The present invention relates to a method for designing a photodetector, and in particular, a method for designing a photodetector having a photoelectric conversion layer mostly composed of amorphous selenium, a method for designing a layer thickness, a photodetector using the photoelectric conversion layer, and a manufacturing method thereof.
Conventionally, the signal amplification effect, which amplifies the electric signal when a high electric field is applied to amorphous selenium has been reported. Recently, a photodetector using amorphous selenium as a photoelectric conversion layer has been developed by utilizing this property. For example, in Patent Literature 1 and Patent Literature 2, a photoelectric conversion layer containing amorphous selenium is formed on a substrate, and a functional layer for removing noise, which might causes less amplification effect is additionally formed, have been disclosed.
However, in the above documents, after forming a photoelectric conversion layer containing amorphous selenium in a prescribed manner, a functional layer that assists the function of the photoelectric conversion layer is added by ingenuity so that the desired effect of a device can be obtained. Therefore, there is a problem that it takes time and man-hours to design the functional layer in order to manufacture a photodetector that realizes desired performance.
The present invention has been made based on the problems of the prior invention, an object of the present invention is to provide a device designing that desired performance can be achieved without adding a functional layer by designing a photoelectric conversion layer (amorphous selenium layer) based on quantum mechanics.
In order to solve the above-described problem, a photodetector designing method according to an aspect of the present invention includes: according to various requirements required by an application equipped with a photodetector including a photoelectric conversion layer having a superlattice structure mostly composed of amorphous selenium, a step of determining a form of the photodetector; a step of determining a type of a substrate suitable for a wavelength to be detected by the photoelectric conversion layer among the requirements; a step of calculating a multiplication factor M representing an amplification gain generated in a process of tunneling in the superlattice structure; and a step of determining, as a layer thickness of the photoelectric conversion layer, a thickness obtained by multiplying a thickness per one layer of the superlattice structure by the number of layers NSL of the superlattice structure on the assumption that the multiplication factor M is approximate to the number of layers NSL.
In the aspect described above, it is also preferable that the multiplication factor M is calculated by the following equation by using a collector current Ic from a light receiving device of the photodetector using the photoelectric conversion layer, a dark current ID of the photodetector, and the number of photons N00 serving as carriers in the tunneling, provided that q is a basic electronic charge.
In the aspect described above, it is also preferable that, when the photodetector has a vacuum gap, a correction coefficient γ for increasing the multiplication factor M is included in the multiplication factor M. In order to solve the above-described problem, a layer thickness designing method according to an aspect of the present invention is a method for designing a thickness of a photoelectric conversion layer having a superlattice structure mostly composed of amorphous selenium, and includes: the number of layers NSL of the superlattice structure is obtained from a multiplication factor M representing an amplification gain generated in a process of tunneling in the superlattice structure, and a layer thickness of the photoelectric conversion layer is determined.
In order to solve the above-described problem, it is also preferable that a photodetector manufacturing method by laminating the photoelectric conversion layer on the substrate by rotational evaporation by matching the number of layers NSL with the number of rotations of a turntable.
In the aspect of described above, in the photodetector manufacturing method, it is also preferable that the superlattice structure is made of selenium and arsenic triselenide.
In order to solve the above-described problem, it is also preferable that a photodetector includes: in order from a light incidence side, a first substrate including a transparent electrode; a photoelectric conversion layer having a superlattice structure mostly composed of amorphous selenium and having a layer thickness obtained by multiplying a thickness per one layer of the superlattice structure by the number of layers NSL of the superlattice structure approximate to a multiplication factor M representing an amplification gain generated in a process of tunneling in the superlattice structure; and a second substrate of a substrate type which is suitable for a requirement for a wavelength to be detected by the photoelectric conversion layer and on which the photoelectric conversion layer is laminated.
In order to solve the above-described problem, it is also preferable that a photodetector includes: in order from a light incidence side, a first substrate including a transparent electrode, an anode layer having a superlattice structure mostly composed of amorphous selenium and having a layer thickness obtained by multiplying a thickness per one layer of the superlattice structure by the number of layers NSL of the superlattice structure approximate to a multiplication factor M representing an amplification gain generated in a process of tunneling in the superlattice structure; a vacuum gap; and a cathode layer.
Also, a storage medium storing a computer program of the photodetector designing method according is also preferable.
The present invention enables to provide electronic devices having desired performance by designing its photoelectric conversion layer (amorphous selenium layer) based on quantum mechanics.
