RU2600381C1 - Method for growing monocrystals of substances with density exceeding density of their melt - Google Patents

Abstract

FIELD: manufacturing technology.

SUBSTANCE: invention relates to growing monocrystals from a liquid melt using Czochralski's method. Growing a crystal with radius r is first performed using the Czochralski's method by drawing from fixed pot with radius R₁, so that

where ρ_hd - density of the crystal, ρ_l - is density of the melt. Produced crystal is tore off the melt and cooled to room temperature in a growth chamber. Then, the growth chamber is opened, the pot is removed from the heater replaced with a pot of smaller radius R₂, such that

thereafter the chamber is closed, temperature is raised to melting point, crystal is lowered to contact with the melt and the crystal is again grown by its permanent downward movement.

EFFECT: technical result is improved structural perfection of grown crystals due to reduction of residual mechanical stresses and reduction of density of dislocations.

1 cl, 6 dwg, 2 ex

Description

The invention relates to methods for growing single crystals from a melt.

The known Czochralski method, which consists in pulling upward from a crucible with a molten crystal onto a rotating seed [Wilke KT. Crystal growing. L .: Subsoil. 1977.S. 600; Modern crystallography // Ed. B.K. Weinstein, A.A. Chernova, L.A. Shuvalova M .: Nauka, 1980.V. 3. 346 p.]. This method is used for crystals whose density ρ _{TV is} greater than the density of their melt ρ _W , for example, for paratellurite crystals [Mochalov I.V. Growing optical crystals // St. Petersburg. 2012. P. 126. 37 pp.], And for crystals whose density is less than the density of their melt, for example, for germanium crystals [Kolesnikov AI, Kaplunov IA, Terentyev IA Defects of various dimensions in large-sized single crystals of paratellurite // Crystallography. 2004.V. 49. No. 2. S. 229-233]. In all well-known patents, technical documents and scientific publications related to the Czochralski method, the direction vertically is indicated as a direction for drawing. Only in a single patent of the Russian Federation (RU No. 2491375, publ. 08.27.2013) is used pulling at an angle to the vertical. There is also known a method of directional crystallization (RU No. 2241792, published December 10, 2004), in which the crystal is first pulled out of the melt upward along the Czochralski, and when a certain diameter is reached, the upward movement ceases and the crystal grows in the horizontal plane due to lowering the temperature until until it reaches the inner walls of the crucible.

The disadvantage of all these options for implementing the Czochralski method is that when pulled up, the crystal moves to a colder region of space than the region near the crystallization front. As a result of this, the vertical (axial) temperature gradient ∇T is, firstly, quite large (in any heating systems, including systems with additional top heaters, its values are at least 10 K cm ^-1 ), and in secondly, as the stretch takes place, the vertical temperature gradient, especially near the upper end of the crystal (in its seed part), increases unevenly. This prevents the production of low-dislocation and dislocation-free crystals due to the failure to fulfill the known [Modern Crystallography // Ed. B.K. Weinstein, A.A. Chernova, L.A. Shuvalova M .: Nauka, 1980. V. 3. 346 p.] Conditions ∇T = const.

In the closest to the claimed prior art method for producing single crystals with a density higher than the melt density, a method for growing paratellurite single crystals from a Czochralski melt from a fixed crucible is used with programming the speed of drawing and rotation of the seed, characterized in that after reaching the required diameter the cylindrical drawing parts of the crystal are carried out at rotational speeds corresponding to the range of Reynolds numbers Re = 100-150, calculated according to the formula Re = ω · r (Rr) / ν, where ω is the speed of rotation of the seed (s ^-1 ); R is the radius of the crucible (cm); r is the radius of the crystal (cm); ν is the kinematic viscosity of the melt (cm ² · s ^-1 ), at which a stable system of two diametrically opposed convective cells of a supercooled melt turning around the crystal is darker in color than the rest of the melt surface (RU 2338816 C1, publ. 20.11.2008 )

The application of the method leads to a partial decomposition of the formed defects in the crystal structure, to a reduction in mechanical stresses in the material, and to an improvement in its structural quality as a whole. However, the prototype is characterized by a drawback common to all variants of the Czochralski method — moving the growing crystal upward — into the cold zone with large non-uniform temperature gradients, leading to an increase in the concentration of various defects in the material structure.

The problem to which the claimed technical solution is directed is to improve the conditions for growing crystals from a crucible with a melt and, as a result, improve their structural quality.

