Method for improving purity of germanium metal to reduce defects of germanium single crystal for solar cell

Abstract

The invention discloses a method for improving purity of germanium metal to reduce defects of germanium single crystals for solar cells, and belongs to the technical field of preparation of high-purity germanium materials. The method comprises the steps of quartz tube cleaning, graphite boat cleaning, zone-melting germanium ingot cleaning, charging into a furnace, high-temperature melting and directional crystallization and germanium ingot cleaning, specifically, after the zone-melting germanium ingot is re-melted at the high temperature of 1300 DEG C, horizontal directional cooling crystallization is conducted, so that micro-trace impurities in germanium melt are enriched at the tail of the germanium ingot, and the germanium ingot is cleaned. Meanwhile, a hydrogen atmosphere with the purity being 99.999% or above is used for protecting the germanium melt from being oxidized by oxygen, and volatilized trace impurities are taken away through hydrogen flow. When germanium single crystal growth is carried out after treatment of the method, compared with a conventional method, the proportion of crystal defects during crystal growth of a Czochralski (CZ) method is reduced to 5% or below from 30% during direct crystal growth, and the proportion of crystal defects during crystal growth of a vertical gradient solidification (VGF) method is reduced to 2% or below from 10% during direct crystal growth.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-purity germanium preparation, and particularly relates to a method for reducing the defects of germanium single crystals for solar cells. Background Art

[0002] At present, the rapid development of the demand for space solar cells has greatly promoted the high-speed growth of the germanium single crystal substrate product market. Therefore, higher requirements are put forward for the production materials of germanium single crystal wafers - germanium single crystals for solar energy. It is not only required that the compensating impurities are evenly distributed and the dislocation density is ultra-low, but also higher technical requirements are put forward for other crystal defects. It is necessary to improve the quality and qualification rate of germanium single crystals and reduce or even eliminate the defect density of germanium single crystals. In the production of germanium single crystals, problems such as polycrystallization and high dislocation density are common problems affecting crystal quality, and these problems have been largely solved through continuous process improvement and technology optimization. However, through years of a large number of production practices, it has been shown that tiny defects such as "pinholes, holes, and inclusions" still exist, directly affecting the quality and photoelectric conversion efficiency of germanium single crystals for solar energy. Summary of the Invention

[0003] The present invention proposes a method of high-temperature melting and directional cooling crystallization of zone-melted germanium ingots to reduce the content of germanium dioxide, germanium monoxide, germanium nitride, and other trace metal impurities in germanium metal materials, thereby reducing the crystal defects generated by germanium single crystals affected by impurities during the growth process, improving the quality of germanium single crystals for solar cells, and increasing the single crystal qualification rate and wafer finished product rate.

[0004] A method for improving the purity of germanium metal to reduce the defects of germanium single crystals for solar cells, the method includes steps of quartz tube cleaning, graphite boat cleaning, zone-melted germanium ingot cleaning, loading into the furnace, high-temperature melting and directional crystallization, and germanium ingot cleaning. It is characterized in that high-temperature melting and directional crystallization is to remelt the zone-melted germanium ingot at a high temperature of 1300 °C and then horizontally directionally cool and crystallize, so that the micro-trace impurities in the germanium melt are enriched at the tail of the germanium ingot. At the same time, the germanium melt is protected from oxidation by oxygen with a hydrogen atmosphere with a purity of more than 99.999%, and the volatilized trace impurities are carried away by the hydrogen flow.

[0005] For the said quartz tube cleaning, after soaking the high-purity quartz tube in an electronic-grade hydrofluoric acid (HF) solution with a mass fraction of 2.5% - 7.5% for 2h - 3h, cleaning the inner and outer walls of the quartz tube with a 1% HF solution without contamination, then repeatedly rinsing with deionized water until there is no fluoride ion, and then cleaning with methanol to volatilize and dehydrate and drying.

[0006] The cleaning of the graphite boat is as follows: First, use sandpaper to polish the inner wall of the graphite boat to make its surface smooth without pits. Then, wipe it clean with methanol until there is no stain. After that, put it into a container and soak it in a mixed solution prepared by mixing 7.5% electronic-grade HCl solution and 2.5% electronic-grade HNO3 solution in a volume ratio of 1:1. Heat the mixed solution to 40°C - 50°C and soak for 5h - 8h. After taking it out, put it into an ultrasonic container filled with deionized water for heating and cleaning. Then, rinse it with deionized water until there is no chloride ion, and then rinse it with methanol to volatilize and dehydrate it, and finally dry it.