Next, preferred embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the embodiments described here, and various modifications can be made based on the knowledge of a person skilled in the art. For example, although the photoelectric conversion layer of the embodiments is mostly composed of amorphous selenium, it is presumed that the same layer thickness design method, photodetector, and manufacturing method can be implemented even if the photoelectric conversion layer is mostly composed of chalcogenide semiconductor. It is understood that such aspects are also included in the scope of the present invention.
In a conventional photodetector using amorphous selenium as a photoelectric conversion layer, or an application equipped with a photodetector, first, an amorphous selenium layer is formed by a method using molecular beam epitaxy in a specified manner, next, a functional layer is additionally designed in order to sublimate the signal amplification to meets requirements of an application.
On the other hand, in the embodiments, photodetectors are each included an amorphous selenium layer, the layer thickness of the amorphous selenium layer (the number of layers) is predicted and formed to obtain the signal amplification effect necessary to meet requirements of the application without adding a functional layer. This layer thickness (the number of layers) is calculated based on quantum mechanics. Then, depending on what kind of substrate the amorphous selenium layer is laminated on, it is possible to detect light in various optical regions, and it is possible to customize the performance according to the requirements of the application. Hereinafter, this device designing will be described.
Form of Photodetector
First, configuration of a photodetector that preferably carries out the present invention will be described.
When a high electric field is applied to the amorphous selenium layer 12 through the through hole 15 provided in the substrate 11, the photodetector 10 amplifies the incident light energy and performs photoelectric conversion. The amorphous selenium layer 12 is a photoelectric conversion layer. The signal amplification effect varies depending on the structure of the amorphous selenium layer 12. A readout circuit (light receiving device) 16 connected to the photodetector 10 reads out the signal current and thus detecting the light.
In the photodetector 10′, when a high electrifield is applied to the amorphous selenium layer 12 through the through hole 15 provided in the glass substrate 11, the incident light energy is signal-amplified and photoelectrically converted. The signal amplification effect varies depending on the structure of the amorphous selenium layer 12. Further, an increase in high input impedance can be obtained by having the vacuum gap 17 and the diamond cold cathode layer 18. A readout circuit (light receiving device) 16 connected to the photodetector 10′ reads out the signal current and thus detecting the light.
Device Designing
Next, the device designing of these embodiments will be described.
Designing of Substrate
Since the second substrate 13 also functions as a photon absorber, it can absorb photons of different energies depending on the type of substrate. Therefore, the photodetector 10 can be extended to a wavelength spectrum in a wide optical region.
From the above, the designing of the photodetector 10 can provide the performance suitable for an optical region required for each application by selecting the type of the second substrate 13 on which the amorphous selenium layer 12 is laminated.
Manufacturing of Amorphous Selenium Layer
The amorphous selenium layer 12 is preferably manufactured by a rotational evaporation method. Selenium (hereinafter, Se) is, for example, 2-3 mm in size and has a purity of 99.999%, and arsenic triselenide (hereinafter, As2Se3) is, for example, in powder form and has a purity of 99.999% (arsenic), is suitable and can be obtained from a drug manufacturer or the like.
Next, a method for manufacturing the amorphous selenium layer 12 using the above Se and As2Se3 will be described.
From the above results, by using the rotational evaporation method of
Designing of Amorphous Selenium Layer
The photodetector 10 realizes a high gain by superlattice Quantum Tunneling Multiplication of the amorphous selenium layer 12. The superlattice structure of Se and As2Se3 forms a mini-band energy level. Carriers such as electrons and holes dominate these energy levels and are conned between superlattice quantum well barriers. There is always a possibility that the carriers will tunnel, but by applying an external electric field, the width of the barrier can be reduced, and the tunneling probability in the direction of the electric field from the quantum well to the quantum well can be increased.
In the holes in the amorphous selenium superlattice, the holes can be tunneled from the As2Se3 quantum well through the Se barrier.
When one or some hole passes through the Se barrier, impact ionization is possible if sufficient kinetic energy is obtained for the holes to collide and release other holes. The multiplication factor M generated in this process is expressed by Equation (1).
M=NSL exp(αSLl) [Numerical Equation 1]
Here, l is the thickness of the Se barrier alone. Each superlattice structure is composed of a Se barrier and an As2Se3 quantum well, and the total thickness of the amorphous selenium superlattice structure is “2 NSL l”. The Ionization coefficient “α SL” represents the probability of causing impact ionization for the holes reached the threshold energy E th. The thickness l is 10 nm or less in order to maintain the quantum characteristics, and the ionization coefficient “α SL” is on the order of 10−4 [m−1]. This means that the exponential term can be “1 (unity)” approximately.
