Influence of hydrogen on the regeneration of boron-oxygen related defects in crystalline silicon

Abstract

When exposed to light, boron doped monocrystalline Czochralski grown silicon suffers from degradation of the minority carrier lifetime due to the formation of recombination active boron-oxygen related defects. The so called regeneration procedure is able to convert these recombination active defects into a new less recombination active state characterized by a higher minority charge carrier lifetime and stability under illumination. However, the exact working principle on microscopic scale is still unknown even though some influencing factors were identified. The role of hydrogen in the regeneration process is investigated in this work. We find that the characteristic regeneration time constant is subject to variation depending on the process parameters of a Plasma Enhanced Chemical Vapor Deposition a-SiNx:H deposition, namely the applied gas flows, as well as on the thermal history of the sample prior to applying the regeneration procedure. The positive effect of a short high temperature (800–900 °C) step leads to the idea that the presence of atomic hydrogen in the silicon bulk is crucial for the regeneration effect to occur. The different regeneration behavior of samples with variable thickness of a hydrogen diffusion barrier, namely an Al2O3 layer capped by SiNx:H, supports those results. Finally, the importance of hydrogen for regeneration is directly shown on samples having different hydrogen bulk concentrations due to direct hydrogenation in a Microwave Induced Remote Hydrogen Plasma reactor. A new model to explain the effect of the regeneration of boron-oxygen related defect centers based on the possible role of atomic hydrogen is presented.

Influence of bound hydrogen states on BO-regeneration kinetics and consequences for high-speed regeneration processes

“…Whereas rates for annealing, degradation and probably destabilization only depend on temperature under typical regeneration conditions, regenera tion can be accelerated by increasing illumination intensity. In addition, the regeneration rate can be increased by improved hydrogenation of the silicon bulk [ 14) and according to Section 3 the ratio of hydrogen being in a favorable bonding state. Altogether, temperature, injection level, and hydrogen will deter mine which transition reactions dominate, and thus what is the occupation probability of the different 80 states in long term equilibrium…”

“…by choosing suitable hydrogenated dielectric surface coatings [14,15] and optimized high temperature steps during sample processing [16,17] the regeneration process becomes faster. This leads to the idea that regeneration of BO related defects might in fact evolve from their passivation by hydrogen [14]. But the question arises how hydrogen can influence regeneration, even though it should mainly exist in bound states at typical regeneration temperatures of below 200 1C [18].…”

“…Recent results suggest that hydrogen might be the key factor to fast regeneration processes [12,13]: It has been shown, that if more hydrogen is introduced into the silicon bulk realized e.g. by choosing suitable hydrogenated dielectric surface coatings [14,15] and optimized high temperature steps during sample processing [16,17] the regeneration process becomes faster. This leads to the idea that regeneration of BO related defects might in fact evolve from their passivation by hydrogen [14].…”

From simulation to experiment: Understanding BO-regeneration kinetics

“…But more data is needed to investigate a possible additional influence of phosphorus or gallium on the regeneration of BO defects. This question will be addressed in a separate publication [32).…”

Eliminating Light-Induced Degradation in Commercial p-Type Czochralski Silicon Solar Cells

“…Wilking et al [103] and Walter et al [104] showed that the firing step mainly responsible for contact formation has a significant impact on the kinetics of passivation most probably as it determines the degree of hydrogenation of the silicon bulk. Also, it was observed that increasing the hydrogen content in dielectric layers can lead to an increase in the hydrogen concentration within the silicon, which correlates well with an acceleration of the passivation [102,103,[105][106][107][108].…”