EP2262932A1

EP2262932A1 - Method for the crystallogenesis of a material electrically conducting at the molten state

Info

Publication number: EP2262932A1
Application number: EP09715571A
Authority: EP
Inventors: Thierry Duffar; Gilbert Vian
Original assignee: Centre National de la Recherche Scientifique CNRS; Institut Polytechnique de Grenoble
Current assignee: Centre National de la Recherche Scientifique CNRS; Institut Polytechnique de Grenoble
Priority date: 2008-02-27
Filing date: 2009-02-27
Publication date: 2010-12-22
Also published as: US9493889B2; JP2011513168A; US20110000424A1; WO2009106625A1; FR2927910A1; JP5777888B2; FR2927910B1

Abstract

The invention relates to a method for the crystallogenesis of a material that is electrically conducting at the molten state, by drawing from a molten mass of said material in a crucible (1), that comprises: progressively subjecting the molten material to a decreasing temperature so that a liquid-solid interface is formed; controlling the flatness of the liquid-solid interface of the material; subjecting the molten material, before and during solidification, to an electromagnetic kneading; said method being characterised in that the electromagnetic kneading is obtained by applying an alternating magnetic field. The invention also relates to a device for implementing said method.

Description

METHOD FOR CRYSTALLOGENING MADE ELECTRICALLY CONDUCTIVE MATERIAL

FIELD OF THE INVENTION The present invention relates to a method of crystallogenesis of an electrically conductive material in the molten state, that is to say the solidification of the molten material by drawing in a crucible, leading to the formation of a crystal .

BACKGROUND OF THE INVENTION The segregation of chemical species during the solidification of an alloy is a long-standing problem.

It systematically leads to obtaining materials whose chemical composition is heterogeneous, which constitutes a major defect for materials whose use properties are directly related to the chemical composition - such as for example that the electronic properties of semiconductors, or the optical properties of the laser materials or scintillators.

In addition, when it is desired to develop monocrystalline concentrated alloys, the composition variations lead to deformations of the crystal which generate defects in the crystal structure, up to the fracture of the sample. Methods for producing alloys and crystals much more homogeneous have therefore been the subject of much research.

To characterize the performance of a crystallogenesis process, we are interested in the following parameters:

- absence of cracks in the crystal - radial homogeneity

- longitudinal homogeneity

- Crystallization or draw rate (expressed in mm / h).

At present, the most commonly used crucible solidification method is the "Bridgman" pull technique. According to this technique, with reference to FIG. 1, the alloy to be crystallized is melted in a crucible 1 located in a vertical furnace

(hatched section) whose temperature is higher in its upper part than in its lower part. Crystallization takes place by slowly moving crucible 1 downwards.

Alternatively, the material can be solidified by gradually decreasing the temperature of the furnace, the crucible being fixed. In the case of concentrated alloys, i.e. where the alloying element has a sufficiently high concentration to change the melting temperature of the material (which typically corresponds to a concentration greater than a few percent), this technique has consequences on the one hand a strong curvature of the interface between the solid S and the liquid L (represented by dashes) which generates crystalline defects sources of cracks, and on the other hand a bad homogeneity of the liquid which produces a heterogeneous material both according to its radius and axis.

As a first approximation, we consider that the shape of the interface is parabolic; the curvature is then defined as being the arrow of the interface at the center of the crucible (that is to say the difference in altitude, on the interface, between the axis and the wall).

Homogeneity is expressed as a percentage of the average concentration: the lower the percentage, the more homogeneous the material. For example, the radial homogeneity of the sample is defined by the ratio: composition at the center - composition at the edge medium composition Similarly, the longitudinal homogeneity is defined by the ratio: composition at the top - composition at the base average composition

With the conventional Bridgman process, the radial homogeneity of the sample is of the order of 100%. Longitudinal heterogeneity results in the rapid loss of the crystal structure of the sample. Optimizations of the Bridgman process with variable crucible displacement rates have been developed (in this respect reference may be made to the article by Stelian et al., "Growth of concentrated GaInSb alloys with improved chemical homogeneity at low and variable pulling rates". ", Journal of Crystal Growth 283 (2005) 124-133), but the homogeneity and crystalline quality of the material, although improved, are not yet optimal and the growth rates are very low.

