Method for growing asymmetric photovoltaic germanium single crystal through czochralski

Abstract

The invention discloses a method for growing an asymmetric photovoltaic germanium single crystal by czochralski, which comprises the following shouldering steps: A, reducing the pulling speed to 12 + / -0.1 mm / hr, increasing the pulling speed to 12.5 + / -0.1 mm / hr when the diameter of the single crystal begins to be enlarged and reaches 20 + / -0.1 mm, heating by 2SP, suspending the growth of the single crystal, and keeping the temperature for 10-30 minutes, so that the latent heat of crystallization generated by the rapid growth in the early stage is fully dissipated, and the growth time of the single crystal is shortened to 12 + / -0.1 mm / hr; enabling a solid-liquid interface to return to be flat again; b, cooling by 4SP to amplify the single crystal again, when the diameter reaches 30 + / -0.1 mm, increasing the pulling speed to 13 + / -0.1 mm / hr, heating by 2SP, suspending the growth of the single crystal and keeping the constant temperature for 10-30 minutes to finish 10mm growth once; and C, repeating the step B until the single crystal reaches the target diameter. According to the method, the monotectic single crystal is directly grown, the crystal bar does not need to be cut in a deviated manner in the subsequent directional rolling process, a large amount of germanium mud can be saved, the diameter of the single crystal can be greatly reduced, and the utilization rate of the germanium material is increased from 65% to about 90%.

Description

Technical Field

[0001] The invention relates to a method for growing an asymmetric photovoltaic germanium single crystal by direct pulling, and belongs to the technical field of an asymmetric photovoltaic germanium single crystal. Background Art

[0002] Wafers made of photovoltaic germanium single crystals are the substrate layer of multi-stage gallium arsenide cells in space. Compared with other materials, germanium substrates have excellent radiation resistance, high temperature resistance and high strength. They are mainly used in spacecraft (satellites, etc.) panels and ground-based high-concentration photovoltaic power generation (CPV). In order to achieve the purpose of photovoltaic power generation, germanium single crystals also need to meet the characteristics of high carrier concentration (2E17-3E18 / ㎝3), high mobility (>250㎝2 / V·s) and low defect density (EPD<300).

[0003] A semiconductor-grade crystal, the main growth methods include VGF (vertical directional solidification method) and Czochralski (Czochralski method). Compared with the VGF method, the Czochralski single crystal can produce 750mm long finished products per furnace, while the VGF method can produce a maximum of 150mm long finished products per furnace, and the dislocation of VGF is relatively large.

[0004] In order to grow compound semiconductor battery layers such as gallium arsenide on the surface of the germanium substrate, the germanium substrate needs to be processed into a specific crystal orientation to match the lattice of the epitaxial layer, generally requiring (100) to be offset by 9° to (111). Due to the low thermal conductivity of the material itself (1 / 2 of silicon), it is very difficult to grow a zero-dislocation single crystal. Czochralski germanium single crystals are usually grown into a (100) positive crystal orientation and then cut by 9° offset through directional processing to obtain the required crystal rod ( Figure 1 ), the processing will produce a lot of germanium mud and waste. In recent years, the price of germanium has been rising steadily. If it is still produced according to the original process, the loss of waste germanium in the process of processing and recycling is already an unbearable loss. However, if the (100) deviation (111) 9° single crystal is directly grown, the uneven heat dissipation caused by the asymmetry of the crystal structure makes it very difficult to grow a single crystal with zero dislocation. Summary of the invention

[0005] The invention provides a method for growing an asymmetric photovoltaic germanium single crystal by direct pulling, and directly grows a zero-dislocation germanium single crystal with a (100) deviation from (111) 9° crystal orientation by direct pulling.

[0006] In order to solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0007] A method for growing an asymmetric zero-dislocation germanium single crystal by direct pulling, the shoulder placement step comprising:

[0008] A. Reduce the pulling speed to 12±0.1㎜ / hr, and the single crystal diameter begins to expand. When the diameter reaches 20±0.1㎜, increase the pulling speed to 12.5±0.1㎜ / hr, increase the temperature by 2SP, and suspend the growth of the single crystal and keep the temperature constant for 10-30 minutes to allow the crystallization latent heat generated by the rapid growth in the early stage to be fully dissipated, so that the solid-liquid interface returns to a flat state;

[0009] B. Cool down by 4SP to enlarge the single crystal again. When the diameter reaches 30±0.1㎜, increase the pulling speed to 13±0.1㎜ / hr, increase the temperature by 2SP, suspend the growth of the single crystal and keep the temperature constant for 10-30 minutes to complete a growth of 10mm.

