Radiation detection element, radiation detector, and production method for radiation detection element

Abstract

Provided are a radiation detection element, a radiation detector, and a production method for the radiation detection element that make it possible to measure the energy of radiation with favorable accuracy. A radiation detection element according to the present invention includes: a compound semiconductor crystal substrate that comprises cadmium zinc telluride to which indium has been added as an impurity; and a metal electrode that is provided on the surface of the compound semiconductor crystal substrate. The energy resolution for gamma rays of 122 keV emitted from 57Co is no more than 7%.

Description

Radiation detection element, radiation detector, and method of manufacturing radiation detection element

[0001] The present invention relates to a radiation detection element, a radiation detector, and a method for manufacturing a radiation detection element.

[0002] Compound semiconductors that offer high efficiency, high resolution, and compact size for use in direct conversion radiation detection elements are being developed. Among these, cadmium zinc telluride (CdZnTe), a II-VI group compound semiconductor, has recently attracted attention as a promising material for use in radiation detection elements. Compared to other semiconductors, CdZnTe has a relatively large atomic number, which gives it a high radiation absorption rate and detection efficiency, and a large band gap energy, which gives it the advantage of low thermal leakage current even at room temperature, eliminating the need for a cooling device (it can operate at room temperature).

[0003] In a radiation detection element using a compound semiconductor crystal substrate made of CdZnTe, it is useful to increase the resistivity of the substrate and form ohmic electrodes on both sides of the substrate to suppress the operating current of the element.

[0004] However, it is difficult to obtain good ohmic characteristics from a compound semiconductor crystal substrate made of CdZnTe, which has a high resistivity, and the operating current increases when a high voltage is applied due to the influence of the bonding condition at the bonding interface between the semiconductor and the electrode. Furthermore, poor ohmic contact affects not only the electrical characteristics but also the radiation spectrum characteristics.

[0005] Patent Document 1 describes a zinc cadmium telluride compound semiconductor crystal with good radiation spectrum characteristics, which is made of zinc cadmium telluride containing 0.2 weight ppm or more and 2.6 weight ppm or less of indium as an impurity, and has a product μτ(e) of the electron mobility μ and the electron lifetime τ of 7.85E-04 cm 2 / V or more, a resistivity of 1.2E+11 Ωcm or more when a voltage of 100 V is applied, and a ratio b / a of the change in leakage current b after 60 seconds from the start of bias voltage application to the leakage current value a at the start of bias voltage application is 56% or less.

[0006] Patent No. 5953116

[0007] One method for forming an ohmic junction on a compound semiconductor crystal substrate made of CdZnTe, which has high resistivity, is to form a Pt electrode by electroless displacement plating. Conventional Pt electrode formation by plating has the problem that even if a device is fabricated from a substrate with good crystallinity, the current-voltage characteristics (hereinafter also referred to as IV characteristics) are poor, and the operating current increases under high voltage application, resulting in poor energy resolution of the radiation spectrum. In such cases, the accuracy of radiation energy measurement becomes poor.

[0008] The embodiments of the present invention have been made to solve the above-mentioned problems, and an object of the present invention is to provide a radiation detection element, a radiation detector, and a method for manufacturing a radiation detection element that are capable of measuring radiation energy with high accuracy.

[0009] The above-mentioned problems are solved by the present invention, which is specified by the following (1) to (11): (1) A compound semiconductor crystal substrate made of zinc cadmium telluride doped with indium as an impurity, comprising: a metal electrode provided on a surface of the compound semiconductor crystal substrate; 57 (2) A radiation detection element having an energy resolution of 7% or less for 122 keV gamma rays emitted from Co. 57 (3) The radiation detection element according to (1), wherein the energy resolution for 122 keV gamma rays emitted from Co is 5% or less. 57 The radiation detection element according to (1) or (2) above, having an energy resolution of 3.4% or less for 122 keV gamma rays emitted from Co. (4) The radiation detection element according to any one of (1) to (3) above, having an ideality factor n of the IV characteristics of 1.0 to 1.5. (5) The radiation detection element according to (1) or (2) above, having a resistivity of 1×10 at an operating voltage of 500 V. 11(6) A radiation detector comprising: the radiation detection element according to any one of (1) to (4), having a resistivity of Ωcm or more. (7) A method for manufacturing a radiation detection element, comprising: a step of cutting a wafer from an ingot of a compound semiconductor crystal containing zinc cadmium telluride doped with indium as an impurity, and polishing the wafer, a step of forming a metal electrode on a surface of the wafer, cutting a radiation detection element from the wafer with the metal electrode formed thereon, and a step of placing the radiation detection element in an annealing furnace and annealing it at 100 to 200°C for 10 minutes to 24 hours. (8) A method for manufacturing a radiation detection element, comprising the steps of: cutting a wafer from an ingot of compound semiconductor crystal containing zinc cadmium telluride doped with indium as an impurity, and polishing the wafer; forming a metal electrode on the surface of the wafer; placing the wafer with the metal electrode formed thereon in an annealing furnace and annealing it at 100 to 200°C for 10 minutes to 24 hours; and cutting a radiation detection element from the annealed wafer. (9) The radiation detection element is 57 (10) The method for manufacturing a radiation detection element according to (7) or (8), wherein the energy resolution for 122 keV gamma rays emitted from Co is 7% or less. (11) In the step of cutting out radiation detection elements from the wafer, 30 radiation detection elements are cut out from one wafer, and after the step of performing the annealing, 57 (11) The method for manufacturing a radiation detection element according to (7), wherein, when the energy resolution of the 30 radiation detection elements is measured under the same conditions for a 122 keV gamma ray emitted from Co, the standard deviation of the energy resolution is 2% or less. 57 The method for manufacturing a radiation detection element according to (8), wherein when the energy resolution for 122 keV gamma rays emitted from Co is measured under the same conditions, the standard deviation of the energy resolution of the 30 radiation detection elements is 2% or less.