Another improvement to the Bridgman process was then developed which consists in placing the crucible 1 in an electromagnetic motor 2, as illustrated in FIG. 2. This process is the subject of the patent application EP 1 167 586 and of the article of Mitric et al., "Growth of Ga (i _-X ) In _x Sb alloys by Vertical Bridgman technique under alternating magnetic field", Journal of Crystal Growth 287 (2006) 224-229. According to this method, the electromagnetic motor 2 may be an alternating field coil or a coil generating a magnetic field either rotating or sliding. The magnetic field creates movements in the liquid that homogenize it effectively. This electromagnetic stirring thus makes it possible to obtain crystals which are much more homogeneous than by the conventional technique. The radial homogeneity obtained is of the order of a few tens of percent as described in the article cited above. However, the solid liquid interface remains curved - although to a lesser extent than in the classical Bridgman process - and the crystalline quality of the material is not optimal. The resulting sample cracks after a few centimeters of growth. In addition, there is a loss of the crystalline structure after a few centimeters of growth, which indicates a longitudinal heterogeneity. In addition, this process operates only discontinuously, that is to say that it can not be added in a novel first way in the crucible during crystallization. Indeed, the addition of raw material would disrupt the flow created towards the interface.

Other researchers have also developed a method of plunging a piston into the crucible. This method, called "AHP" (acronym for the English expression "Axial heat flux close to the phase interface" or "axial heat flux near the interface"), is described in the patent application WO 2007 / 064247. Referring to Figure 3, the piston 3 is fixed relative to the furnace and therefore does not descend with the crucible 1. The piston is equipped with a thermocouple and a heating resistor whose power is regulated so that the Piston temperature 3 is kept constant. In these conditions, the interface solid-liquid remains at a constant distance from the piston 3 and is much flatter than before, because it follows the shape of the piston 3.

This technique also has the advantage of being able to operate continuously, since the addition of new raw material does not disturb the region close to the interface. It therefore looks promising on an industrial level.

However, the small volume of liquid between the piston and the interface is substantially at rest and is therefore not homogenized at all. The radial homogeneity measured on samples obtained by this technique is of the order of 10%.

Other researchers have also studied a method combining a heating piston, the application of an electric field in the molten material and a continuous magnetic field. In this regard, reference can be made to the article by Nancy Ma et al., "Vertical gradient freezing of doped gallium-antimonide semiconductor crystals using submerged heater growth and electromagnetic stirring", Journal of Crystal Growth 259 (2003) 26-35. However, this is a very cumbersome technique since it requires several amperes of current passing through the sample (which may be detrimental to the material) and the installation of a large electromagnet around the furnace. On the other hand, the results presented result from numerical simulations which conclude to a better homogeneity, but no experimental results relating to this technique have been published.

For the reasons explained above, no semiconductor alloy is currently available on the market. However, this type of crystal is of the utmost technological importance since a semiconductor alloy makes it possible to obtain physical parameters intermediate between those of the constituent materials. For example, an alloy comprising 50% silicon and 50% germanium has intermediate electronic properties between those of pure silicon and pure germanium.

Moreover, the known crystallogenesis methods are relatively slow - typically, the faster ones have a crystallization rate of the order of 1 mm / h, ie the average duration of production of a crystal. 'expresses in days. One of the objectives of the invention is therefore to allow the preparation of crystalline alloys whose composition is much more homogeneous than by known techniques and which are free of cracks. It is thus sought to obtain perfect longitudinal homogeneity (ie close to 0%) over most of the sample. Regarding radial homogeneity, homogeneity is a few%.

Another object of the invention is to define an industrial process which allows continuous operation and faster crystallization than in the prior art.

