In particular, the present invention concerns cyclical twinned crystals of semiconductor materials, especially of silicon, crystallizing into a diamond cubic structure, and a method for producing them.

2. Description of the Related Art Main materials which are used for manufacturing solar cells are dislocation-free silicon single crystals produced using the Czochralski method (so-called CZ crystals) and multicrystalline silicon produced by casting. The raw materials used in these methods are semiconductor-grade virgin polysilicon products of semiconductor purity and scrap of monocrystalline silicon ingots produced from virgin polysilicon by Czochralski method or float zone method for microelectronics and power electronics applications.

According to a rough estimate, it would be necessary to have roughly 84,000 m tones of high purity virgin polysilicon for the production of dislocation-free silicon single crystals based on the Czochralski method and cast multicrystalline silicon ingots in order to satisfy world requirements for solar cells amounting to-1400 MW/a. Virgin polysilicon is mainly raw material for producing silicon single crystals designated for microelectronics. The costs of such raw material costs is very high (approx. 50 USD per kg) in order to use it for producing the substrate material for solar cells. At the same time, it is not possible to use low-quality silicon, such as cleaned metallurgical grade silicon, for the production of dislocation-free silicon due to the fact that the dislocation-free crystal growth

process is extremely sensitive to the impurities and foreign particles, even if they are very fine.

In order to use affordable, poor-quality raw materials for producing substrates for solar cells with a sufficiently high efficiency, it is necessary to develop a special method for the production of silicon crystals with satisfactory structure and acceptable physical characteristics.

Another challenge in the production of solar cells is to significantly decrease the thickness of the implemented substrates (wafers) in order to reduce the costs of solar cells and promote the use of silicon crystals with a low lifetime of the minority carriers for the production of solar cells with a high efficiency. In spite of the fact that silicon is sufficiently durable, the silicon single crystals are very fragile due to the presence of four planes (111) in them, which are cleavage planes fully intersecting the single crystal. This is the main reason why thin silicon wafers are very easy to break. That is why it is virtually impossible to slice silicon single crystals in very thin wafers with a high yield.

In this context, it seems reasonable that large-grain silicon crystals having a regular structure could be a suitable material for resolving this problem.

The first attempts to develop a method for producing large-grain silicon crystals with a regular twin structure for use in solar cells were carried out by G. Martinelli and R. Kibizov in 1992 and published in G. Martinelli, R. Kibizov"Growth of stable dislocation-free 3-grain silicon ingots for thinner slicing"Appl. Phys. Letters, Vol.

62, June 21,1993, pp. 3262-3263. The material produced represented a crystalline semiconductor silicon with three adjacent, sectorially positioned monocrystalline zones, the so-called three-grain silicon. This work documented the possibility of ultra-thin slicing three-grain silicon ingots into wafers and revealed the possibility of their implementation for producing highly efficient solar cells.

FIG. 1 shows a cross-sectional view of a crystal produced by this technique. The structure comprises a host crystal 140 and two twinned crystals 150,160. Each of the twinned crystals 150,160 is joined through a coherent first-order {111}- {111}

twin plane 110,120 to the host crystal 140. Between the two twinned crystals 150,160, there is a second-order {221}- {221} twin boundary 130. The structure of FIG. 1 will be referred to in the following as basic cyclical twin structure.

DE 43 43 296 C2 describes a method for producing three-grain silicon crystals.

The process includes the preparation of seed crystals, during which three regular octahedrons with all surfaces aligned to the crystallographic planes {111} are sawed out of a silicon single crystal. Then, a two-grain ingot is grown from the melt using two prepared octahedrons as seed crystal which are arranged one to another in a twin position and which are linked together by a molybdenum wire.

Furthermore, a prismatic sector is sawn out from the grown ingot along the planes {111}, and a third octahedron is placed in twin position in the sawn-out prismatic sector of the grown crystal and is linked by way of a molybdenum wire. The two- grain crystal is shortened to the length of the third inserted octahedron, and finally, a three-grain crystal is grown from the silicon melt by means of the seed crystal prepared in this manner.

