Gettering effect in grain boundaries of multi-crystalline silicon (original) (raw)
Related papers
Materials Science and Engineering: B, 2009
Multicrystalline silicon materials for photovoltaic applications inherently contain extended defects like grain boundaries, dislocations, microdefects and in some cases also second phase precipitates due to high concentrations of light elements (carbon, nitrogen or oxygen) and transition metal impurities. The latter are known to reduce the minority carrier lifetime and hence should be removed by gettering during solar cell processing. This paper discusses the influence of extended defects on the spatial distribution of copper-and nickel-related silicide precipitates for a model system containing a small angle grain boundary and in one part silicon oxide pecipitates partly associated with punched-out dislocations. Phosphorusdiffusion gettering under conditions of mostly precipitated metal impurities is discussed in terms of quantitative simulations. It is shown that two regimes can be distinguished where gettering kinetics are either limited by precipitate dissolution or phosphorus in-diffusion. Finally, binding of metal impurities to dislocations is considered and its effect on gettering kinetics is illustrated in terms of gettering simulations.
Effect of P-induced gettering on extended defects in n-type multicrystalline silicon
Progress in Photovoltaics: Research and Applications, 2007
The electrical properties and the minority charge carrier recombination behaviour of grain boundaries (GBs) and intragrain dislocations in different n-type multicrystalline silicon (mc-Si) ingots were systematically studied through microwave-detected PhotoConductance Decay (mW-PCD), Electron Beam Induced Current (EBIC) and PhotoLuminescence (PL) spectroscopy on as-grown samples and on samples submitted to P-diffusion step. It was confirmed that the overall quality of n-type mc-Si is high, indicating that n-type-Si is a valid source for photovoltaic applications. As expected, the average lifetime increases after the P-diffusion process, which induces impurity gettering effects at the external surfaces, like in the case of p-type samples, but an evident local increase of electrical activity of some GBs after that process was also observed using the EBIC mapping technique. Apparently, a redistribution of impurities occurs at the processing temperature and impurities are captured at the deepest sinks. In fact, while all GBs act as heterogeneous segregation/precipitation sites, some of them will compete with the external surfaces sinks, partly vanishing the effect of P-gettering. Last but not least, it was experimentally demonstrated that the average lifetime values measured with the mW-PCD technique well correlate with the recombination activity of GBs measured with the EBIC technique, showing the extreme importance of GBs on the effective lifetime of this material.
Gettering in silicon photovoltaics: current state and future perspectives
physica status solidi (a), 2006
This paper summarizes current understanding and predictive simulations of gettering processes predominantly applied in silicon photovoltaics. Special emphasis is put on various processes limiting gettering efficiency and kinetics, i.e. the mobility of interstitially dissolved metal species, the formation of the gettering layer, and the effect of immobile metal species. The latter are substitutional metal species, precipitates, complexes with defects related to non-metallic impurities, and finally the interaction with extended defects, in particular dislocations. Finally, alternative annealing schemes involving high-temperature rapid thermal processing are explored by simulations. It is shown that a processing window exists for a two-step process efficient for the removal of precipitates even under the constraints of a fixed thermal budget for phosphorus diffusion.
The increase of the average lifetime in silicon multicrystalline wafers at the end of the cell fabrication process shows that, like in p-type materials, also in n-type materials the average quality can be improved by suitable gettering and passivation treatments. However, it was already reported that different parts of p-doped ingots react in different way to the gettering procedures. In this paper we report on the effect of the phosphorus (P)-gettering process (45 Ohm/sq P-diffusion) on n-and p-type mc Si in order to get more information about the physics of the gettering process in both materials. The detailed analysis of the electrical activity of microstructural defects shows that grain boundaries and precipitates are the more detrimental defects, even if they do not deteriorate the average good quality of the material and the final cell performance. No intragranular electrical active defects could be detected in the EBIC maps at room temperature. Finally, despite the formation of precipitates at grain boundaries, the average diffusion lengths in n-type samples are larger than in p-type samples prepared in similar conditions, confirming that n-type-Si is less sensitive to metallic impurities.
Investigations of impurity gettering in multicrystalline silicon
Semiconductor Physics, Quantum Electronics and Optoelectronics, 2001
The processes of gettering the recombination-active impurities in multicrystalline silicon were investigated using methods of mass spectrometry of neutral atoms with the depth profile analysis and spectroscopy of a surface photovoltage (permitting to determine the diffusion length of nonequilibrium charge carriers). Getters formed by a silicon layer with a developed internal surface, and also combined getter (the mentioned layer covered with a thin film of aluminum) were used. It was shown that the efficiency of gettering depends on annealing temperature and character of Al depth distribution that, in turn, depends on the regimes of structurally modified silicon layer formation. The models of gettering that enabled us to explain obtained results are considered.
