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Daniel Macdonald - One of the best experts on this subject based on the ideXlab platform.
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Gettering of transition metals in high performance multicrystalline silicon by silicon nitride films and phosphorus diffusion
Journal of Applied Physics, 2019Co-Authors: Anyao Liu, Hang Cheong Sio, Chang Sun, Xinyu Zhang, Hao Jin, Daniel MacdonaldAbstract:High-performance multicrystalline silicon (HP mc-Si) from directional solidification has become the mainstream industrial material for fabricating mc-Si based solar cells for photovoltaic applications. Transition metal impurities are inherently contained in HP mc-Si during ingot growth, and they are one of the major efficiency-limiting drawbacks. In this work, we investigate the Gettering of transition metals (Cu, Ni, Fe, and Cr) in HP mc-Si wafers along an industrial-standard p-type HP mc-Si ingot, via examining the metal concentration and distribution in the near-surface Gettering layers using secondary ion mass spectrometry. We applied both conventional phosphorus diffusion Gettering and the recently developed silicon nitride (from plasma-enhanced chemical vapour deposition) Gettering techniques. Both techniques are shown to remove significant quantities of metals from the silicon wafer bulk to the surface Gettering layers. Improvements in the bulk minority carrier lifetimes throughout the ingot height are also observed by lifetime measurements and spatially-resolved photoluminescence imaging. The gettered Cu and Ni concentrations, as well as the as-grown dissolved Fe concentrations in the silicon wafer bulk, along the HP mc-Si ingot height are shown to follow a similar concentration profile as the metals in conventional mc-Si ingots.
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Gettering effects of silicon nitride films from various plasma enhanced chemical vapor deposition conditions
IEEE Journal of Photovoltaics, 2019Co-Authors: Anyao Liu, Ziv Hameiri, Yimao Wan, Chang Sun, Daniel MacdonaldAbstract:This paper investigates and compares the impurity Gettering effects of silicon nitride (SiN x ) films that are synthesized by plasma-enhanced chemical vapor deposition (PECVD) under various conditions. Both industrial- and laboratory-scale PECVD systems are employed to deposit SiN x films with a wide range of properties (with refractive indices from 1.93 to 2.45 at 632 nm), which covers the entire range of SiN x used for silicon solar cells. The Gettering effects are quantified by monitoring the reduction kinetics of the interstitial iron concentration in the silicon wafer bulk as iron becomes gettered to the surface SiN x layers during cumulative annealing at 400 °C. The results show that the very different SiN x films generate similar Gettering kinetics, indicating that the impurity Gettering effect is likely present in most PECVD SiN x films for silicon solar cells. The Gettering kinetics and the SiN x film properties of refractive index, Si–N, Si–H, N–H bond densities, and H content, are found to have no clear correlations.
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effective impurity Gettering by phosphorus and boron diffused polysilicon passivating contacts for silicon solar cells
Solar Energy Materials and Solar Cells, 2017Co-Authors: Anyao Liu, Di Yan, Sieu Pheng Phang, Andres Cuevas, Daniel MacdonaldAbstract:Abstract This paper presents direct experimental evidence for the strong impurity Gettering effects associated with the formation of both phosphorus and boron doped polysilicon/oxide passivating contacts for silicon solar cells, doped via thermal diffusion from POCl3 or BBr3 sources. Ion-implanted iron is used as a marker to quantify the Gettering effectiveness via carrier lifetime measurements. The process conditions for fabricating optimum polysilicon passivating contacts are found to remove more than 99.9% of the iron from the silicon wafer bulk. The Gettering effects of POCl3 and BBr3 diffused polysilicon/oxide contacts mainly arise from the dopant diffusions, as opposed to Gettering by structural defects in the polysilicon films. The thin oxide interlayer hinders the Gettering effectiveness at low diffusion temperatures, although its blocking effect becomes small at the moderate temperatures used to fabricate optimum polysilicon contacts. The Gettering effectiveness increases with increasing diffusion temperature. The Gettering of iron from the silicon wafer bulk to the surface layers is found to have a negligible impact on their ability to suppress recombination at the interface with the silicon wafer. Therefore, the formation of polysilicon/oxide passivating contacts, via thermal diffusion from POCl3 and BBr3 sources, not only achieves high quality surface and contact passivation but also has the net additional benefit of achieving very effective Gettering of unwanted impurities in the silicon wafer bulk.
