JP2021048053A - Light detection device - Google Patents

Abstract

To improve performance of a light detection device.SOLUTION: A light detection device 10 comprises: a tabular photoelectric surface 12 which emits electrons by making light incident thereto; a tabular anode 15 which is disposed so as to be opposed with the photoelectric surface 12; a power source 18 for generating an electric field between the photoelectric surface 12 and the anode 15; a gas chamber 13 into which a gas for amplifying electrons moving in the electric field is introduced with the electrons emitted from the photoelectric surface 12 defined as a trigger; n insulator 14 which is provided between the gas chamber 13 and the anode 15; and a signal electrode 16 for reading electric charge which is induced by the electrons amplified in the electric field, as a signal.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to a photodetector.

Measurement technology using light is used in various fields, and various optical sensors are provided according to the application and purpose. The optical sensor mainly includes a semiconductor optical sensor utilizing the internal photoelectric effect, a photomultiplier tube utilizing the external photoelectric effect, and the like (see, for example, Non-Patent Documents 1 to 3). In each case, weak light can be detected as an electric signal by converting a photon into an electron and multiplying the electron.

The semiconductor photosensor multiplies the electrons converted inside the semiconductor by avalanche amplification or the like. Semiconductor optical sensors are generally characterized by being small, thin, and highly sensitive.

The photomultiplier tube uses a dynode to multiply the electrons converted on the photocathode and emitted into the vacuum. The sensitivity of the photomultiplier tube is mainly determined by the type of photocathode, and the time characteristics are determined by the type of dynode. At present, the dynode that can obtain the best time resolution is a microchannel plate (MCP) type dynode. In the case of MCP having a channel diameter of 10 μm, a time resolution of about 30 picoseconds is obtained with a standard deviation in detecting one photon.

"Optical Semiconductor Device Handbook Chapter 1 Introduction", Hamamatsu Photonics Co., Ltd., https://www.hamamatsu.com/resources/pdf/ssd/01_handbook.pdf "Optical Semiconductor Device Handbook Chapter 3 Si PPD, MPPC", Hamamatsu Photonics Co., Ltd., https://www.hamamatsu.com/resources/pdf/ssd/03_handbook.pdf "Photomultiplier tube, its basics and applications 4th edition", Hamamatsu Photonics Co., Ltd. Editorial Committee, April 1, 2017, Hamamatsu Photonics Co., Ltd., https://www.hamamatsu.com/resources/pdf/etd /PMT_handbook_v4J.pdf

In order to realize an optical sensor with high time resolution, it is indispensable that the noise is small, the propagation distance of electrons does not depend on the incident position of photons, and that a mechanism for multiplying electrons at high speed is provided. In general, photodiodes and photomultiplier tubes tend to sacrifice time resolution when the detection area is increased, and it is difficult to realize a large-area optical sensor with high time resolution.

In a photodiode, the capacitance increases as the area of the electrode increases, so that the noise component due to the capacitance increases. Therefore, in principle, the decrease in time resolution of the photodiode due to the increase in area is unavoidable.

Even in a photomultiplier tube, in general, as the area increases, the variation in the orbital lengths of the secondary electrons emitted from the dynode becomes large, and the spread of the transit time of the secondary electrons (TTS) becomes large. , The time resolution is reduced.

In the MCP type photomultiplier tube, the dynodes are arranged parallel to the photoelectric surface, and the propagation distance of electrons does not depend on the incident position of photons, so it is expected that high time resolution can be obtained even if the area is increased. However, since the inside of the photomultiplier tube is a vacuum, it is necessary to have a structure that can withstand atmospheric pressure, and the larger the size, the more difficult it is to keep the photoelectric surface and the dynode parallel. In addition, since MCP has a complicated structure, it is very expensive, which is a major obstacle to using the MCP type photomultiplier tube.

The present disclosure has been made in view of such problems, and an object of the present disclosure is to provide a technique for improving the performance of a photodetector.