[0010] According to the embodiments of the present invention, it is possible to provide a radiation detection element, a radiation detector, and a method for manufacturing a radiation detection element that are capable of measuring radiation energy with high accuracy.

[0011] 1 is a circuit diagram of a radiation detector according to an embodiment of the present invention; FIG. 2 is a perspective view showing a radiation detection element according to an embodiment of the present invention; FIG. 3 is a cross-sectional view taken along III-III in FIG. 2; FIG. 4 is an IV characteristic analysis result using a thermionic emission model of each radiation detection element before and after annealing according to Example 1; FIG. 5 is an IV characteristic analysis result using a thermionic emission model of each radiation detection element before and after annealing according to Example 2; FIG. 6 is an IV characteristic analysis result using a thermionic emission model of each radiation detection element before and after annealing according to Example 3; FIG. 7 is an IV characteristic analysis result using a thermionic emission model of each radiation detection element before and after annealing according to Comparative Example 1; FIG. 8 is an evaluation result of energy resolution according to Test Example 3; FIG. 9 is an evaluation result of the annealing temperature dependency of resistivity according to Test Example 4; FIG. 10 is an evaluation result of the annealing time dependency of resistivity according to Test Example 5; FIG. 11 is an evaluation result of the annealing temperature dependency of μτ value according to Test Example 6; and FIG. 12 is an evaluation result of the annealing time dependency of μτ value according to Test Example 7.

[0012] Next, the embodiments for carrying out the present invention will be described in detail. It should be understood that the present invention is not limited to the following embodiments, and that appropriate design changes, improvements, etc. may be made based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention.

[0013] [Configuration of Radiation Detector] The schematic configuration of a radiation detector according to an embodiment of the present invention will be described. As shown in Fig. 1, the radiation detector 1 of this embodiment is composed of a radiation detection element 2, a capacitor 3, an amplifier (amplifying section) 4, a multi-channel analyzer (MCA) 5, etc. One electrode (common electrode 7) of the radiation detection element 2 is connected to ground (earthed), and the other electrode (pixel electrode 8) is connected to a negative potential, so that a predetermined bias voltage is applied. The other electrode is also connected to the MCA 5 via the capacitor 3 and the amplifier 4.

[0014] The compound semiconductor crystal substrate 6 of the radiation detection element 2 is formed from crystals of cadmium zinc telluride (CdZnTe (CdTe in which part of the Cd is replaced with Zn)), a II-VI group compound semiconductor. When this CdZnTe is exposed to radiation (hard X-rays or gamma rays), it emits electrons, which become an ionization current when exposed to a bias voltage. The radiation detector 1 according to the embodiment of the present invention converts the ionization current of the radiation detection element 2 into a pulse signal via a capacitor 3 and an amplifier 4. The pulse signal is then analyzed by an MCA 5 to obtain the radiation spectrum.

[0015] [Configuration of Radiation Detector] A specific configuration of a radiation detection element according to an embodiment of the present invention will be described. The radiation detection element 2 according to an embodiment of the present invention may be a planar type in which a common electrode 7 is formed on the entire surface of a compound semiconductor crystal substrate 6. Alternatively, as shown in FIG. 2, the radiation detection element 2 according to an embodiment of the present invention may be a pixel type composed of a compound semiconductor crystal substrate 6, a common electrode 7, a pixel electrode 8, and the like. The compound semiconductor crystal substrate 6 is formed in the shape of a thin plate, and the main surface on which the common electrode 7 and the pixel electrode 8 are formed is parallel to a predetermined crystal plane (e.g., the (111) plane). Here, since the

[111] crystal orientation is the polar axis of CdZnTe, when the predetermined crystal plane is the (111) plane, the surface composition on one main surface (hereinafter referred to as "A-plane") 6a of the surface of the compound semiconductor crystal substrate 6 has a high Cd content, and the surface composition on the other main surface (hereinafter referred to as "B-plane") 6b has a high Te content.

