A gallium arsenide wafer doped with cadmium element and preparation method thereof

Abstract

The present invention relates to the field of semiconductor technology, and specifically discloses a gallium arsenide wafer doped with cadmium elements and a preparation method thereof, wherein the preparation method comprises the following steps: cleaning the gallium arsenide wafer, a mounting frame and a diffusion container; mounting the gallium arsenide wafer on the mounting frame, respectively loading arsenic and cadmium into two loading slots at the top of the diffusion container, and placing the mounting frame in the diffusion container; loading the diffusion container into a vacuum diffusion furnace for baking after vacuuming; regulating the temperature in the vacuum diffusion furnace to diffuse and dope the cadmium element into the gallium arsenide wafer; after the doping is completed, cooling the diffusion container; taking out the gallium arsenide wafer for cleaning; and thinning the gallium arsenide wafer to a target thickness. The present invention diffuses the cadmium element into the gallium arsenide wafer in a vacuum diffusion furnace, and the cadmium element occupies the gallium position, provides holes, and converts the gallium arsenide into a P-type semiconductor, thereby effectively improving the carrier concentration of the wafer, and the head-to-tail deviation of the wafer electrical performance is small, and the situation of excessive head-to-tail electrical performance deviation caused by the segregation phenomenon is effectively avoided.

Description

Technical Field

[0001] The invention belongs to the technical field of semiconductors, and in particular relates to a gallium arsenide wafer doped with cadmium elements and a preparation method thereof. Background Art

[0002] Epitaxial customers use gallium arsenide wafers, usually using the surface layer of the wafer for epitaxial growth, so they have high requirements for the electrical performance parameters of the surface layer of the gallium arsenide wafer.

[0003] In the prior art, most of the gallium arsenide wafers doped with cadmium are doped with cadmium when synthesizing gallium arsenide polycrystals or gallium arsenide single crystals. During the doping, gallium arsenide is in a liquid state, and then cooled from the liquid state to a solid state to grow crystals. Finally, the synthesized cadmium-doped gallium arsenide single crystal is sliced ​​to obtain a cadmium-doped gallium arsenide single crystal. However, when the gallium arsenide crystal grows, there is a phenomenon of solid-liquid segregation, which causes the cadmium element to be unevenly distributed in the crystal, and the head-tail parameter difference can reach 4 times. After the crystal is cut into wafers, the parameter difference within the wafer can reach 1.5 times. The large head-tail parameter difference makes the electrical performance parameters of the wafer surface layer unable to meet the use requirements. At the same time, only the surface of the gallium arsenide wafer is actually used in the epitaxial growth process, and the cadmium element in the cadmium-doped gallium arsenide single crystal obtained by the crystal growth method used in the prior art is distributed in the entire wafer. Therefore, in order to make the wafer surface layer meet the use requirements, it is necessary to increase the overall cadmium element usage, which increases the production cost. Summary of the invention

[0004] In order to overcome the shortcomings of the prior art and solve the problems existing in the prior art, the present invention provides a gallium arsenide wafer doped with cadmium elements and a preparation method thereof, which can allow the cadmium element on the surface of gallium arsenide to replace the gallium element to form a layered structure surface of cadmium-doped gallium arsenide, and the head-to-tail parameter difference in the layered structure surface is small, the parameter consistency is good, and the epitaxial growth conditions are met.

[0005] The present invention is achieved through the following technical solutions.

[0006] The present invention provides a method for preparing a gallium arsenide wafer doped with a cadmium element, comprising the following steps:

[0007] Step 1: Pre-treatment, cleaning the gallium arsenide wafer, the mounting frame for mounting the gallium arsenide wafer, and the diffusion container for placing the mounting frame and sealing the mounting frame;

[0008] Step 2: Loading: Place the cleaned gallium arsenide wafer on the mounting rack, and respectively load arsenic and cadmium into two loading slots provided at the top of the diffusion container, and place the mounting rack in the diffusion container so that the loading slots are higher than the mounting rack;

[0009] Step 3: Doping: placing the sealed diffusion container into a vacuum diffusion furnace, adjusting the temperature in the vacuum diffusion furnace, and allowing the elemental cadmium to diffuse and dope into the gallium arsenide wafer;

[0010] Step 4: Cooling: After the doping is completed, the diffusion container is cooled down. When the temperature in the vacuum diffusion furnace drops below 100°C, the diffusion container is taken out for cooling;

[0011] Step 5: Post-processing, taking out the cadmium-doped gallium arsenide wafer from the diffusion container and cleaning it;

[0012] Step 6: Processing, thinning the cadmium-doped gallium arsenide wafer to the target thickness.