In order to solve the above problems, an optical detection device according to an embodiment of the present invention includes a flat plate-shaped photoelectric surface that emits electrons when light is incident, and a flat plate-shaped anode provided so as to face the photoelectric surface. A power supply for generating an electric field between the photoelectric surface and the anode, a gas chamber in which a gas for amplifying electrons moving in the electric field triggered by electrons emitted from the photoelectric surface is introduced, and a gas chamber. It includes an insulator provided between the anode and a signal electrode for reading a charge induced by electrons amplified in an electric field as a signal.

According to the present disclosure, it is possible to provide a technique for improving the performance of a photodetector.

It is a figure which shows schematic the structure of the photodetector which concerns on embodiment. It is a figure which shows the appearance of the light detection apparatus which concerns on Example. It is a figure which shows the electric signal detected by the photodetector which concerns on Example. It is a figure which shows the charge distribution of the output signal of the photodetector which concerns on Example. It is a figure which shows the distribution of the time when the detection signal was detected by the photodetector which concerns on Example. It is a figure which shows the relationship between the charge of the signal detected by the photodetector which concerns on Example, and the time resolution. It is a figure which shows another example of the structure of the light detection apparatus which concerns on embodiment. It is a figure which shows still another example of the structure of the light detection apparatus which concerns on embodiment.

As the embodiment of the present disclosure, a photodetector having high time resolution, easy to increase the area, and inexpensive to be manufactured will be described. As described above, in order to suppress the decrease in time resolution due to the increase in the area of the photodetector, a structure capable of amplifying the photoelectrons emitted from the large area photoelectric surface at high speed with a uniform and short propagation distance is adopted. There is a need to. Further, in order to make the photodetector inexpensive to manufacture even if the area is increased, it is necessary to adopt a simple structure.

Therefore, in the photodetector of the present embodiment, the photoelectric surface cathode formed in a flat plate shape and the anode formed in a flat plate shape are arranged in parallel, a gas is introduced between them, and a high electric field is generated between the electrodes. Causes. When light is incident on the photoelectric surface, the photoelectrons emitted from the photoelectric surface are accelerated by a high electric field, collide with gas molecules, and are amplified by an electron avalanche or the like. No matter where the light is incident on the photoelectric surface, the emitted photoelectrons are amplified at high speed in a uniform high electric field, so that extremely high time resolution can be obtained. Further, as long as the area is such that the photoelectric surface and the anode can be uniformly produced, a photodetector having an arbitrary area can be realized. Furthermore, since the structure is simple, it can be manufactured at a relatively low cost.

FIG. 1 schematically shows a configuration of a photodetector according to an embodiment. The photodetector 10 includes an incident window 11, a photoelectric surface 12, a gas chamber 13, an insulator 14, an anode 15, a signal electrode 16, an amplifier 17, and a power supply 18.

The incident window 11 constitutes an opening that functions as a window for incident light to be detected on the photoelectric surface 12. The incident window 11 is formed in a flat plate shape by a material that transmits light having a wavelength to be detected by the photodetector 10. The incident window 11 may be formed of, for example, MgF ₂ , alkali halide, sapphire, quartz glass, ultraviolet transmissive glass, borosilicate glass, or the like.

The photoelectric surface 12 is formed in a flat plate shape on the inner surface of the incident window 11 by a material that emits electrons when light of a wavelength to be detected by the photodetector 10 is incident. The photoelectric surface 12 is, for example, Cs-I, Cs-Te, Sb-Cs, bi-alkali (Sb-Rb-Cs, Sb-K-Cs), low dark current bi-alkali (Sb-Na-K), multi-alkali. (Sb-Na-K-Cs), Ag-O-Cs, Cs-activated GaN (Cs), GaAsP (Cs), GaAs (Cs), InGaAs (Cs), InP / InGaAsP (Cs), InP / It may be formed by InGaAs (Cs) or the like. The photoelectric surface 12 is formed by a vapor phase method (VPE: Vapor Phase Epitaxy) such as a physical vapor deposition method (PVD: Physical Vapor Deposition) or a chemical vapor deposition method (CVD: Chemical Vapor Deposition), or a chemical solution method (CSD: CSD:). It may be formed by any film forming technique such as Chemical Solution Deposition). The photoelectric surface 12 is preferably formed of a material having high quantum efficiency with respect to light having a wavelength to be detected. Further, in the photodetector 10 of the present embodiment, since the photoelectric surface 12 is in contact with the gas introduced into the gas chamber 13, the gas introduced into the gas chamber 13 under the conditions such as temperature and pressure in the detection environment. On the other hand, it is preferably formed of a material that is chemically and physically stable.

