High-gain amorphous selenium photomultiplier

Abstract

A photomultiplier containing a solid-state photoconductive film composed of amorphous selenium (a-Se) is provided. In the a-Se-containing photomultiplier, a hole-blocking layer is provided, which maximizes gain and maintains low dark conductivity. Furthermore, the hole-blocking layer enables reliable and repeatable impact ionization without irreversible breakdown. The hole-blocking layer is a non-insulating metal oxide with a dielectric constant (k) greater than 10.

Description

[0001] Government support

[0002] This invention was completed with government support under license number EB025300 granted by the National Institutes of Health. The government holds certain rights to this invention.

[0003] Cross-references to related applications

[0004] This invention claims the benefit of U.S. Provisional Patent Application No. 62 / 899,437, filed September 12, 2019, the entire contents and disclosure of which are incorporated herein by reference. Technical Field

[0005] This invention relates to photomultipliers, and more particularly, to photomultipliers comprising a solid photoconductive film composed of amorphous selenium (a-Se). Background Technology

[0006] The efficient sensing and imaging of low-light signals down to the single-photon level using true solid-state photomultipliers has been a long-standing pursuit, with wide applications in astronomy, spectroscopy, optical communications, medical imaging, and the rapidly developing fields of quantum optics and quantum information science. However, the most popular commercial detector for low-light detection, achieving high dynamic range and linear mode operation, remains the vacuum photomultiplier tube (PMT). PMTs operate based on photoelectric interaction.

[0007] Electrons can be excited from the valence band to vacuum level by high-energy photons, then multiplied by a multiplying electrode, and subsequently collected by the anode. A major advantage of PMTs is their very high gain (typically 10⁻⁶). 5 Up to 10 8 It has low excess noise. However, PMTs are expensive, bulky, fragile, cannot operate in magnetic fields, have poor quantum efficiency in the visible spectrum, cannot operate in the infrared spectral region, cannot be pixelated into 2D imaging arrays, and are not suitable for fast-time applications (such as the recent advanced field of picosecond time-of-flight sensing).

[0008] Avalanche photodiode based on crystalline silicon (c-Si) (reverse bias) pn or p - i - n PMTs (partial junction devices) also amplify photogenerated carriers via a collisional ionization process. The difference is that in PMTs, only electrons exist and are multiplied highly deterministically through multiplication electrodes, while in crystalline semiconductors, both electrons and holes can undergo collisional ionization avalanche in high-field regions. This latter collisional ionization process is highly random and leads to… Excessive noise "By increasing the electric field"F With the multiplication factor in avalanche photodiodes (APDs) M As the avalanche gain increases, the fluctuations gradually worsen. According to... McIntyre theory, M relatively F The slope is the ionization rate of the two charge carriers. k A strong function of the ratio, where 1 ≤ k< 0. High in crystalline semiconductors k Values ​​contribute to uniformity and yield issues in APD.

[0009] In the 1980s, the concept of band structure engineering using multi-gain-level semiconductor heterojunctions was proposed to ensure that only one type of charge carrier (electron or hole) undergoes impact ionization, thus leading to more deterministic ionization behavior. In conventional APDs, charge carriers undergo highly random impact ionization processes with uniform probability in a constant high-electric-field region. By utilizing the conduction band discontinuity of the heterojunction, the probability of an ionization spike immediately after an electron transitions from a wider-bandgap semiconductor to a narrower-bandgap semiconductor is mimicked, thus mimicking the behavior of multiplication electrodes in PMTs. However, there is no conclusive evidence that such an ideal modification of ionization properties could produce... k∼ 0, and also because of the very low gain achieved with these multi-level heterojunctions at the best signal-to-noise ratio (SNR) with low excess noise, interest in the field of strip structure engineering diminished before truly remarkable results emerged.

[0010] Another important development is the development of a near-solid-state device to simulate the behavior of a classic PMT, which is characterized by noiselessness. Non-Markov Branches and F =1 Rockwell Si-based SSPM. Conventionally, carriers in the APD are transported via a process called... Marco husbandThe (i.e., memoryless) branching process undergoes ballistic impact ionization. The carriers travel such a short distance (and for such a very short time) for the next impact ionization that the avalanche process is independent of the history of phonon scattering (and the history of the steps required to build sufficient kinetic energy for impact ionization). However, carriers in a Si SSPM are subjected to a much smaller electric field, and therefore must be accelerated within a finite time period before acquiring sufficient kinetic energy for the next impact ionization event. This “delay time” is expected to reduce (and potentially eliminate) excess noise in such a device (i.e., excess noise coefficient ~1), as multiple scattering events and associated accelerations and decelerations cause an average of the distance traveled within the finite delay time before impact ionization. Therefore, the history of phonon scattering and energy / momentum relaxation events plays a role in the non-Markovian branching process in the SSPM, leading to an internal averaging of the stochastic process to produce a noise-free deterministic gain. Si SSPM is an impurity band avalanche device that operates at low temperatures (i.e., cooled to ~5 K) and can count single photons at wavelengths between 0.4 and 28 μm, making it an important device for low-background, near-IR to mid-IR detection applications. This device also features up to 10 5 The very high single-carrier collisional ionization gain of conduction electrons. Avalanche impact ionization process in F There is no noise in an SSPM with a value of 1. However, SSPMs are very difficult to manufacture and have very low yields to ensure... k =0, and operates at low temperatures that severely limit its applications, and therefore, the technology was eventually forgotten shortly after it was conceived. Summary of the Invention

[0011] A photomultiplier containing a solid-state photoconductive film composed of amorphous selenium (a-Se) is provided. In the a-Se-containing photomultiplier, a hole-blocking layer is provided, which maximizes gain and maintains low dark conductivity. Furthermore, the hole-blocking layer enables reliable and repeatable impact ionization without irreversible breakdown. Moreover, due to the reduced number of layers constituting the a-Se-containing photomultiplier, it exhibits low light scattering.

