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

This paper presents a direct quantitative comparison of the effectiveness of borondiffusion, phosphorus diffusion, and aluminumalloying 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.Aluminumalloying 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 borondiffusion 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

Gettering of Iron in Silicon Solar Cells With Implanted Emitters

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.Peer reviewe

Significant minority carrier lifetime improvement in red edge zone in n-type multicrystalline silicon

We have carried out experiments on both boron diffusion gettering (BDG) and phosphorus diffusion gettering (PDG) in n-type multicrystalline silicon. We have focused our research on the highly contaminated edge areas of the silicon ingot often referred to as the red zone. Due to poor carrier lifetime attributed to these areas, they induce a significant material loss in solar cell manufacturing. In our experiments, the red zone was found to disappear after a specific BDG treatment and a lifetime improvement from 5 μs up to 670 μs was achieved. Outside the red zone, lifetimes even up to 850 μs were measured after gettering. Against the common hypothesis, we found higher dopant in-diffusion temperature beneficial both for the red zone and the good grains making BDG more efficient than PDG. To explain the results we suggest that high temperature leads to more complete dissolution of metal precipitates, which enhances the diffusion gettering to the emitter.Peer reviewe

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

We 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.Peer reviewe

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.Peer reviewe

Increasing minority carrier lifetime in as-grown multicrystalline silicon by low temperature internal gettering

We report a systematic study into the effects of long low temperature (≤500 °C) annealing on the lifetime and interstitial iron distributions in as-grown multicrystalline silicon (mc-Si) from different ingot height positions. Samples are characterised in terms of dislocation density, and lifetime and interstitial iron concentration measurements are made at every stage using a temporary room temperature iodine-ethanol surface passivation scheme. Our measurement procedure allows these properties to be monitored during processing in a pseudo in situ way. Sufficient annealing at 300 °C and 400 °C increases lifetime in all cases studied, and annealing at 500 °C was only found to improve relatively poor wafers from the top and bottom of the block. We demonstrate that lifetime in poor as-grown wafers can be improved substantially by a low cost process in the absence of any bulk passivation which might result from a dielectric surface film. Substantial improvements are found in bottom wafers, for which annealing at 400 °C for 35 h increases lifetime from 5.5 μs to 38.7 μs. The lifetime of top wafers is improved from 12.1 μs to 23.8 μs under the same conditions. A correlation between interstitial iron concentration reduction and lifetime improvement is found in these cases. Surprisingly, although the interstitial iron concentration exceeds the expected solubility values, low temperature annealing seems to result in an initial increase in interstitial iron concentration, and any subsequent decay is a complex process driven not only by diffusion of interstitial iron

Iron gettering in silicon using doped layers and bulk defects

The removal of iron impurities to desired regions in silicon wafers has been studied using phosphorus and boron doped layers and bulk defects as gettering sites. Techniques to remove metal impurities, so-called gettering techniques, are needed for improving the performance of both the microelectronic and photovoltaic silicon devices, although the desired location of impurities may be different in various applications. In this work, both separate and simultaneous influences of the doped layers and bulk defects on the gettering behaviour of iron, e.g. the gettering efficiency and gettering mechanisms, were investigated. The phosphorus diffusion gettering studies at low temperatures enabled the determination of a more accurate segregation coefficient for iron between a phosphorus diffused layer and bulk silicon. Comparison between the phosphorus diffusion gettering experiments and similar experiments with boron showed that boron diffusion gettering can in some cases be nearly as effective as the phosphorus diffusion gettering. The gettering studies with implanted boron layers revealed that the gettering occurs also by precipitation, not only by segregation. Competitive gettering between an implanted boron layer and bulk defects was investigated using specially designed gettering anneals. It was found that depending on the desired location of iron in silicon wafers in different applications, iron can be collected either to the doped layers or the bulk defects. The gettering anneals were also applied to a microelectronic device process and their effect on the electronic device parameters was evaluated. These results contribute to the understanding of iron behaviour in silicon. Thus, they can help when designing the gettering anneals both for microelectronic and photovoltaic fabrication processes

