JP5739763B2 - Photoconductive element and imaging device - Google Patents

Description

  The present invention relates to a photoconductive element and an imaging device capable of obtaining a high-quality image with high sensitivity and high resolution and high S / N.

Conventionally, it is known that when a high electric field of about 1 × 10 ⁸ [V / m] or more is applied to a photoconductive layer composed of a selenium-based amorphous semiconductor, an avalanche multiplication phenomenon occurs inside. A highly sensitive imaging tube using this phenomenon has been proposed (see, for example, Non-Patent Document 1).

In order to generate such an avalanche multiplication phenomenon, it is necessary to apply a high electric field of about 5 × 10 ⁷ [V / m] or more to the photoconductive layer. It is necessary to suppress the dark current by preventing the injection of charges (holes) into the photoconductive layer.

As a technique for preventing such charge (hole) injection, for example, a cerium oxide (CeO ₂ ) thin film is blocked between the positive electrode and the selenium-based amorphous semiconductor (photoconductive layer). A method of providing a layer is known (see, for example, Patent Document 1).

JP 09-050771 A

Proceedings of the annual conference of the Institute of Television Engineers of Japan, 33-34, 1989

However, even if a cerium oxide (CeO ₂ ) thin film is provided as the hole injection blocking layer as described above, depending on the film quality of the hole injection blocking layer and the strength of the applied electric field, the selenium-based amorphous material In some cases, injection of charge into the semiconductor (photoconductive layer) cannot be effectively suppressed, and dark current may be generated due to charge injection that has passed through the hole injection blocking layer.

  In general, in a high-sensitivity image pickup tube using the avalanche multiplication phenomenon, a higher multiplication factor (sensitivity) can be obtained as the electric field applied to the photoconductive layer is higher. The dark current due to the avalanche multiplication may also cause image quality deterioration such as S / N reduction.

  For this reason, there is a limit to the intensity of the electric field that can be applied to the photoconductive layer, and there is a problem that the maximum value of the multiplication factor of the high-sensitivity image pickup tube is limited by the limit of the electric field intensity.

  Accordingly, an object of the present invention is to provide a photoconductive element and an imaging device including a hole injection blocking layer that can suppress dark current and obtain a high-quality image with high sensitivity and high resolution and high S / N. .

The photoconductive element according to one aspect of the present invention includes a translucent substrate, a conductive film formed on the translucent substrate, a hole injection blocking layer formed on the conductive film, and the hole. The hole injection block comprising a photoconductive layer formed on an injection block layer and composed of an amorphous semiconductor layer mainly composed of selenium and causing an avalanche multiplication phenomenon upon application of a predetermined high voltage. The layer is made of gallium oxide and suppresses injection of holes from the conductive film to the photoconductive layer .

  An imaging device according to an aspect of the present invention includes: the photoconductive element; an electron beam source that emits a scanning electron beam to the photoconductive element; and the photoconductive element that is electrically connected to the scanning device. And a signal reading unit for reading an imaging signal obtained by the above.

  According to the present invention, it is possible to provide a photoconductive element and an imaging device including a hole injection blocking layer that can suppress a dark current and obtain a high-sensitivity image with high sensitivity, high resolution, and high S / N. can get.

1 is a diagram illustrating a configuration of an imaging device according to a first embodiment. 3 is a cross-sectional view illustrating in detail a configuration of a photoconductive portion 13 of the imaging device according to Embodiment 1. FIG. It is a figure which shows the characteristic of the signal current and dark current of the imaging device of Embodiment 1, and the characteristic of the signal current and dark current of the conventional imaging device. 6 is a diagram illustrating a cross-sectional structure of an imaging device according to Embodiment 2. FIG.

  Hereinafter, embodiments to which the photoconductive element and the imaging device of the present invention are applied will be described.

[Embodiment 1]
1A and 1B are diagrams illustrating a configuration of an imaging device according to Embodiment 1, in which FIG. 1A is a side cross-sectional view, and FIG. 1B is a diagram illustrating the imaging device in a cross-sectional view taken along line AA in FIG.

  As shown in FIG. 1A, the imaging device 100 according to the first embodiment includes a photoconductive element 10, an indium ring 20, a glass tube 30, an electron beam source 40, a deflection device 50, a mesh electrode 60, and a signal readout device. 70.

