RU2426824C2 - Procedure for growing crystals by crucible-less method and device for its implementation - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: procedure consists in growing crystals by crucible-less method on seed by drawing crystal down from melted zone in gradient of temperature with usage of growing vessel, background multi-sectional heater, additional heater in sealed case - AHB (axial heat background) -heater near front of crystallisation in contacting melted zone maintained with forces of surface tension between bottom of case of AHB-heater and crystal, and also in supply of crystallised material with feeder. Also, height of melted zone is kept within the range from 1 to 20 mm facilitating its different thickness on opposite ends of the AHB-heater within the range from 0.1 to 0.5 mm, while along whole section of growing crystal - from 0.1 to 5 mm at axial gradient of temperature within he range from 5 to 500°C/cm and radial one - within the range from 0.1 to 10°C/cm.

EFFECT: facilitation of close to flat shape of crystallisation front, of specified heat conditions on it along whole section of crystal, and also of specified composition of crystallised material, which finally increases quality of grown crystal and efficiency of mono-crystal production.

21 cl, 5 dwg, 4 ex, 1 tbl

Description

The invention relates to the growth of single crystals from a melt by zone melting at a temperature gradient using a heating element in contact with the molten zone, the shape of which is controlled, and feeding is carried out using a mechanism to move the load.

Widely known are various methods of growing crystals from a melt on the seed, in which crucibles are not used. Among them, the most common are crucible-free zone melting methods and methods using forming elements. The latter are characterized in that the molten zone inherits the cross-sectional shape of the forming element due to capillary effects, and the growing crystal inherits (reproduces) the cross-sectional shape of the molten zone due to the same effects.

In the device [1], based on the method of growing a crystal from a melt, the charge is fed to the forming element in the form of a disk, where it melts and gradually flows out of the hole in the center of the disk. Wetting the seed, the melt forms a melt layer, the diameter of which is controlled simultaneously by the supply of the mixture and the speed of drawing the crystal down into the cold zone. In the crucibleless zone melting method, crystallized material is supplied not in the form of a charge, but in the form of a feeding rod. With the help of an additional heater in the form of a coil located around the rod, a molten zone is formed, which is held between the supply rod and the growing crystal by surface tension forces. The heating element is stationary while the rod and crystal move down. The disadvantage of such approaches is that the shape of the molten zone is not controlled in full. In particular, the shape of the phase boundary is practically uncontrollable: as a rule, it is not possible to create a flat crystallization front over the entire cross section of the crystal; the temperature gradient at the crystallization front, in which the crystal grows, whose magnitude largely determines its quality, is difficult to control.

The ability to control the shape of the front and the temperature gradient during crystallization is achieved by using a heater immersed in the melt [2, 3] or in contact with the molten zone [3, 4] in the so-called axial heat flow method near the crystallization front of crystal growth from the melt ( OTF method). It is also possible to control the shape of the front of the growing crystal in the device (method) to obtain a single crystal according to the patent [5]. This is achieved due to the bottom shape of a well heat-conducting crucible with holes for melt to enter the growing crystal. In this case, the melt is formed upon melting of the charge fed to the heated plate located above the crucible, into which it flows.

Closest to the proposed invention is the method and design of the installation for growing oxides with crucible-free OTP method [4]. To grow crystals by this method, an OTF heater, placed together with thermocouples in a sealed enclosure, is fixed along the axis of the growth chamber near the seed crystal located on a support, which is lowered by the rod into the cold zone of the growth chamber during crystallization. In the stand are thermocouples at points whose temperature is changed according to a certain law using sections of the background heater. In the described device, the OTF heater is fixed in the furnace to the upper rod using a rod connected to the sealed housing of the OTF heater. At high temperatures, the initial orientation of the OTF heater in the furnace and, as a consequence, the height of the molten zone under the OTF heater along its cross section are violated. Therefore, it needs additional adjustment in the attachment site to ensure its alignment with respect to the seed and strict parallelism of the bottom of the housing of the OTF-heater of the surface of the seed (it is necessary to arrange the bottom perpendicular to the gravity vector). In addition, and what turned out to be the most important, the feed of the charge and, accordingly, the feed melt itself does not occur evenly enough [6], which does not allow for constant feeding of the growing crystal. As a result of this, the crystal size begins to change: either the diameter of the crystal decreases, and voids appear along its cross section, or vice versa, the diameter increases and the melt overflows through the crystal, disrupting the thermal picture and the growth process itself.

The technical result of the claimed invention is to provide a crystalline front close to a flat shape, the required thermal conditions on it over the entire cross section of the crystal, as well as a given composition of the crystallized material, i.e. ultimately improving the quality of the grown crystal and the production efficiency of single crystals.

The technical result is achieved by the claimed method of crystal growth by the crucible-free method by pulling the crystal down from the molten zone in a temperature gradient using a growth chamber, a background multi-section heater, an additional heater in an airtight housing (OTP heater) located near the crystallization front in contact with the molten zone held forces of surface tension between the bottom of the housing of the OTF-heater and the crystal, as well as the supply of crystallizable material This is characterized by the fact that the height of the zone is maintained in the range from 1 to 20 mm, providing its thickness at the opposite edges of the OTP heater in the range from 0.1 to 0.5 mm, and over the entire cross section of the growing crystal, from 0.1 to 5 mm with an axial temperature gradient in the range from 5 to 500 ° C / cm and radial in the range from 0.1 to 10 ° C / cm.

Using the proposed method, various crystals are grown, creating a different height of the melt zone under the OTF-heater. Its maximum value, which depends on the capillary constant of the fluid α, is determined by the physical properties of the melt, primarily the surface tension coefficient σ, as well as its density ρ, in accordance with the ratio α = 2 × [σ / (g · ρ)] ^0.5 . The table presents the calculated data on α for some melts of metals, salts and oxides.

