GB2269048A - Photoemitters - Google Patents

Abstract

A III-V photoemitter layer (11) in the form of an array of spaced III-V elements (13) the front faces of which are angled towards the gaps between the elements. Electrons (e* box - middle top *) ejected from the elements' front faces (by impacting photons or electrons) will, under the influence of an appropriate electrical field (E), be swept laterally towards and then through the spaces between the elements, so that though the III-V material is acting in reflective mode, and so has good blue sensitivity, the device itself is acting in transmissive mode, and so has good imaging properties. To form a photocathode/photomultiplier device a plurality of these individual III-V layers may be stacked one above the next, with the elements of each succeeding layer aligned with the gaps in the preceding layer, so that electrons ejected from each layer and passing through the gaps will impact the next adjacent layer without losing their image-defining spatial resolution.

Description

Phot oemitters This invention relates to photoemitters, and concerns in particular a novel form of Group III-V device having both photoemitter and electron multiplier properties.

Mounted within some suitable vacuum envelope with an appropriate input window, photoemitters, usually in the form of photocathodes, are used in a wide variety of devices, such as image intensifiers and photomultipliers utilised as photon counters), to convert received photons of light Cor other electromagnetic rAdiation) into electrons. It is then common to employ an associated electron multiplier to amplify the usually low electron flux into something usable by an imaging or a counting system, or whatever is appropriate.

Until relatively recently the photocathodes employed have almost always been those made from thin layers of the metals antimony, sodium, potassium or caesium, usually evaporated onto a suitable glass substrate window, while the most popular electron multipliers have been either those constructed from microchannel plates Ca closely-packed array of tiny 10 micrometre diameter - glass tubes internally processed by hydrogen reduction and other chemical methods to optimise the secondary emission of the glass or those made from one or more dynode Ca 2 or 3 cm diameter metal plate electrode coated with a good secondary electron emitter such as magnesium oxide).

Because of the small size of the individual glass tubes, the microchannel plate devices have good imaging properties, so they are chosen for use with photoemitters in image intensifiers, while the dynode variety, made from relatively large plates, do not, and so are mostly used in photon counters.

In the last few years, however, great interest has been shown in semiconductor photoemitter and electron multiplier devices based on "mixtures of the Group III and Group V elements gallium and arsenic or phosphorus.

Discovered in 1965 by Scheer and van Laar, these GaAs and GaP surfaces have much higher photoelectric efficiency than any other surfaces discovered before or since 1965. The problem has always been how these surfaces could best be used in a photoelectric tube.

The earliest approach was to grow a thin epitaxial layer of GaAs on a GaP substrate, forming a structure that allowed infrared (IR) light (from around 500 nanometre, being the band gap of GaP, to 900 nm, the band gap of GaAs) to pass through the GaP substrate and to be absorbed in the thin GaAs layer, where it is converted into electrons which are then emitted from the opposite - GaP-distant - surface of the GaAs (so the surface was acting in transmissive mode). Unfortunately, the large lattice mismatch between GaP and GaAs results in poor crystal quality, and this type of photocathode structure has low efficiency.

A more recent structure uses thin layers of AlAs and GaAs to provide a window and buffer between the GaAs and a glass support to which the crystal is bonded.

After bonding, the bulk of the GaAs is etched away to leave a layer of optimum thickness for photoemission (as disclosed in our British Patent Specification No: 2,213,634A CP1OBQ)). Unfortunately, since glass absorbs in the UV, this type of structure has'no sensitivity at all to wavelengths of less-than 350 nm.

In transmissive mode - being used as a transparent device, with photons coming in from one side and electrons leaving from the opposed side - all these III-V materials have high sensitivity to inf ra-red (IR) Cand so are extremely useful in the construction of night vision systems), but have much poorer sensitivity to the shorter wavelength light, and especially to blue and ultra-violet. Unfortunately, there are many applications where high sensitivity in the blue end of the spectrum is required.

Now, while the III-V materials may be relatively blue insensitive in transmissive mode, it turns out that they are much more sensitive to these shorter wavelengths in reflective mode - that is, where light photons come in from one side and electrons leave from that same side. Moreover, in this reflective mode they are especially good as electron multipliers - a single electron impacting the surface may cause as many as a thousand to be ejected from the same surface. The problem is to provide a device operating in this reflective mode with good imaging capability, because in general it is difficult if not impossible accurately to control the ejected electrons sufficiently finely to stop the original image organisation of the incoming photons being lost by those electrons scattering and diverging on ejection.It is primarily for this reason that there are employed the semi-transparent structures such as are exemplified in the aforementioned Patent Specification No: 2,213,634A.