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impact of phosphorous Gettering and hydrogenation on the surface recombination velocity of grain boundaries in p type multicrystalline silicon
IEEE Journal of Photovoltaics, 2015Co-Authors: Hang Cheong Sio, Sieu Pheng Phang, Thorsten Trupke, Daniel MacdonaldAbstract:We compare the recombination properties of a large number of grain boundaries in multicrystalline silicon wafers with different contamination levels and investigate their response to phosphorous Gettering and hydrogenation. The recombination activity of a grain boundary is quantified in terms of the effective surface recombination velocity $S_{{\bf GB}} $ based on photoluminescence imaging and 2-D modeling of the emitted photoluminescence signal. Our results show that varying impurity levels along the ingot significantly impact the grain boundary behavior. Grain boundaries from the middle of the ingot become more recombination active after either Gettering or hydrogenation alone, whereas grain boundaries from the top and bottom of the ingot have a more varied response. Hydrogenation, in general, is much more effective on gettered grain boundaries compared with as-grown grain boundaries. A close inspection of their injection dependence reveals that while some grain boundaries exhibit little injection dependence before Gettering, others show a relatively large injection dependence, with their $S_{{\bf GB}} $ increasing as the injection level decreases. The former type tend not to be recombination active after both Gettering and hydrogenation and are less likely to impact the final cell performance, in comparison with grain boundaries of the latter type.
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effect of boron codoping and phosphorus concentration on phosphorus diffusion Gettering
IEEE Journal of Photovoltaics, 2014Co-Authors: Sieu Pheng Phang, Daniel MacdonaldAbstract:Compared with phosphorus diffusions, conventional boron diffusions for n-type solar cells are not effective at impurity Gettering without the presence of a boron-rich layer. In this paper, we investigate the Gettering effectiveness of light phosphorus diffusions for removing Fe impurities, applied on an underlying boron diffusion, similar to the buried emitter concept, as an option for achieving effective Gettering on boron diffused substrates. Our experimental results on monocrystalline silicon samples demonstrate that the underlying boron diffusion does not affect the Gettering effectiveness of the phosphorus diffusion, even though much of the phosphorus diffused region is overdoped by the boron diffusion. Furthermore, we investigate the Gettering effectiveness of low surface concentration phosphorus diffusions that can result in reduced recombination in the n+ region. Our results show that the Gettering effectiveness decreases when the surface phosphorus concentration is reduced, either through manipulating the deposition gas flows or through subsequent driving in. Driving in the surface phosphorus concentration from 2 × 1020 to 3.5 × 1019 cm-3 decreased the Gettering effectiveness by about one order of magnitude.
Hele Savin - One of the best experts on this subject based on the ideXlab platform.
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black silicon significantly enhances phosphorus diffusion Gettering
Scientific Reports, 2018Co-Authors: Toni P Pasanen, Ville Vahanissi, J Schon, Hannu S Laine, Hele SavinAbstract:Black silicon (b-Si) is currently being adopted by several fields of technology, and its potential has already been demonstrated in various applications. We show here that the increased surface area of b-Si, which has generally been considered as a drawback e.g. in applications that require efficient surface passivation, can be used as an advantage: it enhances Gettering of deleterious metal impurities. We demonstrate experimentally that interstitial iron concentration in intentionally contaminated silicon wafers reduces from 1.7 × 1013 cm−3 to less than 1010 cm−3 via b-Si Gettering coupled with phosphorus diffusion from a POCl3 source. Simultaneously, the minority carrier lifetime increases from less than 2 μs of a contaminated wafer to more than 1.5 ms. A series of different low temperature anneals suggests segregation into the phosphorus-doped layer to be the main Gettering mechanism, a notion which paves the way of adopting these results into predictive process simulators. This conclusion is supported by simulations which show that the b-Si needles are entirely heavily-doped with phosphorus after a typical POCl3 diffusion process, promoting iron segregation. Potential benefits of enhanced Gettering by b-Si include the possibility to use lower quality silicon in high-efficiency photovoltaic devices.