Capillary constant of some melts Melt T, K σ, mJ / m ² ρ, kg / m ³ α, mm B ₂ O ₃ 1373 90 2460 3.8 SiO ₂ 2023 400 2600 7.8 Al ₂ O ₃ T _m 680 3900 8.4 Lbo 1200 1200 2100 15.1 LiF 1102 250 2630 6.2 LiCl 887 138 2070 5.2 NaCl 1076 114 2165 4.6 NaI 1034 106 2790 3.9 Csi 920 one hundred 6500 3.0 Bi 544 474 10500 4.2 Ge 1210 600 5300 7.5 Si 1700 720 2330 11.2

The larger the capillary constant of the melt, the greater the height of the h _pac can create a zone of the melt (see figure 1), the more intense convection can be created in such a layer. This is of particular importance when it comes to the growth of crystals, for example, impurity-doped semiconductors, or solid solutions. In this case, more intense convection provides a more uniform distribution of the second component in the melt, and therefore, obtaining more uniform crystals over the cross section. If salts or simple oxides are grown, when the issue of stoichiometry violation does not arise, then there is no need to strive to create large layers of melt. Another factor limiting the height of the molten zone is the diameter of the growing crystal D. It has been established that the OTP method is satisfactorily implemented with a ratio h _pac / D not exceeding 1/7, i.e. when a near-one-dimensional temperature field is provided in such a thin cylindrical layer. Therefore, for small crystal diameters, a large melt layer cannot be installed. And vice versa, those crystals whose melt constant is large have great prerequisites for their growing large diameter. The fact is that, as a rule, the crystallization front cannot be realized completely flat over the entire cross section of the crystal. There is always some thickness Δ, which is technically difficult to implement less than 0.1 mm. On the other hand, in some cases it is necessary to have at least a small curvature (convex into the melt) of the crystallization front (see Fig. 1, a), which manifests itself more noticeably at the periphery. Since the height of the molten zone is visually controlled only on the outside, i.e. In fact, according to the observed meniscus of the melt, it is not recommended to grow crystals with a melt thickness h _{pac of} less than 1 mm by the proposed crucible-free OTP method. the error in the estimation of h _pac can reach 100% in this case. If the crystal and the OTP heater are installed strictly along the camera axis and horizontally, then in the ideal case, the crystallization front is also symmetrical about the camera axis. In most of the cross section of the crystal, it is flat and only closer to the edges (at the periphery) it becomes convex. The thickness difference observed in this case is defined as Δ ^S = h _max ^S -h _min ^S , where h _max ^S and h _min ^S are the maximum and minimum melt thicknesses over the entire cross section of the growing crystal, respectively.

The choice of layer thickness is also determined by the magnitude of the axial temperature gradient required to be realized for a given crystal. If a small gradient is needed, then we must strive for a large layer of the melt and a small overheating of the OTP heater. On the contrary, if you need to create a very large gradient, then the melt layer must be minimized.

In practice, if the crystal is quite easy to install both in the center of the growth chamber and horizontally, having done this before the start of growth, then the OTP heater, especially after heating the chamber to the growth temperature, is, as a rule, shifted along the chamber axis by Δ ^O. Moreover, its bottom is not parallel to the surface of the growing crystal, i.e. the angle β between the axis of the OTP heater and the axis of the chamber is nonzero. In this case, the molten zone does not occupy the gap between the OTP heater and the crystal, and the molten layer itself has the shape of a wedge (see Fig. 1, b), i.e. characterized by a difference in thickness Δ ^D , determined by different thicknesses of the heights h ^{S of the} melt on opposite sides of the crystal: Δ ^D = h _max ^D -h _min ^D. It is clear that there are critical values of Δ ^D and Δ ^O at which the molten zone cannot be held between the OTP heater and the crystal and drains along the side surface of the crystal. At some lower values of the parameters Δ ^D and Δ ^{O, the} melt is still retained, but does not occupy the entire cross section of the crystal, therefore, only part of it grows. In addition, if the melt layer is not flat, but has a wedge shape, then the growth conditions over the cross section, including thermal, are different, which leads to a decrease in the quality of the growing crystal. In order for the crystal to grow under optimal conditions, it is necessary that the OTF heater also be located along the axis of the chamber, the value of the thickness difference Δ ^{D should} be within the limits indicated in the claimed invention.

Under certain conditions, a single crystal grows by a layer-by-layer mechanism. In this case, a set of faces is formed at the crystallization front. In the OTF method, such thermal conditions can be created when the face growing at the front actually coincides with the isotherm, which is flat when the crystal grows from a thin layer. In this case, the crystal, as described in paragraph 2 of the claims, grows with one face, actually parallel to the bottom of the OTP heater. It is clear that in this case the melt zone over the entire cross section of the crystal is a disk in shape with a very small thickness difference.

As the crystal grows, the molten zone is fueled by fresh melt, which is formed on the upper surface of the OTF-heater body after melting the supply material there. The melt flows down the side surface of the casing and falls into the gap between the bottom of the OTP heater and the crystal. In accordance with claim 3 of the claims, the feed material is supplied in the form of a powder of crystallizable material. In this case, it is necessary to supply a powder of a certain fraction, bearing in mind the following two circumstances. On the one hand, the grain size should not be too large so that when it gets on the hot surface of the OTP heater it has time to melt quickly enough without changing the temperature of the OTP heater. On the other hand, powder particles that are too small and too light will not be able to fall onto the surface of the OTP heater, but will stick near the outlet of the feeder or fall by, i.e. the optimum particle size should be between 0.1 and 0.5 mm.

To avoid the described problems, as well as problems with the placement of the hopper with the powder inside the chamber, which requires additional volume and the use of a special dispenser, in paragraph 4 of the claims, it is proposed to supply the feed material in the form of a rod made from crystallizable material. Upon contact with the upper surface of the OTF-heater body, the core melts, and the fresh melt formed when the rod is lowered flows down the OTF-heater body and enters the molten melt zone from which the crystal grows. If the cross section and density of the feed rod coincide with those for the growing crystal, then the rate U of lowering of the feed rod coincides with the growth rate of the crystal or, which is the same in the OTF method, with the speed of its drawing V. As a rule, the feed rod is not perfect in shape, those. its cross section S _rod differs from the cross section of the crystal S _crist . Therefore, the feed of the rod is carried out at a speed different from the speed of drawing the crystal, ensuring equality of supply of the feeding material to the amount of grown crystal.

