Phosphorus Diffusion Gettering Research Articles

Impact of a p-type Solar Cell Process on the Electrical Quality of Czochralski Silicon

The impact of the three main process steps during p-type solar cell processing, namely phosphorus diffusion, anti- reflection coating and contact formation, on the electrical quality of Cz silicon is investigated. Adjacent wafers from the middle and tail part of three Czochralski grown silicon ingots of varying quality were treated by the process steps as well as process step combinations. The impact of the thermal budget during the process steps without the application of phosphorus dopant, ammonia and silan gases and metal contact pastes on the Cz silicon was examined. The excess charge carrier lifetime and the interstitial iron content were measured separately for each step. Besides other effects, phosphorus diffusion gettering was found to be very efficient in removing interstitial iron from the Cz silicon. The antireflection coating step as well as the contact formation step were found to be most detrimental to the Cz silicon quality. Changes observed in the Cz silicon quality were discussed within the frame of recent models.

Effects of Phosphorus Diffusion on Gettering of Metal Impurities in UMG Silicon Wafers

Fe distributions were simulated by using impurity-to-efficiency simulator for the analysis of phosphorus diffusion gettering and confirmed the dependencies of the electron lifetime on the various gettering conditions. It was observed that iron atoms in the Si wafer could be more efficiently gettered if the temperatures were provided to the wafer in two different steps. That two-step gettering process was applied to an upgraded-metallurgical grade (UMG) Si wafer, and the electron lifetimes of the UMG-Si wafer were 3 μsec by applying the second temperature profile at 600°C for 420 min. It was also confirmed that the efficiency of the UMG-Si solar cell increased 0.53% due to two-step gettering process.

Engineering metal precipitate size distributions to enhance gettering in multicrystalline silicon

AbstractThe extraction of metal impurities during phosphorus diffusion gettering (PDG) is one of the crucial process steps when fabricating high‐efficiency solar cells using low‐cost, lower‐purity silicon wafers. In this work, we show that for a given metal concentration, the size and density of metal silicide precipitates strongly influences the gettering efficacy. Different precipitate size distributions can be already found in silicon wafers grown by different techniques. In our experiment, however, the as‐grown distribution of precipitated metals in multicrystalline Si sister wafers is engineered through different annealing treatments in order to control for the concentration and distribution of other defects. A high density of small precipitates is formed during a homogenization step, and a lower density of larger precipitates is formed during extended annealing at 740 °C. After PDG, homogenized samples show a decreased interstitial iron concentration compared to as‐grown and ripened samples, in agreement with simulations.

Modeling of Boron and Phosphorus Diffusion Gettering of Iron in Silicon

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Gettering of metal impurities by using phosphorus diffusion in UMG silicon wafers

Upgraded metallurgical grade (UMG) silicon is a cost-effective and energy-efficient silicon material for the production of solar cells. UMG silicon wafers with 5N purity include various kinds of metal impurities like Fe, Ni, Cu, and Co, which may limit the electrical performance of UMG silicon wafers and solar cells. Phosphorus diffusion gettering, which can effectively reduce the transition-metal impurities in the bulk of UMG silicon wafers and enhance the minority carrier lifetime (MCLT), is a well-known process to improve the performance of solar cells in the photovoltaic (PV) industry. In this study, phosphorus diffusion gettering was used at a constant temperature of 700 for 5 hours, and the effects of the gettering process were confirmed by measuring the MCLTs and the efficiencies of the solar cells. Depth profiles of the Fe concentration before and after the gettering process were compared using secondary ion mass spectroscopy (SIMS) measurements.

Impact of phosphorus gettering parameters and initial iron level on silicon solar cell properties

ABSTRACTWe have studied experimentally the effect of different initial iron contamination levels on the electrical device properties of p‐type Czochralski‐silicon solar cells. By systematically varying phosphorus diffusion gettering (PDG) parameters, we demonstrate a strong correlation between the open‐circuit voltage (Voc) and the gettering efficiency. Similar correlation is also obtained for the short‐circuit current (Jsc), but phosphorus dependency somewhat complicates the interpretation: the higher the phosphorus content not only the better the gettering efficiency but also the stronger the emitter recombination. With initial bulk iron concentration as high as 2 × 1014 cm−3, conversion efficiencies comparable with non‐contaminated cells were obtained, which demonstrates the enormous potential of PDG. The results also clearly reveal the importance of well‐designed PDG: to achieve best results, the gettering parameters used for high purity silicon should be chosen differently as compared with for a material with high impurity content. Finally we discuss the possibility of achieving efficient gettering without deteriorating the emitter performance by combining a selective emitter with a PDG treatment. Copyright © 2012 John Wiley & Sons, Ltd.

