Influencing Light and Elevated Temperature Induced Degradation and Surface-Related Degradation Kinetics in Float-Zone Silicon by Varying the Initial Sample State
2020-01, Hammann, Benjamin, Engelhardt, Josh, Sperber, David, Herguth, Axel, Hahn, Giso
Light and elevated temperature induced degradation (LeTID) kinetics in float-zone silicon are investigated by varying the initial sample state, composed of different base material, base doping, SiN x :H films, and subsequent firing, and/or annealing steps. The approach of deliberately changing the initial sample state is shown to allow for specific studies of influences of LeTID kinetics. Bulk- and surface-related degradations are examined separately and the influence on the kinetics of bulk- and surface-related degradation is illustrated by a four-state and three-state model, respectively. In case of bulk-related degradation, an increase in defect density because of the firing step is shown, whereas the annealing step has an inverse effect. Both temperature steps—individually and combined—influence the transition rates of bulk-related degradation and regeneration by presumably changing the distribution of a defect precursor. For surface-related degradation, the firing step reduces the transition rate from the initial to the degraded state. In addition, the influence of a comparably humid atmosphere and the absence of UV light are found to be negligible.
Bulk and Surface-Related Degradation in Lifetime Samples Made of Czochralski Silicon Passivated by Plasma-Enhanced Chemical Vapor Deposited Layer Stacks
2018-12, Sperber, David, Schwarz, Anton, Herguth, Axel, Hahn, Giso
Significant bulk-related degradation (BRD) is followed by surface-related degradation (SRD) of effective excess charge carrier lifetime in lifetime samples made of Czochralski silicon during illuminated treatment at 80–150°C. Samples are passivated with either AlOx:H/SiOxNy:H/SiNx:H or SiOxNy:H/SiNx:H stacks stemming entirely from plasma-enhanced chemical vapor deposition. Samples show strong variations in BRD depending on passivation stacks and treatment conditions, and a potential link to light and elevated temperature‐induced degradation (LeTID) is discussed. All samples are fired in a belt furnace, and variations of firing temperature and belt speed are shown to influence SRD slightly. SRD is furthermore accelerated with increasing treatment temperature and an apparent activation energy Eapp=1.07±0.02eV is determined in SiOxNy:H/SiNx:H passivated samples. Interpretation of Eapp is, however, difficult as both changes in interfacial defect and fixed charge density occur in parallel during SRD.
Temperature and Light-Induced Changes in Bulk and Passivation Quality of Boron-Doped Float-Zone Silicon Coated With SiNx:H
2017-03, Sperber, David, Heilemann, Adrian, Herguth, Axel, Hahn, Giso
In this study, it is observed that boron-doped float-zone silicon coated with hydrogenated silicon nitride shows strong instabilities in effective minority carrier lifetime after a fast firing step and subsequent treatment at elevated temperatures and illumination. During such a treatment, both degradation and recovery features are visible over time scales from minutes to months. To further investigate the observed behavior, corona charging series, capacitance voltage measurements, and chemical repassivation methods are applied. It is shown that a first fast degradation and recovery is associated with changes in the bulk lifetime, and it is observed that the fast firing step strongly influences this bulk instability. A subsequent slower degradation and recovery reflects changes in the effective surface recombination velocity that can be attributed to changes in the chemical passivation quality. It can be concluded that care has to be taken when boron-doped float-zone silicon is used as a supposedly stable high lifetime reference material after a fast firing step. Additionally, it can be stated that a silicon nitride related passivation may be far from stable at elevated temperatures and illumination after a fast firing step.
Bulk and surface instabilities in boron doped float-zone samples during light induced degradation treatments
2017, Sperber, David, Graf, Alexander, Heilemann, Adrian, Herguth, Axel, Hahn, Giso
Float-zone silicon is often used as a supposedly stable high lifetime reference material. Here it is shown, however, that boron doped float-zone samples that underwent a fast firing step may suffer from a severe degradation in bulk lifetime during illumination at elevated temperatures. Furthermore, it is observed that silicon nitride related passivation may be affected by a long-term decrease in chemical passivation quality. A time and injection resolved visualization is introduced to quickly distinguish between these degradation features. Both bulk lifetime and chemical passivation quality are shown to recover at the same treatment conditions after longer treatment times.
Does LeTID occur in c-Si even without a firing step?