[0013] As a further improvement of the above scheme, the mounting frame and the diffusion container are both made of quartz material, and the mounting frame is provided with several layers of mounting grooves for mounting gallium arsenide chips; the diffusion container includes a quartz diffusion tube and a quartz cap arranged at the lower port of the quartz diffusion tube, and the inner top of the quartz diffusion tube is provided with two loading troughs, one of which is used to hold elemental arsenic, and the other is used to hold elemental cadmium.

[0014] As a further improvement of the above scheme, the gallium arsenide wafer in step 1 is cleaned with hydrofluoric acid, hydrogen peroxide and deionized water, the mounting frame, quartz diffusion tube and quartz cap are cleaned with aqua regia and deionized water, dried and set aside, and the gallium arsenide wafer doped with cadmium element in step 5 is rinsed with aqua regia and deionized water.

[0015] As a further improvement of the above scheme, in step 2, the purity of arsenic and cadmium is 7N, and the molar ratio is 1:1.

[0016] As a further improvement of the above scheme, in step 3, after the loading is completed, the quartz diffusion tube is evacuated, and after evacuation, the diffusion container is placed in a vacuum diffusion furnace and baked for 2 to 4 hours. The vacuum range of the evacuation is 1×10 -4 Pa~9×10 -2 Pa; after baking, take out the quartz diffusion tube and the quartz cap and weld them together.

[0017] As a further improvement of the above solution, the diameter of the quartz diffusion tube is 100-200 mm, the height is 80-120 cm, and the amount of cadmium added is 0.2 g-15 g.

[0018] As a further improvement of the above scheme, the temperature regulation in the vacuum diffusion furnace in step 3 includes the following steps: first, heating the entire quartz diffusion tube to 620-660°C, then heating the area where arsenic and cadmium are placed to 765-800°C, and finally heating the mounting frame area to 1040-1080°C, keeping warm for 15-20 hours, and diffusing the cadmium element.

[0019] As a further improvement of the above solution, the cooling rate in step 4 is 20-50° C. / hour.

[0020] As a further improvement of the above solution, the diameter of the gallium arsenide wafer in step 1 is any one of 51 mm, 103 mm or 160 mm.

[0021] The present invention also provides a gallium arsenide wafer doped with cadmium element prepared according to the preparation method.

[0022] The beneficial effects achieved by the present invention are:

[0023] Compared with the prior art, the present invention provides a gallium arsenide wafer doped with cadmium and a preparation method thereof. The metal cadmium element is diffused to the surface of the gallium arsenide wafer in a vacuum diffusion furnace. The cadmium element occupies the gallium position and provides holes, so that the gallium arsenide is converted into a P-type semiconductor, and the carrier concentration of the wafer is effectively improved. Since the gallium arsenide wafer is diffused simultaneously under the same environment, the electrical properties of the wafer surface after diffusion have a small deviation between the head and the tail, which can effectively avoid the situation where the electrical properties of the head and the tail are too large due to the segregation phenomenon. At the same time, the electrical properties of the wafer surface after diffusion can be effectively controlled by controlling the diffusion time, the diffusion temperature and the vapor pressure of the doping element, so as to meet the needs of epitaxial customers. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the structure of the mounting frame and the diffusion container in the present invention when in use;

[0025] Figure 2 It is a structural schematic diagram of the mounting frame in the present invention;

[0026] Figure 3 It is a structural schematic diagram of the diffusion container in the present invention.

[0027] In the figure: gallium arsenide wafer 1, mounting frame 2, mounting groove 201, diffusion container 3, charging trough 301, quartz diffusion tube 302, quartz cap 303, vacuum diffusion furnace 4. DETAILED DESCRIPTION

[0028] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. Example 1

[0029] The invention provides a gallium arsenide wafer doped with a cadmium element, which is prepared by the following method.

[0030] The present invention provides a method for preparing a gallium arsenide wafer doped with a cadmium element, comprising the following steps:

[0031] Step 1: Pretreatment: clean the gallium arsenide wafer 1 with hydrofluoric acid, hydrogen peroxide and deionized water to remove stains and impurities on its surface, and clean the mounting frame 2 for mounting the gallium arsenide wafer 1 and the diffusion container 3 for placing the mounting frame 2 and sealing the mounting frame 2 with aqua regia and deionized water. After cleaning, dry them for use.