Chromium distribution in multicrystalline silicon: comparison of simulations and experiments

ABSTRACTWe studied the precipitation of chromium in multicrystalline silicon during the crystallization process and temperature treatments typical for solar cell processing. A model which was already successfully used for simulating heterogeneous precipitation of iron is transferred to chromium, allowing two‐dimensional simulations of dissolved chromium and precipitate density. The observed accordance with spatially resolved measurements demonstrates the similarity of chromium to iron precipitation and the ability of our model to predict the dissolved chromium concentration in multicrystalline silicon. After the crystallization process, a high impact of chromium on the carrier lifetime of wafers originating from an ingot intentionally contaminated with 20 ppma chromium in the melt was observed. The concentration of dissolved chromium was significantly reduced by phosphorus diffusion gettering or oxidation at 815°C. Copyright © 2012 John Wiley & Sons, Ltd.

Effect of extended phosphorus diffusion gettering on chromium impurity in HEM multicrystalline silicon

We have investigated the extended phosphorus diffusion gettering (PDG) effect on chromium impurities (Cr) in p-type multicrystalline silicon (mc-Si) grown by Heat Exchanger Method (HEM). The study was made after phosphorous diffusion and according to different extended annealing temperatures. The secondary ion mass spectrometry (SIMS) analysis revealed a significant accumulation of 52Cr in heavily phosphorus doped (HPD) region. Using quasi-steady state photoconductance (QSSPC) technique, the apparent lifetime dependent minority carrier density curves have been obtained. The results showed an increment of the bulk minority carrier lifetime for specific annealing temperatures. Appropriate calculations based on QSSPC results allowed us to determine the lifetime curves associated to gettered impurities. Their fitting by Shockley-Read-Hall (SRH) model reveal that the origin of the lifetime increment is the reduction of interstitial chromium (Cri) density in the bulk. Furthermore, the estimation of electron to hole capture cross-section ratio (k=σn/σp) through the modelling of apparent lifetime curves using Hornbeck–Haynes model, confirmed the effectiveness of Cri gettering and identified the nature of dominant recombination centres after gettering process.

Evaluation of possible mechanisms behind P gettering of iron

Several possible mechanisms for phosphorus diffusion gettering (PDG) in silicon (Si) are evaluated. Float Zone (FZ) monocrystalline Si samples were intentionally contaminated with iron by ion implantation followed by a 1 h anneal at 900 °C to achieve a homogeneous iron distribution. Phosphorus gettering was then performed at different combinations of time and temperature. Depth-versus-concentration profiles of Fe, P, and O were measured by SIMS. The depth interval of Fe accumulation was found to be independent of the extension of the P profiles, unlike predictions from modeling of FexPy complex formation. The capture capability of P does not play an important role in PDG, still decreasing the P concentration below 6% in the surface source cause poor gettering efficiency. Hence accumulation of Fe close to the surface hinges on the presence of P, which is likely to be due to generation of vacancies during in-diffusion of high concentrations of P. The vacancies cause localized precipitation of oxygen in the highly P doped region. These oxygen precipitates act as gettering centers for Fe, as substantiated by a close correlation between the measured depth profiles of O and Fe after PDG. Our results suggest that interactions between oxygen, vacancies and metal impurities are the most crucial factors in PDG.

Phosphorus and boron diffusion gettering of iron in monocrystalline silicon

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.

Direct comparison of boron, phosphorus, and aluminum gettering of iron in crystalline silicon

This paper presents a direct quantitative comparison of the effectiveness of boron diffusion, phosphorus diffusion, and aluminum alloying in removing interstitial iron in crystalline silicon in the context of silicon solar cells. Phosphorus diffusion gettering was effective in removing more than 90% of the interstitial iron across a range of diffusion temperatures, sheet resistances, and iron doses. Even relatively light phosphorus diffusions (145 Ω/□) were found to give very effective gettering, especially when combined with extended low temperature annealing. Aluminum alloying was extremely effective and removed more than 99% of the implanted iron for a range of alloying temperatures and aluminum film thicknesses. In contrast, our experimental results showed that boron diffusion gettering is very sensitive to the deposition conditions and can change from less than 5% of the Fe being gettered to more than 99.9% gettered by changing only the gas flow ratios and the post-oxidation step.

