CdSe single crystal-based heterojunction semiconductor radiation detector and preparation method thereof

Abstract

The invention relates to the technical field of semiconductor radiation detection, in particular to a CdSe single crystal-based heterojunction semiconductor radiation detector and a preparation method thereof. The heterojunction radiation detector comprises a first metal electrode layer, a CdSe single crystal wafer, an amorphous selenium layer and a second metal electrode layer which are sequentially connected from bottom to top. An amorphous selenium layer is deposited on the surface of a cut, polished and cleaned CdSe single crystal through vacuum thermal evaporation, and forms a heterojunction with CdSe; and depositing the first metal electrode layer on the surface of the CdSe layer and the second metal electrode layer on the surface of the amorphous selenium layer by using vacuum thermal evaporation. The heterojunction radiation detector prepared by the invention can keep stable low dark current under high reverse bias voltage, has high signal-to-noise ratio and low detection limit, shows quick response to X-rays, has a good X-ray imaging effect, and has excellent application potential in the field of radiation detection.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor radiation detection, and in particular relates to a CdSe single crystal based heterojunction semiconductor radiation detector and a preparation method thereof. Background Art

[0002] Radiation detectors are devices that convert high-energy ionizing radiation (such as X-rays, gamma rays, etc.) into electrical signals. They have important application scenarios in environmental monitoring, medical imaging, national defense security, industrial non-destructive testing, basic physics, etc. Compared with the currently widely used scintillator-type (indirect) radiation detectors, direct radiation detectors based on suitable semiconductor materials do not require the two-step conversion from high-energy radiation to visible light and then to electrical signals. Instead, they directly convert high-energy ionizing radiation into electrical signal output. Therefore, they have higher conversion efficiency, better energy resolution and spatial resolution, simpler structure, and easier integration. Therefore, they have become a hot research topic at present.

[0003] Currently, there are not many materials suitable for developing semiconductor radiation detectors, mainly CdTe, CdZnTe (CZT), amorphous selenium, single crystal silicon, high purity germanium, and perovskite semiconductors. However, CdTe and CZT have high application costs due to the complexity of large-size single crystal growth process and low yield; amorphous selenium has an inherent low carrier mobility-lifetime product (10 -7 ~10 -6 cm 2 V -1 ) makes its performance of detecting high-energy radiation with thick film poor, and it is not suitable as an independent radiation detection absorption layer; silicon-based materials have poor efficiency in detecting higher-energy radiation due to their low absorption capacity for ionizing radiation; high-purity germanium-based detectors have excellent performance, but due to the extremely narrow bandgap of germanium (~0.6eV), they need to work at extremely low temperatures to reduce leakage current and noise caused by intrinsic excitation; perovskite materials are temporarily difficult to be widely used because they often have to face problems such as poor stability and serious ion migration polarization.

[0004] CdSe single crystal has high radiation absorption coefficient, high carrier mobility lifetime product (10 -5 ~10 -4 cm 2 V -1 ), high resistivity (10 9 ~10 10 Ωcm) and a suitable band gap (~1.7eV) make it one of the candidates for the ideal radiation absorption layer material of direct room temperature radiation detectors. In previous reports, photoconductive radiation detectors based on CdSe single crystals showed a high absorption rate of up to 2.08×10 5 μC Gyair -1 cm -2 Sensitivity [ACS Appl. Mater. Interfaces, 2020, 12, 56126-56134] and as low as 21.49nGy air s -1 detection limit [Appl.Phys.Lett.,2023,123,032102]. At present, high-quality CdSe single crystals grown by the pressure-assisted vertical Bridgman method can reach a large size of tens of centimeters [Opt.Mater.Express,2018,8,1796–1805], laying the foundation for the development and widespread application of CdSe single crystal-based semiconductor radiation detectors. However, the photoconductive radiation detector prepared by the pressure-assisted vertical Bridgman method based on CdSe single crystals shows a severe hysteresis phenomenon of up to more than ten seconds in response to X-rays [Appl.Phys.Lett.,2023,123,032102], which is also one of the reasons that prevent its detection limit from being further reduced at room temperature (21.49nGy air s -1 ), which limits its application in imaging and other fields that require high response speed and detection limit. The slow response of the detector is mainly attributed to the extension of the lifetime of photogenerated carriers caused by the capture-decapture effect in the photoconductive detector. This effect allows uncombined carriers to be injected and cycled through the electrode many times, which on the one hand leads to photoconductive gain and high sensitivity, but on the other hand causes a slow response speed and often exhibits a high leakage current under high bias, limiting its further performance improvement.