[0007] The cleaning of the zone-melted germanium ingot is to oxidize and remove the trace oxide layer and impurities on the surface of the germanium ingot with a mixed solution of 2.5% electronic-grade H2O2 solution and 5% electronic-grade HF solution in a volume ratio of 1:2. Then, rinse it with deionized water until there is no fluoride ion. Then, clean it with methanol to volatilize and dehydrate it, and finally dry it.

[0008] The charging into the furnace is as follows: Put the high-purity quartz tube into the horizontal tubular temperature-controlled heating furnace body, and tightly fill the gaps at the head and tail of the quartz tube and the temperature-controlled heating furnace body with heat-insulating cotton. Adjust the horizontal direction of the furnace head to maintain an inclination of 2 degrees. Put the dried germanium ingot into the high-purity graphite boat, and then put it into the quartz tube. Then, seal the quartz tube well, connect the air inlet and the air outlet with an ampoule tail gas bottle. After evacuating the vacuum in the furnace to 0.1 Pa, then introduce hydrogen with a purity of 99.999% for 0.5h - 1h to displace the trace air in the tube. After the tube is completely filled with hydrogen, adjust the hydrogen flow rate to be set at 2.5L / min - 5.0L / min. Install temperature-measuring and temperature-controlling thermocouples on the temperature-controlled heating furnace body respectively.

[0009] The high-temperature melting and directional crystallization are specifically as follows: After charging into the furnace and introducing hydrogen, heat up to 950°C and keep it warm for 2h. After the germanium ingot is completely melted, then slowly heat up to 1300°C in 1h - 2h, keep the germanium melt in a molten state at high temperature for 2h, and then cool down. First, cool down the direction of the air outlet. When the temperature at the air inlet drops to 935°C, it solidifies into germanium polycrystals and then slowly cools down to 300°C. Turn off the heating device, naturally cool to room temperature, then stop introducing hydrogen, and displace the hydrogen in the quartz tube with high-purity nitrogen. Take out the graphite boat and then take out the germanium ingot, cut off the head and tail of the germanium ingot, and the qualified germanium ingot part in the middle is for growing single crystals.

[0010] The cleaning of the germanium ingot uses a mixed solution of 5% hydrofluoric acid solution and 2.5% nitric acid solution in a volume ratio of 2:1 to corrode and clean the surface of the qualified germanium ingot to remove the impurities on the oxide layer. Then, rinse it with deionized water until it is clean and dry, and then put it into a single crystal furnace for solar cells for crystal growth.

[0011] In the process of heating and constant temperature melting, trace impurities such as germanium dioxide, germanium monoxide, germanium nitride and the like contained in the zone melting germanium ingot react with hydrogen to generate single germanium and water vapor, as well as volatile non-metallic and low melting point impurities, which evaporate from the liquid surface from the inside of the germanium melt at high temperature and are taken away from the tail gas along with hydrogen. At the same time, in the process of directional movement cooling crystallization of the germanium ingot, non-volatile heavy metal impurities with a distribution coefficient greater than 1 (germanium is 1) migrate and enrich at the head of the germanium ingot, and impurities with a distribution coefficient less than 1 migrate and enrich at the tail of the germanium ingot, and the purpose of removing impurities is achieved by cutting off the head and tail of the germanium ingot.

[0012] The main chemical reaction equations involved in the present invention are as follows:

[0013] (1) GeO2+2H2=Ge+2H2O;

[0014] The trace amount of germanium dioxide contained in it is further reduced to germanium metal and water vapor and volatilizes;

[0015] (2) GeO + H2 = Ge + H2O;

[0016] The trace amount of germanium monoxide contained in it is further reduced to germanium metal and water vapor and volatilizes;

[0017] (3) Ge3N4+6H2=3Ge+4NH3;

[0018] The trace amount of germanium nitride included is reduced to germanium metal and ammonia and volatilizes;

[0019] The present invention aims at the influence of compounds such as germanium dioxide (GeO2), germanium monoxide (GeO), germanium nitride (Ge3N4) and other trace metal impurities on the quality of germanium single crystal growth caused by the presence of germanium dioxide (GeO2), germanium monoxide (GeO), germanium nitride (Ge3N4) and other trace metal impurities in germanium single crystals for solar cells, and aims at the problem of crystal micro defects such as "pinholes, inclusions and holes" produced. The purity of the raw material zone melting germanium ingot is further purified and impurities are separated by high-temperature melting and directional horizontal cooling crystallization of the zone melting germanium ingot, and then the germanium single crystal is grown, thereby significantly improving the quality of the germanium single crystal and reducing the inclusion of the above impurities in the germanium single crystal.

