RU2473719C1 - Method of making silicon crystals - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to production of silicon monocrystals by Czochralski method or silicon multicrystals by method of directed crystallisation to be used in making solar cells and modules with higher operating performances. Proposed method comprises preparing initial mix alloyed with boron and its melting. Note here that aluminium is added to produced melt in amount sufficient for allow ratio between concentrations of aluminium and oxygen equal to 1-10².

EFFECT: p-type conductance silicon with low concentration of oxygen.

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Description

The invention relates to the field of producing single crystals of a semiconductor material, more specifically to the production of silicon single crystals by the Czochralski method or by the method of growing multicrystalline silicon by directional crystallization.

In the production of semiconductor silicon crystals by the Czochralski method (Cz method) or directional crystallization method (multy method) using quartz crucibles (SiO ₂ ), silicon ingots are usually obtained containing a high oxygen concentration reaching (5-10) · 10 ¹⁷ at / cm ³ dissolved in the volume of the ingot.

With a certain use of silicon crystals, the oxygen content in its volume should be minimal in order to prevent the formation of oxygen atomic complexes with dopant atoms (for example, B-O, where B is a dopant atom and O is an oxygen atom) or just oxygen complexes in volume material. A large concentration of such complexes leads to the degradation of the lifetime of minority charge carriers (NEC), that is, to the degradation of silicon material for use as, for example, photoelectric converters (PECs). Therefore, to prevent the formation of such complexes, in some cases, a material containing a minimal amount of dissolved oxygen in its volume is necessary. Such a material can be obtained by directional crucibleless zone melting (Fz), which significantly increases the cost of silicon ingot production technology, and also does not allow to obtain crystals of large diameter (more than 150 mm).

The main source of oxygen in silicon is associated with the dissolution of silicon dioxide from a quartz crucible in a silicon melt. In addition, in the starting material (raw silicon or silicon revolutions), the oxygen content is already at the level of (5-10) · 10 ¹⁷ at / cm ³ .

To obtain p-type silicon crystals of conductivity used for the manufacture of photovoltaic cells, boron-doped silicon crystals are used to make wafers with a size of (156 × 156) mm or more. At a high oxygen concentration in the volume of such plates as a result of primary solar (light) degradation (LID - light induced degradation), the efficiency of the photomultiplier can decrease to (10-15)% of the initial value during several hours of continuous illumination of the photomultiplier with intense light stream.

), что приводит к сильной неоднородности удельного сопротивления вдоль оси кристалла.Existing methods for stabilizing silicon associated with a decrease in oxygen concentration are either a complication of technological equipment, for example, magnetic mixing of the melt, or the use of other acceptors, such as gallium, that do not form stable oxygen complexes instead of boron. Known acceptors (elements of group III of the Periodic system) cannot uniformly compete with boron atoms in the case of the uniform distribution of the concentration of dopant along the axis of the ingot (the effective distribution coefficient of which

), which leads to a strong heterogeneity of the resistivity along the axis of the crystal.

A known method for producing single-crystal silicon, including melting the original silicon in a crucible, introducing a crystal seed, drawing a crystal from a melt in a rotating crucible onto a rotating seed with the same direction of rotation of the crucible and the crystal, while the crystal rotates during the process of obtaining it, the rotation speed of the crucible and the rotation speed of the crystal is gradually increased, while maintaining approximately constant the ratio of the angular velocity of rotation of the crucible and crystal / 1 /. The method allows to obtain single crystals of silicon with a uniform radial and along the length of the crystal distribution of the dopant and oxygen. The disadvantage of this method is the inability to reduce the total amount of oxygen in the volume of the obtained material in this case. In fact, the oxygen content increases with such a process.

There is another method for growing silicon single crystals by the Czochralski method / 2 /. In this case, the growth is carried out with the rotation of the crucible with an angular speed of 0.2-2.0 rpm and with the rotation of the crystal towards it with a speed of 0.2-2.0 rpm and with the arrow of deflection of the crystallization front towards the seed no more than 0.2, which allows to obtain crystals with a reduced density microdefects. The disadvantage of this method is the inability to effectively reduce the oxygen content or its distribution in the volume of single-crystal material.

