Rapid growth method for improving energy resolution of CdZnTe epitaxial single crystal

Abstract

The invention provides a rapid growth method for improving the energy resolution of a CdZnTe epitaxial single crystal, and solves the problems of growth process increase, high cost, limited size and limited energy resolution improvement in the existing method for improving the energy resolution of the CdZnTe epitaxial single crystal. The Te powder is uniformly placed around the growth source, the growth temperature is adjusted to control the sublimation rate of the Te powder (namely, the temperature of the upper heating table and the temperature of the lower heating table), the growth of the CdZnTe epitaxial single crystal in the Te atmosphere is realized under the cooperation of the Te powder and the Te powder, and the free carrier concentration and dark current in the epitaxial single crystal can be remarkably reduced, so that the signal-to-noise ratio is improved, and the growth efficiency is improved. And a relatively good energy resolution is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of crystal growth, and particularly relates to a rapid growth method for improving the energy resolution of CdZnTe epitaxial single crystals. Background Art

[0002] With the development of technology, X-rays have been widely used in various fields of radiation detection. Cd 1-x Zn x Te (abbreviated as CdZnTe or CZT), as a II-VI group compound semiconductor with excellent optoelectronic properties, has received extensive attention in the field of X-ray radiation detection due to its relatively high average atomic number, large band gap width, and high product of intrinsic carrier mobility and lifetime.

[0003] The traditional methods for growing CdZnTe crystals are the melt method or the solution method. However, the CdZnTe crystals obtained by such methods often have a slow growth rate and high cost. At the same time, CdZnTe has low thermal conductivity, small critical shear stress, and Zn component segregation, resulting in easy presence of grain boundaries, inclusion phases, and various cation defects inside the crystal, introducing defect energy levels in the energy band, and leading to a low yield of large-size single crystals. Therefore, it cannot be batch-applied to large-area X-ray detection devices. Carriers will be captured, recombined, and scattered to varying degrees at defects, seriously deteriorating the transport characteristics of carriers and reducing the detection performance of CdZnTe detectors.

[0004] Energy resolution is a key indicator for measuring the application performance of X-ray detectors. High energy resolution means that the detector can distinguish different energy rays in detail, and is expected to improve the accuracy of radiation detection. To solve the above problems, researchers generally adopt methods such as preparing a blocking layer or subsequent heat treatment (i.e., improving the energy resolution by heat treating (Te atmosphere annealing) the CdZnTe epitaxial single crystal) to reduce the defect concentration in the epitaxial single crystal and optimize the transport characteristics of carriers, so as to achieve the purpose of improving the energy resolution of the epitaxial single crystal. However, the above methods all require secondary treatment of the already grown epitaxial single crystal, which not only increases additional growth processes, but also may introduce new influencing factors such as interface states or oxide layers, the regulation of defects is not thorough, the improvement of energy resolution is limited, and at the same time, the preparation time and material cost will be greatly increased. Therefore, there is an urgent need to design a rapid growth method for improving the energy resolution of CdZnTe epitaxial single crystals to solve the above problems. Summary of the Invention

[0005] The object of the present invention is to solve the deficiencies in the existing methods for improving the energy resolution of CdZnTe epitaxial single crystals, such as increasing growth processes, high cost, limited size, and limited improvement in energy resolution. Therefore, a rapid growth method for improving the energy resolution of CdZnTe epitaxial single crystals is provided.

[0006] To achieve the above-mentioned invention object, the technical solution provided by the present invention is as follows:

[0007] A rapid growth method for improving the energy resolution of CdZnTe epitaxial single crystals, which is characterized in that it includes the following steps:

[0008] S1. Pretreat the Te particles

[0009] Process the Te particles into Te powder with a particle size of 0.3 - 1 mm; The reason for grinding the Te particles is that during the research and development period, it was found that the larger the particle size of the Te particles, the less conducive to the evaporation of the Te powder; Therefore, it is necessary to grind the Te particles in advance to reduce the particle size of the Te powder;

