Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
  • H01J9/02—Manufacture of electrodes or electrode systems
  • H01J9/12—Manufacture of electrodes or electrode systems of photo-emissive cathodes; of secondary-emission electrodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
  • H01J1/02—Main electrodes
  • H01J1/34—Photo-emissive cathodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2201/00—Electrodes common to discharge tubes
  • H01J2201/34—Photoemissive electrodes
  • H01J2201/342—Cathodes
  • H01J2201/3421—Composition of the emitting surface
  • H01J2201/3423—Semiconductors, e.g. GaAs, NEA emitters
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J40/00—Photoelectric discharge tubes not involving the ionisation of a gas
  • H01J40/02—Details
  • H01J40/04—Electrodes
  • H01J40/06—Photo-emissive cathodes

Definitions

• the present invention relates to the field of photocathodes, in particular for electromagnetic radiation detectors such as image intensifiers or sensors of EBCMOS (Electron Bombarded CMOS) or EBCDD (Electron Bombarded CDD) type.
• electromagnetic radiation detectors such as image intensifiers or sensors of EBCMOS (Electron Bombarded CMOS) or EBCDD (Electron Bombarded CDD) type.
• Electromagnetic radiation detectors such as, for example, image intensifier tubes and photomultiplier tubes, make it possible to detect electromagnetic radiation by converting it into a light or electrical output signal. They usually comprise a photocathode for receiving the electromagnetic radiation and in response transmitting a photoelectron flux, an electron multiplier device for receiving said photoelectron flux and in response transmitting a flow of so-called secondary electrons, then an output device for receiving said secondary electron flow and in response transmitting the output signal.
• Photocathodes convert an incident photon flux into a photoelectron flux. They are generally composed of a substrate that is transparent to the spectral band of interest and an electro-emissive layer deposited on the rear face of this substrate.
• Photocathodes can be characterized by their quantum efficiency QE (Quantum Efficiency) defined as the average percentage of incident photons converted to photoelectrons or by their sensitivity defined as the photocathode current generated by a given luminous flux. There are two types of photocathodes.
• QE Quantum Efficiency
• the so-called second-generation photocathodes use an electro-emissive layer of multi-alkaline compound such as SbNaK or SbNa 2 KCs deposited by CVD (Chemical Vapor Deposition) on a glass substrate.
• the thickness of the light emitting layer is usually between 50 and 200 nm.
• the sensitivity of these photocathodes is generally 700 to 800 ⁇ ⁇ ⁇ and its quantum efficiency is relatively low (of the order of 15%).
• the so-called third-generation photocathodes use an electro-emissive layer of GaAs, epitaxialized by MOCVD (Metal Organic Chemical Vapor Desposition) and reported on a glass substrate.
• the thickness of the electro-emissive layer is generally of the order of 2 ⁇ m.
• the sensitivity of such a photocathode is of the order of 1500 to 2000 ⁇ ⁇ Im.
• the third-generation photocathodes have a high quantum efficiency, of the order of 30%, but their manufacture is complex and expensive.
• nanostructured photocathodes As described in application WO-A-2003/043045. These photocathodes are obtained by etching a channel pattern in an alumina matrix and filling these channels, by an electroplating technique, with an electro-emissive material such as an alkaline compound or a III-V semiconductor.
• photocathodes can reach high sensitivities but are complex to manufacture.
• the transfer of the emissive layer on a transparent substrate to the spectral band of interest proves particularly difficult because of the fragility of the nanostructure.
• the nanostructure is directly etched in a substrate constituting the input window of the photocathode, an important part of the conversion takes place in the solid part of the semiconductor layer so that the quantum yield is reduced by the recombinations. within it.
• the object of the present invention is therefore to provide a photocathode structure that allows to obtain levels of sensitivity / high quantum efficiency (s) which is very simple to manufacture.
• Another object of the present invention is to propose a method of manufacturing such a photocathode.
• the present invention is defined by a photocathode comprising an amorphous substrate, transparent to the spectral working band of the photocathode and having a first face, said front face and a rear face opposite to the front face, characterized in that it comprises a nanowire mats made of at least one semiconductor material III-V, deposited on said rear face and extending from this face in a direction opposite to the front face.
• the substrate is made of glass.
• the semiconductor material is selected from GaAs, GaN, InGaAs, InGaAs, GaP, InGaP,
• GaSb GaSb, GaAsSb, AIGaAS, AIGaASP, GaBiAs.
• the composition of the nanowires has a radial variation in the ratio of the elements of the material III-V so as to obtain a band gap gradient directed from the core of the nanowires to their periphery.
• the semiconductor material may be doped with a dopant selected from Zn, Be, C or an amphoteric material.
• the nanowires are advantageously covered with a layer of activation material selected from LiO, CsO or N F3.
• the nanowire mat can be electrically connected to a polarization electrode deposited on said substrate.
