Electromagnetic radiation detector with photocathode and improved quantum yield

Description

Technical field

[0001] The present invention relates to the general field of photocathodes such as those used in image intensifier tubes or photomultiplier tubes. State of the prior art

[0002] Electromagnetic radiation detectors, such as image intensifier tubes or photomultiplier tubes, detect electromagnetic radiation in a given spectral band by converting it into a light or electrical output signal.

[0003] These detectors generally comprise a photocathode for receiving electromagnetic radiation and emitting in response a flow of photoelectrons, an electron multiplier device for receiving said flow of photoelectrons and emitting in response a flow of electrons, called secondary electrons, and finally an output device for receiving said flow of electrons and converting it into an output signal.

[0004] There Fig. 1 represents an electromagnetic radiation detector known from the state of the art.

[0005] As illustrated in the figure, such a detector, 100, comprises an input window made of transparent material, 110, generally glass, serving as a support for a photoemissive layer or photocathode, 120, made of a semiconductor material. The input window has a front face, 111, intended to receive the flux of incident photons and a rear face, 112 opposite the front face. The photoemissive layer has an upstream face, 121, in contact with the rear face of the input window and a downstream face 122, from which the photoelectrons are emitted.

[0006] The photocathode is brought to a negative potential relative to that applied to the electron multiplier device 130, the electron multiplier device itself being at a negative potential relative to that applied to the output device 140, for example a phosphor screen or a CCD matrix.

[0007] The photons arriving on the front face 111 pass through the transparent window 110 and enter the photoemissive layer 120, where they generate electron-hole pairs if they have an energy greater than the band gap of the semiconductor material. The photoelectrons migrate to the downstream face, 122, of the photocathode where they are emitted into the vacuum, before being multiplied by the electron multiplier device, 130, and converted into a light or electrical signal by the output device, 140.

[0008] The quantum efficiency of the photocathode is classically defined as the ratio between the number of photoelectrons emitted by the photocathode and the number of photons received. The quantum efficiency of the photocathode is an essential parameter of the detector; it determines both its sensitivity and its signal-to-noise ratio. It depends in particular on the wavelength of the incident photons and the thickness of the photoemissive layer.

[0009] The quantum efficiency can be significantly degraded by the presence of defects at the interface between the photocathode, 120, and the transparent window, 110. More precisely, these defects create surface states trapping the photoelectrons generated in the photocathode. The photoelectrons thus trapped can no longer migrate to the downstream face of the photocathode and therefore do not participate in the photocurrent generated by the photoelectrons emitted by the photocathode.

[0010] This deterioration in the photocathode's quantum efficiency is particularly noticeable at short wavelengths. This is because higher-energy photons interact with the semiconductor earlier along their path within the photocathode. The photoelectrons generated by these photons therefore have a higher probability of being trapped at the interface.

[0011] To combat this deterioration in quantum efficiency, it has been proposed to introduce, at the interface between the input window and the photocathode, an intermediate layer of semiconductor material having a band gap wider than that of the photocathode. Thus, for example, if the photocathode is made of a III-V semiconductor material such as p-type GaAs, an intermediate layer of p-type GaAlAs can be introduced at the interface. The larger band gap of GaAlsAs compared to GaAs creates an upward band bending ( upward band bending ) on the photocathode side as can be seen in the band diagram of the Fig. 2 . Thus, when a photon generates an electron-hole pair near the interface, the photoelectron is extracted out of the recombination zone by the local electric field.

[0012] This solution is however not transposable to all types of photocathode, in particular to those made of a polycrystalline material, for example in a bi- or multi-alkaline compound such as SbK 2 Cs, SbRb 2 Cs, SbRb 2 Cs, SbCs 3 , SbNa 3 , SbNaKRbCs, SbNaKCs, SbNa 2 KCs. Due to their polycrystalline structure, these photocathodes do not have a well-defined band diagram and it is difficult to provide an intermediate layer of a second semiconductor material making it possible to obtain the desired band curvature at the interface with the polycrystalline material.

[0013] More generally, even for photocathodes made of a single-crystal semiconductor material, it is not always easy to find a second suitable semiconductor material that allows both mesh matching and the desired band curvature to be achieved with the semiconductor material constituting the photocathode. This is particularly problematic for photocathodes made of a II-VI semiconductor material, such as CdTe.

[0014] An aim of the present invention is therefore to propose an electromagnetic radiation detector having a photocathode made of a first semiconductor material, having a high quantum efficiency without requiring an intermediate layer made of a second suitable semiconductor material, between the input window and the photocathode.