[0032] Further, such as Figures 1 to 3 As shown, the mounting frame 2 and the diffusion container 3 are both made of quartz material, and the mounting frame 2 is provided with several layers of mounting grooves 201 for mounting the gallium arsenide wafer 1; the diffusion container 3 includes a quartz diffusion tube 302 and a quartz cap 303 arranged at the lower end of the quartz diffusion tube 302, and the inner top of the quartz diffusion tube 302 is provided with two loading grooves 301, one of which is used to hold elemental arsenic, and the other is used to hold elemental cadmium.

[0033] Step 2: Loading. Install the cleaned gallium arsenide wafer 1 on the mounting rack 2. Load elemental arsenic and elemental cadmium into two loading troughs 301 respectively provided at the top of the diffusion container 3. The molar ratio of elemental arsenic to elemental cadmium is 1:1, and the purity is 7N. Place the mounting rack 2 in the diffusion container 3, and make the loading troughs 301 higher than the mounting rack 2.

[0034] Specifically, the mounting frame 2 is placed on the quartz cap 303 , and then the mounting frame 2 is covered with the quartz diffusion tube 302 .

[0035] Step 3: Doping, placing the sealed diffusion container 3 into the vacuum diffusion furnace 4, adjusting the temperature in the vacuum diffusion furnace 4, and allowing the single element cadmium to diffuse and dope into the gallium arsenide wafer 1. Arsenic turns into arsenic gas at high temperature, increasing the arsenic pressure in the diffusion container 3 to prevent the gallium arsenide crystal from decomposing.

[0036] Specifically, before doping, the quartz diffusion tube 302 and the quartz cap 303 are first connected by welding to achieve the purpose of sealing the diffusion container 3 as a whole.

[0037] Specifically, during welding, the quartz diffusion tube 302 is wrapped with wet insulation cotton, gloves are put on, hydrogen is turned on for ignition, oxygen is turned on for flame adjustment, and low flame is preheated for more than 1 minute to completely weld the quartz diffusion tube 302 body and the quartz cap 303 to achieve sealing.

[0038] Furthermore, before welding, the diffusion container 3 is baked as a whole for 2 to 4 hours at a temperature of 200 to 400°C. After baking, the power of the vacuum diffusion furnace 4 is turned off, the temperature is lowered, the furnace cover is opened, the quartz diffusion tube 302 and the quartz cap 303 are taken out, and then the diffusion container 3 is evacuated to a vacuum degree of 1×10 -4 Pa~9×10 -2 Pa, while evacuating, the quartz diffusion tube 302 and the quartz cap 303 are welded, and after the welding is completed, the diffusion container 3 is placed in the vacuum diffusion furnace 4 to perform step 3.

[0039] Specifically, the quartz cap 303 is cylindrical, and there is a 1-2 mm gap between the outer wall of the quartz cap 303 and the inner wall of the quartz diffusion tube 302. In this embodiment, a vacuum pump is used to evacuate the space inside the diffusion container 3 through the gap between the inner wall of the quartz diffusion tube 302 and the quartz cap 303; the wall of the quartz diffusion tube 302 is concave and welded to the quartz cap 303 by high temperature, and the vacuuming is stopped after the welding is completed.

[0040] The purpose of the above-mentioned baking and vacuuming is to deal with the residual moisture inside the quartz diffusion tube 302 to avoid excess moisture remaining in the quartz diffusion tube 302 after welding. If there is too much moisture in the quartz diffusion tube 302 after welding, it will cause the quartz diffusion tube 302 to be directly broken after the moisture turns into water vapor as the temperature of the quartz diffusion tube 302 increases during the doping step, causing an accident.

[0041] Specifically, the temperature control in the vacuum diffusion furnace 4 in the doping step includes the following steps: first, the diffusion container 3 is heated as a whole to 620-660°C, then the area where arsenic and cadmium are located is heated to 765°C, and finally the mounting frame 2 area is heated to 1040°C, and kept warm for 15 hours to diffuse the cadmium element.

[0042] Furthermore, the vacuum diffusion furnace 4 is heated by a built-in resistance wire, and at least two temperature control zones that can independently adjust the temperature are provided in the vacuum diffusion furnace 4, one of which is used to control the temperature of the area where arsenic and cadmium are located, and the other is used to control the temperature of the area where the mounting frame 2 is located.

[0043] Step 4: Cooling. After the diffusion doping is completed, the diffusion container 3 is cooled at a rate of 20-50°C / hour. When the temperature in the vacuum diffusion furnace 4 drops below 100°C, the diffusion container 3 is taken out and naturally cooled to room temperature.