[0020] The single crystal obtained by growing germanium single crystal for solar cell after high temperature melting-directional cooling crystallization purification treatment in the present invention has significantly fewer defects compared with conventional germanium single crystal for solar cell produced without purification treatment, as specifically listed in Table 1.

[0021] Table 1 Defect ratio of germanium ingot treated by the present invention and germanium single crystal for solar cells produced by conventional method

[0022]

[0023] Judging from the test results of GD-MS after purification, the purification effect of the high-temperature melting - directional cooling crystallization method of the present invention is significantly better than that of the conventional purification method, which is specifically listed in Table 2.

[0024] Table 2 Comparison of purification effects between the present method and the conventional method

[0025]

[0026] Description of the Drawings

[0027] Figure 1 It is a micro-defect map on a non-conforming wafer;

[0028] As shown in the figure, after taking a germanium single crystal sampling wafer and observing it under a 500-fold microscope after chemical polishing, defects are found.

[0029] Figure 2 It is a schematic structural diagram of a directional crystallization tubular furnace;

[0030] Among them, 1 is a seal; 2 is a graphite boat; 3 is a temperature-controlled heating furnace body; 4 is an air inlet, 5 is a quartz tube, and 6 is an air outlet. Detailed Embodiments

[0031] Example 1: A method for improving the purity of germanium metal to reduce defects in germanium single crystals for solar cells, the method comprising the following steps:

[0032] Step 1, cleaning the quartz tube:

[0033] Soak the high-purity quartz tube in an electronic-grade hydrofluoric acid (HF) solution with a mass fraction of 2.5% for 3 h, then clean the inner and outer walls of the quartz tube with an HF solution with a mass fraction of 1% until there is no contamination, and then repeatedly rinse with deionized water for about 3 min to completely remove fluoride ions. Then, clean with methanol, volatilize and dehydrate, and place it in a clean infrared lamp cabinet to dry at about 60°C.

[0034] Step 2, cleaning the graphite boat:

[0035] Repeatedly polish the inner wall of the graphite boat with 3000-mesh sandpaper until the surface is smooth and there are no pits, then wipe it clean with methanol, place it in a container, add 3000 mL of a mixed solution prepared by mixing an electronic-grade HCl solution with a mass fraction of 7.5% and an electronic-grade HNO3 solution with a mass fraction of 2.5% in a volume ratio of 1:1, heat to 50°C and soak for 5 h. After taking it out, place it in an ultrasonic container filled with deionized water, heat to 100°C, and ultrasonically clean 3 times, 15 min each time. Then rinse with deionized water until there are no chloride ions, check with a 1% silver nitrate solution to ensure there are no chloride ions, then rinse with methanol, volatilize and dehydrate, and place it in a clean infrared lamp cabinet to dry at about 60°C for later use.

[0036] Step 3, cleaning the zone melting germanium ingot:

[0037] Cut the zone melting germanium ingot into blocks with a length of 100 mm with a cutting machine, mix the pre-prepared electronic grade H2O2 solution with a mass fraction of 2.5% and the electronic grade HF solution with a mass fraction of 5% in a volume ratio of 1:2, take out 5000 mL and put it into a 10L quartz glass container, put in the germanium ingot and heat it to 50℃ for reaction for 60 minutes, take it out and rinse the surface of the germanium ingot with deionized water until there is no fluoride ion, then wash it with methanol for volatilization and dehydration, put it in a clean infrared lamp cabinet for drying and standby use.

[0038] Step 4, loading into the furnace:

[0039] First, load the cleaned high-purity quartz tube into the temperature-controlled heating furnace body, and fill the gaps between the quartz tube and the head and tail of the temperature-controlled heating furnace body with insulation cotton; adjust the furnace head in the heating zone horizontally to maintain a 2-degree inclination; then load 15kg of dried germanium ingots into a high-purity graphite boat with an internal length, width, and height of 1000mm*60mm*50mm, and then load it into a quartz tube with an inner diameter of 80mm and a length of 2000mm, then seal the sealing quartz tube flange, connect the air inlet and the air outlet with an ampoule tail gas bottle (filled with water). Finally, insert the temperature measuring thermocouple and the temperature controlling thermocouple in the independent 4-section controllable temperature zone of the temperature-controlled heating furnace body, and connect the computer and the thermocouple port.