While this process allows for the production of three-grain crystals, it does have a number of disadvantages: 1) It is very difficult to produce octahedron crystals and cut out a prismatic sector with planes having a precise crystallographic orientation {111}. It is also very difficult to produce the mechanical interconnection of the octahedron crystals and the octahedron crystal with the cutout area of the two-grain crystal exactly in the twin position and with precise coincidence of the crystal lattices. This fact involves the creation of tensile forces and structural defects in the grown ingot along the twin boundaries.

2) Technically, the use of crystals in the form of interconnected octahedrons as seed crystals is extremely difficult due to the high value of the diameter/length ratio, which is roughly 2. Seed crystals with a diameter of approx. 12 mm and a length of 100 to 150 mm are implemented in conventional technologies for growing silicon ingots. It is also possible to use seed crystals with a length of 30 to 50 mm, but

not any smaller. If interconnected octahedrons are implemented as seed crystals, then the diameters of said crystals with a length of 30 to 50 mm must be 60 to 100 mm, which complicates the growth process to a considerable extent.

3) It is possible to grow three-grain ingots at relatively higher pulling rates than in the case of monocrystalline ingots due to the generation on the crystallization front of the so-called re-entrant corners, formed by the planes {111} at the outgoing sites of the twin boundaries, but there exists the following limitation. As was shown by R. S. Wagner in Acta Metallurgica, Vol. 8,1960, pp. 57-60 and D. R. Hamilton and R. G. Seidensticker in Journal of Applied Physics, Vol. 31,1960, pp.

1165-1168, these re-entrant corners form areas (sites) of easier nucleation. However, it was also shown in the aforementioned publications that the presence of no less than two or more adjacent twin planes is a prerequisite for the self-reproduction of the re-entrant corners and the rapid crystal growth. Otherwise the re-entrant corners will vanish and the rapid growth ceases.

Ff 0 106 729 B1 suggests, with regard to practical implementation, a simple method for producing seed crystals, ingots and wafers with cyclical twin structure and crystals with cyclical twin structure, from the basic three-grain structure discussed above up to a full twenty-grain structure. Such twin crystals contain coherent twin planes of the first order, twin boundaries of the second order and may contain twin boundaries of the third order.

For example, FIG. 2 shows a cross-sectional view of a crystal having a basic cyclical twin structure and additional co-parallel even number coherent first-order twin planes. Similarly, FIG. 3 shows a cross-sectional view of a crystal having a basic cyclical twin structure and angularly displaced coherent first-order twin planes together with second and third-order twin boundaries. It is to be noted that other cyclical twin structures exist where first-order twin planes and second and third-order twin boundaries are found at different locations and in different directions, and are present in varying numbers. Like in FIG. 1, an undecorated

line in the figures represents a coherent first-order {111}- {111} twin plane, and a line having two adjacent crossing marks represents a second-order {221}- {221} twin boundary. Similarly, a line having three adjacent crossing marks represents a third-order {552}- {552} twin boundary.

While FIG. 1 shows the basic three-grain structure, and FIGs. 2 and 3 depict intermediate cyclical twin structures, a full cyclical twin structure is shown in FIG. 4. Compared to the basic structure, the full structure has angularly displaced additional 18 coherent first-order twin planes, 3 second-order twin boundaries and 6 third-order twin boundaries, together with a plurality of even number coherent first-order twin planes parallel to said 18 angularly displaced first-order twin planes.

The method of FI 0 106 729 B1 for growing silicon crystals with cyclical twin structure possesses a number of advantages which include: 1) The possibility of radically improving growth technology (e. g. utilization of semicontinuous multiple growth), which allows for reducing the costs of these crystals.

2) The possibility of using poor-quality raw materials (more affordable) without impairing the parameters of the devices (solar cells), which are produced on the basis of crystals with cyclical twin structure and thus the possibility of reducing costs.

3) The possibility of ultra-thin slicing of crystals with cyclical twin structure into wafers thanks to the strengthening of the crystals by the twin planes and boundaries and thus the possibility of reducing costs.

The method does not allow however for growing crystals with a sufficient length, which in turn limits the productivity and any further reduction in costs.

It is known that coherent twin planes of the first order are electrically inactive, since such a twin formation does not result in generating broken and deformed bonds. Twin boundaries of the second and third orders are by nature defective.

