Optical structure for floating zone furnace, growth of crystals using the same, annealing method and optical floating zone furnace

Abstract

The application discloses a floating zone furnace optical structure, a crystal growth method using the same, an annealing method and an optical floating zone furnace. The floating zone furnace optical structure comprises a focusing optical reflector and a control pipe. The focusing optical reflector can converge and guide the light energy emitted by a light source to a crystal growth melting zone or the control pipe by adjusting the position of the focusing optical reflector. The control pipe is arranged below the crystal growth melting zone and has an inner hole through which a material passes. The inner wall of the control pipe has at least a diffuse reflection surface. When the crystal is in a growth state, the position of the focusing optical reflector is adjusted so as to guide the light energy to the crystal growth melting zone. When the crystal growth is completed, the position of the focusing optical reflector is adjusted so as to guide the light energy to the control pipe, and the light energy is incident on the diffuse reflection surface, thereby forming a radiation annealing heat field around the crystal after the growth is completed, and in-situ annealing is realized. The application can realize the annealing of the crystal by only using an existing light source and a simple optical structure adjustment.

Description

Technical Field

[0001] This invention relates to the field of crystal growth technology, specifically to a floating zone furnace optical structure, a method for growing crystals using it and annealing them, and an optical floating zone furnace. Background Technology

[0002] The floating zone method is a vertical zone melting crystal growth method that does not require a precious metal crucible. The molten material in the molten zone maintains equilibrium through surface tension and its own gravity. The optical floating zone method focuses the energy emitted by a light source onto a narrow molten zone using a spherical or ellipsoidal mirror. The raw material rod melts upon heating, and the melt grows into a single crystal material under the traction of the moving or rotating seed crystal. Growth temperatures up to approximately 2000℃ can be achieved. Because the floating zone method does not require a metal crucible, no other impurities are introduced during crystal growth, and the heating temperature is not limited by the crucible's melting point. Therefore, the optical floating zone method offers advantages such as low growth cost, fast growth speed, no pollution, and easy observation, making it suitable for growing high-melting-point, volatile, and other crystal materials that are difficult to grow using other methods.

[0003] During crystal growth, the melting zone reaches its highest temperature. After growth, the crystal cools rapidly upon leaving the high-temperature melting zone, easily accumulating thermal stress internally, which can lead to crystal cracking and affect the yield. Therefore, annealing is necessary after crystal growth to eliminate thermal stress.

[0004] To address the aforementioned issues, existing technologies employ a ring-shaped resistance heater as a post-heating device to perform in-situ annealing of the grown crystal. However, this method suffers from several drawbacks: the resistance heater is bulky, has a slow thermal response, and lacks precise temperature control. Furthermore, some technologies utilize induction coils to heat a tubular heating element, creating an axial temperature gradient on the heating element for post-heating of the crystal. However, this approach requires an additional induction power supply and a complex coil control system, resulting in a complex structure and high cost. Therefore, further research is needed to develop an optical structure that is simple in structure, flexible in control, and capable of in-situ annealing of crystals after growth without the need for additional complex external equipment. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the technical problem to be solved by the present invention is: how to provide a floating zone furnace optical structure that is simple in structure, flexible in control, and convenient for in-situ annealing of crystals after growth.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] An optical structure for a floating zone furnace includes a focusing optical reflector and a control tube. The focusing optical reflector is configured to selectively converge and guide light energy emitted from a light source to a crystal growth molten zone or the control tube by adjusting its position. The control tube is located below the crystal growth molten zone and has an inner hole through which material passes. The control tube and the crystal growth molten zone are located on the same central axis. The inner wall of the control tube has at least a diffuse reflection surface. When the crystal is in the growth state, the position of the focusing optical reflector is adjusted to guide light energy to the crystal growth molten zone. After the crystal growth is completed, the position of the focusing optical reflector is adjusted to guide light energy to the control tube, so that the light energy is incident on the diffuse reflection surface, forming a radiation annealing thermal field around the crystal after growth, thereby achieving in-situ annealing.

[0008] In this invention, after crystal growth is complete, the light energy is guided to the control tube by adjusting the reflector and incident on the diffuse reflection surface. The diffuse reflection surface then reflects the light energy, forming a radiation annealing thermal field around the crystal, thus achieving in-situ annealing. This invention eliminates the need for complex external heating equipment; it utilizes only existing light sources and simple optical structure adjustments to anneal the crystal, eliminating thermal stress and preventing cracking, significantly reducing equipment costs and operational complexity.

