Device and method for eliminating head twin crystal indium phosphide single crystal

Abstract

The invention relates to the technical field of crystallization of indium phosphide single crystals, and particularly discloses a device and a method for eliminating head twin crystal indium phosphide single crystals. The device comprises a quartz tube and a hearth tube, wherein the quartz tube comprises a quartz tube body and a quartz cap arranged at a port of the quartz tube body; a supporting part is arranged on the outer wall of the quartz tube body and used for being matched with a hearth tube to enable the quartz tube body to be suspended in the hearth tube. The core of the device is that the supporting part is constructed, and the quartz tube is fixed in the hearth tube in a suspension manner, so that contact points between the quartz tube and the supporting tube are thoroughly eliminated, and the problem of rapid heat conduction of a'quartz tube body-supporting tube 'contact interface in a conventional supporting manner is fundamentally avoided; the path of local temperature loss caused by contact points is effectively blocked, so that the uniformity and the stability of a melt growth temperature field in the quartz tube are guaranteed, and the technical problem of frequent twin crystal defects in the existing indium phosphide single crystal production process is solved.

Description

Technical Field

[0001] This application relates to the field of indium phosphide single crystal crystal technology, and in particular to an apparatus and method for eliminating head twins indium phosphide single crystals. Background Technology

[0002] Indium phosphide (InP) substrates, as a key core material in the III-V compound semiconductor system, are hailed as the "golden platform" in the fields of optoelectronics and microwave devices. Their unique direct bandgap characteristics and excellent high electron mobility make them an ideal substrate material for fabricating high-speed, high-efficiency semiconductor devices.

[0003] However, InP material itself has a low stacking fault energy, which significantly lowers the energy barrier for stacking fault formation in its crystal. Once a stacking fault forms, it easily becomes the nucleation core for twins and further expands to form large-scale twin defects, especially in the crystal head region, where the nucleation and expansion of twin defects are more prominent, severely damaging the integrity of the single crystal. In conventional growth processes, the quartz tube loaded with raw materials is fixed by a quartz support tube to ensure it is within a preset growth temperature range. However, this support method often leads to a contact interface between the quartz tube and the support tube, since the thermal conductivity of quartz material is approximately [missing information - likely a value]. Furthermore, a significant temperature difference exists between the quartz tube and the support tube, leading to rapid heat conduction at the contact interface and causing a disordered temperature field distribution in the melt within the quartz tube. Given the extremely high requirements for temperature field uniformity during InP single crystal growth, this disorder directly results in the loss of stability at the crystal growth interface, leading to dendritic growth or cellular crystal structures. In the boundary regions and intersections of these irregular crystal structures, especially at the head region, atoms are highly susceptible to mis-stacking, inducing twin defects.

[0004] In summary, current processes are insufficient to effectively suppress head twin defects during InP single crystal growth. There is an urgent need to develop new growth technologies to achieve precise suppression and control of head twin defects, thereby significantly improving the product quality of InP single crystals. Summary of the Invention

[0005] In view of this, the purpose of this application is to provide an apparatus and method for eliminating head twins in indium phosphide single crystals, in order to solve the problem of frequent head twin defects in existing indium phosphide single crystal production processes.

[0006] To achieve the above-mentioned technical objectives, this application provides a device for eliminating head twinning indium phosphide single crystals, including a quartz tube and a furnace tube;

[0007] A quartz tube includes a quartz tube body and a quartz cap disposed at the end of the quartz tube body;

[0008] The outer wall of the quartz tube is provided with a support part, which is used to cooperate with the furnace tube to suspend the quartz tube inside the furnace tube.

[0009] Furthermore, the support is provided on the upper part of the outer wall of the quartz tube; the support is used to overlap the upper edge of the furnace tube so that the quartz tube is suspended inside the furnace tube.

[0010] Furthermore, a support is provided on the upper part of the outer wall of the quartz tube, and a support is provided on the inner wall of the furnace tube; the support and the support work together to suspend the quartz tube inside the furnace tube.

[0011] Furthermore, the support section is integrally formed with the quartz tube body.