[0016] In the pixel-type radiation detection element 2 of Figure 2, the common electrode 7 is formed so as to cover the entire B-side 6b of the compound semiconductor crystal substrate 6. A plurality of pixel electrodes 8 are provided on the A-side 6a of the compound semiconductor crystal substrate 6, and are arranged in a matrix (four columns and four rows in the figure). Both the common electrode 7 and the pixel electrodes 8 are formed as thin films of platinum (Pt), gold (Au), indium (In), other metals, or alloys containing these metals. In other words, there is ohmic contact between the compound semiconductor crystal substrate 6 and the common electrode 7, and between the compound semiconductor crystal substrate 6 and the pixel electrodes 8. Hereinafter, when there is no need to distinguish between the common electrode 7 and the pixel electrodes 8, both electrodes will be referred to collectively as the metal electrodes 7, 8.

[0017] Furthermore, as shown in FIG. 3 , an intermediate layer 62 made mainly of tellurium oxide and a metal such as Pt that constitutes the metal electrode may be formed in the surface layer (here, the A-side side) of compound semiconductor crystal substrate 6, in the region where metal electrodes 7 and 8 (here, pixel electrode 8) are formed on the surface, i.e., between bulk crystal 61 of compound semiconductor crystal substrate 6 and metal electrodes 7 and 8.

[0018] The composition of the compound semiconductor crystal substrate 6 (the composition of the bulk crystal 61 when the compound semiconductor crystal substrate 6 has a bulk crystal 61 and an intermediate layer 62) is not particularly limited as long as it contains cadmium zinc telluride, but may be, for example, one represented by the following formula 1: Formula 1: Cd 1-x Zn x Te (In Formula 1, X is a molar ratio, and X = 0 to 0.1.)

[0019] Furthermore, the compound semiconductor crystal substrate 6 is doped with indium (In) (added as an impurity). The doping with In is carried out to increase the resistivity of the compound semiconductor crystal substrate 6, and the amount of doping can be adjusted appropriately depending on the desired resistivity. From this perspective, the concentration of In in the compound semiconductor crystal substrate 6 can be set to, for example, 0.1 to 2.6 ppm by mass.

[0020] The radiation detection element according to an embodiment of the present invention comprises: 57The energy resolution for 122 keV gamma rays emitted from Co is 7% or less. Energy resolution is an index that represents the accuracy of radiation energy measurement. When radiation of a certain energy loses all of its energy in a detector, the energy spectrum of this radiation will, in principle, become a line spectrum. However, in actual detectors, it is detected as a distribution spectrum with a certain spread. The narrower the width of this distribution, the more accurately the radiation energy distribution can be measured. In the present invention, the energy resolution is calculated by ΔE / E, which is the value obtained by dividing the half-width ΔE of the peak of the distribution spectrum by the radiation energy (peak channel value) E. When the energy resolution of a radiation detection element is 7% or less, the radiation energy distribution can be measured with high accuracy by a radiation detector using this radiation detection element. The radiation detection element according to an embodiment of the present invention has the following features: 57 The energy resolution for 122 keV gamma rays emitted from Co is preferably 5% or less, and more preferably 3.4% or less.

[0021] When manufacturing an ohmic radiation detection element from a high-resistivity CdZnTe substrate, it is difficult to form a good ohmic junction, which can result in an increase in operating current at high voltage, resulting in a deterioration in the radiation performance characteristics. Here, the operating current refers to the current value that flows through the radiation detection element when an external voltage is applied to the element. In order to obtain good radiation performance characteristics, it is preferable to apply a high external voltage and have a small operating current at that time. The radiation detection element according to the embodiment of the present invention has the following features: 57 Since the energy resolution for 122 keV gamma rays emitted from Co is 7% or less, it is possible to measure the energy of radiation with good accuracy.

[0022] The energy resolution of the radiation detection element according to the embodiment of the present invention can be measured as follows. That is, the radiation detector shown in FIG. 1 is constructed for the radiation detection element, and the cobalt ( 57The spectrum of 122 keV gamma rays emitted from Co is measured. The half-width ΔE of the peak and the radiation energy (peak channel value) E are found from the radiation spectrum, and ΔE / E is calculated to obtain the energy resolution (%). The energy resolution of the radiation detection element according to the embodiment of the present invention is preferably the average value of the values ​​obtained by performing the measurement multiple times under the same conditions.

