Phosphorus Gettering Research Articles

Enhanced phosphorus gettering of impurities in p-type Czochralski silicon through a variable temperature processing (VTP)

This paper investigates the effect of heat treatment with a phosphorus-rich porous silicon layer on the electrical properties Czochralski (CZ), p-type monocrystalline silicon (c-Si) wafer. Phosphorus diffusion into porous silicon layer can play a crucial role for gettering metallic impurities. Further, the variable temperature process (VTP) was presented to provide higher gettering efficiency than the constant temperature process (CTP). We estimated with a Quasi-Steady-State Photo-Conductance (QSSPC) technique the effective minority carrier lifetime values τeff. The Light-Beam-Induced-Current measurements (LBIC) have been executed to estimate the diffusion length (LD). In the I–V characteristics, we observed a significant increase in Voc and Jsc. Particularly; we can say that the VTP may be required to provide the highest efficiency in devices.

Quantifying carrier recombination at grain boundaries in multicrystalline silicon wafers through photoluminescence imaging

We present a method based on steady state photoluminescence (PL) imaging and modelling of the PL intensity profile across a grain boundary (GB) using 2D finite element analysis, to quantify the recombination strength of a GB in terms of the effective surface recombination velocity (Seff). This quantity is a more meaningful and absolute measure of the recombination activity of a GB compared to the commonly used signal contrast, which can strongly depend on other sample parameters, such as the intra-grain bulk lifetime. The method also allows the injection dependence of the Seff of a given GB to be explicitly determined. The method is particularly useful for studying the responses of GBs to different cell processing steps, such as phosphorus gettering and hydrogenation. The method is demonstrated on double-side passivated multicrystalline wafers, both before and after gettering, and single-side passivated wafers with a strongly non-uniform carrier density profile depth-wise. Good agreement is found between the measured PL profile and the simulated PL profile for both cases. We demonstrate that single-side passivated wafers allow more recombination active grain boundaries to be analysed with less unwanted influence from nearby features. The sensitivity limits and other practical constraints of the method are also discussed.

Efficacy of Phosphorus Gettering and Hydrogenation in Multicrystalline Silicon

The emitter formation step (POCl3 diffusion) in p-type crystalline silicon solar cell processing includes many variables, e.g., peak temperature, gas flows, temperature ramps, which can be optimized in order to improve material quality. Diffusion parameters of an 80-Ω/L1 emitter are varied, and the resulting change in electronic quality of multicrystalline silicon is analyzed. A detailed gettering analysis of multicrystalline material, surface passivated with hydrogen-rich amorphous silicon, after POCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> diffusion, and an additional gettering step combined with hydrogenation from SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H is presented. The industrial-type diffusion leads to material of lower electronic quality than the extended reference diffusion. A major finding of this paper is the fact that results on different 5 × 5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> samples out of one 15.6 × 15.6 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> wafer can vary significantly. Hence, conclusions about which diffusion is most efficient in gettering strongly depend on wafer position. An edge position close to crucible walls, for example, might improve less effectively than another position close to the crucible center. In fact, the opposite can also be shown, and samples originating from edge regions reach their highest lifetimes after gettering. This is explained by the different defect structure of the investigated samples. Structures exhibiting high gettering efficacy contain fewer recombination active grain boundaries and are predominantly free of extended defect clusters.

Enhanced Phosphorus Gettering of Impurities in Multicrystalline Silicon at Low Temperature

We demonstrate the enhancements of phosphorus (P) gettering efficiency to remove impurities in multicrystalline silicon (mc-Si) using multiple cycles of annealing and cooling technique at low temperature. The gettering effect was evaluated by photoluminescence (PL) imaging and microwave photoconductance decay (micro-PCD) measurement. Optimal low temperature annealing was pursued to suppress the defects formation during annealing. Multiple cycles of annealing and cooling technique was utilized to enhance the P gettering efficiency by collecting the dissolved impurities at P-diffused layer. The results show that the proposed annealing technique gives a significantly greater enhancement of P gettering than continuous annealing, particularly in regions of low defect density. This could possibly be explained by the increased collections of dissolved impurities both of remaining impurity atoms and frozen precipitates. The best gettering parameter was achieved at 400°C for 35min of annealing time. These results further confirmed the benefit of P gettering using multiple cycles of annealing and cooling technique in order to improve the electrical properties and yield of mc-Si for solar cells.