The present invention seeks to solve this problem in another way, by providing a novel structure of III-V photoemitter such that, even though the photoemissive material is operating in reflective mode, where its blue sensitivity is highest, the device itself is operating in transmissive mode, so enabling the image properties of the exciting photons to be retained in the ejected electrons. More specifically, the invention proposes that the III-V photoemitter layer be in the form of an array of spaced III-V elements the front faces of which are angled towards the gaps between the elements Cand the invention includes a method of making such an array).The idea of this is that electrons ejected from the elements' front faces (by impacting photons or electrons) will, under the influence of an appropriate electrical field, be swept laterally towards and then through the spaces between the elements, so that though the III-V material is acting in reflective mode, and so has good blue sensitivity, the device itself is acting in transmissive mode, and so has good imaging properties.The invention also proposes that to form a photocathode/photomultiplier device a plurality of these individual III-V layers be stacked Cwithin some suitable windowed vacuum envelope) one above the next, with the elements of each succeeding layer aligned with the gaps in the preceding layer, so that electrons ejected from each layer and passing through the gaps will impact the next adjacent layer without losing their image-defining spatial resolution.

In one aspect, therefore, the invention provides a novel structure of Ill-V photoemitter material capable of operating in reflective mode but in a device itself operating in transmissive mode, in which structure the III-V photoemitter layer is in the form of an array of spaced III-V elements the front faces of which are angled towards the gaps between the elements.

In a second aspect the invention provides a method of making such an element array photoemitter structure, in which a corresponding sheet of photoemitter material is etched away, through a suitable mask, in the central area thereof to form the desired element array supported within a peripheral area of unmodified sheet material In a third aspect the invention provides a photocathode/photomultiplier device comprising a plurality of the individual II1-V element array photoemitter structures stacked as layers one above the next, with the elements of each succeeding layer aligned with the gaps in the preceding layer, so that light passing through the gaps in the first layer, or electrons ejected from each layer and passing through the gaps therein, will impact the next adjacent layer without losing their image-defining spatial resolution.

In its primary aspect the invention provides a novel structure of- Ill-V photoemitter material. This material may be any of those III-V materials used or suggested for use, typically GaAs or GaP, or the mixed materials like InGaAsP and GaAsP (which exhibit longer wavelength response or higher quantum efficiency in the spectrum green region), any of which may be caesium activated (GaP is easter to activate, and is for that reason generally preferred as a secondary emitter).

The novel structure of the invention is one wherein the II I-V photoemitter layer is in the form of an array of spaced Ill-V elements the front faces of which are angled towards the gaps between the elements. The idea of this is that when the layer is built into a photoemitter device, then electrons ejected in operation from the elements' front faces normal thereto will, under the influence of an appropriate applied electrical field, be swept laterally towards and then through the spaces between the elements, so that though the TII-V material is acting in reflective mode, and so has good blue sensitivity, the device itself is acting in transmissive mode, and so has good imaging properties.

Of course, in order to ensure that the imaging capability is high, both the elements themselves and the gaps, or spaces, between those elements should naturally be small. Typical element and gap sizes suitable for this are around 0.25 mm (250 micron), though the smaller the bar/hole the higher the resolution and imaging capability, and thus the better (always assuming the structure has the necessary physical strength).

The elements and the gaps therebetween may take any of a wide variety of geometrical forms. For example, the elements may be parallel spaced bars (the sizes mentioned above are then the bar width and depth, and inter-bar widths), or the elements may be a foraminous mesh-like structure with the spaces the actual '2island" holes therein. Such a mesh can be a reticulated, square mesh, effectively formed from two orthogonal sets of "bars", with correspondingly square holes, or it can be a hexagonal mesh, with three crossed sets of bars and hexagonal holes; it can even be a mesh of circular holes, with no definable bars.

The front faces of the elements are angled so that electrons ejected therefrom travel toward the adjacent spaces (and can then relatively easily be swept by an applied field into and through the spaces, so making the device a transmission one). It will be seen that each element thus has a generally isosceles triangular crosssection. The angle - the triangle's base angle - can be of any significant value; 45 is generally satisfactory, though the actual angle may, as explained hereinafter, depend upon the method employed to form the spaces and shape the elements.