[0010] S2. Pretreat the CdZnTe growth source

[0011] This process aims to remove impurities and oxide layers on the surface of the growth source to prevent them from affecting the growth quality of the epitaxial single crystal. The specific operation is:

[0012] S21. According to the size of the required product, use a spiral rangefinder to select several 2-inch CdZnTe polycrystalline growth source wafers with a thickness of 2.5 ± 0.5 mm, and use a laser cutting machine to cut out the largest inscribed square of each growth source wafer; The main reason for selecting 2-inch polycrystalline growth source wafers here is that polycrystalline growth source wafers with a size of 4 inches and above are not only relatively rare in the market, but also have a high price. Therefore, it is intended to reduce the purchase cost of raw materials by using the method of splicing 2 inches, and at the same time, this cutting method can also reduce raw material waste; For example: if the product requirement is 4 inches, just select 4 2-inch polycrystalline growth source wafers;

[0013] S22. Perform grinding and polishing operations on the top surface and four side surfaces of the cut square growth source wafers to make their thickness 2.5 mm. First, rough polish with 400-mesh sandpaper, and then fine polish with 3000-mesh sandpaper;

[0014] S23. Ultrasonically clean the polished square growth source wafers to remove the growth source powder left on the growth source wafers during grinding and polishing, and then dry them;

[0015] S24. Place a 1 - 4 mm glass plate, an AlN sheet, a first mask plate, and the spliced CdZnTe growth source on the lower heating table of the near-space growth furnace from bottom to top in sequence. Then, heat the upper heating table and the lower heating table to 180 - 200 °C and 740 - 780 °C respectively, and perform an empty burn for 5 - 20 min to complete the pretreatment of the CdZnTe growth source. Then, reserve it in the near-space growth furnace for standby;

[0016] The spliced CdZnTe growth source is square and is composed of multiple square growth source wafers after being cleaned and dried with S23;

[0017] S3. Grow CdZnTe epitaxial single crystal

[0018] S31. Uniformly sprinkle the Te powder obtained in S1 around the CdZnTe growth source pretreated on the lower heating platform in the near-space growth furnace, and ensure that the gram number of the Te powder is not less than the minimum amount. For example, at least 10 g of Te powder is required for a 4-inch epitaxial single crystal, and at least 30 g of Te powder is required for a 6-inch epitaxial single crystal. Those skilled in the art can determine the minimum amount through an energy resolution performance evaluation experiment according to the required size of the product. Generally, a slightly excessive amount will be used during actual spreading;

[0019] Place a 3-6 mm glass plate, a growth substrate (such as a GaAs substrate), and a second mask plate on the upper heating platform of the near-space growth furnace in sequence from top to bottom;

[0020] The diameter of the growth substrate is greater than (slightly greater is fine) or equal to the diagonal length of the spliced CdZnTe growth source to ensure that the growth substrate can be as fully covered as possible by the spliced CdZnTe growth source;

[0021] In addition, since the thickness of the glass plate directly affects the distance and heat conduction effect between the CdZnTe growth source and the growth substrate, in this invention, requirements are imposed on the thickness of the glass plates used on the upper heating platform and the lower heating platform to ensure reliable growth of the single crystal;

[0022] S32. Evacuate the near-space growth furnace;

[0023] S33. Heat the upper heating platform and the lower heating platform to 400 - 450 °C and 670 - 730 °C respectively and hold for 8 - 15 min for the first-step growth;

[0024] S34. Keep the temperature of the lower heating platform unchanged, and continue to slowly heat the upper heating platform to 500 - 600 °C and hold for 150 - 300 min for the second-step growth to obtain the CdZnTe epitaxial single crystal.

[0025] Further, in S1, put the Te particles into a mortar and grind them in a circular motion into fine powder.

[0026] Further, in S23, for environmental protection and convenient use, ultrasonically clean the polished growth source wafers with deionized water and then dry them;

[0027] In S24, the heating rate of the upper heating platform and the lower heating platform is 1 - 3 °C / s.

