Effet getter dans des plaquettes de silicium multicristallin par diffusion de phosphore

The external gettering effect by phosphorus diffusion is used to improve the electrical properties of multicrystalline silicon wafers. After diffusion at 900 °C for 4 h it was found that the effective diffusion lengths $L_{\rm n}$ of minority carriers achieve or overpass the thickness of the wafers. After diffusion at 850 °C for 4 h the improvements are less marked and hydrogenation is needed to obtain the same increase of $L_{\rm n}$. SIMS analysis indicates that the gettered impurities are essentially iron, copper and nickel. Some restricted regions of the wafers are only poorly improved. It was found after chemical etching that these regions contain a high density of subgrain boundaries. The mechanism of the gettering effect used in this work is proposed, taking in account dissolved impurities in the grains and impurities segregated by dislocations. The additivity of the hydrogenation effect might be understood by the neutralisation of the recombination centers related to oxygen atoms segregated by the dislocations.L'effet getter externe par diffusion de phosphore est utilisé pour améliorer les propriétés électriques de plaquettes de silicium multicristallin. Après 4 h à 900 °C les longueurs de diffusion des porteurs minoritaires atteignent ou dépassent l'épaisseur des plaquettes. Après 4 h à 850 °C, les augmentations sont moins spectaculaires et une hydrogénation du matériau est nécessaire pour obtenir un résultat comparable au précédent. Les analyses SIMS indiquent que les impuretés extraites sont surtout du fer, du cuivre et du nickel. Certaines régions du matériau, d'extension limitée, sont toutefois peu améliorées. Elles sont caractérisées par la présence d'un réseau très dense de sous-joints. Une interprétation du mécanisme de l'effet getter observé est proposée faisant intervenir les impuretés métalliques dissoutes et celles ségrégées par les dislocations. L'additivité de l'action de l'hydrogène s'expliquerait par la neutralisation des centres de recombinaison dus à l'oxygène ségrégé par ces défauts

LBIC INVESTIGATION OF PHOSPHORUS GETTERED MULTICRYSTALLINE SILICON WAFERS

In multicrystalline silicon the interaction of dissolved impurities with extended crystallographic defects limits the effective minority carrier diffusion lengths Ln. External gettering by phosphorus diffusion near the surface is used to remove metallic impurities from the bulk of P type silicon wafers. It was found that Ln increases drastically after gettering at 850°C for 120 or 240 mn. SIMS analysis indicates that iron, copper and nickel are accumulated in the phosphorus doped regions and LBIC scan maps at λ = 940 nm indicate that the improvement of Ln results of the neutralization of intragrain and grain boundary recombination centers. It is concluded that phosphorus gettering removes dissolved and segregated impurity atoms

Multicrystalline silicon wafers prepared from upgraded metallurgical feedstock.

International audienc

n-p junction formation in p-type silicon by hydrogen ion implantation

International audienceHydrogen ion implantations at an energy of 250keV and a dose of 3 x 10(16) cm(-2) were applied to float zone, Czochralski grown silicon wafers and to multicrystalline samples. It was found that after annealing at 350degreesC < T < 550degreesC for 1 h a n-p junction is formed and a photovoltaic behaviour is observed. Spectral responses show that the photocurrent in the near infrared part of the spectrum is comparable to that given by a standard silicon solar cell. The depth of the junction is about 2mum and C-V measurements show that the junction is graduated. Hydrogen plasma immersion leads to similar results. The conversion of p- to n-type silicon is explained by the formation of shallow donor levels associated to a high concentration of hydrogen. (C) 2002 Elsevier Science B.V. All rights reserved

Investigation of self interstitial influences in Light and Dark Induced Degradation in p-type compensated Silicon