[0012] Hole blocking layers are non-insulating metal oxides with high dielectric constants (k). "High k" means that the dielectric constant of the metal oxide is greater than 10 when measured in a vacuum. High-k metal oxide hole blocking layers offer improvements over equivalent α-Se-containing photomultipliers in which the high-k hole blocking layer is replaced by an insulating hole blocking layer, a non-stoichiometric non-insulating hole blocking layer, or a stoichiometric non-insulating hole blocking layer with a lower dielectric constant than the high-k hole blocking layer.

[0013] In one aspect of the invention, a photomultiplier with high gain is provided. "High gain" means that the gain of the photomultiplier is 100 or greater. The photomultiplier of this application also exhibits low dark current density. "Low dark current density" means a dark current density of 1000 pA / cm². 2 Or even smaller, such as those measured at the start of an avalanche.

[0014] In one embodiment of the invention, the photomultiplier includes an electron blocking layer located on a first electrode. An amorphous selenium solid-state photoconductive film is located on the electron blocking layer. A hole blocking layer is located on the amorphous selenium solid-state photoconductive film. According to the invention, the hole blocking layer comprises a non-insulating metal oxide. A second electrode is located on the hole blocking layer. In some embodiments, the photomultiplier of the invention includes a passivation buffer layer sandwiched between the amorphous selenium solid-state photoconductive film and the hole blocking layer.

[0015] In some embodiments of the present invention, the electron blocking layer is in direct physical contact with the surface of the first electrode, the amorphous selenium film is in direct physical contact with the surface of the electron blocking layer, the hole blocking layer is in direct physical contact with the surface of the amorphous selenium film, and the second electrode is in direct physical contact with the surface of the hole blocking layer. In other embodiments of the present invention, the electron blocking layer is in direct physical contact with the surface of the first electrode, the amorphous selenium film is in direct physical contact with the surface of the electron blocking layer, the passivation buffer layer is in direct physical contact with the surface of the amorphous selenium film, the hole blocking layer is in direct physical contact with the surface of the passivation buffer layer, and the second electrode is in direct physical contact with the surface of the hole blocking layer.

[0016] In another aspect of the invention, an apparatus, such as a photodetector or imager, is provided. The apparatus of the invention includes at least one photomultiplier comprising an electron blocking layer on a first electrode, an amorphous selenium solid-state photoconductive film on the electron blocking layer, a hole blocking layer on the amorphous selenium photoconductive film (wherein the hole blocking layer comprises a non-insulating metal oxide), and a second electrode on the hole blocking layer.

[0017] In a further aspect of the invention, a method for forming a photomultiplier is provided. In one embodiment, the method includes forming an electron blocking layer on a first electrode. An amorphous selenium solid-state photoconductive film is formed on the electron blocking layer. A hole blocking layer is formed on the amorphous selenium solid-state photoconductive film. According to the invention, the hole blocking layer comprises a non-insulating metal oxide. A second electrode is formed on the hole blocking layer.

[0018] In one embodiment, the hole-blocking layer is formed by preparing a solution-treated material composed of metal oxide nanocrystals or perovskite. Then, the solution-treated material is deposited on the surface of the amorphous selenium solid-state photoconductive film at a temperature below the crystallization initiation temperature of selenium. Attached Figure Description

[0019] Figure 1 This is a cross-sectional view of a high-gain a-Se photomultiplier according to an embodiment of the present invention.

[0020] Figures 2A-2C are electric field diagrams inside various vertical a-Se photomultipliers containing different hole-blocking layers; Figure 2A (not part of this invention) uses a SiO2 hole-blocking layer (k=4); Figure 2B (of this invention) uses a CeO2 quantum dot hole-blocking layer (k=28); and Figure 2C (In this invention) a SrTiO3 hole-blocking layer (k=300); (Note: k=ε) r ).

[0021] Figure 3 This is a graph illustrating the effective quantum efficiency of various a-Se photomultipliers using a 15 μm thick a-Se solid photoconductive film, containing different hole-blocking layers as mentioned in Figures 2A-2C; the graph shows an avalanche gain field exceeding 80 V / μm, with the highest gain achieved using a SrTiO3 hole-blocking layer.

[0022] Figure 4A is a cross-sectional view and corresponding FIB-SEM of a photomultiplier containing an a-Se layer with a CeO2 quantum dot hole blocking layer.

[0023] Figure 4B is a TEM micrograph of CeO2 nanocrystals with an average size of 5.3 nm, used to provide the CeO2 quantum dot hole blocking layer shown in Figure 4A.

[0024] Figure 4C is a size distribution diagram of CeO2 nanocrystals shown in Figure 4B; the inset is a high-resolution TEM of a single CeO2 quantum dot with a lattice fringe spacing of 0.316 nm, corresponding to the (111) plane of cubic fluorite CeO2.

[0025] Figure 4D Explain the working principle of the avalanche a-Se photomultiplier in Figure 4A.

[0026] Figure 5A shows the XRD pattern of 14 nm CeO2 quantum dots, a bulk reference pattern with a cubic fluorite crystal structure.

[0027] Figure 5B shows Ce 3d deconvolution using the Gaussian-Lorentz (Voight) function. 3 / 2 and Ce 3d 5 / 2XPS spectrum.

[0028] Figure 5C shows the absorbance of CeO2 quantum dots using UV-Vis-NIR spectroscopy for 14 nm and 5.3 nm CeO2 quantum dots.

[0029] Figures 5D-5E are Tauc diagrams for 14 nm and 5.3 nm CeO2 quantum dots, respectively. The inset shown in Figure 5D is a high-resolution TEM micrograph of a single 14 nm CeO2 quantum dot, while... Figure 5E The illustration shown is a high-resolution TEM micrograph of a single 5.3 nm CeO2 quantum dot.

[0030] Figure 6A is a graph showing the dark current transients of the a-Se (15 μm) / CeO2 quantum dot (150 nm) device measured over a wide range of applied currents.

[0031] Figure 6B is a graph showing the measured dark current density of the α-Se-containing photomultiplier according to the present invention, which includes a CeO2 quantum dot hole blocking layer.

[0032] Figure 6C The figure shows the measured light response of devices with 40 nm and 150 nm CeO2 quantum dot hole blocking layers achieving avalanche gains of 50 and 7, respectively; the inset is a schematic diagram of an optical TOF experiment. Detailed Implementation

[0033] The invention will now be described in more detail with reference to the following discussion and the accompanying drawings. Note that the drawings provided are for illustrative purposes only and are therefore not drawn to scale. It should also be noted that identical and corresponding elements are indicated by the same reference numerals.

