Direct x-ray detection device and a method of making such a device

Abstract

An X-ray detector 10 comprising an absorber layer 58. The absorber layer 58 is configured to receive X-ray radiation 85 and provide an electrical output by performing a direct conversion from the X-ra

Description

Field The present application relates to an X-ray direct detector and a method of making such a device. Background X-ray imaging is important in many technology areas including medical operations, dentistry, food safety, security, industrial inspection, and so on. At present, most X-ray imaging is performed by using an indirect X-ray detector in which the X-rays are absorbed by a scintillator made from materials such Gadox or Csl. The scintillator then emits visible light photons which are captured by a silicon thin film transistor (TFT) array. This extra conversion step from X-rays to visible light is utilised because: (i) silicon itself is not good at absorbing X-rays, and (ii) Gadox and Csl have poor electronic properties. However, the visible light emission from the scintillator is isotropic, and this therefore limits the resolution of images which are obtained using indirect detection. Direct detectors are also available. These devices directly convert X-rays to electron-hole pairs which are then collected by a TFT array. As a result of the direct detection, resolution is much improved compared to indirect detection. However, direct detection materials need to: (i) have good electronic properties and (ii) efficiently absorb X-rays. It is difficult to find materials which satisfy this requirement. At present, the most widely used direct detection material is based on amorphous selenium (a-Se), but its low atomic number requires a thick layer of material to absorb a sufficient number of X-rays. CdTe and CdZnTe have recently emerged as promising alternative materials, combining high atomic number and good electronic properties. However, the high cost associated with producing CdTe and CdZnTe, originating from the need for high feedstock purity, high processing temperature and low throughput, has so far limited widescale adoption of these materials. A new generation of direct X-ray detectors based on lead halide perovskite (LHP) has the potential to replace existing materials. LHP materials have excellent electronic properties and a high atomic number. Performance levels have been demonstrated to match or outperform existing direct X-ray detectors. In addition, LHPs may be synthesized in a scalable process at low temperature from low purity feedstocks, which significantly decreases the cost of active materials for direct X-ray detection. To efficiently absorb X-rays, the LHP layer typically has to be thick (typically 0.1-3 mm, depending on the X-ray energy of interest). This thickness can be achieved by depositing from solution or by pressing LHP powder into a wafer; both of these techniques produce polycrystalline materials with grain sizes of the order of 1 pm. Another approach is to produce an active material by growing a single crystal. A single crystal is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries. LHP single crystals typically have dimensions of0.1-10mm. Single crystals can further be identified by the narrow fullwidth at half maximum (FWHM) of the rocking curve in X-ray diffraction (<100 arcsec or <0.03°). Single crystal growth and integration is more challenging than using polycrystalline materials to produce active materials. However, the use of single crystals offers the advantage of lower defect densities, which helps in turn to achieve desirable qualities such as lower dark current, faster response times and better long-term stability. Accordingly, there is growing interest in developing LHP single crystals for use in a direct X-ray detector. Examples of documents relating to the background of X-ray detectors include: “Stable perovskite single-crystal X-ray imaging detectors with single-photon sensitivity” by Sakhatskyi et al, Nature photonics, v17, p510-517, published 8 May 2023. “Top-Down Approaches Towards Single Crystal Perovskite Solar Cells” by Schlipf et al, Scientific Reports, v8, n4906, published 20 March 2018. “A Wet-Fusing Assembly Strategy for Forming Low Dark Current 2D / 3D Perovskite X-Ray Detector on a Thin Film Transistor Backplane” by Liao et al, Advanced Materials Technologies, v8, issue 21, published 5 September 2023. “Robust Perovskite X-ray Flat Panel Detector by Anisotropic Conductive Adhesive to Regulate Thermal Stress” by Liu et al, ACS Energy Letters, v9, issue 4, published 5 March 2024. “High-sensitivity high-resolution X-ray imaging with soft-sintered metal halide perovskites” by Deumel et al, Nature electronics, v4, p681-688, published 23 September 2021. “High-performance direct conversion X-ray detectors based on sintered hybrid lead triiodide perovskite wafers” by Ahrestha et al, Nature photonics, v11, p436-440, published 19 June 2017. Summary The invention is defined in the appended claims. An X-ray detector such as described herein comprises an absorber layer configured to receive as input X-ray radiation incident on the absorber layer and to provide an electrical output by performing a direct conversion in the absorber layer from the input X-ray radiation into charge carriers to provide the electrical output. The absorber layer comprises one or more lead halide perovskite crystals. The electrical output is provided by charge carriers in the detector. The absorber layer is bonded within the detector by a (semi)conductor glue. A method of fabricating an X-ray detector stack for use in a direct X-ray detector is also provided. The method includes forming an absorber layer comprising one or more lead halide perovskite crystals; forming first and second transport layers on either side of the absorber layer; and forming a substrate together with a first electrode and attaching the first transport layer to the substrate and the first electrode via a layer of glue. Brief Description of the Figures Various implementations of the claimed invention will now be described by way of example only with reference to the following drawings. Figure 1 is a schematic diagram of an example of a direct X-ray detector as described herein and comprises four sections: (a) a vertical cross-section, (b) a first top view / horizontal cross-section for a detector formed from a single crystal, (c) a second top view / horizontal cross-section for a tiled detector formed from a single crystal for each tile, and (d) a hi-level circuit representation of a photodiode. Figure 2 is a plot showing the current output produced by a direct X-ray detector firstly in the presence of an external X-ray signal and secondly in the absence of an external X-ray signal (i.e. a dark current). Figure 3 is a schematic diagram showing various stages, labelled (a)-(d), of an example procedure for forming a direct X-ray detector such as shown in Figure 1. Detailed Description A significant problem in the production of LHP single crystal X-ray detectors relates to integrating the LHP single crystals with chip substrates. At present, a common method of integrating LHP single crystals with a chip involves growing LHP single crystals directly on the substrate. This approach suffers from low device yields, as it is difficult to control where the single crystals grow. In addition, the surface of the substrate must be