EP2751309A1

EP2751309A1 - System for manufacturing a crystalline material by directional crystallization provided with an additional lateral heat source

Info

Publication number: EP2751309A1
Application number: EP12762324.7A
Authority: EP
Inventors: Jean-Paul Garandet; Anis Jouini; David Pelletier
Original assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2011-08-31
Filing date: 2012-08-31
Publication date: 2014-07-09
Also published as: CN103890240B; KR20140062093A; JP2014525385A; WO2013030470A1; JP6121422B2; FR2979357B1; CA2845068A1; US20140190398A1; BR112014003988A2; US9938633B2; CN103890240A; FR2979357A1; WO2013030470A8

Abstract

The crystallization system comprises a crucible (1) provided with a base (2) and side walls (3) that is intended to contain the material to be solidified and a device (4) for creating a main thermal gradient inside the crucible (1) in a direction perpendicular to the base (2) of the crucible (1). An additional induction heating device (6) is positioned at the side walls (3) of the crucible (1) facing the liquid material and without overlapping with the solid phase. This additional induction heating device (6) is configured in order to heat a portion of the crystalline material located in the vicinity of the triple line between the liquid material, the solidified material and the crucible (1) so that the interface (10) between the liquid material and the solidified material forms a convex meniscus in the vicinity of the triple line.

Description

System for manufacturing crystalline material by directed crystallization with additional lateral heat source

Technical field of the invention

The invention relates to a system and a method for producing crystalline material by directed crystallization. The invention applies in particular to semiconductor materials whose electrical conductivity in the liquid phase is higher than in solid phase.

State of the art

The silicon used in the photovoltaic industry is mainly crystalline silicon of multi-crystalline structure, that is to say with monocrystalline grains without fixed orientation relative to each other and surrounded by grain boundaries. There is also a die using monocrystalline silicon, that is to say that a single grain forms the silicon ingot. The growth of this type of material is carried out, for example, inside a crucible in a Bridgman type crystallization furnace or by means of the Czochralski growth technique.

A significant part of the silicon used in the photovoltaic industry comes from the Czochralski sector. However, it is important to note that the Czochralski growth technique is usually limited to the formation of cylindrical ingots which is particularly problematic for use in the field of photovoltaics where it is important to increase the effective area of the photovoltaic panel.

On the other hand, the Bridgman technology makes it possible to define the shape of the ingot as a function of the shape of the crucible containing the molten material. In the Bridgman technology, the ingots are crystallized in solidification furnaces directed within which the cooling of the bath of molten material is controlled by a mechanical drawing device, and alternatively, in the so-called "Gradient Freeze" technology, the cooling is controlled by decreasing the power delivered to the liquid phase. The displacement of the liquid / solid interface in the crucible comes from the modulation of the heat delivered and the heat extracted in the different parts of the crucible.

WO2009 / 014961 discloses a device for manufacturing silicon in a crucible from a seed. In addition to a primary heating for heating the material present in the crucible, this document teaches the use of additional heating placed around the crucible in order to modify the shape of the solid / liquid interface.

However, the use of a crucible results in an increased difficulty in the management of the heat flows inside the furnace. The side walls of the crucible increase the risk of germination of defects (parasitic crystals, twins) in the final ingot. The presence of crystalline defects which arise from a particular physicochemical environment at the triple crucible / solid material / liquid material line is regularly observed.

Crystalline defects degrade the crystallographic quality of the materials used in photovoltaic panels, which results in a reduction in the energy conversion efficiency of the final photovoltaic device.

Another technique consists in defining an air gap between the material to be crystallized and the crucible, for example by means of an electromagnetic field coming from inductive coils. Such teaching is present in US2010 / 0148403. Object of the invention

It is found that there is a need for making ingots of crystalline material that have a lower amount of crystallographic defects.

This need is solved by means of a system for manufacturing crystalline material by directed crystallization comprising:

- a crucible provided with a bottom and side walls for containing the material to be solidified,

a device for creating a main thermal gradient inside the crucible in a direction perpendicular to the bottom of the crucible,

an additional inductive heating device disposed at the level of the side walls of the crucible and mounted to move relative to the crucible in the direction perpendicular to the bottom of the crucible, and configured to heat a portion of the material located near the triple line between the liquid material , the solidified material and the crucible so that the interface between the liquid material and the solidified material forms in the vicinity of said triple line a convex meniscus.

