WO2021087290A1

WO2021087290A1 - Hard x-ray detectors with photon energy attenuation and electron generation-detection layers with integration capability to cmos image sensor (cis)-based or quanta image sensor (qis)-based devices

Info

Publication number: WO2021087290A1
Application number: PCT/US2020/058257
Authority: WO
Inventors: Eldred LEE; Jifeng Liu; Eric R. Fossum; Kaitlin ANAGNOST
Original assignee: Trustees Of Dartmouth College
Priority date: 2019-10-31
Filing date: 2020-10-30
Publication date: 2021-05-06

Abstract

Aspects of the disclosure provide a radiation detector. The radiation detector includes a plurality of layers including a first layer and a second layer. The first layer includes a first material configured to reduce energy from first photons incident on a first side of the first layer and transmit reduced-energy second photons through a second side of the first layer and into the second layer. The second layer includes a second material configured to convert the reduced-energy second photons to a plurality of photoelectrons. The radiation detector includes readout circuitry configured to transmit electrical signals based on the plurality of photoelectrons. The transmitted electrical signals represent an intensity of the first photons incident on the first side of the first layer as a function of position on the first layer. The first layer can be stacked on the second layer. The first material can include a high atomic number (high-Z) material.

Description

HARD X-RAY DETECTORS WITH PHOTON ENERGY ATTENUATION AND ELECTRON GENERATION-DETECTION LAYERS WITH INTEGRATION CAPABILITY TO CMOS IMAGE SENSOR (CIS)-BASED OR QUANTA IMAGE SENSOR (QIS)-BASED DEVICES

INCORPORATION BY REFERENCE

[0001] This present disclosure claims the benefit of priority to U.S. Provisional Application No. 62/704,068, "Novel light-matter interactions in the X-ray wavelength regime" filed on October 31, 2019, which is incorporated by reference herein in its entirety.

STATEMENT OF ACKNOWLEDGEMENT

[0002] This invention was made with Government support by P-25 Subatomic Physics Group at Alamos National Laboratory under subcontract number 537679 and under basis agreement number 537992 with the Trustees of Dartmouth College. This invention was also made with Government support under DE-NA0003864 awarded by Department of Energy National Nuclear Security Administration Laboratory Residency Graduate Fellowship (DOE NNSA LRGF). The Government has certain rights in the invention.

TECHNICAL FIELD

[0003] The present disclosure describes embodiments generally related to X-ray detection technologies.

BACKGROUND

[0004] The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

[0005] Detecting high-energy X-ray photons is challenging mainly due to low photon-to- photoelectron conversion efficiencies at higher energies. Scintillation is a prevalent method to detect X-rays. Related technologies such as Si direct detection based Si charge coupled device (CCD) can be inefficient for X-ray photons having energies larger than lOkeV.

SUMMARY [0006] Aspects of the disclosure provide a radiation detector. The radiation detector can include a plurality of layers including a first layer and a second layer. The first layer includes a first material and is configured to reduce energy from first photons incident on a first side of the first layer and transmit reduced-energy second photons through a second side of the first layer and into the second layer. The second layer includes a second material and is configured to convert the reduced-energy second photons to a plurality of photoelectrons. The radiation detector can include readout circuitry configured to transmit electrical signals based on the plurality of photoelectrons. The transmitted electrical signals represent an intensity of the first photons incident on the first side of the first layer as a function of position on the first layer. In an example, the first layer is stacked on the second layer.

[0007] In an example, the radiation detector includes pixelated carrier storage wells configured on an opposite side of the second layer with respect to the first layer. The pixelated carrier storage wells are configured to store the plurality of photoelectrons from the second layer. The readout circuitry is further configured to transmit the electrical signals based on the plurality of photoelectrons stored in the pixelated carrier storage wells. The pixelated carrier storage wells can be buried photodetector storage wells or pinned photodiode storage wells.

[0008] In an example, the plurality of layers is arranged in a backside illumination (BSI) configuration.

[0009] The first material can include a high atomic number (high-Z) material. In an example, the high-Z material in the first material comprises a high-Z semiconductor material or a high-Z conductive material. The high-Z material in the first material can include one of tin (Sn), cadmium telluride (CdTe), lead telluride (PbTe), and cadmium zinc telluride (CZT).

[0010] In an example, the second material comprises silicon (Si).

[0011] When the first photons are X-rays having energies between 20 keV and 40 keV, the first material of the first layer is configured to reduce the energy of the X-rays to an average energy below 10 keV.

[0012] A thickness of the first layer can be in a range of 1 micron to 10,000 microns, and a thickness of the second layer can be in a range of 1 micron and 500 microns. In an example, the thickness of the first layer is in a range of 0.25 microns to 10 microns, and the thickness of the second layer is in a range of 5 microns and 750 microns. [0013] In an example, a third layer is arranged between the first layer and the second layer. The third layer can be fabricated using epitaxial growth or ion implantation and can be configured to perform surface pinning.

[0014] In an example, a passivation oxide layer is arranged between the first layer and the third layer.

[0015] The radiation detector according to claim 1, wherein the readout circuitry is further configured in a quanta image sensor configuration, a pump-gate architecture, a 3-T architecture, or a 4T architecture.

[0016] When the first photons are X-rays having energies between 20 keV and 40 keV, the first material of the first layer can be configured to reduce the energy of the X-rays to an energy below 10 keV.

[0017] When the first photons are X-rays having energies between 30 keV and 40 keV, the first material of the first layer can be configured to reduce the energy of the X-rays to an average energy below 10 keV.

[0018] When the first photons are X-rays having energies between 10 keV and 15 keV, the first material of the first layer can be configured to reduce the energy of the X-rays to an average energy of 5 keV or less.

[0019] When the first photons are X-rays having energies between 50 keV and 100 keV, the first material of the first layer can be configured to reduce the energy of the X-rays to an energy below 20 keV.

[0020] In an example, a passivation oxide layer is arranged between the first layer and the second layer.

[0021] In an example, a thickness of the first layer is determined based on a thickness of the second layer and energies of the first photons incident on the first side of the first layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Further features, the nature, and various advantages of the disclosed subject matter will be more apparent from the following detailed description and the accompanying drawings in which:

[0023] FIG. 1 A shows an example of a two-layer X-ray detector structure according to an embodiment of the disclosure; [0024] FIGs. IB- ID show exemplary external photoemission X-ray detector structures according to embodiments of the disclosure;

[0025] FIG. IE shows an example of a metal-oxide-semiconductor (MOS) structure according to an embodiment of the disclosure;

[0026] FIG. 2A shows an exemplary portion of an X-ray detector having a two-layer structure according to an embodiment of the disclosure;

[0027] FIG. 2B shows an exemplary cross-section of a photon attenuation layer (PAL)- electron generation layer (EGL) X-ray detector according to an embodiment of the disclosure;

[0028] FIG. 3 shows an exemplary portion of an X-ray detector according to an embodiment of the disclosure;

[0029] FIG. 4A shows an exemplary photon energy distribution histogram of transmitted X-ray photons according to embodiments of the disclosure;

[0030] FIG. 4B shows peak information of peaks in Fig. 4A according to embodiments of the disclosure;

