Francis Durand

Carbon solubility in solid and liquid silicon—A review with reference to eutectic equilibrium

A review is presented of the direct measurements and observations concerning C in solid and liquid Si, focused on the solidification conditions of electronic-grade materials. After a critical examination of the experimental procedures, a particular set of measures is selected. For the three-phase equilibrium (SiC)+(liquid)+(solid), which is eutectic, the proposed C solubility values are 9 at. ppm in the solid and 261 at. ppm in the liquid.

Electromagnetic continuous pulling process compared to current casting processes with respect to solidification characteristics

Abstract Electromagnetic continuous pulling (EMCP) process combines the advantages of the cold crucible melting (no pollution, no crucible consumption) with those of continuous casting (high material flow rate, uniform material structure and properties). It is compared to the usual casting techniques for massive crystalline silicon, i.e. directional solidification, Czochralski pulling, and floating zone pulling. The presentation is focused on technical productivity parameters, and on longitudinal distribution of impurities, typically oxygen and iron. This allows a semi-quantitative comparison between the 4 processes, having in prospect the use of a cheaper feedstock, together with the photovoltaic quality requirements.

Multicrystalline silicon prepared by electromagnetic continuous pulling: recent results and comparison to directional solidification material

The electromagnetic continuous pulling (EMCP) is a new growth process competing with directional solidification for the production of massive silicon ingots. The advantages of the EMCP are higher production rates, no crucible consumption and a more uniform crystalline structure of the wafers. The present work is devoted to the characterization of the EMCP material by means of minority carrier diffusion length measurements (L) which allow the comparison between raw, annealed, phosphorus-diffused, annealed and gettered or hydrogenated wafers. The EMCP wafers are compared with conventionally cast wafers. It is shown that L is relatively high in the raw material, but L is degraded by annealings at temperatures higher than 600°C, while hydrogenation treatments drastically improve the material. In the present state, the EMCP silicon appears as a particular form of multicrystalline silicon materials with promising abilities. However, it requires low temperature processing steps or additional hydrogenation treatments to make low-cost and efficient solar cells.

Continuous solidification of photovoltaic multicrystalline silicon from an inductive cold crucible

An inductive cold crucible is used to melt photovoltaic granular silicon and to form massive multicrystalline billets by continuous pulling downwards. The cold crucible is noncontaminating. In the billet the impurity content is kept at the same low level as it is in the feed stock, even for copper, and somewhat lower for oxygen. Square billets are as easy to shape as circular ones. Continuous solidification gives the material uniform grain structure and properties, so that a considerable reduction in waste material is expected. Details are given on the experimental procedure and on measurements. For a given pulling rate, the optimal electrical conditions result from a fine compromise between the risk of a liquid leak, and the risk of nonmelting granules. Among the electrical and thermal measurements which were tested, the electrical frequency seems to give a possible gage for estimating the variation of the amount of liquid.

Nature et morphologie des cristaux riches en Ti et en B dans les alliages Al-Ti-B riches en Al

Abstract Al-Ti-B alloys on the Al-rich side, with the Ti/B ratio varying from 17.5 to 0.1, were prepared between 1000 and 700δC to obtain crystals in equilibrium with the liquid. The observations with a scanning transmission electron microscope and with a microprobe, show that: (a) the phase rich in Ti and in B has a composition near stoichiometric TiB 2 , and Al appears only as traces; (b) AlB 2 and TiB 2 could coexist but never form a solid solution; (c) when a B-rich liquid is brought into contact with a Ti-rich liquid, a TiB 2 barrier is formed which isolates domains where local equilibria could exist.

Oxygen and carbon transfer during solidification of semiconductor grade silicon in different processes

Abstract A model is established for comparing the solute distribution resulting from four solidification processes currently applied to semiconductor grade silicon: Czochralski pulling (CZ), floating zone (FZ), 1D solidification and electromagnetic continuous pulling (EMCP). This model takes into account solid–liquid interface exchange, evaporation to or contamination by the gas phase, container dissolution, during steady-state solidification, and in the preliminary preparation of the melt. For simplicity, the transfers are treated in the crude approximation of perfectly mixed liquid and boundary layers. As a consequence, only the axial ( z ) distribution can be represented. Published data on oxygen and carbon transfer give a set of acceptable values for the thickness of the boundary layers. In the FZ and EMCP processes, oxygen evaporation can change the asymptotic behaviour of the reference Pfann law. In CZ and in 1D-solidification, a large variety of solute profile curves can be obtained, because they are very sensitive to the balance between crucible dissolution and evaporation. The CZ process clearly brings supplementary degrees of freedom via the geometry of the crucible, important for the dissolution phenomena, and via the rotation rate of the crystal and of the crucible, important for acting on transfer kinetics.

Morphologie du composé intermétallique Al2Cu

The morphology of the Al2Cu intermetallic compound is derived using the theory of periodic bond chains (PBC) combined with an energetical calculation. The PBC are 〈001〉, 12 〈111〉, 〈100〉, 〈110〉, 〈011〉, 〈0 12〉 and 〈021〉. They determine five F-faces {110}, {200}, {211}, {112} and {022}. The attachment and surface energies are calculated in the approximation of the nearest neighbours bond. They lead to the growth morphology and the equilibrium morphology respectively. Whereas the equilibrium form deviates little from the spherical one, the growth form appears to be limited with {110} planes and elongated along the [001] axe, in agreement with the experimental observations. It is worth noting that the PBC method and the energy calculations complete one another in a useful manner.

Solid-liquid equilibria in the silicon-rich corner of the Si-O-C system

A phase diagram is proposed for the solid-liquid equilibria in the silicon-rich corner of the Si-O-C system. It is calculated from the solubility data in binary systems, using the model of a regular solution to estimate the interaction coefficient from existing standard reaction data. The solubility of carbon in solid and liquid solutions saturated with SiC is slightly increased by the effect of oxygen. A similar conclusion holds for the effect of carbon on oxygen solubility in solutions saturated with SiO2. The effect of the third element on the partition coefficient of carbon or of oxygen is negligible. The main result of the proposed diagram is the finding that the solubility can be limited either by SiO2 saturation or by SiC saturation according to the carbon-to-oxygen ratio.

Utilisation du microscope electronique a balayage pour indexer les facettes de dendrites Al3Ni

Abstract Observations in Scanning Electron Microscopy combined with a geometrical procedure can be used to determine the crystallgraphic direction of facets for small crystals, for which the application of classical methods is hardly conceivable, This method is applied to Al 3 Ni dendrites. The observed facets are: {110}, {101} and {111}.

Modèle structural des liquides mono atomiques: IV. — Estimations des propriétés thermodynamiques et des coefficients d’autodiffusion

Le modele des liquides monoatomiques permet d’estimer a partir d’un parametre unique, les variations avec la temperature de l’entropie, de la chaleur specifique et du coefficient d’autodiffusion des metaux liquides. Les resultats sont generalement en accord satisfaisant avec les donnees experimentales. Il en va de meme pour les semi-metaux a conditions de tenir compte d’une entropie de fusion d’origine electronique.