It is also found that there is a need to provide a process which facilitates the production of crystalline ingots with a low concentration of crystallographic defects. This need is addressed by means of a process for producing a crystalline material by directed crystallization comprising the following steps:

- providing a crucible provided with a bottom and a side wall and at least partially filled with the crystalline material in the liquid phase,

generating a main thermal gradient inside the crucible in a direction perpendicular to the bottom of the crucible so as to obtain a progressive solidification of the material in the perpendicular direction from the bottom of the crucible,

- heating, using an additional inductive heating device disposed at the side walls of the crucible and mounted movably relative to the crucible in said perpendicular direction, a portion of the material located in the vicinity of the triple line between the liquid material , the solidified material and the crucible, so that the interface between the liquid material and the solidified material forms in the vicinity of said triple line a convex meniscus.

Brief description of the drawings

Other advantages and features will emerge more clearly from the following description of the particular embodiments of the invention given as non-restrictive examples and represented in the accompanying drawings, in which:

- Figure 1 shows schematically a cross section of a particular embodiment of a directed crystallization system;

- Figure 2 schematically shows a cross section of a particular embodiment of a melting / crystallization device.

Description of particular embodiments

The directed crystallization system illustrated in Figure 1 comprises a crucible 1 provided with a bottom 2 and side walls 3. The shape of the bottom of the crucible 1 is arbitrary. For example, the section (that is to say the shape drawn by the bottom 2 of the crucible 1) may be square, rectangular or cylindrical. Preferably, the crucible 1 has a rectangular section or square to facilitate the realization of photovoltaic panels having a good occupation of the available surface by the crystalline substrate.

The side walls 3 are perpendicular to the bottom 2 of the crucible 1 or substantially perpendicular to the bottom 2. The crucible 1 is made of a material resistant to the high temperatures experienced during the melting and solidification phases. Preferably, the crucible 1 is made of silica, but it can also be made of graphite, silicon carbide or a mixture of these materials.

The crucible 1 is impervious to the material to be solidified, that is to say that the bottom 2 and the side walls 3 do not allow the output of the molten material. The crucible can be monobloc, even monolithic that is to say made of the same material.

The directed crystallization system comprises a device for generating a main thermal gradient in a direction perpendicular or substantially perpendicular to the bottom 2 of the crucible 1, that is to say deviating a few degrees from the perpendicular direction. The gradient is represented by an arrow X in FIG. The device for generating the main thermal gradient is configured to begin solidification from the bottom 2 of the crucible 1. The interface "liquid material / solidified material" that is to say the interface between the liquid phase and the solid phase of the material moves from the bottom 2 of the crucible 1 to the top of the crucible 1 in the direction of the X arrow.

The device for generating the thermal gradient can be formed by any means adapted, for example, by a main heating device 4 placed above the crucible 1 and associated with a cooling device 5 placed under the bottom 2 of the crucible 1. It is still possible to use a lateral heating device 4 which faces the side walls 3 of the crucible 1. The The heating device is then capable of delivering different powers according to the height in the crucible 1. By way of example, during the crystallization phase, a greater power is delivered in the upper part of the crucible 1 in comparison with the power delivered for the bottom 2 of the crucible 1. The main heating device may also be associated with a cooling device 5 placed under the crucible 1.

In yet another embodiment, the heating device 4 is fixed and oriented vertically and defines a thermal gradient according to the height. The crucible is movably mounted and moves in the thermal gradient imposed by the heater. Such an embodiment is illustrated in FIG.

The main heating device 4 is, for example, made in a resistive technology, a radiative technology or in an inductive technology.

The crucible 1 and the device for generating the main thermal gradient in the crucible are also configured to allow displacement of the liquid / solid interface inside the crucible 1. The displacement of the liquid / solid interface takes place according to the direction X or substantially in the direction X perpendicular to the bottom 2 of the crucible 1. As indicated above, during crystallization, the liquid / solid interface moves away from the bottom 2 of the crucible 1.

In order to reduce or even avoid the germination of parasitic crystals and more particularly parasitic monocrystals from the side walls of the crucible, the directed crystallization system comprises an additional inductive heating device 6 arranged facing at least one of the sidewalls 3 of the crucible 1 and configured to heat a portion of the localized crystalline material in contact with the side wall 3. In other words, the directed crystallization system comprises an additional inductive heating device 6 disposed at the side walls 3 of the crucible 1 and configured to heat a portion of the crystalline material located in the vicinity of the triple line "liquid material / solidified material / crucible",

By triple line is meant the line formed by the intersection between the interface "liquid material / solidified material" and the crucible. The triple line is represented in the various figures by a point representative of the intersection between the crucible, the liquid phase and the solidified material. The triple line runs along the lateral faces of the crucible.