BRIEF DESCRIPTION OF THE INVENTION

According to the invention, there is provided a method of crystallogenesis of an electrically conductive material in the molten state, by drawing from a melt of this material in a crucible, wherein:

the molten material is progressively subjected to a decreasing temperature, so that a liquid-solid interface is formed,

the flatness of the liquid-solid interface of the material is controlled,

the molten material is subjected, before and during the solidification, to an electromagnetic stirring, said method being characterized in that said electromagnetic stirring is obtained by the application of an alternating magnetic field, without passing electrical current through the material .

In a particularly advantageous manner, the control of the flatness of the liquid-solid interface is carried out by maintaining a piston whose temperature is controlled at a determined distance from said interface. The temperature of the piston is substantially equal to the sum of the melting temperature of the material and the product of the thermal gradient in the material by the distance between the interface and the piston.

For electromagnetic stirring, an alternating magnetic field is generated whose intensity is between 1 and 10 mT and the frequency is between 1000 and 10000 Hz. Another object of the invention relates to a device for crystallogenesis of an electrically conductive material in the molten state, by drawing from a melt of this material in a crucible, comprising:

means for cooling the molten material; means for controlling the flatness of the liquid-solid interface of the material;

means for generating an electromagnetic stirring of the molten material, said device being characterized in that said means for generating the electromagnetic stirring comprise means for generating an alternating magnetic field in the material. The means for controlling the flatness of the interface advantageously comprise:

a piston whose temperature is controlled, having a flat underside, and

means for maintaining the lower face of the piston at a determined distance from the liquid-solid interface of the material. The means for generating electromagnetic stirring preferably comprise an electromagnetic coil and means for circulating an alternating electric current in the coil.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings in which: FIG. 1 is a diagram of a conventional Bridgman type installation; FIG. 2 is a diagram of an improved Bridgman type installation using an electromagnetic motor; FIG. 3 is a diagram of a Bridgman type installation incorporating the piston of the AHP method; Figure 4 schematically illustrates an installation according to the invention; Figure 5A schematically illustrates the structure of a Bridgman furnace; Figure 5B illustrates the temperature profile within the material in the oven of Figure 5A; Figure 6 illustrates a heating piston.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have discovered that by combining the two techniques, namely simultaneously:

subjecting the molten material contained in the crucible to electromagnetic stirring, and plunging into the molten material a piston maintained at a controlled temperature and at a constant distance from the liquid / solid interface, surprisingly significantly better results are obtained; those that were discounted simply by combining the best results of each of these methods. The block diagram of the device is illustrated in Figure 4.

Description of the device

The device comprises a conventional Bridgman furnace, in which a crucible is movable in translation, to which electromagnetic inductors and a piston whose temperature is regulated are added. Figure 5 shows, in its part A, a cylindrical crucible 1, at the bottom of which was placed a seed G. Above the seed G is disposed the material from which it is desired to manufacture the single crystal. The crucible 1 is placed in a furnace consisting of two heating parts 4 and 5 separated by a thermally insulating zone 6 so as to obtain inside the crucible 1 the temperature profile T represented in part B of FIG.

The temperature profile established during the melting step of the material is centered on the melting temperature Tf of the material. This is the temperature that prevails in the crucible 1 in the part of the melt L which is just in contact with the seed G. The heating portion 4 produces the T1 temperature lower than the melting temperature of the material. The heating portion 5 produces the temperature T2 which is higher than the melting temperature of the material. At this stage of the process, the material constitutes a melt L having an upper zone called hot zone, of height H and temperature T2 sufficiently far from the melting temperature Tf.

The crucible 1 is made of boron nitride. It has a diameter of 11 mm and a depth greater than 90 mm. More generally, those skilled in the art can choose any conventional crucible material, for example graphite or silica for semiconductor materials, noble metals for oxides. In addition, any dimension is potentially conceivable, knowing that the industrial dimensions are a diameter of about 4 inches and a length of several decimetres.