[0034] In the following description, numerous specific details, such as particular structures, components, materials, dimensions, processing steps, and techniques, are set forth to provide an understanding of various embodiments of the invention. However, those skilled in the art will understand that various embodiments of the invention can be practiced without these specific details. In other instances, well-known structures or processing steps have not been described in detail to avoid obscuring the invention.

[0035] It should be understood that when an element that is a layer, region, or substrate is referred to as being "on" or "above" another element, it may be directly on the other element, or there may be intermediate elements present. Conversely, when an element is referred to as being "directly on" or "directly above" another element, there are no intermediate elements present. It will also be understood that when an element is referred to as being "below" or "under" another element, it may be directly below or under the other element, or there may be intermediate elements present. Conversely, when an element is referred to as being "directly below" or "directly under" another element, there are no intermediate elements present.

[0036] The embodiments of the present invention described herein provide a high-gain amorphous selenium photomultiplier and a method for forming the same. The embodiments of the present invention utilize solution-treated nanocrystals / nanoparticles (including quantum dots) and / or perovskites with high dielectric constants as hole-blocking layers to enable high gain in amorphous selenium. The embodiments of the present invention enable the development of the first true solid-state photomultiplier and hold promise for revolutionizing the application of solid-state photoelectric detection and imaging in the rapidly developing fields of astronomy, spectroscopy, optical communication, medical imaging, and quantum optics and quantum information science.

[0037] Amorphous selenium (a-Se), as a solid-state photoconductive film, paves the way for revolutionizing photoelectric detection through its unique avalanche multiplication process. Two key characteristics of the avalanche phenomenon in a-Se are: first, due to the significant difference between the collisional ionization rates of electrons and holes, only holes become heated and undergo collisional ionization; second, the avalanche process is noiseless and non-Markovian. Furthermore, a-Se is a large-area room-temperature semiconductor with a wide bandgap and ultra-low leakage current even at high fields, thus requiring no cooling.

[0038] Although a-Se can ideally provide PMT-like gain, its avalanche gain as a solid-state detector structure has been severely limited due to (A) an insulating hole-blocking layer (HBL), or (B) a non-stoichiometric non-insulating HBL, or (C) a stoichiometric non-insulating HBL with a low dielectric constant (i.e., 10 or less). The term "stoichiometry" as used herein defines a compound in which its constituent elements are present in precise proportions according to their chemical formulas.

[0039] Project A is insufficient because the insulator induces space charge effects and polarization in the trap. Project B is ineffective because the defect states in the HBL significantly enhance charge injection. Project C also limits the attainment of very high gain due to the presence of field hotspots near the electrode edges and corners. The a-Se photomultiplier requires an alternative to the HBL that maximizes gain while maintaining low dark conductivity. Furthermore, the alternative HBL should achieve reliable and repeatable impact ionization without irreversible breakdown.

[0040] In this invention, a non-insulating n-type hole-blocking / electron-transporting layer (hereinafter "hole-blocking layer") comprising a non-insulating metal oxide is provided. The non-insulating metal oxide has a dielectric constant greater than 10, i.e., the non-insulating metal oxide is a high-k material. The non-insulating metal oxide used as the hole-blocking layer is substantially stoichiometric. "Substantially stoichiometric" means that the non-insulating material oxide is fully stoichiometric or within ±5% of full stoichiometry.

[0041] The use of such a hole-blocking layer in a-Se photomultipliers provides a true solid-state alternative to vacuum PMT. In this invention, solution-treated metal oxide nanocrystals / nanoparticles or solution-treated perovskites are used to provide the hole-blocking layer. The solution-treated material is deposited at a temperature that does not cause any crystallization (surface or bulk phase) of the a-Se layer. Experimental results of the fabricated a-Se photomultiplier containing a high-k non-insulating metal oxide as a hole-blocking layer show the lowest dark current density ever reported in an avalanche electric field, with no defect states and oxygen vacancies. Using a cryogenically solution-treated material as a hole-blocking layer substantially improves the performance of avalanche selenium devices and may ultimately end the long-standing need to develop solid-state photomultipliers that mimic the behavior of classical PMTs.

[0042] Now for reference Figure 1 This describes a high-gain, low-dark-current-density a-Se photomultiplier according to an embodiment of the present invention. Figure 1 The a-Se photomultiplier can be used as a component in photodetectors, imagers, sensors, or any other device that needs to detect photons. Figure 1 The a-Se photomultiplier includes a substrate 10, a first electrode 12, and an electron blocking layer 14 (i.e., p (layer), amorphous selenium solid-state photoconductive film 16 (i.e., i (layer), passivation buffer layer 18, hole blocking layer 20 (i.e., n The first electrode is a passivation buffer layer 18, and the second electrode is a second electrode 22. In some embodiments, the passivation buffer layer 18 may be omitted. In some embodiments, the substrate 10 may be omitted.

[0043] In the implementation plan, and as in Figure 1 As described above, the substrate 10, the first electrode 12, the electron blocking layer 14, the amorphous selenium solid photoconductive film 16, the passivation buffer layer 18, the hole blocking layer 20, and the second electrode 22 are vertically stacked on top of each other.

[0044] When present, substrate 10 is typically a transparent substrate, such as a semiconductor substrate or a glass substrate. The semiconductor substrate includes at least one semiconductor material, such as silicon.

[0045] The first electrode 12 can be made of any transparent conductive material, including, for example, indium tin oxide (ITO). In some embodiments (not shown), the first electrode 12 can be present across the entire substrate 10. In other embodiments, and as in Figure 1 As described, the first electrode 12 is present on a portion of the substrate 10. In some embodiments, the first electrode 12 may be in direct physical contact with the substrate 10, and thus form a material interface therewith. The thickness of the first electrode 12 may be from 10 nm to 1000 nm; although other thicknesses are contemplated for the first electrode 12 and may be used as such. The first electrode 12 may be formed using techniques known to those skilled in the art. For example, the first electrode 12 may be formed using deposition processes such as physical vapor deposition (PVD), atomic layer deposition (ALD), or electroplating. In some embodiments, a patterning process, such as photolithography, may be performed after the deposition of the transparent conductive material providing the first electrode 12.