, где ρ_тв - плотность кристалла, ρ_ж - плотность расплава, полученный кристалл после охлаждения, извлечения из печи и установки другого тигля с меньшим радиусом R₂, таким, что

, используется в качестве затравки при выращивании из этого тигля кристалла радиусом r путем его постоянного перемещения вниз.This problem is achieved due to the fact that in the method of growing single crystals of substances having a density higher than the density of their melt, including preliminary drawing a crystal of radius r by the Czochralski method from a stationary crucible of radius R ₁ , such that

, where ρ _tv is the density of the crystal, ρ _f is the melt density, the obtained crystal after cooling, extraction from the furnace and installation of another crucible with a smaller radius R ₂ such that

, is used as a seed when growing a crystal of radius r from this crucible by constantly moving it down.

The technical result provided by the given set of features is to improve the structural perfection of the grown crystals by reducing the residual mechanical stresses in them and reducing the density of dislocations.

The invention is illustrated by drawings.

In FIG. 1 shows a diagram of the implementation of the classical Czochralski method in analogues and prototype.

In FIG. 2 presents a diagram of the implementation of the proposed method of growing a crystal of supercritical radius by lowering it down.

In FIG. Figure 3 shows a paratellurite crystal with a diameter of d = 71 mm, pulled upward by the Czochralski method from a crucible with a diameter of D = 100 mm.

In FIG. Figure 4 shows a paratellurite single crystal with a diameter of d = 71 mm, grown by the classical Czochralski method from a crucible with a diameter of D = 100 mm.

In FIG. 5 shows a single crystal of paratellurite with a diameter of d = 71 mm, grown according to the invention by lowering into a crucible with a diameter of D = 75 mm.

In FIG. 6 shows a single crystal of paratellurite with a diameter of d = 71 mm, grown according to the invention by lowering into a crucible with a diameter of D = 75 mm.

To achieve this goal with respect to crystals of substances in which the density of the solid phase ρ _{TV is} higher than the density of the liquid phase (melt) ρ _W , it is proposed not to pull upwards, as is done in the classical Czochralski method, but move the crystal in the opposite direction - down , which can also be defined as the “indentation” of a crystal into a melt. This possibility is explained as follows.

When pulled from the crucible the melt upward (FIG. 1) true growth rate in the vertical direction V _ist is not equal to the drawing speed V _in the crystal by lowering the level of the melt, and it is always greater than V _ist> V _in. The ratio between these speeds is derived from the condition of mass balance.

The mass of the formed new solid phase (crystal) during stretching for some time τ (in Fig. 1 corresponds to the shaded volume with a constant velocity V _c ) should be equal to the mass of the volume of the decreased melt in the crucible, determined by the difference between the initial and final levels of the liquid phase.

where S _cr = πr ² is the cross-sectional area of the crystal, r is the radius of the crystal, S _t = πR ² is the surface area of the melt in the crucible, R is the radius of the crucible, V _p is the rate of lowering of the melt relative to the walls of the crucible. Since this speed is equal to the difference between the true growth rate and the drawing speed, i.e. V _p = V _East -V _in , substituting this difference in equation (1) and reducing both its parts by the time τ and π, we obtain the equation for the true growth rate V _East :

. Так, для кристаллов германия (ρ_ж=5,61 г/см³, ρ_тв=5,3 г/см³) скорость роста оказывается в 20 раз больше скорости вытягивания.From equation (2), important technical conclusions follow. For crystals of the substances in which the melt density greater than the density ρ _x ρ _tv solids at low, tending to zero radii crystals 0 r →, V → V _{ist into a} growth rate substantially equal to the drawing speed V _{ist into} ≈V. When a crystal grows to the crucible walls (r → R), the true velocity increases to a finite value with a maximum value

. So, for germanium crystals (ρ _l = 5.61 g / cm ³ , ρ _tv = 5.3 g / cm ³ ), the growth rate is 20 times higher than the drawing speed.

истинная скорость роста согласно уравнению (2) должна стремиться к бесконечности. Поэтому никакое дальнейшее уменьшение мощности нагревательной системы не позволит получить кристалл с радиусом r>r_кр, что и наблюдается на практике.For crystals of the substances in which the melt density less than the density ρ _x ρ _tv solids at low, tending to zero radii r 0 → crystal growth rate is also substantially equal to V _ist ≈V drawing _speed. However, as the radius of the crystal increases to a critical value

the true growth rate according to equation (2) should tend to infinity. Therefore, no further decrease in the power of the heating system will make it possible to obtain a crystal with a radius r> r _cr , which is observed in practice.

For example, a paratellurite crystal (ρ _w = 5 g / cm ³ , ρ _tv = 6.02 g / cm ³ ) with a radius greater than r _k = 4.47 cm cannot be pulled out from a crucible with a radius of R = 5 cm according to Czochralski application of the invention.