Gettering of interstitial iron in silicon by plasma enhanced chemical vapour deposited silicon nitride films

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 250o C to 900o C, via a segregation gettering effect. Further experimental evidence is presented to support this finding. Deep-level transient spectroscopy (DLTS) 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 of 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 700o C. The gettering process is found to become reaction-limited at higher temperatures

Combining low-temperature gettering with phosphorus diffusion gettering for improved multicrystalline silicon

We have investigated low-temperature (≤500 °C) gettering in combination with phosphorus diffusion gettering with a view to improving poor quality multicrystalline silicon. Low-temperature gettering applied after standard phosphorus diffusion gettering is found to provide a >40% improvement in minority carrier lifetime in samples from the top and bottom of an ingot. The best results are achieved at 300 °C with very long annealing times (>24 h). Improvements in the lifetime do not correlate with changes in interstitial iron concentration. Experiments are performed to assess whether the presence of a phosphorus-diffused emitter affects low-temperature gettering, and results from sister samples show the low-temperature gettering behavior is not affected by the existence of an emitter. Further experiments show that low-temperature gettering prior to phosphorus diffusion results in a 20% higher lifetime after phosphorus diffusion. Low-temperature gettering can, therefore, enhance lifetime even when used in conjunction with standard phosphorus diffusion gettering

Metal Impurity Redistribution in Crystalline Silicon for Photovoltaic Application

Multicrystalline silicon, which is among the most common materials for solar cells [3], contains extended defects like grain boundaries and dislocations and a high amount of metal impurities potentially reducing the solar cell efficiency [1,7]. In order to reduce the detrimental effect of metal impurities on the efficiency of multicrystalline solar cells, the metal impurities can be redistributed into electrical inactive areas of the solar cell (gettering) [12] or at few large accumulation sites, e.g. at grain boundaries [16]. In this work the interaction of metal impurities with extended defects is investigated for the purpose of a better understanding of the underlying physical mechanisms, which is necessary to effectively redistribute the metal impurities. Experimental investigations on the atomistic scale with a combination of in-situ EBIC/FIB and TEM are preformed on multicrystalline silicon samples intentionally contaminated with copper and iron. The combination method was developed in this work to investigate the distribution, the atomic structure and the chemical nature of selected extended recombination active defects at high resolution, but low necessary defect density [43,71]. Accumulation of copper at light elements like nitrogen and oxygen located at grain boundaries, and dislocation networks decorated with copper are identified to be the origin of recombination-active defects. Simulations on the wafer scale of the redistribution of the iron concentration during temperature treatments and gettering processes in presence of grain boundaries are performed. Simulations of gettering processes show that kinetics of phosphorus diffusion gettering and aluminium gettering are limited by the dissolution of precipitates at a low temperature regime. For high gettering temperatures aluminum gettering has the advantage of being only limited by the thermodynamic conditions, while phosphorus diffusion gettering is limited by the phosphorus indiffusion. A comparison of simulations of the redistribution of iron during temperature treatments in presence of a grain boundary with experimental LBIC and PL measurements and simulations investigating the influence of segregation and precipitation as mechanisms of impurity accumulation at grain boundaries show that quantitative modeling with precipitation as mechanism of impurity accumulation at grain boundaries is possible, while segregation has only minor effects. The simulations illustrate that grain boundaries can serve as sinks or sources for metal impurities depending on the temperature treatments. A variation of grain size in the simulations imply that there is an optimal grain size in the range of two times the diffusional range to achieve optimal diffusion lengths in solar cells. The simulations show that for some temperature treatments, it is important to take the history of temperature treatments of the sample into account. The comparison of simulations and LBIC/ PL measurements hint that especially for low iron concentrations and in the close vicinity of grain boundaries, interstitial iron is not the limiting factor for the diffusion length. As a result, other recombination processes, like the recombination activity of the grain boundary itself or recombination due to precipitates, have to be taken into account when calculating the interstitial iron concentration from LBIC/ PL measurements