  The photoconductive element 10 includes a translucent substrate 11, a conductive film 12, a photoconductive portion 13, and a metal pin 14. The translucent substrate 11 is a transparent substrate that transmits the reflected light obtained from the subject and guides it to the photoconductive portion 13, and is composed of, for example, a disc-shaped substrate made of translucent glass and having a diameter of 18 mm. . A conductive film 12 having a diameter of 14 mm and a thickness of 30 nm mainly composed of indium oxide is formed on one surface of the translucent substrate 11 by a sputter deposition method.

  A photoconductive portion 13 is formed on the lower surface of the conductive film 12 in the drawing. The metal pin 14 is fitted into a through-hole penetrating the translucent substrate 11, one end is electrically connected to the conductive film 12, and the other end is connected to the signal readout device 70.

  The photoconductive portion 13 becomes a target of an electron beam emitted from the electron beam source 40, and causes an avalanche multiplication phenomenon inside by applying a high electric field. The photoconductive portion 13 has a circular shape in plan view, and has a scanning surface 13A scanned by the electron beam 41, and an incident surface 13B through which light enters from the upper direction in the drawing through the translucent substrate 11 and the conductive film 12. . The incident surface 13 </ b> B is a boundary surface between the photoconductive portion 13 and the conductive film 12.

  Further, as shown in FIG. 1B, the photoconductive portion 13 has a scanning region 13C at the center of the scanning surface 13A facing the electron beam source 40. The scanning region 13C is a region where scanning for taking out an imaging signal is performed by the electron beam 41 deflected and focused by the deflection coil and the focusing coil. The metal pin 14 is disposed at a position outside the scanning region 13C in plan view. The detailed configuration of the photoconductive portion 13 will be described later with reference to FIG.

  The indium ring 20 seals between the translucent substrate 11 and the glass tube 30, and fixes the photoconductive element 10 to the glass tube 30.

  The glass tube 30 is a glass tubular member whose one end is sealed by the photoconductive element 10 and the indium ring 20, and the electron beam source 40 is formed inside the glass tube 30 from the back to the opening. The deflection device 50 and the mesh electrode 60 are disposed.

  The glass tube 30 is sealed (sealed) with the translucent substrate 11 by the indium ring 20 in order to realize scanning with an electron beam, so that the internal space is maintained in a vacuum.

  The electron beam source 40 is an electron emission source disposed in the inner part of the glass tube 30 and is an apparatus that excites an electron cloud by heating a cathode material with a heater. Electrons excited as an electron cloud are emitted as an electron beam 41.

  The deflection device 50 includes a first grid electrode 51, a second grid electrode 52, and a third grid electrode 53, and deflects the electron beam 41. The first grid electrode 51, the second grid electrode 52, and the third grid electrode 53 are electrodes for extracting and accelerating electrons excited by the electron beam source 40 as electron beams 41 in the direction of the mesh electrode 60.

  The mesh electrode 60 is provided in front of the photoconductive portion 13 when viewed from the electron beam source 40, and is a deceleration for reducing the speed of electrons that collide with the photoconductive portion 13 that is the target of the scanning electron beam 41. It is an electrode that generates an electric field.

  The signal readout device 70 is a device for reading out the electric charge generated in the photoconductive portion 13 as an imaging signal, and includes a power source 71 for applying a voltage to the photoconductive portion 13 and a readout portion 72 for reading out the imaging signal. Have

  A positive terminal of the power supply 71 is connected to the photoconductive portion 13 via the metal pin 14 and also connected to the readout portion 72. Further, the negative terminal of the power source 71 is connected to the electron beam source 40, and a closed circuit is formed via the scanning electron beam 41. Here, the voltage applied from the power source 71 to the photoconductive portion 13 is referred to as a target voltage. For convenience of explanation, the line connecting the negative terminal of the power source 71 and the electron beam source 40 is not shown.

  A deflection coil and a focusing coil (not shown) are disposed outside the glass tube 30 to deflect and focus electrons (electron beam 41) accelerated in the third grid electrode 53.

  In the imaging device 100 according to the first embodiment, the voltage applied to each electrode is, for example, the heater of the electron beam source 40: about 6V, the first grid electrode 51: about 20V, and the second grid electrode 52: about 300V, the third grid electrode 53: about 600V, and the mesh electrode 60: about 800V.

  A target voltage of 100 to 5000 V is applied to the light incident surface 13B of the photoconductive portion 13 from the power source 71 via the conductive film 12 according to the thickness of the photoconductive portion 13.