2nd International Conference on Crystalline Silicon Photovoltaics (SiliconPV), IMEC, Leuven, BELGIUM, APR 03-05, 2012International audienceIn this study, based on p-type strongly compensated electronic grade monocrystalline Cz silicon, one tried to find if self interstitials have a direct or an indirect role in light and dark induced degradation (LID and DID respectively). Many studies have been carried out on LID phenomenon due to the formation of boron-oxygen complexes under light exposure. Some of them compare as-cut wafers and wafers which have been treated by phosphorus diffusion. They show that the LID phenomenon is reduced when samples have been phosphorus diffused. This effect has been explained by self interstitial injection in the bulk during the phosphorus diffusion, but the role of these self-interstitials on LID phenomenon is not yet well defined [1] [2] [3]. In this paper, investigations on the degradation kinetic are made in as-cut wafers, in phosphorus diffused wafers and in wafers which were annealed in the same conditions used for the phosphorus diffusion. Comparisons of the results allow us to pinpoint the role of self-interstitials on LID phenomenon. It seems that the presence of a high self interstitial concentration reduce the formation of BsO2i complexes responsible for the first faster degradation. (C) 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientific committee of the SiliconPV 2012 conference

Minority carrier bulk lifetimes through a large multicrystalline silicon ingot and related solar cell properties

International audienceThe bulk lifetime tau(n) and diffusion length L-n of minority carriers vary through the height of a cast multicrystalline silicon ( mc-Si) block. This variation is due to the segregation of metallic impurities during the directional solidification and the native impurity concentrations increase from the bottom to the top of the ingot, which is solidified last, while the ingot bottom, which is solidified first, is contaminated by the contact with the crucible floor. It is of interest to verify if a correlation exists between the bulk lifetime tau of as cut wafers and the conversion eficiency. of solar cells. In a very large ingot (> 310 kg), it was found that tau(0), in raw wafers, tau(dif) in phosphorus diffused ones and Ln in diffused wafers are smaller in the top and in the bottom of the ingot. The same evolution is observed in solar cells, however the diffusion length values L-cel in the central part of the ingot are markedly higher than those found in diffused wafers, due to the in-diffusion of hydrogen from the SiN-H antireflection coating layer. The variations of. and those of tau(0), along the ingot height, are well correlated, suggesting that the evaluation of tau(0) can predict the properties of the devices. In addition, segregation phenomena around the grain boundaries are observed at the bottom of the ingots, due to a marked contamination by the crucible floor, and at its top where impurities are accumulated. These phenomena are linked to the long duration of the solidification process and the large amount of imperfect silicon used to cast the ingot

Growth undercooling in multi-crystalline pure silicon and in silicon containing light impurities (C and O)

International audienceUndercooling during the solidification of silicon is an essential parameter that plays a major role in grain nucleation and growth. In this study, the undercooling of the solid-liquid interface during growth of multi-crystalline silicon samples is measured for two types of silicon: pure, and containing light elements (carbon and oxygen) to assess and compare their impact on crystal growth. The solid-liquid interface undercooling is measured using in situ and real time X-ray synchrotron imaging during solidification. As a subsequent step, ex situ Electron Backscattered Diffraction (EBSD) is performed to obtain information about the crystalline structure, the grain orientation and the grain boundary character. Two main conclusions arise: (i) the undercooling of the global solid-liquid front increases linearly with the growth rate which indicates uniform attachment, i.e. all atoms are equivalent, (ii) the same trend is observed for pure silicon and silicon containing carbon and oxygen. Indeed, the growth law obtained is comparable in both cases, which suggests that the solutal effect is negligible as concern the undercooling in the case of a contamination with carbon (C) and oxygen (O). However, there is a clear effect of the impurity presence on the crystalline structure and grain boundary type distribution. Many grains nucleate during growth in samples containing C and O, which suggests the presence of precipitates on which grain nucleation is favored

Users watch growth of defects in silicon

The ESRF’s ID19 and BM05 beamlines have enabled scientists to watch the growth of carbon- based defects in silicon. The results could help in the development of silicon photovoltaics that are less affected by carbon impurities. Carbon is a common impurity in photovoltaic silicon, entering the crystal as carbon monoxide, which is formed by the contact of oxygen with neighbouring graphite components in industrial furnaces. Once inside, it can make the silicon harder to cut into ingots for wafer preparation, and initiates several types of crystal defects, which impact on both electrical and mechanical performance once the wafers are made into solar cells, reducing photovoltaic efficiency.According to researchers at the CNRS Institute of Materials Microelectronics and Nanosciences of Provence (IM2NP), and Aix- Marseille Université, both in France, “post mortems” of silicon ingots reveal little about how these defects develop