In silicon-based devices copper (Cu) contamination is the cause of a variety of adverse effects, one of them being the so-called light-induced degradation (Cu-LID). This phenomenon results in the progressive reduction of minority carrier lifetime during light-soaking, thus becoming a particular issue in devices sensitive to this parameter, such as state-of-the-art silicon solar cells. The content of this dissertation elucidates various aspects of Cu-LID that have long remained unclear and examines the possibility of leveraging this knowledge to quantitatively detect copper impurities from carrier lifetime measurements. First, experiments carried out on specimens featuring lightly doped phosphorus emitters provide insights into the effect of gettering on the strength of Cu-LID. It is found that in specimens with a phosphorus-diffused emitter significant suppression of Cu-LID is achievable through the addition of a slow cooling tail after high-temperature anneals. On the other hand, in another batch of differently processed specimens none of the tested gettering schemes results in the suppression of Cu-LID. Further investigations into this latter set of specimens indicate oxygen precipitation as the cause for the ineffectiveness of the gettering treatments. The fact that relevant quantities of Cu may persist in the bulk region after phosphorus gettering treatments raises the need for deeper understanding into the mechanisms causing Cu-LID and development of fast and contactless methods for quickly detecting copper impurities in silicon substrates. In this dissertation, multiple evidence towards bulk Cu precipitation being the root-cause of Cu-LID is provided via (i) defect characterization based on the Shockley-Read-Hall theory, (ii) analysis of the experimental data in terms of a Schottky recombination model that specifically describes the effect of metallic precipitates on the carrier lifetime, (iii) modelling of the light-induced precipitation process and comparison of the simulation results with the existing experimental data. These ﬁndings also constitute the theoretical framework for the formulation of an imaging method that provides quantitative and spatially-resolved information on the distribution of Cu species across the silicon wafer. Such method relies on the acceleration of the degradation process through simultaneous illumination and low-temperature annealing and subsequent comparison of lifetime data with the values predicted by the aforementioned Schottky model. Examples of application of this approach to deliberately contaminated mono- and multicrystalline specimens are also discussed.
|Translated title of the contribution||On the light-activation of copper impurities in crystalline silicon: root cause analysis and applications for fast high-resolution imaging|
|Publication status||Published - 2018|
|MoE publication type||G5 Doctoral dissertation (article)|
- light-induced degradation
- defect characterization