Gas phase preparation method of cadmium zinc telluride photon counting detector material

Abstract

The invention provides a vapor phase preparation method of a cadmium zinc telluride photon counting detector material, and solves the technical problems of a CZT epitaxial single crystal film prepared by a near-space sublimation method at present. According to the method, Cd1-xZnxTe polycrystal (x = 0.1-0.2) is selected as a growth source, first-step growth is carried out by controlling the temperature of the substrate to reduce the initial nucleation rate on the GaAs substrate, and a CZT buffer layer with a small number of needle-shaped, hill-shaped and other defects is obtained. And then, replacing the CZT polycrystal growth source subjected to the same pretreatment to carry out second-step rapid growth so as to obtain a thick film with the average thickness of 850 microns. And finally, the prepared CZT epitaxial thick film is put into an annealing furnace to be subjected to gradient annealing treatment for three times, so that the problem of excessive intrinsic point defects such as Cd vacancies is effectively reduced, the quality of the film is improved, and the CZT epitaxial thick film with the saturation count rate reaching 2MCPS / channel and meeting the photon counting imaging requirement is obtained.

Description

Technical Field

[0001] The invention belongs to the technical field of detector materials, and in particular relates to a gas phase preparation method of cadmium zinc telluride photon counting detector materials. Background Art

[0002] Photon counting detectors (PCDs) count X-ray photons with multiple energies, which can achieve high-contrast multi-energy CT imaging and has gradually become a research hotspot in the medical and industrial fields. Compared with CT imaging technology based on energy integrating detectors (EID-CT), PCD-based spectral CT technology (PCD-CT) can provide relatively rich spectral imaging information, which can effectively distinguish the differences in subtle structures, thereby achieving accurate identification of material materials. Some scientists predict that in the next ten years, photon counting technology will replace 90% of X-ray detection technology in the world.

[0003] Cadmium zinc telluride is considered to be the best material for realizing photon counting technology. It has advantages such as high atomic number, low noise, high resolution, adjustable bandgap width and high energy response rate. Its sensitivity and accuracy for radiation detection are much higher than those of traditional materials. It is the best semiconductor material for hard X-ray detection with energy between 10keV-600keV. The traditional melt growth method has problems such as low efficiency, low ingot utilization and waste material cannot be recycled, but the near-space sublimation method (CSS) solves this problem well.

[0004] The equipment structure of the CSS method for growing CZT epitaxial single crystal film is simple and easy to operate. Compared with the traditional melt method, the cost is reduced by one order of magnitude, which greatly reduces the preparation cost of large-size CZT epitaxial single crystal film. However, due to the 13% lattice mismatch between the GaAs substrate and CZT used in the CSS method, nucleation growth often occurs in a three-dimensional island manner in the early stage of growth, resulting in a large number of hillock-shaped, needle-shaped defects and transcrystalline dislocations in the film, which seriously affects the crystallization quality. In addition, a large number of intrinsic point defects such as Cd vacancies will inevitably be generated during the growth of the film, which will cause damage to the resistivity and carrier mobility of CZT, resulting in a decrease in the saturation count rate of the detector. At present, there is also a method for preparing CZT epitaxial single crystal film in two steps. Although this method can reduce the number of point defects such as Cd vacancies to a certain extent, there are still defects such as needle-shaped and hillock-shaped defects, which are difficult to meet the requirements of photon counting imaging, and the quality needs to be improved.

[0005] Therefore, it is urgent to explore a new preparation method to improve the crystallization quality and increase the detector saturation count rate. Summary of the invention

[0006] The purpose of the present invention is to solve the technical problems existing in the CZT epitaxial single crystal film prepared by the close-space sublimation method at present, and to provide a gas phase preparation method of cadmium zinc telluride photon counting detector material, which is a gas phase preparation method of cadmium zinc telluride thick film material that can meet the requirements of photon counting imaging.

[0007] To achieve the above purpose, the technical solution provided by the present invention is:

[0008] A method for preparing a cadmium zinc telluride epitaxial thick film, which is special in that it comprises the following steps:

[0009] 1) Growth source and substrate pretreatment:

[0010] Will Cd 1-x Zn x Te polycrystal is used as a growth source, and a surface pretreatment is performed on the surface of the growth source to remove the oxide layer on the surface of the growth source; wherein x=0.1-0.2;

[0011] The reason for using this ratio of growth source is that the composition of the thin film grown by the vapor phase method is affected by the source composition. The Zn content in the material will affect the resistivity of the thin film. The higher the Zn content, the higher the resistivity of the thin film. However, the higher the Zn content, the more defects there will be in the thin film, which will cause the resistivity to decrease. Therefore, using this ratio of polycrystalline as the growth source can control the Zn content in the thin film within a certain range, thereby meeting the use requirements of the photon counting detector under high pressure.