[0010] C. Repeat step B until the single crystal reaches the target diameter; that is: after each 10mm growth, reduce the temperature by 4SP, the single crystal grows by 10cm, the pulling speed is increased by 0.5㎜ / hr compared to the previous step, and then increase the temperature by 2SP, suspend the growth of the single crystal and keep the temperature constant for 10-30 minutes.

[0011] The above method is applicable to the growth of single crystals of any diameter.

[0012] In the above steps AC, when the diameter of the single crystal is in the range of 10-50 mm (including the endpoints 10 mm and 50 mm), the constant temperature time is 15±1 minutes each time; when the diameter of the single crystal is in the range of 50-100 mm (excluding the endpoint 50 mm and including the endpoint 100 mm), the constant temperature time is 20±1 minutes each time; when the diameter of the single crystal is >100 mm, the constant temperature time is 25±1 minutes each time.

[0013] The present application adopts a vertical pulling method, and in the single crystal shouldering step, a stepped shouldering method is used to offset the uneven heat dissipation caused by the asymmetric crystal structure, so that a zero dislocation crystal can be obtained for use in the semiconductor field.

[0014] SP: In order to intercept the temperature range of 800-1200℃, it is amplified to obtain the analog temperature signal, amplified 4 times, 1SP is equivalent to 0.25℃, and the real temperature of the melt surface is measured by the infrared thermometer on the top of the single crystal furnace. 935℃ is calibrated to 1300SP (the side temperature measurement calibration reference value of the single crystal furnace). Since the top temperature measurement will be blocked during the crystal growth process, the temperature control during the crystal growth process is controlled by the SP value.

[0015] A method for growing an asymmetric photovoltaic germanium single crystal by direct pulling, comprising the following steps:

[0016] 1) Directional processing of special eutectic seed crystals;

[0017] 2) Complete the steps of furnace loading, melting and constant temperature (consistent with the conventional crystal growth process), and the constant temperature is 1300SP;

[0018] 3) Seeding and necking: Cool down 10SP, the starting pulling speed of the eutectic seed crystal automatically shrinks the neck. After running for 30 minutes, it automatically increases by 12±0.1㎜ / hr every 15 minutes until it reaches 72±0.1㎜ / hr. After the diameter continues to shrink to 5㎜, it enters the automatic diameter control state and starts to measure the length. The pulling speed is controlled to 72±12㎜ / hr, and the cumulative length of the neck is increased from 150㎜ to 250㎜, which can effectively avoid the asymmetric crystal ( <100> Bias <111> 9°) and the resulting growth direction and dislocation slip direction <111> The problem of internal defects of seed crystal being difficult to remove after the angle becomes smaller;

[0019] 4) Shoulder release, including:

[0020] A. Reduce the pulling speed to 12±0.1㎜ / hr, and the single crystal diameter begins to expand. When the diameter reaches 20±0.1㎜, increase the pulling speed to 12.5±0.1㎜ / hr, increase the temperature by 2SP, and suspend the growth of the single crystal and keep the temperature constant for 10-30 minutes to allow the crystallization latent heat generated by the rapid growth in the early stage to be fully dissipated, so that the solid-liquid interface returns to a flat state;

[0021] B. Cool down by 4SP to enlarge the single crystal again. When the diameter reaches 30±0.1㎜, increase the pulling speed to 13±0.1㎜ / hr, increase the temperature by 2SP, suspend the growth of the single crystal and keep the temperature constant for 10-30 minutes to complete a growth of 10mm.

[0022] C. Repeat step B until the single crystal reaches the target diameter; that is, after each 10mm growth, the temperature is lowered by 4SP, the single crystal grows by 10cm, the pulling speed is increased by 0.5㎜ / hr compared with the previous step, and then the temperature is increased by 2SP, the growth of the single crystal is suspended and the temperature is kept constant for 10-30 minutes;

[0023] 5) Shoulder rotation and equal diameter steps;

[0024] 6) Closing and shutdown steps.

