JPH07107941B2 - Radiation detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a radiation detection apparatus utilizing a photoelectric absorption effect.

(Prior Art) Conventionally, various configurations of radiation detection devices are known.

FIG. 9 shows an example. In the figure, 31 is a W collimator, 32 is a semiconductor radiation detecting element, and 33 is NaI.
(Tl) It is a scintillator. The radiation detecting element 32 is an Au electrode 3 forming a Schottky barrier on _{one side} of the n-type silicon substrate 32 _1.
2 ₂ and an Al electrode 32 ₃ in ohmic contact with the other are formed. A multi-channel radiation detection device is a multi-channel radiation detection device that is configured by arranging multiple stages at right angles to the X-ray incident direction, and
Used for etc. This device is a semiconductor radiation detection element
The X-ray is directly detected by 32, and at the same time, the detection element 32
The scintillator 33 is caused to emit light by the X-rays that have passed through the semiconductor scintillator 33, and the light is detected by the semiconductor detection element 32 as shown by the broken line to increase the sensitivity.

FIG. 10 shows another conventional example. This device comprises a phosphor layer 42, an Al electrode 43, an amorphous Si (a-Si) film 44 on a substrate 41,
The transparent electrode 45 is sequentially laminated. With this device,
Incident X-rays are converted into light by the phosphor layer 42, and this light is converted into a-Si.
An electric signal is obtained by exciting the membrane 44. This construction method is a
The -Si film takes advantage of the fact that it has a low X-ray detection ability but a high sensitivity to light, and is known as a fluorescence sensitization method.

FIG. 11 shows another conventional example. This is intended to detect X-rays only with a-Si, and is the same as that known as a solar cell. That is, n-type a-Si on the substrate 51
Film 51 _1, i-type a-Si film 52 _2, p-type a-Si film 52 ₃ sequentially laminated form is obtained by forming a transparent electrode 53 on its surface.

(Problems to be Solved by the Invention) The conventional radiation detectors shown in FIGS. 9 to 11 have the following problems. In the device shown in FIG. 9, the depletion layer, which is the sensitive portion of the semiconductor radiation detection element 32, has an Au electrode 32 ₂
Since it is formed only on the side, the utilization efficiency of radiation and light is low. Therefore, in order to sufficiently increase the X-ray detection sensitivity, the size of the detection element 32 in the X-ray incident direction must be, for example, several cm or more.

The device shown in FIG. 10 can be manufactured in a large area and at a low cost because it uses an a-Si film, but the emission intensity of the phosphor layer 42 is small and the X-ray detection sensitivity is limited. Also, the emission spectrum of the phosphor layer 42 and the absorption spectrum of the a-Si film 44 must be matched.

The apparatus shown in FIG. 11 absorbs X-rays only with a-Si, but when the a-Si film has a thickness of about 1 μm, most X-rays are transmitted, and the detection sensitivity is very low. Although some in order to achieve a sensitivity requires several cm thickness of i-type a-Si film 52 _2, it is difficult to form this much thickness. Even if an a-Si film having a thickness of several cm can be formed, it is necessary that the carrier lifetime and the mobility are sufficiently large in order to take out internally generated carriers as an electric signal. -Si film cannot obtain such long carrier life and mobility.

It is an object of the present invention to solve the above problems and to provide a radiation detection apparatus which has a simple structure and can be manufactured at low cost, and which has a very high detection sensitivity.

[Structure of the Invention] (Means for Solving Problems) A radiation detection apparatus according to the present invention has a structure in which a plurality of metal films and semiconductor films are alternately laminated. As the metal film, a metal having a large photoelectric absorption effect, preferably an atomic number
A metal of 30 or more is used, and its film thickness is set to be thinner than the electron range of photoelectrons generated by the photoelectric effect upon incidence of radiation.