[0012] Considering the lattice matching problem, GaAs with a crystal plane orientation of (100) is used as the substrate, and its surface is pretreated to remove the oxide layer on the substrate surface;

[0013] 2) Two-step method for growing CdZnTe thick films:

[0014] 2.1) The first step of growth:

[0015] Place the substrate and growth source pretreated in step 1) into a near-space sublimation furnace, turn on the mechanical pump and the vent valve, and automatically connect the molecular pump when the vacuum degree in the chamber is lower than 5Pa (when the molecular pump is automatically connected, the mechanical pump will be automatically closed), pump the vacuum degree of the chamber to 0.05Pa (the higher the vacuum degree, the better the effect), turn on water cooling (that is, the water cooling should be turned on before running the growth program), and then grow. After the growth is completed, cool it to room temperature with the furnace, and turn off the vent valve, molecular pump and water cooling in turn;

[0016] The process parameters for the first step of growth are:

[0017] The upper heating stage temperature is set to 520-560°C, the lower heating stage temperature is set to 700-720°C, the heating rate is 1-3°C / S, and the growth time is 10-30min;

[0018] In actual operation, the operating program can be set according to the above process parameters first;

[0019] 2.2) The second step of growth:

[0020] Open the near-space sublimation furnace, replace the growth source that has been pretreated in the same way, turn on the mechanical pump and the vent valve, and automatically connect the molecular pump when the vacuum degree in the cavity is lower than 5Pa, pump the vacuum degree of the cavity to 0.05Pa (the higher the vacuum degree, the better the effect), turn on water cooling, and then grow. After the growth is completed, cool it to room temperature with the furnace, turn off the vent valve, molecular pump and water cooling in turn, and obtain and take out the cadmium zinc telluride epitaxial thick film;

[0021] The process parameters for the second step growth are:

[0022] The upper heating stage temperature is set to 380-420℃, the lower heating stage temperature is set to 700-720℃, the heating rate is 1-3℃ / S, and then the growth is 120-140min; time mainly affects the film thickness. As time goes by, the film thickness increases more and more slowly, so when the thickness difference is not big, the time cost can be appropriately reduced, and the growth is 120min;

[0023] In this step, the process parameters may be set before or after the growth source is replaced;

[0024] 3) Annealing treatment:

[0025] The cadmium zinc telluride epitaxial thick film obtained in step 2) is peeled off from the substrate and installed at both ends of the annealing bracket respectively with the Te2 annealing source, and then the annealing bracket is placed in a quartz crucible, and the vacuum is drawn to 0.05 Pa and then the tube is sealed, and then the whole is placed in a multi-stage temperature-controlled furnace, and the temperature in the furnace is accurately controlled by the multi-stage temperature-controlled furnace to perform gradient annealing, and after the annealing is completed, the furnace is cooled to room temperature; then the Te2 annealing source is replaced, and the annealing process is repeated; according to this process, a total of three gradient annealings are performed on one side of the epitaxial thick film, and finally a cadmium zinc telluride epitaxial thick film that meets the requirements of photon counting imaging is obtained;

[0026] Among them, the annealing process parameters are:

[0027] The temperature of the area where the annealing source is located is 400-420°C, the annealing temperature of the area where the CZT epitaxial thick film is located is 360-380°C, the annealing time is 12-14h, and the heating rate is 1-3°C / S.

[0028] It can be seen that the annealing method adopted by the method of the present invention is three-step gradient annealing in Te2 atmosphere. Annealing in Te2 atmosphere can generate Te by introducing Te to react with Cd vacancies. CdTo compensate for the cadmium vacancy defects; the use of a multi-stage temperature-controlled furnace to control the temperature in sections is mainly to form a constant temperature gradient in the furnace. The temperature gradient can be used as a driving force for the migration of intrinsic point defects such as Te gaps to the surface, but too large a temperature gradient will cause the Te gap to produce a stress zone during the migration process. Therefore, the temperature gradient must be strictly controlled according to the aforementioned process parameters; in addition, a single annealing time that is too long will lead to the introduction of Te Cd Too high concentration increases the probability of carrier capture, and a single long annealing may also lead to oxidation of the annealing source, reducing detector performance. Therefore, considering the requirements for the depth of the annealing layer, 12h-14h each annealing is best, and the annealing source should be replaced before each annealing.

