Segregation of chemical species during the solidification of an alloy is a problem which has been known for a long time. It systematically leads to obtaining materials, the chemical composition of which is heterogeneous, which is a major flaw for materials for which the properties of use are directly related to the chemical composition—such as for example electronic properties of semiconductors, or optical properties of laser or scintillator materials. Further, when elaboration of single-crystal concentrated alloys is desired, changes in composition cause deformations of the crystal which generate crystalline structure defects, which may lead as far as fracture of the sample.
Methods allowing the elaboration of alloys and of much more homogeneous crystals have therefore been the subject of many investigations. In order to characterize the performance of a crystallogenesis method, the benefit lies on the following parameters:
absence of cracks in the crystal,
radial homogeneity
longitudinal homogeneity
crystallization or drawing rate (expressed in mm/h).
Presently, the most used solidification method in a crucible is the so-called Bridgman drawing 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 oven (hatched section), the temperature of which is higher in its upper portion than in its lower portion. Crystallization is performed by slowly moving the crucible 1 downwards. Alternatively, the material may be solidified by gradually lowering the temperature of the oven, the crucible being fixed.
In the case of concentrated alloys, i.e. when the alloy element has a sufficiently high concentration for modifying the melting temperature of the material (which typically corresponds to a concentration above a few percent), this technique has the consequences of strong curvature of the interface between the solid S and the liquid L (illustrated by dashed lines) on the one hand, which generates crystalline defects, sources of cracks, and poor homogeneity of the liquid on the other hand which produces a heterogeneous material both along its radius and its axis. As a first approximation, it is considered that the shape of the interface is parabolic; the curvature is then defined as being the deflection of the interface at the centre of the crucible (i.e. 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 this percentage, the more the material is homogenous. For example, the radial homogeneity of the sample is defined by the ratio:
            composition      ⁢                                        ⁢                                      ⁢      at      ⁢                          ⁢      the      ⁢                          ⁢      centre        -          composition      ⁢                          ⁢      at      ⁢                                        ⁢                                      ⁢      the      ⁢                          ⁢      edge            average    ⁢                  ⁢    composition  Also, the longitudinal homogeneity of the sample is defined by the ratio:
            composition      ⁢                          ⁢      at      ⁢                                        ⁢                                      ⁢      the      ⁢                          ⁢      apex        -          composition      ⁢                                        ⁢                                      ⁢      at      ⁢                          ⁢      the      ⁢                          ⁢      base            average    ⁢                  ⁢    composition  
With the standard Bridgman method, the radial homogeneity of the sample is of the order of 100%. The longitudinal heterogeneity is expressed by rapid loss of the crystalline structure of the sample. Optimizations of the Bridgman method with variable displacement velocities of the crucible have been developed (in this matter, reference may be made to the article of 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 homogeneity and crystalline quality of the material, although improved, are not yet optimum and the growth rates are very low.
Another enhancement to the Bridgman method was then developed which consisted of placing the crucible 1 in an electromagnetic motor 2, as illustrated in FIG. 2. This method is the subject of patent application EP 1 167 586 and of the article of Mitric et al., “Growth of Ga(1-x)InxSb 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 a coil with an alternating field or a coil generating a rotating or sliding magnetic field. The magnetic field generates movements in the liquid, which homogenize it efficiently. With this electromagnetic kneading it is therefore possible to obtain much more homogeneous crystals than with the standard technique. The obtained radial homogeneity is of the order of a few tens of percent as described in the article cited above.
However, the liquid-solid interface remains curved—although to a lesser extent than in the standard Bridgman method—and the crystalline quality of the material is not optimum. The obtained sample cracks after a few centimeters of growth. A loss of the crystalline structure is further observed after a few centimeters of growth, which shows longitudinal heterogeneity. Further, this method only operates batchwise, i.e. no new raw material can be added into the crucible during crystallization. Indeed, adding raw material would perturb the flow generated towards the interface.
Other researchers have moreover developed a method consisting of plunging a piston into the crucible. This method, a so-called “AHP” (acronym of Axial Heat flux close to the Phase interface) method, is described in patent application WO 2007/064247. With reference to FIG. 3, the piston 3 is fixed relatively to the oven and therefore does not move down with the crucible 1. The piston is equipped with a thermocouple and a heating resistor, the power of which is controlled so that the temperature of the piston 3 is maintained constant. Under these conditions, the solid-liquid interface remains at a constant distance from the piston 3 and it is much more planar than previously, since it fits the shape of the piston 3.
This technique also has the advantage of being able to operate continuously, since adding new raw material does not perturb the region close to the interface. It therefore seems to be promising industrially. However, the small volume of liquid located between the piston and the interface is practically at rest and is therefore not at all homogenized. The radial homogeneity measured on samples obtained by this technique is of the order of 10%.
Other researchers have moreover studied a method combining a heating piston, the application of an electric field in the molten material and of a continuous magnetic field. In this respect, reference may be made to the article of 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 flowing through the sample (which may be detrimental to the material) and the installation of a big electromagnet around the oven. On the other hand the presented results result from numerical simulations which draw the conclusion of better homogeneity but no experimental result relating to this technique has been published.
For the reasons stated above, no semiconductor alloy is today offered on the market. Now this type of crystals is of greatest technological importance since with a semiconductor alloy, physical parameters intermediate between those of the constitutive materials may be obtained. For example, an alloy comprising 50% silicon and 50% germanium has intermediate electronic properties between those of pure silicon and pure germanium. Moreover, known crystallogenesis methods are relatively slow—typically the most rapid methods have a crystallization rate of the order of 1 mm/h, i.e. the average time for producing a crystal is expressed in days.
One of the goals of the invention is therefore to allow elaboration of crystalline alloys, the composition of which is much more homogenous than with known techniques and which are free of cracks. It is thereby sought to obtain perfect longitudinal homogeneity (i.e. close to 0%) on the major portion of the sample. As regards radial homogeneity, a homogeneity of a few % is targeted. Another object of the invention is to define an industrial method which allows continuous operation and faster crystallization than in the prior art.