(Operation) In the radiation detection apparatus according to the present invention, high-energy photoelectrons generated by photoelectric absorption in the metal film upon incidence of radiation are incident on the semiconductor film, and electron-hole pairs are generated in the semiconductor film. The energy of incident radiation is partially extinguished by this photoelectric absorption, but the radiation transmitted through this metal film is
It is much less efficient than metal films, but it produces photoelectrons by photoelectric absorption. Then, the radiation transmitted through the semiconductor film with almost no attenuation is incident on the next metal film, and photoelectrons are again generated by photoelectric absorption, which then enter the next semiconductor film to generate electron-hole pairs. Similarly, by repeating the generation of photoelectrons by photoelectric absorption in a plurality of metal films and the semiconductor film and the generation of carriers in the semiconductor film by the photoelectrons from the metal films, incident radiation is absorbed very efficiently, and electricity is generated. It can be detected as a signal.

In the present invention, photoelectric absorption of a metal is mainly used, and in this sense, it is preferable to use a metal having a large atomic number, particularly a metal having a atomic number of 30 or more. Further, in this case, it is necessary that the photoelectrons generated by photoelectric absorption are incident on the semiconductor film with sufficient energy. From this point, it is essential that the film thickness of the metal film is thinner than the electron range of photoelectrons. It becomes a requirement. Then, by stacking a large number of metal films and semiconductor films alternately, the metal films sequentially function as a radiation absorption-electron emission unit, and incident radiation can be converted into photoelectrons. If one semiconductor layer sandwiched between the metal films has a thickness of, for example, about several μm and is set to have a total thickness of about several mm, each semiconductor film is generated even if the semiconductor film is morphological. The electron-hole pairs can be efficiently collected.

(Example) Hereinafter, the Example of this invention is described.

1 (a) and 1 (b) are a plan view and a cross-sectional view showing an X-ray detector of one embodiment. 11 is an insulating substrate on which the first metal film 13 (13 ₁ , 13 ₂ ) and non-doped a-Si
Film 12 (12 ₁ , 12 ₂ , 12 ₃ ) and second metal film 14 (14 ₁ , 14 ₁ , 14 ₂ )
Are alternately laminated. Specifically, an insulating substrate
A quartz glass substrate is used as 11, a Mo film that makes ohmic contact with the a-Si film 12 is used as the first metal film 13, and a Schottky is used as the second metal film 14 with the a-Si film 12. An Au film that forms a barrier is used. The Mo film and the Au film are formed by the sputtering method or the vacuum deposition method. The a-Si film is SiH ₄ , S
It is formed by a plasma CVD method utilizing glow discharge decomposition of i ₂ H ₆ or the like or an optical CVD method utilizing photoexcitation decomposition. The film thickness of each portion is, for example, 2 μm for the first metal film 13 and the a-Si film 12
Is set to 5 μm and the second metal film 14 is set to 1 μm.

In this detection device, a unit cell is composed of an a-Si film and first and second metal films sandwiching the a-Si film, and a plurality of unit cells are laminated and electrically connected in parallel to each other. Has become.

The operation of the X-ray detection apparatus configured as above will be described below. This X-ray detector makes X-rays incident perpendicularly to the surface.
When X-ray photon flux incident on the second metal film 14 ₂ of the top surface, wherein interacting with the metal. That is, the photoelectric absorption, Compton scattering, Rayleigh scattering and the like occur in the second metal film 14 within _2, this result photoelectrons are generated. The effects of Compton scattering and Rayleigh scattering are negligibly smaller than the photoelectric effect, and are mainly generated by photoelectric absorption.
Photoelectrons, which have almost the same energy as X-ray photon energy, play a leading role. The photoelectrons emitted incident on the a-Si film 12 ₃ from the second metal film 14 _2, and generates electron-hole pairs excites the a-Si film 12 _3. To photoelectrons generated in the second metal film 14 within ₂ is incident on the a-Si film 12 _3, the film thickness of the second metal film 14 ₂ as will be described in detail later the electron range of photoelectrons It must be thinner. X-ray photons lost some energy interact with the second metal film 14 _2, a-Si
The light is transmitted through the film 12 ₃ with almost no loss of its energy and is incident on the next first metal film 13 ₂ . And it generates photoelectrons mainly by a photoelectric absorption as in the first metal film 13 and the second metal film 14 in the ₂ preceding in the _2, this is the next a-Si
It enters the film 12 ₂ generates electron-hole pairs. The same behavior is repeated thereafter. The reason why X-ray photons pass through the a-Si film with almost no interaction is that the atomic number and density of Si are considerably smaller than those of metals. Moreover, the X-ray photon energy is several tens KeV, which is larger than the photon energy number eV of normal light by about four digits or more, so that resonance absorption due to the forbidden band of a-Si does not occur.