[0025] This application does not make any special improvements to step 1), and existing mature technologies can be used, which will not be described in detail.

[0026] The above 5) shoulder turning and equal diameter steps: the shoulder turning pulling speed is increased by 2mm / hr on the basis of the final pulling speed of shoulder release, and it will automatically enter the equal diameter step after two hours.

[0027] In order to further improve the growth quality, the controlled pulling speed range in the above-mentioned equal-diameter stage is 18-24 mm / hr. The smaller pulling speed range can prevent the edge breakage problem caused by the drastic change in growth rate.

[0028] In order to improve the stability of growth quality, in the equal diameter stage, the temperature curve of each 50mm equal diameter length is set separately. The specific temperature rise and fall values ​​are corrected according to the average growth rate curve in each length interval collected in the early stage. If the average pulling speed runs in the upper half of 18-24mm / hr (18-21mm / hr, including 21mm / hr), the automatic temperature curve parameters in the 50mm length interval need to be positively corrected by 2-3SP, otherwise (21-24mm / hr, excluding 21mm / hr, including 24mm / hr)) it will be reduced by 2-3SP (can be negative). Through multiple iterative updates, a more perfect temperature curve can be obtained to assist in controlling the diameter stability, so as to achieve the purpose of stabilizing the growth rate.

[0029] In order to further improve the growth stability, the above 6) tail length is increased from 150mm to 250mm, and the diameter is reduced to 25mm±1. Increasing the length means that the pulling speed fluctuation will be smaller, completing the crystal growth process.

[0030] The technologies not mentioned in the present invention are all referred to the prior art.

[0031] The method for growing asymmetric photovoltaic germanium single crystal by direct pulling of the present invention has the following beneficial effects:

[0032] 1) Directly grow eccentric single crystals, and the subsequent directional rounding process does not require eccentric cutting of the crystal rod, which can save a lot of germanium mud, and the diameter of the single crystal can also be greatly reduced, and the utilization rate of germanium material is increased from 65% to about 90%;

[0033] 2) No more worries about the waste of germanium mud. The length of a single section of the crystal rod is no longer limited. The segment length is increased from 150mm to 250mm, and the number of sections is changed from 5 to 3, which reduces the waste of EPD test pieces and the waste of head and tail pieces generated by subsequent processing;

[0034] 3) Since the cutting direction of the back-end wire cutting is completely parallel to the solid-liquid interface direction of the crystal growth, the BOW (bending) of the wafer is significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Schematic diagram of processing of single crystal with 9° deviation (left: the intention of existing forward single crystal 9° deviation cutting orientation processing, the shaded part in the figure is processed into germanium mud, and the utilization rate is 63%; right: schematic diagram of direct orientation processing of single crystal with 9° deviation of the present invention, the shaded part in the figure is processed into germanium mud, and the utilization rate is 90%)

[0036] Figure 2 Symmetry comparison of the crystal structure of a normal single crystal (left) and a biased single crystal with shoulder placement (right) (top view);

[0037] Figure 3 This is a schematic diagram of the eutectic seed crystal; DETAILED DESCRIPTION

[0038] In order to better understand the present invention, the content of the present invention is further explained below in conjunction with the embodiments, but the content of the present invention is not limited to the following embodiments.

[0039] Example 1

[0040] A method for growing an asymmetric photovoltaic germanium single crystal by direct pulling, comprising the following steps:

[0041] 1) Directional processing of special eutectic seed crystals, such as Figure 3 As shown;

[0042] 2) Complete the steps of loading the furnace, melting the materials and maintaining the temperature, and the temperature is 1300SP (calibrated to 935°C);

[0043] 3) Seeding and necking, including: cooling 10SP, automatic necking of the eutectic seed crystal, starting pulling speed from 24mm / hr for 30 minutes, automatically increasing by 12mm / hr every 15 minutes until 72mm / hr, the diameter continues to shrink to 5mm, then enters the automatic diameter control state and starts length measurement, controlling the pulling speed to 72±12mm / hr, and the cumulative length of the neck is increased from 150mm to 250mm, which can effectively avoid the asymmetric crystal ( <100> Bias <111> 9°) and the resulting growth direction and dislocation slip direction <111> The problem of internal defects of seed crystal being difficult to remove after the angle becomes smaller;