hydrophobic to allow uniform growth and this limits the choice of charge transport layers. Another method for producing LHP single crystal X-ray detectors involves first growing the LHP single crystal before integrating the crystal with a substrate. This latter approach gives more options for the material choice of charge transport layers, but efficient integration requires smooth interfaces and a (semi)conducting glue with anisotropic charge transport properties. The defect density of such detectors may be relatively low, but if large volumes of the LHP crystals are incorporated into a direct X-ray detector 10, this may lead to a relatively large total number of defects. Similarly, a dark current for such a detector may be relatively high due to the large volume of the single crystal(s). The approach described here provides an X-ray detector architecture, applicable to both single pixels and arrays, which is based on using a (semi)conducting glue. This approach facilitates efficient integration of LHP single crystals with a substrate, and provides hole and electron transporting layers on opposing sides of the LHP single crystal to reduce dark current and increase charge extraction. The top transport layer may also act as a physical barrier against atmospheric degradants, thereby increasing the stability of the device. By way of overview, the approach described herein utilises a (semi)conducting glue which serves as an electrical connection between the substrate and the LHP single crystal. This glue layer is provided by a (semi)conducting material with high conductivity in the vertical direction to promote charge transport, and low conductivity in the horizontal direction to prevent crosstalk between adjacent pixels of the readout chip. (For present purposes the vertical direction is assumed to be perpendicular to the image plane while the image plane is assumed to be horizontal; these directions are adopted for explaining the structure of the image detector and do not reflect the orientation of the image detector in space during use of the image detector). The hole transport layer (HTL) serves as a carrier selective medium at the p-side of the junction and is connected to the anode side of the X-ray detector. This HTL is a p-type semiconductor. The electron transport layer (ETL) serves as a carrier selective medium at the n-side of the junction and is connected to the cathode side of the X-ray detector. This ETL is an n-type semiconductor hybrid structure and may comprises at least two adjacent materials. The first material layer protects the device from processing damage during deposition of the second material layer, while the second material layer protects the device from environmental and operational degradation. The X-ray-sensitive (active) layer comprises electromagnetic radiation absorbing material, also termed a radiation absorber, an absorber layer, a photoactive layer etc, and is provided as a LHP single crystal. This layer may be formed with a single crystal covering the whole active (X-ray sensitive) area or as a tiled assembly of LHP single crystals. LHP single crystals have a common chemical formula of APbXs, where A is an alkali metal cation (for example Cs+, Rb+ or K+) or a small organic cation (for example methylammonium (CHaNH; / ) or formamidinium (CH(NH2)2+)), and X is a halide such as iodine, bromine or chlorine. The electrode layers, namely the anode and cathode, provide an interface to CMOS integrated circuitry for providing a readout representing an acquired X-ray image. Each of the anode and cathode may be formed from (i) a continuous optically transparent conducting layer functioning as a global shutter or grounding contact, or (ii) a dense array of electrically isolated electrodes, allowing for a higher-resolution spatial imager. Figure 1 is a schematic diagram of an example of a direct X-ray detector as described herein. Figure 1 comprises four sections labelled (a)-(d). Figure 1(a) presents a cross-section of the direct X-ray detector 10 which has a layered structure. At the base of the detector is a substrate or insulator 68. An electrode 62 (the anode for this configuration) is formed in the substrate 68 and extends from the bottom through to the top of the substrate 68. The electrode 62 is pixelated (not explicitly shown in Figure 1(a)), whereby each pixel represents a different horizontal location (pixel) within a captured image. In some implementations, the substrate 68 may be utilised to form or support a read-out integrated circuit (ROIC) (not shown in Figure 1). The ROIC generally provides an electrical (electronic) interface between the pixelated electrode 62 and other components of the direct X-ray detector device, such as a processor, user interface, and so on, which are external to the X-ray detector 10 per se formed on the substrate 68. Further details about various type of ROIC can be found, inter alia, at: https: / / en.wikipedia.org / wiki / Readout_integrated_circuit The electrode 62 and the substrate 68 are arranged to provide a planar top for receiving a layer of a semi(conducting) glue 66. A hole layer 64, i.e. a layer which predominantly comprises holes rather than free electrons, is formed on the planar top of the electrode 62 and substrate 68. The hole layer 64 is held in position by the semi-conductor glue 66. In some implementations, the hole layer 64 may also be pixelated, with the pixel locations aligned between the hole layer 64 and the bottom electrode 62. Pixelation of the hole layer 64 may provide better isolation between the set of pixel electrodes 62 and so provide a better spatial sensitivity. However, the additional processing to support pixelation of the hole layer 64 may sometimes be unattractive for other reasons, such as cost and / or increased complexity of fabrication. As discussed above, the terms top and bottom are not representative of a spatial location during use of the direct X-ray detector device, but rather to indicate the structure and configuration within the direct X-ray detector device. In particular, the substrate 68 can be regarded as forming the base of the device while the cathode 52 (as described in more detail below) is located at the top of the device and may be the last layer to be formed during manufacture of the direct X-ray detector device. Another criterion for determining top and bottom within the direct X-ray detector 10 may be based on the interaction between incoming radiation and the direct X-ray detector 10. In particular, the incoming radiation first encounters the electrode 52 which is regarded as the top layer. The radiation progress further (down) into the direct X-ray detector 10 where it is converted into an electrical (electronic) signal by the absorber layer 58. The electronic signal then progresses further down to the base (bottom) of the direct X-ray detector formed by substrate 68 before being transferred into the ROIC which forms (part of) the back end of the device. With this interpretation, the down direction represents depth of the optical (X-ray) path within the X-ray detector 10. Above the hole layer 64 is a photoactive layer 58 which is provided to absorb X-ray radiation received in through from the top of the imaging device 10. It can be seen from Figure 1 (which is schematic only and not to scale) that the photoactive layer 58 is generally thicker compared to the other layers in the detector 