[0015] Document US2014 / 0239157A1 describes an electromagnetic radiation detector comprising a photosensitive layer (PSL) located between the photocathode (PK) and a conductive layer. The role of the photosensitive layer, positively polarized relative to the photocathode, is to reduce the gain of the detector in the event of strong radiation by becoming electrically conductive, the photoelectrons generated at the PSL-PK interface then being evacuated by the electrode connected to the photosensitive layer. A dielectric layer may be located between the photosensitive layer and the photocathode. Presentation of the invention

[0016] The present invention is defined by an electromagnetic radiation detector comprising a glass input window having an upstream face intended to receive a flux of incident photons as well as a downstream face opposite the upstream face, a photocathode in the form of a semiconductor layer, intended to generate photoelectrons from the incident photons and to emit said photoelectrons thus generated, an electron multiplier device configured to receive the photoelectrons emitted by the photocathode and to generate for each photoelectron received a plurality of secondary electrons and an output device configured to generate an output signal from said secondary electrons, the radiation detector being specific in that a transparent conductive layer is deposited on the downstream face of the input window and that a thin insulating layer is arranged between said conductive layer and the semiconductor layer,the conductive layer being electrically connected to a first electrode and the semiconductor layer being electrically connected to a second electrode, the first electrode being intended to be brought to a potential lower than that applied to the second electrode.,

[0017] The semiconductor layer may in particular be made of a polycrystalline semiconductor material. This material may be chosen from SbK 2 Cs, SbRb 2 Cs, SbRb 2 Cs, SbCs 3 , SbNa 3 , SbNaKRbCs, SbNaKCs, SbNa 2 KCs.

[0018] Alternatively, the semiconductor layer can be made of a III-IV or II-VI monocrystalline semiconductor material.

[0019] The transparent conductive layer is typically made of ITO or ZnO.

[0020] The insulating thin layer is advantageously made of a dielectric material having a breakdown voltage greater than 1 V / 10 nm. This insulating thin layer generally has a thickness of 100 to 200 nm. The dielectric material is advantageously chosen from Al 2 O 3 , SiO 2 , HfO 3 .

[0021] The potential difference applied between the second electrode and the first electrode is chosen to be greater than or equal to 4 δ ε s Δ U bb eN a ε d Or ε s And ε d are respectively the dielectric constants of the semiconductor layer and the insulating layer, δ is the thickness of the insulating layer, Δ U bb is the amplitude of the band curvature in the absence of applied potential difference, N / A is the concentration of acceptors in the semiconductor layer and e is the charge of the electron. Brief description of the figures

[0022] Other characteristics and advantages of the invention will appear on reading a preferred embodiment of the invention, described with reference to the attached figures among which: There Fig. 1 , already described, schematically represents the structure of an electromagnetic radiation detector known from the state of the art; The Fig. 2 represents the band diagram of a high quantum efficiency photocathode, known from the state of the art; The Fig. 3 schematically represents the structure of an electromagnetic radiation detector according to one embodiment of the invention; The Fig. 4 represents the band diagram of a photocathode used in the electromagnetic radiation detector of the Fig. 3 . Description of the embodiments

[0023] The idea behind the invention is to introduce between the input window of the electromagnetic detector and the photocathode, a capacitive structure formed of a thin conductive layer serving as a polarization electrode, and a thin dielectric layer. The polarization electrode is intended to be polarized at a potential lower than that applied to the photocathode so as to drive out of the recombination zone the photoelectrons generated near the interface.

[0024] There Fig. 3 schematically represents the structure of an electromagnetic radiation detector according to one embodiment of the invention.

[0025] The detector comprises, as previously, an entrance window, 310, made of a material transparent in the spectral band of interest, for example a quartz or borosilicate glass window.

[0026] The entrance window has an upstream face, 311, intended to receive the flow of incident photons and a downstream face, 312, opposite the upstream face.

[0027] A conductive layer, 316, transparent in the spectral band of interest, is deposited on the downstream face of the input window. It is further electrically connected to a first electrode 315. The transparent conductive layer can advantageously be made of ZnO or ITO. Its thickness is chosen in the range 50 to a few hundred nm and advantageously equal to 150 nm.