[0044] 4) Shoulder release, including:

[0045] A. Reduce the pulling speed to 12mm / hr, the diameter of the single crystal begins to expand. When the diameter reaches 20mm, increase the pulling speed to 12.5mm / hr, increase the temperature by 2SP, suspend the growth of the single crystal and keep the diameter constant for 15 minutes, so that the latent heat of crystallization generated by the rapid growth in the early stage can be fully dissipated and the solid-liquid interface can be flattened again;

[0046] B. Cool down by 4SP to enlarge the single crystal again. When the diameter reaches 30mm, increase the pulling speed to 13mm / hr and increase the temperature by 2SP to stop the growth of the single crystal and keep the temperature constant for t minutes to complete a growth of 10mm.

[0047] C. Repeat step B until the single crystal reaches 150mm; that is: after each 10mm growth, reduce the temperature by 4SP, the single crystal grows 10cm, the pulling speed is increased by 0.5㎜ / hr compared to the previous step, and then increase the temperature by 2SP, suspend the growth of the single crystal and keep the temperature constant for t minutes.

[0048] When the diameter of the single crystal is in the range of 10-50㎜ (including the two end points), the constant temperature time t is 15 minutes each time; when the diameter of the single crystal is in the range of 50-100㎜ (excluding the end point 50mm and including the end point 100mm), the constant temperature time t is 20 minutes each time; when the diameter of the single crystal is in the range of 100-150㎜ (excluding the end point 100mm and including the end point 150mm), the constant temperature time t is 25 minutes each time.

[0049] 5) Shoulder rotation and equal diameter steps, including:

[0050] A. The pulling speed of shoulder turning is increased by 2mm / hr on the basis of the final pulling speed of shoulder releasing, and it will automatically enter the equal diameter step after two hours;

[0051] B. In the stage of equal diameter automatic control, the pulling speed range is controlled to be 18-24 mm / hr. A smaller pulling speed range can prevent the problem of broken edges caused by drastic changes in growth rate.

[0052] Temperature control in the equal diameter stage: according to the changing trend of the heat dissipation amplitude in the equal diameter process, the temperature curve of each 50mm equal diameter length is set separately instead of the conventional temperature curve of each 100mm. The specific temperature rise and fall values ​​are corrected according to the average growth rate curve in each length interval collected in the early stage. If the average pulling speed runs in the upper half of 18-24mm / hr, the automatic temperature curve parameters in the 50mm length interval need to be positively corrected by 2-3SP, otherwise, it needs to be reduced by 2-3SP (can be negative value). Through multiple iterative updates, a more perfect temperature curve is obtained to assist in controlling the diameter stability so as to achieve the purpose of stabilizing the growth rate. The initial temperature curve correction is shown in the following table;

[0053] Length mm SP value correction Length mm SP value correction 0-50 5 450-500 1 50-100 4 500-550 2 100-150 3 550-600 2 150-200 2 600-650 2 200-250 -1 650-700 3 250-300 -2 700-750 3 300-350 -2 750-800 2 350-400 -1 800-850 2 400-450 1 850-900 2

[0054] 6) Finishing and furnace shutdown steps: The finishing length is increased from 150mm to 250mm, and the diameter is also reduced to about 25mm. Increasing the length means that the pulling speed fluctuation will be smaller, completing the crystal growth process.

[0055] The workshop used the above method to produce 14 furnaces of 9° germanium single crystals in September 2024. All of them were crystallized and qualified, with a crystallization rate of 100%. The tracking chip BOW value dropped from an average of about 15μm to an average of about 8μm, and can be reduced to 0.1μm at the lowest.