Normally, they occur as a result of the joining of twin individuals. Such joining

results in the formation of deformed and broken bonds. The higher the order of the twin boundary, the more irregular is the boundary. Moreover, the structure of the twin boundaries of the second and third orders depends, in addition to the crystallographic factors, on the growth conditions and may differ from a equilibrium structure. Irrespective of the relatively stabile and reproducible growth of crystals with cyclical twin structure, growing such crystals of great length without losing the regular structure is complicated in this context. Growing such crystals usually leads to the generation of dislocations by non-coherent twin boundaries (of second and third orders). As a result, polycrystalline inclusions occur, starting from the length of about 250 to 300 mm. This fact limits the productivity of the process to produce crystals with a regular cyclical twin structure without any polycrystalline inclusions.

SUMMARY OF THE INVENTION The object of the present invention is to increase the length of grown silicon crystals with a cyclical twin structure without polycrystalline inclusions and consequently to enhance the productivity of the process of producing such crystals.

Another object of the present invention is to develop a method for producing silicon crystals with improved characteristics to produce highly effective solar cells using the said silicon crystals.

Accordingly, an embodiment of the present invention provides a method of producing silicon with a cyclical twin structure by growth from a silicon melt using a seed having a cyclical twin structure. The cyclical twin structure of the seed comprises at least two coherent twin planes of the first order and at least one twin boundary of the second order. The method comprises the step of adding to the silicon melt at least one isovalent additive having a radius greater than that of silicon.

In another embodiment, a silicon ingot with a cyclical twin structure is provided that is produced by performing the above method.

Still another embodiment provides a silicon wafer with a cyclical twin structure, produced by slicing from the above silicon ingot.

According to yet another embodiment, there is provided a silicon photovoltaic device such as a solar cell, manufactured using a silicon wafer that was produced by performing the method of the above embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings are incorporated into and form a part of the specification for the purpose of explaining the principles of the invention. The drawings are not to be construed as limiting the invention to only the illustrated and described examples of how the invention can be made and used. Further features and advantages will become apparent from the following and more particular description of the invention, as illustrated in the accompanying drawings, wherein: FIG. 1 shows a cross-sectional view of a crystal having a basic cyclical twin structure; FIG. 2 shows a cross-sectional view of a crystal having a basic cyclical twin structure and additional co-parallel even number coherent first-order twin planes ; FIG. 3 shows a cross-sectional view of a crystal having a basic cyclical twin structure and angularly displaced coherent first-order twin planes together with second and third-order twin boundaries; FIG. 4 shows a cross-sectional view of a crystal having a full cyclical twin structure and a high and additional even number coherent first-order twin planes ; and FIG. 5 is a flowchart illustrating the process of producing silicon crystals having a cyclical twin structure according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION The illustrative embodiments of the present invention will be described with reference to the figure drawings wherein like elements and structures are indicated by like reference numbers.

As will be described in more detail below, the present embodiment provides a method to produce silicon crystals with cyclical twin structure by way of growth from a melt by the Czochralski method using the seed crystal with cyclical twin structure from the basic three-grain structure formed by two coherent twin planes of the first order and a twin boundary of the second order, through to the full cyclical twin structure, which is formed by twenty coherent twin planes of the first order, four twin boundaries of the second order, six twin boundaries of the third order and an even number of additional twin planes of the first order parallel to the said twenty planes. Growth is realised according to the present embodiment by adding additives to the silicon melt, selected from a series of germanium (Ge), tin (Sn) and lead (Pb), whereby the concentration of the additives is in relation to the silicon (1. 0x10-7-15 wt. percent).

The present embodiment is based on the idea of growing long (preferably greater than 300 mm) silicon crystals from a melt in the direction <110> with a variable cyclical twin structure: from the"basic"cyclical twin structure, which is formed by two coherent twin planes of the first order and a twin boundary of the second order, up to the"full"cyclical twin structure, which is formed by twenty coherent twin planes of the first order, four twin boundaries of the second order and six twin boundaries of the third order, in relation to the degree of supercooling of the melt. In doing so, passivation additives with larger atomic radii than the radii of silicon atoms are added to the silicon melt (to the initial silicon charge) in order to passivate the non-coherent twin boundaries, i. e. to decrease the probability of generation of dislocations by these boundaries and the formation of polycrystalline inclusions. Such passivation additives may preferably be isovalent impurities like germanium, tin and lead.