A thin film formed of another material may be provided between the incident window 11 and the photoelectric surface 12. This thin film has a role as an antireflection film for reducing reflection at the interface between the incident window 11 and the photoelectric surface 12 and a role as an electrode for uniformly supplying a current to the photoelectric surface 12. In general, since the photoelectric surface 12 is a semiconductor and is a thin film having a film thickness of several tens of nm, it has high resistance and it may be difficult to create a uniform electric field only with the photoelectric surface 12. Even in such a case, a thin film of an arbitrary material that can be used as an electrode, for example, indium tin oxide (ITO), is provided between the incident window 11 and the photoelectric surface 12, and the thin film is grounded to serve as a cathode. By making it work, a uniform electric field can be created.

The anode 15 is provided so as to face the photoelectric surface 12. The anode 15 is formed in a flat plate shape by any material that can be used as an electrode, for example, carbon, colloidal graphite, or the like. If the resistance of the anode 15 is small, all the signals flow to the anode 15 and are not transmitted to the signal electrode 16. Therefore, the anode 15 is formed so that the surface resistance is high to some extent, for example, several hundred Ω / □. As a result, a DC voltage can be supplied to the anode 15 to generate an electric field while suppressing transmission of the pulsed signal to the anode 15. The anode 15 is arranged parallel to the photoelectric surface 12. The photoelectric surface 12 and the anode 15 may be arranged slightly offset from parallel as long as they do not affect the time resolution of the photodetector 10.

The power supply 18 applies a high voltage to the anode 15. Since the photoelectric surface 12 side is grounded, a high electric field is generated between the photoelectric surface 12 and the anode 15. Since the photoelectric surface 12 and the anode 15 are formed in a flat plate shape and arranged in parallel, a uniform high electric field is generated between the photoelectric surface 12 and the anode 15 regardless of the position.

The gas chamber 13 is a space between the photoelectric surface 12 and the insulator 14, and a gas is introduced into the gas chamber 13. Since a high electric field is generated in the gas chamber 13, when electrons are emitted from the photoelectric surface 12, the emitted electrons are accelerated in the high electric field and collide with gas molecules, and the electrons are ejected from the gas molecules. Is done. These electrons are accelerated in a high electric field and collide with another gas molecule, and the electrons are also ejected from another gas molecule, so that an electron avalanche is generated and the number of electrons is amplified at an accelerating rate. A gas suitable for causing an electron avalanche, for example, R134a (C ₂ H ₂ F ₂ ), which is an alternative CFC used as a refrigerant gas, or a rare gas, may be introduced into the gas chamber 13. In particular, since R134a has a high electronegativity and easily adsorbs electrons, it is possible to suppress the generation of further electrons due to photoionization by ultraviolet rays emitted from gas molecules and the generation of streamer discharge. In order to suppress the occurrence of unnecessary streamer discharge, _{a mixed gas to which a small amount of SF 6} , CF ₄ , carbon dioxide, methane, ethane, isobutane and the like is added may be introduced into the gas chamber 13. By suppressing the occurrence of streamer discharge, the time from the occurrence of the electron avalanche to the return to the steady state can be shortened, so that light can be detected at a high rate. If it is not necessary to detect light at a high rate, streamer discharge may be intentionally generated. This makes it possible to obtain higher time resolution. The gas chamber 13 may be sealed with the gas introduced. The gas chamber 13 may be provided with a gas introduction port and a gas discharge port, and gas may be supplied to the gas chamber 13 from a gas supply means (not shown). By adjusting the pressure of the gas in the gas chamber 13, the external pressure applied to the inside of the photodetector 10 can be canceled out, so that it is necessary to have a strong structure like a photomultiplier tube having a vacuum inside. Absent. Therefore, it is possible to realize a photodetector 10 that can be easily manufactured in a large area and can be manufactured at low cost. Further, since the photoelectric surface 12 and the anode 15 can be easily kept in parallel, the photodetector 10 having high time resolution can be realized even if the area is increased. Further, since it is not necessary to have a curved structure like a photomultiplier tube, when a plurality of photodetectors 10 are arranged side by side to increase the detection area, the undetectable region can be reduced.