The photoemitter structure is an array of elements, and these elements need to be supported. A support substrate, on which all the elements are individually mounted, would be suitable were it possible to use a material for the substrate that was transparent to photons and electrons, but in the absence of such a material it is acceptable to provide the array with a peripheral support across which the element array extends. Thus, the array could, either before or after formation, be attached to and mounted on a separate "hoop" support, or it could be provided during its formation with an integral peripheral support formed as an "unholed" boundary area of the Ill-V material.

A typical array is a disc around 10 cm (4 in) diameter, the outer 1.25 cm C0.5 in) of which is the supporting boundary area.

The photoemitter structure of the invention may be made in any convenient way, even by cutting away material from a sheet of photoemitter material. This cutting is best effected by a through-mask etching process, and thus in its second aspect the invention provides a method of making the element array structure in which a corresponding sheet of photoemitter material is etched away through a suitable mask (if the etched area is a central area of the sheet, then the formed array is supported within a peripheral area of unmodified sheet material, as observed above).

In this preferred through-mask etching process, which itself is in principle quite conventional, the sheet of photoemitter material is first provided with a layer of photoresist, this is then exposed to light radiation, through a photographic mask, to generate the mask pattern thereon, the unwanted areas of photoresist are then dissolved away to leave the mask pattern covering the wanted areas of material sheet Cthose areas defining the array elements), and then the exposed, unwanted sheet areas (those defining the "spaces" between array elements) are etched away using some appropriate etchant, and finally the remaining photoresist is removed to leave the finished array.

This is, in itself, all quite conventional, and needs no comment. Even so, it is perhaps worth pointing out that suitable photoresists are readily available in many different grades, from many different manufacturers CShipley AZ 111 S is a typical example), and after exposure to UV light either the exposed or the unexposed portions (as desired) may be removed by the appropriate positive or negative developer Csuch as Shipley 303), The subsequent removal of the exposed sections may be carried out in the liquid or the vapour phase, as appropriate to the photoresist; a typical vapour-phase system uses a strong oxidiser, such as nitrogen oxide in a glow discharge. The actual etching of the underlying III-V material may itself be effected in any convenient way, using either liquid- or vapour-phase systems.

Liquid-phase etchants are commonly aqueous mixtures (1 pbv water) of sulphuric acid (5 pbv) and hydrogen peroxide (1 pbv), whilst vapour-phase etching involves a glow discharge in a mixture of an inert gas or halogen.

Shaping a photoemitter sheet by etching results in the etchant "cutting" down into the material through the exposed surface where the material is unprotected by the remaining photoresist. Under normal circumstances the etchant will cut slightly sideways as well, undercutting the photoresist, with the result that the exposed side surface of the cut will be at an angle rather than normal to the sheet surface, so giving the cut the cross-section of an inverted triangle Cand the remaining material defining the bars that of a right-way-up triangle). The rate of etching/undercutting will depend upon the III-V material and its crystal structure and orientation, upon the etchant, and upon the physical conditions. In any given case it will be relatively easy to work out what is required, and no further comments are needed at this time.

The photoemitter structure of the invention is a Ill-V photoemitter layer in the form of an array of spaced lII-V elements the front faces of which are angled towards the gaps between the elements. When contained within a suitably-windowed vacuum envelope such a structure can in fact be used as a photoemitter device, for photons or electrons impacting the front face of the structure will indeed result in electron emission, and these electrons, suitably directed by an applied electrical field, can be directed through the gaps between the elements for subsequent use either to form an image or to feed a counter.However, the efficiency of a single layer device such as this is poor, much of the original photon or electron flux passing straight through the gaps in the array rather than impacting the elements to cause electron emission, and it is very much preferred to employ at least two layers stacked on on the other, with the elements of the second "blocking" the gaps in the first.Accordingly, the third aspect the invention provides a photocathode/photomult iplier device comprising a plurality of the individual III-V element array photoemitter structures stacked as layers one above the next, with the elements of each succeeding layer aligned with the gaps in the preceding layer; in such a stack not only do few if any original photons/electrons pass straight through - those that go through the gaps in the first layer are intercepted by the elements in the next layer - but most of the electrons ejected from the first layer are then turned Cby the applied electric field) to pass through the gaps in that layer to impact the next adjacent layer, so causing even more electrons to be emitted to add to the total flux and improve the intensity of the final output Though, because of the 1000 times electron multiplying effect the Ill-V materials provide, a twoor three-layered stack is for most purposes satisfactory, more layers can be employed should that be thought desirable. In some devices, especially photon counters, it may be convenient to employ two or three sets of pairs of layers, each at a higher voltage.