Studies of the structure of twin boundaries in crystals with a diamond structure were published in Kohn J. A. , American Mineralogist, 41, No. 9/10,778-784

(1956); Kohn J. A. , American Mineralogist, 43, No. 3/4,263-284 (1958); Hornstra<BR> J. , Physica, 25 (6), 409-422 (1959); and Hornstra J. , Physica, 26 (3), 198-208 (1960). These publications showed that there are deformations, increased centre- to-centre distances and broken bonds in the areas of the twin boundaries of the crystals and that the twin boundaries"choose", during their formation, the sites with the lowest energy and may assume a zigzag shape.

The conditions for crystallization differ from the equilibrium conditions in real growth processes. That is why the twin boundaries are not always able to find the most favorable sites in terms of energy, which must result in increasing the energy of the stress fields formed by them. The relaxation of these stresses during the growth process is precisely the reason for the production of crystal imperfections and the formation of polycrystalline inclusions.

The implementation of additives with atoms of other sizes in the melt provides the growing crystal with expanded possibilities of selection to form twin boundaries with minimum energy and allows for"remedying"boundaries already formed.

The occurrence of impurity segregation in the area of the grain boundaries is a confirmation of the tendency of the crystal to use the atoms of the additives for forming boundaries with minimum energy.

Isovalent additives, such as germanium, tin and lead, may be used for growing silicon crystals, since their addition to the crystal does not result in an uncontrolled change of the charge carrier concentration. These additives can be introduced either individually or jointly to the crystal (the melt).

The preferred level of concentrations of additives for forming stable inactive twin boundaries during the growth process may depend on the specific growth conditions: the parameters of the growth process (i. e. , the rates of movement and rotation of the crucible and the crystal, the construction of the hotzone, the <BR> <BR> consumption and the residual pressure of the inert gas, etc. ), the concentration of the main dopant element, and the degree of purity of the raw material. In the process of this embodiment, the total concentration of germanium and/or tin and/or lead for the purpose of addition to the liquid silicon (the charge) may be

selected from the range of the concentration variables in relation to the silicon (1. Ox10-7-15 wt. percent): C Additive = C Germanium + C Tin + C Lead = (1. 0x10'-15 wt. percent) Adding the aforementioned additives in accordance with the proposed method allows for improving the stability of the twin boundaries and for growing silicon crystals with cyclical twin structure and a length greater than 300 mm without any polycrystalline inclusions.

Moreover, adding germanium, tin and lead to the silicon crystals results in changing the silicon's band structure and to a certain extent in broadening the optical absorption spectrum and in decreasing the energy to generate electron- hole pairs during the irradiation of such crystals with light. This results in increasing the efficiency of solar cells manufactured on the basis of silicon with isovalent additives.

In the following, examples of actual applications are described, noting that these examples do not limit the scope of the invention.

Silicon crystals with cyclical twin structure and with/without the addition of isovalent additives were grown by Czochralski method from a crucible with a diameter of 330 mm with 22 kg charge in a Redmet-30 crystal grower. The parameters of all growth tests carried out were roughly identical. Silicon scrap was used as a raw material with hole conductivity and a resistivity in the range of 0.9 to 1.1 Ohm*cm. Growth was implemented in an argon flow of 1600 I/h at a residual argon pressure of 10 mm mercury column (Torr). The crystals were grown with a pulling rate of 1.2 mm/min at the beginning of the cylindrical part and up to 0.6 mm/min at the end of the growing process. The rates of rotation of the crystal and the crucible were 12 rpm and 5 rpm respectively. The grown crystals had a diameter ranging from 135 to 138 mm, the length of the cylindrical part was 540 to 570 mm.

Specially prepared crystals with a specified cyclical twin structure were used as seed crystals with an orientation in the direction <110>. Used seed crystals had a rectangular shape with a cross-section of 10 mm x 10 mm and a length of 120

mm. The seed crystals exhibited the cyclical twin structure of three different types.

Seed crystal No. 1 possessed a basic cyclical twin structure which was formed by two radially positioned coherent twin planes of the first order {111}- {111} and a twin boundary of the second order {221}- {221}, which are parallel to the growth direction <110> and intersect in the centre of the seed crystal. Moreover, the seed crystal No. 1 contains an even number of additional coherent twin planes of the first order parallel to the coherent main twin planes of the cyclical twin basic structure.