Dependence of phosphorus gettering and hydrogen passivation efficacy on grain boundary type in multicrystalline silicon

This investigation analyzes the dependency of minority charge carrier lifetime values at grain boundaries in multicrystalline silicon on the grain boundary type after P gettering and/or firing of SiNx:H layers deposited by plasma enhanced chemical vapor deposition. To get a broad statistics, a new method to determine the coincidence site lattice grain boundary types on large scale throughout entire 50 × 50 mm2 samples is combined with spatially resolved lifetime-calibrated photoluminescence measurements and mappings of the interstitial iron concentration. As an evaluation of the lifetime data at grain boundaries in comparison to the recombination activity of the bordering grains, lifetime contrast values are calculated. The correlation of this dependency on the grain boundary type with the impurity concentration is analyzed by the investigation of multicrystalline samples from two different ingots grown by directional solidification with different crucible material qualities. A dependency of the efficacy of all applied processes on the grain boundary type is shown based on broad statistics—higher coincidence site lattice indexes correlate with a decrease of median lifetime values after all processes. Hydrogenation of both grains and grain boundaries is found to be more effective in cleaner samples. Extended getter sinks, as a P emitter, are also beneficial to the efficacy of hydrogenation. The lifetime contrast values are dependent on the degree of contamination of the multicrystalline silicon material. In cleaner samples, they rather decrease after the processes; in standard solar-grade material, they increase after POCl3 diffusion and decrease again after subsequent hydrogenation. No correlation with the interstitial iron concentration is found.

Improvements to the solar cell efficiency and production yields of low-lifetime wafers with effective phosphorus gettering

This research focuses on the improvement of solar cell efficiencies in low-lifetime wafers by implementing an appropriate gettering method of the diffusion process. The study also considers a reduction in the value of the reverse current at −12 V, an important electrical parameter related to the hot-spot heating of solar cells and modules, to improve the product's quality during commercial mass production. A practical solar cell production case study is examined to illustrate the use of the proposed method. The results of this case study indicate that variable-temperature gettering significantly improves solar cell efficiencies by 0.14% compared to constant-temperature methods when the wafer quality is poor. Moreover, this study finds that variable-temperature gettering raises production yields of low quality wafers by more than 30% by restraining the measurement value of the reverse current at −12 V during solar cell manufacturing.

Phosphorus diffusion gettering of substitutional metal contaminants at a small-angle grain boundary in n-type silicon: The case of gold

In this paper, we have studied the effect of phosphorus diffusion gettering on the Au impurities at a model small-angle grain boundary (GB) in n-type silicon by combining spreading resistive profiling and current–voltage characteristic techniques. It is found that at a low level contamination, a single-step phosphorus gettering can be effective for most of Au impurities at the GB by extending the diffusion time. However, at a high level contamination, a two-step phosphorus gettering should be utilized to getter out the gold impurities at the GB. After the effective phosphorus gettering for Au impurities, the GB electrical characteristics can be recovered to the original level.

Classification of Defective Regions in p-type Multicrystalline Silicon by Comparing Luminescence Images Measured under Different Conditions

In this contribution, we apply three different camera-based luminescence imaging techniques to mc-Si wafers and solar cells, fabricated on neighboring wafers. On wafer level, we determine the spatially-resolved carrier lifetime using calibrated photoluminescence lifetime imaging. On the solar cell level, we use band-to-band electroluminescence and sub-band-gap electroluminescence imaging for the characterisation. We analyze the differences obtained by the different techniques in specific defective areas. Characteristic regions are additionally examined using deep-level transient spectroscopy (DLTS). Comparing different luminescence images, we find different signal correlations in selected regions of the wafers and the neighboring cells presumably caused by different types of defects, which react more or less effective on the phosphorus gettering during the solar cell process. DLTS spectra show that in the edge region of the wafer close to the crucible, FeB pairs are present in the wafer as well as in the cell. However, the FeB concentration in the cell is, due to phosphorus gettering during the cell process, reduced by one order of magnitude. In regions which appear as very recombination-active defect clusters in the solar cell, we detect ZnB pairs by DLTS analysis. Note that the ZnB itself is a shallow centre and therefore expected to be not strong recombination active. However, our measurements reveal that Zn is present in regions with increased recombination activity, which is also in good agreement with the high total Zn concentration measured in the mc-Si ingot. We hence conjecture that dislocation clusters decorated by Zn are responsible for the non-getterable defect regions.

