Using amosov method for growing monocrystals from melt

FIELD: crystal growing.

SUBSTANCE: invention relates to technology of growing monocrystals seed crystal and can be used for growing monocrystals having different chemical composition, e.g., types A2B6 and A3B5, as well as monocrystals of refractory oxides, for instance sapphire. In a method preparing monocrystals by growing them from melt comprising melting starting material and drawing monocrystal by crystallization of melt on seed crystal at controlled removal of crystallization heat and use of independent heating sources forming own heat zones, according to invention, independent heating sources form two equal-sized coaxially disposed heat zones so that unified thermal melt and grown crystal region is created separated by melt mirror. Starting material is melted in two steps: first upper heat zone is heated by feeding upper heater with 50% power required to produce melt until maximum temperature providing stable state of seed crystal solid phase is attained, after which the rest of power is directed to lower heat zone onto lower heater at unchanged temperature of upper heat zone until batch is completely melted. Enlargement and growth of monocrystal proceed at controlled lowering of temperature in upper heat zone and preserved unchanged power fed into lower heat zone. Furthermore, crystallization heat is removed in crystal enlargement and growth step at a velocity calculated from following formula: g/sec, where Δm denotes crystal mass, g; Δτ denotes mass growth (Δm) time; Tmelt melting temperature of starting material, °C; Tcrit maximum temperature of stable state of seed crystal solid phase, °C; ΔT temperature change in upper heater in the process, °C; ΔHmelt specific melting point, cal/g; ρ pressure const; R crystal radius, cm; A = ΔT/ΔR is radial temperature gradient near crystallization zone, °C/cm; is initial axial temperature gradient in crystal growth zone, °C/cm; Cp specific heat capacity, cal/g-°C'; and λ heat conductivity of crystal, cal/cm-sec-°C.

EFFECT: achieved independence of process of grown monocrystal material, increased productivity, and increased structural perfection of monocrystal due to lack of supercooling in the course of growth.

2 cl

 

The invention relates to the technology of growing single crystals from the melt to the seed crystal.

Technical problem solved by the claimed invention is the creation of a universal method of growing single crystals of different chemical composition, for example, type a2In6and a3In5and of single crystals of refractory oxides, for example, sapphire.

Monocrystalline materials such As3In5and a2In6and on the basis of the oxides used as optical materials. The development of instrumentation on the basis of these materials significantly increases the need for them, and also increases the requirements for quality, performance and cost.

There is a method of growing crystals of sapphire from the melt to the seed crystal, including the values of the temperature gradients in the range of 0.05 to 1.0°C/mm and the ratio of the variance of vertical temperature gradients radial >1, the vacuum melting of the original charge, making priming and pulling the single crystal from the cooling of the melt (see RF patent №2056463, With 30 In 15/00, 29/20, publ. 1996).

The method consists in determining the temperature of depositing the seed for the emergence of a single crystal with a size of 1-3 mm on the surface of the cooled melt and growing a single crystal when the article is pentacom the change rate of withdrawal from 0.1 mm/hour at the beginning of crystallization to 1.0 mm/h in the final stage of the process with simultaneous decrease in the temperature of the melt at a rate of 0.5 to 2.0° /H. the growing Process is completed by cooling the obtained single crystal with a speed of 25-50°/hour. Pulling slowly at the initial stage of the process helps to ensure the correct formation of the crystal lattice, to avoid dislocation and blocks and bubbles. Pulling with a 10-fold increase in speed at the final stage reduces the duration of the process.

This method provides a crystal is grown from the "freezing" of the melt at the crystallization front. Since the radial temperature gradient in the center of the melt, where the seed crystal is always zero, even a small reduction in the temperature of the heater, and then melt creates the solidification front area with a temperature below the crystallization temperature. For diversion released during the growing heat of crystallization in the way that reduces the rate of crystal growth and thereby reduce the amount of generated heat during crystallization and allow time for heat dissipation in the crystal due to thermal conductivity of the material of the grown single crystal.

The disadvantages of this method is the low performance, precluding its use in the production of large single crystals.

There is a method of growing single crystals of gallium arsenide from u is ava on the seed crystal, in which bringing the seed crystal into contact with placed in the crucible with the melt is carried out under a layer of liquid flux with subsequent crystallization of the total volume of the melt with a liquid sealing compound (see patent No. 2054495, With 30 In 17/00, publ. 1996). The method is designed for growing single crystals of gallium arsenide for the manufacture of substrates of integrated circuits, therefore, the thickness of the layer of the melt is chosen equal to the thickness of the substrate. The method cannot be used for growing bulk single crystals of gallium arsenide.