When growing a crystal from a thin layer of a molten zone or a large section, the fresh melt entering this zone along the outer side surface of the OTF-heater body may not penetrate the entire depth of this section, i.e. a stagnant zone appears, which can lead to the growth of a heterogeneous and low-quality crystal. In certain cases, the melt may not reach the central zone at all, and then crystal growth over the entire cross section will become impossible. Therefore, according to claims 3, 4 of the claims, it is proposed to supply the melt also through openings that pass through the body of the OTF heater, allowing the melt to flow from the area where it is formed from the supply material into the molten zone from which the crystal grows, according to its entire cross section.

As a rule, as a seed crystal, a crystal is taken with a size smaller than that which needs to be grown. One has to do the same in the OTF method when there is simply no seed crystal of the required size, i.e. equal to the diameter of the body of the OTF heater. In accordance with paragraph 5 of the claims, the crystal will grow in cross section with a size smaller than the size of the OTP heater, while completely repeating the shape of its bottom in its cross section. After the seed crystal has been grown, the crystal will grow in a size close to or equal to the size of the OTP heater.

The studies [7] showed that crystal growth is more stable when its diameter exceeds the diameter of the OTP heater, which is proposed to be done according to claim 6. In this case, it is possible to ensure the height of the layer of the molten zone h of a larger value than the melt constant α, several times, which contributes to a significant increase in the quality of the crystal due to additional (in addition to diffusion) mixing of the crystallized material in the molten zone by convection. The calculations performed in [8] make it possible to estimate the range of excess of the radius of the crystal R _cr compared with the radius of the TFT heater R _n , ΔR = R _cr -R _n . For it, given in the dimensionless form ΔR / α, i.e. normalized, like all geometric values during calculations, to the melt constant α, the range of values 0.05–0.4 is indicated. This means that ΔR ~ (0.05 ÷ 0.4) × α. Since at ΔR _min values less than 0.05α, the capillary stability of the process is lost, this value is the minimum threshold. When assessing the maximum possible value of ΔR _max, it is necessary to take into account that ΔR grows both with increasing melt layer height h _pac and melt film thickness δ on the side wall of the OTF-heater body. Since h _pac, in turn, is also normalized to α, for a fixed crystal drawing speed V ₀ = 1 mm / h we actually have ΔR _max ~ 0.4 × h _pac . Based on the conditions of hydrostatic equilibrium, in turn, there is a relation for the thickness of the melt film:

,

,

where ρ _races and ρ _cr are the density of the melt and crystal, ν is its kinematic viscosity, g is the gravity, i.e. δ ~ V ^1/3 . Thus, we finally obtain the estimate ΔR _max = 0.4 × h _races × (V / V ₀ ) ^1/3 .

In paragraph 7 of the claims, it is proposed to grow helical crystals by displacing the OTP heater from the axis of the growth chamber parallel to the crystallization front of the growing crystal and rotating the latter around its axis.

The geometry of the growing crystal, as well as its quality, is determined by the shape of the molten zone: its height and shape of the meniscus. In the proposed method according to claim 8 of the formula, it is proposed to control the shape of the zone (in addition to changing the temperature field in the growth chamber) by changing the feed rate of the feed material and the speed of drawing the crystal. By controlling the shape of the molten zone, ultimately provide a given size of the grown crystal. Clause 9 of the claims indicates a method for controlling the size of a crystal, namely, its diameter, if the crystal grows cylindrical in terms of the growth angle φ, which is constant for a given crystal, and the maximum radius of this melt layer R _max , i.e. in fact in the geometric parameters of his meniscus. It should be especially noted that even in the case when the size of the growing crystal is equal to or smaller than the size of the OTP heater, the observed meniscus height only very roughly corresponds to the thickness of the layer of the molten zone. Due to the fact that the melt film significantly thickens on the lower edge of the OTP heater and the meniscus height h _meniscus is actually observed, the error in the estimation of h _{pac by a} visual method in some cases can reach 50-100%.

According to claim 10 of the claims, it is proposed to control the shape of the meniscus of the melt with a camera or video camera. Moreover, the geometric parameters φ and R _max necessary for control are determined by processing the recorded image.

If during the crystallization the meniscus shape is kept constant, then the crystal grows not only constant in cross-sectional size, but also of higher quality, ceteris paribus, namely, under optimal thermal conditions. In this case, it is important to ensure a uniform or in accordance with a given law in time supply of the supply melt. For this, when the crystal grows, a feed control loop must be implemented. In accordance with paragraph 11 of the claims, it is proposed to use the meniscus geometric parameters as feedback in the control loop.

The parameter characterizing the shape of the meniscus, which can be measured by processing the meniscus image recorded by the camera or camera, is primarily the angle θ that forms the melt at the point where the crystal touches the melt, i.e. at the so-called triple point, relative to the vertical axis of the camera. Another parameter that can be determined from the meniscus image processing data is the position of the horizontal extreme points on the curve of the meniscus surface. That is why in paragraph 12 of the claims, it is proposed to use these characteristic meniscus points as feedback for controlling the shape of the molten zone. It should be noted that if the observed angle θ is equal to the angle of growth φ characteristic of a given crystal, then the crystal grows of a constant diameter. This is further evidenced by the situation R _max = const. If θ> φ is observed during crystallization, and constantly, then the seed crystal grows. At the growth stage, such a deviation of the observed angle from the angle of growth indicates a deficiency or excess of the supplied melt (feed material), which is characterized by a concavity or convex shape of the meniscus.

In order to implement control of the shape of the molten zone, in addition to the camera or video camera for monitoring the molten zone and a computer for recording data on the meniscus shape and their subsequent processing, in accordance with paragraph 13 of the claims, an actuator is included in the control circuit. The mechanism provides a change in the feed rate of the feeding material according to an algorithm that is implemented programmatically as a digital controller on the same computer. The input information in the feedback channel is the measured value of the angle θ. In particular, if a rod is used as the feeding material, then in accordance with item 14, such an actuator ensures lowering of the rod with a given speed U.

When the crystal grows at a constant speed or with a given change in time, it is necessary to supply the feed material at a constant speed or, according to some law, changes in time. Usually (for example, for the Czochralski method), the growing crystal is weighed for this, and the signal is fed from the weight sensor to the power and speed controllers of the crystal, which mutually influence each other. In the OTF method, it is possible to independently provide the thermal conditions necessary for growth and supply the necessary amount of crystallizable material. For this purpose, according to paragraph 15 of the claims, it is proposed to organize an additional control cascade, the controller of which maintains the required value of the required amount of supply material. This value is the control action for the meniscus shape control loop, which is calculated at each step of its regulation.

