Photocathode device for sub-nanosecond gating and method for the manufacturing thereof, and visible and ultraviolet radiation detection device

Abstract

Photocathode device for sub-nanosecond gating comprising an active photo-emission region (3) surrounded by an electrode region (5), the device comprising: - a substrate (7); - a photocathode layer (9) deposited on the substrate; wherein the device also comprises: - a conductive underlayer (11) deposited between the substrate and the photocathode layer in the active photo-emission region; - a layer of aluminium oxide (17) deposited between the conductive underlayer and the photocathode layer. In a preferred embodiment, the underlayer comprises a gold layer (15) deposited on a seed layer (13). Method of manufacturing of the photocathode device, and visible and ultraviolet radiation detection device comprising the photocathode device.

Description

TECHNICAL FIELD

[0001] The present invention generally relates to the field of optoelectronic devices, and more particularly to the field of photocathode devices for sub-nanosecond gating.BACKGROUND

[0002] The current state of the art in the field of sub-nanosecond gateable image intensifier tubes involves the use of photocathodes comprising multi-alkali materials such as sodium potassium antimonide (Na 2 KSb). These photocathodes are deposited on substrates to achieve high quantum efficiency in the deep ultraviolet and visible spectrum between 200 nm and 900 nm range. These devices typically employ conductive grid layers made from materials like chromium, which are deposited using photolithography techniques. The relatively transparent grid layers are crucial for providing the necessary electrical conductance to the otherwise highly resistive photocathode materials, which can exhibit square resistance values in the kilo Ohm (kOhm) to Giga Ohm (GOhm) range.

[0003] However, the use of conductive grids for multi-alkali photocathodes presents several significant disadvantages. The fabrication of these grids using photolithography is both complex and costly, significantly increasing the production costs of these devices. Additionally, the grid structures can block a portion of the incoming light, reducing the overall quantum efficiency of the photocathode.

[0004] Document Y. Yang et.al, Nuclear Inst. and Methods in physics research A, 1056, 2023, 168621 discloses a device comprising a layer of alkali antimonide directly deposited onto a gold layer. However, such a device exhibits a very low quantum efficiency.

[0005] It is therefore an object of the present disclosure to provide a device and a method that at least partly overcome one or more of the above-mentioned drawbacks of the prior art.SUMMARY

[0006] In a first aspect, the present disclosure relates to a photocathode device for sub-nanosecond gating comprising an active photo-emission region surrounded by an electrode region, the device comprising: a substrate; a photocathode layer deposited on the substrate; a conductive underlayer deposited between the substrate and the photocathode layer in the active photo-emission region and preferably in the electrode region; a layer of aluminium oxide deposited between the conductive underlayer and the photocathode layer.

[0007] "Deposited on" as used in the present disclosure means that a layer may be deposited directly on an underlying layer, but this it may also be possible that another layer is arranged in between said layer and said underlying layer.

[0008] A multi alkali photocathode device may be understood as an optoelectronic device that is designed to detect visible and / or ultraviolet light.

[0009] The photocathode device may be adapted to detect visible and ultraviolet light.

[0010] The device comprises a substrate. A substrate may be understood as the underlying material or layer upon which other materials are deposited or constructed. In this context, the substrate provides the foundational support for the subsequent layers of the device.

[0011] The device comprises a conductive underlayer disposed between the substrate and the photocathode layer. With "conductive" underlayer as used in the present disclosure is meant that the underlayer is electrically conductive.

[0012] The conductive underlayer provides electrical conductance for the photocathode layer to enable a gating duration below 1 ns, preferably below 0.2 ns. It may thus be provided that the conductive underlayer supports gating duration below 1 nanoseconds (ns), preferably below 0.2 ns. This enables the device to operate at high speeds, which is critical for applications requiring rapid response times and high temporal resolution.

[0013] The aluminium oxide layer forms a beneficial interface in terms of growth and quantum efficiency between the conductive underlayer and photocathode layer. A good growth and high quantum efficiency of the photocathode is obtained. The presence of the aluminium oxide layer is important as it avoids oxidation of the conductive underlayer beneath the aluminium oxide layer.

[0014] Another advantage of this combination of the conductive underlayer and the aluminium oxide layer is that it reduces the overall production costs and complexity compared to traditional grid-based conductive layers, while still delivering high performance.

[0015] The conductive underlayer preferably comprises a seed layer deposited directly on the substrate and a layer of gold deposited directly on the seed layer, wherein the layer of gold is arranged between the seed layer and the layer of aluminium oxide.

[0016] A seed layer may be understood as a thin film of a metal that is applied onto the substrate that provides a sufficient adhesive underlayer for the layer of gold. The use of a thin layer of gold provides a cost-effective and compatible material for the conductive underlayer, which aids in enhancing the overall conductance of the device.

[0017] A layer of gold may be understood as a thin film of gold metal that is applied onto the seed layer. The thickness of this gold layer is configured to be transparent and electrically conductive.

[0018] The use of a thin metal (Au) film structure provides sufficient electrical conductance to prevent voltage drops, or in other words to maintain the same electric potential over the entire surface of the photocathode, and dark zones in the image, thereby enhancing the device's operational reliability and performance. This approach also facilitates a more straightforward manufacturing process, leading to improved production yields and reduced costs.

[0019] Electrical conductivity (σ) is a measure of a material's ability to conduct electric current. It is the reciprocal of electrical resistivity (ρ) and is expressed in siemens per meter (S / m). Materials with high electrical conductivity allow electric charges to flow easily, while materials with low conductivity resist the flow of current. The relationship between conductivity and resistivity is: σ = 1 / ρ where: σ = electrical conductivity (S / m), and ρ = resistivity (Ω·m).

[0020] The relationship between the square resistance R □ (Ω), electrical resistivity ρ, electrical conductivity σ and thickness d (m) of a conductive thin layer is defined by the following formula: R □ = ρ / d = 1 / σ * d

[0021] The conductance G (in S) of an object in a material is calculated from the conductivity σ and the geometry of the material as defined by the following formula: G = σ × A / L wherein A (in m 2< ) is the cross-sectional area of the object and L (in m) is the length of the object.

[0022] A conductivity of 1 × 10 -8< S / m (at 20 °C) can generally be considered a minimal value to label a material as "conductive," depending on context. In the present invention, "conductive" is defined as having a conductivity over 10 5< S / m, preferably over 10 6< S / m. The high conductivity can be associated with a layer of at least 1 nm.

[0023] The conductive underlayer may be arranged to provide electrical conductance for the photocathode while maintaining quantum efficiency of the photocathode device over 15%, preferably over 20%, in a wavelength range of 200 nm to 900 nm.

[0024] In an embodiment, the photocathode device comprises a ring electrode in the electrode region to provide electrical conductance for the photocathode device. Preferably, the ring electrode comprises a further layer of gold at a side of the ring electrode facing towards the substrate and optionally a further layer of chromium and / or silver and / or nickel at another side, i.e., the opposite side, of the ring electrode facing towards the photocathode layer.

[0025] Preferably, the ring electrode has a circumference that is larger than a circumference of the photocathode layer defining a flange arranged for sealing the photocathode device in a photocathode vacuum envelope.

[0026] The photocathode device may have in the active photo-emission region a relative quantum efficiency loss of at most 25%, more preferably at most 20%, in the wavelength range of 200 nm to 900 nm, as compared to a photocathode device not comprising a conductive underlayer.

[0027] The photocathode device may have in the active photo-emission region a quantum efficiency over 20%, preferably over 25%.

[0028] It is preferred that the photocathode device, more specifically the conductive underlayer, has a square resistance of at most 200 Ohm, more preferably at most 100 Ohm. It may be provided that the conductive underlayer has this specific range of square resistance, ensuring adequate conductance without excessive current leakage. This feature provides optimal electrical performance, preventing voltage drops and ensuring consistent image quality.

[0029] In an embodiment, the photocathode layer comprises a multi-alkali material selected from alkali antimonide, alkali telluride, and combinations thereof and / or wherein the multi-alkali of the multi-alkali material is selected from the group consisting of sodium, potassium, rubidium, cesium, and combinations thereof. Preferably, the multi-alkali material is sodium potassium cesium antimonide according to Na x K y Cs z Sb, wherein x is 2, y is 1, and z is at most 0.2. These materials are known for their high quantum efficiency in the visible and ultraviolet spectrum, enhancing the device's sensitivity and performance in detecting visible and ultraviolet light.

[0030] In another embodiment, the layer of gold has a thickness of 0.5 nm to 3 nm, preferably 1 nm to 2 nm. Preferably, total thickness of the seed layer and of the layer of gold is at most 3 nm, preferably at most 1 nm. This has the effect that it provides excellent electrical conductance for the photocathode while maintaining the structural integrity and high quantum efficiency in the 200 nm to 900 nm wavelength spectrum. It may be provided that the layer of gold is deposited with a controlled thickness to balance conductance and transmitted intensity of the layer. This feature ensures adequate electrical conductance while maintaining structural integrity and performance of the photocathode device.

[0031] In yet another embodiment, the layer of aluminium oxide has a thickness of 5 nm to 200 nm, preferably 10 nm to 60 nm. A layer of aluminium oxide of a thickness in the defined range avoids a chemical interface reaction between the layer of gold and the multi alkali photocathode. Also, the defined thickness range results in optimization of both conductance and performance of the photocathode, making this approach particularly suitable for high-precision applications.

[0032] The assembly formed by the substrate, by the conductive underlayer and by the aluminium oxide layer, in the active photo-emission region, may have a transmittance of at least 70%, preferably at least 75%, in a wavelength range of 200 nm to 900 nm. This allows a maximum number of photons to reach the photocathode layer, improving the device's sensitivity and efficiency in detecting visible and ultraviolet radiation.

[0033] The active photo-emission region surrounded by the electrode region may have a surface area of at least 0.25 cm 2< , preferably at least 0.5 cm 2< .