[0009] As an optimization, the inner wall of the control tube also includes a directional reflection arc surface located above the diffuse reflection surface. The focal point of the directional reflection arc surface is located 2-5 mm below the crystal growth molten zone and is on the same central axis as the control tube. The directional reflection arc surface is used to reflect and focus the light energy penetrating the crystal growth molten zone to the focal point when the crystal is in the growth state, thereby forming a stepped temperature gradient downwards from the crystal growth molten zone. The light energy penetrating the molten zone is reflected by this arc surface and focused to the focal point, forming a stepped temperature gradient downwards from the molten zone. This gradient provides auxiliary heating to the crystal below the molten zone, reduces the radial temperature gradient, makes the solid-liquid interface flatter, and thus improves crystal quality.

[0010] As an optimization, the directional reflective surface is an ellipsoid, parabola, or sphere. Using these surfaces ensures secondary focusing of light penetrating the molten zone and also facilitates manufacturing.

[0011] As an optimization, the control tube is composed of two independent components spliced ​​together along the axial direction. The directional reflection arc surface is located on the inner wall of the upper independent component, and the diffuse reflection surface is located on the inner wall of the lower independent component. The two-section splicing structure allows for independent replacement of the upper section (with different curvatures or surface shapes) or the lower section (with different diffuse reflection characteristics) according to different crystal materials and process requirements, which not only reduces maintenance costs but also improves process adaptability.

[0012] As an optimization, a growth chamber is also included, which consists of an outer tube and a control tube lining the outer tube. The outer tube is made of quartz or sapphire, and the control tube is made of ceramic material with an inner reflective coating. The outer tube ensures light path transmittance and chamber sealing, while the inner control tube is made of high-temperature resistant ceramic material and coated with a reflective film, improving the reflection efficiency of infrared light.

[0013] As an optimization, the focusing optical reflector is configured to move along the axis of the control tube to adjust the size and divergence of the focused spot. By moving the focusing optical reflector axially, the energy density and coverage of the spot on the crystal can be continuously and steplessly adjusted. During the annealing stage, appropriately increasing the spot size and making it diverge allows for more uniform illumination of the diffuse reflection surface area, thereby forming a more uniform radiative thermal field and improving the annealing effect.

[0014] A method for crystal growth and in-situ annealing using the above-described floating zone furnace optical structure includes the following steps:

[0015] (1) Adjust the focusing optical reflector to the first working position so that the light energy emitted by the light source is focused on the crystal growth melting zone between the material rod and the seed crystal above the control tube;

[0016] (2) Start crystal growth, melt the rod and form a growth interface until crystal growth is complete;

[0017] (3) After the crystal growth is completed, the crystal position is adjusted to the diffuse reflection surface area inside the control tube, the focusing optical reflector is adjusted to the second working position, the position and size of the focusing spot are adjusted so that the focusing spot irradiates the diffuse reflection surface inside the control tube and is reflected back to the crystal by the diffuse reflection surface, forming a radiation annealing thermal field around the crystal. At the same time, the output power of the light source is reduced to a level lower than that of the crystal growth stage and slowly reduced so that the crystal is slowly cooled.

[0018] As an optimization, the control tube also includes a directional reflection arc surface located above the diffuse reflection surface. During the crystal growth stage, when the focusing optical reflector focuses the light energy onto the crystal growth molten zone, the directional reflection arc surface can reflect the light energy penetrating the crystal growth molten zone and focus it onto a focal point located 2-5 mm below the crystal growth molten zone, forming a stepped temperature gradient from the molten zone downwards.

[0019] As an optimization, the output power of the light source is slowly reduced at a preset cooling rate during the annealing stage. The cooling rate is controlled according to the thermal stress release characteristics of different crystals to avoid cracking caused by excessively rapid cooling.

[0020] An optical floating zone furnace includes the floating zone furnace optical structure as described above.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] (1) By setting an adjustable focusing optical reflector and a control tube with a diffuse reflection surface on the inner wall, after the crystal growth is completed, only the position of the reflector needs to be adjusted to direct the light source energy to the diffuse reflection surface in the control tube, so that the diffuse reflection surface diffusely reflects the incident energy and forms an enclosed radiation annealing thermal field around the crystal.

[0023] (2) A directional reflective arc surface is set above the diffuse reflective surface. During the crystal growth stage, the light energy penetrating the molten zone is focused twice by the directional reflective arc surface, thereby forming an auxiliary heating zone below the molten zone, reducing the radial temperature gradient of the crystal growth interface and improving the growth quality of the crystal.

[0024] (3) No additional complex external equipment such as resistance heaters, induction coils, and dedicated power supplies are required, and low-cost upgrades and modifications can be made to most existing optical floating zone furnaces. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the optical structure of the floating zone furnace in this invention;

[0026] Figure 2 This is a schematic diagram of the optical floating zone furnace in this invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0028] It should be noted that similar reference numerals and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the figures, or the orientation or positional relationship commonly used when the product is in use. They are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance. In addition, the terms "horizontal," "vertical," etc., do not indicate that the component is required to be absolutely horizontal or suspended, but can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal than "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted. In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0029] like Figure 1 and Figure 2 As shown, an optical structure for a floating zone furnace includes a focusing optical reflector 1 and a control tube; the focusing optical reflector 1 is an ellipsoidal reflector with a high-reflectivity film (such as a silver film) coated on its inner side; the focusing optical reflector 1 is installed outside the growth chamber and connected to an electric displacement platform, which allows it to move along the crystal growth axis to adjust the center position of the focusing spot, as well as the size and divergence of the focusing spot.