Since the supply of crystallizable material occurs as the crystal grows, in accordance with paragraph 16 of the claims, it becomes possible to ensure that the melt contains such an impurity or other components that after crystallization in the grown crystal will have a constant composition along the length or change according to the law.

The technical result is also achieved by the device (paragraph 17 of the claims) containing a growth chamber with lower and upper rods, a multi-section background heater, an additional OTF heater in a sealed housing with its mount in the chamber, a crystal located on the lower rod under the OTF heater and pulled down during crystallization, the molten zone held by surface tension forces between the bottom of the OTF-heater body and the crystal, the feeding material and its feeding mechanism from the feeder, in which a molten zone and its position relative to the crystal is set by changing the position of the body OTF-heater and its inclination with respect to the axis of the chamber assembly via its fastening in the chamber, as well as varying the ratio of feed material supply velocity U and the crystal pulling V.

One of the options for placing an OTF heater inside the chamber is to suspend it along the camera axis and fix it in the same way as in the Czochralski method the seed is fixed in the form of a rod on the upper rod. Such a rod has a unit that allows even the seed that is not ideal in cylindricality to enter the melt in the center of the crucible. The node is a composite sphere that provides rotation and tilt of the crystalline rod. If the OTF-heater was fixed on a rod that would be connected to the upper rod through a micrometer table, which ensures the movement of the OTF-heater in a horizontal plane, then, in general, the issue with the adjustment of the OTF-heater could be considered resolved.

However, in practice, it is not enough to install an OTF heater before the start of the growth cycle, providing the desired inclination of the rod and, accordingly, the horizontal location of the bottom of the body of the OTF heater in a horizontal position, and its axis along the camera axis. When heated to high temperatures, the various elements of the OTF heater and the core itself are heated unevenly, and the bottom of the body of the OTF heater tilts, which is unacceptable in this method due to a violation of the necessary geometry of the molten zone from which the crystal grows. In some cases, this can even lead to leakage of the melt from the region between the growing crystal and the OTP heater. Since the adjustment is carried out during visual control over the shape of the molten zone, first the entire OTP heater is displaced in the horizontal plane. Only then can the wedge-shaped melt layer be corrected. However, when trying to correct the tilt by rotating the rod in a spherical node at the top of the camera, the lower end of the rod, and with it the OTP heater itself, will be shifted relative to the crystal. Those. it is now necessary to align the OTF heater in a horizontal plane. Therefore, the procedure has to be repeated many times. In accordance with claim 18, it is proposed to fix the OTF heater with a rod on an arc-shaped guide with a center located on the camera axis and a radius equal to the total length of the rod L and the height of the housing l, with the possibility of movement along it. In this case, the slope of the OTP heater changes when the cold end of the rod moves along the guide, however, the center of the bottom of the OTP heater continues to remain along the camera axis, and the bottom itself is exactly opposite the crystal. The crystallized material is supplied in the form of a powder, which is supplied from the hopper using a dispenser. If the OTF heater needs to be raised or lowered, this is done by moving the upper rod to which the guide is fixed through a micrometer table in the upper cold part of the chamber.

Another option for mounting and adjusting the OTF heater is described in paragraph 19 of the claims. It is proposed that its fastening elements in the form of a plate together with several rods having an arbitrary shape in section (or without them) connected to the body of the OTF heater be placed in the horizontal plane of the chamber. The outer part of the plate or rods are connected to the camera body at least at three points and at an angle of about 120 degrees, so that it is possible to adjust the slope of the OTP heater in the chamber, without which it is impossible to implement the growing method. To this end, the plate or rods are connected to the camera body not rigidly, but with the help of nodes providing independent movement of the external ends of the plate or disks in the vertical direction. These nodes provide joint movement of the fastening elements in a horizontal plane in any direction, i.e. The installation of an OTF heater along the axis of the chamber is definitely necessary with a crystal.

According to claim 20, the plate is made in the form of a disk, in the center of which an OTP heater is installed. The edges of the disc at several, usually at three points, are fixed directly to the side of the camera body or using rods.

Additional adjustment of the OTF heater may be required not only during the initial assembly of the mold. In order to ensure that during the heating of the chamber to operating temperatures, as well as during crystallization, according to paragraph 21 of the claims, the nodes connecting the mounting parts of the OTF heater to the chamber body and providing their movement in the vertical and horizontal directions are equipped with drives. Actuators can be placed inside or outside the camera. In the latter case, they are connected through the airtight motion inputs in the camera body to the said units.

The design of the device, in which the mounting of the OTF heater is carried out in a horizontal plane, allows, along with the supply of charge from the hopper, to implement another option for feeding the feed material. According to paragraph 22 of the claims, the feeder is a node for fastening and moving the rod, consisting of crystallizable material, which is located near the upper surface of the housing of the OTF-heater, to the upper rod. Moving the supply rod is carried out using a drive that provides movement of the upper rod, to which the rod is connected, along the axis of the chamber.

In order to exclude the feeding rod from melting, with the exception of the place where it touches the OTP heater, in accordance with paragraph 23 of the claims, the feeding rod is not directly fixed to the rod, but through the cooling unit. The latter is made of a material with high thermal conductivity or with forced cooling, for example, water or gas.

List of drawings

Figure 1 is a diagram of the formation of the molten zone (a) and its change (b) in case of violation of the alignment of the OTP heater.

Figure 2 illustrates the process of growing a crystal with a size different from the size of the OTP heater: (a) large and (b) smaller.

Figure 3 is a General view (a) of a device that provides growth by a crucible-free method with the supply of feed material in the form of a charge, with a diagram (b) of violation of the shape of the molten zone and alignment of the OTP heater to restore it.

Figure 4 is a General view of the device (a) with the supply of feed material in the form of a rod with a circuit (b) for adjusting the OTF-heater, which provides growth by the crucible-free method.

5 is an image of a molten zone on a computer screen illustrating the formation of a meniscus and a method for determining its geometrical parameters using computer image processing.