[0012] Furthermore, the quartz tube body includes, from top to bottom, a cylindrical part, an inverted truncated conical part, and a cylindrical sealed tube connecting the bottom end of the truncated conical part, and the cylindrical part, the truncated conical part, and the cylindrical sealed tube are integrally formed.

[0013] Furthermore, a wet felt is installed at the bottom of the furnace tube, and the lower part of the cylindrical sealed tube is embedded in the wet felt; the space formed by the upper end face of the wet felt, the outer wall of the quartz tube, and the inner wall of the furnace tube is filled with thermal insulation cotton.

[0014] Furthermore, the insulation cotton has a fiber diameter of 3~10μm and a filling density of 30~100kg / m³. 3 ; and / or, the thickness of the wet felt is 3 to 6 mm.

[0015] Furthermore, multiple vertically arranged support tubes are installed inside the wet felt, and the support tubes do not contact the quartz tube body.

[0016] This application provides a method for eliminating head-twin indium phosphide single crystals, which is implemented using a device for eliminating head-twin indium phosphide single crystals.

[0017] Furthermore, it includes the following steps:

[0018] Step S1: Place the crucible containing the raw materials into the quartz tube, evacuate the quartz tube, and then seal the quartz tube with a quartz cap.

[0019] Step S2: Inert gas is introduced into the external space of the quartz tube so that the external air pressure of the quartz tube is always higher than the internal air pressure of the quartz tube.

[0020] Step S3: Crystals are grown using the vertical gradient solidification method to obtain indium phosphide crystals.

[0021] In summary, this application provides a device for eliminating head-twinned indium phosphide single crystals, comprising a quartz tube and a furnace tube; the quartz tube includes a quartz tube body and a quartz cap disposed at the port of the quartz tube body; a support portion is provided on the outer wall of the quartz tube body, which cooperates with the furnace tube to suspend the quartz tube body inside the furnace tube. The core of this device lies in fixing the quartz tube inside the furnace tube in a suspended manner through the support portion, completely eliminating the contact point between the quartz tube and the support tube. This design fundamentally avoids the problem of rapid heat conduction at the "quartz tube-support tube" contact interface under conventional support methods, effectively blocking the path of local temperature loss caused by the contact point, thereby ensuring the uniformity and stability of the melt growth temperature field inside the quartz tube, and effectively suppressing the formation of head-twinned crystals.

[0022] This application provides a method for eliminating head twins in indium phosphide single crystals. This method is based on the above-mentioned device. By improving the temperature field stability through device structure optimization and combining it with the synergistic control of process parameters during growth, the formation of head twins can be effectively suppressed, and the twin defects can be efficiently eliminated.

[0023] Compared with existing technologies, this invention is the first to propose and apply a quartz tube suspension design in the field of indium phosphide single crystal production. This design keeps the quartz tube and the bottom support tube in a spaced state, which completely solves the problem of the support tube interfering with the local temperature of the quartz tube in the traditional support method, and greatly reduces the temperature field non-uniformity caused by the support structure. This provides a key technical path for improving the quality and yield of indium phosphide single crystals. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 A schematic diagram of a device for eliminating head-twin indium phosphide single crystals provided in an embodiment of this application;

[0026] Figure 2 A schematic diagram of an apparatus for producing indium phosphide single crystals provided in an embodiment of this application;

[0027] Reference numerals: Quartz tube 1; Support part 11; Furnace tube 2; Support tube 21; Wet felt 22; Insulation cotton 23; PBN crucible 3. Detailed Implementation

[0028] The technical solutions of the embodiments of this application 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 this application. Based on the embodiments in this application specification, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection claimed in this application.

[0029] In the description of this application, 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 accompanying drawings. They are used only for the convenience of describing this application 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 this application. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0030] Unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0031] The raw materials used in this invention are not particularly restricted in terms of their source; they can be purchased on the market or prepared using conventional methods known to those skilled in the art.