Predicting Solar Cell Efficiencies from Bulk Lifetime Images of Multicrystalline Silicon Bricks

We present a model for predicting the solar cell efficiency potential of multicrystalline silicon bricks prior to sawing. Three model inputs are required: bulk lifetime images from the side faces of the bricks, the cell manufacturing process, and its gettering action. The model is set up with numerical device and circuit simulations, but may afterwards be parameterized for inline application. In the example shown here, we chose literature data to quantify the increase in bulk lifetime caused by phosphorus gettering of impurities during cell manufacturing. Our proposed model enables manufacturers to (i) assess initial brick quality in relation to their specific cell production line, (ii) to exclude certain parts of the bricks from cell manufacturing, and (iii) to adjust cell manufacturing to initial material quality. The specific gettering efficiency and cell process can be fed into the model dynamically and need to be calibrated ideally for each material manufacturer and each cell production line. The model presented here can be extended to cast mono and dendritically grown bricks.

Iron-rich particles in heavily contaminated multicrystalline silicon wafers and their response to phosphorus gettering

Heavily contaminated multicrystalline silicon wafers have been studied using a scanning synchrotron x-ray fluorescence micro-probe, revealing the presence of iron-rich particles located along grain boundaries. These particles were found to be partially dissolved and removed during phosphorus gettering treatments at 900 or 1000 °C for times of up to 100 min, with increased gettering efficiency at higher temperatures and times. Annealing without the phosphorus gettering layer present at the wafer surfaces resulted in no discernible reduction in particle coverage, showing that the gettering layer is essential in such heavily contaminated wafers, in order to reduce the dissolved Fe concentration below the solubility limit, and therefore allow dissolution to proceed.

Defect engineering during the contact co‐firing step in an industrial belt furnace

AbstractThis work implements an optimization of the phosphorus gettering effect during the contact co‐firing step by means of both simulations and experiments in an industrial belt furnace. An optimized temperature profile, named ‘extended co‐firing step’, is presented. Simulations show that the effect of the short annealing on the final interstitial iron concentration depends strongly on the initial contamination level of the material and that the ‘extended co‐firing’ temperature profile can enhance the gettering effect within a small additional time. Experimental results using sister wafers from the same multicrystalline silicon ingot confirm these trends and show the potential of this new defect engineering tool to improve the solar cell efficiency (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Influences of Phosphorous Gettering on Minority Carrier Lifetime Distribution of Polycrystalline Silicon Wafer

The influences of the phosphorus gettering in PN junction formation on the distribution of minority carrier lifetime of polycrystalline silicon wafer in solar cell production process were studied. The experimental results shows that the distribution of internal defects and impurities in polycrystalline silicon wafer significantly impact on the effectiveness of the phosphorus gettering during preparation of PN junction through the phosphorus diffusion technology. Especially when the impurities were transition metal elements, it caused a large difference of the phosphorus gettering effectiveness in PN junction preparation, which was due to the presence and interaction of defects and impurities on polycrystalline silicon wafer, such as grain boundaries, dislocations, oxygen and carbon, and other metal elements. In addition, the uneven distribution of impurities and defects of also influenced the phosphorus gettering effectiveness.

Phosphorus gettering of iron by screen‐printed emitters in monocrystalline Czochralski silicon wafers

ABSTRACTIn this paper, we demonstrate single‐sided screen‐printed emitters in thin monocrystalline Czochralski silicon (Cz‐Si) wafers with an improved gettering of iron compared with conventional double‐sided POCl3 emitters. The phosphorus dopant pastes used have to be chosen carefully to provide a sufficiently low emitter sheet resistance and to avoid iron contamination. The iron concentration is determined in a non‐destructive way from the minority carrier lifetime obtained by quasi‐steady‐state photoconductance measurements, down to levels not yet demonstrated for screen‐printed emitters. In addition, the well‐known metastable boron–oxygen complexes in Cz‐Si have been transferred into a stable state by light‐induced degradation prior to these measurements. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Gettering improvements of minority-carrier lifetimes in solar grade silicon

The minority-carrier lifetime in p-type solar-grade silicon (SoG-Si) produced by Elkem Solar was investigated after different types of heat treatment. Two groups of samples differing by the as-grown lifetimes were exposed to internal and phosphorus gettering using constant and variable temperature processes. Optimal heat-treatment parameters for each group of samples were then identified which improved the minority-carrier lifetimes to values higher than the minimum value needed for solar cells. Phosphorus gettering using a variable temperature process enhanced in particular the lifetime within each group, increasing in certain cases the lifetime from 3 up to 81μs.