[0029] Further, in step 1), the Cd 1-x Zn x Te polycrystal is grown by Bridgman method, and its pretreatment process is as follows:

[0030] First, the oxide layer on the surface of the polycrystal is polished with sandpaper (e.g., 1000 mesh), and then the polished polycrystal is ultrasonically cleaned in acetone, anhydrous ethanol, and deionized water in turn (e.g., 20 minutes), and finally dried with nitrogen and placed in a drying oven for later use;

[0031] The pretreatment process of the GaAs substrate is as follows:

[0032] First, the GaAs substrate is roughly polished on both sides with an aqueous solution of MgO powder for 20 to 30 minutes. Finally, when no obvious scratches are observed, it is chemically polished with a silica gel solution containing 50% (volume fraction) H2O2 to eliminate small scratches on the surface. It is then ultrasonically cleaned in acetone, anhydrous ethanol and deionized water in turn, and finally blown dry with nitrogen for later use.

[0033] Furthermore, the Cd 1-x Zn x Te polycrystalline to Cd 0.9 Zn 0.1 Te polycrystalline.

[0034] Furthermore, in step 2.1), the process parameters of the first step of growth are:

[0035] The temperature of the upper heating stage was set to 550°C; the temperature of the lower heating stage was set to 710°C; the heating rate was 2°C / S; the program was run and the growth was for 20 minutes.

[0036] Further, in step 2.2), the process parameters of the second step growth are:

[0037] The temperature of the upper heating stage was set to 380°C, and the temperature of the lower heating stage was set to 710°C; the heating rate was 2°C / S; the program was run and the growth time was 120 minutes.

[0038] During the above two growth processes, the temperature of the lower heating stage will affect the composition and film thickness of the material. The performance of the photon counting detector is best within this temperature range. Although CdZnTe epitaxial thick films can be prepared at other temperatures (e.g., as low as 700°C and as high as 740°C), the performance is not very good. The principle of selecting the temperature of the lower heating stage is as follows: the temperature of the lower heating stage will affect the evaporation rate of the source. The lower the temperature, the slower the evaporation rate. Since the saturated vapor pressure of Cd is high, the reverse evaporation of Cd on the substrate is correspondingly stronger, resulting in a higher Zn content on the film surface, but also introducing defects such as reverse evaporation pits. As the source temperature increases, the evaporation rate increases, the reverse evaporation weakens, and the Zn content on the film surface decreases. The Zn content near 710°C is relatively high, and the number of surface defects is also relatively small, which is beneficial to the improvement of the count rate of the photon counting detector. In order to obtain a detector with a high count rate, it is preferred to set the source temperature at 710°C.

[0039] Further, in step 3), the process parameters of annealing are:

[0040] The temperature of the annealing source area is 400°C, the annealing temperature of the CZT epitaxial thick film area is 380°C, the annealing time is 14h, and the heating rate is 2°C / S.

[0041] At the same time, the present invention also provides a CdZnTe epitaxial thick film and the use of the CdZnTe epitaxial thick film as a detection material in a photon counting detector. The CdZnTe epitaxial thick film is prepared by the above preparation method, and the film thickness is 800-900 μm.

[0042] Based on the above application, the present invention further provides a photon counting detector, which is special in that the cadmium zinc telluride epitaxial thick film prepared by the above preparation method is used as the detection material.

[0043] Concept and principle of the present invention:

[0044] With respect to the two-step method mentioned in the background technology, the present invention has conducted an in-depth analysis of the problems existing in the two-step method. The current two-step method is to first adopt a low substrate temperature to quickly form a CZT buffer layer, and then increase the substrate temperature to perform a slow epitaxial growth, and this process does not consider reducing needle-shaped and hillock-shaped defects; the first step is to adopt rapid growth to promote the fusion of growth islands, but these have little effect on the counting performance improvement of photon counting detectors; the second step is to adopt slow growth to reduce the number of point defects such as Cd vacancies, but due to the slow growth rate, it is difficult to grow a thicker film for photon counting, because under the condition of the same film quality, the thicker the film, the greater the dose it can tolerate. For this reason, the present invention considers preparing a "thick film" with an average film thickness of 800+μm and good crystal quality.

[0045] The present invention is a gas phase preparation method, which adopts a two-step growth method. First, in the first step of growth, a CZT buffer layer is formed on the GaAs substrate at a lower nucleation rate to effectively reduce the number of needle-shaped and hillock-shaped defects at the CZT / GaAs interface in the early stage of growth. Then, in the second step of growth, it is re-nucleated for homoepitaxial growth, and a thick film with an average thickness of about 850 μm is obtained relatively quickly. The Te2 atmosphere gradient annealing is combined to form a Te2 atmosphere. cd Compensate for Cd vacancies during the film growth process, promote the migration of intrinsic point defects such as Te gaps under the action of temperature gradient, adjust the point defect concentration during the growth process, replace the annealing source for three annealings, increase the annealing layer depth while preventing the source from oxidizing during long-term annealing, improve the photoelectric properties of the film, and finally obtain a high-performance CZT detector material with a saturation count rate of 2MCPS / Channel that meets the requirements of photon counting imaging.