[0046] In some embodiments, the electron blocking layer 14 may be a high-temperature, high-field electron blocking layer composed of a polymer (e.g., parylene or polyimide (PI)). In other embodiments, the electron blocking layer 14 is composed of an inorganic electron blocking material, such as arsenic triselenide (As₂Se₃) or nickel oxide (NiO). The thickness of the electron blocking layer 14 may be from 10 nm to 6000 nm; although other thicknesses are contemplated for use in the electron blocking layer 14 and may be used as such. The electron blocking layer 14 may be formed using techniques known to those skilled in the art. For example, the electron blocking layer 14 may be formed using deposition processes such as chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECD), solution deposition, thermal evaporation, or spin coating. The electron blocking layer 14 is typically in direct physical contact with the first electrode 12 and thus forms a material interface therewith.

[0047] The amorphous selenium solid-state photoconductive film 16 is a selenium film lacking any crystalline structure; although local atomic ordering may exist in the amorphous selenium solid-state photoconductive film 16, long-term ordering is absent. The amorphous selenium solid-state photoconductive film 16 can be doped (stable) or undoped. Examples of dopants used for the amorphous selenium solid-state photoconductive film 16 include, but are not limited to, arsenic, tellurium, or chlorine. Dopants may be present in the amorphous selenium solid-state photoconductive film 16 in an amount from 0.1 atomic% to 0.5 atomic% of the film. The thickness of the amorphous selenium solid-state photoconductive film 16 can be from 0.5 μm to 100 μm; although other thicknesses are contemplated for use in the amorphous selenium solid-state photoconductive film 16 and can be used as such, the thickness of the amorphous selenium solid-state photoconductive film 16 is not specified. The amorphous selenium solid-state photoconductive film 16 can be formed using techniques known to those skilled in the art. In one example, the amorphous selenium solid-state photoconductive film 16 can be formed by thermally evaporating stable glassy selenium granules. The amorphous selenium solid-state photoconductive film 16 is typically in direct physical contact with the electron blocking layer 14, and thus forms a material interface therewith.

[0048] When present, the passivation buffer layer 18 is composed of any material that protects the underlying amorphous selenium solid-state photoconductive film 16 from oxidation. In one example, the passivation buffer layer 18 may be composed of SiO2. The thickness of the passivation buffer layer 18 can be from 10 nm to 1000 nm; although other thicknesses are contemplated for and can be used for the passivation buffer layer 18. The passivation buffer layer 18 can be formed using deposition (e.g., sputtering). In this invention, the passivation buffer layer 18 is deposited using conditions that do not cause any portion (i.e., surface or bulk phase) of the underlying amorphous selenium solid-state photoconductive film 16 to crystallize. Therefore, after the passivation buffer layer 18 is formed, the amorphous selenium solid-state photoconductive film 16 remains completely amorphous. The passivation buffer layer 18 is typically in direct physical contact with the amorphous selenium solid-state photoconductive film 16 and thus forms a material interface with it.

[0049] To provide high gain and low dark current density to the device, hole blocking layer 20 comprises a non-insulating metal oxide. The non-insulating metal oxide providing hole blocking layer 20 has a high k (i.e., a dielectric constant greater than 10). In some embodiments, the non-insulating metal oxide providing hole blocking layer 20 has a dielectric constant greater than 10 up to 100. In other embodiments, the non-insulating metal oxide providing hole blocking layer 20 has a dielectric constant greater than 10 up to 20,000. The non-insulating metal oxide providing hole blocking layer 20 can be substantially stoichiometric, as defined above.

[0050] In one embodiment of the invention, the non-insulating metal oxide providing the hole-blocking layer 20 is composed of metal oxide nanocrystals / nanoparticles. The terms "nanocrystals" and "nanoparticles" are used interchangeably in this invention. Nanocrystals / nanoparticles typically have a size with a diameter less than 100 nm. Metal oxide nanocrystals can provide metal oxide quantum dots. Quantum dots are nanocrystals that demonstrate quantum confinement. Examples of metal oxides that can be used as metal oxide nanocrystals / nanoparticles in this invention include, but are not limited to, oxides of transition metals from the periodic table. In one embodiment of the invention, the metal oxide nanocrystals / nanoparticles used as the hole-blocking layer 20 comprise cerium oxide (CeO2) quantum dots.

[0051] In other embodiments, the non-insulating metal oxide providing the hole-blocking layer 20 is composed of perovskite. Perovskite is a material having the same crystal structure as the mineral perovskite. Typically, perovskite has the chemical formula ABX3, where A and B represent cations, and X is an anion bonded to both cations. Examples of perovskites that can be used in this invention include, but are not limited to, strontium titanate (i.e., SrTiO3) or barium titanate (BaTiO3). In one embodiment, the perovskite providing the electron-hole blocking layer 20 is SrTiO3. Perovskite is also a nanocrystal.

[0052] Although the type of non-insulating metal oxide used in providing the hole blocking layer 20 is different, the thickness of the hole blocking layer 20 can be from 10 nm to 150 nm; although other thicknesses are expected to be used for the hole blocking layer 20 and can be used as the thickness of the hole blocking layer 20.

[0053] The hole-blocking layer 20 of the present invention is formed by first preparing a solution-treated material as defined above, consisting of metal oxide nanocrystals / nanoparticles or perovskite. The formation of the solution-treated material includes preparing a colloidal dispersion of metal oxide nanocrystals / nanoparticles or perovskite. The colloidal dispersion further includes a solvent or a mixture of solvents. The term "colloidal dispersion" is used in this invention to refer to a heterogeneous system consisting of a dispersed phase (i.e., metal oxide or perovskite) and a dispersion medium (i.e., a solvent or a mixture of solvents). In a colloidal dispersion, a substance (i.e., metal oxide or perovskite) is dispersed as fine particles in the dispersion medium (i.e., a solvent or a mixture of solvents). Therefore, the solvent or mixture of solvents includes substances that disperse but do not dissolve the metal oxide or perovskite. Examples of solvents that can be used to form the solution-treated material include, but are not limited to, organic solvents, including, for example, hexane, octane, heptane, decane, chloroform, or toluene. In one example, the solution used to provide the solution-treated material comprises a solvent mixture of hexane and octane.