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elucidation of iron Gettering mechanisms in boron implanted silicon solar cells
IEEE Journal of Photovoltaics, 2018Co-Authors: Hannu S Laine, Ashley E Morishige, Barry Lai, Ville Vahanissi, Hele Savin, Zhengjun Liu, Ernesto Magana, Jan Krugener, Kristian Salo, D P FenningAbstract:To facilitate cost-effective manufacturing of boron-implanted silicon solar cells as an alternative to BBr3 diffusion, we performed a quantitative test of the Gettering induced by solar-typical boron-implants with the potential for low saturation current density emitters (<50 fA/cm2). We show that depending on the contamination level and the Gettering anneal chosen, such boron-implanted emitters can induce more than a 99.9% reduction in bulk iron point defect concentration. The iron point defect results as well as synchrotron-based nano-X-ray-fluorescence investigations of iron precipitates formed in the implanted layer imply that, with the chosen experimental parameters, iron precipitation is the dominant Gettering mechanism, with segregation-based Gettering playing a smaller role. We reproduce the measured iron point defect and precipitate distributions via kinetics modeling. First, we simulate the structural defect distribution created by the implantation process, and then we model these structural defects as heterogeneous precipitation sites for iron. Unlike previous theoretical work on Gettering via boron- or phosphorus-implantation, our model is free of adjustable simulation parameters. The close agreement between the model and experimental results indicates that the model successfully captures the necessary physics to describe the iron Gettering mechanisms operating in boron-implanted silicon. This modeling capability allows high-performance, cost-effective implanted silicon solar cells to be designed.
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Cu Gettering by phosphorus-doped emitters in p-type silicon: Effect on light-induced degradation
AIP Publishing LLC, 2018Co-Authors: Alessandro Inglese, Ville Vahanissi, Hannu S Laine, Hele SavinAbstract:The presence of copper (Cu) contamination is known to cause relevant light-induced degradation (Cu-LID) effects in p-type silicon. Due to its high diffusivity, Cu is generally regarded as a relatively benign impurity, which can be readily relocated during device fabrication from the wafer bulk, i.e. the region affected by Cu-LID, to the surface phosphorus-doped emitter. This contribution examines in detail the impact of Gettering by industrially relevant phosphorus layers on the strength of Cu-LID effects. We find that phosphorus Gettering does not always prevent the occurrence of Cu-LID. Specifically, air-cooling after an isothermal anneal at 800°C results in only weak impurity segregation to the phosphorus-doped layer, which turns out to be insufficient for effectively mitigating Cu-LID effects. Furthermore, we show that the Gettering efficiency can be enhanced through the addition of a slow cooling ramp (-4°C/min) between 800°C and 600°C, resulting in the nearly complete disappearance of Cu-LID effects
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Gettering of iron in silicon solar cells with implanted emitters
IEEE Journal of Photovoltaics, 2014Co-Authors: Ville Vahanissi, Antti Haarahiltunen, Marko Ylikoski, Hele SavinAbstract:We present here experimental results on the Gettering of iron in Czochralski-grown silicon by phosphorus implantation. The Gettering efficiency and the Gettering mechanisms in a high resistivity implanted emitter are determined as a function of both initial iron level and Gettering anneal. The results show that Gettering in implanted emitters can be efficient if precipitation at the emitter is activated. This requires low Gettering temperatures and/or high initial contamination level. The fastest method to getter iron from the bulk is to rapidly nucleate iron precipitates before the Gettering anneal. Here, this was achieved by a fast ramp to the room temperature in between the implantation anneal and the Gettering anneal.