In order to follow the displacement of the liquid / solid interface of the material as the solidification thereof, the additional heating device is mounted to move relative to the crucible 1 in a direction perpendicular to the bottom 2 of the crucible 1. It is advantageously fixedly mounted relative to the main heating device 4.

The inductive heating device 6 is configured so that the heating of the portion of the material located in the vicinity of the triple line causes the formation by the liquid / solid interface, in the vicinity of the triple line of a convex meniscus. The additional device 6 thus makes it possible to bend locally, towards the bottom of the crucible, the liquid / solid interface of the material at the level of the triple line. Meniscus means a curved portion of the liquid / solid interface of the material considered localized near the triple line.

The meniscus is said to be convex when the interface has a positive curvature, that is to say when the center of curvature is located in the solid phase of the material. The meniscus is then oriented downwards, that is to say towards the bottom of the crucible. Conversely, a concave meniscus is defined by a negative curvature, the center of curvature being located outside the solid phase of the material, especially in the liquid phase thereof. A concave meniscus is then oriented upwards, that is to say in a direction opposite to the bottom of the crucible.

The inductive heating device 6 is configured to make the liquid / solid interface convex close to the side wall, that is to say to have a liquid / solid interface further from the bottom 2 of the crucible 1 in the center in comparison edge when the bottom 2 of the crucible 1 is plane. In other words, the height of the liquid / solid interface along the X axis is greater as one moves away from the side walls 3 in the meniscus. The inductive heating device 6 tends to bring the liquid / solid interface of the bottom of the crucible closer as one approaches the side wall.

The inductive heating device 6 is produced by at least one turn, for example made of graphite or silicon carbide. The device 6 generates an additional thermal gradient that locally changes the main thermal gradient. This additional thermal gradient is perpendicular or substantially perpendicular to the side walls 3.

The inductive heater 6 may be disposed facing the solid phase, facing the liquid phase of the material and / or facing the liquid / solid interface of the material so as to obtain the modification of the temperature field in the crucible and thus obtain the desired curvature of the liquid / solid interface in the immediate vicinity of the side wall 3.

The heating device 6 is preferably facing the liquid part of the crystalline material, which makes it possible to limit the influence of the heat input in the crucible 1. It is particularly interesting to place the inductive coil facing the liquid phase material because the inductive influence characterized by the electromagnetic skin thickness is also smaller which makes it possible to better control the thickness of the heated zone and therefore the extent of the additional thermal gradient. The positioning of the heating device facing the liquid phase takes advantage of the fact that the semiconductor materials have a greater electrical conductivity in the liquid phase than in the solid phase. Preferably, the solid phase is devoid of overlap by the heating device in order to reduce the influence of this additional heating on the main thermal gradient and therefore to limit the influence of this additional gradient on the formation of crystal defects of the type dislocations.

Although convection only exists in the liquid phase, the inventors have observed that the localized heating of the liquid phase has a smaller influence than the localized heating of the solid phase. In the case of silica crucibles, electrically insulating, the temperature field in the crucible is slightly disturbed because mainly the liquid portion of the material facing the coil is heated. This effect is all the more marked as the heating is arranged near the liquid / solid interface. In order to follow the position of the liquid / solid interface, the inductive heating device 6 is associated with a device for displacing the heating device that is advantageously configured to place the heating device 6 facing the liquid material and the interface solid / liquid throughout the duration of the crystallization phase.

The distance separating the heating device 6 from the liquid / solid interface is defined so as to have an effect on the liquid and at the interface in order to obtain the desired curvature. The distance depends on the depth of introduction of the heat into the crystalline material and thus on the feed conditions of the turn and the electrical properties of the heated material. In a preferred embodiment, which can be combined with the preceding modes, the displacement device of the additional heating device 6 is configured to place an inductive coil at a distance of between 1 and 20 mm with the triple line of the liquid interface. solid in the perpendicular direction X at the bottom 2 of the crucible 1.