2019, Sperber, David, Furtwängler, Florian, Herguth, Axel, Hahn, Giso
It is shown that a non-fired B-doped floatzone silicon sample coated with SiNx:H may show severe bulk related degradation and regeneration during illuminated treatment at elevated temperature. It is discussed that the likely cause is light and elevated temperature induced degradation (LeTID) in the silicon bulk. Firing is found to modulate the extent of LeTID so that degradation may either be weaker or stronger compared to the non-fired sample depending on firing parameters. A sample which was annealed instead of fired is found to be stable for up to 1,000 h of treatment time.
Enhanced stability of passivation quality on diffused silicon surfaces under light-induced degradation conditions
2018, Sperber, David, Schwarz, Anton, Herguth, Axel, Hahn, Giso
Significant surface related degradation (SRD) is observed in samples passivated with either SiNx:H or AlOx:H/SiNx:H during treatment at 150 °C and 1 sun equivalent illumination intensity. Degradation of SiNx:H passivation is caused by a decrease of chemical passivation quality whereas degradation of AlOx:H/SiNx:H is caused by a decrease of fixed charge density. SRD is, however, strongly suppressed on highly doped silicon surfaces resulting from a diffusion step. Device simulations indicate that this cannot only be explained by reduced sensitivity to changes at the silicon surface due to the diffused region, and implications for defect formation are discussed.
A 3-state defect model for light-induced degradation in boron-doped float-zone silicon
2017-03, Sperber, David, Herguth, Axel, Hahn, Giso
We report on a light-induced bulk defect activation and subsequent deactivation in boron doped float-zone silicon that can be described by a 3-state model. During treatment at elevated temperature and illumination, a sample first converts from an initial high lifetime state into a degraded low lifetime state and then shows a recovery reaction leading to a third high lifetime state that is then stable under degradation conditions. Furthermore, it is shown that reverse reactions into the initial state appear to be possible both from the degraded as well as the regenerated state. An injection dependent analysis of lifetime data yields a defect capture cross section ratio of ∼20 suggesting a positively charged defect.
Bulk and Surface Related Degradation Phenomena in Monocrystalline Silicon at Elevated Temperature and Illumination
2019, Sperber, David
This work investigates the stability of lifetime samples made of monocrystalline silicon at elevated temperature and illumination. It is shown that samples made of Czochralski (Cz-Si) and float-zone silicon (FZ-Si) are affected by significant bulk related degradation (BRD) and surface related degradation (SRD) phenomena. SRD is observed in samples passivated with SiNx:H, SiO2/SiNx:H, SiOx:H/SiNx:H, SiOxNy:H/SiNx:H, and AlOx/SiNx:H. SRD furthermore occurs after variation of wet-chemical and most other processing steps and thus appears to present a general phenomenon in samples made of crystalline silicon. However, SRD is much weaker on highly doped silicon surfaces, and different degrees of changes in chemical passivation and field effect passivation are observed depending on choice of passivation layers. Frequently, SRD is followed by a recovery of surface passivation quality. A large number of samples is furthermore affected by BRD and subsequent regeneration phenomena. While BRD is expected in Cz-Si samples due to boron oxygen related degradation, even samples made of FZ-Si may show severe BRD. An in-depth analysis of degradation properties leads to the conclusion that BRD in FZ-Si is very likely related to light and elevated temperature induced degradation (LeTID) as originally observed in multicrystalline silicon. It is furthermore shown that samples made of Cz-Si may likely suffer from LeTID in addition to boron oxygen related degradation. Finally, the long-term stability of samples is investigated after a regeneration process. It is shown that regeneration of boron oxygen defects is very stable whereas samples affected by strong LeTID may suffer from a bulk related long-term instability.
Investigating possible causes of light induced degradation in boron-doped Float-Zone silicon
2017, Sperber, David, Herguth, Axel, Hahn, Giso
The use of different silicon nitride deposition tools is found to change the degree of light induced degradation (LID) of B-doped float-zone silicon after a fast firing step. In addition, a thermally grown SiO2 interlayer further suppresses LID after firing. Possible mechanisms and a potential link to Light and elevated Temperature Induced Degradation (LeTID) are discussed. Furthermore, it is shown that LID is not related to an earlier described class of thermally activated defects in float-zone silicon and that phosphorous gettering does not influence the occurrence of LID in B-doped float-zone silicon significantly.