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phosphorus and boron diffusion Gettering of iron in monocrystalline silicon
Journal of Applied Physics, 2011Co-Authors: Heli Talvitie, Ville Vahanissi, Antti Haarahiltunen, Marko Ylikoski, Hele SavinAbstract:We have studied experimentally the phosphorus diffusion Gettering (PDG) of iron in monocrystalline silicon at the temperature range of 650–800 °C. Our results fill the lack of data at low temperatures so that we can obtain a reliable segregation coefficient for iron between a phosphorus diffused layer and bulk silicon. The improved segregation coefficient is verified by time dependent PDG simulations. Comparison of the PDG to boron diffusion Gettering (BDG) in the same temperature range shows PDG to be only slightly more effective than BDG. In general, we found that BDG requires more carefully designed processing conditions than PDG to reach a high Gettering efficiency.
Tonio Buonassisi - One of the best experts on this subject based on the ideXlab platform.
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exceptional Gettering response of epitaxially grown kerfless silicon
Other repository, 2016Co-Authors: Douglas M Powell, Sergio Castellanos, Mallory A Jensen, Ashley E Morishige, Jasmin Hofstetter, Barry Lai, V P Markevich, A R Peaker, Tonio BuonassisiAbstract:The bulk minority-carrier lifetime in p- and n-type kerfless epitaxial (epi) crystalline silicon wafers is shown to increase >500× during phosphorus Gettering. We employ kinetic defect simulations and microstructural characterization techniques to elucidate the root cause of this exceptional Gettering response. Simulations and deep-level transient spectroscopy (DLTS) indicate that a high concentration of point defects (likely Pt) is “locked in” during fast (60 °C/min) cooling during epi wafer growth. The fine dispersion of moderately fast-diffusing recombination-active point defects limits as-grown lifetime but can also be removed during Gettering, confirmed by DLTS measurements. Synchrotron-based X-ray fluorescence microscopy indicates metal agglomerates at structural defects, yet the structural defect density is sufficiently low to enable high lifetimes. Consequently, after phosphorus diffusion Gettering, epi silicon exhibits a higher lifetime than materials with similar bulk impurity contents but highe...
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minority carrier lifetime and defect content of n type silicon grown by the noncontact crucible method
Journal of Crystal Growth, 2014Co-Authors: Maulid Kivambe, Douglas M Powell, Sergio Castellanos, Mallory A Jensen, Ashley E Morishige, Kazuo Nakajima, Kohei Morishita, Ryota Murai, Tonio BuonassisiAbstract:Abstract We evaluate minority-carrier lifetime and defect content of n -type photovoltaic silicon grown by the noncontact crucible method (NOC-Si). Although bulk impurity concentrations are measured by inductively coupled plasma mass spectroscopy to be less than one part per million, homogeneously throughout the as-grown material we observe lifetimes in the ~150 µs range, well below the theoretical entitlement of single-crystalline silicon. These observations suggest the presence of homogeneously distributed recombination-active point defects. We compare an industry-standard Gettering profile to an extended Gettering profile tailored for chromium extraction, to elucidate potential gains and limitations of impurity Gettering. Near the ingot top, Gettering improves lifetimes to 750 and >1800 µs for standard and extended profiles, respectively. Relatively lower gettered lifetimes are observed in wafers extracted from the ingot middle and bottom. In these regions, concentric-swirl patterns of low lifetime are revealed after Gettering. We hypothesize that Gettering removes a large fraction of fast-diffusing recombination-active impurities, while swirl microdefect regions reminiscent of Czochralski silicon can locally limit Gettering efficiency and lifetime. Apart from these swirl microdefects, a low dislocation density of 3 cm −2 is observed. The millisecond lifetimes and low dislocation density suggest that, by applying appropriate bulk microdefect and impurity control during growth and/or Gettering, n -type NOC-Si can readily support solar cells with efficiencies >23%.