[0046] The beneficial effects of the present invention are as follows:

[0047] The method of the present invention selects Cd 1-x Zn x Te polycrystalline (x = 0.1 ~ 0.2) is used as the growth source. The first step of growth is to control the substrate temperature to reduce the initial nucleation rate on the GaAs substrate, and a CZT buffer layer with fewer defects such as needle-shaped and hillock-shaped defects is obtained. After that, the CZT polycrystalline growth source that has undergone the same pretreatment is replaced for the second step of rapid growth to obtain a thick film with an average thickness of up to 850μm. Finally, the prepared CZT epitaxial thick film is placed in an annealing furnace for three gradient annealing treatments, which effectively reduces the problem of excessive intrinsic point defects such as Cd vacancies, improves the quality of the film, and obtains a CZT epitaxial thick film with a saturation count rate of 2MCPS / channel that meets the requirements of photon counting imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 The count rate curve of the CZT single crystal epitaxial film grown in Example 1 of the present invention under different doses of X-ray irradiation with a negative 500V bias is shown.

[0049] Figure 2 The photocurrent step diagram of the CZT single crystal epitaxial film grown in Example 1 of the present invention under different doses of X-ray irradiation with an external negative 500V bias.

[0050] Figure 3 The count rate curve of the CZT single crystal epitaxial film grown in Example 2 of the present invention under different doses of X-ray irradiation with a negative 500V bias is shown.

[0051] Figure 4The photocurrent step diagram of the CZT single crystal epitaxial film grown in Example 2 of the present invention under different doses of X-ray irradiation with a negative 500V bias.

[0052] Figure 5 The count rate curve of the CZT single crystal epitaxial film grown in Example 3 of the present invention under different doses of X-ray irradiation with a negative 500V bias is shown.

[0053] Figure 6 The photocurrent step diagram of the CZT single crystal epitaxial film grown in Example 3 of the present invention under different doses of X-ray irradiation with an external negative 500V bias.

[0054] Figure 7 This is a graph showing the count rate curve of a cadmium zinc telluride film grown by conventional close-space sublimation method under different doses of X-ray irradiation with an external negative 500V bias.

[0055] Figure 8 This is the photocurrent step diagram of the CdZnTe film grown by the traditional close-space sublimation method under different doses of X-ray irradiation with an external negative 500V bias. DETAILED DESCRIPTION

[0056] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0057] Since needle-shaped and hillock-shaped defects are mainly generated at the interface where GaAs and CZT contact at the initial stage of nucleation, the present invention adopts a two-step growth method. The high-temperature nucleation of the first step growth substrate reduces the nucleation rate, increases the atomic migration time at the interface, and forms a CZT buffer layer with fewer defects. The second step is to grow homoepitaxially on this buffer layer. In order not to introduce new needle-shaped and hillock-shaped defects in the second step, a lower substrate temperature is adopted in the second step to increase the growth rate. It can be seen that this two-step method mainly reduces the number of needle-shaped and hillock-shaped defects in the first step of growth, and the increase of intrinsic point defects such as Cd vacancies that may be caused by the rapid growth in the second step is eliminated by subsequent annealing, thereby growing a thick film that meets the requirements.

[0058] Embodiment 1:

[0059] A gas phase preparation method for cadmium zinc telluride photon counting detector material comprises the following steps:

[0060] 1) Growth source and substrate pretreatment:

[0061] Bridgman-grown Cd 0.9 Zn 0.1Te polycrystal was used as the growth source, and the oxide layer on the surface of the CZT polycrystal source was polished with 1000-mesh sandpaper. The polished CZT polycrystal source was then ultrasonically cleaned in acetone, anhydrous ethanol and deionized water for 20 minutes, dried with nitrogen and placed in a drying oven for later use.

[0062] A GaAs substrate with a crystal plane orientation of (100) was selected. The GaAs substrate was first roughly polished on both sides with an aqueous solution of MgO powder for 25 minutes. Finally, when no obvious scratches were observed, a silica gel solution containing 50% H2O2 was used for chemical polishing to eliminate small scratches on the surface. The substrate was then ultrasonically cleaned in acetone, anhydrous ethanol and deionized water in turn. Finally, it was dried with nitrogen and placed in a drying oven for use.