Limiting factors of gettering treatments in mc-Si wafers from the metallurgical route
Materials Science and Engineering: B, 2009
Gettering treatments such as phosphorus diffusion and aluminium-silicon alloying have been applied to multicrystalline silicon wafers prepared from upgraded metallurgical feedstock. Purification of the feedstock results of plasma torch treatment and directional solidification. Minority carrier diffusion lengths L n are close to 40 m in the raw wafers and increase up to 150 m after phosphorus plus Al-Si getterings. Improvements are limited by the presence of residual metallic impurities, mainly slow diffusers like aluminium, and also by the high doping level.
Optimization of gettering processes of metallurgical-grade silicon for solar cell applications
MRS Proceedings, 2009
Conversion efficiency of a solar energy in the electric is substantially determined not only by the total impurity concentration in solar cell element, but also by impurity chemical and physical state. Gettering processes, which are included in the technology of solar cell manufacturing, are usually used for such impurity redistribution. In order to optimize gettering processes we developed a program tool based on the fundamental physical and chemical laws. The description of physical and chemical behaviour of impurities in silicon is based both on known experimental data, and on calculations of necessary parameters by means of present-day thermodynamic and quantum-chemical methods. Developed tool helps to choose a gettering regime (a temperature profile, time, getter layer thickness) for optimization of these processes for the given initial chemical composition of the silicon wafer. Possibility of analysis of recombination activity of various types of defects in silicon on the basi...
Journal of Applied Physics, 2006
We have investigated the gettering of transition metals in multicrystalline silicon wafers during a phosphorus emitter diffusion for solar cell processing. The results show that mainly regions of high initial recombination lifetime exhibit a significant lifetime enhancement upon phosphorus diffusion gettering. Nevertheless, transition metal profiles extracted by secondary ion mass spectrometry in a region of low initial lifetime reveal significant gradients in Cr, Fe, and Cu concentrations towards the surface after the emitter diffusion, without exhibiting a significant enhancement in the lifetime. In a region of higher initial lifetime, however, diminutive concentration gradients of the transition metal impurities are revealed, indicating a significantly lower initial concentration in these regions. From spatial maps of the dislocation density in the wafers, we find that lifetime enhancements mainly occur in regions of low dislocation density. Thus, it is believed that a generally higher concentration of transition metals combined with an impurity decoration of dislocations in regions of high dislocation density limit the initial lifetime and the lifetime after the phosphorus diffusion, in spite of the notable gettering of transition metal impurities towards the surface in these regions. Furthermore, after a hydrogen release from overlying silicon nitride layers, we observe that only regions of low dislocation density experience a significant lifetime enhancement. This is attributed to impurity decoration of the dislocations in the regions of both high dislocation density and high transition metal impurity concentration, reducing the ability of hydrogen to passivate dislocations in these regions.
Combination of gettering and etching in multicrystalline silicon used in solar cells processing
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2006
Undesired impurities can be removed away from multicrystalline silicon (mc-Si) wafers by combining porous silicon (PS) formation and heat treatments. The gettering procedure used in this work is based on the formation of a PS film at both back and front sides of the mc-Si wafers, followed by a heat treatment. The latter was achieved in an infrared furnace at different temperatures and during various periods. We show that when the based material undergoes such a gettering, the electrical properties (short-circuit current, open-circuit voltage, serial and shunt resistances) and the electronic parameters (diffusion length and grain boundary recombination velocity) of the corresponding solar cells can be improved only if some regions of the wafers are etched. Compared to reference cells based on untreated wafers, the diffusion length and grain boundary recombination velocity of solar cells fabricated from gettered and etched samples was improved by about 30% and reduced by a factor of 10, respectively.
2012
The features of the recombination-active impurity gettering in polycrystalline silicon have been studied. The research method included the formation of a porous silicon layer 0.5–2 μm in thickness on the backside of a silicon wafer, the deposition of aluminum layer 0.5–1 μm in thickness, and the thermal annealing at 700– 950 ◦C during 30–60 min. The corresponding gettering model has been proposed, which includes the diffusion of iron atoms by means of two most probable independent channels: in the wafer bulk and along the grain boundaries. By comparing the theoretical results and experimental data, we established that 30% of gettered impurity atoms diffuse with a high rate along the grain boundaries, and 70% of them in the grain bulk.