime curves of the Group IV samples (not shown) indicate an additional lifetime limitation due to other recombination active defects. This is in accordance with the observations of Grant et al 1 and indicates that the defects observed in this study are an additional defect species.…”

“…The phosphorus doped n-type samples featured a nominal thickness of 200 lm, while the boron doped p-type samples were 250 lm thick. After wet chemical cleaning, the samples were subjected to a thermal pretreatment in the form of an oxidation step at 1050 C for 60 min in order to homogenize thermal history and avoid influence of the bulk defect class discussed by Grant et al 1 We then address the wafer bulk as "stabilized" (e.g., in Fig. 1) as no changes of the wafer bulk lifetime due to temperature steps are expected after this treatment.…”

“…However, recent studies have revealed that even FZ wafers of high purity and electronic quality can be prone to the formation of recombination active defects during moderate temperature treatments, e.g., Ref. 1. Additional recombination activity in the wafer bulk hinders the optimization of surface passivation schemes and overlooking the underlying defects complicates their consideration.…”

Superacid-Treated Silicon Surfaces: Extending the Limit of Carrier Lifetime for Photovoltaic Applications

“…After removal of the phosphosilicate glass in buffered HF and subsequent SC cleaning, the wafers were thermally oxidized in oxygen (with background dichloroethylene) for 60 min at 1050°C. The oxidation annihilates grown-in defects [35], [36] while the phosphorus diffusion getters impurities and prevents external contamination during the high-temperature oxidation. Following the high-temperature processing, the SiO 2 film and phosphorus diffused region were chemically etched away.…”

A novel source of atomic hydrogen for passivation of defects in silicon

“…The implied assumption is that the material is free of any defects, which may not always be the case [20]. This assumption was nonetheless deemed preferable to the use of an arbitrary or variable bulk lifetime for comparison of surface passivation.…”