Let it be necessary to grow from a melt in a crucible with a radius R a long crystal with a radius r that is greater than the critical radius r _k for a given crucible: r _k <r <R. Then, first, from another crucible with a radius R ₁ greater than the radius R, a small crystal is grown in the form of a disk with the required radius r, which is smaller than the critical radius for a larger crucible. After separation from the melt and cooling, the resulting disk-shaped crystal of the required radius r is used as a seed crystal for growing a long crystal. To do this, instead of a crucible with a radius R ₁ , a crucible of a smaller radius R _{2 is} installed in the heating system, the substance is melted in the crucible, a disk-shaped crystal is brought to the surface of the formed melt, and then the crystal is grown by constantly lowering it, as shown in FIG. 2. The true crystal growth rate V _ist is associated with the lowering (indentation) rate of crystal V _{in the} dependence resulting from the equality of the masses of the grown part of the crystal and the decreased melt volume m _tv = m _w

In contrast to the case when the radius of the crystal is less than the critical radius r _k , when indenting the crystal at a speed of V _c , the true growth rate is not the sum, but the difference between the rate of decrease of the melt V _p and the speed of indentation V _c ;

V _East = V _p -V _in , whence it follows (after reducing both parts of (3) by πr:

Formula (4) differs from (2) only in the sign of the terms in the denominator. From (4) it follows that with a decrease in the radius r of the crystal to a value of r _{k, the} true growth rate V _ist tends to infinity, and with an increase in the radius of a crystal r to a value of R, i.e. when the crystal approaches the walls of the crucible, it tends to a final value of V _source = V _in ρ _W / (ρ _TV -ρ _W ). For example, for crystals paratellurite V _{ist into} ≈V · 5.25, i.e. 5.25 times the indentation speed. With this method, the crystal is constantly lowered into the crucible space freed from the melt, i.e. into a warmer region of the growth space with lower temperature gradients than with the classical extension according to Czochralski, when the crystal rises into the cold region. This provides an improvement in the structural perfection of the grown crystals by reducing the residual mechanical stresses in them and reducing the density of dislocations.

Examples of the implementation of the proposed method when growing a single crystal of paratellurite (α-TeO ₂ ) with a density of the solid phase of 6.0 g / cm ³ and a melt density of 5.0 g / cm ³ .

Example 1. A single crystal of paratellurite without the use of the invention, obtained in the usual Czochralski method (according to RU No. 2338816, publ. 20.10.2008):

- crucible diameter - 100 mm;

- diameter - 71 mm; height - 54 mm;

- growth direction - [110];

- rotation speed - 13 rpm;

- pulling speed - 0.25 mm / h;

- true vertical growth rate - 0.63 mm / h;

- the average density of dislocations is 12000 cm ^-2 .

Example 2. A single crystal using the invention.

a) First, a paratellurite crystal was grown from a crucible with a diameter of 100 mm in the usual Czochralski method (according to the patent for the invention of the Russian Federation No. 2338816 of November 20, 2008):

- diameter - 71 mm; height 19 mm;

- growth direction - [110];

- rotation speed - 13 rpm;

- pulling speed - 0.25 mm / h;

- true vertical growth rate - 0.63 mm / h.

The process of drawing a seed crystal with a diameter of d = 71 mm, used further for growing in a crucible with a diameter of D = 75 mm, is shown in FIG. 3.

A crystal grown in the usual Czochralski method with a diameter of 71 mm, used hereinafter as a seed, is shown in FIG. four.

b) The obtained crystal with a diameter of 71 mm after cooling and annealing was placed in a chamber of the same setup for growing, but with a crucible with a diameter of not 100 mm, but 75 mm, for which the diameter of 75 mm is supercritical, where it was grown by extrusion rather than extrusion into the melt.

Process and crystal parameters:

- growth direction - [110];

- rotation speed - 8 rpm;

- indentation speed (lowering the rod with the crystal down) - 0.048 mm / h;

- true vertical growth rate - 0.63 mm / h;

- the average density of dislocations is 5600 cm ^-2 .

The process of pressing a crystal into a melt is shown in FIG. 5.

The paratellurite crystal grown by the proposed method is shown in FIG. 6.

The application of the method made it possible to significantly reduce the density of dislocations in paratellurite single crystals used in optics, acousto-optics, laser technology and nuclear physics (as a material for detecting double beta decay events). The presented method of growing is suitable for introduction into industrial production on any plants for drawing by the Czochralski method in the preparation of single crystals of substances in which the density of the solid phase is higher than the density of the melt.

Claims (1)

A method of growing single crystals of substances having a density exceeding the density of their melt, including preliminary drawing a crystal of radius r by the Czochralski method from a stationary crucible of radius R ₁ such that

where ρ _TV is the density of the crystal, ρ _W is the density of the melt, characterized in that the obtained crystal after cooling, removing from the furnace and installing another crucible with a smaller radius R ₂ such that

It is used as a seed for growing a crystal of radius r from this crucible by constantly moving it down.

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