[0005] In order to improve the response speed and suppress the leakage current, an effective method is to block the injection of carriers from the electrode by constructing a heterojunction with a rectifying effect. However, there are still few studies on junction semiconductor radiation detectors based on CdSe single crystals. Therefore, in order to further improve the response speed, reduce the detection limit and leakage current, it is of great significance to find suitable materials and processes based on CdSe single crystals to develop a heterojunction detector with a rectifying structure. Summary of the invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the CdSe single crystal based semiconductor radiation detector prepared by the prior art, such as slow response speed, low signal-to-noise ratio and high detection limit, and to provide a CdSe single crystal based heterojunction semiconductor radiation detector.

[0007] To achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a CdSe single crystal based heterojunction type semiconductor radiation detector, which structure comprises from bottom to top: a first metal electrode layer, a CdSe single crystal, an amorphous selenium layer and a second metal electrode layer.

[0008] As a further improvement of CdSe single crystal based heterojunction semiconductor radiation detector:

[0009] Preferably, the horizontal size of the CdSe single crystal wafer is 0.5 mm×0.5 mm to 50 mm×50 mm, and the thickness is 0.5 to 10 mm.

[0010] Preferably, the thickness of the first metal electrode layer is 10 to 500 nm, and the material is one or a combination of two or more of Au, Ag, Pt, Ti, Al, Ni, and In.

[0011] Preferably, the thickness of the amorphous selenium layer is 0.1-100 μm.

[0012] Preferably, the second metal electrode layer has a thickness of 10 to 500 nm, and its material is one or a combination of two or more of Au, Ag, Pt, Ti, Al, Ni, and In.

[0013] The second object of the present invention is to provide a method for preparing the above-mentioned CdSe single crystal based heterojunction semiconductor radiation detector, comprising the following steps:

[0014] S1, polishing and cleaning the CdSe single crystal after cutting to obtain a CdSe single crystal with a smooth surface;

[0015] S2, depositing a first metal electrode layer (1) on a plane of the CdSe single crystal wafer by using a vacuum thermal evaporation method;

[0016] S3. Using vacuum thermal evaporation, an amorphous selenium layer and a second metal electrode layer are sequentially deposited on another plane of the CdSe single crystal away from the first metal electrode layer, thereby obtaining a CdSe single crystal-based heterojunction semiconductor radiation detector.

[0017] As a further improvement of the preparation method of CdSe single crystal based heterojunction semiconductor radiation detector:

[0018] Preferably, in step S1, the CdSe single crystal is oriented about the (001) crystal plane, cut along the crystal plane, and then polished and cleaned to obtain a CdSe single crystal wafer with a smooth surface.

[0019] Preferably, the vacuum thermal evaporation process of depositing the first metal electrode layer and the second metal electrode layer in steps S2 and S3 is performed under a pressure in the deposition chamber that is not higher than 1×10-4 Pa under vacuum conditions, the evaporation rate is The deposition time is 1 to 50 minutes.

[0020] Preferably, in the process of depositing the amorphous selenium layer by vacuum thermal evaporation in step S3, the pressure in the deposition chamber is not higher than 1×10 -4 Pa under vacuum conditions, the evaporation rate is The deposition time is 10 to 100 minutes, and then the film is naturally cooled to room temperature, and a uniform amorphous selenium layer is deposited on the CdSe single crystal wafer.

[0021] Preferably, the preparation method of the evaporation source required for depositing the amorphous selenium layer by vacuum thermal evaporation in step S3 is as follows: using selenium particles with a purity higher than 5N and a particle size of 0.2 to 5 mm, loading the selenium particles into an evaporation boat, and keeping the pressure in the deposition chamber no higher than 1×10 -4 Pa under vacuum conditions until completely melted and then naturally cooled to form a selenium ingot, which is the evaporation source required for depositing an amorphous selenium layer by vacuum thermal evaporation.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1) The present invention provides a CdSe single crystal-based heterojunction semiconductor radiation detector, which structurally comprises a first metal electrode layer, a CdSe single crystal, an amorphous selenium layer and a second metal electrode layer from bottom to top. The first metal electrode layer and the second metal electrode layer serve as the anode and cathode of the detector respectively; the CdSe single crystal serves as the main radiation absorption and electrical signal conversion functional layer; the amorphous selenium layer and the CdSe layer form a heterojunction, which has the function of blocking electrons from being injected from the cathode and extracting holes generated in the CdSe layer.