10. The photoactive layer 58 therefore acts as a radiation absorber 58 in which X-ray radiation arriving at the photoactive layer 58 generates a corresponding electronic signal. This additional thickness or depth of the absorber (photoactive) layer 58 supports increased conversion of the incoming X-ray radiation by the direct X-ray detector 10 and so provides a stronger electrical signal at the anode 62. A first electron layer 56 and a second electron layer 54 are located above (on) the radiation absorber 58. In particular, the first electron layer 56 is formed directly on the radiation absorbing layer 58 and the second electron layer 54 is then formed on the first electron layer 56. The top electrode 52 (cathode) is located on top of the second electron layer 54. The pairing of bottom electrode 62 and top electrode 52 allows the imaging detector 10 to be inserted into an electric circuit (not shown in Figure 1). The electron layers 56 and 64 may be used to control the directionality of electron flow (or hole flow) through the relevant layers. Thus an electron transport layer allows electrons to flow through the layer, while an electron blocking layer has the opposite functionality and prevents electrons from flowing through the layer. In some cases, an electron blocking layer may be implemented as a hole transport layer and an electron transport layer may be implemented as a hole blocking layer. In some configurations, the first electron layer 56 is further used to protect the device from processing damage during deposition of the second electrode layer 54, whereby the second material layer 54 in turn protects the device 10 from environmental and operational degradation. Accordingly, the second electrode layer 54 may be a conformal layer that protects the direct X-ray detection device 10 against ingress of impurities, such as environmental molecules. The second electrode layer may also (or alternatively) prevent metal ion migration from the top electrode 62 (such migration may reduce the performance and lifetime of the device 10). Further encapsulation layers may be included in the direct X-ray device structure 10 such as to provide addition protection from ingress of environmental molecules. In the configuration shown in Figure 1(a), each of the first electron layer 56 and the second electrode layer 54 is formed as an electron transport layer and the hole layer 64 is formed as a hole transport layer. In other implementations, a reverse ordering may be utilised, so that the first electron layer 56 is formed as a hole transport layer (likewise for the second electron layer 54) and the hole layer 64 is formed as an electron transport layer. The combination and operation of the hole layer 64 and the first electron layer 56 on opposing sides of the radiation absorber 58 help to provide increase device performance (such as better signal-to-noise ratio). The provision of the second electron transport layer 54 may be utilised, inter alia, to achieve increased stability for the direct X-ray detector 10. In some implementations of the direct X-ray detector 10, there are no intervening layers with respect to the layers shown in Figure 1(a). In other words, the substrate 68 may be in direct contact with the electrode 62 and the glue 66; the glue 66 is further in direct contact with the hole layer 64; the hole layer 64 is further in direct contact with the absorber layer 58; the absorber layer 58 is further in direct contact with the first electron layer 56; the first electron layer 56 is further in direct contact with the second electron layer 54; and the second electron layer is in direct contact with the electrode 52. In other implementation, the direct X-ray detector 10 may have one or more additional intervening layers located between electrode 52 and electrode 62 (such as an extra blocking or transport layer, not shown in Figure 1). Note that the X-ray range is typically understood as extending approximately from ~150eV to ~125keV. In the present context, the X-ray range may be considered to extend where appropriate to higher energies such as normally associated with gamma (y) rays, for example up to around ~10MeV (according to the circumstances of any given implementation). Figure 1 (b) is a top view of one implementation of the direct X-ray detector 10 as described herein. More particularly, this view may be obtained prior to formation of the upper layers, namely the first and second electron layers 56, 54 and the top electrode (cathode) 52. Accordingly, Figure 1(b) illustrates the substrate 68 having the broadest footprint (when looking down from the top as in Figure 1(b)) and within this footprint, and formed on top of the substrate layer 68, is the radiation absorber 58. It will be appreciated that the direct X-ray detector 10 further includes the conducting (or semiconducting) glue layer 66 and the hole layer 64, but these are not visible in the top view of Figure 1(b) because they are overlain and therefore obscured by the photoactive layer 58. Also visible in the top view Figure 1(b) are portions of the bottom electrode (anode) 62. This electrode has a pixelated structure for capturing the image received by the direct X-ray detector 10. For example, Figure 1(b) shows the electrode layer 62 comprising a linear pattern of contacts 72 providing individual contacts 72A, 72B, and so on. In some implementations, each contact 72A, 72B may correspond to a row of pixels for read-out; in other implementations, each contact 72 may be configured to receive electrical read-out from multiple rows of pixels. It will be appreciated that for clarity Figure 1(b) shows only a limited number of contacts (nine), with only the first two contacts explicitly indicated as 72A and 72B. However, in a typical implementation the number of contacts in a row of pixelated electrodes may be significantly larger, for example hundreds or potentially thousands. In Figure 1(b), the photoactive layer 58 is formed from a single LHP crystal. Utilising a single LHP crystal in this manner may help to provide the best electrical properties for the direct X-ray detector, however, there may be lower yields (and hence enhanced costs) for such a single LHP crystal. Accordingly, Figure 1 (c) shows an example of another configuration for the direct X-ray detector 10. The configuration of Figure 1 (c) represents a top view broadly similar to the configuration of Figure 1(b), but in Figure 1(c) the radiation absorber is subdivided into a pattern of tiles 20. Each of these tiles (20A, 20B, and so on) is typically implemented as having a square or rectangular shape. Each tile 20 is formed from a single LHP crystal. This use of tiles 20 allows the direct X-ray detector 10 to be formed from smaller single LHP crystals (compared to the configuration of Figure 1 (b)) because the single crystals only need to be large enough to fill a tile 20A, 20V rather than having to occupy the full horizontal spread of the detector 10 itself. Accordingly, the use of tiles 20 may improve yield and help to reduce costs in some implementations. Figure 1(d) is a high-level circuit diagram showing a photodiode 80 which may be implemented using the direct X-ray detector 10 illustrated in Figures 1(a)-(c). The photodiode 80 generally does not support current flow through the direct X-ray detector 10 in the absence of light (X-rays). In particular, this non-conductive behaviour is