A known method of growing optical single crystals from the melt by Czochralski method using three heaters (HF heating of the platinum crucible with the melt, a heater for heating the bottom of the crucible and managed heater resistance conical shape for heating the seed crystal, its holder and stem), including the melting of the source heavily powdered oxides and growing a single crystal from the melt on a rotating seed crystal; the cultivation is carried out at the establishment of thermal equilibrium and achieve flat or slightly convex surface of the phase boundary of the melt-crystal. This process is carried out with additional heating during the whole process of the seed crystal, structurally and stem temperatures exceed is it the temperature of the growing crystal by an amount providing the ratio of infrared radiation in the melt and the solid phase, i.e. λmeltcrystal=0,25. At this stage in razresevanja pulling of the crystal, the heating is not carried out (see patent DD No. 290226, A5, With 30 In 15/22, publ. 1991).

The essence of the method is that the temperature field is created in which the temperature of the seed crystal holder and the rod is equal to the temperature ˜TPLcultivated material. Seeding, razresevanje and further growth of the crystal based on the difference of the optical conductivity of infrared radiation from the front of crystallization through the crystal and through the melt. Since the crystal has a higher transmittance than the melt, it naturally has a somewhat lower temperature than the melt. This results in the growth of a crystal on a seed crystal. This thermodynamic equilibrium automatically support throughout the crystal growth additional heating of the seed of tetrastearate and stock. The method adopted for the prototype.

The method has some significant drawbacks that do not allow it to be used for a large range of materials grown by Czochralski methods or kyropoulos method.

1. The method cannot be used for decomposing materials, for example, a3In5where the pressure of the vapor pressure of onethe components at T PLup to 40 ATM and more.

2. The method cannot be used for evaporating materials, for example, A2B6where the vapor pressure of both components is about 3 ATM and more at TPLconnections.

3. The method cannot be used for a range of materials with high ductility at TPLsince plastic deformation occurs in the growing crystal from its own weight (for example, α-Al2About3), when plastic deformation is observed at a temperature of 1600°C.

4. The method cannot be used for a range of materials with small values of supercooling of the melt (for example, CdTe ΔT per ≅1° (C)when a small temperature fluctuations heater ±0,5°With either lead to melting of the seed crystal or spontaneous crystallization of the melt in the crucible.

5. The method cannot be used for materials in which the degree of blackness of the melt and the crystal, as the absorption of the infrared rays from the heater, close, for example, Ge, Si, InSb, and other

6. The difference and the magnitude of the noise of the IR radiation from the melt and the crystal is much lower than the allocated crystallization heat. For example, α-Al2About3=255 cal/g, so the method can be implemented only at very low speeds crystallization, i.e. industrial n is applicable.

The technical result of the claimed invention is the versatility in relation to the material of the grown single crystal, improving performance and increasing structural perfection of the obtained single crystals due to the exclusion of supercooling of the melt in the growth process.

The technical result is achieved in that in the method of producing single crystals grown from the melt, including the melting of the source material and the pulling of the single crystal solidification of the melt on a seed crystal with adjustable exhaust heat of crystallization and the use of independent sources of heat, forming a thermal zone, according to the invention independent sources of heat form two equal spaced coaxial heat zone by creating a single thermal region to melt and the grown single crystal and a partial mirror of the melt, and the melt of the source material are in two stages: first, by heating the upper thermal zone filing for upper heater 30-50% of its capacity, necessary for obtaining the melt to achieve in the upper thermal zone of maximum temperature, ensuring stability of the solid phase of the seed crystal; then the remaining power is served in the lower thermal area on the lower heater when TF is the neigh constant temperature of the upper heat zone until complete melting of the charge; the process of razresevanja and single crystal growth is conducted at a regulated temperature decrease in the upper thermal zone while maintaining the same value of the supplied power in the lower thermal zone.