[0031] FIG. 5A shows an exemplary photon energy distribution histogram of transmitted X-ray photons according to embodiments of the disclosure;

[0032] FIG. 5B shows peak information of peaks in Fig. 5A according to embodiments of the disclosure;

[0033] FIG. 6 shows quantum yields (QY) as a function of an incident X-ray photon energy with a PAL (solid lines) and without a PAL (dashed lines) according to embodiments of the disclosure;

[0034] FIG. 7 shows a QY enhancement by a PAL according to an embodiment of the disclosure;

[0035] FIG. 8 shows an total number of electrons after impact ionization according to an embodiment of the disclosure;

[0036] FIG. 9 shows exemplary QYs as functions of PAL thickness for a 5 pm Si EGL at 20keV, 30keV, and 50keV incident X-ray photon energies, respectively, according to embodiments of the disclosure; and

[0037] FIG. 10 shows an exemplary QY as a function of a PAL thickness for a 200pm Si EGL at 20keV, 30keV, and 50keV incident X-ray photon energies, respectively, according to an embodiment of the disclosure. DETAILED DESCRIPTION OF EMBODIMENTS

[0038] High-energy X-ray photon (or X-ray) detection at a high quantum yield, a high spatial resolution and a short response time are important. Scintillation is a prevalent method but can be limited. Directly detecting high-energy X-ray photons is challenging mainly due to low photon-to-photoelectron conversion efficiencies for the high-energy X-ray photons. For example, silicon (Si) direct detection based Si charge coupled device (CCD) can be inefficient for X-ray photons having energies larger than 10 keV.

[0039] According to aspects of the disclosure, an X-ray detector including a photon attenuation layer (PAL) (also referred to as a PAL-based X-ray detector) (e.g., a PAL-based multi-layer X-ray detector) can be used for efficient detection of high-energy X-ray photons.

The PAL is configured to down-convert incident X-ray photons (or first X-ray photons) into second X-ray photons. The second X-ray photons can have lower energies than the first X-ray photons. The PAL can include high atomic number (Z) (high-Z) material(s) (e.g., high-Z semiconductor materials, high-Z conductive materials). In an example, the first X-ray photons detected by the PAL-based X-ray detectors can have high energies or high photon (X-ray) energies in a range of 10 to 50 kilo electron volt (eV) (keV), larger than 20keV, or the like. Aspects of the disclosure are also directed to methods of manufacturing the PAL-based multi layer X-ray detector.

[0040] The PAL-based X-ray detector can include a plurality of layers: a top high-Z PAL (or a first layer) and a bottom layer or a second layer (e.g., a layer including Si) that can detect the second X-ray photons and thus the PAL-based X-ray detector is the PAL-based multi-layer X-ray detector, such as the PAL-based two-layer X-ray detector. The bottom layer can be an electron generation layer (EGL). In an embodiment, the main mechanism for the PAL-based X- ray detector is an internal photoemission and photoelectric effect, unlike related detection technologies that rely on an external photoemission (e.g., as shown below in Figs. 1B-1D).

Monte Carlo simulation show that the PAL-based multi-layer X-Ray detector can be highly effective with significantly enhanced photon-to-electron conversion efficiencies and can have simple structure, and thus suitable for high-energy X-ray detection. Si can be used in the EGL to absorb the second X-ray photons and generate electrons. A mass attenuation coefficient of Si is relatively large when the photon energy is less than 10 keV and is significantly reduced when the photon energy is larger than 10 eV (see Figs. 4A and 5 A). According to aspects of the disclosure, based on the principle of photon energy down conversion, the high-energy first X-ray photons can be attenuated down to the second first X-ray photons (e.g., having energies less than or equal to lOkeV) in the PAL via inelastic scattering and thus the second first X-ray photons can be suitable for efficient photoelectric absorption by the EGL (e.g., the Si EGL).

[0041] The Monte Carlo simulation shows that an approximate 10 to 30 times increase in the quantum yield can be achieved using a PbTe PAL on the Si EGL, and thus the PAL-based X- Ray detector may advance high resolution, high-efficiency X-ray detection using PAL-enhanced Si CMOS image sensors.

[0042] Since the discovery of X-rays by Roentgen, the continuous expansion of X-ray technology has transformed our society from materials science to biomedical applications. High- energy X-rays detection (e.g., efficient and direct detection of high-energy X-ray photons in the 10-50 keV range and beyond) can be challenging, for example, due to unwanted effects such as inter-pixel scattering, crosstalk, inefficient energy deposition, and a low photon-to-electron conversion efficiency. The photon-to-electron conversion efficiency can be referred to as a quantum yield (QY) or an electron generation rate. The quantum yield (QY) can be determined based on a number (N_pre) of primary photoelectrons collected and a number (N_x.ray) of incident photons. In an example, the quantum yield (QY) is determined as a ratio of the number (N_pre) of primary photoelectrons collected over the number (Nx._ray) of incident photons using Eq. 1 as below.

QY = N_pre/ Nc __ray Eq. 1

[0043] Therefore, methods to design and/or manufacture efficient high-energy X-ray detectors that can overcome the undesirable features, for example, in a 10 to 50keV X-ray photon energy range (or a range of 10 to 50keV) are desirable. Furthermore, alleviating a capability gap for high-energy X-ray (e.g., hard X-ray) detectors, for example, in the range of 10 to 50keV or higher, can be effective in a next generation of light source facilities, high-energy X- ray imaging technologies, and/or the like.

[0044] Related X-ray detection technologies include scintillator technologies that are widely used, such as in high-energy X-ray imaging. In some examples, the scintillator technologies are limited in decay time responses and light yield. The spatial resolution in the scintillator technologies can be limited due to a thickness (e.g., relatively thick) of absorber material(s) in a scintillator and/or light propagation in the scintillator. In various examples, it is challenging to use scintillator technologies in advanced small-form-factor imaging sensor technology platforms, quantum image computing, and next generation of light source facilities. [0045] X-ray detectors can be based on direct detection using Si, such as used in Si charge-coupled device (CCD) and CMOS image sensors. X-ray detectors based on Si direct detection can be efficient when X-ray photon energies are less than or equal to 10 keV (e.g., in a 100 eV to 10 keV range). However, Si direct detection can be inefficient in detecting high- energy X rays (e.g., larger than 10 keV).

[0046] Certain limitations, for example, of the scintillator technologies can be overcome by related technologies. In an embodiment, a two-layer X-ray detector is used. The two-layer X-ray detector can include a metal buildup layer (e.g., copper or Cu) and a semiconductor thin film (e.g., Cadmium Telluride (CdTe)).