Seed crystal No. 2 possessed a basic cyclical twin structure (as described above) with additional, radially positioned, parallel and nonparallel coherent twin planes of the first order and three twin boundaries of the second order.

Seed crystal No. 3 possessed a complete cyclical twin structure, including a basic cyclical twin structure with additional radially positioned parallel and nonparallel coherent twin planes of the first order and four twin boundaries of the second order and six twin boundaries of the third order.

Altogether twelve crystals were grown, three of which without additives and nine of which with additives. The stability of the growth process of the silicon crystal with specified cyclical twin structure was analyzed based on the critical value of the crystal length (Lcr), as of which the polycrystalline structure begins to form.

The larger the critical length, the greater the stability of the growth process. The results of the growth process are presented in the following table. Seq. Seed Composition by Weight of Components in the Charge Critical No. crystal Silicon Additives Length, No. Germanium Tin Lead Total (Lcr), mm 1 No. 1 22. 0 kg 285 2 No. 2 22. 0 kg 195 3 No. 3 22. 0 kg 145 4 No. 1 22. 0 kg--0. 022 mg 0. 022 mg 325 (1x10-7 wt. %) (1x10-7 wt. %) 5 No. 3 22. 0 kg 0. 11 mg 0. 11 mg 0. 22 mg 310 (5x10-wt. %) (5x. 10' ut. %) (1x10-wt. %) 6 No. 1 22. 0 kg 6. 6 mg 11 mg 17. 6 mg 380 (3x10-5 wt. %) (5x10-5 wt. %) (8x10-5 wt. %) 7 No. 3 22. 0 kg 1. 1 g--1. 1 g 375 (5x10-3 wt. %) (5x10-3 wt. %) 8 No. 1 22. 0 kg-11 g 11 g 495 (5x10-2 wt. %) (5x10-2 wt. %) 9 No. 3 22. 0 kg 110 g-110 g 220 g 430 (0. 5 wt. %) (0. 5 wt. %) (1. 0 wt. %) 10 No. 3 20. 952 kg-628. 8 g 419. 2 g 1. 048 kg 425 (3.0 wt. %) (2.0 wt. %) (5.0 wt. %) 11 No. 1 20 kg 0. 6 kg 0. 4 kg 1 kg 2 kg 480 (3.0 wt. %) (2.0 wt. %) (5.0 wt. %) (10.0 wt. %) 12 No. 3 19. 130 kg 0. 957 kg 0. 957 kg 0. 956 kg 2. 870 kg 390 (5.0 wt. %) (5.0 wt. %) (5.0 wt. %) (15.0 wt. %)

The data indicated in the table clearly shows that the stability of the growth process is considerably greater during the growth of silicon crystals with cyclical twin structure with the addition of germanium, tin and lead than in the case of growing silicon crystals without any additives.

Given the description above, a method for producing silicon crystals with cyclical twin structure is provided. Referring to the flowchart of FIG. 5, the method may for instance comprise the steps 510 and 520 of preparing the melt and adding the additives. Then, the seed is dipped into the melt in step 530, and the crystal is fabricated in step 540. Finally, wafers are sliced from the grown material (step 550). Of course, the invention is not limited to the depicted order of process steps but may differ therefrom.

Compared to other known methods, the utilization of the proposed method for producing silicon crystals with cyclical twin structure offers the following advantages: 1. The possibility of significantly increasing the length of the grown silicon crystal with cyclical twin structure without polycrystalline inclusions and thus considerably enhancing the productivity of the technology to produce these crystals.

2. Broadening the optical absorption spectrum for silicon by adding isovalent additives with larger atomic radii. Reduction of the energy to generate electron-hole pairs during the irradiation of such crystals with light and improvement of the efficiency of the solar cells fabricated on the basis of such materials.

3. Reduction of the costs for silicon crystals with cyclical twin structure and solar cells which are fabricated on the basis of such crystals.

While the invention has been described with respect to the physical embodiments constructed in accordance therewith, it will be apparent to those skilled in the art that various modifications, variations and improvements of the present invention may be made in the light of the above teachings and within the purview of the appended claims without departing from the scope of the invention. In addition, those areas in which it is believed that those of ordinary skill in the art are familiar, have not been described herein in order to not unnecessarily obscure the invention described herein. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrative embodiments, but only by the scope of the appended claims.