[0054] The preparation of a colloidal dispersion of nanocrystals / nanoparticles or perovskite and a solvent or solvent mixture involves adding nanocrystals / nanoparticles or perovskite and a solvent or solvent mixture in any order. Metal oxide nanocrystals / nanoparticles can be prepared using techniques known to those skilled in the art. Following the addition step, the metal oxide nanocrystals / nanoparticles or perovskite and a mixture of one or more solvents are mixed under conditions that promote the preparation of the colloidal dispersion.

[0055] After preparing the solution-treated material, the solution-treated material is deposited on the surface of the amorphous selenium solid-state photoconductive film 16 at a temperature below the crystallization initiation temperature of selenium. "A temperature below the crystallization initiation temperature of selenium" refers to, for example, 80°C or lower, a temperature where the underlying layer of the amorphous selenium solid-state photoconductive film 16 does not undergo any crystallization on the surface of the amorphous selenium solid-state photoconductive film 16 or in the bulk phase of the amorphous selenium solid-state photoconductive film 16. Therefore, after depositing the solution-treated material providing the hole-blocking layer 20, the amorphous selenium solid-state photoconductive film 16 remains completely amorphous. In one embodiment, the deposition of the solution-treated material providing the hole-blocking layer 20 is performed at a temperature of 60°C or lower. In yet another embodiment, the deposition of the solution-treated material providing the hole-blocking layer 20 is performed at nominal room temperature (i.e., a temperature of 20°C to 30°C).

[0056] The deposition of the solution-treated material providing the cavity blocking layer 20 may include any known deposition technique, including but not limited to spraying, spin coating, inkjet printing, doctor blade coating, roll-to-roll printing, dip coating, screen printing, drop casting, brush coating, embossing, zone casting, hollow pen writing, slot die printing, or solution shearing.

[0057] In some embodiments of the invention, a ligand exchange process can be performed on the solution-treated material to provide a hole-blocking layer 20. The ligand exchange process involves replacing the natural ligands used during synthesis and deposition with new organic and inorganic ligands; typically, but not always, the new organic and inorganic ligands are tight-chain short-chain ligands. This ligand exchange process can be performed before and after deposition. Typical short-chain ligands can be both organic ligands (e.g., ethylenedithiol, ethylenediamine, pyridine, hydrazine) and inorganic ligands (e.g., sulfides, hydroxides, selenides, tellurides, thiocyanates, hydrosulfides, etc.).

[0058] The second electrode 22 can be made of any transparent conductive material, including, for example, indium tin oxide (ITO). The transparent conductive material providing the second electrode 22 can be the same as or different in composition from the transparent conductive material providing the first electrode 12. In one embodiment, both the first electrode 12 and the second electrode are made of indium tin oxide. In some embodiments (not shown), the second electrode 22 can be present throughout the hole blocking layer 20. In other embodiments, and as in... Figure 1 As described, the second electrode 22 is present on a portion of the hole-blocking layer 20. In some embodiments, the second electrode 22 may be in direct physical contact with the hole-blocking layer 20, and thus form a material interface therewith. The thickness of the second electrode 22 may be from 10 nm to 1000 nm; although other thicknesses are contemplated for the second electrode 22 and may be used as such. The second electrode 22 may be formed using techniques known to those skilled in the art. For example, the second electrode 22 may be formed using deposition processes such as physical vapor deposition (PVD), atomic layer deposition (ALD), or electroplating. In some embodiments, a patterning process, such as photolithography, may be performed after depositing the transparent conductive material providing the second electrode 22.

[0059] The following embodiments are provided to illustrate some aspects of the invention. However, the invention is not limited to these embodiments.

[0060] Example 1: Amorphous selenium (a-Se) photomultiplier containing different hole blocking layers.

[0061] In this embodiment, the electric field and effective quantum efficiency of various vertical a-Se photomultipliers containing different hole-blocking layers were investigated. Notably, the various vertical a-Se photomultipliers include amorphous selenium as a photoconductive film 16, different hole-blocking layers 20 as defined below, and an indium tin oxide second electrode 22.

[0062] Different hole blocking layers 20 include: for Figure 2A (not part of this invention): a 100 nm SiO2 hole blocking layer (k=4); for Figure 2B (of this invention): a 100 nm CeO2 quantum dot hole blocking layer (k=28); for... Figure 2C (In this invention): 100 nm SrTiO3 hole blocking layer (k=300); (Note: k=ε) r The SiO2 hole-blocking layer was deposited by sputtering, while the CeO2 quantum dot hole-blocking layer and the SrTiO3 hole-blocking layer were prepared by first providing the solution-treated material and then depositing the solution-treated material at a temperature below the crystallization initiation temperature of selenium.

[0063] Referring now to Figures 2A, 2B, and 2C, electric field diagrams are shown in various a-Se photomultipliers. Note that in all cases, the electric field within the bulk phase is 100 V / μm. As shown in Figure 2A, hot spots exist using a SiO2 hole-blocking layer. In Figure 2B, hot spots exist using a CeO2 hole-blocking layer, although to a lesser degree than with a SiO2 hole-blocking layer. For example, using a CeO2 hole-blocking layer, when the bulk phase is biased only at 100 V / μm, the electric fields near the electrode / oxide and oxide / a-Se interfaces reach 300 V / μm and 200 V / μm, respectively. Figure 2C As shown, when a SrTiO3 hole blocking layer is used, the field hotspots are completely erased.

[0064] Now for reference Figure 3 The effective quantum efficiency of various a-Se photomultipliers is shown, where the avalanche gain is severely limited by the presence of field hotspots. However, 10-1 is achieved using a SrTiO3 hole blocking layer. 6 High gain.

[0065] Example II: Study on the use of solution-treated CeO2 quantum dots as hole blocking layers.