[0034] In the active photo-emission region, the substrate and the aluminium oxide layer may be devoid of contact. Indeed, the conductive layer allows optical transmission that avoids the use of a conductive grid under the photocathode layer. The conductive layer can be uniformly deposited under an entire surface where the photocathode layer is deposited.

[0035] The device may have no conductive grid arranged between arranged between the substrate and the photocathode layer.

[0036] In an embodiment, the ring electrode has a thickness of 20 nm to 800 nm, preferably 80 nm to 400 nm.

[0037] In another embodiment, the further layer of gold and the optional further layer of chromium and / or nickel of the ring electrode each have a thickness of 10 nm to 400 nm, preferably 30 nm to 200 nm.

[0038] In an embodiment, the substrate is a transparent substrate and / or a diffractive substrate in a wavelength range of 200 nm to 900 nm.

[0039] In another embodiment, the substrate is made of a material selected from the group consisting of glass, quartz, sapphire, fused silica, magnesium fluoride, calcium fluoride and lithium fluoride, which are known for their excellent transmittance and stability in the ultraviolet spectrum. This feature ensures minimal light absorption and high structural integrity, enhancing the overall performance of the device.

[0040] Preferably, the substrate has a thickness of 1 mm to 100 mm, preferably 2 mm to 10 mm.

[0041] In an embodiment, the seed layer comprises particles selected from the group consisting of nickel, titanium, chromium, copper, silver, molybdenum, hafnium, aluminium, silicon, zirconium, niobium, tantalum, and combinations of two or more thereof, preferably the seed layer comprises nickel particles.

[0042] It is preferred that the seed layer has a thickness of at most 2 nm, preferably at most 1 nm. Preferably, the total thickness of the seed layer and of the layer of gold is at most 3 nm, preferably at most 1 nm. This provides an optimal balance between electrical conductance, structural integrity and transmittance of the device.

[0043] In a second aspect, the present disclosure relates to a method of manufacturing a photocathode device for sub-nanosecond gating comprising an active photo-emission region surrounded by an electrode region, the method comprising the steps of: 1) providing a substrate, preferably a transparent substrate and / or a diffractive substrate and / or a fibre optic substrate; 2) depositing a conductive underlayer on the substrate in the active photo-emission region; 3) depositing or forming a layer of aluminium oxide on the conductive underlayer; 4) depositing a photocathode layer on the layer of aluminium oxide.

[0044] The step of deposition a conductive underlayer preferably comprises the steps of: a) depositing a seed layer on the substrate; b) depositing a layer of gold on the seed layer.

[0045] The step of depositing or forming the layer of aluminium oxide may comprise the sub-step(s) of: i) depositing aluminium oxide on the conductive underlayer in the active photo-emission region, and / or ii) depositing aluminium on the conductive underlayer in the active photo-emission region and subsequently exposing the deposited aluminium to an oxidizing gas, such as air, thereby forming the layer of aluminium oxide on the conductive underlayer.

[0046] The conductive underlayer may be arranged to provide electrical conductance for the photocathode while maintaining quantum efficiency of the photocathode device over 20%, preferably over 25%, in a wavelength range of 200 nm to 900 nm.

[0047] The conductive layer is at least deposited in the active photo-emission region and may also be deposited in the electrode region. This is beneficial as it provides a good electrical contact between the ring electrode and the thin gold film (i.e., the layer of gold).

[0048] Prior to step 1), the substrate is preferably polished such that a polished substrate is provided in step 1). Preferably, the substrate to be provided is also cleaned and / or treated with ozone in an air atmosphere and / or provided with a glow discharge treatment in an evaporator.

[0049] Furthermore, the depositing of the layers may be done by evaporation, preferably at a high temperature (200 °C to 300 °C). This is beneficial to obtain the desired properties for conductance, transmittance, transmitted intensity, and diffusion, which minimalizes the formation of alkali oxides.

[0050] The term "transmittance" of an optical object as used in the present disclosure means the ratio between the intensity of a light beam transmitted by the object and the intensity of the light beam incident on the object.

[0051] The method may further comprise a step of depositing a ring electrode in the electrode region in between steps 2) and 3) to provide electrical conductance for the photocathode layer. Preferably, the step of depositing the ring electrode in the electrode region comprises depositing a further layer of gold on the layer of gold, and optionally depositing a further layer of chromium and / or silver and / or nickel on the further layer of gold. This has the effects that the adhesion of the ring electrode to the layer of gold will be guaranteed and because the same metal is used, there will be no interface reactions between the layer of gold and the electrode ring.

[0052] The depositing of the ring electrode is preferably done in a manner such that the ring electrode has a circumference that is larger than a circumference of the photocathode layer defining a flange arranged for sealing the photocathode device in a photocathode vacuum envelope.

[0053] In an embodiment, the step of depositing the layer of gold comprises depositing a layer of gold with a thickness of 0.5 to 3 nm, preferably 1 to 2 nm.

[0054] In another embodiment, the step of depositing the further layer of gold of the ring electrode comprises depositing a layer of gold with a thickness of 10 nm to 400 nm, preferably 30 nm to 200 nm, such as 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 120 nm, 140 nm, 160 nm, or 180 nm, and / or the step of depositing the further layer of chromium and / or nickel of the ring electrode comprises depositing a layer with a thickness of 10 nm to 400 nm, preferably 30 nm to 200 nm, such as 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 120 nm, 140 nm, 160 nm, or 180 nm.

[0055] The seed layer that is deposited on the substrate preferably comprises particles selected from the group consisting of nickel, titanium, chromium, copper, silver, molybdenum, hafnium, aluminium, silicon, zirconium, niobium, tantalum, scandium, yttrium, and combinations of two or more thereof, more preferably the seed layer comprises particles that provide after oxidation an ultraviolet-transparent oxide, even more preferably the seed layer comprises nickel particles.

[0056] A heat treatment in air (or oxygen) at for example 290 °C after depositing of the seed layer, the layer of gold, and the layer of aluminium oxide may provide extra transmittance. This effect is especially present in case the seed layer comprises hafnium, aluminium, scandium, yttrium, titanium, silicon, or combinations of two or more thereof.

[0057] In an embodiment, the step of depositing the seed layer comprises depositing a layer with a thickness of at most 2 nm, preferably at most 1 nm.

[0058] The depositing of each of the seed layer, the layer of gold, and the ring electrode may be done by an e-beam evaporation process.

[0059] The depositing of the layer aluminium oxide or aluminium may be done by an e-beam evaporation process, a sputtering process, or an atomic layer deposition process.

[0060] The depositing of the photocathode layer may be done by chemical vapour deposition (CVD). The CVD process allows precise control over the thickness and uniformity of the layer, which ensures high-quality deposition, enhancing the performance and reliability of the photocathode device.

[0061] The method according to the second aspect of the present disclosure may be used in the manufacturing of a photocathode device according to the first aspect of the present disclosure.

[0062] Embodiments of the photocathode device according to the first aspect of the present disclosure as presented herein are also applicable to the method according to the second aspect of the present disclosure, and vice versa.

[0063] Effects of the photocathode device according to the first aspect of the present disclosure as presented herein correspond to or are similar to effects of the method according to the second aspect of the present disclosure.

[0064] In a third aspect, the present disclosure relates to a visible and ultraviolet radiation detection device, comprising: a photocathode device for sub-nanosecond gating comprising an active photo-emission region surrounded by an electrode region, the photocathode device comprising: a substrate; a photocathode layer deposited on the substrate; a conductive underlayer deposited between the substrate and the photocathode layer in the active photo-emission region and preferably in the electrode region; a layer of aluminium oxide deposited between the conductive underlayer and the photocathode layer, wherein the visible and ultraviolet radiation detection device is configured for use in applications requiring selective sensitivity to visible radiation and / or ultraviolet radiation and insensitivity to infrared radiation.

[0065] Optionally, the visible and ultraviolet radiation detection device further comprises a ring electrode in the electrode region being in electrical contact with the conductive underlayer and configured to provide electrical conductance for the photocathode device.

[0066] The visible and ultraviolet radiation detection device may be implemented in one or more of the following applications: UV and solar-blind image intensifiers; visible light image intensifiers; photon counting detectors and modules; night vision and low-light imaging devices; ultraviolet spectroscopy instruments; radiation monitoring equipment; scientific and medical imaging cameras; aerospace and satellite-based UV detectors; UV detection in fire and explosion sensors; fast-gated photocathode detectors for laser-induced fluorescence; range gated imaging devices; and UV astronomy instruments.

[0067] The visible and ultraviolet radiation detection device according to the third aspect of the present disclosure may comprise a photocathode device according to the first aspect of the present disclosure or a photocathode device manufactured according to the method according to the second aspect of the present disclosure.

[0068] Embodiments of the photocathode device according to the first aspect of the present disclosure as presented herein are also applicable to the ultraviolet detection device according to the third aspect of the present disclosure, and vice versa.

[0069] Effects of the photocathode device according to the first aspect of the present disclosure as presented herein correspond to or are similar to effects of the ultraviolet detection device according to the third aspect of the present disclosure.

[0070] The present disclosure is hereinafter explained in more detail with reference to the accompanying drawings in which embodiments of the present disclosure are shown and in which like reference numbers indicate the same or similar elements. The present disclosure is by no means limited to the embodiments described therein.BRIEF DESCRIPTION OF THE DRAWINGS

[0071] The present disclosure will be explained in more detail below by means of examples of a device and method according to the present disclosure shown in the drawings, in which: Fig. 1 shows a photocathode device for sub-nanosecond gating according to a first aspect of the present disclosure; Fig. 2 shows a method step of manufacturing a photocathode device for sub-nanosecond gating according to a second aspect of the present disclosure. DETAILED DESCRIPTION

[0072] The present disclosure will now be described in detail with reference to the accompanying drawings, which illustrate exemplary embodiments of the photocathode device 1 for sub-nanosecond gating and the method 101 of manufacturing the same. It is to be understood that these embodiments are provided by way of example only and are not intended to limit the scope of the present disclosure. The following detailed description should be read in conjunction with the claims and the figures.

[0073] As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. Expressions such as "at least one of," when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list.