[0024] The CdSe single crystal based heterojunction semiconductor radiation detector of the present invention has the rectification characteristics of a PN junction, and can significantly suppress the leakage current when working under a relatively high reverse bias. When performing X-ray detection under reverse bias, compared with the CdSe single crystal based semiconductor radiation detector of the traditional structure, it has the advantages of fast response speed, high signal-to-noise ratio and low detection limit, and exhibits good X-ray imaging effect.

[0025] 2) The present invention provides a method for preparing a CdSe single crystal-based heterojunction semiconductor radiation detector. The technical principles underlying each preparation step are as follows:

[0026] The present invention uses a CdSe single crystal cut along the (001) crystal plane as a radiation absorption and signal conversion layer, which can realize efficient conversion of X-rays into electronic signals at room temperature;

[0027] The present invention uses vacuum thermal evaporation to deposit the first metal electrode layer and the second metal electrode layer, which can achieve uniform and dense contact between the metal electrode and the functional layer;

[0028] The present invention adopts vacuum thermal evaporation method to deposit amorphous selenium layer. The evaporation source adopts selenium particles with purity of 5N and particle size of 0.2-5 mm. In order to make the evaporation source heated evenly to ensure the stability of evaporation rate and uniformity of deposition area during deposition, the selenium particles need to be loaded into the evaporation boat first. The pressure in the deposition chamber is not higher than 1×10 -4 Pa vacuum conditions, heated until completely melted and then naturally cooled to form a selenium ingot as the final evaporation source to ensure the stability of the deposition rate and the quality and uniformity of the deposited film;

[0029] The invention prepares a heterojunction structure by depositing an amorphous selenium layer on a CdSe single crystal by a vacuum thermal evaporation method. The method has simple operation, good controllability and high repetition rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the structure of the CdSe single crystal based heterojunction semiconductor radiation detector in Example 1;

[0031] Figure 2 (a)-(c) are respectively the XRD test results of the amorphous selenium layer deposited on the CdSe single crystal in Examples 2, 3, and 4;

[0032] Figure 3 is a current-voltage curve of the CdSe single crystal-based heterojunction semiconductor radiation detector in a dark environment in Example 2;

[0033] Figure 4 This is a current-time curve diagram of the CdSe single crystal-based heterojunction semiconductor radiation detector in Example 2 in response to X-rays;

[0034] Figure 5 is a signal-to-noise ratio-dose rate curve of the CdSe single crystal-based heterojunction semiconductor radiation detector in Example 2;

[0035] Figure 6 (a) is an optical photograph of the object to be imaged, and (b) is the X-ray imaging result of the object by a CdSe single crystal-based heterojunction semiconductor radiation detector.

[0036] The meanings of the symbols in the accompanying drawings are as follows:

[0037] 1. First metal electrode layer; 2. CdSe single crystal; 3. Amorphous selenium layer; 4. Second metal electrode layer. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention.

[0039] Example 1

[0040] This embodiment provides a CdSe single crystal based heterojunction semiconductor radiation detector, the structure of which is as follows: Figure 1 As shown, it includes from bottom to top: a first metal electrode layer 1, a CdSe single crystal crystal 2, an amorphous selenium layer 3 and a second metal electrode layer 4.

[0041] The thickness of the first metal electrode layer 1 is 10-500 nm, and the material is one or a combination of two or more of Au, Ag, Pt, Ti, Al, Ni, and In.

[0042] The horizontal size of the CdSe single crystal crystal 2 is 0.5 mm×0.5 mm to 50 mm×50 mm, and the thickness is 0.5 to 10 mm.

[0043] The thickness of the amorphous selenium layer 3 is 0.1-100 μm.

[0044] The thickness of the second metal electrode layer 4 is 10-500 nm, and the material is one or a combination of two or more of Au, Ag, Pt, Ti, Al, Ni, and In.

[0045] Example 2

[0046] This embodiment provides a method for preparing a CdSe single crystal-based heterojunction semiconductor radiation detector, comprising the following steps:

[0047] S1. Orient the CdSe single crystal grown by the vertical Bridgman method along the (001) crystal plane, cut it into wafers with a horizontal size of 5 mm×5 mm and a thickness of 1 mm along the crystal plane, mechanically polish the surface of the wafer using diamond polishing pastes with particle sizes of 3.5 μm and 0.5 μm, then scrub it with analytically pure acetone and dry it under vacuum conditions to obtain a CdSe wafer with a thickness of 1 mm and a smooth surface as a CdSe single crystal wafer;

[0048] S2. Using vacuum thermal evaporation, deposit the first metal electrode layer on one side of the CdSe single crystal. The pressure in the deposition chamber is not higher than 1×10 -4 Pa under vacuum conditions, the evaporation rate is The deposition time is 8 minutes, the material of the first metal electrode layer is Au, and the thickness is 50 nm;

[0049] S3, use selenium particles with a purity of 6N and a particle size of 2-3mm, load them into the evaporation boat, and the pressure in the deposition chamber is not higher than 1×10 -4 Pa under vacuum conditions until it is completely melted, and then naturally cooled to form a selenium ingot as the final evaporation source.