In a photoemitter device of the invention an electric field is applied to turn the electrons emitted from the elements in a layer towards and through the adjacent gaps. It is one of the characteristics of negative electron affinity emitters of the III-V type that the electrons are ejected both normal to the surface and with a fairly precise electron energy Cthe negative affinity). Knowing this, and knowing also the mechanical dimensions of the array of elements, it is possible to calculate the optimum field needed to ensure that all the emitted electrons are indeed swept into and through the gaps.By way of example, with element bars 0.2 mm thick the field would be about 50 volts/ An electric field is also employed to drive the electrons emitted from one layer though to the next, where they impinge to cause many more electrons to be emitted. The minimum energy required to achieve this is perhaps 10 to 20 volts, so for an applied field of 50 soltsfmm the necessary minimum separation between the two layers is a small fraction of a millimetre.

However, since the device can be optimised for either opacity Cand thus imaging capability) or counting efficiency - the closer the layers the better the second blocks the gaps in the first, and higher the opacity, while the more distant the layers the higher the voltage between the two, and the greater the secondary emission and thus the counting efficiency.

The photoemitter device of the invention employs one or more array structure suitably mounted within an appropriately windowed vacuum envelope. This may take any form used or suggested for use in the Art, and needs no further comment here.

The invention enables the construction of a lII-V photoemitter device having a good short wavelength response limited only by the cut-off of the vacuum window. The device has excellent potential as a photomultiplier/counter, where the good III-V response enables a satisfactory result from a relatively low number of layers, and thus compares well with presentday devices employing from 9 to 15 stages, with considerable improvements in transit time, transit time spread, pulse height distribution, and spatial resolution.Moreover, while, in its imaging guise, the device, with its submillimetre element size Con which its resolution primarily depends), is hardly up to the imaging capability of systems utilising microchannel plates Cwith their 10 micrometre tubes), it is nevertheless acceptable for a number of fairly coarse, low resolution uses.

An embodiment of the invention is now described, though by way of illustration only, with reference to the accompanying highly diagrammatic Drawings in which: Figures 1A a B show respectively plan and cross sect ion views of a photoemitter structure according to the invent ion; Figure 2 shows a stage in the manufacture of a structure of Figure 1; and Figure 3 shows a photoemitter device made from a stacked pair of structures as in Figure 1.

The photoemitter structure of Figure 1 is a circular sheet of III-V material (generally 11) the central area (generally 12) of which has been etched away to form a plurality of parallel spaced bars Cas 13) extending across the sheet and integral with a peripheral unetched area C14) that supports the bars and by which the whole structure can be mounted in some sort of holder (not shown). The sectional view of Figure 1B Ctaken on the line A-A in Figure 1A) helps to show the general layout, shape and relative size of the various parts; the bars 13 in particular are shown with a triangular cross-section, apex to the front (the top as viewed).

The photoemitter structure of Figure 1 can best be made by masking and etching a plane sheet of III-V material, and this is represented in Figure 2. The sheet is provided Con both sides) with a protective layer of photoresist, which is exposed to light through an appropriate mask (not shown) and then selectively removed to leave on one side the required pattern of bar and peripheral portions Cas 23 and 24 respectively).

The whole is then bathed in an etchant, which eats away the exposed photoemitter material (25) to form the gaps between the bars 13, but also undercuts the protecting strips 23 so that the bars 13 end up with the required triangular section. Once a sufficient amount of etching has been carried out the process is halted, and the remaining protective resist Con both sides) is removed to leave the finished structure.

A stacked pair of structures as in Figure 1 is shown very diagrammatically in the device of Figure 3, which also shows the way in which the pair are staggered laterally so that the bars 13 of the lower sheet (111) are aligned with the spaces between the bars of the upper sheet (1111), thus rendering the device as a whole more or less opaque. Photons Chu) impacting the front Cupper as viewed) face of the device strike either a bar 13 in the upper sheet 11u or a bar in the lower one 11 while electrons emitted by either strike are turned by the applied field E towards and through the adjacent space. Electrons Cho~) emitted from the upper sheet 11u and driven through the gaps therein are accelerated towards the lower sheet 111, striking the bars 13 therein to cause multiple secondary emission.