[0030] The control tube is located below the crystal growth melting zone 2 and has an inner hole for material to pass through. The control tube and the crystal growth melting zone 2 are located on the same central axis. The control tube is composed of two independent components spliced ​​together along the axial direction: the inner wall of the upper component 3 is a directional reflective arc surface, and the inner wall of the lower component 4 is a diffuse reflective surface. The directional reflective arc surface of the upper component 3 is part of an ellipsoid, and its focal point is located 2-5 mm below the crystal growth melting zone 2 (3 mm in this embodiment), and coincides with the central axis of the control tube. The diffuse reflective surface of the lower component 4 is treated to form a macroscopic roughness, so that it has Lambertian reflection characteristics.

[0031] The growth chamber consists of an outer tube 5 and a control tube lined inside it; the outer tube 5 is a sapphire tube with high light transmittance and high temperature resistance; the control tube is made of alumina ceramic with an inner layer coated with a reflective film to improve the reflectivity of infrared light.

[0032] It also includes a material bar and a seed crystal lifting device, which controls the movement of the material bar 6 and the seed crystal 7 respectively, and is also used to move the crystal to the diffuse reflection surface area of ​​the control tube after the crystal growth is completed.

[0033] Crystal growth and in-situ annealing using an optical floating zone furnace with the aforementioned floating zone furnace optical structure include the following steps:

[0034] (1) Preparation stage: Install the material bar 6 on the material bar lifting device located above the melting zone, and install the seed crystal 7 on the crystal lifting device located below the melting zone; adjust the focusing optical reflector 1 to the first working position so that the focal plane of the focusing optical reflector 1 is located at the predetermined melting zone position between the material bar 6 and the seed crystal 7; start the light source 8 (xenon lamp), and the light emitted by the light source 8 is converged by the focusing optical reflector 1 to form a high energy density light spot, which is focused on the melting zone between the material bar 6 and the seed crystal 7;

[0035] (2) Growth stage: control the position of the material rod 6 and the seed crystal 7 entering the melting zone, the end of the material rod 6 melts and contacts the seed crystal 7 to form a growth interface. During crystal growth, the feed rod 6 and crystal 7 rotate and move relative to each other at a constant speed (feeding rod 6 downwards, stretching crystal 7 downwards). Simultaneously, some light rays penetrating the molten zone (mainly infrared) continue to propagate downwards and are incident on the directional reflective arc surface of the upper section of the control tube component 3 located below the molten zone. This directional reflective arc surface reflects the light and refocuses it onto a focal point located 3mm below the molten zone. Because the focal point provides additional auxiliary heating to the crystal below the molten zone, the temperature decreases slowly and stepwise from the molten zone downwards (e.g., molten zone temperature 2000℃, focal point temperature approximately 1800℃, further decreasing to 1500℃). If the temperature drops too quickly during the solidification process from the high-temperature melt to the low-temperature zone, it can lead to significant thermal stress within the crystal. This slow, stepwise cooling effectively alleviates this thermal stress, preventing defects such as cracking and breakage during growth or cooling. Furthermore, this non-linear cooling path facilitates sufficient molecular rearrangement, thereby shortening the crystallization time and obtaining a superior final crystal configuration.

[0036] (3) Annealing stage: When the crystal growth reaches the preset length, the feeding of the bar 6 is stopped and the bar 6 is quickly lifted upward away from the melting zone; then, the grown crystal is moved downward by the crystal lifting device so that it completely enters the diffuse reflection surface area of ​​the lower section component 4 of the control tube; then, the focusing optical reflector 1 is adjusted to the second working position: the reflector is slowly moved in the vertical direction (moving speed is 0.3mm / min), while the relative distance between the focusing optical reflector 1 and the light source 8 is slightly increased, so that the focused spot is enlarged and the energy is dispersed; the adjusted spot irradiates the crystal located in the diffuse reflection surface area of ​​the control tube, part of the energy is absorbed by the crystal, and part of the energy penetrates the crystal and is incident on the diffuse reflection surface; the diffuse reflection surface reflects the incident light in all directions (Lambertian reflection), forming a uniform radiative thermal field surrounding the crystal; at the same time, the output power of the light source 8 is gradually reduced (for example, reduced from the growth power of 2000W to 0 at an equivalent cooling rate of 50℃ / h), so that the crystal is slowly cooled in the radiative thermal field; the entire annealing process lasts for several hours until the crystal temperature drops to room temperature;

[0037] (4) Take out the crystal: turn off the light source 8, open the growth chamber and take out the crystal; the surface of the obtained crystal is free of cracks, and the internal stress test shows that its thermal stress is reduced by more than 60% compared with the traditional process.