Closest to the claimed essence (hereinafter the prototype) is a method for producing doped single crystals and multicrystals of silicon, which consists in preparing the initial mixture containing 50% polycrystalline silicon doped with phosphorus or boron, with the addition of 50% of scrub (waste) produced by polycrystalline silicon and production waste single-crystal silicon, its melting and subsequent crystal growth from a melt, into which elements of the fourth group of the periodic table are additionally introduced, into which ETS use germanium, titanium, zirconium or hafnium in concentrations October ¹⁷ -7 × 10 ¹⁹ cm ^-3 / 3 /. This allows you to get crystals with high values of the lifetime of the NZ, high uniformity of resistivity along the axis of crystal growth and high uniformity of the distribution of oxygen along the length of the ingot with increased thermal stability and radiation resistance / 3 /. The disadvantage of this solution is the lack of the ability to effectively reduce the oxygen concentration in the ingot during the application of this technology, only the uniformity of the oxygen distribution in the grown ingot is improved.

The objective of the present invention is to obtain single crystals or multicrystals of silicon p-type conductivity with low oxygen concentrations in the volume of the material grown by the Czochralski method, or multicrystals of silicon by the method of directional crystallization.

The method of producing silicon crystals includes preparing the initial mixture doped with boron, melting the mixture, characterized in that aluminum is introduced into the melt in an amount sufficient to satisfy the ratio of aluminum and oxygen concentrations in the silicon melt in the range of 1.0-10 ² .

The concentration of aluminum added to the initial silicon-based charge is determined by the oxygen content in the starting material (raw silicon or revolutions of silicon doped with boron) and the conditions of the crystallization process (dissolution of a quartz crucible, oxygen content in an inert gas, and time) the range from 10 ¹⁸ to 10 ²⁰ at / cm ³ .

For the successful application of the method, alloying additives must satisfy the following requirements:

- must have low solubility in a silicon crystal, that is, a low distribution coefficient in the solid phase - K _eff <<1;

- must have a high affinity for oxygen;

- should not impair the properties of semiconductor silicon (structure, uniformity of electrophysical properties, high lifetime of minority charge carriers);

- during the production of devices and their practical operation, the properties of the material must meet the technical conditions;

- in relation to the production of solar cells, additives in the melt should reduce (or eliminate) the effect of primary light degradation.

As additives, in addition to the main dopant (boron), active metals can be added to the silicon melt, forming strong oxides, such as aluminum or electronically neutral atomic impurities in the bulk of the crystal, for example, magnesium, calcium, strontium, barium, zirconium hafnium.

When growing a p-type crystal of conductivity doped with boron, metal aluminum was used as an additive in the melt. The use of aluminum as the main acceptor is hindered by the fact that, unlike boron, aluminum has a low distribution coefficient (K _eff ~ 0.003), which does not allow obtaining a uniform distribution of the impurity in the bulk of the single crystal. The use of aluminum to bind oxygen in the melt makes it possible to co-alloy the crystal in the Si: B: Al system.

Concrete example

The calculated amount of aluminum corresponded to the oxygen content in silicon - 1 · 10 ¹⁸ at / cm ³ and for 50 kg of the initial charge was 10 g (aluminum with a purity of 99.99 weight percent of the basic substance).

The control sample Si: B contained in the initial part 9 · 10 ¹⁷ at / cm ³ oxygen, in the final part - 6 · 10 ¹⁷ at / cm ³ .

When doping with boron and aluminum (Si: B: Al), the distribution of resistivity along the length of the ingot grown by the Czochralski method was 1-2 Ω · cm, the oxygen content varied from 1 · 10 ¹⁶ at / cm ³ at the beginning of the ingot to 5 · 10 ¹⁷ at / cm ³ in its end part. Plates (125 × 125) mm were cut out of Si: B: Al single crystals, from which solar cells with an average efficiency of ~ 17.5% were made. The lifetime of the SC of a Si: B: Al single crystal was 60 μs - top, 40 μs - bottom.

The study of the effect of primary solar degradation in the resulting solar cells under the influence of intense light radiation (~ 1000 W / m ² ) for 6 hours of continuous exposure either did not show a change in the solar cell efficiency at all, or these changes were no more than 1-3% relative to the initial value Efficiency, while control samples doped only with boron atoms showed a change in efficiency by 10-15% in the direction of decreasing this parameter during similar tests.

Information sources

1. RF patent No. 2177513 from 09.20.2000

2. RF patent No. 2278912 of 01.20.2006,

3. RF patent No. 2250275 of 06/30/2003 - a prototype.

Claims (1)

A method of producing silicon crystals, including the preparation of the initial mixture doped with boron, melting the mixture, characterized in that aluminum is introduced into the melt in an amount sufficient to satisfy the ratio of aluminum and oxygen concentrations in the silicon melt in the range of 1.0-10 ² .