• the photocathode may have a transparent contact layer in the working spectral band of the photocathode, connected to the polarization electrode, the contact layer being located between the nanowire mat and said substrate.
• the contact layer may be a layer of ITO, graphene or a polycrystalline layer of highly doped III-V semiconductor material.
• the photocathode may include an antireflection layer located between the contact layer and said substrate.
• the diameter of the nanowires is typically between 50 to 300 nm, preferably between 50 to 150 min.
• the density of nanowires can be 10 5 to 10 cm 2 and preferably 10 8 to 10 cm 2 .
• the present invention also relates to a method for manufacturing a photocathode as defined above, wherein the nanowires are grown on said substrate by means of molecular beam epitaxy in an MBE frame.
• a gold film Prior to the growth of nanowires, within the same frame of MBE, it is advantageous to deposit on said substrate a gold film at a temperature of 0 to 1200 ° C for a period of 1 to 30 min and then it is left to dewake to a temperature between 400 ° C and 700 ° C for 1 to 30 min so as to create gold particles of 5 to 50 nm in diameter.
• a colloidal solution of gold particles 5 to 50 nm in diameter can be dispersed on the surface of the substrate, prior to the growth of nanowires.
• the temperature of the substrate during the growth phase of the nanowires is advantageously between 400 ° C. and 700 ° C.
• the atomic fluxes are advantageously calibrated so as to obtain a nanowires growth rate of between 0.5 ⁇ s and 10 ⁇ s.
• the flows of the materials making up the semiconductor material III-V are varied in such a way as to increase a material having a wider band gap at the beginning of the growth phase than at the beginning of the growth phase. at the end of this same phase.
• an activation layer is deposited in LiO, CsO or N F3, within the same MBE frame or without breaking the vacuum.
• FIG. 1A schematically shows a nanowire photocathode structure according to a first embodiment of the invention
• Fig. 1B schematically shows a nanowire photocathode structure according to a second embodiment of the invention
• Fig. 1C schematically shows a nanowire photocathode structure according to a third embodiment of the invention
• Fig. 2 represents an image obtained by scanning electron microscopy of a photocathode according to one embodiment of the invention.
• the present invention is based on the surprising finding that it is possible, under certain conditions, to directly epitaxize semiconductor nanowires III-V with a high crystalline quality on an amorphous substrate such as a glass substrate.
• the research carried out so far in nanowire growth involved either crystalline substrates or amorphous substrates undergoing a prior step of surface crystallization.
• Cohin et al. entitled "Growth of vertical GaAs nanowires on an amorphous susbtrate via a fiber-textures Si platform” published in Nanoletters, 13 May 2013, 13, pp. 2743-2747.
• Fig. 1A schematically shows the structure of a nanowire photocathode, according to a first embodiment of the invention.
• the photocathode comprises an amorphous substrate such as a glass substrate, 110, constituting the input window of the image intensifier or the sensor.
• the material of the amorphous substrate is chosen to be transparent in the spectral working band of the photocathode. If necessary, the amorphous substrate may be nano-structured to allow a more even distribution of the nanowires at the cost of greater complexity. The growth then starts in the wells of the nanostructure.
• the substrate is covered with a nanowire mat of III-V semiconductor material, for example GaN, InGaN, InGaAs, GaP, InGaP, InAs, GaSb, GaAsSb, AIGaAS, AIGaASP, GaBiAs and more generally their ternary and quaternary alloys.
• III-V semiconductor material for example GaN, InGaN, InGaAs, GaP, InGaP, InAs, GaSb, GaAsSb, AIGaAS, AIGaASP, GaBiAs and more generally their ternary and quaternary alloys.
• the nanowires are doped with a P-type material, for example Zn, Be, C, or an amphoteric material such as Si.
• a P-type material for example Zn, Be, C, or an amphoteric material such as Si.
• the nanowire stack 120 is grown directly on the amorphous substrate by molecular beam epitaxy (MBE) as described below.
• MBE molecular beam epitaxy
• the nanowires have a diameter of from 20 to 500 nm, preferably from 50 to 150 nm.
• the nanowire mat has a density of 10 5 to 10 cm 2 , preferably 10 8 to 10 9 cm -2 .
• a metal layer, 130 serves as an electrode and makes it possible to apply a polarization to the nanowire mat.
• This polarization is negative with respect to a remote anode (not shown), opposite to the photocathode.
• the photons arriving on the input face of the substrate, transparent at the wavelength of interest generate electron-hole pairs within the nanowires.
• the holes are removed by recombination with the electrons provided by the biasing electrode 130.
• the electrons generated can be emitted along the length of the nanowires.
• the nanowires are covered with a layer for lowering the output work, for example LiO, CsO or N F3 and thus to facilitate the extraction of electrons in a vacuum.
• the electrons extracted from the nanowires can then be multiplied by an electron multiplier 140, such as a microchannel slab or a nanodiamond layer (NDs).