[0028] Further, in S31, the thickness of the glass plate placed on the lower heating table is 2 mm; the thickness of the glass plate placed on the upper heating table is 5 mm;

[0029] In S32, a vacuum pump is used to pump the vacuum degree of the near-space growth furnace to 5 Pa;

[0030] In S33, the heating rate of the upper heating table and the lower heating table is 1 - 2 °C / s;

[0031] In S34, the heating rate of the upper heating table is 1 - 3 °C / s.

[0032] Further, in S33, after the upper heating table and the lower heating table are heated to the target temperature, they are maintained for 10 min;

[0033] In S34, after the upper heating table is heated to the target temperature, it is maintained for 200 min.

[0034] The present invention also provides a CdZnTe epitaxial single crystal grown by the above method, and an X-ray detector using the CdZnTe epitaxial single crystal as a detection material.

[0035] Principle of the present invention:

[0036] Since CdZnTe itself is a ternary compound, the saturated vapor pressures of its components vary greatly. During the growth process, the saturated vapor pressure of Cd is relatively large, and some of the Cd atoms deposited on the substrate will sublimate back and become gaseous again. However, the saturated vapor pressures of Zn and Te are small, and the sublimated Zn and Te atoms are very few. Eventually, a large number of shallow energy level point defects - Cd vacancies (V cd ) are contained in the grown CdZnTe epitaxial single crystal. This defect is the main factor affecting the reduction of energy resolution. By setting Te powder around the growth source, the epitaxial single crystal can grow in a Te-rich atmosphere. During the diffusion process, the Cd vacancies will be compensated by Te atoms to form Te antisites (Te Cd ). Te antisites are deep energy level point defects, which can significantly reduce the free carrier concentration and dark current in the epitaxial single crystal, thereby significantly improving the signal-to-noise ratio and obtaining better energy resolution.

[0037] Advantages of the present invention:

[0038] 1. Considering the complex post - processing problems generally involved in the current methods for optimizing the energy resolution of CdZnTe crystals, the present invention attempts to achieve the growth of CdZnTe epitaxial single crystals in a Te atmosphere by uniformly placing Te powder around the growth source and controlling the sublimation rate of the Te powder (i.e., controlling the temperatures of the upper and lower heating platforms) through adjusting the growth temperature. Through the synergistic cooperation of the two, shallow - level defects inside the epitaxial single crystals can be rapidly compensated into deep - level defects during the growth stage, thereby significantly reducing the dark current of the epitaxial single crystals themselves, increasing the resistivity, achieving the purpose of improving the signal - to - noise ratio, and enhancing the energy resolution without secondary treatment. The growth rate of the size of the epitaxial single crystals obtained by this method is more than 20 times that of the traditional growth method, greatly shortening the growth time. At the same time, the present invention also grows large - size epitaxial single crystals by splicing multiple small - size polycrystalline source wafers, which not only greatly reduces the growth cost but also realizes the growth of large - area epitaxial single crystals, making the size of the epitaxial single crystals only limited by the size of the substrate. Through this invention, CdZnTe epitaxial single crystals with better quality can be obtained, and an X - ray detector with an all - energy peak resolution of 7.21% for 241 Amγ rays and a relatively large mobility - lifetime product can be obtained after simple packaging. 241 Amγ rays and a relatively large mobility - lifetime product.

[0039] 2. The method of the present invention can be applied to other II - VI group compound semiconductors to promote their high - quality and rapid growth, but the growth process needs to be adaptively adjusted according to different elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is the top - view of the operation of the growth device for the CdZnTe epitaxial single crystals in Example 1 and the comparative example of the present invention; (a) is for Example 1, and (b) is for the comparative example.

[0041] Figure 2 It is the I - V curve graph of the Au / CdZnTe / p - GaAs / Au detector made of the CdZnTe epitaxial single crystal in Example 1 of the present invention and the Au / CdZnTe / p - GaAs / Au detector made of the CdZnTe epitaxial single crystal in the comparative example; (a) is for Example 1, and (b) is for the comparative example.