[0074] As the inventive concept allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in more detail in the written description. Effects, features, and a method of achieving the inventive concept will be obvious by referring to exemplary embodiments of the inventive concept with reference to the accompanying drawings. The inventive concept may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein.

[0075] In the embodiments described in the present specification, an expression utilized in the singular encompasses the expression of the plural, unless it has a clearly different meaning in the context. Also, it is to be understood that the terms such as "including," "having," and / or "comprising" are intended to indicate the presence of the stated features or components, and are not intended to preclude the presence or addition of one or more other features or components.

[0076] It will be understood that when a layer, region, or component is referred to as being "on" or "onto" another layer, region, or component, it may be directly or indirectly formed on the other layer, region, or component. That is, for example, intervening layer(s), region(s), or component(s) may be present.

[0077] Sizes of components in the drawings may be exaggerated for convenience of explanation. In other words, since sizes and thicknesses of components in the drawings are arbitrarily illustrated for convenience of explanation, the following embodiments of the present disclosure are not limited thereto.

[0078] Fig. 1 shows schematically an example of a photocathode device 1 for sub-nanosecond gating according to the first aspect of the present disclosure. The photocathode device 1 comprises two regions: an active photo-emission region 3 that is surrounded by an electrode region 5. The device 1 further comprises a substrate 7, a conductive underlayer 11, a layer of aluminium oxide 17, and a photocathode layer 9. As can be seen, the layer of aluminium oxide 17 is deposited between the conductive underlayer 11 and the photocathode layer 9. As can further be seen, the conductive underlayer 11 is deposited between the substrate 7 and the photocathode layer 9 in the active photo-emission region 3 and in the electrode region 5. More specifically, the conductive underlayer 11 is deposited between the substrate 7 and the aluminium oxide layer 17 in the active photo-emission region 3 and in the electrode region 5. The conductive underlayer 11 is arranged to provide electrical conductance for the photocathode while maintaining quantum efficiency of the photocathode device over 20%, preferably over 25%, in a wavelength range of 200 nm to 900 nm and comprises a seed layer 13, deposited on the substrate 7, and a layer of gold 15, deposited on the seed layer 13 and arranged between the seed layer 13 and the layer of aluminium oxide 17.

[0079] The device 1 shown in Fig. 1 further comprises a ring electrode 19, which provides conductance for the photocathode device 1. The ring electrode 19 comprises a further layer of gold 21 at a side of the ring electrode 19 that faces towards the substrate 7 and a further layer of chromium 23 (which may also be silver, nickel, or a combination of two or more of chromium, silver, and nickel) at another side, i.e., the opposite side, of the ring electrode 19 facing towards the photocathode layer 9.

[0080] The example of the device 1 shown in Fig. 1 has in the active photo-emission region 3 a relative quantum efficiency loss of at most 25%, preferably at most 20%, in a wavelength range of 200 to 900 nm as compared to a photocathode device not having a conductive underlayer, and a square resistance of at most 200 Ohm, preferably at most 100 Ohm.

[0081] Furthermore, the example shown in Fig. 1 comprises a photocathode layer that comprises a multi-alkali material selected from alkali antimonide, alkali telluride, and combinations thereof. The multi-alkali of the multi-alkali material is selected from the group consisting of sodium, potassium, rubidium, cesium, and combinations thereof. As an example, the photocathode layer may comprise or consist of sodium potassium cesium antimonide according to Na x K y Cs z Sb, wherein x is 2, y is 1, and z is at most 0.2.

[0082] In the photocathode device 1 shown in Fig. 1, the substrate 7 has a thickness of 1 to 100 mm, preferably 2 to 10 mm. The seed layer 13 has a thickness of at most 2 nm, preferably at most 1 nm. The layer of gold 15 has a thickness of 0.5 to 3 nm, preferably 1 to 2 nm. The layer of aluminium oxide 17 has a thickness of 5 to 100 nm, preferably 10 to 20 nm. The ring electrode 19 has a thickness of 20 to 800 nm, preferably 80 to 400 nm, of which the further layer of gold 21 and the further layer of chromium 23 each have a thickness of 10 to 400 nm, preferably 40 to 200 nm.

[0083] As illustrated in Fig. 1, the assembly formed by the substrate, by the conductive underlayer and by the aluminium oxide layer 1, has, in the active photo-emission region 3, a transmittance of at least 70%, preferably at least 75%, in a wavelength range of 200 to 900 nm. The conductive underlayer 11 provides electrical conductance for the photocathode layer 9 to enable a gating duration below 1 ns, preferably below 0.2 ns. Furthermore, the active photo-emission region 3 surrounded by the electrode region 5 has a surface area of at least 0.25 cm 2< , preferably at least 0.5 cm 2< .

[0084] The substrate 7 is a transparent substrate and diffractive substrate in a wavelength range of 200 to 900 nm. The substrate 7 can be a fibre optic substrate. However, in other examples, the substrate 7 may be a transparent substrate and / or a diffractive substrate and / or a fibre optic substrate in a wavelength range of 200 to 900 nm. The substrate is made of a material selected from the group consisting of glass, quartz, sapphire, fused silica, magnesiumfluoride, calciumfluoride and lithiumfluoride.

[0085] The seed layer 13 of the device 1 as exemplified in Fig. 1 comprises particles selected from the group consisting of nickel, titanium, chromium, copper, silver, molybdenum, hafnium, aluminium, silicon, zirconium, niobium, tantalum, and combinations of two or more thereof, preferably the seed layer 13 comprises nickel particles.

[0086] Fig. 2 shows an example of a method 101 according to the second aspect of the present disclosure of manufacturing of a photocathode device 1 for sub-nanosecond gating comprising an active photo-emission region 3 surrounded by an electrode region 5, such as the device 1 according to the first aspect of the present disclosure. The method 101 comprises the steps of 1) providing 103 a substrate 7, preferably a transparent substrate and / or a diffractive substrate and / or a fibre optic substrate; 2) depositing 105 a conductive underlayer 11 on the substrate 7 in the active photo-emission region 3; 3) depositing or forming 107 a layer of aluminium oxide 17 on the conductive underlayer 11; and 4) depositing 109 a photocathode layer 9 on the layer of aluminium oxide 17.

[0087] The step of depositing 105 the conductive underlayer 11 comprises the steps of a) depositing 105-a a seed layer 13 on the substrate 7 and b) depositing 105-b a layer of gold 15 on the seed layer 13.

[0088] The step of depositing or forming the layer of aluminium oxide 17 comprises the sub-step(s) of i) depositing 107-1 aluminium oxide on the conductive underlayer 11 or ii) depositing 107-iia aluminium on the conductive underlayer 11 and subsequently exposing 107-iib the deposited aluminium to an oxidizing gas, such as air, thereby forming the layer of aluminium oxide 17 on the conductive underlayer 11. It is also possible to first perform step i) and then step ii) or vice versa.

[0089] The method 101 further comprises the step of depositing 111 a ring electrode 19 in the electrode region 5 of the photocathode device 1 in between step 2) of depositing 105 the conductive underlayer 11 and step 3) of depositing or forming 107 the layer of aluminium oxide 17.

[0090] The step of depositing 111 the ring electrode 19 comprises depositing a further layer of gold 21 on the layer of gold 15, and depositing a further layer of chromium 23 (which may also be silver, nickel or a combination of two or more of chromium, silver, and nickel) on the further layer of gold 21. The depositing of the further layer of gold 21 during the step of depositing 111 the ring electrode 19 comprises depositing a layer with a thickness of 10 to 400 nm, preferably 40 to 200 nm and the depositing of the further layer of chromium 23 during the step of depositing 111 the ring electrode 19 comprises depositing a layer with a thickness of 10 to 400 nm, preferably 40 to 200 nm.

[0091] In Fig. 2, the step of depositing 105-a the seed layer 13 comprises depositing a layer with a thickness of at most 2 nm, preferably at most 1 nm. The step of depositing 105-b the layer of gold 15 comprises depositing a layer with a thickness of 0.5 to 3 nm, preferably 1 to 2 nm.

[0092] In said step of depositing 105-a the seed layer 13, the seed layer 13 comprises particles selected from the group consisting of nickel, titanium, chromium, copper, silver, molybdenum, hafnium, aluminium, silicon, zirconium, niobium, tantalum, and combinations of two or more thereof, preferably the seed layer 13 comprises nickel particles.

[0093] Furthermore, the depositing 105-a,105-b,111, of each of the layer of gold 15, the ring electrode 19, and the seed layer 13 are done by an e-beam evaporation process. The depositing 107-i, 107-iia of each of the layer of aluminium oxide and the layer of aluminium is done by an e-beam evaporation process, a sputtering process, or an atomic layer deposition process. The depositing 109 of the photocathode layer 9 is done by chemical vapour deposition.

[0094] The foregoing description of the embodiments of the disclosure has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the claimed disclosure to the precise form disclosed. Those skilled in the art will readily appreciate that many modifications and variations to the claimed disclosure are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present disclosure and its practical application to thereby enable others skilled in the art to best utilize the present disclosure in various photocathode embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the present disclosure be defined exclusively by the following claims, and equivalents thereof.

[0095] A single unit or component may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope thereof.EXAMPLES

[0096] Several different combinations of material layers and some additional variations were tested to achieve a suitable replacement of the current gating technology with a chromium grid. Current solutions for fast gating are listed below: 100nm Chromium Grid + 40 nm Al 2 O 3 . Standard on quartz. 17 nm indium tin oxide (ITO) + 40 nm (NB: 80 nm was also used and also worked well) Atomic Layer Deposition (ALD) Al 2 O 3 . Standard on diffractive substrate.

