Gettering and Defect Engineering in Semiconductor Technology XV

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Authors: Vladimir V. Voronkov, Robert Falster
Abstract: Lifetime-degrading recombination centres those that emerge in the presence of excess carriers in boron and oxygen containing silicon - show a peculiar dependence on the concentrations of the relevant impurities, B and O, and on the hole concentration p0 (net doping) in materials that contain compensating donors (phosphorus or Thermal Donors) or added Ga acceptors. The data indicate involvement of both substitutional (Bs) and interstitial (Bi) boron atoms in the major recombination centres observed in p-Si. A suggested model ascribes degradation to the presence of a BiBsO latent defect inherited from the thermal history in a recombination-inactive atomic configuration. In the presence of excess electrons, this latent defect reconfigures into a recombination-active centre. The defect concentration dependence on the material parameters is reduced, in some special cases, to a proportionality to p0 [2 or to [ [2. The essential feature is an involvement of a fast-diffusing species Bi in the defect. This species can be removed to the boron nanoprecipitates thus eliminating the defects responsible for the degradation.
Authors: Jasmin Hofstetter, David P. Fenning, Douglas M. Powell, Ashley E. Morishige, Tonio Buonassisi
Abstract: In multicrystalline silicon for photovoltaic applications, high concentrations of iron are usually found, which deteriorate material performance. Due to the limited solubility of iron in silicon, only a small fraction of the total iron concentration is present as interstitial solute atoms while the vast majority is present as iron silicide precipates. The concentration of iron interstitials can be effectively reduced during phosphorus diffusion gettering (PDG), but this strongly depends on the size and density of iron precipitates, which partly dissolve during high-temperature processing. The distribution of precipitated iron varies along the height of a mc-Si ingot and is not significantly reduced during standard PDG steps. However, the removal of both iron interstitials and precipitates can be enhanced by controlling their kinetics through carefully engineered time-temperature profiles, guided by simulations.
Authors: Daniel Macdonald, An Yao Liu, Sieu Pheng Phang
Abstract: The removal of dissolved iron from the wafer bulk is important for the performance of p-type multicrystalline silicon solar cells. In this paper we review some recent progress in understanding both external and internal gettering of iron. Internal gettering at grain boundaries and dislocations occurs naturally during ingot cooling, and can also be driven further during cell processing, especially by moderate temperature anneals (usually below 700 °C). Internal gettering at intra-grain defects plays key a role during such precipitation annealing. External gettering to phosphorus diffused regions is crucial in reducing the dissolved iron concentration during cell processing, although its effectiveness depends strongly on the diffusion temperature and profile. Gettering of Fe by boron and aluminum diffusions is also found to be very effective under certain conditions.
Authors: An Yao Liu, Daniel Macdonald
Abstract: The internal gettering of iron in silicon via iron precipitation at low processing temperatures is known to improve solar cell efficiencies. Studies have found that the optimal temperature lies in the range of 500°C-600°C. In this paper, we present experimental results on quantitatively analysing the precipitation of interstitial Fe in multicrystalline silicon wafers during the 500°C-600°C thermal annealing processes. The concentration and the spatial distribution of interstitial Fe in mc-Si were measured by the photoluminescence imaging technique. It was found that, apart from the processing temperature, the Fe precipitation time constant is highly dependent on the supersaturation ratio and the density and types of the precipitation sites.
Authors: Yutaka Yoshida, Yoshinori Tsukamoto, Masahiro Ichino, Kiyotaka Tanaka
Abstract: n-situ Mössbauer studies on 57Fe solute atoms in Si solar cells are performed: (1) GeV-57Mn/57Fe implantation into Si solar cells, (2) 57Fe diffused n-type Si under light illumination; (3) 57Fe diffused solar cells under applying external voltages. The carrier trapping cross sections for the interstitial components with different charge states, Fei+ and Fei2+, can be successfully obtained by evaluating the dynamical charge fluctuations within a time scale of 100ns between Fei+ and Fei2+ which appear in the Mössbauer spectra of 57Fe doped mc-Si solar cells. We further measure the distributions of Fei+ and Fei2+ by a Mössbauer Microscope, which we have been developing. The present results provide us a possibility to clarify the carrier trapping process on an atomistic scale directly on the Fe impurities in Si-solar cells.
Authors: Abdelazize Laades, Uta Stürzebecher, Hans Peter Sperlich, Christian Möller, Kevin Lauer, Alexander Lawerenz
Abstract: We are investigating the effect of different wet chemical surface preconditioning sequences for silicon wafers prior to the deposition of aluminum oxide based passivation layers coated by plasma enhanced chemical vapor deposition. We are focusing on the development of a simple and industrially feasible preconditioning process to achieve a high level of interface passivation after the firing process applied to industrial solar cells. Our process optimization is monitored by characterizing the passivation quality before and after a firing process. We are also investigating the effectiveness of the removal of residual surface iron concentrations by the wet chemical process.
Authors: Bhushan Sopori, Vishal Mehta, Srinivas Devayajanam, Mike Seacrist, Gang Shi, J Chen, Aditya Deshpande, Jeff Binns, Jesse Appel
Abstract: This paper describes results of our study aimed at understanding mechanism (s) of dislocation generation and propagation in multi-crystalline silicon (mc-Si) ingots, and evaluating their influence on the solar cell performance. This work was done in two parts: (i) Measurement of dislocation distributions along various bricks, selected from strategic locations within several ingots; and (ii) Theoretical modeling of the cell performance corresponding to the measured dislocation distributions. Solar cells were fabricated on wafers of known dislocation distribution, and the results were compared with the theory. These results show that cell performance can be accurately predicted from the dislocation distribution, and the changes in the dislocation distribution are the primary cause for variations in the cell-to-cell performance. The dislocation generation and propagation mechanisms, suggested by our results, are described in this paper.
Authors: Ali Ghaderi, Semih Senkader
Abstract: A major performance limiting factor of multicrystalline silicon wafers is structural defects, mainly dislocations, reducing solar cell efficiency. Dislocations are formed during crystallisation process. Characterization of dislocation-content is necessary both to optimise the crystallisation and to eliminate bad wafers before cell processing. We developed two techniques to characterise dislocations: conventional etch-pit counting modified for full size wafers using a new etch-recipe and a novel etch-pit counting algorithm. Secondly we developed a technique to estimate the dislocation content directly from photoluminescence images of as-cut wafers.
Authors: Maulid Kivambe, Gaute Stokkan, Torunn Ervik, Sergio Castellanos, Jasmin Hofstetter, Tonio Buonassisi
Abstract: Light microscopy, electron backscatter diffraction and transmission electron microscopy is employed to investigate dislocation structure and impurity precipitation in commonly occurring dislocation clusters as observed on defect-etched directionally solidified multicrystalline silicon wafers. The investigation shows that poligonised structures consist of parallel mostly similar, straight, well-ordered dislocations, with minimal contact-interaction and no evidence of precipitate decoration. On the other hand, disordered structures consist of various dislocation types, with interactions being common. Decoration of dislocations by second phase particles is observed in some cases. Enhanced recombination activity of dislocations may therefore be a result of dislocation interaction forming tangles, microscopic kinks and jogs, which can serve as heterogeneous nucleation sites that enhance metallic decoration.

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