[0066] In this embodiment, a solution-treated CeO2 quantum dot layer with a large band gap of 3.77 eV was deposited on an a-Se photoconductor at room temperature without any surface or bulk crystallization. Figure 4A illustrates the fabricated prototype. p - i - n A schematic diagram of the structure and cross-section obtained using focused ion beam scanning electron microscopy (FIB-SEM), showing... p - i - nAnd ITO electrode layer. At sizes below the exciton Bohr radius of 7-9 mm, CeO2 quantum dots exhibit quantum confinement and size-tunable band gaps, further increasing the hole barrier beyond 2.8 eV. Many previously reported CeO2 quantum dot syntheses involve high-temperature calcination (500°C to 600°C), which produces non-stoichiometric CeO2 and is prone to morphological instability and uncontrolled aggregation. Here, in this embodiment, substantially stoichiometric non-aggregated CeO2 quantum dots were obtained using an easy colloidal method, which were passivated by ligand surfaces within a quantum confinement range of 14 nm to as low as 5.3 nm, demonstrating band gaps ranging from 3.66 eV to 3.77 eV. Colloidal CeO2 quantum dots were synthesized using the high-temperature decomposition technique proposed by Runnerstrom et al. in their paper "Colloidal Nanocrystals Films Reveal the Mechanism for Intermediate Room TEMperature Proton Conductivity in Porous Ceramics" (J. Phys. Chem. C208, 122, 13624-13635).

[0067] Figure 4B shows a transmission electron microscope (TEM) image of the entirety of a fairly monodisperse quantum dot with an average size of 5.3 ± 0.7 nm (Figure 4C). The inset of Figure 4C shows a high-resolution TEM micrograph of a single CeO2 quantum dot measured from lattice fringes, which has an interplanar spacing of 0.316 nm, corresponding to the (111) plane of cubic fluorite.

[0068] Device-level simulations were performed using SILVACO TCAD (ATLAS version 5.25.1.R) to establish a clearer physical picture of light interactions in an a-Se avalanche photodetector with a bulk CeO2 quantum dot hole-blocking layer. Figure 4D The simulated band diagrams of the detector structure are shown under different electric fields, where transport shifts from localized to extended states after the avalanche threshold voltage (≈80 V / μm) is crossed, resulting in hole-induced avalanche gain.

[0069] The powder X-ray diffraction pattern of CeO2 quantum dots shown in Figure 5A matches the powder X-ray diffraction pattern of the cubic fluorite structure of CeO2, where Scherrer broadening results in broad peaks. The diffraction peaks are as sharp as expected when the average particle size increases from 5.3 nm to 14 nm. To elucidate the oxidation state and stoichiometry of CeO2, the emission spectrum of Ce 3d was studied using X-ray photoelectron spectroscopy (XPS) (Figure 5B, which shows Ce...). 4+ and Ce3+ (The coexistence of the two). Because Ce 3+ and Ce 4+ The peaks approximately overlap each other. Deconvolve the spectrum in Figure 5B and thus estimate Ce. 3+ The nontrivial concentration of 18.5% is nearly 10% to 20% lower than previously reported values ​​for CeO2 quantum dots synthesized by other methods. The crystal contains a large amount of Ce. 3+ This leads to oxygen vacancies and thus induces defect states in the quantum dot. In the colloidal method of the present invention, ligands are used to provide both colloidal stability and passivation of defects. Furthermore, these surfactants can induce quantum dot-ligand interfacial dipoles, which contribute to the energy level shift of the valence band maximum, and thus potentially even further increase hole blocking.

[0070] The band gap of the synthesized CeO2 quantum dots was measured by UV-Vis-NIR absorption spectroscopy (Fig. 5C). The absorption spectrum of the quantum dots dispersed in carbon tetrachloride did not show a distinct absorption peak at the band edge. Therefore, the band gap was calculated from the experimental data using Tauc plots, which showed that when the size of the quantum dots decreased from 14 nm to 5.3 nm, the direct optical band gap increased from 3.66 eV to 3.77 eV (Figs. 5D and 5E), as expected from the quantum confinement effect.

[0071] Because ligands enhance the colloidal dispersion of the quantum dots synthesized in this method, devices can be fabricated at room temperature using inexpensive deposition techniques (such as inkjet printing or spin coating). At room temperature, 5.3 nm CeO2 quantum dots with a band gap of 3.77 eV were spin-coated onto an a-Se substrate using 110 nm SiO2 to achieve CeO2 quantum dot layers of 40 nm and 150 nm thickness.

[0072] The quality of quantum dot films deposited by the colloidal method of this invention was compared with that obtained using a custom-designed precipitation method for CeO2 quantum dots, performed according to the procedure described by Arul et al., entitled “Strong Quantum Confinement Effect In Nanocrystalline Cerium Oxide”, Mater. Lett. 2011, 65, 2635-2638. The ligand-containing CeO2 quantum dots synthesized colloidally achieved uniform deposition without any microcracks, voids, or aggregations, clearly highlighting the importance of suitable surfactants in achieving better colloidal dispersions for film fabrication.

[0073] In the deposited film, post-synthetic CeO2 quantum dots with long-chain ligands were exchanged with short-chain NH4SCN ligands to increase electronic coupling between quantum dots on the true solid-state device, and the ligand exchange was tracked using Fourier transform-infrared spectroscopy (FT-IR). The ligand exchange was observed at 2048 cm⁻¹ corresponding to the NH4SCN ligand exchange. -1 A distinct peak appears at [value missing], indicating that approximately 88% ligand exchange has occurred. XRD analysis suggests that the solution treatment of this invention did not induce any α-Se crystallization.

[0074] Figure 6A shows the applied electric field over a wide range. E Below, a 150 nm thick CeO2 quantum dot-based structure p - i - n The device measures the transient dark current density. In each case, the transient is fitted with a bivariate exponential curve (solid line) until a steady state (dashed line) is reached after 25 minutes. The rapid initial drop in dark current is due to injected carriers being trapped within the hole-blocking layer, reducing the effective density at the interface. E The second gradual decay to steady-state equilibrium is due to the release of bulk space charge.