The molten zone 1, from which crystal 2 grows with a crucible-free OTP method, is held by surface tension forces between the crystal and the OTP heater 3 (Fig. 1). Inside the sealed housing 4 of the OTP heater, a heating element 5 and thermocouples (not shown) are located. Moreover, depending on the growth stage, the crystal size can be both smaller and larger in size of the OTF heater (Fig. 2), repeating the shape of its sealed housing 4 of height l. The crystal is placed on a heat sink 6, mounted on the lower rod 7 (figure 3). To heat the crystal and the melt, a multi-sectional background heater 8 is used, protected by thermal insulation 9. All of the above-mentioned parts are located inside the growth chamber 10. In the upper part of the chamber, an upper rod 11 is mounted, which has a micrometer stage 12, which allows its lower part 13 to move freely in a plane perpendicular the axis of the growth chamber, using a micrometer screw 14. On top of the body of the OTF-heater is a layer of fresh melt, which is formed by melting the charge coming from feeder 15 to OTF- agrevatel. If the feed material is supplied in powder form, the feeder is a hopper 16 with a dispenser (not shown), mounted on the housing of the growth chamber. In this case, the body of the OTF heater is suspended from the upper rod using a rod 17 of length L using a special unit. The node is a guide 18 in the form of an arc of radius R = l + L, along which a slider 19 slides connected to the rod 17. In the upper part, the guide is connected to the lower part 13 of the table 12. The entire assembly is located in the cold part at the top of the chamber. If the supply material is a rod 20 (Fig. 4), then it is fixed with the upper rod 11 also through the stage 12 using a heat-removing part 21, similar in design to the stand 6. To improve cooling, an additional cooling unit 22. OTP heater 3a is installed fixed to the side of the growth chamber using a plate 23, which depending on the design of the camera can be made in the form of a disk and rods 24 as shown in figure 4, b. The disk and rods are located in a plane perpendicular to the axis of the growth chamber. At the same time, they are not fixed to the camera body rigidly, but with the ability to freely move vertically along the camera axis and in a horizontal plane using, for example, screws 25, which are supported by ring 26, and screws 27. For adjustment, screws 25 and 27 are coupled to actuators (not shown) that may be located inside the chamber or externally. In the latter case, the entries into the chamber can be vacuum sealed. To observe the meniscus of the molten zone 1, as well as the molten part 28 of the supply rod 20, a window 29 is provided, next to which, if necessary, a digital camera 30 is placed. To provide access to monitor the growth zone, a slit 31 is made in the side part of the thermal insulation and the background heater For additional supply of fresh melt 28 to the molten zone 1 near the growing crystal 2, the housing of the OTP heater is provided with holes 32. The heating element 5 is located not only in the lower part of the housing of the OTP heater, but also in top 5a, so that it is possible to realize independent heating of the molten zones above and below the OTP heater. Monitoring the weight of the feed material is carried out using a weight sensor 33.

The device operates as follows. The seed crystal 2a (see Fig. 3, a), in cross section repeating the shape of an OTF heater 3, the opposite end surfaces of which are plane parallel and ground, is mounted on a stand 6 so that its center coincides with the axis of the growth chamber 10 passing through the lower 7 and the top 11 stocks. Then they achieve (using the level) that the upper end of the crystal is in the horizontal plane. With small initial deviations, it is sufficient to align only the stem 7. Otherwise, it is necessary to tilt the entire growth chamber 10. The OTP heater mounted on the upper stem 11 is fed to the seed crystal, leaving a gap of several millimeters. This is necessary to perform the procedure for adjusting the OTF heater relative to the crystal; the gap allows us to ensure that the bottom of the housing 4 of the OTP heater becomes strictly parallel to the end of the crystal and in its center. When adjusting, it may be necessary to offset the OTF heater using a micrometer table 12, if the upper rod is not on the same axis as the lower one, as well as its tilt, for which the slider 19 is moved along the arcuate guide 18. Due to the radius of the arc and the length of the OTF heater (together with the rod 17) coincide, the rotation of the OTF-heater is carried out without its displacement relative to the crystal.

The working zone in which the seed crystal and the OTP heater are located is heated with the help of a background multi-section heater to temperatures close to the melting of the crystal, so that the required axial temperature gradient is established in the furnace. The OTP heater is lowered until it contacts the seed crystal. Then the upper part of the crystal melts, forming a molten zone 1, from which the crystal will subsequently grow. During heating, as a rule, due to the difference in the values of the coefficient of thermal expansion of various materials of which the OTP heater is made, both the housing 4 of the OTP heater and the rod are elongated unevenly, i.e. distortions occur. In addition, with uneven heating, the growth chamber itself can also change its shape. For this reason, the OTF heater turns out to be offset from the camera axis by Δ ^about ₁ , and the bottom of its body loses parallelism to the end face of the crystal (see Fig. 3, b). The molten zone takes the form of a wedge and shifts relative to the crystal. To correct the situation, first the OTF heater is placed opposite the crystal, i.e. along the axis of the lower rod. Finally, at the final stage of adjustment, it remains to change the angle of inclination of the rod so that the bottom of the OTP heater and the end face of the crystal are parallel. For this, the slider 19 is moved along the guide 18 until the melt layer becomes plane-parallel. In this case, the guide along its axis of attachment to the lower part of the table 13 will be offset from the axis of the lower rod 7 by Δ ^о ₂ <Δ ^о ₁ . Visual adjustment control is carried out through a window 29 in the camera body and a slot 31 in thermal insulation and a background heater.

As soon as they were able to achieve the required shape of the molten zone, the heating element 5 of the OTP heater is turned on, providing a given value of the axial gradient at the crystallization front and a small radial temperature gradient, which is necessary to obtain a crystallization front close to flat. Crystallized material is fed to the upper surface of the housing of the OTF heater from a feeder, which is a hopper 16 with a charge and a dispenser. It melts and flows both along the lateral surface of the OTP heater and through the through holes 32 in its body to the molten zone 1, from which crystal 2 grows when the lower rod 7 is pulled down.