Although FIG. 1 shows a structure in which three unit cells are stacked, the incident X-rays can be almost absorbed by stacking more unit cells. Then, by extracting the carriers generated in each a-Si film 12 as a current in parallel in the case of FIG. 1, an electric signal corresponding to the incident X-ray can be obtained.

The theoretical analysis result of the detection sensitivity of the X-ray detection apparatus of this embodiment will be described in detail below.

FIG. 2 is a model diagram used for this theoretical analysis.
The film thickness of the metal film 14 is Wa, the film thickness of the a-Si film 12 is Ws, and the metal film 13 is
The film thickness of Wb is Wb, and N layers of unit cells are laminated by repeating the same film thickness. The attenuation coefficient of X-ray photons, energy absorption coefficient, and electron range at electron energy of 50 KeV in each layer are shown in the table below.

First, the probability that the X-ray photons interact in the second metal film 14 is Pa, the probability that the X-ray photons interact in the first metal film 13 is Pb, and the X-ray photon in the a-Si film 12 is Pb. When but a probability of interacting with _{Ps, Pa = (P 0 -P} 1) + (P 4 -P 5) + ...... + (P 2N-2 -P 2N-1) Ps = (P 1 -P 2 _{) + (P 3 -P 4)} + ...... + (P 2N-1 -P 2N) Pb = (P 2 -P 3) + (P 6 -P 7) + ...... + (P 2N -P 2N + ₁ ) Here, assuming P ₀ = 1, P ₁ = exp {−μt (a) Wa} P ₂ = P ₁ exp {−μt (s) Ws} P ₃ = P ₂ exp {−μt (b) Wb} And can be similarly expressed for P _{4 and} below.
Note that the above-mentioned interaction is mainly photoelectric absorption as described above, and here, too, the effect of only photoelectric absorption is considered.
The effective energy Ef in which photoelectrons generated by the above-mentioned interaction in each layer, that is, photoelectric absorption is used to generate electron-hole pairs in the a-Si film is expressed as follows, where photoemission energy is Ee.

The first term and the second term on the right side of the above equation generate photoelectrons in the second metal 14 and the first metal 13, respectively.
This is energy that is effectively used for generating electron-hole pairs in the -Si film 12, and the third term is energy that is generated by photoelectrons in the a-Si film 12 and consumed by electron-hole pairs. This is obtained as follows. First, the probability that the X-ray photons interact with each other in the second metal 14 and it is photoelectric absorption is Pa · μe (a) / μt (a). Next, photoelectrons generated in this metal 14 are a-Si film.
It loses energy by the time it reaches 12, and on average it has decreased to (1-Wa / Ra) Ee. Explaining the reason with reference to FIG. 3, the photoelectrons generated in the center of the metal 14 due to the incidence of X-ray photons are detected at an angle of about 60 ° when Ee≈50 KeV, and the photoelectron in the metal film 14 is about Wa. Lost about (Wa / Ra) Ee of energy by traveling the distance. Ra is the electron range, and the photoelectron energy becomes zero when traveling this distance. Photoelectrons reaching the boundary between the metal film 14 and the a-Si film 12 are a-
The Si film 12 has a range of Rs {Ee (1-Wa / Ra)}. This is a range function that depends on energy. Therefore, the energy used to generate electron-hole pairs is (1-Wa / Ra) Ee during the distance Ws in which the photoelectrons travel straight at an angle of about 60 ° and travel before colliding with the next metal film 13. 2Ws / Rs {E (1-Wa / Ra)} times. Here, when 2Ws / Rs {E (1-Wa / Ra)} >> 1, the photoelectrons stop in a-Si, and this value is fixed to 1. The effective energy used by the photoelectrons thus generated in the second metal 14 to generate electron-hole pairs in the a-Si film is represented by the first term.