generally due to a band gap which results in a lack of charge carriers, i.e. (free) electrons or holes, in the radiation absorber layer 58. However, in the presence of radiation 85 incident on the radiation absorber layer 58, as indicated by the arrows 80 in Figure 1(d), charge carriers in the radiation absorber 58 are created (or promoted across the band gap) to provide electrical conductivity through the single crystal X-ray detector 10. This conductivity may, for example, arise from incoming X-rays impacting or interacting with the atoms and charge carriers in the radiation absorber layer 58 such as to produce a pair of free (unbound) charge carriers comprising a hole and an electron. (It will be appreciated that the X-rays may also have more complex interactions with the radiation absorber layer 58, including multiple interactions in succession, but these interactions generally still serve (inter alia) to create charge carriers in the radiation absorber layer 58). In operation, an electric field (voltage) is applied to the direct X-ray detector 10 using the pixelated anode 62 and the top cathode 52. This electric field causes a current flow in the X-ray detector 10 in the presence of radiation (X-rays) 85, compared with the absence of a current flow if there is currently no illumination of the direct X-ray detector 10. As indicated by the photodiode symbol of Figure 1 (d), the X-ray imaging detector 10 generally supports current flow in one direction, namely from the anode 62 through the imaging detector 10 to the top electrode 52. However, the direct X-ray detector device 10 described herein may generally be operated in reverse bias - in other words, the voltage applied to the top electrode 52 is greater than (positive with respect to) the pixelated bottom electrode 62. Other implementations may operate the photodiode (X-ray detector device) with forward bias. The final choice of forward or reverse bias may depend, for example, on which direction of bias provides better long-term stability, absolute dark current, and / or signal-to-noise ratio. In implementations which use reverse bias, the hole layer 64 may act as a hole transport layer to allow holes created by the absorption of X-ray illumination (85) in the radiation absorber 58 to exit the radiation absorber into the hole layer 64 (and then through the glue layer 66 into the anode 62). Conversely, electrons created by X-ray illumination and absorption in the radiation absorber 58 exit the radiation absorber 58 via the first hole layer 56 en route to the cathode electrode 52. In this situation the first electron layer 56 acts as a hole blocking layer and likewise as an electron transport layer. Accordingly, the overall flow of charge carriers when using the device 10 in reverse bias has the hole layer 64 acts as a hole transport layer (and an electron blocking layer) and the first electron layer 56 act as an electron transport layer (and a hole blocking layer). The use of transport and blocking layers as described above can help to control the flow of charge carriers (holes and electrons) through the direct X-ray detector 10. For example, the transport and blocking layers may enhance the signal to noise ratio in the presence of illumination 85 and / or to reduce the level of dark current, which is the (unwanted) flow of current through the direct X-ray detector in the absence of radiation 85. A direct X-ray detector 10 such as illustrated in Figure 1 may be implemented using (for example) a p-i-n junction architecture, where p indicates a semiconductor material with positive doping, n indicates a semiconductive material with negative doping, and / indicates an intrinsic semiconductor material (little or no doping). In an example of photodiode 80 and with further reference to direct X-ray detector 10, the hole layer 64 may be p doped with positive charge carriers (holes) while the first electron layer 56 may be n doped with negative charge carriers (electrons) to form a p-i-n junction with the i representing the intrinsic semiconductor material of the radiation absorption layer 58. In operation of the direct X-ray detection system 10, holes created in the radiation absorber 58 flow towards the anode 62 (due to the reverse bias). This flow of holes passes through the hole layer 64 which is therefore a hole transport layer. Conversely, the first electron layer 56 is a hole blocking layer which in effect prevents the holes from flowing upwards out of the radiation absorber 58. In effect, the flow of holes is constrained to follow a downward direction from the radiation absorber 58 to the anode 62, thereby helping to maximise the hole signal that arrives at the anode 62 for analysis via the ROIC. With regard to the electrons created in the radiation absorber 58, these electrons flow towards the cathode (top electrode) 52 (due to the reverse bias). This flow of electrons passes through the first electron layer 56 which is therefore an electron transport layer. Conversely, the hole transport layer 64 also acts as an electron blocking layer to prevent electrons from travelling downwards towards the substrate 58. In effect, the flow of electrons is constrained to follow an upward direction from the radiation absorber 58 to the cathode 52. The p-i-n junction architecture therefore helps to improve the sensitivity and efficiency of the direct X-ray detector by encouraging the charge carriers created within the radiation absorber 58 to flow in the appropriate direction to the anode 62 for the electrons and to the cathode 52 for the holes. Accordingly, a p-i-n junction architecture such as shown in Figure 1 helps to achieve various advantages in comparison with detectors which have a different configuration. For example, the p-i-n junction architecture may help to reduce dark current and / or to achieve a larger absorption cross-section area for incoming radiation 85, which leads in turn to improved photodiode (detector) characteristics. This has particular relevance for direct X-ray detection devices 10, 80 as disclosed herein which are made from bulky single crystals. Although the defect density in such devices may be relatively low, the large volume of the radiation absorber 58 incorporated into a direct X-ray detector 10 as described herein may lead in some case to a relatively large total of defects and associated dark current. A typical p-i-n photodiode may generally be characterized by the following features: (i) operation at reverse bias - a higher (more positive) voltage is applied to the cathode 52 compared to the anode 62, (ii) energy equilibrium is upset upon irradiation 85, (iii) excitons and electron-hole pairs are generated in the radiation absorber 58, (iv) the electrons are attracted to the n-side and the holes are attracted to the p-side (given the reverse biasing), so X-ray generated current flows from the n-side (such as hole layer 64) to the p-side (such as the first electron layer 56), (v) X-ray current is further facilitated by an external electric field applied by the two electrodes 52, 62 provided as part of the X-ray detector 10, and (vi) the X-ray induced current is received and measured as an electrical signal for each pixel of the image to be captured. We now consider in more detail the operation of various functional layers which may be present in a direct X-ray detector 20 such as described herein. 