[0044] Furthermore, the quartz diffusion tube 302 is broken, the gallium arsenide wafer 1 doped with the cadmium element is taken out, and the mounting frame 2 can be recycled and reused.

[0045] Step 5: Post-processing: After taking out the cadmium-doped gallium arsenide wafer 1 from the diffusion container 3, rinse it with aqua regia and deionized water to remove surface contamination.

[0046] Step 6: Wafer processing: thinning the cadmium-doped gallium arsenide wafer 1 to a target thickness through equipment.

[0047] Furthermore, the diameter of the quartz diffusion tube 302 is 100-200 mm, the height is 80-120 cm, and the amount of cadmium added is 0.2 g-15 g.

[0048] Specifically, the amount of cadmium added is calculated based on the internal space of the diffusion container 3, mainly considering the ideal gas equation, PV=nRT. The installation quantity will affect the volume inside the tube, so the volume of the installed wafer needs to be subtracted.

[0049] Among them, P is the pressure inside the quartz tube; V is the volume inside the quartz tube (the volume after subtracting the installed chip); n is the amount of cadmium substance; R‌ is the molar gas constant; T‌ is the temperature.

[0050] Furthermore, in the present embodiment, the gallium arsenide wafer 1 raw material in step 1 is selected to have a diameter of 51 mm, 103 mm or 160 mm, and other sizes may also be selected according to production requirements. Example 2

[0051] The difference between Example 2 and Example 1 is that in step 2, the molar ratio of elemental arsenic to elemental cadmium is 1:0.9, and the rest is the same as Example 1. Example 3

[0052] The difference between this embodiment 3 and embodiment 1 is that in step 3, the area where arsenic and cadmium are located is heated to 800° C., and the rest is the same as embodiment 1. Example 4

[0053] The difference between the present embodiment 4 and the embodiment 1 is that in step 3, the area where the mounting frame 2 is located is heated to 1080° C., and the rest is the same as the embodiment 1. Example 5

[0054] The difference between Example 5 and Example 1 is that in step 3, the whole is kept warm for 20 hours, and the rest is the same as Example 1. Example 6

[0055] The difference between this embodiment 6 and embodiment 1 is that in step 3, the area where arsenic and cadmium are located is heated to 800° C., and the area where the mounting frame 2 is located is heated to 1080° C., and the rest is the same as embodiment 1. Example 7

[0056] The difference between Example 7 and Example 1 is that in step 3, the area where arsenic and cadmium are located is heated to 800° C. and the whole area is kept warm for 20 hours. The rest is the same as Example 1. Example 8

[0057] The difference between this embodiment 8 and embodiment 1 is that in step 3, the area where the mounting frame 2 is located is heated to 1080° C. and the whole is kept warm for 20 hours. The rest is the same as embodiment 1. Example 9

[0058] The difference between this embodiment 9 and embodiment 1 is that:

[0059] In step 3, the area where arsenic and cadmium are located is heated to 800° C., the area where the mounting frame 2 is located is heated to 1080° C., and the whole is kept warm for 20 hours. The rest is the same as in embodiment 1.

[0060] Comparative Example 1

[0061] The difference between this comparative example 1 and embodiment 1 is:

[0062] In step 2, the molar ratio of elemental arsenic to elemental cadmium is 1:1.1, and the rest is the same as in Example 1.

[0063] Comparative Example 2

[0064] The performance of cadmium-doped wafers on the market was tested.

[0065] Comparative Example 3

[0066] The performance of silicon-doped wafers on the market was tested.

[0067] Performance test: Take the wafers prepared in Examples 1-9 and Comparative Examples 1-3, and test the carrier concentration of the wafers at different parts of the crystal according to the national standard GB / T4326-2006 "Measurement method for Hall mobility and Hall coefficient of extrinsic semiconductor single crystals", where the head is 10% of the wafer number, the middle is 45-55% of the wafer number, and the tail is 95-100% of the wafer number. The test results are shown in Table 1:

[0068] Table 1 Carrier concentration of wafers in the embodiments and comparative examples