[0047] Fig. 1 A shows an example of a two-layer X-ray detector structure 220 according to an embodiment of the disclosure. The two layers X-ray detector structure 220 can include a layer 221 having a first material and a layer 222 having a second material. The layer 221 has a side 226 and a side 225. Electrons generated in the layer 222 can be detected when the electrons reach the side 225. The two layers X-ray detector structure 220 can have a relatively simple design, is relatively simple to be fabricated, and can be scaled. Referring to Fig. 1 A, the first material can be Cu and the second material can be CdTe. A large-area detector design includes the layer 222 (e.g., a Cu metal buildup layer) on top of the layer 221 (e.g., a thin-film CdTe semiconductor detector layer). The layer 222 (e.g., the Cu layer) can be considered as an absorber layer and the layer 221 (e.g., the CdTe layer) can be considered as a detector layer. In an example, X-ray photons (e.g., having an energy of 6 mega eV (MeV)) 223 can incident onto the layer 222 (e.g., the Cu layer) and can be converted to secondary electrons 224, for example, via Compton interaction in the Cu layer 222. The secondary electrons 224 can be transported to the CdTe detector layer 221. The Cu-CdTe two-layer configuration can have a high energy deposition (or a high dose delivery) and low inter-pixel scattering. When thicknesses of the layer 222 (e.g., the Cu layer) and the layer 221 (e.g., the CdTe layer) are relatively large, such as a thickness of 3 millimeters (mms) for the layer 222 (e.g., the Cu layer) and a thickness of 300 microns for the layer 221 (e.g., the CdTe layer), the Cu-CdTe two-layer configuration can be suitable for large-scale (e.g., centimeters) and a target area can be relatively large, such as 250 mm².

[0048] The two-layer X-ray detector structure 220 can be efficient and have a large area (e.g., on a centimeter or cm scale) and a relatively thick metal buildup layer, such as the mm- scale-thick layer 222 on the layer 221 (e.g., a semiconductor thin-film). The two-layer X-ray detector can have a high energy deposition, a high dose delivery (e.g., for radiation therapy), and low inter-pixel scattering effects. However, in some examples, when dimensions of the two- layer X-ray detector are decreased to a microscale (e.g., less than 300 microns), the two-layer X- ray detector can exhibit unwanted effects such as a low or inefficient energy deposition and a high inter-pixel scattering. The two-layer X-ray detector structure 220 may not be suitable with small-form-factor commercial-off-the-shelf technologies, such as CMOS image sensors (CIS) and quanta image sensors (QIS) as a cm-scale metal layer with a mm- thickness can cause significant X-ray crosstalk to adjacent pixels when a pixel pitch is relatively small (e.g., 20 to 50 microns). Further, the high-energy X-ray photons 223 are converted to the secondary electrons

224 through Compton interaction in the top mm-scale metal buildup layer 222 followed by the transport of the secondary electrons 224 into the semiconductor thin-film layer 221 for electron detection. If photon energies of the incident X-ray 223 are relatively low compared to energies intended for certain medical applications (e.g., in a MeV range), the low-energy secondary electrons (e.g., a 1-10 keV range) 224 may not be able to escape the thick metal buildup layer 222 as electron mean free paths are small (e.g., less than 1 micron) in various materials that can be used to fabricate the layer 222. Therefore, the secondary electrons 224 may not reach the side

225 of the semiconductor thin-film layer 221 for electron detection. Even if the thickness of the semiconductor thin-film layer 221 for electron detection is decreased to a pm-scale, certain secondary electrons 224 can still remain in the mm-scale-thick metal buildup layer 222 because of a relatively small travel range of electrons. Thus, the two-layer X-ray detector 220 has a relatively low spatial resolution and may not be suitable for X-ray energies in certain ranges, such as 10 to 50 keV.

[0049] In another related technology, a structured photocathode X-ray detector design is used. In an example, the structured photocathode X-ray detector is based on external photoemission, an angle of X-ray photon absorption, and collection of electrons converted from X-ray photons using an external electric field. Figs. IB- ID show exemplary external photoemission X-ray detector structures 214-216. The external photoemission X-ray detector structures 214-216 can be conical 3D cavity structures and correspond to a shallow depth (e.g., 4 microns), a middle depth (e.g., 8 microns), and a full depth (e.g., 16 microns) in bulk structures 211-213, respectively. The bulk structures 211-213 can include semiconductor material(s), such as Si. Referring to Figs. 1B-1D, a dimension (e.g., a length, a width, a diameter) of 6 microns and an angle (e.g., 10°) can be used in the external photoemission X-ray detector structures 214- 216.

[0050] The full-depth conical 3D cavity structures 216 can have a higher photon-to- electron conversion efficiency when compared with the shallow depth structure 214 and the middle depth structure 215. In an example, the full-depth conical 3D cavity structures 216 has a 5% photon-to-electron conversion efficiency or quantum yield (QY) for X-ray photon energies around 7.5keV. The quantum yield (QY) can be defined as the ratio of the number (N_pre) of primary photoelectrons collected over the number (N_x.ray) of incident photons using Eq. 1. The 5% quantum yield (QY) is higher than those (e.g., about 1-1.5%) from other external photoemission for nanometer (nm) and micron-scale X-ray detectors. In general, the X-ray photon absorption coefficient can decay rapidly with an increase of incident photon energies, and thus resulting in a lower QY at higher incident photon energies. Accordingly, it is challenging for the structured photocathode X-ray detector to achieve a high (e.g., 5%) efficiency for X-ray photon energies that are higher than 7.5 keV, such as above 20keV.

[0051] Fig. IE shows an example of a metal-oxide-semiconductor (MOS) structure 200 according to an embodiment of the disclosure. In an embodiment, the MOS structure 200 includes a Tin (Sn)-Si02-Si structure 200, i.e., a Sn absorber 204, a Si02 layer 203, and a Si layer 202 on a conductive plate (e.g., a copper plate 201). The MOS structure 200 can be used to transport UV-excited hot electrons from the Sn absorber 204 through a metal-oxide interfacial barrier to a conduction band of the Si layer 202 for a solar-blind UV detector that can be efficient. The metal Sn absorber 204 can absorb high-energy X-rays and thus generate hot electrons and transfer the hot electrons to the semiconductor material (e.g., Si) 202. A lock-in amplifier 206 can be used to measure the hot electrons. In an example, the hot electron generation (e.g., in the Sn absorber 204)-transport method can be applied to a high-performance CMOS image sensor platform to create a high-energy X-ray detector.

[0052] According to aspects of the disclosure, the PAL-based multi-layer X-ray detector including a two-layer structure can be used. Fig. 2A shows an exemplary portion of the X-ray detector 100 A having the two-layer structure 110A according to an embodiment of the disclosure. The two-layer structure 110A can include a first layer 101 and a second layer 104. The first layer 101 can have a high-atomic number (Z) material (e.g., a high-Z semiconductor material, a high-Z metal material) and can be a PAL. The second layer 104 can be an EGL or an EGL. In an example, the two-layer X-ray detector is referred to as a PAL-EGL structure or a PAL-EGL radiation detector. The first layer can be stacked on the second layer.