[0063] 2) Two-step method for growing CdZnTe thick films:

[0064] 2.1) The first step of growth:

[0065] Set the first step of the growth near-space sublimation furnace process to set the upper heating stage temperature to 550°C, the lower heating stage temperature to 710°C, and the heating rate to 2°C / S. 0.9 Zn 0.1 Te polycrystalline source was placed in the upper and lower heating stages of the near-space sublimation furnace, respectively. The mechanical pump and vent valve were turned on to pump the vacuum of the chamber to 5Pa. The molecular pump was automatically connected to pump the vacuum of the chamber to 0.05Pa. Water cooling was turned on, and then the program was run. The growth was carried out for 20 minutes. After the growth was completed and the furnace was slowly cooled to room temperature, the vent valve, molecular pump and water cooling were turned off in turn.

[0066] Using atomic force microscopy (AFM) for characterization, it can be clearly observed that the number of needle-shaped and hillock-shaped defects is relatively small in the early stage of film growth.

[0067] 2.2) The second step of growth:

[0068] The near-space sublimation furnace was opened, and the polycrystalline source pretreated in the same manner was replaced; the process parameters for the second step of growth were set, the temperature of the upper heating stage was set to 400°C, the temperature of the lower heating stage was set to 710°C, the heating rate was 2°C / S, the mechanical pump and the vent valve were turned on, and when the vacuum degree in the cavity was lower than 5Pa, the molecular pump was automatically connected, and the vacuum degree of the cavity was pumped to 0.05Pa, and water cooling was turned on. Then the program was run, and the growth was carried out for 120min. After the growth was completed and the furnace was slowly cooled to room temperature, the vent valve, molecular pump and water cooling were closed in turn to obtain a CZT epitaxial thick film with a thickness of 805μm;

[0069] 3) Annealing treatment:

[0070] After removing the substrate from the CZT epitaxial thick film obtained in step 2), the CZT epitaxial thick film and the Te2 annealing source are installed on the annealing bracket, and then the annealing bracket is placed as a whole in a quartz crucible, and after evacuating to 0.05 Pa, the tube is sealed and placed in a multi-stage temperature-controlled furnace, and the temperature of the area where the annealing source is located is set to 400°C, the temperature of the area where the CZT epitaxial thick film is located is set to 380°C, and the heating rate is 2°C / S; the temperature in the furnace is accurately controlled by the multi-stage temperature-controlled furnace, and gradient annealing is performed for 14 hours, and then slowly cooled to room temperature, the Te2 annealing source is replaced, and the annealing process is repeated again, and finally three annealings are performed on one side of the thick film to obtain a detection material that meets the requirements of photon counting imaging.

[0071] In order to verify the performance of the detection material prepared by the present invention, it was applied to a photon counting detector for detection:

[0072] from Figure 1 It can be seen that the photon counting rate of the detector increases with the increase of radiation dose, and the saturation counting rate can reach 2.3MCPS / channel, which meets the counting rate requirements of the bone densitometer for the detector.

[0073] from Figure 2 It can be seen from the (It image) that as the dose (X-ray tube current) increases linearly, the detector photocurrent also increases linearly, which can reflect that the number of detector point defects is small. At the same time, the It image X-ray dose corresponds to the dose of the count rate measurement. According to the linear change law of the photocurrent, the linear change area of ​​the count rate should be larger in theory. The reason why the linear area is only up to 1.5MCPS / channel at this time may be due to the limitation of the readout chip.

[0074] Embodiment 2:

[0075] A gas phase preparation method for cadmium zinc telluride photon counting detector material comprises the following steps:

[0076] 1) Growth source and substrate pretreatment:

[0077] Bridgman-grown Cd 0.9 Zn 0.1 Te polycrystal was used as the growth source, and the oxide layer on the surface of the CZT polycrystal source was polished with 1000-mesh sandpaper. The polished CZT polycrystal source was then ultrasonically cleaned in acetone, anhydrous ethanol and deionized water for 20 minutes, dried with nitrogen and placed in a drying oven for later use.

[0078] A GaAs substrate with a crystal plane orientation of (100) was selected. The GaAs substrate was first roughly polished on both sides with an aqueous solution of MgO powder for 25 minutes. Finally, when no obvious scratches were observed, a silica gel solution containing 50% H2O2 was used for chemical polishing to eliminate small scratches on the surface. The substrate was then ultrasonically cleaned in acetone, anhydrous ethanol and deionized water in turn. Finally, it was dried with nitrogen and placed in a drying oven for use.