[0032] This application provides an apparatus for eliminating head twinning of indium phosphide single crystals, including a quartz tube and a furnace tube; the quartz tube includes a quartz tube body and a quartz cap disposed at the port of the quartz tube body; a support portion is provided on the outer wall of the quartz tube body, the support portion being used to cooperate with the furnace tube to suspend the quartz tube body inside the furnace tube.

[0033] In some embodiments, a support is disposed on the upper part of the outer wall of the quartz tube; the support is used to overlap the upper edge of the furnace tube so that the quartz tube is suspended inside the furnace tube.

[0034] In some embodiments, a support portion is provided on the upper part of the outer wall of the quartz tube, and a support portion is provided on the inner wall of the furnace tube; the support portion and the support portion cooperate to suspend the quartz tube inside the furnace tube.

[0035] In some embodiments, the support portion is integrally formed with the quartz tube body.

[0036] In some specific embodiments, the support is a ring-shaped component, which is coaxially disposed at the upper end of the outer wall of the quartz tube and is integrally formed with the quartz tube body.

[0037] In some specific embodiments, the support portion consists of multiple protruding structures, each protruding structure being distributed circumferentially along the upper end of the outer wall of the quartz tube, and the protruding structures being integrally formed with the quartz tube.

[0038] In some embodiments, the quartz tube body includes, from top to bottom, a cylindrical portion, an inverted truncated conical portion, and a cylindrical sealed tube communicating with the bottom end of the truncated conical portion, and the cylindrical portion, the truncated conical portion, and the cylindrical sealed tube are integrally formed.

[0039] In some embodiments, a wet felt is provided at the bottom of the furnace tube, and the lower part of the cylindrical sealed tube is embedded in the wet felt; the space formed by the upper end face of the wet felt, the outer wall of the quartz tube, and the inner wall of the furnace tube is filled with thermal insulation cotton.

[0040] It should be noted that the ultra-low thermal conductivity of the insulation cotton stems from its porous structure. It blocks gas convection by creating tiny air pockets, while the fine fibers construct tortuous heat transfer paths, further inhibiting gas convection and reducing the effective thermal conductivity. This solves the problem of gas thermal convection disrupting the temperature field when the quartz tube is suspended. The wet felt is filled between the cylindrical sealed tube and the support tube; the thermal conductivity of the wet felt... Between thermal insulation cotton By avoiding the problem of seed crystal overheating and melting due to excessive insulation and non-directional nucleation caused by excessive heat from the insulation cotton, this invention also solves the problem of undercooling of melt components in the cylindrical sealed tube caused by gas convection in conventional processes. Therefore, this invention solves the temperature field problem of indium phosphide single crystal growth by using insulation cotton and wet felt in synergy: the insulation cotton fills the space between the quartz tube and the wet felt and the furnace tube, locking in the gas to block heat convection and stabilize the overall temperature field; the wet felt precisely controls the heat conduction efficiency of the cylindrical sealed tube and the support tube, ensuring the local temperature field gradient, and providing key material support for suppressing twin defects and improving single crystal quality and yield.

[0041] In some preferred embodiments, the insulation cotton has a fiber diameter of 3-10 μm and a packing density of 30-100 kg / m³. 3 ; and / or, the thickness of the wet felt is 3 to 6 mm.

[0042] It should be noted that the filling density of the thermal insulation cotton is between 30 and 100 kg / m³. 3 The insulation material must completely fill the target space. If the density is too high, the porosity will decrease, easily damaging the insulation structure and leading to an increase in thermal conductivity. If the density is too low or the space is not completely filled, the fiber content will be insufficient and the air content will be too high, failing to effectively suppress air convection and heat radiation, which will also lead to an increase in thermal conductivity. On the other hand, the insulation material must be kept dry. If it is damp, moisture will replace the internal stagnant gas, and water has low thermal conductivity. The thermal conductivity of the fiber is much higher than that of air, which will form continuous high thermal bridges in the pores, completely destroying the physical basis of the insulation cotton's reliance on gas for insulation. In addition, the fiber diameter needs to be maintained in the range of 3~10μm. Although the diameter is too small, it can increase the number of fibers and create a more tortuous heat transfer path, but it will lead to a decrease in the mechanical strength of the insulation cotton, making it prone to powdering and breakage. If the diameter is too large, it will not be able to form a dense fiber network, the pore size will be larger, it will be difficult to suppress gas convection, and the insulation effect will be significantly reduced. The wet felt with a thickness of 3~6mm can effectively isolate the direct heat exchange between the cylindrical sealed tube and the support tube, and ensure that the heat is orderly discharged from the cylindrical sealed tube to the quartz support tube through the wet felt.