[0097] The proposed solution based on a thin gold film has more potential to improve image quality issues. Chemical attack is very unlikely due to the small layer thickness and protective aluminium oxide layer.Process Au + Ni / Ti thin film

[0098] Cleaning (spinning isopropyl alcohol (IPA) 30 s, 3000 rpm) + ozon treatment (10 min). Pump down overnight. background pressure = 1,6*10 -6< mbar. E-beam evaporation titanium or nickel 0,5 nm (0.25 A / s). Jig1 static. E-beam evaporation gold 1.6 nm (0.45 A / s). Jig1 static. Aeration evaporator. Windows placed in Jig 2. E-beam evaporation ring electrode. 20 nm gold (0.36 A / s) + 188 nm chromium (1.56 A / s). Jig 2 planetary. Aeration evaporator. Windows placed in Jig 3. E-beam evaporation Al2O3 5 nm (0,55A / s). Jig 3 static. Aeration evaporator. Windows placed in Jig 3. Optional: Bake in an oven at 290 °C for 90 minutes (2 hours including a ramp up from 25 °C) for the titanium seed layer. After cool down in Jig 4 planetary. E-beam evaporation 20 nm (0.8 A / s) chromium + silver. Jig 4 planetary.

[0099] A Jig is a type of evaporation fixture having specific size dimensions.Measurement square resistance

[0100] The square resistance was measured in situ during deposition with microscope slides (76 mm * 25 mm). These microscope slides were equipped with two chromium terminals using a masking cover of 25 mm width positioned in the middle of the slides. Four slides were evaporated simultaneously.

[0101] A slide is a microscopic slide having two evaporated metal end parts, which is very useful for resistance measurements of very thin evaporated layers like gold.Measurement in situ slide

[0102] The resistance was measured during the evaporation of titanium / nickel and gold after one-night pump down and resulting pressure of 1*10 -6< mbar. The resistance was around 5 kOhm after 0,5 nm titanium or nickel and dropped further to around 200 Ohm after 1,6 nm gold was deposited.Measurement window resistance

[0103] A practical approach is to measure the film resistance directly on the product substrates itself after the process. An adjusted shadow mask was used to evaporate a ring metal electrode 18 mm and metal circle electrode 6 mm in the centre of the windows. This design can be used to witness samples in each evaporation run. The resistance has to be recalculated to a square resistance in this case: Mathematical derivation square resistance of 6-18 mm shadowed metallized windows: Rsquare = ρ / d Rwindow = ρ / d * ∫ dr / 2 π r = ρ / 2 π * ∫ dr / r Rwindow = ρ / 2 π d * Ln 18 − Ln 6 = ρ / 2 π d * Ln 3 Rwindow = Ln 3 / 2 π * Rsquare ∼ Rsquare / 5.7 ρ: Resistivity, d: Thickness of the layer. Results

[0104] Measurements of the resistance I and the gating were performed on all relevant windows that were prepared. Two types of ring electrodes were used to make the contact with the conductive film. Chromium (Cr) and chromium with a gold sublayer (Au / Cr).

[0105] With "windows" as used in the present disclosure is meant a substrate having a region or layer that allows incoming light to reach the photocathode's active area. The windows used were quartz windows and diffractive windows. With "relevant" is meant that these windows are used or sold frequently.

[0106] Table 1 shows the resistance and gating data of the tested samples. Table 1. Resistance and gating data# Substrate window Ring electrode Rsquare (Ohm) Gating time (ns) 1 QuartzGrid 1Cr2000.32 QuartzGrid 2Cr2580.33 DiffractiveNi / AuCr4113.04 QuartzNi / AuAu / Cr2050.45 QuartzTi / AuCr62716 QuartzTi / AuAu / Cr1840.47 DiffractiveTi / AuCr74118 DiffractiveTi / AuAu / Cr1220.49 Microscope SlideTi / AuCr260n / a210 Microscope SlideTi / AuAu / Cr130n / a211 Microscope SlideITOCr6270.6 1< empty cell = not measured for various reasons, e.g., tube having a gas leak. 2< n / a = not measured, because a slide is unsuitable for a circular product making it impossible for measuring the gating.

[0107] The resistance is measured on a circular substrate having a 6 mm metal dot in the middle and a 18 mm thick outer edge. This is used to calculate the Rsquare, also known as the square resistance for circular windows. The microscope slides with metallised terminals allow direct Rsquare measurement without recalculation.

[0108] It becomes clear from the results of sample 6 in Table 1 that the combination of a Ti seed layer with a Au / Cr ring electrode results in a gating speed of below 1 ns in combination with a low square resistance (Rsquare).

[0109] From Table 1 a remarkable difference is seen between the two types: the transition from a 90 nm chromium electrode to a 90 nm chromium electrode with a 20 nm gold layer (compare sample 5 with sample 6, and sample 7 with sample 8), the Rsquare is strongly decreased. Direct contact of chromium onto the layer of gold seems to be an inferior solution compared to contact via the further layer of gold onto the layer of gold and demonstrated an extra contact resistance on the edge. The resistance is a factor of five higher. The Rsquare values are still in line with the corresponding data on the grids and even better for the samples comprising the thin gold layer (which samples are according to the present disclosure). Furthermore, the last column of Table 1 shows that the sub-nanosecond gating is sufficient for the sample having an ITO substrate and a square resistance of around 600 Ohm. This seems to indicate that with an ITO substrate a higher Rsquare may be acceptable as upper limit because sub-nanosecond behaviour is still observed. However, this is not relevant for the present disclosure.

[0110] An interesting result was also seen on a test run with Na2KSb photocathode processed windows with nickel seed layer underneath gold and chromium electrode which showed reasonable resistance but a lower gating duration compared to other embodiments of the invention. The S25 processed window is a multi-alkali antimonide Na2KSb photocathode having a thickness of more than 100 nm. The extra contact resistance and / or increase of resistance during the process were measured and below the average resistance value measured on the Na2KSb photocathode itself (around 1 kOhm). Therefore, the relatively small positive effect on gating speed is still visible and consistent with the lower resistance values, see Table 2. The application of the gold-chromium (AuCr) electrode (see Table 1) shows that nanosecond gating now becomes achievable. Table 2. Results on Ni / Au-Al 2 O 3 windows with Cr contact electrode.# 14 15 16 17 Photoresponse White light (µ / lm) 878818612665Gating (ns) 5.85.131.8Visual okspotsokspotsRwindow before process (Ohm) 72Rwindow after opening + evaporated contact dot centre (Ohm) 700170Rwindow S25 (in situ qeneral) (Ohm) -1000

[0111] From the results of Table 2 it becomes clear that the conventional solution to use a ring electrode made of chromium cannot be used in combination with an ultrathin layer of gold in de conductive underlayer as there is a risk of possible interface reactions. Samples 14 and 15 are devices having a chromium ring electrode with no ultrathin gold layer, whereas samples 16 and 17 are devices having a chromium ring electrode and an ultrathin gold layer. The gating for samples 16 and 17 is lower than for samples 14 and 15, but not below 1 ns.

[0112] Table 2 further shows that the photoresponse for white light (2850K), directly related to quantum efficiency (QE) is around 600-700 for samples 16 and 17 (comprising an ultrathin gold layer), whereas it is around 800-900 for samples 14 and 15 (not comprising an ultrathin gold layer). Hence, the ratio between samples 16 / 17 and samples 14 / 15 is about 0.75 that corresponds to a relative transmitted intensity of the device in the active photo-emission region of 75%.

Examples

examples

[0096]Several different combinations of material layers and some additional variations were tested to achieve a suitable replacement of the current gating technology with a chromium grid. Current solutions for fast gating are listed below:

100nm Chromium Grid + 40 nm Al 2 O 3 . Standard on quartz. 17 nm indium tin oxide (ITO) + 40 nm (NB: 80 nm was also used and also worked well) Atomic Layer Deposition (ALD) Al 2 O 3 . Standard on diffractive substrate.

[0097]The proposed solution based on a thin gold film has more potential to improve image quality issues. Chemical attack is very unlikely due to the small layer thickness and protective aluminium oxide layer.

Process Au + Ni / Ti thin film

[0098] Cleaning (spinning isopropyl alcohol (IPA) 30 s, 3000 rpm) + ozon treatment (10 min). Pump down overnight. background pressure = 1,6*10 -6< mbar. E-beam evaporation titanium or nickel 0,5 nm (0.25 A / s). Jig1 static. E-beam evaporation gold 1.6 nm (0.45 A / s). Jig1 static. Aeration evaporator....

TECHNICAL FIELD

[0001] The present invention generally relates to the field of optoelectronic devices, and more particularly to the field of photocathode devices for sub-nanosecond gating.BACKGROUND

[0002] The current state of the art in the field of sub-nanosecond gateable image intensifier tubes involves the use of photocathodes comprising multi-alkali materials such as sodium potassium antimonide (Na 2 KSb). These photocathodes are deposited on substrates to achieve high quantum efficiency in the deep ultraviolet and visible spectrum between 200 nm and 900 nm range. These devices typically employ conductive grid layers made from materials like chromium, which are deposited using photolithography techniques. The relatively transparent grid layers are crucial for providing the necessary electrical conductance to the otherwise highly resistive photocathode materials, which can exhibit square resistance values in the kilo Ohm (kOhm) to Giga Ohm (GOhm) range.

[0003] However, the use of conductive grids for multi-alkali photocathodes presents several significant disadvantages. The fabrication of these grids using photolithography is both complex and costly, significantly increasing the production costs of these devices. Additionally, the grid structures can block a portion of the incoming light, reducing the overall quantum efficiency of the photocathode.

[0004] Document Y. Yang et.al, Nuclear Inst. and Methods in physics research A, 1056, 2023, 168621 discloses a device comprising a layer of alkali antimonide directly deposited onto a gold layer. However, such a device exhibits a very low quantum efficiency.

[0005] It is therefore an object of the present disclosure to provide a device and a method that at least partly overcome one or more of the above-mentioned drawbacks of the prior art.SUMMARY

[0006] In a first aspect, the present disclosure relates to a photocathode device for sub-nanosecond gating comprising an active photo-emission region surrounded by an electrode region, the device comprising: a substrate; a photocathode layer deposited on the substrate; a conductive underlayer deposited between the substrate and the photocathode layer in the active photo-emission region and preferably in the electrode region; a layer of aluminium oxide deposited between the conductive underlayer and the photocathode layer.