[0042] Figure 3 It is the mobility - lifetime product of electrons fitted by the Hecht equation when the Au / CdZnTe / p - GaAs / Au detector made of the CdZnTe epitaxial single crystal in Example 1 of the present invention and the Au / CdZnTe / p - GaAs / Au detector made of the CdZnTe epitaxial single crystal in the comparative example are under the action of 241 Am@5.48MeV α particles; (a) is for Example 1, and (b) is for the comparative example. 241 Am@5.48MeV α particles; (a) is for Example 1, and (b) is for the comparative example.

[0043] Figure 4For the Au / CdZnTe / p-GaAs / Au detector made of the CdZnTe epitaxial single crystal in Embodiment 1 of the present invention, in 241 the Am@59.5KeV γ-ray energy spectrum response diagram. Specific embodiments

[0044] The content of the present invention will be further described in detail below in conjunction with the accompanying drawings and Specific Embodiment 1:

[0045] Taking the preparation of the Au / CdZnTe / p-GaAs / Au detector as an example, the CdZnTe epitaxial single crystal used as the detection material is grown on a 4-inch GaAs substrate by the rapid growth method for improving the energy resolution of the CdZnTe epitaxial single crystal provided by the present invention. The thickness of the CdZnTe epitaxial single crystal is 805 μm.

[0046] Embodiment 1

[0047] The specific steps of this embodiment are as follows:

[0048] S1. Pretreatment of Te powder:

[0049] Put the Te particles into a mortar and grind them in a circular motion into Te powder. The particle size of the powder can be in the range of 0.3 - 1 mm, and weigh the required grams of Te powder with an electronic weighing device.

[0050] S2. Pretreatment of the single crystal source:

[0051] S21. Use a spiral rangefinder to select four 2-inch CdZnTe polycrystalline growth source wafers with thicknesses of 2.64 mm, 2.58 mm, 2.71 mm, and 2.51 mm, and use a laser cutting machine to cut out the largest inscribed square of each growth source wafer (specification: 33×33 mm 2 );

[0052] S22. Polish the top surface and four side surfaces of each square growth source wafer (polish successively with 400-mesh and 3000-mesh) to make its thickness 2.5 mm;

[0053] S23. Ultrasonically clean the square growth source wafers with deionized water and then dry them;

[0054] S24. Place a 2-mm glass plate, an AlN sheet, a first mask plate, and a 4-inch CdZnTe growth source composed of four square growth source wafers spliced together on the lower heating table of the near-space growth furnace from bottom to top in sequence. Then, heat the upper heating table and the lower heating table to 200 °C and 740 °C respectively at a heating rate of 2 °C / s, and burn in air for 5 min to complete the pretreatment of the CdZnTe growth source. Then, reserve it in the near-space growth furnace for standby;

[0055] Growth of S3 CdZnTe Epitaxial Single Crystal:

[0056] S31. Uniformly sprinkle the Te powder obtained in S1 on the periphery of the pre-treated CdZnTe growth source on the lower heating platform reserved in the near-space growth furnace. The top view of the specific growth device operation is as shown in Figure 1 (a) of

[0057] Place a 5-mm glass plate, a 4-inch GaAs substrate, and a second mask plate on the upper heating platform of the near-space growth furnace in sequence from top to bottom.

[0058] S32. Use a vacuum pump to pump the vacuum degree of the near-space growth furnace to 5 Pa.

[0059] S33. Heat the upper heating platform and the lower heating platform to 450 °C and 730 °C respectively at a heating rate of 2 °C / s, and keep for 10 min for the first-step growth.

[0060] S34. Keep the temperature of the lower heating platform unchanged, and slowly heat the upper heating platform to 600 °C at a heating rate of 1 °C / s for the second-step growth. The growth time is 200 min to obtain a 4-inch CdZnTe epitaxial single crystal.

[0061] For subsequent performance testing, encapsulate the CdZnTe epitaxial single crystal:

[0062] First, use a laser cutting machine to cut the above-obtained CdZnTe epitaxial single crystal into 10×10 mm 2 .

[0063] Secondly, load the cut CdZnTe epitaxial single crystal into a vapor deposition mask plate and place it in a vacuum vapor deposition machine.