[0038] To meet the thermal field requirements of different crystal materials, control tube components with different surface shapes can be replaced. For example, for crystals that require a gentler temperature gradient, the upper component can be a parabolic arc surface with a long focal length, and the focal point is located 5 mm below the molten zone. For crystals with high thermal conductivity, the upper component can be an ellipsoidal arc surface with a short focal length, and the focal point is located 2 mm below the molten zone. The roughness of the diffuse reflection surface of the lower component can also be selected according to the required annealing rate. The greater the roughness, the more uniform the diffuse reflection, and the slower the annealing rate.

[0039] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described with reference to preferred embodiments, those skilled in the art should understand that various changes in form and detail can be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. An optical structure for a floating zone furnace, characterized in that: The system includes a focusing optical reflector and a control tube. The focusing optical reflector is configured to selectively converge and guide the light energy emitted by the light source to the crystal growth melting zone or the control tube by adjusting its own position. The control tube is located below the crystal growth melting zone and has an inner hole through which the material passes. The control tube and the crystal growth melting zone are located on the same central axis. The inner wall of the control tube has at least a diffuse reflection surface. When the crystal is in the growth state, the position of the focusing optical reflector is adjusted to guide the light energy to the crystal growth melting zone. After the crystal growth is completed, the position of the focusing optical reflector is adjusted to guide the light energy to the control tube, so that the light energy is incident on the diffuse reflection surface, forming a radiation annealing thermal field around the crystal after growth, thereby achieving in-situ annealing.

2. The optical structure of the floating zone furnace according to claim 1, characterized in that: The inner wall of the control tube also includes a directional reflection arc surface located above the diffuse reflection surface. The focal point of the directional reflection arc surface is located 2-5 mm below the crystal growth melting zone and is on the same central axis as the control tube. The directional reflection arc surface is used to reflect and focus the light energy penetrating the crystal growth melting zone to the focal point of the directional reflection arc surface when the crystal is in the growth state, so as to form a stepped temperature gradient from the crystal growth melting zone downward.

3. The optical structure of the floating zone furnace according to claim 2, characterized in that: The directional reflective arc surface is an ellipsoid, parabola, or sphere.

4. The optical structure of the floating zone furnace according to claim 2, characterized in that: The control tube is composed of two independent components spliced ​​together along the axial direction. The directional reflection arc surface is located on the inner wall of the upper independent component, and the diffuse reflection surface is located on the inner wall of the lower independent component.

5. The optical structure of the floating zone furnace according to claim 1, characterized in that: It also includes a growth chamber, which consists of an outer tube and a control tube lined inside it. The outer tube is a quartz or sapphire tube, and the control tube is made of ceramic material with an inner layer coated with a reflective film.

6. The optical structure of the floating zone furnace according to claim 1, characterized in that: The focusing optical reflector is configured to move along the axis of the control tube to adjust the size and divergence of the focused spot.

7. A method for crystal growth and in-situ annealing using the floating zone furnace optical structure described in claim 1, characterized in that: Includes the following steps: (1) Adjust the focusing optical reflector to the first working position so that the light energy emitted by the light source is focused on the crystal growth melting zone between the material rod and the seed crystal above the control tube; (2) Start crystal growth, melt the rod and form a growth interface until crystal growth is complete; (3) After the crystal growth is completed, the crystal position is adjusted to the diffuse reflection surface area inside the control tube, the focusing optical reflector is adjusted to the second working position, the position and size of the focusing spot are adjusted so that the focusing spot irradiates the diffuse reflection surface inside the control tube and is reflected back to the crystal by the diffuse reflection surface, forming a radiation annealing thermal field around the crystal. At the same time, the output power of the light source is reduced to a level lower than that of the crystal growth stage and slowly reduced so that the crystal is slowly cooled.

8. The crystal growth and in-situ annealing method according to claim 7, characterized in that: The control tube also includes a directional reflection arc surface located above the diffuse reflection surface. During the crystal growth stage, when the focusing optical reflector focuses the light energy onto the crystal growth molten zone, the directional reflection arc surface can reflect the light energy penetrating the crystal growth molten zone and focus it onto a focal point located 2-5 mm below the crystal growth molten zone, forming a stepped temperature gradient from the molten zone downwards.

9. The crystal growth and in-situ annealing method according to claim 7, characterized in that: The output power of the light source decreases slowly during the annealing stage according to a preset cooling rate.

10. An optical floating zone furnace, characterized in that: It includes the optical structure of the floating zone furnace as described in any one of claims 1 to 6.

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