[0053] The first layer 101 and the second layer 104 can have any suitable thicknesses. The thickness of the first layer 101 can be in a range of 0.25 microns to 10,000 microns, and the thickness of the second layer 104 can be in a range of 1 micron and 500 microns. In an example, the thickness of the first layer 101 is from 1 to 10000 microns. In an example, the thickness of the first layer 101 is from 0.25 to 10 microns. In an example, the thickness of the second layer 104 is from 1 to 500 microns. In an example, the thickness of the second layer (or EGL) 104 can be from 5 microns to 750 microns, which corresponds to a typical range between CIS/QIS and a wafer (e.g., a commercial wafer). As shown below (e.g., Figs. 4A, 5A, 6-12), the PAL-EGL structure can have a total thickness in a micron scale. For example, the thickness of the PAL 101 is less than or equal to 3 microns and the thickness of the EGL 104 is less than or equal to 200 microns. The thickness of the PAL 101 can be in a range of 0.25 microns to 3 microns, and the thickness of the second layer 104 can be in a range of 1 micron to 200 microns. The electron generate rate of the PAL-EGL structure can be relatively high when a dimension (e.g., a dimension LI or L2 shown in Fig. 2A in a direction that is perpendicular to the thickness of the EGL) of the PAL-EGL structure decreases. The PAL-EGL structure 110A or the PAL-based multi-layer X-ray detector 100 A can have any suitable dimension(s) and shape. In an example, the PAL-EGL structure 110A or the PAL-based multi-layer X-ray detector 100 A has a rectangular shape shown in Fig. 2A. Referring to Fig. 2A, the width LI and the length L2 of the rectangular shape can be 50 microns. The width LI and the length L2 can be 25 microns, 100 microns, or the like. The width LI and the length L2 can be identical or different. The PAL- EGL structure can have low inter-pixel and high intra-pixel scattering with a relatively large electron generation rate and high energy depositions. The EGL can include Si, and using Si in the EGL can facilitate integration of the PAL-based X-ray detector into CMOS image sensor technology.

[0054] Two types of primary interaction mechanisms in the PAL-EGL structure 110A are photon energy down conversion (or red-shift) due to inelastic collisions in the PAL 101, followed by photoelectric absorption in the EGL 104. According to aspects of the disclosure, the first layer 101 (also referred to as a top layer that first X-ray photons 121 (e.g., higher energy X- ray photons) are incident onto) is a PAL instead of an absorber layer as used in the two-layer X- ray detector structure 220. [0055] Because X-ray absorption and scattering cross-sections can increase as atomic numbers increase, high-Z materials can be chosen for the PAL layer 101 and thus the PAL 101 can include a high-Z thin film. In the PAL 101, a photon can lose energy via single and multiple inelastic collision events and then undergo a notable red-shift from an incident X-ray wavelength, leading to an effective X-ray photon energy attenuation down to an energy that is less than 10 keV. Subsequently, photoelectric absorption can occur in the EGL 104 (e.g., a Si EGL) with relatively high absorption coefficient, thereby significantly improving the QY. Depending on a thickness of the EGL 104, additional photon energy attenuation may occur within the EGL 104 (e.g., the Si EGL) prior to the photon-to-electron conversion. Cascade processes, such as impact ionization, that lead to further multiplication gain can occur following the photoelectric absorption because an average energy of X-ray excited primary photoelectrons can be on an order of keV, while an average energy to create an electron-hole pair (EHP) in Si is 3.65eV. Though a main mechanism in the PAL can be photon energy attenuation, an X-ray photon may be absorbed and then converted to a photoelectron in the PAL.

[0056] Referring to Figs. 1 A and 2A, in the PAL-EGL structure 110A, the first layer 101 (e.g., the metal layer) can act to reduce energies of the first X-ray photons 121, for example, through collisions rather than acting as a direct absorber. Accordingly, the first layer 101 (e.g., the metal layer) can convert (or down-convert) the first X-ray photons 121 having first energies to second X-ray photons 122 (e.g., lower energy X-ray photons) having second energies. One of the second energies can be smaller than one of the first energies. An average of the second energies can be smaller than an average of the first energies.

[0057] In various examples, the second X-ray photons 122 still remain in the X-ray spectral regime as opposed to the UV and visible regime. Thus, unlike scintillators, the down conversion primarily relies on inelastic scattering with high-Z atoms, therefore no expensive bulk crystal (as in the case of scintillators) is necessary for the PAL layer 101. The PAL layer 101 can be poly crystalline or amorphous thin films. Thus, the PAL layer can be easier and cheaper to fabricate than bulk crystal scintillators. A response time is not limited by an optical spontaneous emission lifetime (e.g., in scintillators) and thus may allow for a faster response (e.g., an ultrafast response) than that in scintillators since X-ray photon energy down conversion time via X-ray fluorescence and/or inelastic scattering is typically much shorter than the optical fluorescence time in scintillators. The PAL-EGL structure 110A can offer integration capabilities to CIS- or QlS-based devices (e.g., Si CIS- or QlS-based devices) for high resolution X-ray imaging.

[0058] Electrons can be generated by the second X-ray photons 122 that are resulted from inelastic scattering and other collision events. The electrons can be detected in the Si EGL 104. The electrons that are detected in the EGL 104 can have energies in an order of 10² and 10³ eV. Thus, the principle for the PAL-EGL structure 110A can be photon down conversion where the first X-ray photons 121 having higher energies are converted to the second X-ray photons 122 having lower energies.

[0059] As described above, detection of high-energy X-ray photons can be implemented by using the PAL 101 (e.g., a high-Z thin film) to attenuate the incident photon energy, for example, to below 10 keV. Thus, more efficient absorption of the down-converted X-ray photons 122 by the EGL (e.g., a Si detector) 104 can be implemented. Comparing the PAL- based multi-layer X-ray detector 100 A with related technologies using direct Si detection, the PAL-based multi-layer X-ray detector 100A is configured to down-convert high energy X-ray photons to lower energy X-ray photons (e.g., below 10 keV) and subsequently use the EGL (e.g., the Si EGL) to direct detect the lower energy X-ray photons. In contract, the related technologies using direct Si detection directly detect the high energy X-ray photons as the related technologies using direct Si detection do not have the PAL. Accordingly, the PAL-based multi layer X-ray detector 100 A can be significantly more efficient (e.g., 9-29 times increase of the quantum yield as shown in Fig. 7) than the direct detection of the high energy photons. Further, given the micron scale thickness of the PAL-EGL structure 100 A, the spatial resolution can be on the micron scale, such as less than 5 microns.

[0060] In an example, a Monte Carlo simulation (e.g., using Monte Carlo N-Particle Software (MCNP6.2)) demonstrates a significant QY enhancement (e.g., about 9 to 29 times) in the X-ray photon energy range of 20-50keV with a 1 micron PAL and a 5 to 200 microns Si EGL, as indicated below in Fig. 7.

[0061] Fig. 2B shows an exemplary cross-section of a PAL-EGL X-ray detector 100B according to an embodiment of the disclosure. The PAL-EGL multi-layer X-ray detector 100B can include a multi-layer structure 110B. The multi-layer structure 110B can include the first layer 101 and the second layer 104 that are described above. Further, the multi-layer structure 110B can include additional layer(s) that do not affect or minimally affect the electron generation rate (e.g., down-converted X-ray photons can readily penetrate through the additional layer(s)).

In general, the additional layer(s) can be optional.