[0043] This application provides a method for eliminating head-twin indium phosphide single crystals, which is implemented using a device for eliminating head-twin indium phosphide single crystals.

[0044] In some embodiments, a plurality of vertically arranged support tubes are provided inside the wet felt, and the support tubes do not contact the quartz tube body.

[0045] In some specific embodiments, multiple vertically arranged support tubes are arranged inside the wet felt. Each support tube is evenly distributed along the circumference of the cylindrical sealed tube, and the top of the support tube does not contact the quartz tube body.

[0046] In some embodiments, a method for eliminating head-twin indium phosphide single crystals includes the following steps:

[0047] Step S1: Place the crucible containing the raw materials into the quartz tube, evacuate the quartz tube, and then seal the quartz tube with a quartz cap.

[0048] Step S2: Inert gas is introduced into the external space of the quartz tube so that the external air pressure of the quartz tube is always higher than the internal air pressure of the quartz tube.

[0049] Step S3: Crystals are grown using the vertical gradient solidification method to obtain indium phosphide crystals.

[0050] The applicant further provides the following specific embodiments to describe the present invention. It should be noted that these embodiments are merely descriptive and do not limit the present invention in any way.

[0051] Example 1

[0052] See Figure 1 This embodiment provides a device for eliminating head twin indium phosphide single crystals, including a quartz tube 1 and a furnace tube 2 (the thickness of the tube wall on one side is about 3 to 5 mm).

[0053] Quartz tube 1 includes a quartz cap and a quartz tube body; the quartz cap is located at the top end of the quartz tube body to seal the quartz tube. The quartz tube body includes, from top to bottom, a cylindrical part, an inverted truncated cone part, and a cylindrical sealed tube connecting the bottom end of the truncated cone part, and the cylindrical part, the truncated cone part, and the cylindrical sealed tube are integrally formed. The PBN crucible 3 is installed inside the quartz tube body; an annular quartz baffle (with a radial thickness of 4-7 mm on one side) is integrally formed on the upper end of the outer wall of the quartz tube body; the annular quartz baffle overlaps the edge of the upper end of the furnace tube, so that the quartz tube body is suspended inside the furnace tube, avoiding direct contact between the quartz tube body and the inner wall of the furnace tube 2.

[0054] Multiple evenly distributed, vertically arranged quartz support tubes 21 are installed at the bottom of the furnace tube 2. A 3mm thick wet felt 22 is filled between adjacent quartz support tubes 21. The lower part of the cylindrical sealed tube of the quartz tube is embedded in the wet felt and does not contact the quartz support tubes 21. Insulation cotton 23 is filled in the space formed by the upper end faces of the quartz support tubes 21 and the wet felt 22, the outer wall of the quartz tube, and the inner wall of the furnace tube 2. The insulation cotton 23 has a fiber diameter of 5μm and a filling density of 50kg / m³. 3 .

[0055] Example 2

[0056] This embodiment provides a device for eliminating head-twinned indium phosphide single crystals. The difference from Embodiment 1 is that the insulation cotton has a fiber diameter of 10 μm and a filling density of 80 kg / m³. 3 The thickness of the wet felt is 6mm.

[0057] Comparative Example 1

[0058] This comparative example provides a device for eliminating head-twinned indium phosphide single crystals. The difference from Example 1 is that the fiber diameter of the insulation cotton is 20 μm and the filling density is 80 kg / m³. 3 The thickness of the wet felt is 6mm.