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sacrificial high temperature phosphorus diffusion Gettering process for lifetime improvement of multicrystalline silicon wafers
Photovoltaic Specialists Conference, 2014Co-Authors: Stephanie Morgan Scott, Ashley E Morishige, Jasmin Hofstetter, Tonio BuonassisiAbstract:Iron is among the most deleterious lifetime-limiting impurities in crystalline silicon solar cells. In as-grown material, iron is present in precipitates and as point defects. To achieve solar cell conversion efficiencies above 20%, bulk minority-carrier lifetimes in excess of 300 µs (p-type) and 900 µs (n-type) are required [1]. For cost-effective multi-crystalline silicon wafers, achieving this lifetime requires Gettering. Gettering at higher temperatures for longer times is often necessary to fully dissolve and remove precipitated impurities. However, such time-temperature profiles can result in unacceptably deep emitters, affecting the blue response of the finished device. Here, we explore a “sacrificial” Gettering step in which Gettering and emitter-formation are decoupled and optimized independently. The optimization is guided by the Impurity-to-Efficiency simulation tool [2] and explores high-temperature regimes. While models predict that increasing the Gettering temperature decreases total iron concentration resulting in an increased lifetime, experimental results show that for the highest temperatures tested, the minority carrier lifetime is reduced.
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investigation of lifetime limiting defects after high temperature phosphorus diffusion in high iron content multicrystalline silicon
IEEE Journal of Photovoltaics, 2014Co-Authors: David P Fenning, Jasmin Hofstetter, Mariana I Bertoni, Annika Zuschlag, Alexander Frey, Giso Hahn, Tonio BuonassisiAbstract:Phosphorus diffusion Gettering of multicrystalline silicon solar cell materials generally fails to produce material with minority-carrier lifetimes that approach that of gettered monocrystalline wafers, due largely to higher levels of contamination with metal impurities and a higher density of structural defects. Higher Gettering temperatures should speed the dissolution of precipitated metals by increasing their diffusivity and solubility in the bulk, potentially allowing for improved Gettering. In this paper, we investigate the impact of Gettering at higher temperatures on low-purity multicrystalline samples. To analyze the Gettering response, we measure the spatially resolved lifetime and interstitial iron concentration by microwave photoconductance decay and photoluminescence imaging, and the structural defect density by Sopori etching and large-area automated quantification. Higher temperature phosphorus diffusion Gettering is seen to improve metal-limited multicrystalline materials dramatically, especially in areas of low etch pit density. In areas of high as-grown dislocation density in the multicrystalline materials, it appears that higher temperature phosphorus diffusion Gettering reduces the etch pit density, but leaves higher local concentrations of interstitial iron, which degrade lifetime.
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improved iron Gettering of contaminated multicrystalline silicon by high temperature phosphorus diffusion
Journal of Applied Physics, 2013Co-Authors: David P Fenning, Mariana I Bertoni, Barry Lai, Annika Zuschlag, Giso Hahn, Tonio BuonassisiAbstract:The efficacy of higher-temperature Gettering processes in reducing precipitated iron concentrations is assessed by synchrotron-based micro-X-ray fluorescence. By measuring the same grain boundary before and after phosphorus diffusion in a set of wafers from adjacent ingot heights, the reduction in size of individual precipitates is measured as a function of Gettering temperature in samples from the top of an ingot intentionally contaminated with iron in the melt. Compared to a baseline 820 °C phosphorus diffusion, 870 °C and 920 °C diffusions result in a larger reduction in iron-silicide precipitate size. Minority carrier lifetimes measured on wafers from the same ingot heights processed with the same treatments show that the greater reduction in precipitated metals is associated with a strong increase in lifetime. In a sample contaminated with both copper and iron in the melt, significant iron Gettering and complete dissolution of detectable copper precipitates is observed despite the higher total metal concentration. Finally, a homogenization pre-anneal in N2 at 920 °C followed by an 820 °C phosphorus diffusion produces precipitate size reductions and lifetimes similar to an 870 °C phosphorus diffusion without lowering the emitter sheet resistance.
Anyao Liu - One of the best experts on this subject based on the ideXlab platform.