[0064] Then, perform Au evaporation on both sides of GaAs and CdZnTe at a vacuum degree of 5×10 -3 Pa to prepare double-sided Au electrodes.

[0065] Finally, use a dispensing machine to encapsulate the epitaxial single crystal onto a PCB board through conductive silver glue, and then place it in an oven and dry it at 90 °C for 15 min (it can be dried at 70 - 90 °C for 15 - 45 min), complete the encapsulation, and fabricate an Au / CdZnTe / p-GaAs / Au detector.

[0066] Comparative Example:

[0067] In this comparative example, the specific implementation method is basically the same as that of Example 1, except that: do not sprinkle Te powder on the lower heating platform. The top view of the specific growth device operation is as shown in Figure 1 (b) of

[0068] Example 2

[0069] The difference from Example 1 is as follows:

[0070] In S33, the upper heating table and the lower heating table are heated to 440 °C and 720 °C at a heating rate of 1 °C / s respectively, and maintained for 8 min to perform the first-step growth;

[0071] In S34, with the temperature of the lower heating table remaining unchanged, the upper heating table is slowly heated to 580 °C at a heating rate of 3 °C / s to perform the second-step growth, and the growth time is 150 min.

[0072] Example 3

[0073] The difference from Example 1 is as follows:

[0074] In S33, the upper heating table and the lower heating table are heated to 430 °C and 700 °C at a heating rate of 1 °C / s respectively, and maintained for 15 min to perform the first-step growth;

[0075] In S34, with the temperature of the lower heating table remaining unchanged, the upper heating table is slowly heated to 560 °C at a heating rate of 2 °C / s to perform the second-step growth, and the growth time is 300 min.

[0076] Example 4

[0077] The difference from Example 1 is as follows:

[0078] In S33, the upper heating table and the lower heating table are heated to 420 °C and 680 °C at a heating rate of 1 °C / s respectively, and maintained for 10 min to perform the first-step growth;

[0079] In S34, with the temperature of the lower heating table remaining unchanged, the upper heating table is slowly heated to 530 °C at a heating rate of 2 °C / s to perform the second-step growth, and the growth time is 200 min.

[0080] Example 5

[0081] The difference from Example 1 is as follows:

[0082] In S33, the upper heating table and the lower heating table are heated to 400 °C and 670 °C at a heating rate of 1 °C / s respectively, and maintained for 10 min to perform the first-step growth;

[0083] In S34, with the temperature of the lower heating table remaining unchanged, the upper heating table is slowly heated to 500 °C at a heating rate of 2 °C / s to perform the second-step growth, and the growth time is 200 min.

[0084] To verify the effect, the present invention also conducted the following performance test comparisons:

[0085] As shown Figure 2 in the figure, the I-V curve graphs of the Au / CdZnTe / p-GaAs / Au detector made of the CdZnTe epitaxial single crystal in Example 1 of the present invention and the Au / CdZnTe / p-GaAs / Au detector made of the CdZnTe epitaxial single crystal in the comparative example. The resistivity of the CdZnTe epitaxial single crystal in Experimental Example 1 is 2.83×10 10 Ω·cm, which is significantly higher than the resistivity of 1.25×10 9 Ω·cm in the comparative example.

[0086] As shown Figure 3 in the figure, the electron mobility-lifetime product fitted by the Hecht equation of the Au / CdZnTe / p-GaAs / Au detector made of the CdZnTe epitaxial single crystal in Example 1 of the present invention and the Au / CdZnTe / p-GaAs / Au detector made of the CdZnTe epitaxial single crystal in the comparative example under the action of 241 Am@5.49MeV α particles. The electron mobility-lifetime product of Example 1 reaches 3.66×10 -4 cm 2 ·V -1 , which is significantly higher than the electron mobility-lifetime product of 1.41×10 -5 cm 2 ·V -1 in the comparative example.

[0087] As shown Figure 4 in the figure, the full energy peak of 241 Am@59.5KeV γ rays is measured for the Au / CdZnTe / p-GaAs / Au detector made of the CdZnTe epitaxial single crystal annealed in Example 1 of the present invention. The resolution of Example 1 for 241 Am@59.5keV reaches 7.21%.