Understanding the distribution of iron in multicrystalline silicon after emitter formation: Theoretical model and experiments

We studied the behavior of iron in multicrystalline silicon during phosphorus diffusion by spatially resolved measurements and physical modeling. We present improvements to the previously used models for internal gettering in multicrystalline silicon and phosphorus diffusion gettering. 2-dimensional simulations are used for optimization of the phosphorus diffusion processes for intentionally contaminated wafers regarding the iron distribution, without changing the emitter characteristics. Simulations and experimental results show a reduced interstitial iron concentration after an additional low temperature step at the end of the phosphorus diffusion. The concentration of iron precipitates was reduced by a short annealing at 900°C before the phosphorus diffusion, leading to a carrier lifetime three times higher than compared to the standard process.

Analysis of simultaneous boron and phosphorus diffusion gettering in silicon

AbstractWe studied the simultaneous phosphorus and boron diffusion gettering (BDG) of iron in silicon by physical modeling. We present improvements to the previously used models for BDG and phosphorus diffusion gettering (PDG). We show that the improved model is suitable for analyzing the gettering efficiency in advanced solar cell structures with phosphorus doped (n+) and boron doped (p+) layers by comparing simulations with experimental results. The simulations indicate that during a low temperature step with high iron concentration, an additional p+ layer reduces the time needed for an effective gettering, whereas under low supersaturation segregation to the n+ layer is the dominating gettering mechanism.

Phosphorus gettering of precipitated Cu in single crystalline silicon based on rapid thermal process

In this paper, we have investigated the effect of phosphorus diffusion gettering on the precipitated Cu in silicon via rapid thermal process (RTP). It is found that, for dot-like or star-like precipitates, the RTP-based phosphorus diffusion technique is efficient for gettering out the precipitated Cu. A two-step RTP gettering process is much more effective than a single-step RTP process. Furthermore, the choice of oxygen ambient can enhance the Cu gettering efficiency due to the involvement of considerable self-interstitial silicon atoms. The minority carrier lifetime of the sample subjected to RTP-based phosphorus gettering has also been verified to be significantly enhanced. These results are of interest for the gettering engineering of high-efficiency silicon solar cells.

Application of phosphorus diffusion gettering process on upgraded metallurgical grade Si wafers and solar cells

Although phosphorus (P) diffusion gettering process has been wildly used to improve the performance of Si solar cells in photovoltaic technology, it is a new attempt to apply P diffusion gettering process to upgraded metallurgical grade silicon (UMG-Si) wafers with the purity of 99.999%. In this paper, improvements on the electrical properties of UMG-Si wafers and solar cells were investigated with the application of P diffusion gettering process. To enhance the improvements, the gettering parameters were optimized on the aspects of gettering temperature, gettering duration and POCl 3 flow rate, respectively. As we expected, the electrical properties of both multicrystalline Si (multi-Si) and monocrystalline Si (mono-Si) wafers were significantly improved. The average minority carrier lifetime increased from 0.35 μs to nearly about 2.7 μs for multi-Si wafers and from 4.21 μs to 5.75 μs for mono-Si wafers, respectively. Accordingly, the average conversion efficiency of the UMG-Si solar cells increased from 5.69% to 7.03% for multi-Si solar cells (without surface texturization) and from 13.55% to 14.55% for mono-Si solar cells, respectively. The impurity concentrations of as-grown and P-gettered UMG-Si wafers were determined quantitively so that the mechanism of P diffusion gettering process on UMG-Si wafers and solar cells could be further understood. The results show that application of P diffusion gettering process has a great potential to improve the electrical properties of UMG-Si wafers and thus the conversion efficiencies of UMG-Si solar cells.