[0023] In the radiation detection element according to the embodiment of the present invention, the ideality factor n of the IV characteristics is preferably 1.0 to 1.5. This configuration improves the state of the junction interface of the radiation detection element, reduces the operating current when a high voltage is applied, and improves the spectral characteristics. The closer the ideality factor n of the IV characteristics is to 1.0, the more ideal the state of the junction interface of the radiation detection element is, and the closer it is to 2.0, the worse the state of the junction interface of the radiation detection element is. The ideality factor n of the IV characteristics of the radiation detection element according to the embodiment of the present invention is more preferably 1.0 to 1.3, and even more preferably 1.0 to 1.2.

[0024] The ideality factor n of the IV characteristics of the radiation detection element in this embodiment of the present invention can be measured as follows. That is, by gradually applying a voltage to the radiation detection element, an IV curve (IV characteristics) is plotted on a graph based on the applied voltage (V) and the resulting current (I). Next, the IV characteristics are analyzed using a thermionic emission model (electron conduction model at the metal (conductor)-semiconductor interface) shown in the following equations (1) and (2).

[0025]

[0026] In the formulas (1) and (2), q is the elementary charge, V is the applied voltage, n is the ideality factor, k is the Boltzmann constant, T is the absolute temperature, S is the element area, A is the Richardson constant, and φ is the B : energy barrier height. The ideality factor n of the IV characteristics is calculated using the formulas (1) and (2).

[0027] The radiation detection element according to the embodiment of the present invention has a resistivity of 1×10 at an operating voltage of 500 V. 11It is preferable that the resistivity is 1×10 Ωcm or more. Since the resistivity is determined by the current value relative to the applied voltage, the resistivity generally decreases as the operating current increases. Therefore, a high resistivity means that the operating current is constant (does not increase). If the resistivity of the radiation detection element at an operating voltage of 500 V is 1×10 11 When the resistivity is 3×10 Ωcm or more, the operating current does not increase even under high voltage application, and the IV characteristics of the radiation detection element become better. 11 It is more preferable that the resistivity is Ωcm or more.

[0028] The resistivity (ρ) of the radiation detection element according to the embodiment of the present invention at an operating voltage of 500 V can be calculated by calculating the resistance (R) using the current value (I) and voltage value (V) when an operating voltage of 500 V is applied to the radiation detection element, using the formula R = V / I, and then calculating ρ from the cross-sectional area (S) and thickness (L) of the element using the formula ρ = R × S / L.

[0029] [Manufacturing Method of Radiation Detector Element] Next, a method for manufacturing a radiation detector element according to an embodiment of the present invention will be described. First, a single crystal ingot (a compound semiconductor crystal ingot) made of cadmium zinc telluride doped with indium as an impurity is cut along a predetermined crystal plane (e.g., the (111) crystal plane) to obtain a thin, disk-shaped wafer (a compound semiconductor crystal substrate). The single crystal ingot made of cadmium zinc telluride can be manufactured by a vertical gradient freezing (VGF) method as described in Patent Document 1. A crucible filled with cadmium, zinc, and tellurium is placed in a vacuum-sealed quartz ampoule. A reservoir for filling cadmium can also be provided at the bottom of the quartz ampoule so that the vapor pressure of cadmium is applied to the quartz ampoule during single crystal growth. Then, while controlling the position of the crucible filled with the raw material and the reservoir filled with Cd at predetermined temperatures, the raw material for the single crystal is melted and Cd is volatilized from the reservoir, and a single crystal made of zinc cadmium telluride can be grown while controlling the Cd vapor pressure. After the single crystal growth is completed, ingot annealing can be performed at 940 to 960°C for approximately 20 hours. Next, the cut surfaces of the cut wafers are physically mirror-polished using an abrasive such as alumina powder. The polishing process may be repeated multiple times.

[0030] Next, if the radiation detection element is a planar type, a metal electrode formation process by plating, as described below, is carried out. On the other hand, if the radiation detection element is a pixel type, after the polishing process described above, a resist film patterned into a predetermined shape is formed on the wafer in areas other than the planned electrode formation areas. Specifically, first, the wafer is immersed in methanol and ultrasonically cleaned at room temperature for a predetermined time to remove foreign matter adhering to the wafer. Next, photoresist is applied to the surface of the wafer, and the photoresist is exposed using a photomask with a pixel electrode pattern. Next, the exposed photoresist is removed by development. Next, the wafer is immersed in an etching solution such as methanol mixed with bromine (bromethane solution), and the polished surface of the wafer is etched at room temperature for a predetermined time to remove the process-affected layer from the wafer surface. Next, the etching solution is removed from the wafer using methanol or the like, and the methanol or the like is removed from the wafer using pure water.