In addition, the exhaust heat of crystallization under razresevanja and single crystal growth lead with the speed of crystallization of the single crystal, calculated by the formula:

g/s, where

Δm is the mass of the crystal, g;

Δτ the time of mass increment (Δm);

TPLthe melting temperature of the source material, grad.;

TCretethe maximum temperature stable solid phase of the seed crystal, grad.;

ΔT - temperature of the upper heater in the process, grad.;

ΔNPL- specific heat of fusion, cal/g;

p is pressure, const;

R is the radius of the crystal, sm;

the radial temperature gradient at the solidification front, DK deg./cm;

- initial axial temperature gradient in the growing crystal, deg/cm;

Withp- specific heat of the crystal cal/g·deg;

λ - thermal conductivity of the crystal, cal/cm··grad.

The invention consists in the following.

For the cultivation of "perfect" on the structure of single crystals is Alla in the known methods it is necessary to find a "middle ground" between the temperature gradient, created t° melt, and t° in the area of the growing crystal for removal of heat of crystallization.

In the patent of the Russian Federation 2056463 for removal of heat of crystallization decreases the rate of crystallization, giving the opportunity due to thermal conductivity of the material of the seed crystal to provide relief heat the crystal.

Or are reducing the supply of power to the heater in the zone of the melt crucible, thereby reducing the temperature of the melt (RF patent No. 2054495).

The use of these techniques has drawbacks. The decrease in crystallization rate dramatically reduces the efficiency of the process. Lowering the temperature of the melt by reducing the heating power leads to the supercooling of the melt at the crystallization front and, as a consequence, the formation of structural defects (small angle boundaries, polycrystalline structure).

In the claimed invention allocated to the crystallization heat is discharged (drained) crystal by increasing the axial temperature gradient in the area of the growing crystal from its minimum value.

To achieve this effect, the invention is used in a fundamentally new technique.

In the reaction zone, where the source material and the seed crystal with holder and rod, two independent heaters create two equal thermal zone, are placed the one above the other and forming a single thermal region. To melt the source of the charge, first warm up the upper thermal zone by feeding on the upper heater parts power (30-50%)needed to melt the source material. This value is power, means to heat the upper thermal zone to the maximum temperature that can save the seed crystal in practically stable solid state (TCrete). For example, for connections And3In5practical absence of dissociation, for connections And2In6- the absence of evaporation, for refractory oxides α-Al2About3- no plastic deformation. Then serve the remaining capacity on the lower heater to heat bottom heat zone. In this case, the temperature of the upper heater equal to TCretesupport ongoing until complete melting of the source charge and achieve a dynamic equilibrium between the liquid (melt), and solid (seed crystal) phases.

After reaching a dynamic equilibrium capacity of the lower heater stabilize.

The temperature difference between TPLmelt and TCretecreates a uniform thermal field formed by the upper and lower heat zones, minimum axial gradients above the melt and the melt. Razresevanje and the growing single crystal lead to the betrayal of the receiving axial temperature gradient above the melt by reducing the supplied power to the upper heater and thereby lowering the temperature of the upper heat zone. This temperature reduction is carried out with maintaining the supply of power to the lower heater.

The crystal growth carried out by removal of heat of crystallization by increasing the temperature gradient above the melt by lowering the temperature of the upper heater. When the crystallization rate (g/s) is calculated using formula (1).

With a decrease in temperature of the upper heater and the lower temperature of the upper heat zone increases the axial temperature gradient and the crystal grows.

Simultaneously, while maintaining the magnitude of the supplied power to the lower heater during the whole growing process is a decrease in the temperature of the crystal-melt from over-heated body to a less heated by the scheme: lower heater → the crucible → the periphery of the melt → center for melt → crystal → upper heater.

The temperature of the melt is reduced through the crystallization zone in proportion to the decrease in the temperature of the upper heater, excluding the supercooling of the melt at the crystallization front. Thus, the grown crystal is always growing only in the direction of the field more hot melt, heat always goes through the center of the melt in the direction of the growing crystal.

In all known ways of reducing the temperature to which istall-melt goes in the opposite direction.

The process of the claimed methods provides no in-grown single crystals of small angle boundaries and a low density of dislocations. And, very importantly, the efficiency of the process increases with the decrease of temperature of the upper heater, as there is no supercooling of the melt up to off the top of the heater.

The claimed methods characterize a brand new technology of growing single crystal, in which the crystals receive not of "supercooled" melts, and "overheated", since the heat flux is constant, according to the above scheme.