[0062] The multi-layer structure 110B can include an oxide layer 102 between the PAL layer 101 and the EGL layer 104. The oxide layer 102 can be used for passivation purpose and can be referred to as a passivation oxide layer (e.g., a backside passivation oxide layer). The oxide layer 102 can be a thin film (e.g., a thickness of less than 100 nm) and include Si0_2. In an example, the thickness of the oxide layer 102 is from 100 nm to 7.5 microns. The multi-layer structure 110B can include a layer 103 (e.g., a third layer) for surface pinning, for example, between the oxide layer 102 and the EGL 104. The layer 103 can be arranged between the first layer 101 and the second layer 104. The third layer 103 can be fabricated using epitaxial growth or ion implantation. In an example, the oxide layer 102 is positioned between the PAL layer 101 and the third layer 103.

[0063] The PAL-EGL multi-layer X-ray detector 100B can be integrated to a CMOS image sensor (ClS)-based device or a quanta image sensor (QlS)-based device. In an example, referring to Fig. 2B, the CIS-based device or the QlS-based device can have a backside illumination (BSI) configuration, and thus is referred to as the BSI CIS-based devices or the QIS- based device. Fig. 2B shows the cross-section of the PAL-EGL multi-layer X-ray detector 100B after the CIS- or the QlS-based integration. The PAL-EGL multi-layer X-ray detector 100B can include front side pixel readout circuitry 106 for pixel readout. The EGL 104 can include pixelated carrier storage wells 105. The pixelated carrier storage wells 105 can include buried photodetector storage wells, pinned photodiode storage wells, and/or the like. In an example, the buried photodetector storage well is advantageous over the pinned photodiode storage well where the buried photodetector storage well can have a reduced dark current and a greater resistance to radiation damage relative to the pinned photodiode storage well. The readout circuitry 106 can be configured in a QIS configuration, a pump-gate architecture, a 3-T architecture, 4T architecture, or the like.

[0064] The PAL-EGL multi-layer X-ray detector 100B can include a handle layer or additional stacked readout circuitry layer 107.

[0065] In an example, the PAL-based X-ray detector 100 A further includes the oxide layer 102, the layer 103, the pixelated carrier storage wells 105, the readout circuitry 106, and/or the like. The PAL-based X-ray detector 100A shows a perspective view of a PAL-based X-ray detector while the PAL-based X-ray detector 100B shows a cross-section view of the PAL-based X-ray detector. The cross-section view shown in Fig. 2B is sectioned along CC’ of Fig. 2A.

[0066] Referring to Fig. 3, in an example, an X-ray detector 300 can include a two-layer structure 310 including a PAL 301 (e.g., having a thickness of 1 micron) and an EGL 304. The X-ray detector 300 can be referred to as the PAL-based multi-layer X-ray detector. The PAL 301 and the EGL 304 can have a cylindrical shape with any suitable diameter, such as from 5 microns to 200 microns. Thicknesses of the PAL 301 and the EGL 304 can be identical or similar to the thicknesses of the PAL 101 and the EGL 104, respectively. In an example, the thickness of the PAL 301 is from 1 to 10000 microns. In an example, the thickness of the PAL 301 is from 0.25 to 10 microns. In an example, the thickness of the EGL 304 is from 1 to 500 microns. In an example, the thickness of the EGL 304 is from 5 to 750 microns.

[0067] Materials and functions of the PAL 301 and the EGL 304 can be identical or similar to those of the PAL 101 and the EGL 104, respectively and thus detailed descriptions are omitted for purposes of brevity. Incident high energy X-ray photons 321 can be down-converted in the PAL 301 to lower energy X-ray photons 322. Subsequently, the lower energy X-ray photons 322 can be absorbed by the EGL 304 to generate photoelectrons. The photoelectrons can be further detected by other components in the X-ray detector 300, such as readout circuitry. The X-ray detector 300 can include any suitable additional layer(s), such as an oxide layer, a layer for surface pinning, pixelated carrier storage wells, readout circuitry that are similar or identical to the oxide layer 102, the layer 103, the pixelated carrier storage wells 105, the readout circuitry 106 shown in Fig. 2B.

[0068] Layers in the PAL-EGL multi-layer X-ray detector 100B, such as the PAL layer 101, the oxide layer 102, the layer 103, the EGL 104, and the like can have any suitable dimensions and thicknesses for specific detector requirements and applications. Thus, the dimensions and thicknesses of the layers in the PAL-EGL multi-layer X-ray detector 100B can be scaled up or down depending on applications. In various examples, the dimensions and thicknesses of the layers in the PAL-EGL X-ray detector 100 A or 100B are in a micrometer- scale. For example, referring to Figs. 2A-2B, the thickness of the oxide layer 102 can be from 100 nm to 7.5 microns. Referring to Figs. 2A-2B and 3, the thickness of the EGL 104 or the EGL 304 can be from 5 microns to 750 microns, which is a typical range between CIS/QIS and a wafer (e.g., a commercial wafer). Thus, the PAL-EGL X-ray detector 100A, 100B, or 300 can be applicable for an X-ray energy range from 10 to 100 keV. Embodiments and examples of the disclosure are based on an energy range of X-ray photons being 10 to 50keV. In general, the PAL-EGL multi-layer X-ray detector (e.g., 100 A, 100B) including the two-layer structure (e.g.,

110 A) or the multi-layer structure (e.g., 110B) including a PAL and an EGL can be suitable for a lower energy range (e.g., 10 to 50 keV) or a higher energy range (e.g., 50 to 100 keV), for example, within the hard X-ray regime (e.g., X-ray photon energies in the hard x-ray regime are in a range from 10 to 100 keV). Thus, the disclosure should not have energy range limitations within the hard X-ray regime.

[0069] In related technologies, X-ray detectors can be based on direct detection using Si, such as Si photodiode array with scintillators. X-rays can be directed at an object and can be converted into light by the scintillators, as described above. In general, there is an inverse relationship between an amount of photon absorption and photon energies. Thus, there can be a significant decrease of the electron generation rate at higher X-ray energies.

[0070] Simulations, such as Monte Carlo simulations using Monte Carlo N-Particle Software (MCNP6.2) are implemented to determine quantum yields (QYs) and QY enhancement factors in various scenarios. In various examples, the thickness of the high-Z PAL 101 can be set to be lpm to demonstrate that a thin high-Z material layer can significantly enhance high- energy X-ray photon energy attenuation, leading to efficient photoelectric absorption in the EGL layer 104 (e.g., the Si EGL layer). On the other hand, the thickness of the high-Z PAL 101 can be determined (e.g., optimized) based on the thickness of the EGL layer 104 (e.g., the Si EGL layer) and photon energies of the incident X-ray photons (e.g., the incident X-ray photons 121).

[0071] In an example, the thickness of the EGL layer 104 (e.g., the Si EGL layer) is determined (or chosen) based on a typical range (e.g., 5 microns to 750 microns) between CIS/QIS and commercial Si wafers. Thus, the thickness of the EGL layer 104 can be from 5 microns to 750 microns.