Similarly, a-Si is generated by the photoelectrons generated in the first metal film 13.
The effective energy spent in electron-hole pair production in the film 12 is
It becomes like the second term.

Further, X-ray photons interacting in the a-Si film 12 consume energy of (Ws / Ra) Ee on average to generate electron-hole pairs. Since the probability of photoelectric absorption in the a-Si film is Rs · μe (s) / μt (s), photoelectrons are eventually generated in this a-Si film and consumed for the generation of electron-hole pairs. Energy is as in the third term.

FIG. 4 is a result of obtaining the effective energy Ef in the specific example 1 with respect to the number N of stacks of unit cells by using the above equation of the effective energy. In this Example 1, the second metal film
14 is an Au film, the film thickness is Wa = 1 μm, and the total attenuation coefficient is μ
t (a) = 96.5 / cm, ratio of photoelectric absorption coefficient μe (a) to total attenuation coefficient μt (a) is 14.4 / 16.5, electron range is Ra = 5.11
μm. The first metal film 13 is a Mo film and has a film thickness of Wb.
= 2 μm, total attenuation coefficient μt (b) = 69.7 / cm, ratio of photoelectric absorption coefficient μe (b) and total attenuation coefficient μt (b) is 10.3 / 1
0.9, the electron range is Rb = 7.35 μm. The a-Si film has a film thickness Ws = 5 μm, a total attenuation coefficient μt (s) = 0.985 / cm,
The ratio between the photoelectric absorption coefficient μe (s) and the total attenuation coefficient μt (s) is
9.9 / 19.6, electron range is Rs = 23.85 μm. Comparative Example 1 in FIG. 4 shows the case where only the a-Si film was used to give the same thickness as in Example 1. The incident X-ray photon energy is assumed to be 50 KeV.

As is clear from FIG. 4, according to this example, the photoelectrons generated by photoelectric absorption are effectively used for the generation of electron-hole pairs in the a-Si film, and the photoelectrons are increased by selecting the number of stacked unit cells. X-ray detection sensitivity is obtained. The main reason for the small effective energy in Comparative Example 1 is that both the total attenuation coefficient and photoelectric absorption coefficient in the a-Si film are small, and the probability Ps that X-ray photons interact in the a-Si film is Because it is much smaller than the probabilities Pa and Pb.

FIG. 5 shows the configuration when the same material as in Example 1 is used.
It is the result of obtaining the effective energy Ef in the example in which the film thickness is changed and the number of stacked layers is changed to a large range. That is, in Example 2, the film thickness of the Au film as the second metal film 14 was Wa = 1 μm.
m, the film thickness of the Mo film as the first metal film 13 is Wb = 2 μm,
This is the case where the film thickness of the a-Si film 12 is Ws = 25 μm. Similarly, in Example 3, Wa = 0.5 μm, Wb = 1.0 μm, Ws = 25 μm
That is the case. Comparative Examples 2 and 3 correspond to Examples 2 and 3, respectively, and correspond to the laminated film thickness of a-Si.
This is data when the detection device is composed of only the film.

As is clear from FIG. 5, in Examples 2 and 3, the number of stacked layers N = 1.
When it is 00, half of X-ray photon energy of 5 KeV is more than 29 ~
A large energy of 30 KeV is effectively used to generate electron-hole pairs in the a-Si film. Number of stacks like this
The effective energy at 100 is equal to or higher than that of the image intensifier which is currently used for medical purposes and is considered to have the highest effective energy. Moreover, since the thickness of the a-Si film of the unit cell is 25 μm,
The electron-hole pairs generated in each a-Si film can be taken out as an electric signal with high collection efficiency. The number of stacks is 10
At 0, the total thickness of the a-Si film is 2.5 mm in both Examples 2 and 3. This is a great advantage, for example, considering that an X-ray detector using single crystal Si needs a thickness of several cm in the incident X-ray direction in order to obtain a certain degree of detection sensitivity.