[0079] 2) Two-step method for growing CdZnTe thick films:

[0080] 2.1) The first step of growth:

[0081] Set the first step of the growth near-space sublimation furnace process to set the upper heating stage temperature to 520°C, the lower heating stage temperature to 710°C, and the heating rate to 2°C / S. 0.9 Zn 0.1 Te polycrystalline source was placed in the upper and lower heating stages of the near-space sublimation furnace, respectively. The mechanical pump and vent valve were turned on to pump the vacuum of the cavity to 5Pa. The molecular pump was automatically connected to pump the vacuum of the cavity to 0.05Pa. Water cooling was turned on and the program was run. The growth was performed for 10 minutes. After the growth was completed and the furnace was slowly cooled to room temperature, the vent valve, molecular pump and water cooling were turned off in sequence.

[0082] Using atomic force microscopy (AFM) for characterization, it can be clearly observed that the number of needle-shaped and hillock-shaped defects is relatively small in the early stage of film growth.

[0083] 2.2) The second step of growth:

[0084] Open the near-space sublimation furnace, replace the polycrystalline source pretreated in the same way; set the second step growth process parameters, the upper heating stage temperature is set to 420°C, the lower heating stage temperature is set to 710°C, the heating rate is 2°C / S, turn on the mechanical pump and the vent valve, and when the vacuum degree in the cavity is lower than 5Pa, the molecular pump is automatically connected, and the vacuum degree of the cavity is pumped to 0.05Pa, turn on water cooling, run the program, grow for 120min, and when the growth is completed and the furnace is slowly cooled to room temperature, turn off the vent valve, molecular pump and water cooling in turn to obtain a CZT epitaxial thick film with a thickness of 820μm;

[0085] 3) Annealing treatment:

[0086] After removing the substrate from the CZT epitaxial thick film obtained in step 2), the CZT epitaxial thick film and the Te2 annealing source are installed on the annealing bracket, and then the annealing bracket is placed as a whole in a quartz crucible, and after evacuating to 0.05 Pa, the tube is sealed and placed in a multi-stage temperature-controlled furnace, and the temperature of the area where the annealing source is located is set to 400°C, the temperature of the area where the CZT epitaxial thick film is located is set to 360°C, and the heating rate is 2°C / S; the temperature range in the furnace is accurately controlled by the multi-stage temperature-controlled furnace, and gradient annealing is performed for 14 hours, and then slowly cooled to room temperature, the Te2 annealing source is replaced, and the annealing process is repeated again, and finally three annealings are performed on one side of the thick film to obtain a detection material that meets the requirements of photon counting imaging.

[0087] In order to verify the performance of the detection material prepared by the present invention, it was applied to a photon counting detector for detection:

[0088] from Figure 3 It can be seen that the photon counting rate of the detector increases with the increase of radiation dose, and the saturation counting rate can reach 1.98MCPS / channel, which meets the counting rate requirements of the bone densitometer for the detector.

[0089] from Figure 4 It can be seen from (It image) that as the dose (X-ray tube current) increases linearly, the detector photocurrent also increases linearly. The reason why the linear region is only up to 1.5MCPS / channel may be due to the limitation of the readout chip.

[0090] Embodiment 3:

[0091] A gas phase preparation method for cadmium zinc telluride photon counting detector material comprises the following steps:

[0092] 1) Growth source and substrate pretreatment:

[0093] Bridgman-grown Cd 0.9 Zn 0.1 Te polycrystal was used as the growth source, and the oxide layer on the surface of the CZT polycrystal source was polished with 1000-mesh sandpaper. The polished CZT polycrystal source was then ultrasonically cleaned in acetone, anhydrous ethanol and deionized water for 20 minutes, dried with nitrogen and placed in a drying oven for later use.

[0094] A GaAs substrate with a crystal plane orientation of (100) was selected. The GaAs substrate was first roughly polished on both sides with an aqueous solution of MgO powder for 25 minutes. Finally, when no obvious scratches were observed, a silica gel solution containing 50% H2O2 was used for chemical polishing to eliminate small scratches on the surface. The substrate was then ultrasonically cleaned in acetone, anhydrous ethanol and deionized water in turn. Finally, it was dried with nitrogen and placed in a drying oven for use.

[0095] 2) Two-step method for growing CdZnTe thick films:

[0096] 2.1) The first step of growth:

[0097] Set the first step of the growth near-space sublimation furnace process to set the upper heating stage temperature to 560°C, the lower heating stage temperature to 710°C, and the heating rate to 2°C / S. 0.9 Zn 0.1 Te polycrystalline source was placed in the upper and lower heating stages of the near-space sublimation furnace, respectively. The mechanical pump and vent valve were turned on to pump the vacuum of the chamber to 5Pa. The molecular pump was automatically connected to pump the vacuum of the chamber to 0.05Pa. Water cooling was turned on and the program was run. The growth was continued for 30 minutes. After the growth was completed and the furnace was slowly cooled to room temperature, the vent valve, molecular pump and water cooling were turned off in sequence.