[0069] serial number <![CDATA[Head carrier concentration ( / cm 3 )]]> <![CDATA[Central carrier concentration ( / cm 3 ).]]> <![CDATA[Tail carrier concentration ( / cm 3 )]]> Example 1 <![CDATA[1.72×10 19 ]]> <![CDATA[1.79×10 19 ]]> <![CDATA[1.87×10 19 ]]> Example 2 <![CDATA[1.12×10 19 ]]> <![CDATA[1.03×10 19 ]]> <![CDATA[0.92×10 19 ]]> Example 3 <![CDATA[2.89×10 19 ]]> <![CDATA[2.99×10 19 ]]> <![CDATA[3.03×10 19 ]]> Example 4 <![CDATA[1.85×10 19 ]]> <![CDATA[1.92×10 19 ]]> <![CDATA[1.97×10 19 ]]> Example 5 <![CDATA[1.58×10 19 ]]> <![CDATA[1.51×10 19 ]]> <![CDATA[1.42×10 19 ]]> Example 6 <![CDATA[5.88×10 19 ]]> <![CDATA[5.92×10 19 ]]> <![CDATA[6.0×10 19 ]]> Example 7 <![CDATA[2.42×10 19 ]]> <![CDATA[2.35×10 19 ]]> <![CDATA[2.29×10 19 ]]> Example 8 <![CDATA[2.12×10 19 ]]> <![CDATA[2.20×10 19 ]]> <![CDATA[2.25×10 19 ]]> Example 9 <![CDATA[6.01×10 19 ]]> <![CDATA[6.03×10 19 ]]> <![CDATA[6.07×10 19 ]]> Comparative Example 1 <![CDATA[1.42×10 19 ]]> <![CDATA[1.39×10 19 ]]> <![CDATA[1.21×10 19 ]]> Comparative Example 2 <![CDATA[0.75×10 19 ]]> <![CDATA[1.56×10 19 ]]> <![CDATA[3.21×10 19 ]]> Comparative Example 3 <![CDATA[0.61×10 18 ]]> <![CDATA[1.48×10 18 ]]> <![CDATA[3.56×10 18 ]]>

[0070] By analyzing Examples 1 and 2 and combining them with the above table, it can be seen that increasing the ratio of the added arsenic element to the cadmium element will cause the gas partial pressure of the cadmium element in space to be too small, affecting the effective diffusion of the cadmium element and being disadvantageous to increasing the carrier concentration.

[0071] By analyzing Examples 1, 3, 4, 6, 5, 7, 8, 9 and combining with the above table, it can be seen that increasing the temperature of the cadmium region is conducive to the volatilization of the cadmium element, plays a positive role in the diffusion of the cadmium element, and effectively increases the carrier concentration of the chip.

[0072] By analyzing Examples 1, 4, 3, 6, 5, 8, 7, 9 and combining them with the above table, it can be seen that increasing the temperature in the mounting frame 2 area is beneficial to increasing the surface activity of the chip, allowing the cadmium element to fully occupy the position of the gallium element, playing a positive role in the diffusion of the cadmium element, and effectively increasing the carrier concentration of the chip.

[0073] By analyzing Examples 1, 5, 3, 7, 4, 8, 6, 9 and combining with the above table, it can be seen that increasing the constant temperature time can have an impact on the carrier concentration of the wafer. When the cadmium element is insufficient, more cadmium elements diffuse into the wafer at the beginning, but as time goes by, the cadmium elements in the external environment are consumed, and the cadmium elements that have entered the wafer will be freed again, resulting in a decrease in the carrier concentration. When the cadmium element is sufficient, increasing the constant temperature time is conducive to the diffusion of the cadmium element and increases the carrier concentration of the wafer.

[0074] By analyzing Example 1 and Comparative Example 1, it can be seen that increasing the spatial cadmium content should be conducive to the diffusion of cadmium elements. However, the actual test data is the opposite. Further analysis shows that small spots appear on the surface of the wafer of Comparative Example 1, and element analysis shows that cadmium elements gather in pits. The results show that the excess cadmium element does not occupy the position of the gallium element, but exists in the interstitial position, which is not conducive to the conductivity of the wafer. It is also consistent with the test data of Example 1 and Comparative Example 1 in Table 1.

[0075] By analyzing Examples 1 to 9 and Comparative Example 2, it can be seen that the gallium arsenide wafer 1 made by cadmium doping using the diffusion method has a more uniform head-to-tail carrier concentration, which is reduced from the original head-to-tail ratio of 1:4 to 1:1.01.

[0076] By analyzing Examples 1 to 9 and Comparative Example 3, it can be seen that the gallium arsenide wafer 1 made by cadmium doping using the diffusion method can effectively increase the crystal carrier concentration.