FIG. 6 shows the result of obtaining the effective energy Ef in the same manner as in FIG. 5 when the Ta film is used as the second metal film 14 instead of the Au film in the configuration of FIG. In Example 4, the film thickness of the Ta film as the second metal film 14 was Wa = 1 μm, and the first metal film
The Mo film thickness of 13 is Wb = 2 μm, and the film thickness of the a-Si film 12 is
This is the case when Ws = 25 μm. Similarly, in Example 5, Wa =
This is the case where 0.5 μm, Wb = 1.0 μm, and Ws = 25 μm.
Further, Comparative Examples 4 and 5 are cases in which the detector is constituted only by the a-Si film having a film thickness corresponding to the number of laminated layers of Examples 4 and 5, respectively. Note that the total attenuation coefficient of Ta is μt (a) =
83.8 / cm, photoelectric absorption coefficient μe (a) and total attenuation coefficient μt
The ratio of (a) was 11.6 / 13.4, and the electron range was Ra = 5.06 μm.

Since Ta has a smaller atomic number than Au, the effective energy Ef is smaller than that in the previous embodiment by that amount.
Even in this case, the effectiveness of the present invention is clear.

Here, the optimum value of the film thickness of the metal film in the present invention will be examined. When the film thickness of the metal film is made larger than the electron range of photoelectrons generated here, most of the photoelectrons generated are lost in the metal film and naturally do not contribute to the effective energy Ef. On the other hand, when the number N of stacked layers is fixed and the film thickness of the metal film is too thin, the probability of interaction with the X-ray photons as a whole decreases, and the contribution to the effective energy Ef also decreases. Therefore, there should be an appropriate thickness of the metal film in relation to the number of stacked layers.

FIG. 7 shows the first metal film 13 in the structure of FIG.
A Ta film is used as the Mo film and the second metal film 14, the film thickness of the a-Si film 12 is 25 μm, the film thicknesses of the Mo film and the Ta film are equal to x, this is taken on the horizontal axis, and the number of layers N is used as a parameter Is the result of obtaining the effective energy Ef. As shown in the figure, the effective energy Ef has a peak value with respect to the film thickness of the metal film, and in this calculation example, the optimum metal film thicknesses are 0.9 μm, 1.2 μm and N = 200, 100 and 50, respectively. It is 1.8 μm.

FIG. 8 shows the construction of an X-ray detection apparatus according to another embodiment of the present invention. In this embodiment, the first metal film 22 is formed on the insulating substrate 21.
(22 ₁ to 22 ₃₎ are successively stacked a-Si film 23 (23 ₁ to 23 ₅₎ and the second metal film 24 (24 _{1-24 3)} alternately. The first metal film 22 is, for example, a Mo film, and the second metal film 24 is, for example, an Au film. This basic configuration is the same as that shown in FIG. 1, but the difference from FIG. 1 is that the laminated structure of the unit cells is electrically connected in series. That is, in FIG. 1, the current obtained in each unit cell is taken out in parallel, whereas in this embodiment, the electromotive force generated in the unit cell is added in series and taken out.

It is clear that this embodiment can also obtain the same effect as the previous embodiment.

The present invention is not limited to the above-described embodiments, but can be implemented with various modifications as listed below.

As the substrate (a), in addition to the quartz glass substrate, various other glass substrates or insulating substrates such as alumina substrates and sapphire substrates can be used. It is also possible to use a metal substrate.

(B) As the semiconductor film, other amorphous semiconductors such as Ge, Se, and Ga can be used in addition to the a-Si film. In addition to the amorphous semiconductor, a microcrystalline semiconductor, a polycrystalline semiconductor, or a combination thereof can be used.

Further, the semiconductor film does not necessarily have to be non-doped,
Impurities may be doped. Furthermore, pn structure and pin, pi, n
The i structure can be adopted, or these can be stacked in multiple stages to form a tandem structure.