[0050] The amorphous selenium layer is deposited on the other side of the CdSe single crystal away from the first metal electrode layer by vacuum thermal evaporation. The pressure in the deposition chamber is not higher than 1×10 -4 Pa under vacuum conditions, the evaporation rate is The deposition time was 60 min, and a uniform amorphous selenium layer with a thickness of 2 μm was obtained;

[0051] The second metal electrode layer is deposited on the surface of the amorphous selenium layer by vacuum thermal evaporation. The pressure in the deposition chamber is not higher than 1×10 -4 Pa under vacuum conditions, the evaporation rate is The deposition time is 8 minutes, and an In electrode with a thickness of 50 nm is obtained as the second metal electrode layer, that is, a CdSe single crystal-based heterojunction semiconductor radiation detector is manufactured.

[0052] Example 3

[0053] This embodiment provides a method for preparing a CdSe single crystal-based heterojunction semiconductor radiation detector. The specific steps are referred to in Example 2, except that: in step S3, when the amorphous selenium layer is deposited by vacuum thermal evaporation, the evaporation rate is The deposition time was 60 min and the thickness of the amorphous selenium layer was 1 μm.

[0054] Example 4

[0055] This embodiment provides a method for preparing a CdSe single crystal-based heterojunction semiconductor radiation detector. The specific steps are referred to in Example 2, except that: in step S3, when the amorphous selenium layer is deposited by vacuum thermal evaporation, the evaporation rate is The deposition time was 60 min and the thickness of the amorphous selenium layer was 7 μm.

[0056] Performance Test:

[0057] Figure 2 Figures ac show the XRD test results of the amorphous selenium layer deposited on the CdSe single crystal in Example 2-4. It can be seen that the XRD diffraction peak mainly comes from the CdSe single crystal, and the amorphous selenium layer only has diffuse scattering packets in the range of 15° to 35°, which is consistent with the characteristics of the amorphous structure.

[0058] Figure 3 The current-voltage curve of the CdSe single crystal based heterojunction semiconductor radiation detector prepared in Example 2 in the range of -200 to 200 V. Figure 3 It can be seen that after constructing the CdSe single crystal-amorphous selenium heterojunction in Example 2, its leakage current shows an asymmetric increase with increasing bias under positive and negative bias, and the leakage current under negative bias is significantly smaller than the leakage current under positive bias. This is attributed to the fact that the CdSe single crystal-amorphous selenium heterojunction is essentially a PN junction, composed of N-type CdSe and P-type amorphous selenium. This structure has a rectifying characteristic, and the leakage current under reverse bias is much smaller than the leakage current under forward bias.

[0059] Figure 4 The response speed test results of the CdSe single crystal based heterojunction semiconductor radiation detector prepared in Example 2 to X-rays. The X-ray irradiation dose rate of the detector cathode is 4.66 μGy air s -1 .according to Figure 4 It can be seen that the photocurrent rise time t corresponding to the response of the detector described in Example 1 to the turning on and off of X-rays is r and the fall time t d They are 1.85s and 1.12s respectively, which has a faster response speed.

[0060] Figure 5 The signal-to-noise ratio and detection limit of the CdSe single crystal-based heterojunction semiconductor radiation detector prepared in Example 2 in response to X-rays at different dose rates are shown. Figure 5 It can be seen that the detection limit of the detector for X-ray dose rate in this embodiment is 0.26nGy air s -1 , which is much lower than the 5.5μGy of commercial detectors currently used for medical monitoring air s -1 requirements.

[0061] The CdSe single crystal-based heterojunction semiconductor radiation detector prepared in Example 2 was subjected to an X-ray imaging test, and the X-ray imaging was performed in a single-point scanning manner. During the test, the relative positions of the X-ray tube and the detector were fixed, and the imaging object moved horizontally in the X-axis and Y-axis directions between the X-ray tube and the detector. Imaging was performed by continuously collecting the photocurrent signal generated by the detector. Figure 6 Figure a shows an optical photograph of the object to be imaged, and figure b shows the X-ray imaging test results of the CdSe single crystal based heterojunction semiconductor radiation detector in Example 2. The X-ray dose rate used for imaging is 4.66 μGy air s -1 .according to Figure 6 As can be seen in b, the imaging result of the detector clearly shows the outline of the imaging object, demonstrating the good X-ray imaging capability of the CdSe single crystal-based heterojunction semiconductor radiation detector.