Physical mechanisms of boron diffusion gettering of iron in silicon

AbstractWe have studied the boron diffusion gettering (BDG) of iron in single crystalline silicon. The results show that iron is gettered efficiently by electrically inactive boron, which leads to gettering efficiencies comparable to phosphorus diffusion gettering (PDG). In addition we discuss the different physical mechanisms behind BDG. We also consider the possibilities of using boron diffusion gettering in solar cell fabrication and discuss the role of boron and iron concentration in the optimization of gettering efficiency. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of performance limiting material properties of multicrystalline silicon

We present progress of the analysis of multicrystalline silicon and of improvements of our understanding of material related performance limits of solar cells. Aspects covered are advanced numerical simulation, experimental results on carrier lifetime distributions, detailed analysis of the defect luminescence spectrum, and findings about breakdown behaviour. Modelling of impurity redistribution, specifically iron, with respect to high temperature steps enlightens mechanisms governing the phosphorus diffusion and aluminium gettering in heterogeneous systems. Evidence of outdiffusion of metallic impurities from grain boundaries, after high temperature steps, is given by analysing spatially resolved carrier lifetime measurements. In addition to lifetime, we present defect luminescence spectra measured by photoluminescence spectroscopy, including an interpretation of the origin of a remarkably temperature stable emission line at 838 meV. Finally, defect luminescence is connected to the origin of pre-breakdown of solar cells at reverse bias, a topic which is of high interest especially in context with the increased usage of purified metallurgical grade silicon.

Comparison of Efficiency and Kinetics of Phosphorus-Diffusion and Aluminum Gettering of Metal Impurities in Silicon: a Simulation study

The efficiency of solar cells produced from crystalline silicon materials is considerably affected by the presence of metal impurities. In order to reduce the concentration of metal impurities, gettering processes as phosphorus diffusion gettering (PDG) and aluminum gettering (AlG) are routinely included in solar cell processing. Further development and optimization of gettering schemes has to ground on physics-based simulations of gettering processes. In this contribution we use quantitative simulations to compare the efficiency and kinetics of PDG and AlG in the presence of precipitates for interstitially dissolved metals, like iron, at different gettering conditions. Recently measured segregation coefficients of iron in liquid AlSi with respect to crystalline silicon are used in order to compare with PDG under typical conditions. It is shown that kinetics of both, PDG and AlG, can be separated into two regimes: (i) at low temperatures kinetics are limited by precipitate dissolution, and (ii) at high temperatures kinetics of AlG is mainly limited by metal impurity diffusion while phosphorus in-diffusion is the limiting factor of PDG.

Analysis of Heterogeneous Iron Precipitation in Multicrystalline Silicon

Iron precipitation in multicrystalline silicon has been modeled aiming at the optimization of intrinsic gettering of iron in multicrystalline silicon. Iron precipitation during both crystal growth and following phosphorus diffusion gettering (PDG) are simulated and compared to experimental results as the iron precipitate density after these processes is essential in the modeling of intrinsic gettering in multicrystalline silicon solar cell processing. The PDG decreases the density of iron precipitates compared to the as-grown state and as expected the effect is larger at lower initial iron concentrations. Due to this effect the iron precipitation is significantly reduced almost throughout the whole ingot height and it can be concluded that intrinsic gettering has a beneficial effect only in the case of high initial iron concentration, in accordance with the experimental results. The simulated change in interstitial iron concentration as a function of intrinsic gettering temperature suggests the same optimum intrinsic gettering temperature as the experiments. With the given model it is however much easier to find optimal parameters compared to expensive and time consuming experiments.

Structure, chemistry and electrical properties of extended defects in crystalline silicon for photovoltaics

AbstractThe electronic properties of present‐day multicrystalline silicon (mc‐Si) materials for photovoltaic applications are strongly influenced by point defects, their mutual interaction and their interaction with dislocations and grain boundaries. This paper presents results from fundamental investigations of metal impurity interaction with extended defects, namely a small‐angle grain boundary and bulk microdefects. It is shown that the distribution of copper silicide precipitates closely follows the density of bulk microdefects indicating the underlying physics of ‘good’ and ‘bad’ grains frequently observed in mc‐Si. Co‐precipitation of copper and nickel in the same samples leads to virtually the same distribution of multimetal silicide precipitates which according to light‐beam induced current measurements show the same recombination activity as single‐metal silicide particles. Transmission electron microscopy is used to show that for copper‐rich and nickel‐rich conditions two types of silicides co‐exist, i.e. Cu3Si precipitates containing a small amount of nickel and NiSi2 precipitates containing some copper. Finally, phosphorus‐diffusion gettering (PDG) is discussed as the main gettering process used in presentday silicon photovoltaics. Special emphasis is put on the effect of extended defects and their interaction with metal impurities on PDG kinetics. It is shown that different limiting processes will be simultaneously operative in mc‐Si as a result of inhomogeneous bulk defect distributions (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)