1. (Semi)conducting glue 66. The functional role of the conducting (or semiconducting) glue 66 is to transport charge carriers (holes) from the hole layer 64 to the substrate 68, more particularly to the anode comprising the pixelated electrode 62 which is structurally linked to the substrate 68. This glue layer 66 is a (semi)conducting material, or a hybrid material containing such a (semi)conducting material, that changes phase from liquid to solid upon exposure to an external stimulus such as heating, cooling, (UV) light or the evaporation of a solvent. This glue on solidifying acts to bind the LHP single crystal (or crystals in the case of a tiled implementation, see Figure 1 (c)) to the substrate 68 via the hole layer 64 which is located between the glue 66 and the radiation absorber 58. The conducting or semiconducting nature of the glue 66 allows charge carriers to flow from the pixelated electrode 62 through to the hole layer 64 (or vice versa according to the bias, etc). The thickness of the (semi)conducting glue layer 66 is typically in the range 50-10000 nm. 2. Hole transport layer (p-type semiconductor) 64. The functional role of the hole layer 64 is to provide a carrier selective layer which offers a hole transport or an electron blocking medium (HTL or EBL). In particular, hole layer 64 generally comprises a wide-bandgap semiconductor which predominantly transmits holes and blocks electrons. Accordingly, holes created in the radiation absorber 58 are drawn by the reverse bias to pass through the hole layer 64 (acting as a hole transport layer) and the (semi)conducing glue layer 66 to arrive at the electrode 62 supported by the substrate 68. This current may then flow into the ROIC for further processing. The operation of the hole layer 64 is augmented by acting as an electron blocking layer; this reduces the noise level arising from electrons that might otherwise progress to the electrode 62. In some implementations, the energy bandgap (such as between the valence band and the conduction band) may typically lie in the range between 1 eV and 4 eV. For example, the conduction band may have an energy level for the lowest unoccupied molecular orbit (LUMO) that typically lies in the range 2.0-4.0 eV, while the valence band may have an energy level for the highest occupied molecular orbit (HOMO) that typically lies in the range 4.0-6.0 eV. The charge carrier density for the hole transport layer 64 may typically lie in the range 1016-1020 cm-3. The thickness of the hole transport layer 64 may typically lie in the range 5-150 nm. 3. X-ray absorber 58. The functional role of the radiation absorber (photoactive layer) 58 is the absorption of X-rays. In particular, the radiation absorber 58 acts to generate excited charge carriers upon X-ray irradiation (illumination) 85. The charge carriers are then separated into two types, electrons and holes which flow in opposite directions to the cathode 52 and the anode 62 respectively (in view of the reverse bias). The X-ray absorber layer 58 is nearly (quasi) intrinsic with eliminated minority charge carriers. The carrier density for the X-ray absorption layer may typically line in the range 1014-1018 cm 3. The energy bandgap (such as between the valence band and the conduction band) may typically lie in the range 0.01-3.0 eV. The X-ray absorption range of the radiation absorber 58 may typically extend across some or all of the X-ray range 0.1-100keV (from soft X-rays to hard X-rays). The y-ray absorption range of the radiation absorber may typically extend across some or all of the y-ray range 0.1-8MeV. The thickness of the hole X-ray absorption layer 58 may typically lie in the range 100-5000 pm. It will be appreciated that the thickness of the absorption layer 58 is significantly greater than the thickness of the hole layer 64 and other similar layers. This larger thickness of the absorber layer provides an increased cross-section for interaction between the high energy radiation (X-rays and / or y-rays) and the material of the X-ray absorption material. 4. Electron transport layer (n-type semiconductor) 56. The functional role of the first electron layer 56 is to provide a carrier selective electron transport or hole blocking medium (ETL or HBL). In particular, the layer 56 acts as an electron transport medium (layer) so that electrons may exit the absorption layer 58 and are drawn by the reverse bias through the first and second electron layers 56, 54 to the cathode (top electrode) 52 having regard to the reverse bias applied to the direct X-ray detector device 10. The layer 56 further acts as a hole blocking medium to help prevent stray holes exiting the X-ray absorption material 58 and trying to move towards the top electrode 52 (this can help to reduce noise in the direct X-ray detector device 10. Accordingly, the first electron layer 56 comprises a wide-bandgap semiconductor which predominantly transmits the electrons and blocks the holes. The energy bandgap (such as between the valence band and the conduction band) may typically lie in the range between 1 eV and 4 eV. The conduction band energy level for the LUMO may typically lie in the range 3.0-5.0 eV. The valence band energy level for the HOMO may typically lie in the range 5.0-8.0 eV. The charge carrier density for the first electron layer 56 may typically lie in the range 1O16-1O20 cm'3. The thickness of the first electron layer may typically lie in the range 5-150 nm (like for the hole layer 64, this is much thinner than the layer of X-ray absorption material 58). 5. Electrodes (anode 62 and cathode 52). The functional role of the electrodes (the anode 62 and the cathode 52) is to provide electrical contact between the stack (such as shown in Figure 1) and an external interface such as the ROIC (not shown in Figure 1). This electrical contact within the direct X-ray detection device 10 aligns with the (semi)conducting glue 66 (for the anode 62) and with the second electron layer 54 (for the cathode 52). The electrodes may be built into (or otherwise connected with) CMOS circuitry which may provide an interface between the X-ray sensitive device (detector) 10 and signal readout electronics (not shown in Figure 1). The electrodes 52, 62 are typically formed of metal which provides (inter alia) a high electrical conductivity and mechanical strength. The metal electrodes may have a suitable work function which typically lies in the range 4.5-6.5 eV for the anode 62 and in the range 3.5-5.5 eV for the cathode 52. One or both of the electrodes may be arrayed (formed in the configuration of an array). In some implementations, the array may have a pitch (spacing) down to an optical diffraction limit size of <2 pm. In other implementations, the anode 62 and / or the cathode 52 may be continuous across the entire image array. The thickness of the electrodes typically lies in the range 5-500 nm. The direct X-ray detector described herein may be configured as a multi-layer device stack (such as shown in Figure 1). In some implementation, there may be further vertical integration to form a super (extended) stack with repeated (alternating) p and n layers. In some cases, the charge carrier density may differ between the larger and smaller bandgap layers, while in other cases they may be similar. The valence band energy level (HOMO) or the work function of the (semi)conducting glue 66 should be aligned as closely as possible (within 0.01 - 