[0059] Comparative Example 2

[0060] This comparative example provides a device for eliminating head-twinned indium phosphide single crystals. The difference from Example 1 is that the fiber diameter of the insulation cotton is 10 μm and the filling density is 120 kg / m³. 3 The thickness of the wet felt is 6mm.

[0061] Comparative Example 3

[0062] This comparative example provides a device for eliminating head-twinned indium phosphide single crystals. The difference from Example 1 is that the fiber diameter of the insulation cotton is 10 μm and the filling density is 80 kg / m³. 3 The thickness of the wet felt is 9mm.

[0063] Comparative Example 4

[0064] See Figure 2 This comparative example provides an apparatus for producing indium phosphide single crystals, including a quartz tube 1 and a furnace tube 2;

[0065] The quartz tube 1 includes a quartz cap and a quartz tube body; the quartz cap is located at the top end of the quartz tube body to seal the quartz tube; the quartz tube body includes a cylindrical part, an inverted truncated cone part, and a cylindrical sealed tube connecting the bottom end of the truncated cone part from top to bottom, and the cylindrical part, the truncated cone part, and the cylindrical sealed tube are integrally formed, and the quartz tube body contains a PBN crucible 3.

[0066] Multiple evenly distributed and vertically arranged support pipes 21 are installed at the bottom of the furnace tube 2. Wet felt 22 is filled between adjacent support pipes 21. The truncated conical part of the quartz tube is mounted on the upper end of the support pipe 21. Through the support of the support pipe 21, the quartz tube 1 is suspended inside the furnace tube 2.

[0067] Application Example 1

[0068] This application example provides a method for eliminating head-twinned indium phosphide single crystals. This method is implemented in conjunction with the apparatus for eliminating head-twinned indium phosphide single crystals provided in Example 1, and includes the following steps:

[0069] Step S1, Raw material loading and crucible positioning: Calculate the required mass of phosphorus sample based on the target yield of indium phosphide single crystal, and load the phosphorus sample into the PBN crucible; then transfer the PBN crucible containing the phosphorus sample into the quartz tube of the device shown in Example 1.

[0070] Step S2, Quartz Tube Sealing and Vacuuming Operation: Place the quartz cap at the top end of the quartz tube, then precisely align and tighten the vacuum unit's clamp with the end of the quartz tube to ensure communication between the vacuum unit and the internal cavity of the quartz tube; start the vacuum pump to evacuate the inside of the quartz tube. After the evacuation process is complete, completely cover the end of the quartz tube with the quartz cap to achieve a seal inside the quartz tube.

[0071] Step S3, Pressure Control: Based on the Antoine equation (lgP=AB / (T+C), where P is pressure, T is temperature, and A / B / C are material property constants), the real-time pressure inside the quartz tube at the corresponding temperature is calculated. Based on the calculated internal pressure, an inert gas with a pressure of "P+1atm" is introduced into the gap between the furnace tube and the quartz tube body in the device shown in Example 1, so that the external pressure of the quartz tube is always greater than its internal pressure.

[0072] Step S4, Crystal Growth and Post-processing: The vertical gradient solidification method (VGF method) is used to grow the phosphorus sample in the quartz tube. When the internal pressure of the quartz tube drops to 1 bar, heating is stopped and the growth process is completed. The quartz tube is then removed from the furnace tube. A quartz-specific cutting machine is used to cut the connection between the quartz cap and the quartz tube body, and the indium phosphide crystal that has been grown in the cylindrical sealed tube is removed. The indium phosphide crystal is demolded and then sent to the processing workshop for subsequent cutting, grinding and other fine processing.

[0073] Step S5, Performance Testing and Data Statistics: Multiple performance tests are performed on the processed indium phosphide wafers, including electrical properties (ASTM F678-16), EPD (SEMIM37-0302), yield (SEMIM1-0302), and twin defects (ASTM F2260-19). The test data and statistical results are detailed in Table 1.

[0074] Application Example 2

[0075] Application Example 2 uses the same "method for eliminating head twins indium phosphide single crystals" as Application Example 1, only replacing the apparatus with the "apparatus for producing indium phosphide single crystals" provided in Example 2.