Therefore,
M≈NSL [Numerical Equation 2]
As shown in Equation (2), the multiplication factor M depends on the number of layers NSL of the superlattice structure. Therefore, by controlling the number of layers NSL of the amorphous selenium superlattice structure, it represents the performance of the photodetector 10 can be customized.
Indices such as sensitivity, responsiveness, and quantum efficiency are examples of numbers used to evaluate the performance of the photodetector 10. The sensitivity S at the wavelength λ is calculated as in Equation (3).
Here, P is an incident light power at that wavelength λ, the Ic is a collector current from the light receiving device. Sensitivity and quantum efficiency η are related as in Equation (4).
Here, λ has a unit of [nm].
The incident light having the wavelength λ and the incident light power P [W/m2] carries the photon amount N0 calculated as in Equation (5).
Here, h is Planck's constant and c is speed of light. These photons produce a pair of electrons N00 and holes in the active region A [m2] of the photodetector 10. Relation of electron N00 and amount of photons N0 is shown in Equation (6).
N00=N0A [Numerical Equation 6]
The resulting current “Iph” resulting from these carriers is described as Equation (7).
Iph=qN00 [Numerical Equation 7]
Here, q is a basic electronic charge.
In general, any detector has a characteristic dark current ID. The photodetector 10 of the embodiment, which has a signal amplification effect due to the amorphous selenium layer 12, multiplies the photocurrent when a bias voltage is applied to the device. Therefore, the collector current Ic has a multiplication factor M as in Equation (8).
IC=MIph+ID [Numerical Equation 8]
Then, according to the requirements of desired application, the wavelength λ and the incident light power P at that wavelength are known, so that the current Iph can be obtained. Depending on application requirements, also the dark current ID of the device is determined, using the current required for the external circuit, the collector current Ic is also selected. Therefore, from the Equation (8), the multiplication factor M, which is the required gain, is determined by Equation (9).
Using the Equation (7), the Equation (9) can be written as Equation (10).
Here, since there is a relation of the Equation (2), the number of layers NSL of the amorphous selenium superlattice structure can be determined by obtaining the multiplication factor M.
In addition, it is necessary to apply voltage in order to operate a device. In the photodetectors 10 and 10′ of the embodiments, the following three types of voltages are required.
As described above, the Equation (10) was obtained by considering Superlattice Quantum Tunneling Multiplication in quantum mechanics Thus, depending on application requirements, as the factor IC, ID, and N00, in the Equation (10), can be determined, it is possible to obtain a multiplication factor M required. Then, by obtaining the multiplication factor M, the number of layers NSL in the amorphous selenium superlattice structure forming the amorphous selenium layer 12 can be determined so that it is possible to meet the application requirements.
In the case of the photodetector 10′ according to the second embodiment, an experimental multiplication factor M (exp) may be larger than a calculated (designed) multiplication factor M (cal). This is because of the low dark current due to high impedance originated from the structure having the diamond cold cathode layer 18 and the vacuum gap 17. In detail, a field emission current is negligibly small unless there is a threshold potential difference between the amorphous selenium layer (amorphous Se anode) 12 and the diamond cold cathode layer 18. Therefore, when the diamond cold cathode layer 18 and the vacuum gap 17 are used, the calculated multiplication factor M (cal) needs to be multiplied by the correction coefficient γ as shown in Equation (11) in consideration of the increase in the input impedance.
M(exp)≈γM(cal) [Numerical Equation 11]
Next, device designings were performed assuming some application requirements.
These device designing may be implemented by software, for example, electronic circuits such as a CPU (Central Processing Unit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device) such as an FPGA (Field Programmable Gate Array). A program, which consists of electronic circuits, reads numerical values from a memory device that stores the numerical values of each factor necessary for the above shown calculations according to the application requirements. Then, the program combines photodetector form, layer design, and substrate selection, implements preferable device designing to achieve performance desired by desired application.
(Green Light Detector)
Requirements: It is assumed that this application needs to detect green light (visible light) with a wavelength of 520 nm with a minimum incident light power of 5 μW/cm2. As the photodetector, the photodetector 10 (imaging device form) of the first embodiment is adopted. The external readout circuit connected to the photodetector requires a current of at least 150 nA. The application allows dark current up to 50 nA.
The energy E of a single photon having λ=520 nm is obtained as shown in Equation (12).
When the photon energy E, having P=5 μW/cm2, is incident on a photodetector with an active area of 1 mm2, the electron (the number of photons) N00 is obtained as shown in Equation (13).