[0098] Using atomic force microscopy (AFM) for characterization, it can be clearly observed that the number of needle-shaped and hillock-shaped defects is relatively small in the early stage of film growth.

[0099] 2.2) The second step of growth:

[0100] Open the near-space sublimation furnace, replace the polycrystalline source pretreated in the same way; set the second step growth process parameters, set the upper heating stage temperature to 380°C, the lower heating stage temperature to 710°C, the heating rate to 2°C / S, turn on the mechanical pump and the vent valve, and when the vacuum degree in the cavity is lower than 5Pa, the molecular pump is automatically connected, and the vacuum degree of the cavity is pumped to 0.05Pa, turn on water cooling, run the program, grow for 120min, and when the growth is completed and the furnace is slowly cooled to room temperature, turn off the vent valve, molecular pump and water cooling in turn to obtain a CZT epitaxial thick film with a thickness of 850μm;

[0101] 3) Annealing treatment:

[0102] After removing the substrate from the CZT epitaxial thick film obtained in step 2), the CZT epitaxial thick film and the Te2 annealing source are installed on the annealing bracket, and then the annealing bracket is placed as a whole in a quartz crucible, and after evacuating to 0.05 Pa, the tube is sealed and placed in a multi-stage temperature-controlled furnace, and the temperature of the area where the annealing source is located is set to 400°C, the temperature of the area where the CZT epitaxial thick film is located is set to 370°C, and the heating rate is 2°C / S; the temperature range in the furnace is accurately controlled by the multi-stage temperature-controlled furnace, and gradient annealing is performed for 14 hours, and then slowly cooled to room temperature, the Te2 annealing source is replaced, and the annealing process is repeated again, and finally three annealings are performed on one side of the thick film to obtain a detection material that meets the requirements of photon counting imaging.

[0103] In order to verify the performance of the detection material prepared by the present invention, it was applied to a photon counting detector for detection:

[0104] from Figure 5It can be seen that the photon counting rate of the detector increases with the increase of radiation dose, and the saturation counting rate can reach 1.83MCPS / channel, which meets the counting rate requirements of the bone densitometer for the detector.

[0105] from Figure 6 It can be seen from (It image) that as the dose (X-ray tube current) increases linearly, the detector photocurrent also increases linearly, and the linear region of the photocurrent is also limited by the readout chip.

[0106] Within the range of the process parameters described above, the present invention also prepares detection materials that meet the requirements of photon counting imaging by using other process conditions.

[0107] In addition, the present invention also compares the performance of the thin film prepared in the embodiment with that of the cadmium zinc telluride thin film grown by the traditional close-space sublimation method. Figure 7 and Figure 8 The performance diagram of the CdZnTe thin film grown by the traditional close-space sublimation method is shown in Figure 1. The thickness of the CdZnTe thin film after 2 hours of growth is only 400 μm, which is twice the thickness of the film in the example. Figure 7 The saturation count rate of the detector at 500V bias is only 1.26MCPS / channel, and as the photoelectric tube current exceeds 0.04ma, the detector count rate decreases. At this time, the detector has been polarized. Comparing the It image of the detector ( Figure 8 ) It can be seen that when the photoelectric tube current increases to 0.04ma, the photocurrent jitter is obvious and increases nonlinearly, indicating that there are many internal defects in the detector material and the carrier transport process is seriously disturbed, which cannot meet the use requirements of the photon counting detector.

[0108] It can be seen that the performance of the film prepared by the embodiment of the present invention is significantly better than that of the cadmium zinc telluride film grown by the traditional close-space sublimation method, and has a good application prospect.

[0109] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should be included in the protection scope of the present invention.