If the feed material is supplied not in the form of a charge powder, but as a rod 20 (Fig. 4, a), then the adjustment of the OTF-heater is carried out according to another scheme. After the seed crystal 2a is mounted properly on the stand 6, it is lifted upward by moving the lower rod 7 until it contacts the bottom of the OTP heater. If it is found that the OTP heater is biased or tilted, then this can be easily corrected by shifting it in the horizontal plane and leveling it with screws 25 and 27 (Fig. 4, b). When heated to operating temperatures, only a small additional adjustment of the OTP heater may be required, as a large change in its position and inclination with this design of the device does not occur. This takes place, since the height l of its body (compared with the value of L) is small, the body itself warms up more evenly and the thermal elongation of its elements is small. After the heating element 5 is turned on and the seed crystal, the upper part of which is melted, is raised to the OTP heater, the molten zone 1 is finally formed. Similarly, when the element 5a is heated, the upper molten zone of the supply material is formed after lowering the supply rod 20 to the upper surface of the OTP heater . In the process of crystallization, both rods 7 and 11 are simultaneously lowered: a fresh melt is formed from above and flows into the molten zone under the OTP heater, from which the crystal grows.

Upon reaching the crystal growth regime of a constant cross-section, repeating the shape of the body of the OTF-heater, the side surface of the growing crystal is vertical. Accordingly, the angle between the vertical and the tangent at the triple point (Fig. 2, a) just makes up the growth angle φ. When there is no seed crystal of the required cross-sectional size (close to the size of an OTP heater) or if a larger crystal than a seed crystal is required, then such a crystal must first be grown. In this case, the angle between the vertical and the tangent, i.e. the angle at the base of the meniscus is θ, greater than the angle of growth. The meniscus shape of the molten zone 1 is recorded using a digital camera 29, and the resulting image (Fig. 5) is processed using a computer program. The program allows you to exclude secondary objects, including the melt film 35 on the wall of the housing 4 of the OTP heater and accurately restore the profile 34 of the formed meniscus. This allows you to determine the angle θ, as well as the maximum or minimum radius R _max (R _min ) of the molten zone.

To ensure more stable growth, it is conducted at a crystal radius slightly larger than the radius of the OTP heater by an amount of at least 0.05 of the capillary constant α. A further increase (growth) of the diameter of the crystal is carried out in order to realize a molten zone of a greater height than the value of the capillary constant. In this case, depending on the desired value of h _pac , a crystal is grown with a radius exceeding the radius of the OTP heater by the value described by the ratio ΔR _max = 0.4 × h _pac × (V / V ₀ ) ^1/3 , including depending on the selected speed stretching V.

When a crystal of constant size grows, both parameters: the angle at the base of the meniscus and its maximum radius, must be constant. Therefore, the data on θ and R _max values measured during the crystallization process are used as feedback in the feed supply control loop to compensate for the shortage or excess of the melt in the molten zone 1. The optimum feed rate found from these data is provided by the work of the feeder, which in turn, in its control loop, it can have a weight sensor and autonomously provide the necessary flow of feed material. In the case of feeding the molten zone with a rod, the optimal amount of feeding material is supplied without a dispenser by varying the lowering of the rod U relative to the drawing speed of crystal V. Moreover, if the feeding rod is smaller than the growing crystal, then on average its feed rate U should be faster than crystal V.

Specific examples of crystal growth by the inventive method using the inventive device

, где ρ - плотность кристаллического материала, λ - теплопроводность, С - теплоемкость, H - теплота плавления, ΔT - разница в температуре падающей шихты и ОТФ-нагревателя,

- объем частицы, a

- ее сечение. На практике ΔT=200-400°С, а частицы расплавлялись за 1,5-2,5 с. Через 10-15 мин расплав с верхней поверхности ОТФ-нагревателя начинал стекать к расплавленной зоне, после чего включали вытягивание кристалла вниз со скоростью 2 мм/ч. Выращенный кристалл высотой около 40 мм охлаждали и снимали с подставки. Измерение его диаметра показало, что он рос с превышением диаметра ОТФ-нагревателя на 0.5 мм. После разрезания кристалла вдоль оси роста и декорирования фронта кристаллизации было установлено, что разброс по толщине слоя расплавленной зоны был в пределах 0.5÷0.7 мм за счет небольшого завала (выпуклости) фронта кристаллизации на периферии кристалла.Example 1. The cultivation of a single crystal of bismuth germanate BGO. The seed crystal 2a in the form of a disk with a diameter of 82 mm and a height of 10 mm with well-polished ends was mounted on a stand 6 made of a heat-resistant alloy. Thermocouples were placed in the upper part of the stand to control the temperature of the crystal bottom. To exclude the interaction of BGO with the alloy at high temperatures, a thin spacer in the form of a disk of beryllium oxide was additionally placed between the crystal and the support. The inclination of the lower rod 7 together with the chamber 10 was adjusted so that the upper end of the crystal was strictly horizontal. An OTF heater 3 containing thermocouples for monitoring the temperature on the lower and upper surfaces of its housing 4 and made of platinum to work in air, also having a diameter of 82 mm, was suspended from the upper rod 11 using a guide 18 and a micrometer table 12. Upper the rod was lowered until the bottom of the OTF heater touched the end surface of the seed crystal. The OTP heater was centered and its slope was adjusted to ensure that its bottom was horizontal. The working zone was heated to a temperature of 1050 ° C, corresponding to the melting temperature of BGO. Moreover, due to the fact that the background heater had 2 sections, the axial temperature gradient in the chamber 10 was set to an average of about 100 ° C / cm. After the crystal was melted, the OTP heater was started to be lowered down with the help of the upper rod, while at the same time removing power from its heating element 5. Thus, the upper part of the seed crystal was melted on average about 2 mm high. Then, the desired shape of the molten zone was achieved, including the thickness (on opposite edges of the OTP heater) within 0.3–0.4 mm, adjusting the position of the OTP heater. In this case, the axial temperature gradient on the melt layer itself was set to ~ 200 ° C / cm, and the radial - 4 ÷ 5 ° C / cm. From the hopper 16 began feeding the mixture, prepared with a grain size of not more than 0.5 mm in diameter in accordance with the estimate of the amount of heat that must be reported to the particle of the feed material in order to have time to melt in t s:

where ρ is the density of the crystalline material, λ is the thermal conductivity, C is the specific heat, H is the heat of fusion, ΔT is the difference in temperature of the incident charge and the OTP heater,

is the particle volume, a

- its section. In practice, ΔT = 200-400 ° C, and the particles melted in 1.5-2.5 s. After 10-15 minutes, the melt from the upper surface of the OTP heater began to flow to the molten zone, after which the crystal was pulled down at a speed of 2 mm / h. The grown crystal with a height of about 40 mm was cooled and removed from the stand. Measurement of its diameter showed that it grew with an excess of the diameter of the OTP heater by 0.5 mm. After cutting the crystal along the growth axis and decorating the crystallization front, it was found that the spread in the thickness of the layer of the molten zone was within 0.5–0.7 mm due to a small obstruction (convexity) of the crystallization front at the periphery of the crystal.