[0077] The above describes in detail the implementation modes of the present invention in conjunction with the accompanying drawings, but the present invention is not limited to the above implementation modes. The implementation modes can still be changed within the knowledge scope of ordinary technicians in the field. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a gallium arsenide wafer doped with cadmium, characterized in that: The following steps are involved: Step 1: pre-treatment, cleaning the gallium arsenide wafer (1), a mounting frame (2) for mounting the gallium arsenide wafer (1), and a diffusion container (3) for placing the mounting frame (2) and sealing the mounting frame (2); Step 2: loading, mounting the cleaned gallium arsenide wafer (1) on the mounting frame (2), respectively loading arsenic and cadmium into two loading slots (301) provided at the top of the diffusion container (3), placing the mounting frame (2) in the diffusion container (3), and making the loading slots (301) higher than the mounting frame (2); Step 3: doping, placing the sealed diffusion container (3) into a vacuum diffusion furnace (4), and adjusting the temperature in the vacuum diffusion furnace (4) to diffuse and dope the elemental cadmium into the gallium arsenide wafer (1); Step 4: Cooling, after the doping is completed, the diffusion container (3) is cooled down, and when the temperature in the vacuum diffusion furnace (4) drops to below 100° C., the diffusion container (3) is taken out for cooling; Step 5: post-processing, taking out the cadmium-doped gallium arsenide wafer (1) from the diffusion container (3) and cleaning it; Step 6: Processing, thinning the cadmium-doped gallium arsenide wafer (1) to a target thickness.

2. The method for preparing a gallium arsenide wafer doped with cadmium according to claim 1, characterized in that: The mounting frame (2) and the diffusion container (3) are both made of quartz material, and the mounting frame (2) is provided with a plurality of layers of mounting grooves (201) for mounting gallium arsenide wafers (1); the diffusion container (3) comprises a quartz diffusion tube (302) and a quartz cap (303) arranged at the lower end of the quartz diffusion tube (302), and the inner top of the quartz diffusion tube (302) is provided with two loading grooves (301), wherein one loading groove (301) is used for holding elemental arsenic, and the other loading groove (301) is used for holding elemental cadmium.

3. The method for preparing a gallium arsenide wafer doped with cadmium according to claim 2, characterized in that: The gallium arsenide wafer (1) in step 1 is cleaned with hydrofluoric acid, hydrogen peroxide and deionized water, the mounting frame (2), quartz diffusion tube (302) and quartz cap (303) are cleaned with aqua regia and deionized water, dried and set aside, and the gallium arsenide wafer (1) doped with cadmium in step 5 is rinsed with aqua regia and deionized water.

4. The method for preparing a gallium arsenide wafer doped with cadmium according to claim 1, characterized in that: In the step 2, the purity of arsenic and cadmium is 7N, and the molar ratio is 1:

1.

5. The method for preparing a gallium arsenide wafer doped with cadmium according to claim 2, characterized in that: In step 3, after the loading is completed, the quartz diffusion tube (302) is evacuated, and after evacuation, the diffusion container (3) is placed in a vacuum diffusion furnace (4) and baked for 2 to 4 hours. The vacuum degree of the evacuation is in the range of 1×10 -4 Pa~9×10 -2 Pa; after baking, the quartz diffusion tube (302) and the quartz cap (303) are taken out and welded together.

6. The method for preparing a gallium arsenide wafer doped with cadmium according to claim 2, characterized in that: The quartz diffusion tube (302) has a diameter of 100-200 mm and a height of 80-120 cm, and the amount of cadmium added is 0.2 g-15 g.

7. The method for preparing a gallium arsenide wafer doped with cadmium according to claim 6, characterized in that: The temperature control in the vacuum diffusion furnace (4) in step 3 comprises the following steps: firstly heating the entire quartz diffusion tube (302) to 620-660°C, then heating the area where arsenic and cadmium are placed to 765-800°C, and finally heating the area of ​​the mounting frame (2) to 1040-1080°C, and keeping the temperature for 15-20 hours to diffuse the cadmium element.

8. The method for preparing a gallium arsenide wafer doped with cadmium according to claim 6, characterized in that: The cooling rate in step 4 is 20-50°C / hour.

9. The method for preparing a gallium arsenide wafer doped with cadmium according to claim 1, characterized in that: The diameter of the gallium arsenide wafer (1) in step 1 is any one of 51 mm, 103 mm or 160 mm.

10. A gallium arsenide wafer (1) doped with cadmium element prepared according to the preparation method according to any one of claims 1 to 9.

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