[0056] Ingot number 9-1 (first batch in September), single crystal was grown directly using 9° seed crystal, and the fast and slow alternating step-by-step shouldering method was adopted. The shouldering time was 22 hours and 17 minutes, and the pulling speed was alternating from 12mm / hr to 17mm / hr, and the temperature was alternating and finally dropped by 15SP to complete the shouldering process. The starting pulling speed for shoulder equalization was 19mm / hr, the starting temperature was 1275SP, the equal diameter length of the crystal ingot was 900mm, the equal diameter time was 43 hours and 27 minutes, and the equal diameter cumulative temperature was 30 SP, average pulling speed 20.7㎜ / hr, qualified length 772.7㎜, crystal rod sample piece measured crystal rod head resistivity 0.012Ω·㎝, EPD is 6 / ㎝2, tail resistivity 0.0082Ω·㎝, EPD is 10 / ㎝2, after processing into wafer, sample carrier concentration (0.545-1.832) E+18 / ㎝3, mobility (3.498-6.207) E+2㎝2 / V·s, BOW value minimum can reach 0.7μm, average 8.3μm.

[0057] The crystal rod number is 9-3 (the third batch in September). The single crystal is directly grown using a 9° seed crystal. The shoulder release time is 24 hours and 02 minutes. The pulling speed is alternating from 12mm / hr to 17mm / hr. The temperature is alternating and the temperature drops by 17SP. The shoulder release process is completed. The starting pulling speed for shoulder equalization is 19mm / hr, the starting temperature is 1273SP, the crystal rod equal diameter length is 900mm, the equal diameter time is 42 hours and 29 minutes, and the equal diameter cumulative temperature rises by 28SP. The average pulling speed is 21.2㎜ / hr, the qualified length is 764.6㎜, and the crystal rod sample piece measures the resistivity of the crystal rod head to be 0.0142Ω·㎝, the EPD is 9 / ㎝2, the resistivity of the tail is 0.0087Ω·㎝, the EPD is 13 / ㎝2. After being processed into wafers, the carrier concentration is (0.376-2.015)E+18, the mobility is (3.872-7.071)E+2, the minimum BOW value can reach 0.3μm, and the average is 7.9μm.

[0058] Comparative Example

[0059] The difference from the first embodiment is that the shoulder placement is not stepped.

[0060] A method for growing an asymmetric photovoltaic germanium single crystal by direct pulling, comprising the following steps:

[0061] 1) Directional processing of special eutectic seed crystals;

[0062] 2) The steps of loading the furnace, melting the materials and maintaining the temperature are the same as the conventional crystal growth process;

[0063] 3) Seeding and necking, the automatic necking starting pulling speed runs from 24mm / hr for 30 minutes, and then automatically increases to 72mm / hr and enters the automatic diameter control state of 5mm. The cumulative length of the neck is completed after 150mm;

[0064] 4) Shoulder release, including: reducing the pulling speed to 12 mm / hr, the diameter of the single crystal begins to increase, and the temperature is reduced by 2SP every 60 minutes to maintain the diameter enlargement growth state until it reaches 150 mm;

[0065] 5) Shoulder rotation and equal diameter steps, including:

[0066] A. When the pulling speed increases to 20 mm / hr, the shoulder starts to turn and the equal diameter step will be automatically entered after 1 hour.

[0067] B. The initial pulling speed of equal diameter is automatically increased to 24mm / hr. On the console, one-click turns on the equal-diameter automatic control, the process drawing speed setting control range is 18-30㎜ / hr, and the equal-diameter length is 900㎜.

[0068] 6) Finishing and furnace shutdown steps: At the end, the pulling speed is automatically increased to 50mm / hr, the finishing length is 150mm, and the diameter is reduced to about 25mm to complete the crystal growth process.

[0069] The crystal rod is numbered 5-14-4. After the crystal growth was completed according to the above process, the EPD of the whole crystal rod was detected to be excessive, and a large number of slip dislocation lines appeared. The crystal rod was scrapped and did not flow into the next process.

[0070] The main problem is concentrated in the shoulder release step, that is, in the process of enlarging the crystal diameter of the eccentric orientation from 10㎜ to 150㎜, it is easy to break the edge line and it is almost impossible to grow a zero dislocation single crystal. Therefore, the accumulation of latent heat of crystallization should be strictly controlled through a step-by-step shoulder release method, so as to achieve timely heat transfer and ensure the relative flatness and stability of the solid-liquid interface.