[0028] An insulating layer, 317, made of a dielectric material, is arranged between the conductive layer 316 and the semiconductor layer of the photocathode, 320. The dielectric material is chosen to have a high breakdown voltage, for example greater than 1V / 10 nm. The thickness of the dielectric layer is typically 100 to 200 nm. The dielectric material may in particular be alumina (Al 2 O 3 ), silica (SiO 2 ), or a Hafnium oxide (HFO 3 ). According to a variant, the insulating layer may be made in the form of a multilayer dielectric structure using the aforementioned dielectric materials.

[0029] The photocathode 320 is produced in the form of a semiconductor layer deposited on the insulating layer 317. The semiconductor may be monocrystalline, for example a III-V semiconductor, such as GaAs or II-VI, such as CdTe. Alternatively, it may have a polycrystalline structure, as may be the case in particular for bi- or multi-alkali compounds such as SbK 2 Cs, SbRb 2 Cs, SbRb 2 Cs, SbCs 3 , SbNa 3 , SbNaKRbCs, SbNaKCs, SbNa 2 KCs.

[0030] In all cases, the photocathode is electrically connected to a second electrode, 325.

[0031] The primary electrons emitted by the photocathode are emitted into a vacuum and multiplied by an electron multiplication device, 330, for example a microchannel plate (MCP) or a layer of nanocrystalline diamond as described in published application FR-A-2961628 filed in the name of the present Applicant, or even a discrete dynode multiplier in the case of conventional photomultipliers.

[0032] The electron multiplication device is connected to a third electrode (not shown).

[0033] The photoelectrons thus multiplied, called secondary electrons, are received by the output device, 340. The output device may comprise a phosphor screen, ensuring direct conversion into an image as in an image intensifier or a CCD or CMOS matrix to provide an electrical signal representative of the distribution of the flux of incident photons, as in an EB-CCD system. (Electron Bombarded CCD)or EBCMOS (Electron Bombarded CMOS), or a simple metal anode in the case of classic photomultipliers.

[0034] The output device is connected to a fourth electrode acting as an anode.

[0035] The input window 310, the photocathode 320, the electron multiplication device 330 and the output device are advantageously mounted in a compact tube body, the electrical connections of the electrodes with the external power supply being ensured by connection rings separated by dielectric spacers. According to an advantageous embodiment variant, the tube body may be in the form of a multilayer ceramic substrate on which the electron multiplication device will be fixed as described in published application FR-A-2925218 filed in the name of the present Applicant.

[0036] Of course, as in a conventional photocathode, the extraction of photoelectrons and their acceleration are ensured by applying a high voltage between the anode and the cathode. However, in an original way, a negative voltage is applied between the first electrode and the second electrode so that the conductive layer is brought to a potential lower than that of the photocathode. More precisely, if we note respectively V m , V pk , V a the respective potentials of the conductive layer, the photocathode and the anode, we have V m < V pk << Go . In other words, the potential difference between the conductive layer and the anode is essentially due to that between the photocathode and the anode. In practice, the potential difference V pk -V m will be between 1 and 50 V while the potential difference Go - V pk is of the order of several hundred V.

[0037] The application of this voltage to the first electrode has the effect of driving the photoelectrons generated in the recombination zone 321 towards the emission surface 322 of the photocathode. The recombination zone of the photocathode is located at the interface with the dielectric layer. Those skilled in the art will understand that the dislocations and defects at the interface with the dielectric layer act as a recombination center for the photoelectrons. The residence time of the photoelectrons in the recombination zone is very short due to the electric field applied between the conductive layer and the photocathode and reduces their probability of recombination accordingly.

[0038] Furthermore, the transport of photoelectrons within the photocathode is no longer due primarily to diffusion but also to the internal electric field. This results in a reduction of the average travel time of electrons in the photocathode and an improvement in the response time of the photodetector.

[0039] There Fig. 4 represents the band diagram of a photocathode used in the electromagnetic radiation detector of the Fig. 3 .

[0040] The conductive layer is indicated by 410, the insulating layer by 420 and the semiconductor layer of the photocathode by 430.

[0041] The upper part of the figure, designated by (A), corresponds to the situation where no potential difference is applied between the conductive layer and the semiconductor (p-type) layer.

[0042] It is noted that the conduction and valence bands of the semiconductor layer are curved downwards at the interface with the insulating layer. In other words, in such a situation, a photoelectron gas is formed at the interface, in the potential basin 424. The recombination zone where the surface states are located has also been designated by 425.

[0043] Photoelectrons generated at or near the interface have a high probability of recombination with the surface states, especially since a photoelectron present in the potential trough will tend to migrate towards the recombination zone.