[0031] Next, the wafer is immersed in a plating solution such as a solution of chloroplatinic acid (IV) hexahydrate mixed with hydrochloric acid to form a metal electrode (Pt electrode) in the area where the electrode portion is to be formed (metal electrode formation process). If an Au electrode, an In electrode, or the like is to be formed instead of a Pt electrode, an appropriate plating solution is prepared accordingly. After the metal electrode is formed, the wafer is left as is if the radiation detection element is a planar type, or after removing unnecessary photoresist if the radiation detection element is a pixel type, the wafer is washed with pure water. Next, the wafer and metal electrode are dried by spraying an inert gas such as nitrogen gas onto them.

[0032] Next, the wafer with the metal electrode formed on the polished surface is cut out and divided into a plurality of substrates, and individual radiation detection elements are cut out from the wafer. Through the above steps, a radiation detection element is manufactured, in which a metal electrode is formed on the surface of a compound semiconductor crystal substrate made of zinc cadmium telluride doped with indium as an impurity. At this time, an intermediate layer composed mainly of tellurium oxide and a metal such as Pt that constitutes the metal electrode is formed on the surface of the compound semiconductor crystal substrate, i.e., between the bulk crystal of the compound semiconductor crystal substrate and the metal electrode, as shown in FIG. 3 .

[0033] Next, the radiation detection element is placed in an annealing furnace and annealed at 100 to 200°C for 10 minutes to 24 hours. The annealing temperature is preferably 110 to 190°C, more preferably 115 to 185°C, even more preferably 120 to 180°C, even more preferably 125 to 175°C, even more preferably 130 to 170°C, even more preferably 135 to 165°C, even more preferably 140 to 160°C, and even more preferably 150°C. The annealing atmosphere is preferably an inert gas atmosphere such as N2. By annealing the radiation detection element under these conditions, the state of the junction interface is improved, the operating current of the element is reduced without deteriorating the μτ value (the product of mobility and carrier lifetime), and the operating characteristics (energy resolution) for radiation can be improved. In this way, a radiation detection element according to an embodiment of the present invention is obtained. In the above-described embodiment, the wafer having the metal electrodes formed on the polished surface is cut out and divided into a plurality of substrates, and the individual radiation detection elements are cut out from the wafer, and then the radiation detection elements are annealed, but this is not limiting. That is, the wafer having the metal electrodes formed on the polished surface may be annealed, and then the radiation detection elements may be cut out from the annealed wafer.

[0034] The radiation detection element manufactured according to the embodiment of the present invention, as described above, 57The energy resolution for 122 keV gamma rays emitted from Co is 7% or less. In addition, in the process of cutting out radiation detection elements from a wafer, 30 radiation detection elements are cut out from one wafer, and after the process of annealing, 57 When the energy resolution for 122 keV gamma rays emitted from Co is measured under the same conditions, the standard deviation of the energy resolution of 30 radiation detection elements is preferably 2% or less. This configuration allows multiple radiation detection elements with well-suppressed variations in energy resolution to be obtained from a single wafer, improving manufacturing efficiency. Here, the "standard deviation of the energy resolution of 30 radiation detection elements" is the positive square root of the sum of the squares of the differences between the target data value and the average value of the 30 data, divided by the total number of data (30). The standard deviation of the energy resolution of the 30 radiation detection elements is more preferably 1.5% or less, and even more preferably 1% or less. The standard deviation of the energy resolution of the radiation detection elements described above is also affected by conditions such as the growth conditions of the compound semiconductor crystal, the ingot annealing after ingot production, and the plating conditions in the process of forming the metal electrodes of the elements. However, in embodiments of the present invention, it is particularly important to optimize the annealing conditions after electrode formation, thereby controlling the standard deviation of the energy resolution.

[0035] The following examples are provided to provide a better understanding of the present invention and its advantages, but the present invention is not limited to these examples.