The insulator 14 is provided between the gas chamber 13 and the anode 15. The insulator 14 is ^{formed in a flat plate shape by a material having a high electrical resistivity of about 10 10} to ¹³ Ωcm, for example, glass, ceramics such as ferrite, resin such as bakelite, and the like. When an electron avalanche occurs in the gas chamber 13, it is offset by the positive charge induced on the surface of the insulator 14 in the vicinity thereof, and the electric field in the vicinity is temporarily weakened. As a result, it is possible to prevent an excessive electron avalanche from occurring. Further, since the change in the electric field caused by the electron avalanche can be prevented from spreading in the horizontal direction, it is possible to prevent the detection of light at another position from being affected. By inserting the insulator 14 between the gas chamber 13 and the anode 15, the distance between the photoelectric surface 12 and the anode 15 can be widened, and the capacitance can be reduced even if the gas chamber 13 is thin. Therefore, the noise component of the detection signal can be suppressed, and the time resolution of the photodetector 10 can be improved.

The signal electrode 16 is an electrode for reading the electric charge induced by the electron avalanche in the gas chamber 13 as a signal. The signal electrode 16 is formed in a flat plate shape by any material that can be used as an electrode, such as copper. The photoelectric surface 12 may also serve as the signal electrode 16.

The amplifier 17 amplifies the electric signal from the signal electrode 16. The amplifier 17 amplifies the electric signal from the signal electrode 16 to a voltage that can be processed by a signal processing circuit (not shown). The electric signal amplified by the amplifier 17 is processed in the signal processing circuit. For example, the signal processing circuit counts the pulse of the output signal by the counter circuit when the peak value exceeds a predetermined reference voltage by the photon counting method.

According to the technique of the present embodiment, it is possible to realize a photodetector having high time resolution, easy to increase the area, and can be manufactured at low cost. Further, since a thin photodetector can be realized, it is also useful in applications that require space saving.

FIG. 2 shows the appearance of the photodetector 10 according to the embodiment. The photodetector 10 shown in FIG. 2 was prototyped by the present inventor. In the prototype, the incident window 11 was formed of quartz glass, the photoelectric surface 12 was formed of LaB ₆ , the thin film between the incident window 11 and the photoelectric surface 12 was formed of ITO, the insulator 14 was formed of glass, and the anode 15 was formed of colloidal graphite. _{A gas obtained by mixing R134a and SF 6} at a ratio of 9: 1 was introduced into the gas chamber 13 at a flow rate of 2.5 cc per minute. The thickness of the incident window 11 is 2.0 mm, the thickness of the ITO film is about 20 nm, the thickness of the photoelectric surface 12 is about 20 nm, the size of the photoelectric surface 12 is 36 mm × 36 mm, and the size of the signal electrode 16 (feeling region) is about 30. × 30 mm, the thickness of the gas chamber 13 was 210 μm, the thickness of the insulator 14 was 2.75 mm, the amplification factor of the amplifier 17 was 43 dB, the voltage of the power supply 18 was 4.3 kV, and the gas pressure in the gas chamber 13 was atmospheric pressure. .. This prototype was irradiated with light from a picosecond pulse laser having a wavelength of 405 nm, and the performance of the light detection device 10 for one photon was measured.