[0075] Figure 6B shows the 1-minute (dashed line) and 30-minute (solid line) time points as... E The dark current density measured by the device of the present invention (40 nm green and 150 nm blue) is a function of the dark current, comparing transient and steady-state dark currents. In a sub-avalanche field ( E < At 70 V / μm, the steady-state dark current deposition of the device of the present invention with a 150 nm CeO2 quantum dot layer was found to be reduced to less than 30 pA / cm. 2 At the high field required for avalanche gain, the measured dark current is extremely low. E It reaches approximately 50 pA / cm at 88V / μm. 2 .

[0076] Figure 6B also compares the following: n - i - pThe dark current density of sequentially fabricated solid-state a-Se avalanche devices is adapted from Ohshima et al.'s "Excess Noise in Amorphous Selenium Avalanche Photodiodes", Appl. Phys., Part 2 1991, 30, L1071-L1074 and Abbaszadeh et al.'s "Investigation of Hole-Blocking Contacts for High-Conversion-Gain Amorphous Selenium Detectors for X-Ray Imaging", IEEE Trans., Electron Devices 2021, 59, 2403-2409. In each case, the steady-state dark current (measured after the avalanche begins) is... E >70 V / μm) compared to solid-state n - i - p a-Se avalanche devices are at least two orders of magnitude lower. Additionally, this includes devices capable of achieving approximately 10... 3 The dark current value of the vacuum HARP camera tube with the gain was modified for comparison. Although no low-field data is available for vacuum HARP devices, the results were extended (dashed line) using the same collisional ionization curves as those in Park et al., entitled “Avalanche-type High Sensitive Image Pickup Tube using an a-Sephotoconductive target”, Jpn. J. Appl. Phys. 2003, 42, L209-L211, showing a significant potential improvement in high-field sensitivity if similar gain is achieved with the device of the present invention. Each modification shown in Figure 6B... n - i - p HARP results utilize a CeO2 hole-blocking layer, which is deposited via high-temperature vacuum deposition. n - i - p The CeO2 membrane used in the device and p - i - n Incompatible manufacturing sequences result in defect energy levels due to oxygen vacancies, and typically exhibit poor performance as a function of film thickness. The present invention... p - i - nExperimental results from the device show that it has the lowest reported dark current density under avalanche electric fields, which is more than 300% better than the best solid-state vertical device of its kind, and even close to 200% better than vacuum devices.

[0077] Unlike bulk CeO2, the results provided in Figure 6B show that the dark current decreases significantly with increasing CeO2 film thickness. This can be attributed to the following possibilities: (i) the colloidal synthesis method induces discrete local defect levels only on a small number of CeO2 quantum dots at the surface, such as the relatively low intensity CeO2 in the XPS spectrum. 3+ The peaks are shown. This result is consistent with the fact that the formation energy of “inherent” defects in quantum dots is generally much larger than that in the bulk phase, thus suppressing defect formation. (ii) The presence of sparse, isolated defects in the thin film does not interact strongly with each other and forms a continuous defect band within the band gap, even with increasing film thickness. Most defects in these quantum dots migrate to the surface, and each quantum will have a defect energy level of random energy state (if any), resulting in a dispersion of random energy states with an extremely low density of states, rather than a coherent band. (iii) Ammonium thiocyanate ligands can potentially passivate surface defect states caused by oxygen vacancies through coordination bonding. However, the significant reduction in dark current clearly foreshadows the potential application of quantum dots treated with low-temperature solutions as hole-blocking layers.

[0078] Figure 6C The device, which has a 40 nm (green) and a 150 nm (blue) CeO2 quantum dot hole blocking layer, shows an avalanche gain of 50 and 7, respectively. Figure 6C The illustration schematically represents an optical time-of-flight (TOF) photoconductivity experiment used to measure the light response of each sample, which serves as... E A function of. In a wide range of electric fields ( E The effective quantum efficiency was measured at 6-100 V / μm. η* In sub-avalanche fields, photogeneration efficiency is limited by paired recombination; however, with... E The likelihood of recombination decreases as electron-hole pairs are pulled apart more efficiently. η* Unified stability is achieved only after the Onsager dissociation model. Once the avalanche threshold is crossed, the hole drift mechanism shifts from the mobility constrained by local traps to banded transport in the extended state. η* The rapid increase causes hole shock ionization, which releases additional... E HP, thus amplifying the signal current. The maximum field applied before electrical breakdown ( E (≈120 V / μm) results in an avalanche gain of ≈50.

[0079] In summary, this embodiment demonstrates that using quantum dots treated with a low-temperature solution as a hole-blocking layer can substantially improve...p - i - n Improve the performance of avalanche a-Se detectors and end the long-standing need to develop solid-state photomultipliers that simulate classical PMT behavior.

[0080] Synthesis of CeO2 quantum dots. All chemicals were used as purchased and without any purification. 5.3 nm CeO2 colloidal quantum dots (QDs) were synthesized using a modified procedure proposed by Runnerstrom et al. (see J. Phys. Chem. C208, 122, 13624-13635). In a typical synthesis, 8 ml of oleylamine and 5 ml of 1-octadecene were added to 868.4 mg of Ce(NO3)3·6H2O in a 250 ml round-bottom three-necked flask at room temperature. The temperature was then raised to 80 °C and held constant for 30 minutes to homogenize the reaction mixture. Subsequently, the temperature was raised again to 250 °C and held at 250 °C for 2 hours, resulting in the decomposition of the precursor and the nucleation and growth of the quantum dots. During this process, the stopper was punctured with a needle to prevent excessive pressure buildup in the flask due to the release of nitrogen dioxide (NO2) gas during the decomposition of Ce(NO3)3. Two hours later, oleic acid, a commonly used surfactant (typically about 1 volume of the reaction mixture), was added to improve colloidal stability and minimize aggregation. The reaction mixture was then further annealed at 250 °C for 5 minutes, followed by air cooling to room temperature. Ethanol was added as an antisolvent to the obtained reaction mixture, and the mixture was centrifuged at 8500 rpm for 5 minutes to break up the quantum dots. The collected quantum dots were dispersed in 5 ml of hexane, and then oleic acid (about 10 volume of the mixture) was added and stirred overnight. The purification process was repeated three times using ethanol, and the mixture was redispersed in hexane. The synthesized quantum dots were dispersed as is in an 8:2 solvent ratio of hexane and octane to form 30 mg / ml and 50 mg / ml colloidal dispersions for further use in film fabrication. To investigate the role of quantum confinement, larger CeO2 colloidal quantum dots (about 14 nm) were synthesized according to the procedure of Runnerstrom et al. (see J. Phys. Chem. C208, 122, 13624-13635). Under a nitrogen atmosphere, 40 mg of octadecylamine, 4.9 mL of oleylamine, and 5 mL of octadecene were added to a 250 mL round-bottom three-necked flask, and the mixture was heated to 70 °C to melt the octadecylamine. 4342.4 mg of Ce(NO3)3·6H2O and 0.725 mL of deionized water were added to the reaction mixture. The mixture was heated to 175 °C and then annealed for 30 minutes to achieve homogeneity, followed by raising the temperature to 230 °C. The release of a large amount of NO2 vapor after 2 minutes at 230 °C indicated the completion of the reaction. The resulting solution was then air-cooled. To avoid over-pressurization of the flask, the stopper was punctured with a needle, as previously described. The collected colloidal solution was washed with ethanol and hexane, similar to the procedure using 5.3 nm CeO2 quantum dots.