[0036] (Test Example 1: Fabrication of Radiation Detector) A single crystal of cadmium zinc telluride (Cd 0.965 Zn 0.035Thin disk-shaped wafers (compound semiconductor crystal substrates) were cut from an ingot of CdTe along a predetermined crystal plane (111). The growth method for the single crystal of cadmium zinc telluride was performed by the vertical gradient freeze (VGF) method described in the aforementioned Patent Document 1. Specifically, a crucible filled with Cd, Zn, and Te was placed in a vacuum-sealed quartz ampoule, and a reservoir for filling Cd was provided at the bottom of the quartz ampoule so that Cd vapor pressure was applied to the quartz ampoule during single crystal growth. Then, while controlling the position of the crucible filled with the raw materials and the reservoir filled with Cd at predetermined temperatures, the single crystal raw materials were melted and Cd was volatilized from the reservoir, and a single crystal of cadmium zinc telluride was grown while controlling the Cd vapor pressure. After the single crystal growth was completed, the ingot was annealed at 945°C for 20 hours. Next, the cut surface of the wafer was physically mirror-polished using alumina powder as an abrasive. Next, the wafer was immersed in a plating solution containing a mixture of chloroplatinic acid (IV) hexahydrate and hydrochloric acid to form a Pt electrode on the entire surface of one of the wafer's surfaces (the polished surface), and then the wafer was washed with pure water. The wafer and Pt electrode were then dried by spraying nitrogen gas onto them. The wafer measured approximately 50 mm x 50 mm x 1.4 mm in length, width, and thickness. The wafer with the Pt electrode formed thereon was then cut into multiple substrates each measuring 4 mm x 4 mm x 1.4 mm in length, width, and thickness, and individual radiation detection elements were cut from the wafer. Through the above steps, a radiation detection element was manufactured in which a metal electrode was formed on the surface of a compound semiconductor crystal substrate made of zinc cadmium telluride doped with indium as an impurity. At this time, an intermediate layer mainly made of tellurium oxide was formed between the bulk crystal of the compound semiconductor crystal substrate and the metal electrode.

[0037] (Test Example 2: Evaluation of I-V Characteristics) Four radiation detection elements manufactured in Test Example 1 were taken and designated Examples 1 to 3 and Comparative Example 1. Next, each was placed in an annealing furnace and annealed (electrode annealing) under the following conditions. The annealing atmosphere was N2 flow gas at a flow rate of 10 L / min. Example 1 (150°C, 24 hours), Example 2 (150°C, 30 minutes), Example 3 (150°C, 10 minutes), Comparative Example 1 (300°C, 24 hours). Next, a voltage was applied to each radiation detection element and the current was measured. Furthermore, a voltage was applied to each radiation detection element before annealing, and the current was measured. Here, by gradually applying a voltage to the radiation detection element, an I-V curve (I-V characteristics) was plotted on a graph based on the applied voltage (V) and the resulting current (I). Next, the I-V characteristics were analyzed using the thermionic emission model (electron conduction model at the metal (conductor)-semiconductor interface) shown in the following equations (1) and (2).

[0038]

[0039] In the formulas (1) and (2), q is the elementary charge, V is the applied voltage, n is the ideality factor, k is the Boltzmann constant, T is the absolute temperature, S is the element area, A is the Richardson constant, and φ is the B : energy barrier height. The ideality factor n of the IV characteristics is calculated using equations (1) and (2). The IV characteristics analysis results obtained in this manner using the thermionic emission model for each radiation detection element before and after annealing are shown in Figure 4 (Example 1), Figure 5 (Example 2), Figure 6 (Example 3), and Figure 7 (Comparative Example 1). For Examples 1 to 3, the ideality factor n of the IV characteristics was 1.4. Therefore, the junction interface state of the radiation detection elements of Examples 1 to 3 is good, and it is expected that the operating current during high voltage application will be reduced and the spectral characteristics will be improved. On the other hand, for Comparative Example 1, the ideality factor n of the IV characteristics could not be measured.

[0040] (Test Example 3: Evaluation of energy resolution) A charge amplifier (manufactured by Clear Pulse, 581 type), a waveform shaping amplifier (manufactured by Clear Pulse, 4417 type), and an MCA (manufactured by Clear Pulse, ADC 1125 type) were connected to the radiation detection element manufactured in Test Example 1 to configure the radiation detector shown in FIG. 1. A bias voltage of 500 V was applied to the radiation detector using a bias power supply (manufactured by Clear Pulse, 6671PN type). 57 The spectrum of 122 keV gamma rays emitted from Co was measured. The peak half-width ΔE and the radiation energy (peak channel value) E were determined from the radiation spectrum, and the energy resolution (%) was calculated by calculating ΔE / E. Furthermore, for 30 radiation detection elements manufactured in Test Example 1, the energy resolution (%) of each element was evaluated using the above procedure, and the average value and standard deviation were calculated. Note that the values ​​for samples for which spectrum measurement was not possible were excluded from the calculation. Furthermore, these 30 radiation detection elements were each placed in an annealing furnace and annealed (electrode annealing) at 150°C for 1 hour. The annealing atmosphere was N2 flow gas at a flow rate of 10 L / min. The energy resolution (%) of the 30 radiation detection elements after annealing was evaluated under the same conditions as above. The average value and standard deviation of the energy resolution (%) of these 30 radiation detection elements were calculated. Note that the values ​​for samples for which spectrum measurement was not possible were excluded from the calculation. The evaluation results are shown in Table 1 and FIG. 8. As shown in Table 1 and FIG. 8, the radiation detection element after electrode annealing at 150° C. for 1 hour had the following characteristics: 57The average energy resolution for 122 keV gamma rays emitted from Co was 6.1%. The standard deviation of the energy resolution for 30 radiation detection elements was 1.26%. Therefore, multiple radiation detection elements with well-controlled energy resolution variations could be obtained from a single wafer, improving manufacturing efficiency. For Samples No. 20 (before annealing) and No. 26 (before annealing), the interface between the metal electrode and the wafer was in poor condition, resulting in an abnormal increase in operating current and preventing normal spectrum measurement. It is believed that the interface condition improved after annealing, making it possible to measure the spectrum. For Sample No. 30 (before and after annealing), the crystalline characteristics of the wafer region were poor, making it impossible to measure the spectrum.