0.5 eV) to the valence band level of the HTL 64. The valence band energy level (HOMO) of the HTL 64 should be aligned as closely as possible (within 0.01 - 0.5 eV) to the valence band level of the X-ray absorber 58. The conduction band energy level (LUMO) of the first and / or second ETL 56, 54, should be aligned as closely as possible (within 0.01 - 0.5 eV) to the conduction band level of the X-ray absorber 58. The work function of the first electrode (anode) 62 should be aligned as closely as possible (within 0.01 - 0.5 eV) to the valence band energy level (HOMO) or work function of the (semi)conducting glue 66. The work function of the second electrode (cathode) 52 should be aligned as closely as possible (within 0.01 -0.5 eV) to the conduction band energy level (LUMO) of ETL 54 and / or ETL 56 (wherein each of ETL 54 or ETL 56 may be wide-bandgap or narrow-bandgap. The specified layers decrease or increase the energy barrier for a certain charge carrier type. For example, an electron transport layer (ETL) or a hole blocking layer (HBL) favours electron transport. In contrast, a hole transport layer (HTL) or an electron blocking layer (EBL) favours the transport of positively charged carriers (holes). The specified layers may also be referred to as electron / hole transport material (ETM / HTM), or as an electron / hole injection layer (EIL / HIL), or as an electron / hole injection material (EIM / HIM). The carrier selective layers, such as hole layer 64, or first and second electron layers 56, 54, may be implemented using a conducting or semiconducting layer having a thickness in the range of 5-200 nm. In some implementations, the introduction of suitable HTL and ETL layers into a photodiode architecture may reduce the leakage of dark current from around 1 mA / cm2 to around 1 pA / cm2. In some implementations, the introduction of suitable HTL and ETL layers into a photodiode architecture may reduce the leakage of dark current from around 1 pA / cm2 to around 1 nA / cm2. The introduction of suitable HTL and ETL layers may (also) increase the detective quantum efficiency (DQE) figure of merit in some implementations from around 5% to around 50%. (Further information about detective quantum efficiency can be found, inter alia, at: https: / / en.wikipedia.org / wiki / Detective_quantum_efficiency. In other implementations, the introduction of suitable HTL and ETL layers may increase the DQE figure of merit from around 50% to around 90%. It will be appreciated that the above numerical improvements to dark current and / or DQE are provided by way of example for certain implementations; other implementations may provide different levels of improvement for dark current and / or for DQE according to the particular circumstances of any given device. In some implementations, the direct X-ray detection system 10 may comprise an array of photodetectors grouped in proximity with each other. Each photodetector may be similar to or the same as a direct X-ray detection system 10 such as shown in Figure 1. In some configurations, the photodetectors may share the same p-i-n layers and / or at least one common electrode. In other configurations, the junction doping may be inverted from p-i-n to n-i-p. In this latter arrangement, the cathode is the first electrode (corresponding in position to anode 62 in Figure 1) and the anode is the second electrode (corresponding to the cathode 52 in Figure 1). In this inverted configuration, the architecture of the device becomes n-i-p type. The following table sets out some materials for use in producing a direct X-ray detection system 10. It will be appreciated that the materials presented in the table are provided by way of example only, and the skilled person may be aware of additional materials for use according to the particular circumstances of any given implementation. Functional layer Example Materials (Semi)conducting glue 66 Any one or more of the materials listed below for the HTL 64, ETL 56, 54 and electrode 52, 62 layers, suspended or dissolved in a solvent. Any one or more of the materials listed below for the HTL, ETL and electrode layers hybridized with insulating organic polymers that solidify upon cooling down (PE - polyethylene, PEG - polyethylene glycol, PLA - polylactic acid, PP - polypropylene, PU - polyurethane) or upon heat curing (epoxy, acrylic) where the temperature does not exceed 200°C, or upon exposure to (UV)light (epoxy, acrylic). Semiconductor materials with melting temperatures between 100-200°C, such as Mn or Sn based hybrid organic-metal halide materials (MTP2MnBr4, 1-MHA2Snl4), metal halides (Seh), or organic molecules (P3HT). HTL or p-type transport layer 64 Any one or more of the following materials: NiO, CU2O, WO3, M0O3, SnO, CuSCN, Cui, Ag2S, Ag2Se, Ag2Te, NiO:Cu, NiO:Ag, PbS, InAs, InSb, semiconductor nanocrystals (PbS, CdSe, Ag2Te) with thiol or other organic ligands, organic polymers (PEDOT:PSS, PDPP3T, PSBTBT, P3HT, PTAA, PVK, F8, F8BT, PCDTBT, N2200, PFO, PF10TBT, PSF-TAD, PTPD, MEH-PPV, MDMO-PPV, Poly-TPD, PDTPBT, PDFT), single molecules (pentacene, CoPc, CuPc, ZnPc, DTS(FBTTh2)2, TPB, TPD, TFB, Spiro-MeOTAD, m-MTDATA, 2-TNATA, NTCDA, CBP, NPB, TCTA, ICBA, PCBM, TPBi, m-MTDATA, MeO-TPD, a-NPD, Spiro-TTB, Di-NPD, 4P-NPD, 4P-TPD, NPNPB, F4TCNQ, HAT-CN) self-assembled molecules (Me-2PACz, Me-4PACz, MeO-2PACz, MeO-4PACz, Br-2PACz), graphene-based materials such as graphene oxide. Any of the above materials can be hybridized with insulating organic polymers. ETL or n-type transport layer 56, 54 Any one or more of the following materials: ZnO, TiO2, SnO2, Zn2SnO4 (ZTO), SrTiOs, Zn2SnO4, PbS, InAs, InSb, CdS, CdSe, Bi2Se3, organic polymers (PCNEPV, PF1CVTP, PF10TBT, NRS-PPV), single molecules (Alq3, C60, C70, PCBM, BCP, BPhen, TPBi, PTCDA, Butyl-PBD, BND, ITIC-2F, PDBPyBT, PTCDI-C7, ICBA, 4CzlPN, B3PYPPM, TmPyPB, NBP). Additional materials may include ZnO:AI, ZnO:ln, ZnO:Mg, ZnO:Y, ZnO:Nb, ZnO:Ti, ZnO:Sn, TiO2:Nb, SnO2:Nb, TiO2:AI, SnO2:AI, ZrO2, AI2O3, MgO, CS2CO3, LiF, cobaltocene (CP2C0), benzyl viologen (BV). Any of the above materials can be hybridized with insulating organic polymers. Absorber layer 58 Lead halide perovskite single crystals or polycrystalline wafers selected from the group APbXs: where A is caesium, methylammonium, formamidinium, guanidinium or other ammonium containing molecules, or a combination of two or more of these molecules, and where X is iodide, bromide or chloride or a combination of two or more of these halides. The component 'A', i.e. caesium, methyl ammonium, etc is utilised to provide better electronic properties than a simple PB halide. Electrode 52, 62 Metals (Ti, W, Pd, Ni, Cr, Ti, Au, Ag, Cu, Mg), alloys (AICu), metal nitrides (TiN, NbN), carbon (graphene, nanotubes), transparent conducting oxides (TCO): IZO, ITO, FTO, AZO, ATO, organic / polymer conductors. The work function of electrodes may be suitably chosen to favour the charge carrier transport through the interfaces with the electrodes. Note that a hole blocking layer and / or an electron blocking layer may be formed from the same materials as the electron transport layer 56, 54 and the hole transport layer 64 as respectively listed in the above table. Fabrication methods for producing a direct X-ray detector device such as described herein and / or as illustrated in Figure 1 may include solution processes such as spin-coating, printing, slotdie, coating, blade