In an even more advantageous embodiment, the displacement device of the additional heating device 6 is configured to position the inductive coil, in operation, at a distance of between 1 and 10 mm from the triple line, to maintain the convex shape. meniscus. The distance can be measured between the center of the inductive turn and the triple line, for example in the direction perpendicular to the bottom of the crucible. It should thus be noted that, without additional inductive heating, the liquid / solid interface of the material may present locally in the vicinity of the triple line a form of concave meniscus, that is to say oriented upwards. Preferably, the inductive turn of the additional device 6 is then initially positioned facing the solid phase of the material, at a distance of between 1 and 20 mm from the triple line. The inductive turn, once activated, heats the part of the solid material that forms the concave meniscus and causes it to melt. The curvature of the liquid / solid interface of the material in the vicinity of the triple line is then modified and becomes positive. Of course, the position of the triple line is changed and goes down. The interface thus forms a convex meniscus, that is to say, oriented downwards. The inductive coil of the additional device 6 is then positioned facing the liquid phase of material. It is advantageously located at a distance of between 1 and 20 mm from the triple line, and preferably at a distance of between 1 and 10 mm from the latter in the X direction. The inductive heating device 6 makes it possible to directly heat the material without first heating the crucible 1 in an electrically insulating crucible, as is the case with other heating techniques, for example resistive techniques. The influence on the main thermal gradient is then reduced.

The amount of heat provided in the crystalline material and the extent of this heat input into the crucible 1 are defined by means of the intensity of the current delivered, the frequency and the power flowing in the turn. The location of the heat input in the crystalline material is related to the electromagnetic skin thickness. The skin thickness varies according to (r ¹ /) ^"1 ' ² with σ the electrical conductivity of the material considered and / the frequency of the electromagnetic field applied by the inductive coil.

By way of example, for liquid silicon, the skin thickness is substantially equal to 1 cm for a frequency of 1 kHz and it is of the order of 1 mm for a frequency of 100 kHz. In this way, by modulating the frequency of the electric field traveling through the inductive turn, it is possible to modulate the spatial distribution of the heat input. Under the same conditions, the skin thickness is six times greater in the solid phase which complicates the supply conditions of the inductive coil. In this case, the directed crystallization system comprises a circuit for applying a current in the heating device with a frequency of between 1 kHz and 100 kHz when the crystalline material is silicon. However, the frequency range can be adapted according to the electrical conductivity of the materials and so as to work on a heat delivery in the crucible so that the skin thickness remains between 1 mm and 1 cm. Particularly advantageously, the inductive turns used are uncooled turns. This configuration avoids the introduction of a cold point near the crucible and avoids more difficult management of a cold point in a hot zone of the device.

In a preferred embodiment, the directed crystallization furnace comprises a device 8 for distributing the power delivered in the additional heating device 6 with respect to the device for creating the main thermal gradient. This distribution device 8 is configured so that the additional heating device 6 receives between 5% and 35% of the power delivered in the device for creating the main thermal gradient.

The ratio between the power delivered in the inductive heating 6 and the power delivered in the main heating device 4 of the device for generating the thermal gradient is between 5% and 35%. In this particular range, the effect of the additional thermal gradient is limited relative to the main thermal gradient while being large enough to significantly reduce the problems of parasitic germination from the side faces. Even more preferably, the power delivered in the inductive heating 6 represents between 10% and 20% of the power delivered in the main heating device 4 of the device for generating the thermal gradient in order to have an almost complete reduction of the parasite germination while maintaining a good control of crystal growth according to the thermal gradient.

In a particularly preferred embodiment, the power delivered in the inductive heater 6 represents 15% of the power delivered in the main heating device 4 of the device for generating the thermal gradient. Under these conditions, the main heating device 4 is powerful enough to generate a main thermal gradient suitable for orienting the crystalline growth of the molten material in the case of monocrystalline or multicrystalline growth throughout the volume of the crucible. In parallel, the additional thermal gradient is also important enough to reduce the generation of equiaxial crystals on the edges or even to prevent the propagation of any equiaxial crystals having germinated on the edges of the crucible, because of the local curvature of the interface.

In order to have the displacement of the inductive heating 6 mobile with the liquid / solid interface, it is possible in a first embodiment to use one or more turns which are all displaced along the axis perpendicular to the bottom 2 of the crucible 1 as a function of the temperatures measured in the crucible 1 and therefore as a function of the position of the liquid / solid interface (FIG. D). In an alternative embodiment, it is also conceivable to have a set of fixed turns facing the side walls. In this case, the feed device of the different turns is configured to provide a variable power to the different turns so as to simulate the movement of the moving coil with the liquid / solid interface.