As the metal film (c), those having an atomic number of 30 or more can be suitably selected and used. In each embodiment, a−
It was selected that one metal film sandwiching the Si film constitutes a Schottky barrier with the a-Si film and the other metal film makes ohmic contact with the a-Si film. This is for reading the signal. On the other hand, as a material that makes ohmic contact with any metal film, a voltage can be applied to read out a current. The metal film between adjacent a-Si films may not be a single layer, but may be a laminated film of a metal film that makes ohmic contact with the a-Si film and a metal film that forms a Schottky barrier. Furthermore, the metal film may include a metal film having a small atomic number, which is merely used as an ohmic electrode.

Furthermore, by doping the metal film with, for example, B, it can be effectively used as a neutron detector.

[Effect of the Invention] As described above, according to the present invention, a highly sensitive radiation detector is realized with a simple structure in which a plurality of metal films and semiconductor films are alternately laminated. For example, as described with reference to numerical examples, by setting the number of laminated layers to about 100, sensitivity equal to or higher than that of the imaging intensifier can be obtained. In addition, the structure is simple, the metal film can be easily formed into a large area by the sputtering method or the vacuum deposition method, and the semiconductor film can be easily formed by the plasma decomposition method.

[Brief description of drawings]

1 (a) and 1 (b) are a plan view and a cross-sectional view showing an X-ray detection device according to an embodiment of the present invention, and FIG. 2 is a model diagram used for theoretically analyzing the operation of the device. 3 is the same as X
Diagram for explaining the effect of photoelectric absorption by line photons,
FIG. 4 is a diagram showing a numerical calculation result of obtaining an effective energy of a concrete embodiment, FIGS. 5 and 6 are diagrams showing a numerical calculation result of obtaining an effective energy of another embodiment, and FIG. FIG. 8 is a diagram showing the result of numerical calculation for obtaining the relationship between the metal film thickness and the effective energy, FIG. 8 is a sectional view showing the radiation detecting device of another embodiment, and FIGS. 9 to 11 are conventional radiation detecting devices. It is a figure which shows the structural example. 11 ... Insulating substrate, 12 ... a-Si film, 13 ... First metal film, 14 ... Second metal film, 21 ... Insulating substrate, 22 ... First metal film, 23 ... a-Si film, 24 ... second metal film.

Claims (10)

[Claims] 1. A radiation detecting apparatus comprising a plurality of metal films and semiconductor films, which are thinner than the electron range of photoelectrons generated by the photoelectric absorption effect, and are alternately laminated. 2. The radiation detecting apparatus according to claim 1, wherein the metal film is made of a metal having an atomic number of 30 or more. 3. The radiation detector according to claim 1, wherein the semiconductor film is made of an amorphous semiconductor. 4. The radiation detecting apparatus according to claim 1, wherein the semiconductor film and the metal film form a Schottky barrier or ohmic contact. 5. The radiation detecting apparatus according to claim 1, wherein the semiconductor film and the metal film sandwiching the semiconductor film form a unit cell, and a plurality of unit cells are stacked and electrically connected in parallel. 6. The radiation detecting apparatus according to claim 1, wherein a unit cell is composed of a semiconductor film and a metal film sandwiching the semiconductor film, and a plurality of unit cells are laminated and electrically connected in series. 7. The radiation detecting apparatus according to claim 3, wherein the amorphous semiconductor is any one of an intrinsic semiconductor, a p-type semiconductor, an n-type semiconductor, or a combination thereof. 8. The radiation detecting apparatus according to claim 3, wherein the amorphous semiconductor is a combination of an amorphous phase and a microcrystalline phase. 9. The radiation according to claim 3, wherein the amorphous semiconductor has a pin structure, a pn structure, a ni structure, a pi structure, or a structure formed by a combination of these structures stacked in multiple stages. Detection device. 10. The radiation detecting apparatus according to claim 1, wherein the metal film has a structure in which different metal layers are laminated.