coating, dip coating, drop casting, and so on. Fabrication methods may also include vacuum deposition techniques such as chemical vapour deposition (CVD), atomic layer deposition (ALD), physical vapour deposition (PVD), thermal evaporation, magnetron, radio frequency (RF) or direct current (DC) sputtering, and so on. It will be appreciated that the skilled person is familiar with the above fabrication methods and the direct X-ray detection system described herein may be formed by commercially available implementations of such fabrication methods. Figure 2 is a graph showing the current output produced by a direct X-ray detector 10 firstly in the presence of an external X-ray signal and secondly in the absence of an external X-ray signal - i.e. a dark current. In particular, the X-axis in Figure 2 represents time and the Y-axis represents the current density produced by a direct X-ray detector 10 such as described herein. (Note that the plot of current density against time produced by the external X-ray signal 85 in Figure 2 is interpolated for the period between approximately 9-18 hours. The first, lower (red / lighter) plot in Figure 2 represents an internal dark current, in other words the current density produced within the direct X-ray detector 10 even in the absence of any incoming X-ray radiation. The dark current is an important metric for an X-ray detector because having a lower dark current provides better sensitivity for detecting, imaging and measuring a relatively weak X-ray signal 85 incident on the direct X-ray detector 10. The second, higher (black, darker) plot in Figure 2 represents the reverse bias current density generated by an external X-ray signal 85 which is incident on the X-ray detector 10, 80. The external X-ray signal shown in Figure 2 has an energy (wavelength) corresponding to 160 keV and a power corresponding to 20 Watts. It can be seen in Figure 2 that the current density generated by the external (top) X-ray signal is significantly greater than the current density generated by the internal (dark) X-ray signal, thereby supporting a good signal to noise ratio for detecting, imaging and measuring the external X-ray signal 85 incident on the direct X-ray detector 10. A direct X-ray detector (photodiode device 80) such as described herein may exhibit a strong rectifying behaviour of l-V characteristics, with a clearly distinguished forward bias region and a clearly distinguished reverse bias region. The reverse bias current increases upon illumination by X-rays 85. The response speed of X-ray detectors such as described herein is mainly limited by a carrier drift time (which depends on the layer thicknesses), electrical mobility, and internal / external electric field strength. The absence or reduced level of a dark (leakage) current is important for low noise and high signal-to-noise ratio. The DQE characteristics of such a direct X-ray detector is dependent on the quality of semiconductor layers, their X-ray absorption behaviour, carrier transport properties of specialised layers, and series resistance of contact electrodes. The approach described herein supports the fabrication of a LHP single crystal direct X-ray detector with charge transport layers that are configured to enhance performance and stability (for example with respect to the dark current and DQE). The approach described herein therefore supports the integration of LHP single crystals into a high-performance direct X-ray detector and may outperform existing direct X-ray detectors such as those based on LHPs or amorphous selenium (a-Se) and CdZnTe direct X-ray detectors. The approach described herein provides an integrated LHP single crystal direct X-ray detector device which may be optimized for performance and stability. The LHP single crystal may be placed in the correct position on the substrate while still allowing a wide flexibility in the choice of the hole and electron transport layers. In this regard, the LHP crystal 58 may be first grown from solution, followed by deposition of an HTL 64 and a first ETL 56 before integration with a substrate 68 using a (semi)conducting glue 66. This is followed by deposition of the second ETL 54. Such a manufacturing process helps to provide a device 10 having good performance and stability. Figure 3 is a schematic diagram (not to scale) showing various stages, labelled (a)-(d), of a procedure for forming a direct X-ray detector such as shown in Figure 1. In stage 3(a) the lead halide perovskite crystal 58 is formed. It will be appreciated that the crystal is a relatively thick and therefore sturdy layer (compared with some of the other layers in the direct X-ray detector 10). In stage 3(b) a first ETL layer 56 is formed on one side of the LHP crystal and a HTL layer 64 is formed on the opposite side of the LHP crystal. In stage 3(c), the three layers in stage 3(b) are now attached to the substrate 68 and contact 62 by a layer of glue 66 (the substrate 68 and the anode 62 are attached to each other first and then added together to the stack). In stage 3(d), a second ETL 54 is provided on the first ETL 56 to provide environmental protection, and electrodes 52 and 62 are formed at the top and bottom respectively of the X-ray detection device 10. It will be appreciated that this method of formation is provided by way of example, and the nature and ordering of the layers included in the direct X-ray detector may be varied as appropriate according to the particular circumstances of any given implementation. The provision of a pre-grown LHP single crystal (such as shown in stage 3(a)) supports the use of a wide range of HTL and ETL materials in stage 3(b), followed by using glue 66 to provide an integration with (or attachment to) the substrate 68 at stage 3(c). This approach may help to decrease dark current and to increase charge extraction, thereby augmenting or maximizing the X-ray detective capability of such a direct X-ray detector device 10. Furthermore, the provision in stage 3(d) of a second ETL material 54 may help to improve the long-term stability and environmental protection of the device and further reduce dark current. Accordingly, an X-ray direct detection device as described herein utilises an integrated LHP single crystal onto which the charge transport layers (HTL and ETL) may be deposited (as discussed above). The LHP single crystal may then be attached to a substrate via the HTL 64 using a (semi)conducting glue. The HTL layer and the first ETL layer help to provide increased device performance, while the second ETL helps to provide increased stability. *** In conclusion, while various implementations and examples have been described herein, they are provided by way of illustration, and many potential modifications will be apparent to the skilled person having regard to the specifics of any given implementation. Accordingly, the scope of the present case should be determined from the appended claims and their equivalents. Furthermore, unless the context clearly indicates to the contrary, it is specifically disclosed herein that the features of any independent claim and / or its associated dependent claims may be combined with the features 5 of any other independent claim and / or its associated dependent claims (irrespective of whether such a combination is explicitly claimed, since the claims are used to determine the scope of protection.