[0072] Any suitable high-Z materials, such as high-Z metals or semiconductors, can be used as PAL materials in the PAL 101. The high-Z material can include one of Sn, CdTe, lead telluride (PbTe), and cadmium zinc telluride (CdZnTe or CZT). High-Z semiconductor materials such as CdTe, CZT, and PbTe can be used as PAL materials in the PAL 101 due to chemical stability and material availability of the above high-Z semiconductor materials. In an example, PbTe is used in the PAL 101 because Pb has a higher atomic number than Cd. Further, a PbTe thin film can be easier to fabricate than CdTe and CZT thin films. Thus, in the simulations,

PbTe is used in the PAL 101. [0073] The X-ray detector described in Figs. 2A, 2B, and 3 can also be referred to as an X-ray detection device or a radiation detector. Further, the X-ray detectors can be suitably adapted to other radiation wavelengths that are outside the X-ray wavelengths by making suitable modifications to the materials, dimensions, and thicknesses of the multiple layers in the radiation detector.

[0074] To verify photon energy attenuation in a thin-film PAL (e.g., the PAL 101) using high-Z semiconductor material(s) (e.g., a high-Z semiconductor material), energy distributions of photons transmitted through the PAL are modeled with Monte Carlo simulations (e.g., MCNP simulations using MCNP6.2). The MCNP simulations can be conducted with 10⁵ incident X-ray photons (e.g., a number of the first X-ray photons 121 is 10⁵). Figs. 4A and 5A show exemplary photon energy distribution histograms of transmitted X-ray photons (e.g., the down-converted X- ray photons or the second X-ray photons 122) plotted in 0. IkeV bins according to embodiments of the disclosure. The PAL-based multi-layer X-ray detector (e.g., 110A) can be used in the MCNP simulations. The total number of incident photons is 10⁵. The PAL (e.g., the PbTe PAL) is 1 micron thick.

[0075] Further, Figs. 4A and 5A show a mass attenuation coefficient of Si as a function of photon energy overlapped with the photon energy histograms of the down-converted X-ray photons. Within the range of 1 to 30 keV shown in Figs. 4A and 5A, the mass attenuation coefficient of Si has a peak around 3 keV and decrease with the X-ray photon energy when the X-ray photon energy is larger than 3 keV. The mass attenuation coefficient spectrum indicates that X-ray photon energies below 10 keV can be efficiently absorbed by Si, and thus it is desirable to attenuate the X-ray photon energy to below or equal to 10 keV.

[0076] The incident X-ray photons (e.g., the first X-ray photons 121) in Fig. 4A have a 20 keV energy. The incident X-ray photons (e.g., the first X-ray photons 121) in Fig. 5A have a 30 keV energy.

[0077] Referring to Figs. 4A and 5A, the transmitted X-ray photons (e.g., the second X- ray photons 122) can have lower energies than the incident X-ray photons (e.g., the first X-ray photons 121), and a notable fraction of the transmitted X-ray photons has energies that is less than 10 keV to facilitate absorption by the EGL 104 (e.g., the Si EGL) after transmitting through the lpm PbTe PAL.

[0078] Figs. 4A and 5A do not show an energy range of 0-lkeV since there is a default artificial photon energy cutoff around IkeV in the MCNP simulation and effects for scattering leading to lower energies (e.g., 0-lkeV) are not included in the MCNP simulation. Thus, photon energies below the cutoff are not calculated. As the mass attenuation coefficient of Si can decrease with the X-ray photon energy, and thus the transmitted X-ray photons in the 0-1 keV range can have higher mass attenuation coefficients than those of X-ray photons at higher energies (e.g., energies larger than 1 keV). Accordingly, the photon absorption in the EGL can be underestimated when the transmitted X-ray photons in the 0-1 keV range are not considered.

[0079] Referring to Figs. 4A and 5A, the photon energies can be down-converted mainly into two regimes after transmitting through the lpm-thick PbTe PAL: a first regime and a second regime. The first regime can include a nearly continuous low energy spectrum from 1 to 5 keV where Si has a large mass attenuation coefficient for efficient absorption. The first regime can be induced by multiple inelastic scatterings of the incident X-ray photons. The second regime can include sharp and discrete energy peaks (e.g., peaks 1-6 in Fig. 4A and peaks 1-10 in Fig.

5A) corresponding to (i) characteristic X-ray emissions of Pb or Te atoms or (ii) the incident photon energy subtracted by energy losses from the absorption edges of Pb or Te. The discrete energy peaks in the second regime can be induced by X-ray photons that have experienced one to a few inelastic scattering events. Fig. 4B includes peak information of the peaks 1-6 in Fig. 4A. Fig. 5B includes peak information of the peaks 1-10 in Fig. 5 A.

[0080] Referring to Figs. 4A-4B, for the 20 keV incident X-ray photons, the peaks 1-3 in the second regime are located at an energy range (e.g., a low energy less than 10 keV) where X- ray photons can be effectively absorbed by Si in the EGL. Therefore, both the first regime and the second regime can contribute significantly to enhanced X-ray absorption in Si.

[0081] Referring to Figs. 5A-5B, as the incident photon energy increases to 30 keV, most (e.g., the peaks 4-10) of the characteristic X-ray peaks are located at an energy range that is larger than 10 keV. Thus, efficient absorption of down-converted photons at an energy range of 1 to 5keV in the first regime can be the dominant mechanism of QY enhancement for Si detectors, for example, when the incident photon energy increases to 30 keV and above.

[0082] As shown below in Figs. 6-7, the PAL (e.g., the PAL 101) in the two-layer structure (e.g., 110A) or the multi-layer structure (e.g., 110B) can enhance a quantum yield of the two-layer structure or the multi-layer structure. Fig. 6 shows quantum yields (%) in the EGL (e.g., the Si EGL) as a function of an incident X-ray photon energy with a lpm PbTe PAL (solid lines) and without PbTe PAL (dashed lines). The thicknesses of the EGL are 5pm, 50pm, and 200pm, respectively. The Monte Carlo simulation (e.g., the MCNP simulation) described above can be used to obtain the quantum yields in Fig. 6. In an example, the total number of incident X-ray photons is 10⁵.

[0083] Figs. 6-7 compare the quantum yields of Si hard X-ray detectors including the EGL (e.g., the Si EGL) with and without the PAL. The quantum yield can be determined using Eq. 1. As described above, the quantum yield decreases with an increase of the incident photon energy. For the same Si EGL thickness, Figs. 6-7 show that incorporating the lpm-thick PAL layer can effectively increase the quantum yield of the Si detectors (e.g., the Si EGL) by, for example, approximately 9 to 20 times depending on the incident photon energy and the thickness of the Si EGL. For the 5pm, 50pm, and 200pm Si EGL , the QY with 1pm PbTe PAL (solid lines) ranges between 6.54% and 33.48% for 20keV X-ray photons. Even the thinnest 5pm Si EGL with the PAL demonstrates about 2 times higher QY than the thickest 200pm Si EGL without the PAL. Furthermore, the QYs with the PAL are all higher than the about 5% QY at 7.5keV incident X-ray photon energy that is described above.

[0084] Fig. 7 shows QY enhancement factors as a function of an incident photon energy for the three Si thicknesses (e.g., 5, 50, and 200pm), respectively. Fig. 7 shows that the QY enhancement by the PbTe PAL can increase with the incident X-ray photon energy. Thus, devices (e.g., X-ray detectors) with the PAL can have less QY degradation at higher X-ray photon energies. The above feature can be advantageous for high-energy X-ray detection. Figs. 6-7 show that the 1pm PbTe PAL can significantly increase the QY.