[0007] "Deposited on" as used in the present disclosure means that a layer may be deposited directly on an underlying layer, but this it may also be possible that another layer is arranged in between said layer and said underlying layer.

[0008] A multi alkali photocathode device may be understood as an optoelectronic device that is designed to detect visible and / or ultraviolet light.

[0009] The photocathode device may be adapted to detect visible and ultraviolet light.

[0010] The device comprises a substrate. A substrate may be understood as the underlying material or layer upon which other materials are deposited or constructed. In this context, the substrate provides the foundational support for the subsequent layers of the device.

[0011] The device comprises a conductive underlayer disposed between the substrate and the photocathode layer. With "conductive" underlayer as used in the present disclosure is meant that the underlayer is electrically conductive.

[0012] The conductive underlayer provides electrical conductance for the photocathode layer to enable a gating duration below 1 ns, preferably below 0.2 ns. It may thus be provided that the conductive underlayer supports gating duration below 1 nanoseconds (ns), preferably below 0.2 ns. This enables the device to operate at high speeds, which is critical for applications requiring rapid response times and high temporal resolution.

[0013] The aluminium oxide layer forms a beneficial interface in terms of growth and quantum efficiency between the conductive underlayer and photocathode layer. A good growth and high quantum efficiency of the photocathode is obtained. The presence of the aluminium oxide layer is important as it avoids oxidation of the conductive underlayer beneath the aluminium oxide layer.

[0014] Another advantage of this combination of the conductive underlayer and the aluminium oxide layer is that it reduces the overall production costs and complexity compared to traditional grid-based conductive layers, while still delivering high performance.

[0015] The conductive underlayer preferably comprises a seed layer deposited directly on the substrate and a layer of gold deposited directly on the seed layer, wherein the layer of gold is arranged between the seed layer and the layer of aluminium oxide.

[0016] A seed layer may be understood as a thin film of a metal that is applied onto the substrate that provides a sufficient adhesive underlayer for the layer of gold. The use of a thin layer of gold provides a cost-effective and compatible material for the conductive underlayer, which aids in enhancing the overall conductance of the device.

[0017] A layer of gold may be understood as a thin film of gold metal that is applied onto the seed layer. The thickness of this gold layer is configured to be transparent and electrically conductive.

[0018] The use of a thin metal (Au) film structure provides sufficient electrical conductance to prevent voltage drops, or in other words to maintain the same electric potential over the entire surface of the photocathode, and dark zones in the image, thereby enhancing the device's operational reliability and performance. This approach also facilitates a more straightforward manufacturing process, leading to improved production yields and reduced costs.

[0019] Electrical conductivity (σ) is a measure of a material's ability to conduct electric current. It is the reciprocal of electrical resistivity (ρ) and is expressed in siemens per meter (S / m). Materials with high electrical conductivity allow electric charges to flow easily, while materials with low conductivity resist the flow of current. The relationship between conductivity and resistivity is: σ = 1 / ρ where: σ = electrical conductivity (S / m), and ρ = resistivity (Ω·m).

[0020] The relationship between the square resistance R □ (Ω), electrical resistivity ρ, electrical conductivity σ and thickness d (m) of a conductive thin layer is defined by the following formula: R □ = ρ / d = 1 / σ * d

[0021] The conductance G (in S) of an object in a material is calculated from the conductivity σ and the geometry of the material as defined by the following formula: G = σ × A / L wherein A (in m 2< ) is the cross-sectional area of the object and L (in m) is the length of the object.

[0022] A conductivity of 1 × 10 -8< S / m (at 20 °C) can generally be considered a minimal value to label a material as "conductive," depending on context. In the present invention, "conductive" is defined as having a conductivity over 10 5< S / m, preferably over 10 6< S / m. The high conductivity can be associated with a layer of at least 1 nm.

[0023] The conductive underlayer may be arranged to provide electrical conductance for the photocathode while maintaining quantum efficiency of the photocathode device over 15%, preferably over 20%, in a wavelength range of 200 nm to 900 nm.

[0024] In an embodiment, the photocathode device comprises a ring electrode in the electrode region to provide electrical conductance for the photocathode device. Preferably, the ring electrode comprises a further layer of gold at a side of the ring electrode facing towards the substrate and optionally a further layer of chromium and / or silver and / or nickel at another side, i.e., the opposite side, of the ring electrode facing towards the photocathode layer.

[0025] Preferably, the ring electrode has a circumference that is larger than a circumference of the photocathode layer defining a flange arranged for sealing the photocathode device in a photocathode vacuum envelope.

[0026] The photocathode device may have in the active photo-emission region a relative quantum efficiency loss of at most 25%, more preferably at most 20%, in the wavelength range of 200 nm to 900 nm, as compared to a photocathode device not comprising a conductive underlayer.

[0027] The photocathode device may have in the active photo-emission region a quantum efficiency over 20%, preferably over 25%.

[0028] It is preferred that the photocathode device, more specifically the conductive underlayer, has a square resistance of at most 200 Ohm, more preferably at most 100 Ohm. It may be provided that the conductive underlayer has this specific range of square resistance, ensuring adequate conductance without excessive current leakage. This feature provides optimal electrical performance, preventing voltage drops and ensuring consistent image quality.

[0029] In an embodiment, the photocathode layer comprises a multi-alkali material selected from alkali antimonide, alkali telluride, and combinations thereof and / or wherein the multi-alkali of the multi-alkali material is selected from the group consisting of sodium, potassium, rubidium, cesium, and combinations thereof. Preferably, the multi-alkali material is sodium potassium cesium antimonide according to Na x K y Cs z Sb, wherein x is 2, y is 1, and z is at most 0.2. These materials are known for their high quantum efficiency in the visible and ultraviolet spectrum, enhancing the device's sensitivity and performance in detecting visible and ultraviolet light.

[0030] In another embodiment, the layer of gold has a thickness of 0.5 nm to 3 nm, preferably 1 nm to 2 nm. Preferably, total thickness of the seed layer and of the layer of gold is at most 3 nm, preferably at most 1 nm. This has the effect that it provides excellent electrical conductance for the photocathode while maintaining the structural integrity and high quantum efficiency in the 200 nm to 900 nm wavelength spectrum. It may be provided that the layer of gold is deposited with a controlled thickness to balance conductance and transmitted intensity of the layer. This feature ensures adequate electrical conductance while maintaining structural integrity and performance of the photocathode device.

[0031] In yet another embodiment, the layer of aluminium oxide has a thickness of 5 nm to 200 nm, preferably 10 nm to 60 nm. A layer of aluminium oxide of a thickness in the defined range avoids a chemical interface reaction between the layer of gold and the multi alkali photocathode. Also, the defined thickness range results in optimization of both conductance and performance of the photocathode, making this approach particularly suitable for high-precision applications.

[0032] The assembly formed by the substrate, by the conductive underlayer and by the aluminium oxide layer, in the active photo-emission region, may have a transmittance of at least 70%, preferably at least 75%, in a wavelength range of 200 nm to 900 nm. This allows a maximum number of photons to reach the photocathode layer, improving the device's sensitivity and efficiency in detecting visible and ultraviolet radiation.

[0033] The active photo-emission region surrounded by the electrode region may have a surface area of at least 0.25 cm 2< , preferably at least 0.5 cm 2< .

[0034] In the active photo-emission region, the substrate and the aluminium oxide layer may be devoid of contact. Indeed, the conductive layer allows optical transmission that avoids the use of a conductive grid under the photocathode layer. The conductive layer can be uniformly deposited under an entire surface where the photocathode layer is deposited.

[0035] The device may have no conductive grid arranged between arranged between the substrate and the photocathode layer.

[0036] In an embodiment, the ring electrode has a thickness of 20 nm to 800 nm, preferably 80 nm to 400 nm.

[0037] In another embodiment, the further layer of gold and the optional further layer of chromium and / or nickel of the ring electrode each have a thickness of 10 nm to 400 nm, preferably 30 nm to 200 nm.

[0038] In an embodiment, the substrate is a transparent substrate and / or a diffractive substrate in a wavelength range of 200 nm to 900 nm.

[0039] In another embodiment, the substrate is made of a material selected from the group consisting of glass, quartz, sapphire, fused silica, magnesium fluoride, calcium fluoride and lithium fluoride, which are known for their excellent transmittance and stability in the ultraviolet spectrum. This feature ensures minimal light absorption and high structural integrity, enhancing the overall performance of the device.

[0040] Preferably, the substrate has a thickness of 1 mm to 100 mm, preferably 2 mm to 10 mm.

[0041] In an embodiment, the seed layer comprises particles selected from the group consisting of nickel, titanium, chromium, copper, silver, molybdenum, hafnium, aluminium, silicon, zirconium, niobium, tantalum, and combinations of two or more thereof, preferably the seed layer comprises nickel particles.

[0042] It is preferred that the seed layer has a thickness of at most 2 nm, preferably at most 1 nm. Preferably, the total thickness of the seed layer and of the layer of gold is at most 3 nm, preferably at most 1 nm. This provides an optimal balance between electrical conductance, structural integrity and transmittance of the device.

[0043] In a second aspect, the present disclosure relates to a method of manufacturing a photocathode device for sub-nanosecond gating comprising an active photo-emission region surrounded by an electrode region, the method comprising the steps of: 1) providing a substrate, preferably a transparent substrate and / or a diffractive substrate and / or a fibre optic substrate; 2) depositing a conductive underlayer on the substrate in the active photo-emission region; 3) depositing or forming a layer of aluminium oxide on the conductive underlayer; 4) depositing a photocathode layer on the layer of aluminium oxide.

[0044] The step of deposition a conductive underlayer preferably comprises the steps of: a) depositing a seed layer on the substrate; b) depositing a layer of gold on the seed layer.

[0045] The step of depositing or forming the layer of aluminium oxide may comprise the sub-step(s) of: i) depositing aluminium oxide on the conductive underlayer in the active photo-emission region, and / or ii) depositing aluminium on the conductive underlayer in the active photo-emission region and subsequently exposing the deposited aluminium to an oxidizing gas, such as air, thereby forming the layer of aluminium oxide on the conductive underlayer.