Claims

1. An X-ray detector comprising an absorber layer configured to receive as input X-ray radiation incident on the absorber layer and to provide an electrical output by performing a direct conversion in the absorber layer from the input X-ray radiation into charge carriers, wherein the absorber layer comprises one or more lead halide perovskite crystals, wherein the electrical output is provided by the charge carriers, and wherein the absorber layer is bonded within the detector by a (semi)conductor glue layer.

2. The X-ray detector of claim 1, wherein the absorber layer comprises a single crystal of lead halide perovskite.

3. The X-ray detector of claim 1, wherein the lead halide perovskite comprises polycrystalline wafers.

4. The X-ray detector of claim 1, wherein the absorber layer has an image plane formed from multiple tiles, and each tile comprises a single crystal of lead halide perovskite.

5. The X-ray detector of any preceding claim, where the lead halide perovskite comprises APbXs, where A is caesium, methylammonium, formamidinium, guanidinium or other ammonium containing molecules, or a combination thereof, and where X is iodide, bromide or chloride or a combination of two or more of these halides.

6. The X-ray detector of any preceding claim, wherein the glue layer is a (semi)conducting material or a hybrid material containing a (semi)conducting material, wherein the glue layer changes phase from liquid to solid upon exposure to an external stimulus comprising heating, cooling, (UV) light or the evaporation of a solvent.

7. The X-ray detector of any preceding claim, the detector having an image plane formed of pixels, wherein the glue layer has high conductivity in a first direction perpendicular to the image plane to promote charge transport in this first direction, and low conductivity in a second direction parallel to the image plane to prevent crosstalk in the second direction between adjacent pixels.

8. The X-ray detector of any preceding claim, wherein a first doped semiconductor layer is located between the absorber layer and the glue layer.

9. The X-ray detector of claim 8, wherein the first doped semiconductor layer is a hole transport layer.

10. The X-ray detector of claim 9, wherein the hole transport layer is pixelated.

11. The X-ray detector of any of claims 8 to 10, further comprising a first electrode and a substrate which are arranged to provide a planar surface for receiving the layer of semi(conducting) glue 66.

12. The X-ray detector of any of claims 8 to 11, further comprising a second doped semiconductor layer located on the opposite side of the absorber layer from the first doped semiconductor layer, whereby the first doped semiconductor layer, the absorber layer and the second doped semiconductor layer form a p-i-n configuration.

13. The X-ray detector of claim 9 or any claim dependent thereon, wherein the second doped semiconductor layer comprises a first electron transport layer.

14. The X-ray detector of claim 12 or 13, further comprising a second electrode, wherein the first electrode and the second electrode are configured to be operated in reverse bias.

15. The X-ray detector of claim 14, further comprising a third doped semiconductor layer which is configured to provide a physical barrier against atmospheric degradation.

16. The X-ray detector of claim 15, wherein the third doped semiconductor comprises a second electron transport layer.

17. The X-ray detector of any of claims 14 to 16, wherein each of the electrodes is formed from (i) a continuous optically transparent conducting layer functioning as a global shutter or grounding contact, or (ii) a dense array of electrically isolated electrodes.

18. The X-ray detector of any of claims 8 to 17, wherein each of the first, second and / or third doped semiconductors has a thickness in the range 5-150 nm.

19. The X-ray detector of any preceding claim, wherein the absorber layer has a thickness in the range 100-5000 pm, optionally in the range 250-2500 pm.

20. An X-ray detector comprising an absorber layer configured to receive as input X-ray radiation incident on the absorber layer and to provide an electrical output, wherein the absorber layer is configured to perform a direct conversion of the input X-ray radiation into the electrical output, wherein the absorber layer comprises one or more lead halide perovskite crystals, and wherein the X-ray detector further comprises first and second transport layers on opposing sides of the absorber layer such that the first and second transport layers and the absorber layer form a p-i-n configuration.

21. The X-ray detector of claim 20, further comprising a layer of (semi)conductor glue, wherein the first transport layer is located between the absorber layer and the glue layer.

22. The X-ray detector of claim 20 or 21, further comprising the features of any of claims 2 to 19.

523. A method of fabricating an X-ray detector stack for use in a direct X-ray detector comprising: forming an absorber layer comprising one or more lead halide perovskite crystals; forming first and second transport layers on either side of the absorber layer; and forming a substrate together with a first electrode and attaching the first transport layer to the 10 substrate and first electrode via a layer of glue.

24. The method of claim 23, further comprising incorporating a second electrode such that the first and second electrodes form opposing ends of the stack and optionally including a third transport layer.1525. The method of any of claim 23 or 24, wherein the method is performed in relation to the X-ray detector of any of claims 1 -22.s

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