A single thermal region, made up of equal thermal zones arranged one above the other and separated by a mirror melt, includes the area of the crucible and the future of the crystal. The melting of the charge carried out with a total power delivery in both heat zone so that the reduction supplied power to one of thermal zones would lead to a solidification of the melt in the crucible. Creation of a uniform thermal field specifies the creation of a single minimum for a particular material, the axial and radial temperature gradients. Because in the process of razresevanja and crystal growth the change in the temperature gradient does not lead to the supercooling of the melt, the method allows to grow monocrystal is s, have its own value for the temperature of the supercooling of the melt can be in the range from 70°and ˜ 0°C.

And, in addition, the creation of a single thermal field, the two equal thermal zones allows the grown crystal to cool to room temperature in isothermal conditions by equalizing the temperatures in the heating zones by simultaneously reducing the power of the upper and lower heaters.

Because in the process there is no supercooling of the melt at the crystallization front, this way can be grown single crystals of materials that have the ability to melt supercooling is close to zero and far could not be grown by the Czochralski method (CdTe, α-Al2About3- in the direction of [0001], GaAs is in the direction [100]) or had certain structural abnormalities.

An example of the method

Serves power at the top, above the crucible with the charge, the resistive heater and bring to a temperature close to T critical to make this material.

Critical temperatures for the material is the temperature above which irreversible and uncontrollable processes on the surface of the solid phase of the crystal: the processes of dissociation, evaporation, plastic deformation, and the like, to the GDS further practical application of the process of crystallization does not make sense. For example, the temperature at which there is a marked dissociation of a growing crystal GaP above the flux is ˜1300°at TPL=1467°S; the temperature at which there is a noticeable evaporation of the growing crystal CdTe flux over ˜700°at TPL=1092°S; the temperature at which noticeable plastic deformation of a growing crystal α-Al2About3˜1600°at TPL=2050°With, a plastic deformation of the crystal Si >1100°C at TPL=1420°etc.

After exiting the top of the resistive heater to a temperature close to the critical temperature of the upper heat zone stabilize. The sensor is a thermocouple, installed in the upper part of the upper heater for the least impact on its readings lower resistive heater.

After heating the entire mass of the inner tooling furnace serves power to the lower element, which is used for melting the charge in the crucible. The temperature of the upper heater remains constant and equal to ≤TCrete. As melting of the charge and stabilize the temperature of the melt is automatically created critical (minimum) axial temperature gradient for a given material. Then carry out the persecution. As you reach a stable dinamicheskoj the equilibrium between the solid (seed crystal) and liquid (melt) phase (permanent bright halo around the seed) lower the temperature of the upper heater, thereby increasing axial, and hence the radial gradients of the temperature of the seed crystal and melt, respectively, with stable power bottom heater.

Thus creating the conditions under which a crystal grows from the superheated melt. The field of supercooling of the melt at the solidification front are absent during the whole crystallization process. As the crystal grows constantly from the "overheated"and not "freezing" of the melt, it is possible not only to eliminate the undesirable formation of structural defects at the crystallization front, but also gives the possibility of obtaining the Czochralski method has not previously received or difficult to obtain materials. For example, obtaining a Czochralski single crystal CdTe, GaAs - orientation (100), Al2O3- orientation (0001) and other materials.

When the melt secretariats as grown on a seed crystal of the single crystal, the temperature of the lower heater to reduce the temperature of the upper heater, and then the power of the heaters is reduced synchronously until reaching room temperature, thereby creating isothermal conditions for removal of residual thermal stresses in the whole volume of the crystal.

So, when growing the single crystal α-Al2About3from the "superheated" melt after preparing loveley camera to process (batch charging, installation of heaters round or profiled form, setting appropriate based on the profile of the heater of the seed crystal, evacuation, and creating a certain atmosphere in the melting chamber and other operations) include upper heater and display the temperature on thermocouple (TA WIP) within TCrete˜1600°C. Upon reaching the TCreteall the attachments in the melting chamber is heated for several hours until a stable heat transfer. Then include lower heater and power output at the temperature equal to TPLAl2About3=2050°With, and can stand up to steady state, which is determined visually on the behavior of the surface of the melt. By manipulating the power of the lower heater, produce persecution. The stable state of the system is determined on sustainable halo around the seed crystal at the boundary of the liquid - solid phase. After the establishment of dynamic equilibrium capacity of the lower heater stabilize, and maintain stability during the whole crystallization process. By lowering the temperature at thermocouple (TC) with an accuracy of ±0,5°C upper heater produce razresevanje crystal. The exhaust heat of crystallization occurs in the crystal due to an increasing temperature gradient in time.