[0062] Those skilled in the art should understand that the above are only some specific embodiments of the present invention, rather than all embodiments. It should be noted that for those of ordinary skill in the art, many modifications and improvements can be made, and all modifications or improvements that do not exceed the scope of protection of the present invention should be regarded as the scope of protection of the present invention.

Claims

1. A CdSe single crystal based heterojunction semiconductor radiation detector, characterized in that: The CdSe single crystal-based heterojunction semiconductor radiation detector structure comprises, from bottom to top, a first metal electrode layer (1), a CdSe single crystal wafer (2), an amorphous selenium layer (3) and a second metal electrode layer (4).

2. The CdSe single crystal based heterojunction semiconductor radiation detector according to claim 1, characterized in that: The CdSe single crystal crystal (2) has a horizontal size of 0.5 mm×0.5 mm to 50 mm×50 mm, and a thickness of 0.5 to 10 mm.

3. The CdSe single crystal based heterojunction semiconductor radiation detector according to claim 1, characterized in that: The thickness of the first metal electrode layer (1) is 10 to 500 nm, and the material is one or a combination of two or more of Au, Ag, Pt, Ti, Al, Ni, and In.

4. The CdSe single crystal based heterojunction semiconductor radiation detector according to claim 1, characterized in that: The thickness of the amorphous selenium layer (3) is 0.1-100 μm.

5. The CdSe single crystal based heterojunction semiconductor radiation detector according to claim 1, characterized in that: The thickness of the second metal electrode layer (4) is 10-500 nm, and the material is one or a combination of two or more of Au, Ag, Pt, Ti, Al, Ni, and In.

6. A method for preparing a CdSe single crystal based heterojunction semiconductor radiation detector according to any one of claims 1 to 5, characterized in that: The steps include: S1, polishing and cleaning the CdSe single crystal after cutting to obtain a CdSe single crystal wafer with a smooth surface (2); S2, using a vacuum thermal evaporation method to deposit a first metal electrode layer (1) on a plane of the CdSe single crystal wafer (2); S3. Using a vacuum thermal evaporation method, an amorphous selenium layer (3) and a second metal electrode layer (4) are sequentially deposited on another plane of the CdSe single crystal wafer (2) away from the first metal electrode layer (1), thereby obtaining a CdSe single crystal-based heterojunction semiconductor radiation detector.

7. The method for preparing a CdSe single crystal based heterojunction semiconductor radiation detector according to claim 6, characterized in that: In step S1, the CdSe single crystal is oriented along the (001) crystal plane, cut along the crystal plane, and then polished and cleaned to obtain a CdSe single crystal wafer (2) with a smooth surface.

8. The method for preparing a CdSe single crystal based heterojunction semiconductor radiation detector according to claim 6, characterized in that: The vacuum thermal evaporation method in steps S2 and S3 for depositing the first metal electrode layer (1) and the second metal electrode layer (4) is performed under a pressure in the deposition chamber not higher than 1×10 -4 Pa under vacuum conditions, the evaporation rate is The deposition time is 1 to 50 minutes.

9. The method for preparing a CdSe single crystal based heterojunction semiconductor radiation detector according to claim 6, characterized in that: In the process of depositing the amorphous selenium layer (3) by vacuum thermal evaporation in step S3, the pressure in the deposition chamber is not higher than 1×10 -4 Pa under vacuum conditions, the evaporation rate is s -1 The deposition time is 10 to 100 minutes, and then the mixture is naturally cooled to room temperature to deposit a uniform amorphous selenium layer (3) on the CdSe single crystal wafer (2).

10. The CdSe single crystal based heterojunction semiconductor radiation detector according to claim 6 or 9, characterized in that: The preparation method of the evaporation source required for depositing the amorphous selenium layer (3) by vacuum thermal evaporation in step S3 is as follows: Selenium particles with a purity higher than 5N and a particle size of 0.2 to 5 mm are used, and the selenium particles are loaded into an evaporation boat, and the pressure in the deposition chamber is not higher than 1×10 -4 Pa under vacuum conditions until completely melted, and then naturally cooled to form a selenium ingot, which is the evaporation source required for depositing an amorphous selenium layer (3) by vacuum thermal evaporation.

Images