• the secondary electrons thus generated can then form an image on a phosphorescent screen or on a matrix of CMOS transistors or even a CCD matrix (EBCCD), in a manner known per se.
• the electrons extracted from the nanowires can directly impact the rear face of an EBCMOS (Electron Bombarded CMOS) sensor.
• EBCMOS Electrode Bombarded CMOS
• the phosphorescent screen, the CCD, CMOS or EBCMOS matrix constitute the output window of the detector.
• FIG. 1B schematically shows the structure of a nanowire photocathode, according to a second embodiment of the invention.
• the elements identical to those of FIG. 1A have the same reference numbers and will not be described again.
• This second embodiment differs from the first by the presence of a contact layer 135 which is transparent in the spectral band of interest and which is conductive, for example an ITO layer, a graphene layer or even a thin polycrystalline layer. of heavily doped P-lll-V semiconductor material deposited on the substrate prior to growth of the nanowire mat.
• the contact layer 135 is electrically connected to the bias electrode 130.
• FIG. 1C schematically shows the structure of a nanowire photocathode, according to a third embodiment of the invention.
• the elements identical to those of FIG. 1B bear the same reference numbers and will not be described again.
• This second embodiment differs from the first by the presence of an antireflection layer, 125.
• This antireflection layer is deposited on the surface of the substrate before the deposition of the contact layer, 135. It prevents the light in the the spectral band of the photocathode is reflected by the interface between the substrate 110 and the contact layer 135.
• Figs. 1A-1C illustrate embodiments in which the photocathodes operate in transmission in the sense that they are located between the input window and the output window of the detector.
• these photocathodes can operate in reflection. More precisely, the photon flux in this case is incident on the rear face of the photocathode (with an angle of incidence determined by an input optic) and the photoelectrons generated in the nanowires are emitted by this same rear face.
• the detector input and output windows are therefore here on the same side of the photocathode.
• nanowire growth method on amorphous substrate such as a glass substrate, where appropriate after deposition of an antireflection layer and a contact layer, will be described below.
• the growth of the nanowires is carried out by molecular beam epitaxy (MBE) of the semiconductor material III-V on the amorphous substrate.
• MBE molecular beam epitaxy
• the gold is deposited beforehand on the substrate a gold film.
• the gold is deposited at a temperature of between 800 and 1200 ° C. (temperature of the MBE cell) on the substrate at room temperature or warm, preferably between 400 ° C. and 700 ° C., for a period of 1 to 30 hours. min.
• At the end of the deposition of the gold film one waits for a duration of 30s to 30mn, so that the gold de-wets on the substrate.
• Gold particles 5 to 50 nm in diameter are then formed on the glass substrate.
• the gold film is deposited or dispersed on the contact layer.
• the dewetting and nucleation phenomenon is substantially the same as on the glass substrate.
• the growth of the nanowires is then carried out in the same MBE frame, which avoids any contamination by the ambient air. It is carried out in a temperature range of 400 to 700 ° C. The temperature is measured by means of a pyrometer adapted to the wavelength of the materials III-V composing the nanowires.
• Atomic fluxes are chosen to correspond to growth rates of between 0.5 ⁇ s and 10 ⁇ s.
• the fluxes are calibrated by high-energy electron diffraction grazing incidence or RHEED (Reflecting High Energy Electron Diffraction) by observing the RHEED observations corresponding to the deposition of successive layers, in a manner known per se. After a few seconds of growth, the diffraction pattern reveals semicircles indicating the growth of monocrystalline nanowires in a multitude of directions.
• Fig. 2 is a scanning electron microscope (SEM) image of a GaAs nanowire carpet grown by MBE epitaxy on a glass substrate (Corning TM 7056).
• the ratio of the flows of the materials III-V during the growth can be varied so that the nanowires have a wider band gap. at their base (and at their periphery) than at their summit (and in their heart). More precisely, for a material 11 lV of the type X " 1 ⁇ 1 " Y where ⁇ '", ..., X'" are the materials II I and y the material V, it will be possible to vary the flows of the materials X '", ..., X ! " With respect to the material flow V during the epitaxy so as to obtain a bandgap gradient directed from the core of the nanowires to their periphery.
• the concentration x can be varied during epitaxy.
• composition that is to say the variation of the flows of II I materials during epitaxy
• the variation of composition can be carried out in stages in time. Alternatively, it may be gradual so as to obtain a positive gradient bandgap directed from the heart of the nanowires to their periphery. Whatever the law of composition variation envisaged, this variant will absorb a wider spectral band with a simple homogeneous composition.
• the diameter of the nanowires is substantially less than the average free path length of the electrons in the II-V material, the electrons generated in the nanowires have a high probability of being emitted into vacuum before being recombined.
• the emission of photoelectrons can take place all along the nanowires.
• the high electric field due to the peak effect also increases the probability of emission compared with a conventional planar photocathode configuration.

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• 2017
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