The crucible 1 is movable in translation inside the oven. The means for moving the crucible are known to those skilled in the art and will therefore not be described in more detail. According to a variant of the invention, the crucible is fixed and the furnace is gradually cooled while the coil and the piston rise together with the liquid-solid interface.

The electromagnetic inductors for mixing the liquid can be of any type allowing the application of a rotating, sliding or alternating magnetic field.

The use of a rotating magnetic field has been described, for example, in patent application EP 1 167 586. An electromagnet is used for this purpose, for example the stator of an electric motor.

However, according to a preferred embodiment, an alternating magnetic field will be generated, because this solution has the advantage of requiring a much simpler device, namely an electromagnetic coil in which an alternating electric current is circulated. In this respect reference may be made to the publication of A. Mitric et al. mentioned above.

The electromagnetic coil 2 (illustrated in FIG. 4) is in the form of a coil consisting of 20 turns of copper wire 1 mm in diameter.

The piston 3, whose lower face is flat, is made of graphite. It is fixed relative to the oven. It is equipped with thermocouples and heating means - for example, a heating resistor - able to regulate its temperature to a value determined according to the material to be crystallized.

In a complex version, as illustrated in FIG. 6, the piston 3 comprises four thermocouples referenced Th1 to Th4 and two annular resistors R1 and R2 inside the piston. For this purpose, the piston 3 is hollow and removable. The wires of the thermocouples and the power supply wires of the resistors pass through the tube which holds the piston. The thermocouples serve to regulate the temperature of the heating resistors at a temperature slightly above the melting temperature of the material. A radial temperature difference can be imposed by regulating the resistances to different values.

However, other simpler configurations of the piston are conceivable, such as for example one or two thermocouples and a single resistor. Alternatively, the piston may comprise a single thermocouple which regulates the electric current flowing in the furnace resistor surrounding the crucible to maintain the piston at a desired temperature. Finally, it is also conceivable to have no thermocouple in the piston: it then controls the temperature of the piston by positioning it in the oven at a height whose temperature is known approximately. The present invention of course covers all these embodiments.

Process description

The implementation of the method will now be described with reference to the manufacture of a crystal of an InGaSb concentrated alloy comprising 80% GaSb and 20% InSb. At the bottom of the crucible 1 there is a seed G for initiating the crystallization, and then the raw material, which is melted. The temperature of the junction between the seed G and the molten charge L is the melting temperature of the material. When one moves away from the germ, the temperature of the liquid increases from a few degrees to a few tens of degrees per centimeter, thus defining a thermal gradient expressed in ° C / cm. This thermal gradient is known to those skilled in the art. The crucible 1 is then lowered into the furnace, with a thermal gradient of the order of 40 K / cm and a growth rate of 1 micrometer per second (ie 3.6 mm / h). The material cools gradually crystallizing.

Before and during the solidification, an alternating current of 6 A and frequency 5000 Hz is applied to the coil 2, which generates an alternating magnetic field of

3 mT of intensity. The intensity and frequency of the magnetic field are calculated by the classical equations of electromagnetism. The intensity is calculated by the laws of electromagnetism so as to ensure maximum mixing of the liquid without disturbing the temperature field or the shape or the position of the solid-liquid interface. The frequency is calculated to have an electromagnetic penetration

(skin thickness) of the order of a quarter of the diameter of the sample (inner diameter of the crucible). These quantities are susceptible to large variations from one device to another, in particular as a function of the diameter of the sample. Typically, the alternating magnetic field has an intensity of between 1 and 10 mT and a frequency of between 1000 and 10000 Hz.

The effect of this magnetic field is to generate convection movements inside the molten material that allow to homogenize it.

Furthermore, the piston 3 is brought to a temperature substantially equal to the sum of the melting temperature and the product of the aforementioned thermal gradient by the distance that is desired between the piston and the solid-liquid interface. By substantially equal, it is understood in this text that the actual temperature of the piston may differ by a few degrees (for example ± 10 ° C) from the indicated temperature. On the other hand, the temperature of the piston must not fluctuate during the process, otherwise the interface oscillates. Referring to Figure 6, the piston 3 is maintained at a distance h typically between 5 and 10 mm of the liquid-solid interface.