[0040] Step 5, high temperature melting and directional crystallization:

[0041] After loading the materials, evacuate the furnace to below 0.1 Pa, and then introduce 99.999% pure hydrogen for 0.5h. After the tube is completely filled with hydrogen, adjust the flow rate to 2.5L / min, so that the ampoule tail gas bottle containing water continues to bubble. After passing hydrogen for 30min according to step 4, turn on the power to start the heating program, and heat from room temperature to 950℃ in 2h and keep warm for 2h. After the germanium ingot is completely melted, slowly heat to 1300℃ in 1.5h to keep the germanium melt molten at high temperature for 2h; start the segmented cooling program, The temperature is lowered first in the direction of the air outlet and last in the direction of the air inlet. When the temperature in the direction of the air inlet drops to 935°C, it is slowly lowered to 300°C. Then the temperature control program is stopped and the power is turned off. After naturally cooling to room temperature, the hydrogen is stopped and the hydrogen in the quartz tube is replaced with high-purity nitrogen. Then, the flange of the quartz tube is opened, the graphite boat is taken out, and then the germanium ingot is taken out. A cutting machine is used to cut off 10mm of the head length and 30mm of the tail length of the germanium ingot as unqualified products. The qualified part in the middle is then cut into products with a length of 50mm-150mm for use in growing single crystals.

[0042] Step 6, cleaning the germanium ingot:

[0043] Take a 1-mm-thick test sample from the germanium ingot cut in Step 5, etch and clean it with a mixed solution of 5% hydrofluoric acid solution and 2.5% nitric acid solution in a volume ratio of 2:1, measure its impurity content by GD-MS, calculate the purity to be 99.99999999% (10N), measure its carrier concentration by the Hall measurement method, etch and clean the surface of the qualified germanium ingot produced to remove the impurities in the oxide layer, then rinse it with deionized water, dry it, and load it into a single crystal furnace for solar cells for crystal growth.

[0044] The data statistics of defects such as pinholes, holes, and inclusions after growing germanium single crystals by this method are shown in Table 3.

[0045] Table 3

[0046] Carrier concentration Proportion of pinholes, micropores, and inclusions 5.15E13(10N) Approximately 5% (CZ); approximately 2% (VGF)

[0047] Comparative Example 1: A germanium purification method. After the germanium ingot is completely melted, the constant temperature in the furnace is maintained at 937 - 1000 °C, and other steps are the same as those in Example 1. The data statistics of the carrier concentration of the germanium single crystal prepared and the defects such as pinholes, voids, and inclusions after crystal growth are shown in Table 4.

[0048] Table 4

[0049] Carrier concentration Proportion of pinholes, micropores, and inclusions 5.51E17(6N) Approximately 60% (CZ); approximately 10% (VGF)

[0050] Comparative Example 2: A germanium purification method. After the germanium ingot is completely melted, the constant temperature in the furnace is maintained at 1000 - 1100 °C, and other steps are the same as those in Example 1. The data statistics of the carrier concentration of the germanium single crystal prepared and the defects such as pinholes, voids, and inclusions after crystal growth are shown in Table 5.

[0051] Table 5

[0052] Carrier concentration Proportion of pinholes, micropores, and inclusions 2.27E16(7N) Approximately 30% (CZ); approximately 10% (VGF)

[0053] Comparative Example 3: A germanium purification method. After the germanium ingot is completely melted, the constant temperature in the furnace is maintained at 1300 - 1400 °C, and other steps are the same as those in Example 1. The data statistics of the carrier concentration of the germanium single crystal prepared and the data such as pinholes, voids, and inclusions after crystal growth are shown in Table 6.

[0054] Table 6

[0055] Carrier concentration Proportion of pinholes, micropores, and inclusions 4.17E14(9N) Approximately 30% (CZ); approximately 8% (VGF)

[0056] The germanium ingot produced in Example 1 has the highest purity and the smallest proportion of defects after being applied to germanium single crystal growth. Although there is a certain increase in purity and a certain reduction in the proportion of defects in Example 4. However, there is data indicating that after the quartz material exceeds 1300 °C for a long time, the impurities contained in the quartz material will volatilize with the increase in temperature.