Claims

1. A method for growing an asymmetric zero-dislocation germanium single crystal by Czochralski method, characterized in that: Shoulder release steps include: A. Reduce the pulling speed to 12±0.1㎜ / hr, the diameter of the single crystal begins to expand, and when the diameter reaches 20±0.1㎜, increase the pulling speed to 12.5±0.1㎜ / hr, increase the temperature by 2SP, suspend the growth of the single crystal and keep the temperature constant for 10-30 minutes, and the solid-liquid interface returns to flatness again; B. Cool down by 4SP to enlarge the single crystal again. When the diameter reaches 30±0.1㎜, increase the pulling speed to 13±0.1㎜ / hr, increase the temperature by 2SP, suspend the growth of the single crystal and keep the temperature constant for 10-30 minutes to complete a growth of 10mm. C. Repeat step B until the single crystal reaches the target diameter.

2. The method for growing asymmetric photovoltaic germanium single crystal by direct pulling as claimed in claim 1, characterized in that: In step AC, when the single crystal diameter is in the range of 10-50 mm, the constant temperature time is 15±1 minutes each time; when the single crystal diameter is in the range of 50-100 mm, the constant temperature time is 20±1 minutes each time; when the single crystal diameter is greater than 100 mm, the constant temperature time is 25±1 minutes each time.

3. The method for growing asymmetric photovoltaic germanium single crystal by direct pulling as claimed in claim 1 or 2, characterized in that: The steps include: 1) Directional processing to produce eutectic seed crystals; 2) Complete the steps of loading the furnace, melting the materials and maintaining the temperature, with the temperature being 1300SP; 3) Seeding and necking: Cool down 10SP, the starting pulling speed of the automatic necking of the seed crystal towards the eutectic starts from 24±0.1㎜ / hr and runs for 30 minutes, then automatically increases by 12±0.1㎜ / hr every 15 minutes until it reaches 72±0.1㎜ / hr. After the diameter continues to shrink to 5㎜, it enters the automatic diameter control state and starts to measure the length. The pulling speed is controlled to 72±12㎜ / hr, and the cumulative length of the neck is 250㎜. 4) Shoulder release, including: A. Reduce the pulling speed to 12±0.1㎜ / hr, the diameter of the single crystal begins to expand, and when the diameter reaches 20±0.1㎜, increase the pulling speed to 12.5±0.1㎜ / hr, increase the temperature by 2SP, suspend the growth of the single crystal and keep the temperature constant for 10-30 minutes, and the solid-liquid interface returns to flatness again; B. Cool down by 4SP to enlarge the single crystal again. When the diameter reaches 30±0.1㎜, increase the pulling speed to 13±0.1㎜ / hr, increase the temperature by 2SP, suspend the growth of the single crystal and keep the temperature constant for 10-30 minutes to complete a growth of 10mm. C. Repeat step B until the single crystal reaches the target diameter; 5) Shoulder rotation and equal diameter; 6) Finishing and shutdown.

4. The method for growing asymmetric photovoltaic germanium single crystal by direct pulling as claimed in claim 3, characterized in that: 5) Shoulder turning and equal diameter steps: The shoulder turning pulling speed is increased by 2mm / hr based on the final pulling speed of shoulder release, and it will automatically enter the equal diameter step after two hours.

5. The method for growing asymmetric photovoltaic germanium single crystal by direct pulling as claimed in claim 4, characterized in that: The controlled pulling speed range in the equal diameter stage is 18-24 mm / hr.

6. The method for growing asymmetric photovoltaic germanium single crystal by direct pulling as claimed in claim 4, characterized in that: In the equal diameter stage, the temperature curve of each 50mm equal diameter length is set separately. The specific temperature rise and fall values ​​are corrected according to the average growth rate curve in each length interval collected in the early stage. If the average pulling speed runs in the upper half of 18-24mm / hr, the automatic temperature curve parameters in the 50mm length interval need to be positively corrected by 2-3SP, otherwise they need to be reduced by 2-3SP.

7. The method for growing asymmetric photovoltaic germanium single crystal by direct pulling as claimed in claim 3, characterized in that: 6) The finishing length is 250mm and the diameter is reduced to 25mm±1.

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