[0046] The conductive underlayer may be arranged to provide electrical conductance for the photocathode while maintaining quantum efficiency of the photocathode device over 20%, preferably over 25%, in a wavelength range of 200 nm to 900 nm.

[0047] The conductive layer is at least deposited in the active photo-emission region and may also be deposited in the electrode region. This is beneficial as it provides a good electrical contact between the ring electrode and the thin gold film (i.e., the layer of gold).

[0048] Prior to step 1), the substrate is preferably polished such that a polished substrate is provided in step 1). Preferably, the substrate to be provided is also cleaned and / or treated with ozone in an air atmosphere and / or provided with a glow discharge treatment in an evaporator.

[0049] Furthermore, the depositing of the layers may be done by evaporation, preferably at a high temperature (200 °C to 300 °C). This is beneficial to obtain the desired properties for conductance, transmittance, transmitted intensity, and diffusion, which minimalizes the formation of alkali oxides.

[0050] The term "transmittance" of an optical object as used in the present disclosure means the ratio between the intensity of a light beam transmitted by the object and the intensity of the light beam incident on the object.

[0051] The method may further comprise a step of depositing a ring electrode in the electrode region in between steps 2) and 3) to provide electrical conductance for the photocathode layer. Preferably, the step of depositing the ring electrode in the electrode region comprises depositing a further layer of gold on the layer of gold, and optionally depositing a further layer of chromium and / or silver and / or nickel on the further layer of gold. This has the effects that the adhesion of the ring electrode to the layer of gold will be guaranteed and because the same metal is used, there will be no interface reactions between the layer of gold and the electrode ring.

[0052] The depositing of the ring electrode is preferably done in a manner such that the ring electrode has a circumference that is larger than a circumference of the photocathode layer defining a flange arranged for sealing the photocathode device in a photocathode vacuum envelope.

[0053] In an embodiment, the step of depositing the layer of gold comprises depositing a layer of gold with a thickness of 0.5 to 3 nm, preferably 1 to 2 nm.

[0054] In another embodiment, the step of depositing the further layer of gold of the ring electrode comprises depositing a layer of gold with a thickness of 10 nm to 400 nm, preferably 30 nm to 200 nm, such as 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 120 nm, 140 nm, 160 nm, or 180 nm, and / or the step of depositing the further layer of chromium and / or nickel of the ring electrode comprises depositing a layer with a thickness of 10 nm to 400 nm, preferably 30 nm to 200 nm, such as 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 120 nm, 140 nm, 160 nm, or 180 nm.

[0055] The seed layer that is deposited on the substrate preferably comprises particles selected from the group consisting of nickel, titanium, chromium, copper, silver, molybdenum, hafnium, aluminium, silicon, zirconium, niobium, tantalum, scandium, yttrium, and combinations of two or more thereof, more preferably the seed layer comprises particles that provide after oxidation an ultraviolet-transparent oxide, even more preferably the seed layer comprises nickel particles.

[0056] A heat treatment in air (or oxygen) at for example 290 °C after depositing of the seed layer, the layer of gold, and the layer of aluminium oxide may provide extra transmittance. This effect is especially present in case the seed layer comprises hafnium, aluminium, scandium, yttrium, titanium, silicon, or combinations of two or more thereof.

[0057] In an embodiment, the step of depositing the seed layer comprises depositing a layer with a thickness of at most 2 nm, preferably at most 1 nm.

[0058] The depositing of each of the seed layer, the layer of gold, and the ring electrode may be done by an e-beam evaporation process.

[0059] The depositing of the layer aluminium oxide or aluminium may be done by an e-beam evaporation process, a sputtering process, or an atomic layer deposition process.

[0060] The depositing of the photocathode layer may be done by chemical vapour deposition (CVD). The CVD process allows precise control over the thickness and uniformity of the layer, which ensures high-quality deposition, enhancing the performance and reliability of the photocathode device.

[0061] The method according to the second aspect of the present disclosure may be used in the manufacturing of a photocathode device according to the first aspect of the present disclosure.

[0062] Embodiments of the photocathode device according to the first aspect of the present disclosure as presented herein are also applicable to the method according to the second aspect of the present disclosure, and vice versa.

[0063] Effects of the photocathode device according to the first aspect of the present disclosure as presented herein correspond to or are similar to effects of the method according to the second aspect of the present disclosure.

[0064] In a third aspect, the present disclosure relates to a visible and ultraviolet radiation detection device, comprising: a photocathode device for sub-nanosecond gating comprising an active photo-emission region surrounded by an electrode region, the photocathode device comprising: a substrate; a photocathode layer deposited on the substrate; a conductive underlayer deposited between the substrate and the photocathode layer in the active photo-emission region and preferably in the electrode region; a layer of aluminium oxide deposited between the conductive underlayer and the photocathode layer, wherein the visible and ultraviolet radiation detection device is configured for use in applications requiring selective sensitivity to visible radiation and / or ultraviolet radiation and insensitivity to infrared radiation.

[0065] Optionally, the visible and ultraviolet radiation detection device further comprises a ring electrode in the electrode region being in electrical contact with the conductive underlayer and configured to provide electrical conductance for the photocathode device.

[0066] The visible and ultraviolet radiation detection device may be implemented in one or more of the following applications: UV and solar-blind image intensifiers; visible light image intensifiers; photon counting detectors and modules; night vision and low-light imaging devices; ultraviolet spectroscopy instruments; radiation monitoring equipment; scientific and medical imaging cameras; aerospace and satellite-based UV detectors; UV detection in fire and explosion sensors; fast-gated photocathode detectors for laser-induced fluorescence; range gated imaging devices; and UV astronomy instruments.

[0067] The visible and ultraviolet radiation detection device according to the third aspect of the present disclosure may comprise a photocathode device according to the first aspect of the present disclosure or a photocathode device manufactured according to the method according to the second aspect of the present disclosure.

[0068] Embodiments of the photocathode device according to the first aspect of the present disclosure as presented herein are also applicable to the ultraviolet detection device according to the third aspect of the present disclosure, and vice versa.

[0069] Effects of the photocathode device according to the first aspect of the present disclosure as presented herein correspond to or are similar to effects of the ultraviolet detection device according to the third aspect of the present disclosure.

[0070] The present disclosure is hereinafter explained in more detail with reference to the accompanying drawings in which embodiments of the present disclosure are shown and in which like reference numbers indicate the same or similar elements. The present disclosure is by no means limited to the embodiments described therein.BRIEF DESCRIPTION OF THE DRAWINGS

[0071] The present disclosure will be explained in more detail below by means of examples of a device and method according to the present disclosure shown in the drawings, in which: Fig. 1 shows a photocathode device for sub-nanosecond gating according to a first aspect of the present disclosure; Fig. 2 shows a method step of manufacturing a photocathode device for sub-nanosecond gating according to a second aspect of the present disclosure. DETAILED DESCRIPTION

[0072] The present disclosure will now be described in detail with reference to the accompanying drawings, which illustrate exemplary embodiments of the photocathode device 1 for sub-nanosecond gating and the method 101 of manufacturing the same. It is to be understood that these embodiments are provided by way of example only and are not intended to limit the scope of the present disclosure. The following detailed description should be read in conjunction with the claims and the figures.

[0073] As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. Expressions such as "at least one of," when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list.

[0074] As the inventive concept allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in more detail in the written description. Effects, features, and a method of achieving the inventive concept will be obvious by referring to exemplary embodiments of the inventive concept with reference to the accompanying drawings. The inventive concept may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein.

[0075] In the embodiments described in the present specification, an expression utilized in the singular encompasses the expression of the plural, unless it has a clearly different meaning in the context. Also, it is to be understood that the terms such as "including," "having," and / or "comprising" are intended to indicate the presence of the stated features or components, and are not intended to preclude the presence or addition of one or more other features or components.

[0076] It will be understood that when a layer, region, or component is referred to as being "on" or "onto" another layer, region, or component, it may be directly or indirectly formed on the other layer, region, or component. That is, for example, intervening layer(s), region(s), or component(s) may be present.

[0077] Sizes of components in the drawings may be exaggerated for convenience of explanation. In other words, since sizes and thicknesses of components in the drawings are arbitrarily illustrated for convenience of explanation, the following embodiments of the present disclosure are not limited thereto.

[0078] Fig. 1 shows schematically an example of a photocathode device 1 for sub-nanosecond gating according to the first aspect of the present disclosure. The photocathode device 1 comprises two regions: an active photo-emission region 3 that is surrounded by an electrode region 5. The device 1 further comprises a substrate 7, a conductive underlayer 11, a layer of aluminium oxide 17, and a photocathode layer 9. As can be seen, the layer of aluminium oxide 17 is deposited between the conductive underlayer 11 and the photocathode layer 9. As can further be seen, the conductive underlayer 11 is deposited between the substrate 7 and the photocathode layer 9 in the active photo-emission region 3 and in the electrode region 5. More specifically, the conductive underlayer 11 is deposited between the substrate 7 and the aluminium oxide layer 17 in the active photo-emission region 3 and in the electrode region 5. The conductive underlayer 11 is arranged to provide electrical conductance for the photocathode while maintaining quantum efficiency of the photocathode device over 20%, preferably over 25%, in a wavelength range of 200 nm to 900 nm and comprises a seed layer 13, deposited on the substrate 7, and a layer of gold 15, deposited on the seed layer 13 and arranged between the seed layer 13 and the layer of aluminium oxide 17.

[0079] The device 1 shown in Fig. 1 further comprises a ring electrode 19, which provides conductance for the photocathode device 1. The ring electrode 19 comprises a further layer of gold 21 at a side of the ring electrode 19 that faces towards the substrate 7 and a further layer of chromium 23 (which may also be silver, nickel, or a combination of two or more of chromium, silver, and nickel) at another side, i.e., the opposite side, of the ring electrode 19 facing towards the photocathode layer 9.