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Gettering of transition metals in high performance multicrystalline silicon by silicon nitride films and phosphorus diffusion
Journal of Applied Physics, 2019Co-Authors: Anyao Liu, Hang Cheong Sio, Chang Sun, Xinyu Zhang, Hao Jin, Daniel MacdonaldAbstract:High-performance multicrystalline silicon (HP mc-Si) from directional solidification has become the mainstream industrial material for fabricating mc-Si based solar cells for photovoltaic applications. Transition metal impurities are inherently contained in HP mc-Si during ingot growth, and they are one of the major efficiency-limiting drawbacks. In this work, we investigate the Gettering of transition metals (Cu, Ni, Fe, and Cr) in HP mc-Si wafers along an industrial-standard p-type HP mc-Si ingot, via examining the metal concentration and distribution in the near-surface Gettering layers using secondary ion mass spectrometry. We applied both conventional phosphorus diffusion Gettering and the recently developed silicon nitride (from plasma-enhanced chemical vapour deposition) Gettering techniques. Both techniques are shown to remove significant quantities of metals from the silicon wafer bulk to the surface Gettering layers. Improvements in the bulk minority carrier lifetimes throughout the ingot height are also observed by lifetime measurements and spatially-resolved photoluminescence imaging. The gettered Cu and Ni concentrations, as well as the as-grown dissolved Fe concentrations in the silicon wafer bulk, along the HP mc-Si ingot height are shown to follow a similar concentration profile as the metals in conventional mc-Si ingots.
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Gettering effects of silicon nitride films from various plasma enhanced chemical vapor deposition conditions
IEEE Journal of Photovoltaics, 2019Co-Authors: Anyao Liu, Ziv Hameiri, Yimao Wan, Chang Sun, Daniel MacdonaldAbstract:This paper investigates and compares the impurity Gettering effects of silicon nitride (SiN x ) films that are synthesized by plasma-enhanced chemical vapor deposition (PECVD) under various conditions. Both industrial- and laboratory-scale PECVD systems are employed to deposit SiN x films with a wide range of properties (with refractive indices from 1.93 to 2.45 at 632 nm), which covers the entire range of SiN x used for silicon solar cells. The Gettering effects are quantified by monitoring the reduction kinetics of the interstitial iron concentration in the silicon wafer bulk as iron becomes gettered to the surface SiN x layers during cumulative annealing at 400 °C. The results show that the very different SiN x films generate similar Gettering kinetics, indicating that the impurity Gettering effect is likely present in most PECVD SiN x films for silicon solar cells. The Gettering kinetics and the SiN x film properties of refractive index, Si–N, Si–H, N–H bond densities, and H content, are found to have no clear correlations.
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effective impurity Gettering by phosphorus and boron diffused polysilicon passivating contacts for silicon solar cells
Solar Energy Materials and Solar Cells, 2017Co-Authors: Anyao Liu, Di Yan, Sieu Pheng Phang, Andres Cuevas, Daniel MacdonaldAbstract:Abstract This paper presents direct experimental evidence for the strong impurity Gettering effects associated with the formation of both phosphorus and boron doped polysilicon/oxide passivating contacts for silicon solar cells, doped via thermal diffusion from POCl3 or BBr3 sources. Ion-implanted iron is used as a marker to quantify the Gettering effectiveness via carrier lifetime measurements. The process conditions for fabricating optimum polysilicon passivating contacts are found to remove more than 99.9% of the iron from the silicon wafer bulk. The Gettering effects of POCl3 and BBr3 diffused polysilicon/oxide contacts mainly arise from the dopant diffusions, as opposed to Gettering by structural defects in the polysilicon films. The thin oxide interlayer hinders the Gettering effectiveness at low diffusion temperatures, although its blocking effect becomes small at the moderate temperatures used to fabricate optimum polysilicon contacts. The Gettering effectiveness increases with increasing diffusion temperature. The Gettering of iron from the silicon wafer bulk to the surface layers is found to have a negligible impact on their ability to suppress recombination at the interface with the silicon wafer. Therefore, the formation of polysilicon/oxide passivating contacts, via thermal diffusion from POCl3 and BBr3 sources, not only achieves high quality surface and contact passivation but also has the net additional benefit of achieving very effective Gettering of unwanted impurities in the silicon wafer bulk.