Because the lower heater stable power, the temperature of the crucible with the melt will also be reduced accordingly, but to remain always above TPLi.e. the melt at the crystallization front is always "overheated". Maintaining the diameter of the growing crystal are carried out by the increase in weight per unit time, i.e. the speed of crystallization, which is pre-calculated based on (1) and put into the program. After completion of the process of single crystal growth capacity of the lower heater to reduce the temperatures in the crucible and in the area of the grown single crystal. The crystal is cooled to room temperature in isothermal conditions, while reducing the capacity of the two heaters.

A specific example of the method.

The value of the formula Δm/Δτ is the definition of temperature conditions for growing single crystals of "overheated" melt method, Amosov maximum allowable speed. When using this formula should be taken into account that in the process of razresevanja the radius of the grown crystal is constantly increasing in time R1=R0+ΔT/A, R2=R1+ΔT/A, etc.

With increase in the radius of the crystal, approaching the radius of the crucible (completion stage of razresevanja) by increasing the radial gradient of the walls of the crucible Δ T/A→0, and continuing to reduce the temperature of the upper heater practically does not increase the diameter of the crystal. The crystal continues to grow at the expense of increasing the axial temperature gradient in the crystal.

The original settings.

Material: corundum (α-Al2About3);

MP=2050°C;

Tcrit ˜1600°at the point h=20 cm from the surface of the melt;

λ ˜0,008 cal/cm··deg;

Ro=0.5 cm (the radius of the seed crystal);

ΔNPL=255 cal/g;

Cf=0.3 cal/g·grad.

Substitute these values into the formula (1) crystallization rate (Δm/Δτ). Asked by lowering the temperature of the upper heater, for example, ΔT=-5°C. under razresevanja the increase of mass of the crystal will be Δm/Δτ=8,02 g/hour.

The continuation of the process of razresevanja crystal is carried out by further lowering the temperature of the upper heater to obtain a given diameter of the crystal, for example, 120 mm

The growing process is carried out at a constant value increment of mass of the crystal per unit time and by reducing the temperature of the upper heater. When it Δm/Δτ when the diameter of the grown crystal 120 mm 1149 g/hour.

The duration of the growth process of the crystal, 30 kg is ˜26 hours.

In grown the om crystal completely absent thermal stress, small-angle boundaries and structural defects.

Thus, the claimed method allows you to grow with high performance bulk single crystals without restrictions on the chemical composition and perfect structure.

1. A method of producing single crystals grown from the melt, including the melting of the source material and the pulling of the single crystal solidification of the melt on a seed crystal with adjustable exhaust heat of crystallization and the use of independent sources of heat, forming a thermal zone, characterized in that an independent heating source form two equal located coaxially heat zone by creating a single thermal region to melt and the grown single crystal and a partial mirror of the melt, and the melt of the source material are in two stages: first, by heating the upper thermal zone filing for upper heater 30-50% of its capacity, necessary for obtaining the melt to achieve in the upper thermal zone of maximum temperature ensuring stability of the solid phase of the seed crystal, then the remaining power is served in the lower thermal zone at the bottom of the heater while maintaining constant the temperature of the upper heat zone until complete melting of the charge, and the process of razresevanja and wires is of the single crystal is conducted at a regulated temperature decrease in the upper thermal zone while maintaining unchanged the magnitude of the supplied power in the lower thermal zone.

2. The method according to claim 1, characterized in that the exhaust heat of crystallization under razresevanja and single crystal growth lead to crystallization rate, calculated by the formula

where Δm is the mass of the crystal, g;

Δτ the time of mass increment (Δm);

TPLthe melting temperature of the source material, deg;

TCretethe maximum temperature stable solid phase of the seed crystal, deg;

ΔT - temperature of the upper heater in the process, deg;

ΔNPL- specific heat of fusion, cal/g;

p is pressure, const;

R is the radius of the crystal, sm;

radial temperature gradient at the solidification front, deg/cm;

- initial axial temperature gradient in the growing crystal, deg/cm;

Withp- specific heat of the crystal, cal/g·deg;

λ - thermal conductivity of the crystal, cal/cm··grad.



 

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