Example 2. The growth of a single crystal of lithium tetraborate (LBO). A sample with a diameter of 55 mm and a height of 12 mm was taken as a seed crystal. The preparatory stages are the same as with the growth of BGO. The difference was in the crystallization conditions that were created to realize the morphological instability of growth. In particular, the working zone was heated to a temperature only slightly higher than the melting temperature of 922 ° C so that the axial temperature gradient was 10 ° C / cm. For this, the height of the molten zone was set to about 7 mm. The radial gradient did not exceed 0.2 ° C / cm. The drawing speed was 0.5 mm / h. Another difference was due to a significantly lower density of the LBO crystal (2100 kg / m ³ ) compared with BGO (7150 kg / m ³ ). In this case, particles that are too small in size with a mass of less than 10 ^-3 mg, even due to small convective flows, would drift apart and not fall on the upper surface of the OTP heater. According to the estimate for the mass of a spherical particle, the minimum allowable grain size for LBO was 0.1 mm in diameter. On the other hand, the fraction used up to 1 mm in diameter was quickly melted upon contact with the OTF heater on the hot surface. For crystal growth, the melt was supplied with excess for a given cross section of the crystal in such a way that the angle θ equal to 45 ° was kept constant during crystallization. The grown crystal with a diameter of 80 mm in its upper part had a morphology typical of conditions of unstable growth: a cellular structure of a hexagonal shape with a cell size of 6-8 mm.

Example 3. Growth of a bismuth crystal. An OTP heater 3a with a diameter of 40 mm, the case of which is made of stainless steel and after etching with phosphoric acid, is coated with a bismuth layer to ensure good wetting, was placed along the camera axis using a plate 23 fixed along the edges on the frame 26 and screws 27. Using screws 25 OTF-heater horizontal. The seed crystal 2a in the form of a disk with a diameter of 40 mm and a height of 15 mm was mounted on a copper water-cooled stand with a diameter of 50 and a length of 80 mm strictly along the camera axis. A bismuth feed cylinder 20 with a diameter of 40 and a height of 60 mm was fixed to a similar copper heat sink cylinder 21, also cooled by water using a block 22, which is connected to the rod 11 in the upper part of the chamber. Then, moving both rods, the bismuth samples were brought into contact with the OTF -heater, after which the background heaters warmed up the working area to 270 ° C. When the heating elements 5 and 5a were turned on and the bismuth was melted, an overheat of 5 ° C was achieved, i.e. respectively axial gradient of 20 ° C / cm. The molten zone 28 above the OTP heater was held by surface tension forces similarly to zone 1 under the OTP heater. The crystal was grown with a diameter of 45 mm (ΔR = 2.5 mm) with a drawing speed of 10 mm / h. In this case, the feed rod in accordance with the ratio U = V × (S _crist / S _rod ) was lowered at a speed of U = 12.7 mm / h. Fresh bismuth melt came both through the side surface of the OTF heater and the hole in its body 32. Although the feed rod was prepared in the form of a cylinder, deviations from the ideal took place in geometry. To compensate for the non-uniformity of the melt supply, which certainly would have occurred if the rod was lowered uniformly, its speed was varied depending on the angle θ data coming from the computer to the control circuit of the upper rod drive 11; the angle θ was maintained equal to the growth angle φ≈18 °.

Example 4. Obtaining an alloy of bismuth-antimony. To obtain bismuth containing about 12-17% of antimony, the feed material was supplied in the form of two rods of bismuth and antimony, and the cross section of the rod of antimony was approximately one sixth of the cross section of the rod of bismuth. Both rods simultaneously fell down, melted, and fresh melt of constant composition entered the molten zone. The crystal was grown at a temperature gradient of 60 ° C / cm from a melt layer 1 mm thick with a drawing speed of 2 mm / h. The grown crystal turned out to contain 15.5% Sb in composition with a scatter of values of ± 1.8% in height.

Information sources

1. V.I. Goriletsky et al. “Crystal growth: alkali metal halides”. - Kharkov: Act, 2002 .-- 536 p.

2. Device for growing crystals (RU No. 1800854), С30В 11/00, 1990.

3. V. D. Golyshev, M. A. Gonik, A temperature field investigation in case of crystal growth from the melt with a plane interface on exact determination thermal conditions, Cryst. Prop. and Preparation, 36-38 (1991) 623.

4. VD Golyshev, MAGonik, VBTsvetovsky, Problems of Bi ₄ Ge ₃ O ₁₂ and Li ₂ B ₄ O ₇ single crystal growth by crucibleless variant of AHP method J. Crystal Growth, 198/199, 1999, p.501-506.

5. A device for producing a single crystal (options), a method for producing a single crystal (options) and a single crystal (options), RU 2215070 C2, 10.27.2003.

6. Golyshev VD, Gonik M.A., Polyansky EV, Tsvetovsky VB Features of the charge during the cultivation of Germanoevlitin by the crucible-free method, Exploration and protection of mineral resources, 1996, No. 1, pp. 33-35.

7. M.A. Gonik, M.M. Gonik, T.V. Low. Control over the shape of the meniscus of the melt when controlling crystal growth using a crucible-free OTP method. XVIII St. Petersburg readings on the problem of strength and crystal growth (Stepanov's method). St. Petersburg, October 21-24, 2008, p. 83-86.

8. B.C. Yufer. Capillary shaping of crystals pulled down from a melt by a crucible-free OTF method. Crystallography, 2008, vol. 53, pp. 1214-1220.