In a particular embodiment, the additional inductive heating device 6 is fixedly mounted relative to the device for creating a main thermal gradient inside the crucible. The position of the additional heating device is fixed within the thermal gradient. The device for creating the main thermal gradient and the additional inductive heating device advantageously move identically relative to the crucible.

In another variant embodiment, the device for generating the gradient is fixed just like the inductive heating device 6. The heating device 6 is placed at a given isotherm, which imposes the position of the inductive heating compared to the liquid / solid interface. The distance is fixed between the liquid / solid interface and the device 6 for a given crystalline material. In this case, it is the crucible that moves as shown in Figure 2 which facilitates the implementation.

The directed crystallization system is particularly advantageous when the crucible 1 has an edge between two successive side walls, for example in the case of a square or rectangular crucible. The probability of obtaining parasitic grains is diminished on the edges and especially in the edges.

For this type of architecture, it is preferable to modify the turn of the device 6 to modulate the power delivered by the turn in the crucible 1. The section of the turn is reduced in the vicinity of the corners of the crucible 1 in comparison with the section which faces the flat or slightly curved parts of the lateral faces. In this way, the current density is increased which has the effect of increasing the curvature of the liquid / solid interface in the corners of the crucible 1. The parasitic effects of crystallization related to the corners are lessened.

The inductive heater introduces a lateral thermal gradient from the walls of the crucible. According to the various studies carried out in this field, the lateral thermal gradient must generate stresses which results in the formation of crystallographic defects such as dislocations. The inventors have observed that, contrary to commonly accepted ideas, the few existing defects are located at the extreme periphery of the ingot in a zone that is in any case unusable because it is systematically contaminated chemically by the impurities in the crucible. The introduction of additional inductive heating thus makes it possible to improve the overall crystallographic quality of the ingot while locating defects on the periphery of the ingot in an unusable area. In the end, the crystallographic quality of the actual ingot is increased.

In a particular embodiment, the directed crystallization system comprises a vertical furnace illustrated in FIG. 2. The furnace comprises three zones, a hot zone at 1480 ° C., a cold zone at 1300 ° C. and the intermediate zone defining the gradient. thermal.

The main heating is obtained by means of a resistive device. The power required to obtain the thermal gradient between the hot and cold zones is equal to 10kW. The temperature control is carried out by means of type C thermocouples. The distance separating the hot zone from the cold zone is equal to 10cm. The crucible is square section type 35 ^* 35cm ² . The height of the side walls is equal to 80cm. The drawing speed of the ingot is equal to 25mm / h.

The inductive heating device 6 is formed by a graphite coil having a diameter equal to 1 cm. The turn has a circular section. The center of the disc is placed 5mm above the liquid / solid interface. The coil is connected to a current generator which delivers a power equal to 1.5 kW. The frequency of the current is equal to 10kHz. In an alternative embodiment, the diameter of the coil is reduced to 8 mm in front of the four corners of the crucible over a distance of 1 cm.

Thus, by means of this type of crucible, it is possible to crystallize a bath of molten material by reducing the amount of crystalline defects. The crucible provided with a bottom and side walls is at least partially filled with a material in the liquid phase 9. The material may be melted in the device or in another device and then decanted. A main thermal gradient is generated inside the crucible in the X direction perpendicular to the bottom 2 of the crucible 1 so as to have a displacement of the liquid / solid interface from the bottom 2 of the crucible 1.

An additional lateral thermal gradient is generated in the crucible in a direction parallel to the bottom 2 of the crucible 1. The additional thermal gradient comes from at least one turn of the heating device 6. The turn faces the liquid / solid interface and to the liquid phase to effectively bend the interface 9 by limiting the changes of the main thermal gradient in the rest of the material. The lateral thermal gradient is located immediately after the side walls and moves with the liquid / solid interface so as to be disposed at the liquid / solid interface and in the liquid phase 9.

As the crystallization progresses, the amount of solid phase 11 increases in the crucible 1.

This type of process is compatible for producing monocrystalline or polycrystalline ingots. It can be used to form silicon ingots or other semiconductor materials. The reduction of the crystallographic defects is obtained by means of the additional inductive heating 6 which can be placed on an edge of the crucible, on several edges of the crucible or on all the edges of the crucible according to the needs of the user. It is also very simple to change the shape of the ingot between two stages of crystallization, it is enough to change the crucible and if necessary the shape of the inductive coil of the heater 6. The manufacturing method is particularly suitable for semiconductor materials which have a higher electrical conductivity in the liquid phase than in solid phase which limits the effect of inductive heating on the solidified material.