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investigating internal Gettering of iron at grain boundaries in multicrystalline silicon via photoluminescence imaging
IEEE Journal of Photovoltaics, 2012Co-Authors: Anyao Liu, Sieu Pheng Phang, Daniel Walter, Daniel MacdonaldAbstract:In this paper, we present measurements and modeling of the reduction in dissolved iron Fe; concentrations near grain boundaries in multicrystalline silicon (mc-Si) wafers. The measurements of the interstitial Fe concentrations are obtained via photoluminescence images taken before and after iron-boron pair dissociation. A simple diffusion-capture model was developed to characterize the removal of interstitial Fe by the Gettering sites. The model is based on a numerical solution to the 1-D diffusion equation with two fitting parameters: the diffusion length of dissolved Fe atoms and the effective Gettering velocity at the Gettering site. By comparing the simulation with a controlled phosphorous Gettering process, the model is shown to give good estimation of the diffusion length of Fe atoms. For as-cut multicrystalline silicon wafers from different parts of the ingot, that is, wafers with different average dissolved Fe concentrations [Fei], the diffusion lengths of Fe atoms are found to decrease with decreasing average [Fei] This suggests the presence of relaxation precipitation during the internal Gettering of dissolved Fe by the grain boundaries in mc-Si during ingot cooling.
Macdonald Daniel - One of the best experts on this subject based on the ideXlab platform.
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Impurity Gettering by Diffusion-doped Polysilicon Passivating Contacts for Silicon Solar Cells
IEEE, 2019Co-Authors: Liu, An Yao, Di Yan, Wong-leung Yin-yin, Phang, Sieu Pheng, Cuevas Andres, Macdonald DanielAbstract:We report direct experimental evidence for the strong impurity Gettering effects associated with the formation of diffusion-doped polysilicon passivating contacts. Iron is used as a marker impurity in silicon to quantify the Gettering effectiveness. By monitoring the iron redistribution from the silicon wafer bulk to the polysilicon surface layers, via a combination of carrier lifetime, secondary ion mass spectrometry (SIMS), and transmission electron microscopy (TEM) techniques, the respective Gettering sites in the phosphorus and boron diffusiondoped polysilicon contacts are identified. In phosphorus-doped polysilicon, iron moves to the heavily doped polysilicon layer; and in the boron-doped structure, iron is gettered to the boron-rich layer. Both Gettering processes occur via an impurity segregation mechanism. Lastly, the Gettering of iron to the polysilicon surface layers is found to have no impact on the passivation quality of the polysilicon contacts
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Direct observation of the impurity Gettering layers in polysilicon-based passivating contacts for silicon solar cells
'American Chemical Society (ACS)', 2019Co-Authors: Liu Anyao, Di Yan, Wong-leung Yin-yin, Phang, Sieu Pheng, Cuevas Andres, Macdonald DanielAbstract:The formation of certain types of doped polysilicon passivating contacts for silicon solar cells is recently reported to generate very strong impurity Gettering effects, revealing an important additional benefit of this passivating contact structure. This work investigates the underlying Gettering mechanisms by directly monitoring the impurity redistribution during the contact formation and subsequent processes, via a combination of secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), and minority carrier lifetime techniques. Microscopic features of the phosphorus and boron diffusion-doped polysilicon passivating contacts are also presented. Iron is used as a marker impurity in silicon to enable direct quantification of its concentration change in the bulk of the silicon wafers and in the surface layers that compose the contact structure. The results conclusively show that, for phosphorus-doped polysilicon passivating contacts, impurities are relocated from the silicon wafer bulk to the heavily phosphorus-doped polysilicon layer; while for the boron diffusion-doped polysilicon, the boron-rich layer (a silicon−boron compound) accounts for the majority of the Gettering action.This work has been supported by the Australian Renewable Energy Agency (ARENA) through Project RND009. A.Y.L. is supported by the Australian Centre for Advanced Photovoltaics (ACAP) Postdoctoral Fellowship