Claims (21)

1. A method of growing crystals with a crucibleless method by seeding by pulling the crystal down from the molten zone in a temperature gradient using a growth chamber, a background multi-section heater, an additional heater in a sealed enclosure — an OTF heater located near the crystallization front in contact with the molten zone held by forces surface tension between the bottom of the housing of the OTF-heater and the crystal, as well as the supply of crystallizable material by the feeder, characterized in that This molten zone is maintained in the range from 1 to 20 mm, ensuring its thickness at opposite edges of the OTP heater in the range from 0.1 to 0.5 mm, and over the entire cross section of the growing crystal, from 0.1 to 5 mm with an axial gradient temperatures in the range from 5 to 500 ° C / cm and radial in the range from 0.1 to 10 ° C / cm. 2. The method according to claim 1, characterized in that the thickness difference over the entire cross section of the growing crystal does not exceed 0.5 mm due to the growth of a crystal with a single flat face at the crystallization front. 3. The method according to claim 1 or 2, characterized in that the crystallized material is supplied in the form of a powder, before being loaded into the feeder pre-screened to exclude particles of a size of not more than 0.5 mm and not less than 0.1 mm, which melts when hit on the upper surface of the OTF-heater body, heated above the melting temperature of the crystal, while the resulting melt flows into the molten zone to the growing crystal both along the lateral outer wall of the OTF-heater body and through holes in it. 4. The method according to claim 1 or 2, characterized in that the crystallized material is supplied in the form of a rod lowered onto the upper surface of the OTF-heater body, heated above the melting temperature of the crystal, with a speed U, mm / h, associated with the speed of drawing the crystal V, mm / h, by the formula U = V · (S _crist / S _rod ), where S _crist is the cross section of the growing crystal, mm ² , and S _rod is the cross section of the feed rod, mm, while the core melt drains into the molten during melting zone to the growing crystal as on the side outer wall of the body CA OTF-heater, and through holes in it. 5. The method according to claim 1 or 2, characterized in that when the seed is grown, the crystal in its cross section is grown in a form corresponding to the shape of the bottom of the body of the OTF heater, and a size less than or equal to the size of the bottom of the body of the OTF heater. 6. The method according to claim 1 or 2, characterized in that the crystal in its cross section is grown in a form corresponding to the shape of the bottom of the body of the OTF heater, and a size exceeding the size of the bottom of the body of the OTF heater by 0.05α, where α = 2 · [Σ / (g · ρ)] ^0.5 , mm - melt constant, but not more than 0.4 · h _pac · (V / V ₀ ) ^{l / 3} , h _pac - height of the molten zone, mm, V - speed crystal elongation, V ₀ = 1 mm / h, g = 9.8 m / s ² , σ is the surface tension coefficient, mJ / m ² , ρ is the melt density, kg / m ³ . 7. The method according to claim 1, characterized in that due to the shift of the OTP heater from the axis of the growth chamber parallel to the crystallization front of the growing crystal and the rotation of the latter around its axis, helical crystals are grown. 8. The method according to claim 1 or 2, characterized in that the shape of the molten zone is controlled by changing the feed rate of the feed material and the speed of drawing the crystal. 9. The method according to claim 8, characterized in that the control over the size of the growing crystal, for example, its diameter, if the crystal grows cylindrical, is carried out according to the shape of the meniscus of the molten zone from which the crystal grows, in particular, according to the growth angle φ, which is constant for a given crystal, and the maximum value of the radius of the molten zone R _max . 10. The method according to claim 9, characterized in that the control over the shape of the meniscus of the melt from which the crystal grows is carried out using a camera or video camera, and the geometric parameters necessary for control are determined by processing the recorded image. 11. The method according to claim 9, characterized in that the data on the shape of the meniscus is used as feedback in the feed control of the crystallizable material. 12. The method according to claim 10, characterized in that the characteristic meniscus points are used as feedback for controlling the shape of the molten zone: a triple point at which the melt touches the crystal and makes an angle θ relative to the vertical, extreme points of the meniscus along the horizontal axis characterizing concavity and the bulge of its shape. 13. The method according to p. 12, characterized in that the control circuit of the shape of the molten zone includes a photo or video camera for monitoring the shape of the meniscus of the melt between the OTP heater and the crystal, a computer for processing recorded data on the shape of the meniscus and an actuator providing changing the feed rate of the feed material using a digital controller implemented on the computer. 14. The method according to item 13, characterized in that to control the shape of the molten zone when used as a feed material of the rod change the speed of its lowering U. 15. The method according to item 13, characterized in that the feed rate of the feed material is kept constant or is changed according to the weighing results, and the change in the magnitude of this speed is carried out using an additional control loop, forming a two-stage system with the main meniscus shape controller. 16. The method according to claim 1 or 2, characterized in that a crystal is grown that is constant in composition or with a change in composition set at a height due to the supply of crystallizable material of an appropriate composition. 17. The crystal-free crystal growing device for implementing the method according to claim 1, comprising a growth chamber with lower and upper rods, a multi-section background heater, an additional OTF heater in an airtight housing with its mount in the chamber, a crystal located on the lower rod under the OTF -heater and pulled down to the seed during crystallization, the molten zone held by surface tension forces between the bottom of the OTF-heater body and the crystal, the feeding material and its feeding mechanism from atelier, characterized in that when using the feed material in the form of a powder to be filled, an OTF heater with a housing height of 1 is attached to the upper rod using a vertically arranged rod of length L, the cold end of which is connected to the upper part of the chamber with a guide in the form of an arc of radius R equal to L + 1, with the possibility of sliding and changing the slope of the bottom of the body of the OTF heater relative to the upper end surface of the crystal, and the guide itself is connected to the lower part of the table, providing the movement of the OTF heating eating perpendicular to the camera axis using, for example, micrometric screws on the adjacent sides of the table, and when using feed material in the form of a rod attached to the upper rod, the OTP heater is fixed along the camera axis using horizontally arranged parts in the form of a plate and several rods having in cross section an arbitrary shape that is connected to the camera body in its cold zone at least three points located at an angle of approximately 120 ° relative to each other, using nodes that provide joint e moving said parts in a horizontal direction and independently - vertically. 18. The device according to 17, characterized in that the plate is a disk in shape. 19. The device according to 17, characterized in that the nodes are equipped with drives that provide alignment of the OTP heater before and during crystal growth. 20. The device according to 17, characterized in that the feeder is a node for attaching a rod of crystallizable material located near the upper surface of the OTF-heater body to the upper rod, and the supply mechanism of the supply material is a drive that moves the upper rod together rod along the axis of the camera. 21. The device according to claim 20, characterized in that the attachment point of the supply rod to the upper rod has a cooling unit.

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