It will be seen that, although there is some spreading of the information and energy, nevertheless to some degree, suitable for a low resolution device, the emitted electrons retain the grouping, and thus the resolution, of the original photons.

Claims (7)

1. A structure of III-V photoemitter material capable of operating in reflective mode but in a device itself operating in transmissive mode, in which structure the III-V photoemitter layer is in the form of an array of spaced III-V elements the front faces of which are angled towards the gaps between the elements.

2. A structure as claimed in Claim 1, wherein the III-V photoemitter material is GaP, caesium activated.

3. A structure as claimed in either of the preceding Claims, wherein, in order to ensure that the imaging capability is high, both the elements themselves and the gaps, or spaces, between those elements are around 0.25 mm (250 micron) wide.

4. A structure as claimed in any of the preceding Claims, wherein the elements are parallel spaced bars.

5. A structure as claimed in any of the preceding Claims, wherein the front faces of the elements are angled so that each element has a generally isosceles triangular cross-section, and the angle - the triangle's base angle - is 45".

6. A structure as claimed in any of the preceding Claims, wherein the element array is attached to and mounted on a separate hoop support, or is provided during its formation with an integral peripheral support formed as an unholed boundary area of the III-V material.

7. A structure as claimed in Claim 6, wherein the array is a disc of 10 cm (4 in) diameter, the.outer

7. A structure as claimed in Claim 6, wherein the array is a disc of 10 cm (4 in) diameter, the outer 1.25 cm (0.5 in) of which is the supporting boundary area.

8. A structure as claimed in any of the preceding Claims and substantially as described hereinbefore.

9. A method of making an element array photoemitter structure as defined in any of the preceding Claims1 in which method a corresponding sheet of photoemitter material is etched away, through a suitable mask, in the central area thereof to form the desired element array supported within a peripheral area of unmodified sheet material.

10. A method as claimed in Claim 9, in which the sheet of photoemitter material is first provided with a layer of photoresist, this is then exposed to light radiation, through a photographic mask, to generate the mask pattern thereon, the unwanted areas of photoresist are then dissolved away to leave the mask pattern covering the wanted areas of material sheet (those areas defining the array elements), and then the exposed, unwanted sheet areas (those defining the spaces between array elements) are etched away using some appropriate etchant, and finally the remaining photoresist is removed to leave the finished array.

11. A method as claimed in either of Claims 9 and 10 and substantially as described hereinbefore.

12. An element array III-IV photoemitter structure whenever made by a method as claimed in any of Claims 9 to 11.

13 A photocathode/photomultiPlier device comprising a plurality of the individual III-V element array photoemitter structures as defined in any of Claims 1 to 8 and 12, stacked as layers one above the next, with the elements of each succeeding layer aligned with the gaps in the preceding layer, so that light passing through the gaps in the first layer, or electrons ejected from each layer and passing through the gaps therein, will impact the next adjacent layer without losing their image-defining spatial resolution.

14. A layered device as claimed in Claim 13 which is a two- or three-layered stack.

15. A layered device as claimed in either of Claims 13 and 14 and substantially as described hereinbefore.

Amendments to the claims have been filed as follows 1. A structure of III-V photoemitter material capable of operating in reflective mode but in a device itself operating in transmissive mode, in which structure the III-V photoemitter material is in the form of an unbacked planar array of spaced III-V elements the front faces of which are angled towards the gaps between the elements.

2. A structure as claimed in Claim 1, wherein the III-V photoemitter material is GaP, caesium activated.

3. A structure as claimed in either of the preceding Claims, wherein, in order to ensure that the imaging capability is high, both the elements themselves and the gaps, or spaces, between those elements are around 0.25 mm (250 micron) wide.

4. A structure as claimed in any of the preceding Claims, wherein the elements are parallel spaced bars.

5. A structure as claimed in any of the preceding Claims, wherein the front faces of the elements are angled so that each element has a generally isosceles triangular cross-section, and the angle - the triangle's base angle - is 45".

6. A structure as claimed in any of the preceding Claims, wherein the element array is attached to and mounted on a separate hoop support, or is provided during its formation with an integral peripheral support formed as an unholed boundary area of the III-V material.