FIG. 3 shows an electric signal detected by the photodetector 10 according to the embodiment. The upper signal in FIG. 3 is a detection signal of the photodetector 10, and the lower signal in FIG. 3 is a signal synchronized with the oscillation of the picosecond pulse laser. The multiple detection signals are superimposed and displayed according to the synchronization signal in the lower row. The detection signals of the photodetector 10 rise sharply at about 1 ns regardless of the gain. Therefore, it was shown that the electrons could be multiplied at high speed. Further, due to this steep rise, it is possible to suppress variations in detection timing due to noise components.

FIG. 4 shows the charge distribution of the output signal of the photodetector 10 according to the embodiment. The horizontal axis is the output signal after being amplified by the amplifier 17. The average wave height of the output signal at 4887 detections was about 12 pC. Accordingly, the photodetector 10 of the present embodiment, the gain of about 1 × 10 ⁶ was indicated to be obtained.

FIG. 5 shows the distribution of the time when the detection signal was detected by the photodetector 10 according to the embodiment. The standard deviation of the detection timing in the 4887 detections was 254.5 ps, and the full width at half maximum (FWHM) of the distribution was 210 ps.

FIG. 6 shows the relationship between the charge of the signal detected by the photodetector 10 according to the embodiment and the time resolution. It was shown that the larger the gain of the detection signal, the higher the time resolution, and the maximum time resolution of about 50 ps can be obtained. This value includes the pulse width (about 17 ps) of the picosecond pulse laser and the time resolution (about 35 ps) of the time measurement circuit (Time-to-Digital Converter: TDC). The specific time resolution of the photodetector 10 was calculated to be about 31 ps, and it was shown that a time resolution equivalent to that of the MCP type photomultiplier tube can be obtained. As shown in FIG. 6, it is expected that a higher time resolution can be obtained by increasing the gain of the gas multiplication of the photodetector 10.

As a method of increasing the gain of the gas multiplication and improving the time resolution of the photodetector 10, the following can be mentioned.

The voltage applied from the power source to the anode 15 may be increased to generate a higher electric field. As a result, more secondary electrons can be generated, so that the gain of gas multiplication can be increased.

The thickness of the gas chamber 13 may be increased. When an electric field of the same intensity is generated, secondary electrons can be generated from more gas molecules when the gas chamber 13 is thicker than when it is thin, so that the gain of gas multiplication is increased. can do.

The thickness of the gas chamber 13 may be reduced. When the same voltage is applied to the anode 15, the electric field generated in the gas chamber 13 is higher when the gas chamber 13 is thinner than when it is thick, so that more secondary electrons can be generated. , The gain of gas multiplication can be increased.

The thickness of the insulator 14 may be reduced. When the same voltage is applied to the anode 15, the electric field generated in the gas chamber 13 is higher when the insulator 14 is thinner than when it is thick, so that more secondary electrons can be generated. , The gain of gas multiplication can be increased.

Of the gas introduced into the gas chamber 13, the mixing ratio of _{SF 6 or the like to be mixed in order to suppress the streamer discharge may be reduced.} As a result, although the probability that streamer discharge occurs is slightly increased, more secondary electrons can be generated, so that the gain of gas multiplication can be increased.

The flow rate of the gas introduced into the gas chamber 13 may be increased. As a result, the purity of the gas introduced into the gas chamber 13 can be increased, so that the generated secondary electrons can be prevented from being adsorbed by impurities and lost, and the gain of gas multiplication can be increased. Can be done. The gas flow rate may be adjusted according to the volume of the gas chamber 13 and the degree of gas leak.

In addition to increasing the gain of photomultiplier tube, the following methods can be mentioned as a method of improving the time resolution of the photodetector 10.