Here, from the requirements, the collector current Ic is 150, the dark current ID is 50. Using the Equation (10), the calculated multiplication factor M (cal) can be obtained from Equation (14).
Therefore, from the Equation (2), the photodetector 10 satisfying the above requirements needs to have at least “5” layers in the amorphous selenium superlattice structure. Here, the thickness l of the Se barrier alone is 8 nm. Multiplication only occurs in the Se barrier. Since each superlattice is composed of a Se barrier (8 nm) and an As2Se3 quantum well (also 8 nm), the thickness of the amorphous selenium superlattice structure per layer is 16 nm. From the above, the layer thickness of the amorphous selenium layer 12 (total thickness of the amorphous selenium superlattice structure) required for the application is at least 16 nm×5 layers (or, 2×8 nm×5 layers)=80 nm.
(Blue Light Detector)
Requirements: This application needs to detect blue light (visible light) with a wavelength of 470 nm with a minimum incident light power of 5 μW/cm2. As the photodetector, the photodetector 10 (imaging device form) of the first embodiment is adopted. The external readout circuit connected to the photodetector requires a current of at least 200 nA. The application allows dark currents up to 50 nA.
The energy E of a single photon at λ=470 nm is obtained as shown in Equation (15).
When the photon energy E, having P=5 μW/cm2, is incident on a photodetector with an active area of 1 mm2, the number of photons N00 can be obtained according to Equation (16).
Here, from the requirements, the collector current Ic is 200, the dark current ID is 50. Using the Equation (10), the calculated multiplication factor M (cal) can be obtained from Equation (17).
Therefore, from the Equation (2), the photodetector 10 satisfying the above requirements needs to have at least “8” layers in the amorphous selenium superlattice structure. Since the thickness of the amorphous selenium superlattice structure per layer is 16 nm, the layer thickness of the amorphous selenium layer 12 (total thickness of the amorphous selenium superlattice structure) required for the application is at least 16 nm×8 layers (or, 2×8 nm×8 layers)=128 nm.
(Ultraviolet Light Detector)
Requirements: In this application, the UV LED provides an incident power of 15 mW/m2 at a wavelength of 375 nm. As the photodetector, the photodetector 10′ (vacuum type imaging device form) of the second embodiment is adopted. The external readout circuit connected to the photodetector requires a current of at least 100 nA. The application allows dark currents up to 1 nA. The active region A from the diamond cold cathode layer 18 is a 100 μm square.
The photon energy E at wavelength λ=375 nm is obtained according to Equation (18).
When the photon energy E, having P=15 mW/m2, is incident on a photodetector with an active area of 0.01 mm2, the number of photons N00 is calculated as shown in Equation (19).
Here, from the requirements, the collector current Ic is 100, the dark current ID is 1. Using the Equation (10), the calculated multiplication factor M (cal) can be obtained from Equation (20).
Therefore, from the Equation (2), the photodetector 10′ satisfying the above requirements needs to have at least “104” layers in the amorphous selenium superlattice structure. Since the thickness of the amorphous selenium superlattice structure per layer is 16 nm, the layer thickness of the amorphous selenium layer 12 (total thickness of the amorphous selenium superlattice structure) required for the application is at least 16 nm×104 layers (or, 2×8 nm×104 layers)=1664 nm.
(X-Ray Photodetector)
Requirements: This application needs to detect X-rays with a minimum incident light power of 7.5 mW/m2. As the photodetector, the photodetector 10 (imaging device form) of the first embodiment is adopted. The external readout circuit connected to the photodetector requires a current of at least 5 nA. This application allows dark currents up to 3 nA. The absorption coefficient of the Si layer having a thickness of 500 μm at 30 keV is 0.14, and the ionization coefficient α SL of Si is 3.6 eV.
In the case of X-rays, the energy E has already been given in eV and needs to be converted to joules. The X-ray energy E in joules is obtained according to Equation (21).
E[J]=E[eV]*1.6022×10−19
E[J]=3.6×1.6022×10−19
E[J]=5.8×10−19[J] [Numerical Equation 21]
By adjusting the ionization coefficient, the absorption coefficient, and the active area of 100 μm2 from the tungsten tip, the number of photons N00 generated in the Si absorber can be obtained as shown in Equation (22).
Here, from the requirements, the collector current Ic is 5, the dark current ID is 3. Using the Equation (10), the calculated multiplication factor M (cal) can be obtained from Equation (23).