The heating piston 3 divides the molten material into two zones, respectively lower Z1 and upper Z2.

These zones are connected by a narrow annular space (whose width δ is of the order of 0.5 mm) between the crucible 1 and the piston 3. When the crucible moves relative to the piston, the molten material passes from the upper zone Z2 (ie the zone situated above the piston) to the lower zone Z1 (ie the zone situated between the solidification interface and the piston) .

The effect of the piston 3 is to control the liquid-solid interface by keeping it flat.

Experimental results

The comparative table below can be established for an InGaSb concentrated alloy (comprising 80% Ga and 20% In):

It is thus found that, unexpectedly, much better results (especially in terms of crystallization speed) are obtained with the process of the invention than the results that could have been expected by combining the best performances of the electromagnetic stirring and of the heating piston taken alone.

Possible Applications of the Invention The invention which has just been described with reference to an InGaSb alloy is in no way limited to this alloy.

The invention applies in fact to the crystallogenesis of all semiconductor alloys, such as:

- binary alloys of germanium and silicon, for applications in microelectronics; the ternary alloys of the IM-V family, that is to say based on antimonides (GaSb, AISb and InSb), arsenides (GaAs and InAs) or phosphides (GaP and InP), for applications in fast electronics and optoelectronics;

- ternary alloys of the M-IV family, based on tellurides (CdTe, ZnTe, HgTe) or selenides (CdSe or ZnSe), for applications in the field of detectors for all the range of gamma, X, UV radiation , visible and IR.

The invention can also be applied to the solidification of silicon for photovoltaic applications: it makes it possible to obtain silicon of satisfactory quality from less pure raw material and therefore available in greater quantity and less expensive.

More generally, the invention applies to any type of crucible solidification and may therefore relate to metal alloys, glasses, oxide crystals or halides, provided they are electrically conductive. in the molten state. Finally, it is recalled that the invention is not limited to a device where the piston and the coil are fixed and the crucible movable relative to the furnace; the opposite configuration, where the crucible is fixed relative to the furnace whose temperature is gradually decreased, is a possible embodiment of the invention. In this case, the coil and the piston are movable in translation upwards so as to follow the liquid-solid interface.

Claims

Process for the crystallogenesis of an electrically conductive material in the molten state by drawing from a melt of this material in a crucible (1), in which:

the flatness of the liquid-solid interface of the material is controlled,

- The molten material is subjected, before and during the solidification, electromagnetic stirring, said method being characterized in that said electromagnetic stirring is obtained by the application of an alternating magnetic field.

2. Method according to claim 1, characterized in that the control of the flatness of the liquid-solid interface is performed by maintaining a piston (3) whose temperature is controlled at a determined distance (h) from said interface.

3. Method according to claim 2, characterized in that the temperature of the piston (3) is substantially equal to the sum of the melting temperature of the material and the product of the thermal gradient in the material by the distance between the interface and the piston.

4. Method according to one of the preceding claims, characterized in that, for the electromagnetic stirring, generates an alternating magnetic field whose intensity is between 1 and 10 mT and the frequency is between 1000 and 10000 Hz.

5. Device for crystallogenesis of an electrically conductive material in the molten state, by drawing from a melt of this material in a crucible (1), comprising:

means for cooling the molten material, means for controlling the flatness of the liquid-solid interface of the material,

means (2) for generating an electromagnetic stirring of the molten material, said device being characterized in that said means for generating the electromagnetic stirring comprise means for generating an alternating magnetic field in the material.

6. Device according to claim 5, characterized in that the means for controlling the flatness of the interface comprise:

- a piston (3) whose temperature is controlled, having a flat underside, and

means for maintaining the lower face of the piston at a determined distance (h) from the liquid-solid interface of the material.

7. Device according to one of claims 5 or 6, characterized in that the means for generating the electromagnetic stirring comprises an electromagnetic coil (2) and means for circulating an alternating electric current in the coil.