Claims

1. A method for preparing a cadmium zinc telluride epitaxial thick film, characterized in that: The following steps are involved: 1) Growth source and substrate pretreatment: Will Cd 1-x Zn x Te polycrystal is used as a growth source, and a surface pretreatment is performed on the surface of the growth source to remove the oxide layer on the surface of the growth source; wherein x=0.1-0.2; A GaAs substrate with a crystal plane orientation of (100) is used as the substrate, and a surface pretreatment is performed on the substrate to remove an oxide layer on the surface of the substrate; 2) Two-step method for growing CdZnTe thick films: 2.1) The first step of growth: Place the substrate and growth source pretreated in step 1) into a near-space sublimation furnace, turn on the mechanical pump and the vent valve, and automatically connect the molecular pump when the vacuum degree in the chamber is lower than 5 Pa, pump the vacuum degree of the chamber to 0.05 Pa, turn on the water cooling, and then grow. After the growth is completed, cool it to room temperature with the furnace, and turn off the vent valve, molecular pump and water cooling in turn; The process parameters for the first step of growth are: The upper heating stage temperature is set to 520-560°C, the lower heating stage temperature is set to 700-720°C, the heating rate is 1-3°C / S, and the growth time is 10-30min; 2.2) The second step of growth: Open the near-space sublimation furnace, replace the growth source pretreated in the same way, turn on the mechanical pump and the vent valve, automatically connect the molecular pump when the vacuum degree in the cavity is lower than 5Pa, pump the vacuum degree of the cavity to 0.05Pa, turn on the water cooling, and then grow. After the growth is completed, cool it to room temperature with the furnace, turn off the vent valve, the molecular pump and the water cooling in turn, and obtain and take out the cadmium zinc telluride epitaxial thick film; The process parameters for the second step growth are: The upper heating stage temperature is set to 380-420°C, the lower heating stage temperature is set to 700-720°C, the heating rate is 1-3°C / S, and then the growth is 120-140min; 3) Annealing treatment: The cadmium zinc telluride epitaxial thick film obtained in step 2) is peeled off from the substrate and installed at two ends of the annealing bracket respectively with the Te2 annealing source, and then the annealing bracket is placed in a quartz crucible, and the tube is sealed after the vacuum is drawn, and then the whole is placed in a multi-stage temperature-controlled furnace, and the temperature in the furnace is controlled by the multi-stage temperature-controlled furnace to perform gradient annealing, and after the annealing is completed, the furnace is cooled to room temperature; then the Te2 annealing source is replaced, and the annealing process is repeated; according to this process, the epitaxial thick film is subjected to gradient annealing for a total of three times, and finally a cadmium zinc telluride epitaxial thick film that meets the requirements of photon counting imaging is obtained; Among them, the annealing process parameters are: The temperature of the area where the annealing source is located is 400-420°C, the annealing temperature of the area where the CZT epitaxial thick film is located is 360-380°C, the annealing time is 12-14h, and the heating rate is 1-3°C / S.

2. The method for preparing a CdZnTe epitaxial thick film according to claim 1, characterized in that: In step 1), the Cd 1-x Zn x Te polycrystal is grown by Bridgman method, and its pretreatment process is as follows: First, the oxide layer on the surface of the polycrystal is polished with sandpaper, and then the polished polycrystal is ultrasonically cleaned in acetone, anhydrous ethanol and deionized water in turn, and finally dried with nitrogen and placed in a drying oven for use; The pretreatment process of GaAs substrate is: First, polish both sides of the GaAs substrate, then ultrasonically clean it in acetone, anhydrous ethanol and deionized water in sequence, and finally blow dry it with nitrogen for later use.

3. The method for preparing a CdZnTe epitaxial thick film according to claim 2, characterized in that: The Cd 1-x Zn x Te polycrystalline to Cd 0.9 Zn 0.1 Te polycrystalline.

4. The method for preparing a cadmium zinc telluride epitaxial thick film according to any one of claims 1 to 3, characterized in that: In step 2.1), the process parameters for the first step of growth are: The temperature of the upper heating stage was set to 550°C; the temperature of the lower heating stage was set to 710°C; the heating rate was 2°C / S; the program was run and the growth was for 20 minutes.

5. The method for preparing a CdZnTe epitaxial thick film according to claim 4, characterized in that: In step 2.2), the process parameters for the second step of growth are: The temperature of the upper heating stage was set to 380°C, and the temperature of the lower heating stage was set to 710°C; the heating rate was 2°C / S; the program was run and the growth time was 120 minutes.

6. The method for preparing a CdZnTe epitaxial thick film according to claim 5, characterized in that: In step 3), the process parameters of annealing are: The temperature of the annealing source area is 400°C, the annealing temperature of the CZT epitaxial thick film area is 380°C, the annealing time is 14h, and the heating rate is 2°C / S.

7. A cadmium zinc telluride epitaxial thick film, characterized in that: The film is prepared by the preparation method according to any one of claims 1 to 6, and the thickness of the film is 800-900 μm.

8. Use of the cadmium zinc telluride epitaxial thick film prepared by the preparation method according to any one of claims 1 to 6 as a detection material in a photon counting detector.

9. A photon counting detector, characterized in that: The cadmium zinc telluride epitaxial thick film prepared by the preparation method described in any one of claims 1 to 6 is used as a detection material.

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