[0080] The example of the device 1 shown in Fig. 1 has in the active photo-emission region 3 a relative quantum efficiency loss of at most 25%, preferably at most 20%, in a wavelength range of 200 to 900 nm as compared to a photocathode device not having a conductive underlayer, and a square resistance of at most 200 Ohm, preferably at most 100 Ohm.

[0081] Furthermore, the example shown in Fig. 1 comprises a photocathode layer that comprises a multi-alkali material selected from alkali antimonide, alkali telluride, and combinations thereof. The multi-alkali of the multi-alkali material is selected from the group consisting of sodium, potassium, rubidium, cesium, and combinations thereof. As an example, the photocathode layer may comprise or consist of sodium potassium cesium antimonide according to Na x K y Cs z Sb, wherein x is 2, y is 1, and z is at most 0.2.

[0082] In the photocathode device 1 shown in Fig. 1, the substrate 7 has a thickness of 1 to 100 mm, preferably 2 to 10 mm. The seed layer 13 has a thickness of at most 2 nm, preferably at most 1 nm. The layer of gold 15 has a thickness of 0.5 to 3 nm, preferably 1 to 2 nm. The layer of aluminium oxide 17 has a thickness of 5 to 100 nm, preferably 10 to 20 nm. The ring electrode 19 has a thickness of 20 to 800 nm, preferably 80 to 400 nm, of which the further layer of gold 21 and the further layer of chromium 23 each have a thickness of 10 to 400 nm, preferably 40 to 200 nm.

[0083] As illustrated in Fig. 1, the assembly formed by the substrate, by the conductive underlayer and by the aluminium oxide layer 1, has, in the active photo-emission region 3, a transmittance of at least 70%, preferably at least 75%, in a wavelength range of 200 to 900 nm. The conductive underlayer 11 provides electrical conductance for the photocathode layer 9 to enable a gating duration below 1 ns, preferably below 0.2 ns. Furthermore, the active photo-emission region 3 surrounded by the electrode region 5 has a surface area of at least 0.25 cm 2< , preferably at least 0.5 cm 2< .

[0084] The substrate 7 is a transparent substrate and diffractive substrate in a wavelength range of 200 to 900 nm. The substrate 7 can be a fibre optic substrate. However, in other examples, the substrate 7 may be a transparent substrate and / or a diffractive substrate and / or a fibre optic substrate in a wavelength range of 200 to 900 nm. The substrate is made of a material selected from the group consisting of glass, quartz, sapphire, fused silica, magnesiumfluoride, calciumfluoride and lithiumfluoride.

[0085] The seed layer 13 of the device 1 as exemplified in Fig. 1 comprises particles selected from the group consisting of nickel, titanium, chromium, copper, silver, molybdenum, hafnium, aluminium, silicon, zirconium, niobium, tantalum, and combinations of two or more thereof, preferably the seed layer 13 comprises nickel particles.

[0086] Fig. 2 shows an example of a method 101 according to the second aspect of the present disclosure of manufacturing of a photocathode device 1 for sub-nanosecond gating comprising an active photo-emission region 3 surrounded by an electrode region 5, such as the device 1 according to the first aspect of the present disclosure. The method 101 comprises the steps of 1) providing 103 a substrate 7, preferably a transparent substrate and / or a diffractive substrate and / or a fibre optic substrate; 2) depositing 105 a conductive underlayer 11 on the substrate 7 in the active photo-emission region 3; 3) depositing or forming 107 a layer of aluminium oxide 17 on the conductive underlayer 11; and 4) depositing 109 a photocathode layer 9 on the layer of aluminium oxide 17.

[0087] The step of depositing 105 the conductive underlayer 11 comprises the steps of a) depositing 105-a a seed layer 13 on the substrate 7 and b) depositing 105-b a layer of gold 15 on the seed layer 13.

[0088] The step of depositing or forming the layer of aluminium oxide 17 comprises the sub-step(s) of i) depositing 107-1 aluminium oxide on the conductive underlayer 11 or ii) depositing 107-iia aluminium on the conductive underlayer 11 and subsequently exposing 107-iib the deposited aluminium to an oxidizing gas, such as air, thereby forming the layer of aluminium oxide 17 on the conductive underlayer 11. It is also possible to first perform step i) and then step ii) or vice versa.

[0089] The method 101 further comprises the step of depositing 111 a ring electrode 19 in the electrode region 5 of the photocathode device 1 in between step 2) of depositing 105 the conductive underlayer 11 and step 3) of depositing or forming 107 the layer of aluminium oxide 17.

[0090] The step of depositing 111 the ring electrode 19 comprises depositing a further layer of gold 21 on the layer of gold 15, and depositing a further layer of chromium 23 (which may also be silver, nickel or a combination of two or more of chromium, silver, and nickel) on the further layer of gold 21. The depositing of the further layer of gold 21 during the step of depositing 111 the ring electrode 19 comprises depositing a layer with a thickness of 10 to 400 nm, preferably 40 to 200 nm and the depositing of the further layer of chromium 23 during the step of depositing 111 the ring electrode 19 comprises depositing a layer with a thickness of 10 to 400 nm, preferably 40 to 200 nm.

[0091] In Fig. 2, the step of depositing 105-a the seed layer 13 comprises depositing a layer with a thickness of at most 2 nm, preferably at most 1 nm. The step of depositing 105-b the layer of gold 15 comprises depositing a layer with a thickness of 0.5 to 3 nm, preferably 1 to 2 nm.

[0092] In said step of depositing 105-a the seed layer 13, the seed layer 13 comprises particles selected from the group consisting of nickel, titanium, chromium, copper, silver, molybdenum, hafnium, aluminium, silicon, zirconium, niobium, tantalum, and combinations of two or more thereof, preferably the seed layer 13 comprises nickel particles.

[0093] Furthermore, the depositing 105-a,105-b,111, of each of the layer of gold 15, the ring electrode 19, and the seed layer 13 are done by an e-beam evaporation process. The depositing 107-i, 107-iia of each of the layer of aluminium oxide and the layer of aluminium is done by an e-beam evaporation process, a sputtering process, or an atomic layer deposition process. The depositing 109 of the photocathode layer 9 is done by chemical vapour deposition.

[0094] The foregoing description of the embodiments of the disclosure has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the claimed disclosure to the precise form disclosed. Those skilled in the art will readily appreciate that many modifications and variations to the claimed disclosure are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present disclosure and its practical application to thereby enable others skilled in the art to best utilize the present disclosure in various photocathode embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the present disclosure be defined exclusively by the following claims, and equivalents thereof.

[0095] A single unit or component may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope thereof.EXAMPLES

[0096] Several different combinations of material layers and some additional variations were tested to achieve a suitable replacement of the current gating technology with a chromium grid. Current solutions for fast gating are listed below: 100nm Chromium Grid + 40 nm Al 2 O 3 . Standard on quartz. 17 nm indium tin oxide (ITO) + 40 nm (NB: 80 nm was also used and also worked well) Atomic Layer Deposition (ALD) Al 2 O 3 . Standard on diffractive substrate.

[0097] The proposed solution based on a thin gold film has more potential to improve image quality issues. Chemical attack is very unlikely due to the small layer thickness and protective aluminium oxide layer.Process Au + Ni / Ti thin film

[0098] Cleaning (spinning isopropyl alcohol (IPA) 30 s, 3000 rpm) + ozon treatment (10 min). Pump down overnight. background pressure = 1,6*10 -6< mbar. E-beam evaporation titanium or nickel 0,5 nm (0.25 A / s). Jig1 static. E-beam evaporation gold 1.6 nm (0.45 A / s). Jig1 static. Aeration evaporator. Windows placed in Jig 2. E-beam evaporation ring electrode. 20 nm gold (0.36 A / s) + 188 nm chromium (1.56 A / s). Jig 2 planetary. Aeration evaporator. Windows placed in Jig 3. E-beam evaporation Al2O3 5 nm (0,55A / s). Jig 3 static. Aeration evaporator. Windows placed in Jig 3. Optional: Bake in an oven at 290 °C for 90 minutes (2 hours including a ramp up from 25 °C) for the titanium seed layer. After cool down in Jig 4 planetary. E-beam evaporation 20 nm (0.8 A / s) chromium + silver. Jig 4 planetary.

[0099] A Jig is a type of evaporation fixture having specific size dimensions.Measurement square resistance

[0100] The square resistance was measured in situ during deposition with microscope slides (76 mm * 25 mm). These microscope slides were equipped with two chromium terminals using a masking cover of 25 mm width positioned in the middle of the slides. Four slides were evaporated simultaneously.

[0101] A slide is a microscopic slide having two evaporated metal end parts, which is very useful for resistance measurements of very thin evaporated layers like gold.Measurement in situ slide

[0102] The resistance was measured during the evaporation of titanium / nickel and gold after one-night pump down and resulting pressure of 1*10 -6< mbar. The resistance was around 5 kOhm after 0,5 nm titanium or nickel and dropped further to around 200 Ohm after 1,6 nm gold was deposited.Measurement window resistance

[0103] A practical approach is to measure the film resistance directly on the product substrates itself after the process. An adjusted shadow mask was used to evaporate a ring metal electrode 18 mm and metal circle electrode 6 mm in the centre of the windows. This design can be used to witness samples in each evaporation run. The resistance has to be recalculated to a square resistance in this case: Mathematical derivation square resistance of 6-18 mm shadowed metallized windows: Rsquare = ρ / d Rwindow = ρ / d * ∫ dr / 2 π r = ρ / 2 π * ∫ dr / r Rwindow = ρ / 2 π d * Ln 18 − Ln 6 = ρ / 2 π d * Ln 3 Rwindow = Ln 3 / 2 π * Rsquare ∼ Rsquare / 5.7 ρ: Resistivity, d: Thickness of the layer. Results

[0104] Measurements of the resistance I and the gating were performed on all relevant windows that were prepared. Two types of ring electrodes were used to make the contact with the conductive film. Chromium (Cr) and chromium with a gold sublayer (Au / Cr).