In a particular embodiment, during the solidification solidification process, the solid / liquid interface is observed in order to determine its shape. If the latter is concave, the coil approaches the liquid / solid interface or is placed at the level of the triple line so that upon activation of the additional heating device, the solid / liquid interface becomes convex and the inductive coil is facing the liquid material without recovery with the solid phase.

Claims

claims

A crystalline material manufacturing system by directed crystallization comprising:

- a crucible (1) provided with a bottom (2) and side walls (3) for containing the material to be solidified,

a device for creating a main thermal gradient inside the crucible (1) in a direction substantially perpendicular (X) to the bottom (2) of the crucible (1),

an additional inductive heating device (6) arranged at the level of the side walls (3) of the crucible (1) and configured to heat a part of the material located near the triple line between the liquid material, the solidified material and the crucible (1) so that the interface (10) between the liquid material and the solidified material forms in the vicinity of said triple line a convex meniscus,

characterized in that it comprises a device for moving the additional heating device (6) in the substantially perpendicular direction (X) to the bottom (2) of the crucible (1) and configured to place the additional heating device (6) facing the liquid material and without recovery with the solid phase throughout the crystallization period.

2. System according to claim 1, characterized in that the displacement device of the additional heating device (6) is configured to place the additional heating device (6) near the interface (10) solid / liquid.

3. System according to claim 2, characterized in that the displacement device of the additional heating device (6) is configured to place an inductive coil at a distance of between 1 and 20 mm from said triple line in the perpendicular direction ( X).

4. System according to claim 3, characterized in that the displacement device of the additional heating device (6) is configured to position the inductive coil at a distance of between 1 and 10 mm with respect to said triple line.

5. System according to any one of claims 1 to 4, characterized in that it comprises a device (8) for distribution of the power delivered in the additional heating device (6) relative to the device for creating the thermal gradient main configured for the additional heating device (6) to receive between 5% and 35% of the power delivered in the device for creating the main thermal gradient.

6. System according to any one of claims 1 to 5, characterized in that the crucible (1) has a shape such that two successive side faces (3) define an edge.

7. System according to claim 6, characterized in that the crucible (1) is square or rectangular section.

8. System according to any one of claims 1 to 7, characterized in that it comprises a circuit for applying a current in the heating device with a frequency between 1 kHz and 100 kHz when the crystalline material is silicon.

9. System according to any one of claims 1 to 8, characterized in that the additional inductive heating device (6) is fixedly mounted relative to the device for creating a main thermal gradient inside the crucible (1). ).

Process for the production of crystalline material by directed crystallization comprising the following steps:

- providing a crucible (1) provided with a bottom (2) and a side wall (3) and at least partially filled with the crystalline material in the liquid phase,

- Generating a main thermal gradient inside the crucible in a direction substantially perpendicular (X) to the bottom (2) of the crucible (1) so as to obtain a progressive solidification of the material in the substantially perpendicular direction (X) from the bottom (2) crucible

(1).

- heating, using an additional inductive heating device (6) disposed at the side walls (3) of the crucible (1) and mounted movably relative to the crucible (1) in said substantially perpendicular direction (X) a part of the material located near the triple line between the liquid material, the solidified material and the crucible (1), so that the interface between the liquid material and the solidified material forms in the vicinity of said triple line a convex meniscus, the additional heating device (6) being arranged facing the liquid material and without overlap with the solid phase.

11. The method of claim 10, characterized in that the ratio between the power delivered in the additional heating device (6) and the power delivered in a main heating device (4) generating the main thermal gradient is between 5% and 35%.

12. Method according to one of claims 10 and 1 1, characterized in that the additional heating device (6) is formed by an inductive coil (6) positioned at the interface (10) liquid / solid and face to the liquid phase (9) when the interface between the liquid material and the solidified material forms in the vicinity of said triple line a convex meniscus.

13. The method of claim 12, characterized in that the inductive coil (6) is positioned at a distance between 1 and 20mm relative to said triple line in the perpendicular direction (X).

14. The method of claim 12, characterized in that said crystalline material is a semiconductor material which has a higher electrical conductivity in the liquid phase than in the solid phase.