  The surface potential of the scanning region 13C becomes the same potential as that of the electron beam source 40 immediately after the end of scanning, and the same voltage as the target voltage is applied between the incident surface 13B of the photoconductive portion 13 and the scanning surface 13A. Thereafter, when light is incident, electron-hole pairs are generated in the photoconductive portion 13 by the absorbed light, and the holes travel to the scanning surface 13A along the electric field in the photoconductive portion 13, and the scanning surface 13A. The surface potential changes in the positive direction. The surface potential of the scanning surface 13A that has risen in the positive direction returns to the same potential as that of the electron beam source 40 again by the next electron beam scanning. .

  Electrons generated by the electron beam source 40 are extracted by the first grid electrode 51, emitted as the electron beam 41 by the second grid electrode 52, deflected and focused by an external magnetic field, and accelerated by the third grid electrode 53. The The accelerated electrons are decelerated when passing through the mesh electrode 60 and reach the surface of the photoconductive portion 13 at a substantially zero speed. Thereby, it can suppress that an electron collides with the photoconductive part 13 at high speed, and discharge | release of the secondary electron by collision is suppressed.

  Next, the configuration of the photoconductive portion 13 will be described with reference to FIG.

  FIG. 2 is a cross-sectional view illustrating in detail the configuration of the photoconductive portion 13 of the imaging device according to the first embodiment.

  The photoconductive portion 13 is a stacked body of a hole injection blocking layer 131, a photoconductive layer 132, and an electron beam landing layer 133, and is stacked on the surface of the conductive film 12 of the translucent substrate 11.

The hole injection blocking layer 131 is a layer for blocking the injection of electric charges (holes) from the conductive film 12 to the photoconductive layer 132 and is formed to suppress dark current (Ga ₂ O). ₃ ) A thin film layer made of. The hole injection blocking layer 131 is formed on the surface of the conductive film 12 by a vacuum deposition method.

  The photoconductive layer 132 is an amorphous semiconductor layer containing selenium (Se) as a main material, and is a semiconductor layer that causes an avalanche multiplication phenomenon when a high voltage is applied. This photoconductive layer 132 is formed on the surface of the hole injection blocking layer 131 by vacuum deposition. The reason why the photoconductive layer 132 is formed of an amorphous semiconductor layer containing selenium (Se) as a main material is to use it as, for example, a HARP (High Gain Avalanche Rushing amorphous Photoconductor) photoelectric conversion film (HARP film).

  The electron beam landing layer 133 is a layer that is emitted from the electron beam source 40, accelerated by the first grid electrode 51, the second grid electrode 52, and the third grid electrode 53, and reaches the electrons decelerated by the mesh electrode 60. is there.

  Next, a method for producing the photoconductive portion 13 will be described.

Such a photoconductive portion 13 is manufactured as follows. First, a hole injection blocking layer 131 made of gallium oxide (Ga ₂ O ₃ ) having a diameter of 14 mm and a film thickness of 10 to 30 nm is formed on the conductive film 12 by a sputtering method. The gallium oxide (Ga ₂ O ₃ ) film may be formed by sputtering, for example, while supplying oxygen at room temperature.

Next, a photoconductive layer 132 made of an amorphous semiconductor mainly composed of selenium (Se) having a diameter of 14 mm and a film thickness of 2 to 50 μm is formed on the hole injection blocking layer 131 by a vacuum deposition method. Further, antimony trisulfide (Sb ₂ S ₃ ) is vapor-deposited in an argon (Ar) gas atmosphere at a pressure of 0.1 to 0.4 Torr, and an electron beam landing layer 133 having a diameter of 14 mm and a film thickness of 0.1 to 0.3 μm. Form. The photoconductive portion 13 is obtained as described above.

  By sealing the glass substrate 30 containing the translucent substrate 11 with the photoconductive portion 13 formed in this way, the electron beam source 40, the mesh electrode 60 and the like with the indium ring 20, and vacuum-sealing the inside. The imaging device of Embodiment 1 is obtained.

  Next, characteristics of the signal current and dark current of the imaging device 100 according to the first embodiment will be described.

  FIG. 3 is a diagram illustrating the characteristics of the signal current and the dark current with respect to the applied voltage in the imaging device 100 according to the first embodiment, and the characteristics of the signal current and the dark current of the conventional imaging device.

The conventional imaging device includes a hole injection blocking layer made of cerium oxide (CeO ₂ ) instead of the hole injection blocking layer 131 made of gallium oxide (Ga ₂ O ₃ ) in the imaging device 100 of the first embodiment. . The applied voltage is a voltage applied to the photoconductive portion 13 through the metal pin 14 and the conductive film 12. The signal current is indicated by an absolute value (au (arbitrary unit): arbitrary unit), and the dark current is indicated by (nA).