Therefore, from the Equation (2), the photodetector 10 satisfying the above requirements needs to have at least “83” layers in the amorphous selenium superlattice structure. Since the thickness of the amorphous selenium superlattice structure per layer is 16 nm, the layer thickness of the amorphous selenium layer 12 (total thickness of the amorphous selenium superlattice structure) required for the application is at least 16 nm×83 layers (or, 2×8 nm×83 layers)=1330 nm.
(Infrared Light Detector)
Requirements: It is assumed that this application needs to detect infrared light with a wavelength of 940 nm with a minimum incident light power of 5 μW/cm2. As the photodetector, the photodetector 10 (imaging device form) of the first embodiment is adopted. The external readout circuit connected to the photodetector requires a current of at least 250 nA. The application allows dark currents up to 50 nA.
The energy E of a single photon at λ=940 nm is obtained according to Equation (24).
When the photon energy E, having P=5 μW/cm2, is incident on a photodetector with an active area of 1 mm2, the number of photons N00 can be obtained according to the Equation (25).
Here, from the requirements, the collector current Ic is 250, the dark current ID is 50. Using the Equation (10), the calculated multiplication factor M (cal) can be obtained from Equation (26).
Therefore, from the Equation (2), the photodetector 10 satisfying the above requirements needs to have at least “5” layers in the amorphous selenium superlattice structure. Since the thickness of the amorphous selenium superlattice structure per layer is 16 nm, the layer thickness of the amorphous selenium layer 12 (total thickness of the amorphous selenium superlattice structure) required for the application is at least 16 nm×5 layers (or, 2×8 nm×5 layers)=80 nm.
Experiments were performed on several patterns to prove the above examples.
Photodetector for green light: A configuration of the device is shown in
From
The experimental M (exp) obtained from the above factors was 5.5 to 7.5. These values are in very good agreement with the calculated M (cal), which predicted M≈5.
Blue light photodetector: A configuration of the device is shown in
From
The experimental M (exp) obtained from the above factors was 7 to 12. These values are in very good agreement with the calculated M (cal), which predicted M≈8.
Ultraviolet light photodetector: A configuration of the device is shown in
From
The experimental M (exp) obtained from the above factors was 1100. This is significantly larger than the calculated M (cal) of 104. However, since the photodetector 10′ has the vacuum gap 17, the multiplication factor M is corrected by the Equation (11). The dependence of emission current on the vacuum gap 17 of the cold cathode layer 18 of diamond was reported in, for example, K. Okano et al., Electron emission from N-doped homoepitaxially grown diamond, Journal of Applied Physics 92: 4, 2194-2197. From this result, in the vacuum gap 17 of 60 um, the input impedance increases γ≈10 times, and the calculated M (cal) becomes 1040. This is in good agreement with the experimental M (exp) of 1100.
X-ray photodetector: A configuration of the device is shown in
From
The experimental M (exp) obtained from the above factors was 68 to 106. These values are reasonably consistent with the calculated M (cal) that predicted M≈83.
From the above verification experiments, it was confirmed that the quantum mechanical device designings proposed in the embodiments are well realized.
Therefore, according to the present invention, the thickness of the photoelectric conversion layer (amorphous selenium layer) that meets the requirements for any application can be easily designed quantum mechanically. In addition, by combining the photoelectric conversion layer with a substrate, it is possible to detect light well in a wide wavelength region of X-ray, ultraviolet light, visible light, and infrared light, thus, any devices with performance according to application requirements can be easily designed.
In the above embodiment, as an alternative to selenium, trigonal selenium, monoclinic selenium, rhombohedric selenium, or vitreous selenium may be used, as an alternative to arsenic triselenide, As2S3 (arsenic trisulfide), Sb2Se3 (diantimony triselenide), As2Te3 (diadenic tritelluride), P2Se3 (diphosphorus triselenide), CdTe (cadmium telluride), CdSe (cadmium selenide), ZnSe (zinc selenide), ZnTe (zinc telluride), PbSe (lead selenide), GaAs (gallium arsenide), GaP (gallium phosphorus), GaSe (gallium selenide), AlAs (aluminum arsenide), In2Se3 (Indium selenide), Se3Tl2 (Dithallium selenide), AlP (aluminum phosphorus), InP (indium phosphorus), AlSb (aluminum antimony), GaSb (gallium antimony), InAs (indium arsenic), InSb (indium antimony), it is also suitable to use these materials, and the amorphous selenium layer and the photodetector used by these are also understood to be included in the scope of the present invention.
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Number | Date | Country | |
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20230095246 A1 | Mar 2023 | US |
Number | Date | Country | |
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Parent | PCT/JP2020/032600 | Aug 2020 | US |
Child | 18074009 | US |