[0041]

[0042] Test Example 4: Evaluation of Annealing Temperature Dependence of Resistivity A plurality of radiation detection elements manufactured in Test Example 1 were taken and placed in an annealing furnace. They were annealed (electrode annealed) at 100°C for 24 hours, 150°C for 24 hours, 200°C for 24 hours, 250°C for 24 hours, and 300°C for 24 hours. The annealing atmosphere was N2 flow gas at a flow rate of 10 L / min. Next, the resistivity (ρ) at an operating voltage of 500 V was measured for each radiation detection element before annealing and after the above-mentioned annealing. Specifically, the current value (I) and voltage value (V) were measured when an operating voltage of 500 V was applied to the radiation detection element, and the resistance value (R) was calculated using the formula R = V / I. The resistance value was also calculated using the formula ρ = R × S / L, where S is the cross-sectional area of ​​the element (4 mm × 4 mm) and L is the thickness (L) of the element (1.4 mm). The evaluation results are shown in Table 2 and FIG. 9. According to Table 2 and FIG. 9, the resistivity of the sample annealed at 150° C. for 24 hours and at 200° C. for 24 hours was 1×10 11 It can also be seen that the resistivity increased by low-temperature annealing at 200° C. or less, and that the resistivity decreased compared to before annealing by annealing at 250° C. or more.

[0043]

[0044] (Test Example 5: Evaluation of Annealing Time Dependence of Resistivity) Several radiation detection elements manufactured in Test Example 1 were taken and placed in an annealing furnace. They were annealed (electrode annealed) at 150°C for 30 minutes, 1 hour, 6 hours, and 24 hours, respectively. The annealing atmosphere was N2 flow gas at a flow rate of 10 L / min. Next, the resistivity (ρ) at an operating voltage of 500 V was measured for each radiation detection element before annealing and after the above-mentioned annealing. Specifically, the current value (I) and voltage value (V) were measured when an operating voltage of 500 V was applied to the radiation detection element, and the resistance value (R) was calculated using the formula R = V / I. The resistance value was also calculated using the formula ρ = R × S / L, where S is the cross-sectional area of ​​the element: 4 mm × 4 mm, and L is the thickness: 1.4 mm. The evaluation results are shown in Table 3 and FIG. 10. According to Table 3 and FIG. 10, the samples annealed at 150° C. for 30 minutes, 1 hour, 6 hours, and 24 hours had a resistivity of 1×10 at an operating voltage of 500 V. 11 It can also be seen that the resistivity increases and tends to saturate after annealing for a short period of time, such as 30 minutes.

[0045]

[0046] (Test Example 6: Evaluation of Annealing Temperature Dependence of μτ Value) A plurality of radiation detection elements manufactured in Test Example 1 were taken and placed in an annealing furnace, and annealed (electrode annealing) was carried out at 100°C for 24 hours, 150°C for 24 hours, 200°C for 24 hours, and 250°C for 24 hours, respectively. The annealing atmosphere was N2 flow gas with a flow rate of 10 L / min. Next, the radiation detectors shown in FIG. 1 were constructed for each of the radiation detection elements before annealing and after the above-mentioned annealing, and the cobalt ( 57 The spectrum of 122 keV gamma rays emitted from Co was measured. Next, the peak channel (peak position) of the spectrum for each bias voltage was determined, the bias voltage dependency of the peak channel was graphed, and the correlation of the peak channel was fitted with the model formula "Hecht's formula" shown in the following formula (3) (μτ is a feedback parameter). In formula (3), CHpeak : Peak channel, V out : signal voltage, V bias : bias voltage, D: element thickness.