[0081] Device manufacturing. Each avalanche device utilizes... p - i -n The manufacturing process involved the development of a 2.5 x 2.5 in [size unit]. 2 ITO-coated glass substrate. Under high vacuum... pi The layer is deposited directly onto the ITO-coated glass. p The layer consists of a 2 μm thermally evaporated inorganic electron-blocking layer, followed by thermally evaporated stable glassy selenium particles to form a 15 μm α-Se. i A 110 nm cryogenically sputtered SiO2 film (provided by Hionix Inc.) was deposited on the a-Se surface as a passivation buffer layer. CeO2 quantum dots synthesized in the above solution were spin-coated at room temperature to form a passivation buffer layer. n Layering. A dispersion of 30 mg / ml CeO2 quantum dots was spin-coated at 2000 rpm for 45 seconds, and similarly, a dispersion of 50 mg / ml was spin-coated at 1700 rpm for 45 seconds. The assembled CeO2 quantum dot film was ligand-exchanged with a solution of 130 mM (1% w / v) ammonium thiocyanate (NH4SCN) in acetone, followed by spin-coating with pure acetone using the same spin parameters as CeO2 to remove unbound NH4SCN.

[0082] The CeO2 quantum dot thin film deposition and ligand exchange were repeated twice to achieve the desired thickness. A transparent ITO high-voltage electrode was deposited via shadow mask patterning and oxygen-assisted electron beam deposition. After wire bonding with the ITO-based readout and high-voltage electrode was completed, the entire device was encapsulated in parylene to prevent the high voltage from affecting the top surface.

[0083] Photoconductivity measurements. For each measurement, a CAEN N1471A programmable high-voltage power supply was used to positively bias the top electrode of the avalanche a-Se device. Time (It) and voltage (IV) dependent dark current characteristics were measured using a Keithley 6514 electrometer. Charge transport characteristics and avalanche gain were measured via optical TOF transient experiments using a 450 nm pulsed LED source with a 170 ns FWHM driven by a Tektronix AFG 3021B function generator. Induced photocurrents were captured using a Tektronix TDS 7104 digital oscilloscope. A Tektronix P6245 active probe was used to protect the device from damage at high fields. For all photocurrent measurements of avalanche gain, the a-Se device was mounted in an opaque, grounded metal box.

[0084] Although the invention has been specifically shown and described with respect to preferred embodiments thereof, those skilled in the art will understand that the above and other changes in form and detail may be made without departing from the spirit and scope of the invention. Therefore, this application is not intended to be limited to the exact forms and details described and illustrated, but rather falls within the scope of the appended claims.

Claims

1. A photomultiplier, comprising: An electron blocking layer located on the first electrode; An amorphous selenium solid-state photoconductive film located on the electron blocking layer; A hole-blocking layer is located on the amorphous selenium solid-state photoconductive film, wherein the hole-blocking layer comprises a non-insulating metal oxide having a dielectric constant greater than 10 and selected from cerium oxide quantum dots or perovskite, wherein the perovskite is a nanocrystal; and The second electrode is located on the hole blocking layer.

2. The photomultiplier of claim 1, further comprising a passivation buffer layer sandwiched between the amorphous selenium solid photoconductive film and the hole blocking layer.

3. The photomultiplier of claim 1, wherein the first electrode is located on the surface of the substrate.

4. The photomultiplier of claim 1, wherein the first electrode, the electron blocking layer, the amorphous selenium solid-state photoconductive film, the hole blocking layer and the second electrode are vertically stacked on top of each other.

5. The photomultiplier of claim 1, wherein the amorphous selenium solid-state photoconductive film is completely amorphous.

6. The photomultiplier of claim 1, wherein the perovskite comprises strontium titanate or barium titanate.

7. Equipment, comprising: At least one photomultiplier according to any one of claims 1-6.

8. A method for forming a photomultiplier, the method comprising: An electron blocking layer is formed on the first electrode; An amorphous selenium solid-state photoconductive film is formed on the electron blocking layer; A hole-blocking layer is formed on the amorphous selenium solid-state photoconductive film, wherein the hole-blocking layer comprises a non-insulating metal oxide having a dielectric constant greater than 10 and selected from cerium oxide quantum dots or perovskite, wherein the perovskite is a nanocrystal; and A second electrode is formed on the hole-blocking layer.

9. The method of claim 8, wherein forming the hole-blocking layer comprises: Prepare solution-treated materials containing cerium oxide quantum dots or perovskite; and The solution-treated material is deposited on the surface of the amorphous selenium solid-state photoconductive film at a temperature below the crystallization initiation temperature of selenium.

10. The method of claim 9, wherein the solution-treated material comprises a colloidal dispersion of cerium oxide nanocrystals, and the deposition of the colloidal dispersion of the cerium oxide nanocrystals provides a cerium oxide quantum dot layer, and wherein the cerium oxide quantum dot layer further undergoes a ligand exchange process to form the hole-blocking layer.

11. The method of claim 9, wherein the deposition temperature is performed at nominal room temperature.

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