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Impurity Gettering by Atomic-Layer-Deposited Aluminium Oxide Films on Silicon at Contact Firing Temperatures
'Wiley', 2019Co-Authors: Liu Anyao, Macdonald DanielAbstract:Aluminium oxide (Al2O3) thin films deposited on silicon surfaces, synthesised by plasma‐assisted atomic layer deposition, are recently reported to possess impurity Gettering effects for the silicon wafer bulk during annealing at 425 °C, a typical temperature used for activating the surface passivation quality of the Al2O3 films. This paper investigates the Gettering effects of Al2O3 films at higher temperatures of 700–900 °C, which are commonly used for contact firing in silicon solar cell fabrication. Iron is used as a marker impurity in silicon to study the Gettering effectiveness. Results show that Al2O3 films also generate strong impurity Gettering effects at 700–900 °C, through a segregation Gettering mechanism. The as‐deposited Al2O3 films are found to be more effective at Gettering than the 425 °C‐activated Al2O3 films, demonstrating Gettering processes that are largely limited by the impurity diffusivity in silicon. For both as‐deposited and activated Al2O3 films, Gettering during high temperature annealing occurs by impurity accumulation at the Al2O3/Si interfaces, similar to the Gettering action at 425 °C. However, some iron is found to redistribute into the bulk of the Al2O3 films after long annealing at a high temperature.This work has been supported by the Australian Renewable Energy Agency (ARENA) through project RND009
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Gettering of interstitial iron in silicon by plasma-enhanced chemical vapour deposited silicon nitride films
'AIP Publishing', 2018Co-Authors: Liu, An Yao, Sun Chang, Markevich, Vladimir P, Peaker, Anthony R, Murphy John, Macdonald DanielAbstract:It is known that the interstitial iron concentration in silicon is reduced after annealing silicon wafers coated with plasma-enhanced chemical vapour deposited (PECVD) silicon nitride films. The underlying mechanism for the significant iron reduction has remained unclear and is investigated in this work. Secondary ion mass spectrometry (SIMS) depth profiling of iron is performed on annealed iron-contaminated single-crystalline silicon wafers passivated with PECVD silicon nitride films. SIMS measurements reveal a high concentration of iron uniformly distributed in the annealed silicon nitride films. This accumulation of iron in the silicon nitride film matches the interstitial iron loss in the silicon bulk. This finding conclusively shows that the interstitial iron is gettered by the silicon nitride films during annealing over a wide temperature range from 250 °C to 900 °C, via a segregation Gettering effect. Further experimental evidence is presented to support this finding. Deep-level transient spectroscopy analysis shows that no new electrically active defects are formed in the silicon bulk after annealing iron-containing silicon with silicon nitride films, confirming that the interstitial iron loss is not due to a change in the chemical structure of iron related defects in the silicon bulk. In addition, once the annealed silicon nitride films are removed, subsequent high temperature processes do not result in any reappearance of iron. Finally, the experimentally measured iron decay kinetics are shown to agree with a model of iron diffusion to the surface Gettering sites, indicating a diffusion-limited iron Gettering process for temperatures below 700 °C. The Gettering process is found to become reaction-limited at higher temperatures
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Imaging and modelling the internal Gettering of interstitial iron by grain boundaries in multicrystalline silicon
Curran Associates Inc., 2016Co-Authors: Liu, An Yao, Phang, Sieu Pheng, Walters Daniel, Macdonald DanielAbstract:In this paper a simple one-dimensional diffusion-capture model is used to effectively characterise the reduction in interstitial Fe concentrations near grain boundaries in multicrystalline silicon by two fitting parameters: the diffusion length of Fe atoms and the Gettering velocity at the grain boundary. The measurements are achieved by photoluminescence images taken before and after dissociating FeB pairs in silicon. The measurement artefacts of lateral photon scattering and lateral carrier diffusion are discussed. The method and the model are verified by a multicrystalline silicon wafer annealed at low temperatures which are known to result in diffusion-limited internal Gettering of interstitial Fe