[0044] The lower part of the figure, designated by (B), corresponds to the situation where the conductive layer is brought to a potential lower than that of the semiconductor layer. More precisely, the potential difference V pk - V m is chosen here to be greater than a threshold value Δ V thas explained further below.

[0045] It is noted that the conduction and valence bands of the semiconductor layer are this time curved upwards at the interface with the insulating layer. In other words, in such a situation, the photoelectrons generated at the interface are driven out of the recombination zone 425 by the electric field present in the band curvature zone.

[0046] The potential difference V pk - V m to be applied can be estimated as follows: in the absence of applied voltage (situation (A)), the (negative) space charge corresponding to the band curvature balances the (positive) charge of the surface states. This space charge can be approximated by: Q b ≃ − eN a x dt where e is the charge of the electron, N / A is the concentration of acceptors in the photocathode (p-type) and x dt is the width of the depletion zone.

[0047] The width of the depletion zone can be estimated by: x d ≃ 4 ε s Δ U bb eN a Or ε s is the dielectric constant of the semiconductor and Δ U bb is the band curvature in the absence of potential difference. It follows that Q b ≃ − 4 ε s Δ U bb eN a .

[0048] The potential difference to be applied between the conductive layer and the photocathode to simply balance this charge by capacitive effect in the photocathode is therefore: V m − V pk FF = − Q b C = − Q b ε d δ where the index FF corresponds to a situation where the bands are flat at the interface, δ is the thickness of the insulating layer and ε d its dielectric constant. If we want to at least reverse the band curvature, it will then be necessary to apply a potential difference ( V m - V pk ) ≤ -ΔV th with: Δ V th ≃ 4 δ ε s Δ U bb eN a ε d

[0049] The person skilled in the art will, however, understand that an improvement in quantum efficiency will be obtained when V< m < V pk ,since any reduction in band curvature, even before its inversion, will reduce the width of the potential trough at the interface and consequently decrease the probability of photoelectron recombination.

Claims

1. An electromagnetic radiation detector comprising: • a glass inlet window (310) having an upstream face (311) intended to receive a flux of incident photons as well as a downstream face (312) opposite to the upstream face, • a photocathode in the form of a semiconductor layer (320), intended to generate photoelectrons from the incident photons and to emit said photoelectrons thus generated, • an electron multiplier device (330) configured to receive the photoelectrons emitted by the photocathode and to generate for each received photoelectron a plurality of secondary electrons, • an output device (340) configured to generate an output signal from said secondary electrons, • a transparent conductive layer (316) deposited over the downstream face (312) of the inlet window, and • a thin insulating layer (317) disposed between said conductive layer (316) and the semiconductor layer (320), the conductive layer being electrically connected to a first electrode (315) and the semiconductor layer (320) being electrically connected to a second electrode (325), the first electrode being intended to be set at a potential lower than that applied at the second electrode, characterized in that the potential difference applied between the second electrode (325) and the first electrode (315) is selected higher than or equal to 4 δ ε s ΔU bb eN a ε d where εs and εd are respectively the dielectric constants of the semiconductor layer (320) and of the thin insulating layer (317), δ is the thickness of the insulating layer, ΔUbb is the amplitude of the band bending in the absence of any applied potential difference, Na is the concentration of acceptors in the semiconductor layer (320) and e is the charge of the electron.

2. The electromagnetic radiation detector according to claim 1, characterised in that the semiconductor layer is made of a polycrystalline semiconductor material.

3. The electromagnetic radiation detector according to claim 2, characterised in that the polycrystalline semiconductor material is selected from SbK2Cs, SbRb2Cs, SbRb2Cs, SbCs3, SbNa3, SbNaKRbCs, SbNaKCs, SbNa2KCs.

4. The electromagnetic radiation detector according to claim 1, characterised in that the semiconductor layer is made of a III-IV or II-VI monocrystalline semiconductor material.

5. The electromagnetic radiation detector according to one of the preceding claims, characterised in that the transparent conductive layer is made of ITO or ZnO.

6. The electromagnetic radiation detector according to one of the preceding claims, characterised in that the thin insulating layer is made of a dielectric material having a breakdown voltage higher than 1 V / 10 nm.

7. The electromagnetic radiation detector according to claim 6, characterised in that the thin insulating layer has a thickness of 100 to 200 nm.

8. The electromagnetic radiation detector according to one of the preceding claims, characterised in that the dielectric material is selected from Al2O3, SiO2, HfO3.