Kinetics of Bulk Lifetime Degradation in Float‐Zone Silicon: Fast Activation and Annihilation of Grown‐In Defects and the Role of Hydrogen versus Light

Abstract

Float‐zone (FZ) silicon often has grown‐in defects that are thermally activated in a broad temperature window (≈300–800 °C). These defects cause efficient electron‐hole pair recombination, which deteriorates the bulk minority carrier lifetime and thereby possible photovoltaic conversion efficiencies. Little is known so far about these defects which are possibly Si‐vacancy/nitrogen‐related (VxNy). Herein, it is shown that the defect activation takes place on sub‐second timescales, as does the destruction of the defects at higher temperatures. Complete defect annihilation, however, is not achieved until nitrogen impurities are effused from the wafer, as confirmed by secondary ion mass spectrometry. Hydrogenation experiments reveal the temporary and only partial passivation of recombination centers. In combination with deep‐level transient spectroscopy, at least two possible defect states are revealed, only one of which interacts with H. With the help of density functional theory V1N1‐centers, which induce Si dangling bonds (DBs), are proposed as one possible defect candidate. Such DBs can be passivated by H. The associated formation energy, as well as their sensitivity to light‐induced free carriers, is consistent with the experimental results. These results are anticipated to contribute to a deeper understanding of bulk‐Si defects, which are pivotal for the mitigation of solar cell degradation processes.

Electrical Characterization of Thermally Activated Defects in n-Type Float-Zone Silicon

“…Further annealing at lower temperatures does not result in re-activation of the defects [18]. A recent study showed that the defect activation takes places on sub-second time scale and illumination with photon energy above the bandgap can decrease the onset temperature of degradation [19]. The defect deactivation is also found to occur within a short period of time (1 min at 1000 °C).…”

“…The defect deactivation is also found to occur within a short period of time (1 min at 1000 °C). However, a prolonged high temperature annealing is required for a complete and irreversible defect annihilation [19].…”

“…Activation anneals for aluminum oxide surface passivation are also often performed in this temperature range [20]. Previous studies found that these thermally activated defects can be partially passivated via hydrogenation [19], [21], [22]. Therefore, these defects could be activated during the surface passivation process, but also be partially passivated by the hydrogen introduced during the dielectric deposition process.…”

“…Therefore, these defects could be activated during the surface passivation process, but also be partially passivated by the hydrogen introduced during the dielectric deposition process. It was also shown that the partial passivation provided by hydrogen is only temporary [19]. The assumption that FZ silicon is bulk defect-free is therefore not always a correct one.…”

“…Moreover, a few studies have also indicated that nitrogen doping during the FZ silicon growth plays a significant role in the formation of recombination active defects [18], [21], [23]. Via secondary ion mass spectrometry, a recent study showed that the permanent defect annihilation requires effusion of nitrogen impurities from the wafer, indicating the involvement of nitrogen in the defect formation [19].…”