[0085] In various examples, Figs. 6-7 are based on primary photoelectrons (also referred to as primary photoelectrons pre-multiplication gain) in the Si EGL prior to multiplication gain processes. Referring to the solid lines in Fig. 6, average electron energies of the primary photoelectrons in the Si EGL with the PAL are determined to be in a keV range. Therefore, multiplication gain processes due to regenerative actions can occur. Impact ionization processes to promote electrons in the valence band to the conduction band can further provide a multiplication gain to the number of electrons generated within the EGL (e.g., the Si EGL). Fig. 8 shows the number N_pre (a fourth column in Fig. 8) of primary photoelectrons upon X-ray excitation associated with the QY as described above and an approximate total number N_post of electrons (also referred to as total number of electrons post-multiplication gain) after impact ionization. The total number (N_post) of electrons post-multiplication gain processes can be determined based on Eq. 2 as below. Eq. 2 where E_avg represents an average electron energy of the primary photoelectrons in the EGL (e.g., the Si EGL) and N_pre represents the number of the primary photoelectrons generated in the EGL (e.g., the Si EGL) prior to additional multiplication gain processes. E_avg and N_pre can be determined using MCNP6.2 for devices with the lpm PbTe PAL (i.e. corresponding to the solid lines in Fig. 6). E_EHP is an average energy required to generate an EHP in Si and can be 3.65eV.

[0086] In general, the total number (N_post) of electrons post-multiplication gain processes can be determined based on the average energy ( E_avg ) in the EGL, the number N_pre of the primary photoelectrons generated in the EGL (e.g., the Si EGL) prior to additional multiplication gain processes, and the average electron energy E_EHP used to generate an EHP in Si. Based on Eq. 2, the total number (N_post) of electrons post-multiplication gain processes can be approximated as a multiplication of E_avg and N_pre divided by E_ehp.

[0087] Referring to Fig. 8, the number (Nx._ray) of the incident X-ray photons is 10⁵, the first column represents the incident X-ray energies (e.g., 20, 30, and 50 keV), the second column represents thicknesses (e.g., 5, 50, and 200 microns) of the Si EGL, the third column represents the average electron energy ( E_avg ) in the EGL (e.g., 4.83, 6.79, and 11.00 keV for the EGL thickness of 5 microns), the fourth column represents the number N_pre of the primary photoelectrons generated in the EGL (e.g., the Si EGL) pre-multiplication gain processes (or before impact ionization/multiplication), and the fifth column represents the total number (N_post) of electrons post-multiplication gain (or after impact ionization/multiplication), for example, determined using Eq. 2. N_pre (the number of X-ray excited primary photoelectrons before multiplication gain) in the fourth column can be used to determine the QY in Fig. 6. Comparing N_pre (the number of X-ray excited primary photoelectrons before multiplication gain) in the fourth column and the number N_post of electrons post-multiplication gains in the fifth column, there is an approximate post-multiplication gain (G) of 10³-10⁴. The post-multiplication gain G can be determined as a ratio of N_post over N_pre. As seen in the third column, the average electron energies ( E_avg ) in the EGL can range from 4.39 to 11.00 keV and thus further multiplication gain processes can occur to provide at least three orders of magnitude gain in the number N_post of electrons post-multiplication gains in the fifth column. As expected, a thicker Si EGL can lead to a higher number of electrons (e.g., as shown in the fourth column) and a higher QY. In an example, a thinner Si EGL may lead to a higher spatial resolution. [0088] The thickness of the PAL in the two-layer structure 110A or the multi-structural 110B can be any suitable thickness. In the above description, the thickness of the PAL is 1 micron. The thickness of the PAL can be optimized based on the incident X-ray photon energy and the thickness of the EGL (e.g., the Si EGL).

[0089] In Figs. 9-10, the two-layer structure includes the PAL and the Si EGL. The Si EGLs in Figs. 9-10 have a thickness of 5 pm and 200 pm, respectively. The PAL can be positioned on top of the Si EGL.

[0090] FIG. 9 shows exemplary QYs as functions of the PAL thickness (e.g., from 0 to 3 microns) for the 5pm Si EGL at 20keV, 30keV, and 50keV incident X-ray photon energies, respectively. In the example of Fig. 9, the PAL includes PbTe. In Fig. 9, the QYs peak approximately between 1pm and 1.5pm PbTe PAL and thus an optimal PbTe PAL thickness is approximately between 1pm and 1.5pm for the 5pm Si EGL.

[0091] FIG. 10 shows an exemplary QY as a function of the PAL thickness (e.g., from 0 to 1.5 microns) for the 200pm Si EGL at 20keV, 30keV, and 50keV incident X-ray photon energies, respectively. In the example of Fig. 10, the PAL includes PbTe. In Fig. 10, the QYs peak approximately between 0.5pm and 0.75pm PbTe PAL and thus an optimal PbTe PAL thickness is approximately between 0.5pm and 0.75pm for the 200pm Si EGL. In Fig. 10, the QYs approaching 40% and 16% can be achieved for the 20keV and 30 keV incident photons. Figs. 8-9 indicate that the PAL can be integrated with Si-based high-energy X-ray detectors and the thickness of the PAL can be optimized based on the incident X-ray photon energy and the thickness of the EGL.

[0092] According to aspects of the disclosure, a high-energy X-ray direct detection method and detector can enhance the QY by approximately 10-30 times for Si-based X-ray detectors including a high-Z PAL (Fig. 7). The high-energy PAL-based multi-layer X-ray direct detection method and detector may surpass the performance of related X-ray detectors based on Si CCD or photocathodes without the PAL. The high-Z PAL-based multi-layer X-ray detector is based on X-ray photon energy down-conversion followed by a direct detection of the lower energy (or down-converted) X-ray photons. The high-Z PAL-based multi-layer X-ray detector has a simple and a highly effective device structure. The high-Z PAL-based multi-layer X-ray detector can have high spatial resolution (for example, due to a micron scale thickness of the PAL-EGL structure) and fast response time. The PAL layer material can be optimized. Further, the high-Z PAL-based multi-layer X-ray detector can be monolithic integrated with an image sensor (e.g., a Si CIS) and the PAL enhanced image sensor can be used in a wide field-of-view X-ray camera designs for synchrotron and X-ray free electron laser light source applications.

[0093] According to aspects of the disclosure, a radiation detector (e.g., an X-ray detector) can include a plurality of layers including a first layer (e.g., the PAL 101) and a second layer (e.g., the EGL 104). The first layer can include a first material (e.g., a high-Z material) and can be configured to reduce energy from first photons (e.g., the first X-ray photons 121) incident on a first side of the first layer (e.g., the PAL 101) and transmit reduced-energy second photons (e.g., the second X-ray photons 122) through a second side of the first layer (e.g., the PAL 101) and into the second layer (e.g., the EGL 104). The second layer (e.g., the EGL 104) can include a second material (e.g., Si) and can be configured to convert the reduced-energy second photons (e.g., the second X-ray photons 122) to a plurality of photoelectrons (e.g., primary photoelectrons). In an example, the plurality of layers is arranged in a BSI configuration. The first layer can be stacked on the second layer.