The thickness of the insulator 14 may be increased. As a result, the distance between the photoelectric surface 12 and the anode 15 can be widened and the capacitance can be reduced, so that the SN ratio of the detection signal can be improved and the time resolution of the photodetector 10 can be improved. ..

The signal electrode 16 may be divided into a plurality of regions. As a result, it is possible to suppress variations in the propagation time of the detection signal depending on the detection position of the signal electrode 16, so that the time resolution of the photodetector 10 can be improved.

As the amplifier 17, a low noise or high gain amplifier may be used. As a result, the SN ratio of the detection signal can be improved, so that the time resolution of the photodetector 10 can be improved.

The photodetector 10 may be provided with a configuration for reducing noise from the outside. As a result, the SN ratio of the detection signal can be improved, so that the time resolution of the photodetector 10 can be improved.

FIG. 7 shows another example of the configuration of the photodetector 10 according to the embodiment. In the photodetector 10 shown in FIG. 7, the signal electrode 16 is divided into a plurality of regions 16a, 16b, 16c, and the charges induced in the respective regions 16a, 16b, 16c can be read as different signals. It is composed. As a result, the photodetector 10 can also detect the position where the light is incident. Further, as described above, since the variation in the propagation time of the detection signal depending on the detection position of the signal electrode 16 can be suppressed, the time resolution of the photodetector 10 can be improved. In the prototype shown in FIG. 2, when the signal electrode 16 is divided into 3 × 3 regions, it is expected that a time resolution of about 24 ps can be obtained by calculation.

FIG. 8 shows yet another example of the configuration of the photodetector 10 according to the embodiment. In the photodetector 10 shown in FIG. 8, a negative high voltage is applied to the photoelectric surface 12 from the power supply 18. In this case, since a high voltage is not applied to the anode, the anode can also serve as the signal electrode 16. As a result, the configuration of the photodetector 10 can be further simplified, so that the manufacturing cost can be further reduced.

The present disclosure has been described above based on the examples. This embodiment is an example, and it will be understood by those skilled in the art that various modifications are possible for each of these components and combinations of each processing process, and that such modifications are also within the scope of the present disclosure. ..

One aspect of the present disclosure is a photodetector. This optical detection device generates an electric field between a flat plate-shaped photoelectric surface that emits electrons when light is incident, a flat plate-shaped anode provided so as to face the photoelectric surface, and the photoelectric surface and the electrode. A gas chamber in which a gas for amplifying electrons moving in an electric field triggered by electrons emitted from the photoelectric surface is introduced, an insulator provided between the gas chamber and the anode, and an electric field. It is provided with a signal electrode for reading the charge induced by the electrons amplified in the signal as a signal. According to this aspect, it is possible to provide a photodetector having a high time resolution. Further, it is possible to provide a photodetector that can be manufactured in a large area at low cost.

The photoelectric surface or the anode may also serve as a signal electrode. According to this aspect, the configuration of the photodetector can be simplified.

The signal electrode may be divided into a plurality of regions, and the charges induced in each region may be readable as separate signals. According to this aspect, the position resolution of the photodetector can be improved.

10 photodetector, 11 incident window, 12 photoelectric surface, 13 gas chamber, 14 insulator, 15 anode, 16 signal electrode, 17 amplifier, 18 power supply.

Claims (3)

A flat plate-shaped photoelectric surface that emits electrons when light is incident,
A flat plate-shaped anode provided so as to face the photoelectric surface, and
A power source for generating an electric field between the photoelectric surface and the anode,
A gas chamber into which a gas for amplifying the electrons moving in the electric field triggered by the electrons emitted from the photoelectric surface is introduced, and
An insulator provided between the gas chamber and the anode,
A signal electrode for reading the electric charge induced by the electrons amplified in the electric field as a signal, and
A photodetector comprising. The photodetector according to claim 1, wherein the photoelectric surface or the anode also serves as the signal electrode. The photodetector according to claim 1 or 2, wherein the signal electrode is divided into a plurality of regions, and the charge induced in each region can be read as another signal.

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