[0076] Compare and contrast application examples 1 to 4

[0077] Comparative Application Examples 1 to 4 all use the same "method for eliminating head twin indium phosphide single crystals" as Application Example 1, only the apparatus is replaced with the "apparatus for eliminating head twin indium phosphide single crystals" provided in Comparative Examples 1 to 4.

[0078] Table 1. Characterization parameters of indium phosphide yield, carrier concentration, and EPD

[0079]

[0080] As shown in Table 1, Examples 1 and 2 significantly outperformed Comparative Examples 1 to 4 in key electrical performance indicators such as yield, carrier concentration, and EPD. Furthermore, no twinning defects were detected in the head crystals of Examples 1 and 2. These results demonstrate that the present invention can effectively improve problems such as excessive temperature fluctuations and uneven temperature field distribution during crystal growth, and can precisely suppress the formation of head twinning defects.

[0081] The above are merely preferred embodiments of this application and are not intended to limit the present invention. Although this application has been described in detail with reference to examples, those skilled in the art can still modify the technical solutions described in the foregoing examples or make equivalent substitutions for some of the technical features. However, any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A device for eliminating head-twinned indium phosphide single crystals, characterized in that, Including quartz tubes and furnace tubes; The quartz tube includes a quartz tube body and a quartz cap disposed at the port of the quartz tube body. The outer wall of the quartz tube is provided with a support portion, which is used to cooperate with the furnace tube to suspend the quartz tube inside the furnace tube.

2. The apparatus for eliminating head-twinned indium phosphide single crystals according to claim 1, characterized in that: The support is disposed on the upper part of the outer wall of the quartz tube; the support is used to overlap the upper edge of the furnace tube so that the quartz tube is suspended inside the furnace tube.

3. The apparatus for eliminating head-twinned indium phosphide single crystals according to claim 1, characterized in that: The support is located on the upper part of the outer wall of the quartz tube, and the inner wall of the furnace tube is provided with a support portion; the support and the support portion cooperate to suspend the quartz tube inside the furnace tube.

4. The apparatus for eliminating head-twinned indium phosphide single crystals according to any one of claims 1 to 3, characterized in that, The support portion is integrally formed with the quartz tube body.

5. The apparatus for eliminating head-twinned indium phosphide single crystals according to claim 1, characterized in that: The quartz tube body comprises, from top to bottom, a cylindrical part, an inverted truncated conical part, and a cylindrical sealed tube connecting the bottom end of the truncated conical part, and the cylindrical part, the truncated conical part, and the cylindrical sealed tube are integrally formed.

6. The apparatus for eliminating head-twinned indium phosphide single crystals according to claim 5, characterized in that: A wet felt is provided at the bottom of the furnace tube, and the lower part of the cylindrical sealed tube is embedded in the wet felt; the space formed by the upper end face of the wet felt, the outer wall of the quartz tube and the inner wall of the furnace tube is filled with heat insulation cotton.

7. The apparatus for eliminating head-twinned indium phosphide single crystals according to claim 6, characterized in that, The insulation cotton has a fiber diameter of 3~10μm and a filling density of 30~100kg / m³. 3 ; and / or, the thickness of the wet felt is 3 to 6 mm.

8. The apparatus for eliminating head-twinned indium phosphide single crystals according to claim 6, characterized in that: The wet felt is provided with multiple vertically arranged support tubes, which do not contact the quartz tube body.

9. A method for eliminating head-twin indium phosphide single crystals, characterized in that, The device for eliminating head twin indium phosphide single crystals as described in any one of claims 1 to 8 is used.

10. The method for eliminating head-twinned indium phosphide single crystals according to claim 9, characterized in that, Includes the following steps: Step S1: Place the crucible containing the raw materials into the quartz tube, evacuate the quartz tube, and then seal the quartz tube with a quartz cap. Step S2: Inert gas is introduced into the external space of the quartz tube so that the external air pressure of the quartz tube is always higher than the internal air pressure of the quartz tube. Step S3: Crystals are grown using the vertical gradient solidification method to obtain indium phosphide crystals.

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