[0105] With "windows" as used in the present disclosure is meant a substrate having a region or layer that allows incoming light to reach the photocathode's active area. The windows used were quartz windows and diffractive windows. With "relevant" is meant that these windows are used or sold frequently.

[0106] Table 1 shows the resistance and gating data of the tested samples. Table 1. Resistance and gating data# Substrate window Ring electrode Rsquare (Ohm) Gating time (ns) 1 QuartzGrid 1Cr2000.32 QuartzGrid 2Cr2580.33 DiffractiveNi / AuCr4113.04 QuartzNi / AuAu / Cr2050.45 QuartzTi / AuCr62716 QuartzTi / AuAu / Cr1840.47 DiffractiveTi / AuCr74118 DiffractiveTi / AuAu / Cr1220.49 Microscope SlideTi / AuCr260n / a210 Microscope SlideTi / AuAu / Cr130n / a211 Microscope SlideITOCr6270.6 1< empty cell = not measured for various reasons, e.g., tube having a gas leak. 2< n / a = not measured, because a slide is unsuitable for a circular product making it impossible for measuring the gating.

[0107] The resistance is measured on a circular substrate having a 6 mm metal dot in the middle and a 18 mm thick outer edge. This is used to calculate the Rsquare, also known as the square resistance for circular windows. The microscope slides with metallised terminals allow direct Rsquare measurement without recalculation.

[0108] It becomes clear from the results of sample 6 in Table 1 that the combination of a Ti seed layer with a Au / Cr ring electrode results in a gating speed of below 1 ns in combination with a low square resistance (Rsquare).

[0109] From Table 1 a remarkable difference is seen between the two types: the transition from a 90 nm chromium electrode to a 90 nm chromium electrode with a 20 nm gold layer (compare sample 5 with sample 6, and sample 7 with sample 8), the Rsquare is strongly decreased. Direct contact of chromium onto the layer of gold seems to be an inferior solution compared to contact via the further layer of gold onto the layer of gold and demonstrated an extra contact resistance on the edge. The resistance is a factor of five higher. The Rsquare values are still in line with the corresponding data on the grids and even better for the samples comprising the thin gold layer (which samples are according to the present disclosure). Furthermore, the last column of Table 1 shows that the sub-nanosecond gating is sufficient for the sample having an ITO substrate and a square resistance of around 600 Ohm. This seems to indicate that with an ITO substrate a higher Rsquare may be acceptable as upper limit because sub-nanosecond behaviour is still observed. However, this is not relevant for the present disclosure.

[0110] An interesting result was also seen on a test run with Na2KSb photocathode processed windows with nickel seed layer underneath gold and chromium electrode which showed reasonable resistance but a lower gating duration compared to other embodiments of the invention. The S25 processed window is a multi-alkali antimonide Na2KSb photocathode having a thickness of more than 100 nm. The extra contact resistance and / or increase of resistance during the process were measured and below the average resistance value measured on the Na2KSb photocathode itself (around 1 kOhm). Therefore, the relatively small positive effect on gating speed is still visible and consistent with the lower resistance values, see Table 2. The application of the gold-chromium (AuCr) electrode (see Table 1) shows that nanosecond gating now becomes achievable. Table 2. Results on Ni / Au-Al 2 O 3 windows with Cr contact electrode.# 14 15 16 17 Photoresponse White light (µ / lm) 878818612665Gating (ns) 5.85.131.8Visual okspotsokspotsRwindow before process (Ohm) 72Rwindow after opening + evaporated contact dot centre (Ohm) 700170Rwindow S25 (in situ qeneral) (Ohm) -1000

[0111] From the results of Table 2 it becomes clear that the conventional solution to use a ring electrode made of chromium cannot be used in combination with an ultrathin layer of gold in de conductive underlayer as there is a risk of possible interface reactions. Samples 14 and 15 are devices having a chromium ring electrode with no ultrathin gold layer, whereas samples 16 and 17 are devices having a chromium ring electrode and an ultrathin gold layer. The gating for samples 16 and 17 is lower than for samples 14 and 15, but not below 1 ns.

[0112] Table 2 further shows that the photoresponse for white light (2850K), directly related to quantum efficiency (QE) is around 600-700 for samples 16 and 17 (comprising an ultrathin gold layer), whereas it is around 800-900 for samples 14 and 15 (not comprising an ultrathin gold layer). Hence, the ratio between samples 16 / 17 and samples 14 / 15 is about 0.75 that corresponds to a relative transmitted intensity of the device in the active photo-emission region of 75%.

Examples

examples

[0096]Several different combinations of material layers and some additional variations were tested to achieve a suitable replacement of the current gating technology with a chromium grid. Current solutions for fast gating are listed below:

100nm Chromium Grid + 40 nm Al 2 O 3 . Standard on quartz. 17 nm indium tin oxide (ITO) + 40 nm (NB: 80 nm was also used and also worked well) Atomic Layer Deposition (ALD) Al 2 O 3 . Standard on diffractive substrate.

[0097]The proposed solution based on a thin gold film has more potential to improve image quality issues. Chemical attack is very unlikely due to the small layer thickness and protective aluminium oxide layer.

Process Au + Ni / Ti thin film

[0098] Cleaning (spinning isopropyl alcohol (IPA) 30 s, 3000 rpm) + ozon treatment (10 min). Pump down overnight. background pressure = 1,6*10 -6< mbar. E-beam evaporation titanium or nickel 0,5 nm (0.25 A / s). Jig1 static. E-beam evaporation gold 1.6 nm (0.45 A / s). Jig1 static. Aeration evaporator....

Claims

1. Photocathode device (1) for sub-nanosecond gating comprising an active photo-emission region (3) surrounded by an electrode region (5), the device comprising: - a substrate (7); - a photocathode layer (9) deposited on the substrate; wherein the device also comprises: - a conductive underlayer (11) deposited between the substrate and the photocathode layer in the active photo-emission region and preferably in the electrode region; - a layer of aluminium oxide (17) deposited between the conductive underlayer and the photocathode layer.

2. Photocathode device according to claim 1, wherein the conductive underlayer comprises: - a seed layer (13), deposited on the substrate; - a layer of gold (15), deposited on the seed layer, the layer of gold being arranged between the seed layer and the layer of aluminium oxide.

3. Photocathode device according to claim 1 or 2, wherein the conductive underlayer is arranged to provide electrical conductance for the photocathode while maintaining quantum efficiency of the photocathode device over 15%, preferably over 20%, in a wavelength range of 200 nm to 900 nm.

4. Photocathode device according to any of the preceding claims, wherein the photocathode device comprises a ring electrode (19) in the electrode region to provide electrical conductance for the photocathode device, preferably wherein the ring electrode comprises a further layer of gold (21) at a side of the ring electrode facing towards the substrate and optionally a further layer of chromium and / or silver and / or nickel (23) at another side of the ring electrode facing towards the photocathode layer.

5. Photocathode device according to any of the preceding claims, wherein the photocathode device has a square resistance of at most 200 Ohm, preferably at most 100 Ohm.

6. Photocathode device according to any of the preceding claims, wherein the photocathode layer comprises a multi-alkali material selected from alkali antimonide, alkali telluride, and combinations thereof and / or wherein the multi-alkali of the multi-alkali material is selected from the group consisting of sodium, potassium, rubidium, cesium, and combinations thereof, preferably wherein the multi-alkali material is sodium potassium cesium antimonide according to NaxKyCszSb, wherein x is 2, y is 1, and z is at most 0.2.

7. Photocathode device according to any of the preceding claims, wherein the layer of aluminium oxide has a thickness of 5 nm to 200 nm, preferably 10 nm to 60 nm.

8. Photocathode device according to any of the preceding claims, wherein, in the active photo-emission region, the assembly formed by the substrate, by the conductive underlayer and by the aluminium oxide layer has a transmittance of at least 70%, preferably at least 75%, in a wavelength range of 200 nm to 900 nm.

9. Photocathode device according to any of the preceding claims, wherein the active photo-emission region surrounded by the electrode region has a surface area of at least 0.25 cm2, preferably at least 0.5 cm2.

10. Photocathode device according to any of the preceding claims, wherein the total thickness of the seed layer and of the layer of gold is at most 3 nm, preferably at most 1 nm.

11. Method (101) of manufacturing a photocathode device (1) for sub-nanosecond gating comprising an active photo-emission region (3) surrounded by an electrode region (5), the method comprising the steps of: 1) providing (103) a substrate (7), preferably a transparent substrate and / or a diffractive substrate and / or a fibre optic substrate; 2) depositing (105) a conductive underlayer (11) on the substrate in the active photo-emission region; 3) depositing or forming (107) a layer of aluminium oxide (17) on the conductive underlayer; 4) depositing (109) a photocathode layer (9) on the layer of aluminium oxide.

12. Method according to claim 11, wherein the step of depositing the conductive underlayer comprises the steps of: a) depositing (105-a) a seed layer (13) on the substrate; b) depositing (105-b) a layer of gold (15) on the seed layer.

13. Method according to claim 11 or 12, wherein the step of depositing or forming the layer of aluminium oxide comprises the sub-step(s) of: i) depositing (107-i) aluminium oxide on the conductive underlayer in the active photo-emission region, and / or ii) depositing (107-iia) aluminium on the conductive underlayer in the active photo-emission region and subsequently exposing (107-iib) the deposited aluminium to an oxidizing gas, such as air, thereby forming the layer of aluminium oxide on the conductive underlayer.

14. Method according to any of the claims 11 to 13, wherein the conductive underlayer is arranged to provide electrical conductance for the photocathode while maintaining quantum efficiency of the photocathode device over 20%, preferably over 25%, in a wavelength range of 200 nm to 900 nm.

15. Method according to any of the claims 11 to 14, further comprising a step of depositing (111) a ring electrode (19) in the electrode region in between steps 2) and 3) to provide electrical conductance for the photocathode layer, preferably wherein the step of depositing the ring electrode in the electrode region comprises depositing a further layer of gold (21) on the conductive underlayer, and optionally depositing a further layer of chromium and / or silver and / or nickel (23) on the further layer of gold.

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