[0088] In summary, through this invention, a CdZnTe epitaxial single crystal with better quality and significantly improved energy resolution can be obtained.

[0089] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of various equivalent modifications or substitutions, and these modifications or substitutions should all be covered within the protection scope of the present invention.

Claims

1. A rapid growth method for improving the energy resolution of CdZnTe epitaxial single crystals, characterized in that: The following steps are involved: S1. Pretreatment of Te particles Processing Te particles into Te powder with a particle size of 0.3-1 mm; S2. Pretreatment of CdZnTe growth source S21. According to the size of the desired product, select several 2-inch CdZnTe polycrystalline growth source sheets with a thickness of 2.5±0.5mm, and cut out the largest inscribed square of each growth source sheet; S22. Grinding and polishing the top surface and four side surfaces of the cut square growth source sheet; S23. The square growth source sheet after grinding and polishing is ultrasonically cleaned and dried; S24. Place a 1-4 mm glass plate, an AlN sheet, a first mask, and a spliced ​​CdZnTe growth source on the lower heating stage of the near-space growth furnace from bottom to top, and then heat the upper and lower heating stages to 180-200° C. and 740-780° C., respectively, and burn them for 5-20 minutes to complete the pretreatment of the CdZnTe growth source, and then reserve them in the near-space growth furnace for standby use; The spliced ​​CdZnTe growth source is a square, which is spliced ​​by a plurality of square growth source sheets that have been cleaned and dried by S23; S3. Growth of CdZnTe epitaxial single crystal S31. The Te powder obtained in S1 is evenly spread around the CdZnTe growth source pretreated on the lower heating table reserved in the near-space growth furnace; On the upper heating platform of the near-space growth furnace, a 3-6 mm glass plate, a growth substrate and a second mask are placed in order from top to bottom; The diameter of the growth substrate is greater than or equal to the diagonal length of the spliced ​​CdZnTe growth sources; S32. The near-space growth furnace is evacuated to 5 Pa; S33. The upper heating stage and the lower heating stage are heated to 400-450°C and 670-730°C respectively and maintained for 8-15 minutes for the first step of growth; S34. Keep the temperature of the lower heating stage unchanged, continue to slowly heat the upper heating stage to 500-600°C and maintain it for 150-300 minutes, and perform the second step of growth to obtain a CdZnTe epitaxial single crystal.

2. A rapid growth method for improving the energy resolution of CdZnTe epitaxial single crystal according to claim 1, characterized in that: In S1, Te particles are placed in a grinding mortar and ground into fine powder by circular motion.

3. A rapid growth method for improving the energy resolution of CdZnTe epitaxial single crystal according to claim 1 or 2, characterized in that: In S23, the polished growth source sheet is ultrasonically cleaned with deionized water and then blown dry; In S24, the heating rates of the upper heating stage and the lower heating stage are 1-3°C / s.

4. A rapid growth method for improving the energy resolution of CdZnTe epitaxial single crystal according to claim 3, characterized in that: In S31, the thickness of the glass plate placed on the lower heating table is 2 mm; the thickness of the glass plate placed on the upper heating table is 5 mm; In S32, a vacuum pump is used to evacuate the vacuum degree of the near-space growth furnace to 5 Pa; In S33, the heating rate of the upper heating stage and the lower heating stage is 1-2°C / s; In S34, the heating rate of the upper heating stage is 1-3°C / s.

5. A rapid growth method for improving the energy resolution of CdZnTe epitaxial single crystal according to claim 4, characterized in that: In S33, the upper heating stage and the lower heating stage are heated to the target temperature and maintained for 10 minutes; In S34, the upper heating stage is heated to the target temperature and maintained at the target temperature for 200 minutes.

6. A CdZnTe epitaxial single crystal, characterized in that: The method according to claim 1 is used for rapid growth.

7. An X-ray detector, characterized in that: The CdZnTe epitaxial single crystal rapidly grown by the method described in claim 1 is used as the detection material.

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