Effect of the Phosphorus Gettering on Si Heterojunction Solar Cells

To improve the efficiency of crystalline silicon solar cells, should be collected the excess carrier as much as possible. Therefore, minimizing the recombination both at the bulk and surface regions is important. Impurities make recombination sites and they are the major reason for recombination. Phosphorus (P) gettering was introduced to reduce metal impurities in the bulk region of Si wafers and then to improve the efficiency of Si heterojunction solar cells fabricated on the wafers. Resistivity of wafers was measured by a four-point probe method. Fill factor of solar cells was measured by a solar simulator. Saturation current and ideality factor were calculated from a dark current density-voltage graph. External quantum efficiency was analyzed to assess the effect of P gettering on the performance of solar cells. Minority bulk lifetime measured by microwave photoconductance decay increases from 368.3 to 660.8 μs. Open-circuit voltage and short-circuit current density increase from 577 to 598 mV and 27.8 to 29.8 mA/cm2, respectively. The efficiency of solar cells increases from 11.9 to 13.4%. P gettering will be feasible to improve the efficiency of Si heterojunction solar cells fabricated on P-doped Si wafers.

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.

Improving the Quality of the Deteriorated Regions of Multicrystalline Silicon Ingots during General Solar Cell Processes

The behavior of wafers and solar cells from the border of a multicrystalline silicon (mc-Si) ingot, which contain deteriorated regions, is investigated. It is found that the diffusion length distribution of minority carriers in the cells is uniform, and high efficiency of the solar cells (about 16%) is achieved. It is considered that the quality of the deteriorated regions could be improved to be similar to that of adjacent regions. Moreover, it is indicated that during general solar cell fabrication, phosphorus gettering and hydrogen passivation could significantly improve the quality of deteriorated regions, while aluminum gettering by RTP could not. Therefore, it is suggested that the border of a mc-Si ingot could be used to fabricate high efficiency solar cells, which will increase mc-Si utilization effectively.

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.

Properties of Multicrystalline Silicon Wafers Based on UMG Material

In this paper, we have studied various properties of UMG silicon wafers by combining four-point probe, microwave photo-conductance decay (μ-PCD), Fourier transform infrared (FTIR) spectroscopy,Inductively coupled plasma-mass spectrometry (ICP-MS) and optical microscopy techniques. The results show that the resistivity and the minority-carrier lifetime of UMG silicon are lower than that of standard multicrystalline material, while the detrimental interstitial iron concentration is larger. The concentration of substitutional carbon, interstitial oxygen and dislocation density are close to that in the standard multi-crystalline silicon. Furthermore, a phosphorous gettering has been used to improve the quality of UMG samples. It is found that phosphorus gettering in UMG silicon can increase the minority carrier lifetime and reduce the interstitial iron concentration. But, the minority-carrier lifetime is still not as high as the conventional/standard silicon counterpart after gettering. These results will help us to better understand the properties of UMG silicon wafer.

Precipitation and gettering behaviors of copper in multicrystalline silicon used for solar cells

The precipitation and gettering behaviors of copper (Cu) at different defective regions in multicrystalline silicon were investigated by combining scanning infrared microscopy, optical microscopy, inductively coupled plasma mass spectrometry and microwave photo-conductance decay. It is found that the behaviors of Cu precipitation are strongly dependent on the defect density. Most of the Cu contaminants tend to form precipitates homogeneously in the low density defect region, while they mostly segregate at the defects and form precipitates heterogeneously in the high density defect region. In the case of heavy contamination, the Cu precipitate can significantly reduce the carrier lifetime of multicrystalline silicon due to their Schottkydiode behavior in the silicon substrate. A 900 °C rap thermal process (RTP) phosphorus gettering anneal cannot be sufficiently effective to remove the Cu precipitates in these two regions.