[0047]

[0048] The evaluation results are shown in Table 4 and Fig. 11. Table 4 and Fig. 11 show that the μτ value hardly changes before and after annealing (no change up to an annealing temperature of 250°C). Therefore, it is expected that the operating current of the device can be reduced without deteriorating the μτ value (product of mobility and carrier lifetime), and that the operating characteristics (energy resolution) against radiation can be improved.

[0049]

[0050] (Test Example 7: Evaluation of Annealing Time Dependence of μτ Value) A plurality of radiation detection elements manufactured in Test Example 1 were taken and placed in an annealing furnace, and annealed (electrode annealing) was carried out at 150°C for 30 minutes, 1 hour, 6 hours, and 24 hours, respectively. The annealing atmosphere was N2 flow gas with a flow rate of 10 L / min. Next, the radiation detectors shown in Figure 1 were constructed for each of the radiation detection elements before annealing and after the above-mentioned annealing, and the cobalt ( 57 The spectrum of 122 keV gamma rays emitted from Co was measured. Next, the peak channel (peak position) of the spectrum for each bias voltage was determined, the bias voltage dependence of the peak channel was graphed, and the correlation of the peak channel was fitted using the model formula "Hecht's formula" shown in the above formula (3) (μτ is the feedback parameter). The evaluation results are shown in Table 5 and FIG. 12. Table 5 and FIG. 12 show that the μτ value hardly changes at least until 24 hours of annealing. Therefore, it is expected that the operating current of the device can be reduced without deteriorating the μτ value (the product of mobility and carrier lifetime), and that the operating characteristics (energy resolution) against radiation can be improved.

[0051]

[0052] REFERENCE SIGNS LIST 1 Radiation detector 2 Radiation detection element 3 Capacitor 4 Amplifier (amplifying section) 5 Multi-channel analyzer 6 Compound semiconductor crystal substrate 6a Main surface (A surface) 6b Main surface (B surface) 7 Common electrode (metal electrode) 8 Pixel electrode (metal electrode) 61 Bulk crystal 62 Intermediate layer

Claims

1. A compound semiconductor crystal substrate composed of cadmium zinc telluride with indium added as an impurity, and a metal electrode provided on the surface of the compound semiconductor crystal substrate, and 57 A radiation detection element having an energy resolution of 7% or less with respect to 122 keV gamma rays emitted from Co.

2. The 57 radiation detection element according to claim 1, wherein the energy resolution for 122 keV gamma rays emitted from Co is 5% or less.

3. The 57 radiation detection element according to claim 1, having an energy resolution of 3.4% or less with respect to 122 keV gamma rays emitted from Co.

4. The radiation detection element according to claim 1, wherein the ideality factor n of the IV characteristics is 1.0 to 1.

5.

5. The resistivity at an operating voltage of 500 V is 1 × 10 11 Ω·cm or more, and the radiation detection element according to claim 1.

6. A radiation detector comprising: the radiation detection element according to any one of claims 1 to 5; and an amplification unit connected to the radiation detection element and amplifying an electrical signal output from the radiation detection element.

7. A method for manufacturing a radiation detection element, comprising: a step of cutting out a wafer from an ingot of a compound semiconductor crystal containing cadmium zinc telluride added with indium as an impurity and performing polishing; a step of forming a metal electrode on the surface of the wafer; a step of cutting out a radiation detection element from the wafer on which the metal electrode is formed; and a step of putting the radiation detection element into an annealing furnace and performing annealing at 100 to 200 ° C for 10 minutes to 24 hours.

8. A method for manufacturing a radiation detection element, comprising: a step of cutting out a wafer from an ingot of a compound semiconductor crystal containing cadmium zinc telluride added with indium as an impurity and performing polishing; a step of forming a metal electrode on the surface of the wafer; a step of putting the wafer on which the metal electrode is formed into an annealing furnace and performing annealing at 100 to 200 ° C for 10 minutes to 24 hours; and a step of cutting out a radiation detection element from the annealed wafer.

9. The radiation detection element is 57 The method for manufacturing a radiation detection element according to claim 7 or 8, having an energy resolution of 7% or less with respect to 122 keV gamma rays emitted from Co.

10. In the step of cutting out the radiation detection element from the wafer, 30 radiation detection elements are cut out from one wafer, and after the annealing step, 57 The manufacturing method of a radiation detection element according to claim 7, wherein a standard deviation of the energy resolution of the 30 radiation detection elements is 2% or less when the energy resolution with respect to 122 keV gamma rays emitted from Co is measured under the same conditions.

11. In the step of cutting out the radiation detection element from the wafer, 30 of the radiation detection elements are cut out from one wafer, 57 The method for manufacturing a radiation detection element according to claim 8, wherein a standard deviation of the energy resolution of the 30 radiation detection elements is 2% or less when the energy resolution with respect to 122 keV gamma rays emitted from Co is measured under the same conditions.

Images