  As shown in FIG. 3, the signal current is about 9000 (au) at the maximum in the conventional imaging device, whereas it increases to about 14000 (au) in the imaging device 100 of the first embodiment. I understand that.

  The dark current is about 1 (nA) at the minimum in the conventional imaging device, and the dark current value in the range up to about 10 (nA) with the increase of the applied voltage is shown. It can be seen that the imaging device 100 of aspect 1 shows a dark current value in a range of about 0.1 (nA) at the minimum and up to about 3 (nA) as the applied voltage increases.

Thus, according to the imaging device 100 of the first embodiment using the gallium oxide (Ga ₂ O ₃ ) layer as the hole injection blocking layer 131, the signal current is increased as compared with the conventional imaging device (FIG. 3). In this example, the dark current is drastically reduced (about 1/10 to about 1/3). For this reason, the S / N dramatically increases. The dark current was reduced in this way because the gallium oxide (Ga ₂ O ₃ ) layer having a hole barrier larger than that of cerium oxide (CeO ₂ ) was used as the hole injection blocking layer 131. One of the reasons is considered to be that the blocking characteristics are improved.

The energy gap of cerium oxide (CeO ₂ ) is about 3.2 to 3.4 eV, whereas the energy gap of gallium oxide is about 4.9 eV. In addition, an electron barrier is not formed at the junction with an n-type semiconductor and selenium, and the barrier against holes is increased.

  In addition, it is considered that the hole barrier can be further increased because the hole injection blocking layer 131 with few defect levels can be formed by forming a gallium oxide layer by a sputtering method performed while supplying oxygen at room temperature.

  Thus, it is considered that the dark current can be significantly reduced particularly in the imaging device 100 of the first embodiment.

  Here, in FIG. 3, when the applied voltage of the imaging device 100 is gradually increased, the signal current increases rapidly from around 1200 to 1300 (V). This is because the photoconductive layer 132 is composed of an amorphous semiconductor layer containing selenium (Se) as a main raw material, and is a HARP film.

  The signal current in the case where the photoconductive layer 132 is not a HARP film is considered to have a characteristic obtained by linearly extending the characteristic of the signal current having an applied voltage of 1200 to 1300 (V) or less as indicated by a broken line A. In the broken line A, the signal current when the applied voltage is about 1600 (V) is about 20, but the signal current in the first embodiment is about 10500 (a.u.).

  For this reason, in the imaging device 100 of the first embodiment, a signal multiplication factor of about 525 (= 10500/20) times is obtained by using the photoconductive layer 132 as a HARP film.

As described above, according to the first embodiment, by using a gallium oxide (Ga ₂ O ₃ ) layer as the hole injection blocking layer 131, dark current is significantly reduced, and high sensitivity / high resolution and high S / N ratio are achieved. A photoconductive imaging device capable of obtaining a high-quality image can be provided.

Among imaging devices, particularly in an imaging device such as a high-sensitivity imaging tube using the avalanche multiplication phenomenon, it is necessary to apply a high electric field of, for example, about 1 × 10 ⁸ [V / m] or more to the photoconductive layer. is there. For this reason, particularly in an imaging device such as a high-sensitivity imaging tube using the avalanche multiplication phenomenon, the use of a gallium oxide (Ga ₂ O ₃ ) layer as a hole injection blocking layer has a remarkable effect of suppressing dark current. And a high-quality image with high sensitivity and high resolution and high S / N can be obtained.

In the above, the imaging device using the photoconductive layer made of an amorphous semiconductor mainly composed of selenium (Se) has been described. However, the present invention does not place any restrictions on the material of the photoconductive layer, The present invention can also be applied to an imaging device using a photoconductive layer composed of Si, PbO, CdS, CdSe, CdTe, Sb ₂ S ₃ or the like.

  The translucent substrate 11 is not limited to a glass substrate, and may be a translucent resin substrate or an optical fiber plate. Further, if a thin plate such as beryllium (Be), crystalline silicon (Si), or boron nitride (BN) having a high X-ray transmittance is used, the dark current of the X-ray imaging device can be reduced. .

  In the above description, the image pickup tube that is generally widely used has been described. However, any device that accelerates an electron beam in a vacuum and lands on a photoconductive layer to read out accumulated charges can be used. It may be an imaging device of the form.