[0057] Example Carrier concentration Purity Proportion of pinholes, micropores, and inclusions 1 5.15E13 10N Approximately 5% (CZ); approximately 2% (VGF) 2 5.51E17 6N Approximately 60% (CZ); approximately 10% (VGF) 3 2.27E16 7N Approximately 30% (CZ); approximately 10% (VGF) 4 4.17E14 9N Approximately 30% (CZ); approximately 8% (VGF)

Claims

1. A method for improving the purity of germanium metal to reduce the defects of germanium single crystals used in solar cells. The method includes steps of cleaning quartz tubes, cleaning graphite boats, cleaning zone-melted germanium ingots, loading materials into the furnace, high-temperature melting and directional crystallization, and cleaning germanium ingots, and is characterized in that High-temperature melting and directional crystallization involve remelting the zone-melted germanium ingot at a high temperature of 1300°C and then horizontally directionally cooling and crystallizing it, so that trace impurities in the germanium melt are enriched at the tail of the germanium ingot. At the same time, the germanium melt is protected from oxidation by oxygen in a hydrogen atmosphere with a purity above 99.999%, and trace volatilized impurities are carried away by the hydrogen gas flow.

2. A method for improving the purity of germanium metal to reduce the defects of germanium single crystals used in solar cells as described in claim 1, characterized in that: For the cleaning of the quartz tube, the high-purity quartz tube is soaked in an electronic-grade hydrofluoric acid (HF) solution with a mass fraction of 2.5% - 7.5% for 2h - 3h. After that, the inner and outer walls of the quartz tube are cleaned with a 1% HF solution without contamination, then repeatedly rinsed with deionized water until there are no fluoride ions, and then cleaned with methanol, volatilized to remove water, and dried. For the cleaning of the graphite boat, first, the inner wall of the graphite boat is polished with sandpaper to make its surface smooth without pits. Then, it is wiped clean with methanol without stains and placed in a container. It is soaked in a mixed solution prepared by mixing an electronic-grade HCl solution with a mass fraction of 7.5% and an electronic-grade HNO3 solution with a mass fraction of 2.5% in a volume ratio of 1:1, heated to 40°C - 50°C, and soaked for 5h - 8h. After taking it out, it is placed in an ultrasonic container filled with deionized water for heating and cleaning, and then rinsed with deionized water until there are no chloride ions, and then rinsed with methanol, volatilized to remove water, and dried. For the cleaning of the zone-melted germanium ingot, the zone-melted germanium ingot is oxidized with a mixed solution of an electronic-grade H2O2 solution with a mass fraction of 2.5% and an electronic-grade HF solution with a mass fraction of 5% in a volume ratio of 1:2 to remove the trace oxide layer and impurities on the surface of the germanium ingot. Then, it is rinsed clean with deionized water until there are no fluoride ions, and then cleaned with methanol, volatilized to remove water, and dried. For loading the materials into the furnace, the high-purity quartz tube is loaded into a horizontally tubular temperature-controlled heating furnace body, and the gaps at the head and tail between the quartz tube and the temperature-controlled heating furnace body are tightly filled with heat-insulating cotton. The furnace head is adjusted horizontally to maintain an inclination of 2 degrees. The dried germanium ingot is loaded into a high-purity graphite boat, and then the graphite boat is loaded into the quartz tube. Then, the quartz tube is sealed well, and the inlet and the outlet connected to an ampoule tail gas bottle are connected. After evacuating the furnace to 0.1 Pa, hydrogen with a purity of 99.999% is introduced for 0.5h - 1h to displace the trace air in the tube. After the tube is completely filled with hydrogen, the hydrogen flow rate is adjusted to be set at 2.5 L / min - 5.0 L / min. Temperature-measuring and temperature-controlling thermocouples are respectively installed on the temperature-controlled heating furnace body. The specific process of high-temperature melting and directional crystallization is as follows: After loading the materials into the furnace and introducing hydrogen, the temperature is raised to 950°C and held for 2h. After the germanium ingot is completely melted, it is slowly heated to 1300°C in 1h - 2h, and the germanium melt is kept molten at a high temperature for 2h and then cooled. The temperature at the outlet is cooled first. When the temperature at the inlet drops to 935°C, it solidifies into germanium polycrystals and then is slowly cooled to 300°C. The heating device is turned off, and it is naturally cooled to room temperature. Then, the hydrogen supply is stopped, and the hydrogen in the quartz tube is replaced with high-purity nitrogen. After taking out the graphite boat, the germanium ingot is taken out, the head and tail of the germanium ingot are cut off, and the qualified part in the middle of the germanium ingot is reserved for growing single crystals. The germanium ingot is cleaned with a mixed solution of 5% hydrofluoric acid solution and 2.5% nitric acid solution in a volume ratio of 2:1 to corrode and clean the surface of the qualified germanium ingot, so as to remove the impurities in the oxide layer. After rinsing with deionized water, drying, it is loaded into a single crystal furnace for solar cells for crystal growth.

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