[0094] The radiation detector (e.g., the X-ray detector) can further include readout circuitry (e.g., the readout circuitry 106) configured to transmit electrical signals based on the plurality of photoelectrons. The transmitted electrical signals can represent an intensity of the first photons (e.g., the first X-ray photons 121) incident on the first side of the first layer (e.g., the PAL 101) as a function of position on the first layer. The radiation detector (e.g., the X-ray detector) can further include pixelated carrier storage wells (e.g., the pixelated carrier storage wells 105) configured on an opposite side of the second layer (e.g., the EGL 104) with respect to the first layer (e.g., the PAL 101). The pixelated carrier storage wells can be configured to store the plurality of photoelectrons from the second layer where the readout circuitry is further configured to transmit the electrical signals based on the plurality of photoelectrons stored in the pixelated carrier storage wells.

[0095] In an example, when the first photons are X-rays having energies between 20 keV and 40 keV, the first material of the first layer is configured to reduce the energy of the X-rays to an average energy below 10 keV.

[0096] In an example, when the first photons are X-rays having energies between 20 keV and 40 keV, the first material of the first layer is configured to reduce the energy of the X-rays to an energy below 10 keV. [0097] In an example, when the first photons are X-rays having energies between 30 keV and 40 keV, the first material of the first layer is configured to reduce the energy of the X-rays to an average energy below 10 keV.

[0098] In an example, when the first photons are X-rays having energies between 10 keV and 15 keV, the first material of the first layer is configured to reduce the energy of the X-rays to an average energy of 5 keV or less.

[0099] In an example, when the first photons are X-rays having energies between 50 keV and 100 keV, the first material of the first layer is configured to reduce the energy of the X-rays to an energy below 20 keV.

[0100] In various examples, related technologies and methods for the high-energy X-ray detection rely on scintillation methods and external photoemissions and are not suitable for the X-ray photon energies of interest (e.g., 10-50keV or above 20 keV). For example, the electron generation rate with respect to the number of incident photon energies can be low. As stated above with reference to Figs. 1B-1D, in related technologies, X-ray detectors have an up to 5% electron generation rate at an X-ray energy of 7.5keV and when the X-ray photon energy increases, the electron generation rate decreases.

[0101] As described above, the PAL-based multi-layer X-ray detectors can be based on photon energy attenuation followed by an absorption of the energy attenuated photons to generate electrons (by the means of internal photoemission and photoelectric effect). The PAL- based multi-layer X-ray detectors disclosed in the disclosure can significantly increase the electron generation rate from the Si direct detection method that is prevalent for related technologies (e.g., photomultipliers). For example, adding a high-Z PAL layer can lead up to approximately 10 to 30 times higher electron generation rate when compared with the Si direct detection method at high X-ray energies (e.g., 20-50keV) (Fig. 7). Accordingly, the PAL-based multi-layer X-ray detectors may be used in a generic platform of X-ray imaging sensor technology, quantum computing, and a next generation of synchrotron light source facilities.

[0102] While this disclosure has described several exemplary embodiments, there are alterations, permutations, and various substitute equivalents, which fall within the scope of the disclosure. It will thus be appreciated that those skilled in the art will be able to devise numerous apparatuses and methods which, although not explicitly shown or described herein, embody the principles of the disclosure and are thus within the spirit and scope thereof.

Claims

1. A radiation detector, comprising: a plurality of layers including a first layer and a second layer, the first layer including a first material configured to reduce energy from first photons incident on a first side of the first layer and transmit reduced-energy second photons through a second side of the first layer and into the second layer, the second layer including a second material configured to convert the reduced-energy second photons to a plurality of photoelectrons; and readout circuitry configured to transmit electrical signals based on the plurality of photoelectrons, the transmitted electrical signals representing an intensity of the first photons incident on the first side of the first layer as a function of position on the first layer.

2. The radiation detector according to claim 1, further comprising: pixelated carrier storage wells configured on an opposite side of the second layer with respect to the first layer, the pixelated carrier storage wells being configured to store the plurality of photoelectrons from the second layer, wherein the readout circuitry is further configured to transmit the electrical signals based on the plurality of photoelectrons stored in the pixelated carrier storage wells, and the first layer is stacked on the second layer.

3. The radiation detector according to claim 1, wherein the plurality of layers are arranged in a backside illumination (BSI) configuration.

4. The radiation detector according to claim 1, wherein the first material comprises a high atomic number (high-Z) material.

5. The radiation detector according to claim 4, wherein the high-Z material in the first material comprises a high-Z semiconductor material or a high-Z conductive material.

6. The radiation detector according to claim 4, wherein the high-Z material in the first material comprises one of tin (Sn), cadmium telluride (CdTe), lead telluride (PbTe), and cadmium zinc telluride (CZT).

7. The radiation detector according to claim 1, wherein the second material comprises silicon (Si).

8. The radiation detector according to claim 1, wherein, when the first photons are X- rays having energies between 20 keV and 40 keV, the first material of the first layer is configured to reduce the energy of the X-rays to an average energy below 10 keV.

9. The radiation detector according to claim 1, wherein a thickness of the first layer is in a range of 1 micron to 10,000 microns, and a thickness of the second layer is in a range of 1 micron and 500 microns.

10. The radiation detector according to claim 1, wherein the thickness of the first layer is in a range of 0.25 microns to 10 microns, and the thickness of the second layer is in a range of 5 microns and 750 microns.

11. The radiation detector according to claim 1, further comprising a third layer arranged between the first layer and the second layer, the third layer being fabricated using epitaxial growth or ion implantation, and the third layer being configured to perform surface pinning.

12. The radiation detector according to claim 11, further comprising a passivation oxide layer arranged between the first layer and the third layer.

13. The radiation detector according to claim 2, wherein the pixelated carrier storage wells are buried photodetector storage wells or pinned photodiode storage wells.

14. The radiation detector according to claim 1, wherein the readout circuitry is further configured in a quanta image sensor configuration, a pump-gate architecture, a 3-T architecture, or a 4T architecture.

15. The radiation detector according to claim 1, wherein, when the first photons are X- rays having energies between 20 keV and 40 keV, the first material of the first layer is configured to reduce the energy of the X-rays to an energy below 10 keV.

16. The radiation detector according to claim 1, wherein, when the first photons are X- rays having energies between 30 keV and 40 keV, the first material of the first layer is configured to reduce the energy of the X-rays to an average energy below 10 keV.

17. The radiation detector according to claim 1, wherein, when the first photons are X- rays having energies between 10 keV and 15 keV, the first material of the first layer is configured to reduce the energy of the X-rays to an average energy of 5 keV or less.

18. The radiation detector according to claim 1, wherein, when the first photons are X- rays having energies between 50 keV and 100 keV, the first material of the first layer is configured to reduce the energy of the X-rays to an energy below 20 keV.

19. The radiation detector according to claim 1, further comprising a passivation oxide layer arranged between the first layer and the second layer.

20. The radiation detector according to claim 1, wherein a thickness of the first layer is determined based on a thickness of the second layer and energies of the first photons incident on the first side of the first layer.