[Embodiment 2]
FIG. 4 is a diagram illustrating a cross-sectional structure of the imaging device according to the second embodiment. The imaging device 200 of the second embodiment is suitable for an X-ray imaging device, and is mainly different from the imaging device of the first embodiment in that an electron emission source array 240 is provided as an electron beam source. The electron emission source array 240 has a plurality of minute cathodes arranged in a matrix. An arbitrary cathode can be selected to emit an electron beam. Therefore, unlike the imaging device 100 according to the first embodiment, the first grid electrode 51, the second grid electrode 52, the third grid electrode 53, the deflection coil, and the focusing coil are not provided.

  In the second embodiment, the glass tube 230 has a cup shape. The electron emission source array 240 is disposed on the bottom surface of the glass tube 230.

  Other configurations are basically the same as those of the imaging device according to the first embodiment. Therefore, the same or equivalent components are denoted by the same reference numerals, and description thereof is omitted.

  In the imaging device of the second embodiment, a thin plate-like conductive substrate made of beryllium (Be) having a thickness of 0.5 mm and a diameter of 18 mm is used as the translucent substrate 11. Since the thin plate-like translucent substrate 11 made of beryllium (Be) has conductivity, it is not necessary to form the conductive film 12 on one surface as in the first embodiment.

  The photoconductive portion 13 is formed on one surface of the translucent substrate 11 as in the first embodiment. The translucent substrate 11 is sealed with the glass tube 230 through the indium ring 20.

  Moreover, as described above, the glass tube 230 of the second embodiment is cup-shaped. An electron emission source array 240 is disposed on the bottom surface inside the cup-shaped glass tube 230, and a mesh electrode 60 is disposed in the vicinity of the opening 230A.

  The electron emission source array 240 is a flat electron beam source in which a plurality of minute cathodes are arranged in a matrix. By controlling the voltage value applied to the scan line and the selection line, the electron beam 241 can be emitted from the desired cathode to the photoconductive portion 13.

  The translucent substrate 11 having the photoconductive portion 13 formed on the surface as described above and the glass tube 230 containing the electron emission source array 240, the mesh electrode 60, etc. are sealed using the indium ring 20, and the inside is sealed. By performing vacuum sealing, the imaging device of Embodiment 2 can be manufactured.

The relationship between the voltage applied to the photoconductive portion 13 and the imaging signal current and dark current of the imaging device depends on the structure and film thickness of the photoconductive portion 13. For this reason, also in the imaging device of the second embodiment using the same gallium oxide (Ga ₂ O ₃ ) hole injection blocking layer 131 and the photoconductive layer 132 as in the first embodiment, As with the imaging device of the first embodiment, dark current can be reduced as compared with the imaging device according to the prior art.

  In the above description, the glass tube 230 has a cup shape. However, instead of the cup-shaped glass tube 230, a metal vacuum chamber having the same shape may be used.

  The imaging devices of Embodiments 1 and 2 described above are most suitable for television cameras that require high image quality, particularly high-definition cameras, and if applied to image analysis systems in the industrial, medical, physics and chemistry fields, Effects such as signal processing with a high S / N are obtained.

  Although the photoconductive element and the imaging device according to the exemplary embodiments of the present invention have been described above, the present invention is not limited to the specifically disclosed embodiments and deviates from the scope of the claims. Various modifications and changes can be made without this.

DESCRIPTION OF SYMBOLS 100 Imaging device 10 Photoconductive element 11 Translucent substrate 12 Conductive film 13 Photoconductive part 13A Scanning surface 13B Incident surface 13C Scanning region 131 Hole injection blocking layer 132 Photoconductive layer 133 Electron beam landing layer 14 Metal pin 20 Indium ring DESCRIPTION OF SYMBOLS 30 Glass tube 40 Electron beam source 41 Electron beam 50 Deflector 51 1st grid electrode 52 2nd grid electrode 53 3rd grid electrode 60 Mesh electrode 70 Signal read-out device 71 Power supply 72 Read-out part 200 Imaging device 230 Glass tube 240 Electron emission source Array 241 Electron beam

Claims (2)

A translucent substrate;
A conductive film formed on the translucent substrate;
A hole injection blocking layer formed on the conductive film;
A photoconductive layer formed on the hole injection blocking layer, composed of an amorphous semiconductor layer mainly composed of selenium, and causing an avalanche multiplication phenomenon by application of a predetermined high voltage ;
The hole injection blocking layer is a photoconductive element made of gallium oxide and suppressing injection of holes from the conductive film into the photoconductive layer . A photoconductive element according to claim 1 ;
An electron beam source for emitting a scanning electron beam to the photoconductive element;
An image pickup device comprising: a signal readout unit that is electrically connected to the photoconductive element and reads out an image pickup signal obtained by scanning the electron beam.

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