GB2213634A - Photocathode structures - Google Patents

Abstract

Modern image intensifiers employing transmission-mode photocathodes of the III-V semiconductor type do not always work effectively particularly at longer wavelengths. Moreover, they suffer from problems associated with impurities diffusing out of the glass substrate, and the propagation of crystal dislocations from the glass/semiconductor interface to the photocathode. Viable devices, with longer wavelength sensitivity can be made from other III-V materials, and dislocations and/or impurities in the glass substrate can be more effectively trapped therein, by the relatively simple expedient of using not single layer single composition etch stop and window layers but instead laminar layers - that is to say, layers which are themselves layered, to form a "super lattice" structure - wherein alternate epitaxially grown laminae within a layer have different III-V material compositions. The laminae may be of, for example Al As alternating with Ga As or with In Ga As, or Al Ga As alternating with In Ga As. The laminae may be of equal or different thicknesses, e.g. successive laminae of one material may progressively increase in thickness through the layer as the alternate laminae of the other material decrease in thickness. The window layer may have anti-reflection properties.

Description

Photocathode Structures This invention concerns photocathode structures, and relates in particular to the structure of transmission-mode photocathodes of the III-V semiconductor type used in image intensifiers and other similar devices.

The term "image intensifier" is commonly used in the Art to describe a device for providing a bright pictlJre of a dimly-lit scene. The device gathers radiation - which may be visible light or, perhaps, Infra-Red (IR) radiation - from the scene, and by means usually involving the conversion of the formed radiation image into an equivalent. electron image, the electrical amplification of this latter, and the subsequent reconversion of the amplified electron image into a visible-light image, it provides a much brighter view of the scene than could possibly be obtained by the naked eye.

Over the past twenty years or so the mode of construction and operation of image intensifiers has changed, resulting in smaller, more efficient devices.

In the earliest intensifiers - what are now called the first generation devices - the incoming radiation was imaged via a fibre-optic faceplate onto a photocathode layer (carried by the other side of the plates, and the streams of generated electrons were accelerated (via the intensifier section) through an electron-focussin & field onto a photoanode - a phosphor, or luminescent, screen where they were converted to visible light.At a later date, there were devised the second-generation intensifiers, in which instead of the extremely complicated and bulky fibre-optic facepiate ad electron-optic intensifier sections there was used a simple input window (again carrying the photocathode layer) together with what is now known as a "-microchannel plate" in which, essentially, the generated electrons ricochet along the individual tubes (the channels) of a bundle of tubes, each tube-wall contact generating a cascade of electrons so that each single input electron results in many leaving the tubes and being directed to the photoanode/luminescent screen to provide the required bright visible-light image.

Presently, much work is being done on the thirdgeneration devices, which are very similar to the second-generation microchannel plate devices save that they employ thin semiconductor films (typical materials are gallium arsenide, aluminium gallium arsenide and indium gallium arsenide) - the so-called "glass boded cathode", or "third generation photocathode" - to generate electrons from the incoming radiation rather than using the more conventional type of photocathode (commonly a thin layer of metals such as sodium, potassium, caesium and antimony).

Photocathodes of the semiconductor variety, as used in these third generation devices, are by now well known. They are formed by binary compounds of Periodic Table Group III and Group V elements, most commonly gallium (Group III) and arsenic (Group V), and are described (though not always in the context of third generation devices) in a large number of Patent Specifications, including early Varian Specifications such as 1973 Antypas' US Patent No. 3,769,536 (which proposes the idea of using a glass substrate carrying a III-V layer with an optical passivating layer of silicon oxide) and Antypas' 1976 US No. 3,959,045 (which explsins in detail the mechanism of producing such a glass-supported photocathode by a process involviE a temporary substrate and the judicious use of chemical etches).

In essence, the modern III-V photocathode has a supporting glass substrate to which is secured (by a thermal bonding process, and often via silicon oxide/silicon nitride [SiO2/Si3N4] passivating and/or anti-reflection coating layers) a "window" or "high bandgap" layer and, distant from the glass substrate, the photocathode layer of epitaxial gallium arsenide (GaAs) a few hundreds of Angstroms thick. However, in order to ensure the desired crystal nature and alignment of the cathode layer it is usually produced from, as it were, the "other" side.

First, a relatively thick temporary substrate of single crystal GaAs is prepared. On this there is epitaxially grown a thin (around 100O atoms, or about 10,000 Kngstroms 1000 nanometer), thick) "etch stop" layer of gallium aluminium arsenide (GaAlAs), and on top of that is grown the desired thin photocathode layer of epitaxial GaAs.The thin window (high bandgap) layer (usually GaAlAs; about 1000 to 5000 atoms, or 10,000 to 50,000 A (1000 to 50,000 nm), thick) is then grown on that, and the whole multilayer structure - GaAs temporary substrate/GaAlAs etch stop layer/GaAs photocathode layer/GaAlAs window layer - is then thermally bonded, window layer down, onto the glass support via a SiO/Si,Na anti-reflection coating (this latter is usually X of Siena optimised for 7,500 A 750 nm) since the SiO2 bonds/merges into the glass).

Now the support-distant GaAs temporary substrate and GaAlAs etch stop layers are removed, one at a time.

Aqueous ammoniacal hydrogen peroxide etches away the GaAs substrate, down to the GaAlAs etch stop layer, and then aqueous hydrofluoric acid etches away the Galas etch stop layer down to the GaAs cathode layer, which is in this way revealed, securely mounted (via the window layer and passivating/ anti-reflection layers) on the glass support.

Though the specific devices - for example, GaAs devices - described in these early Varian Specifications do in fact work (though not always as well as one might wish), there is little explanation of how the broad concepts there disclosed might be applicable to the use of other lII-V materials. More importantly. there is no discussion of how there might be prepared in this general way a photocathode having a longer wavelength response than exhibited by GaAs. Moreover, the later of the two Specifications glosses over the various functions served by the window layer. Specifically, it does not disclose and explain the following: - a) The window layer acts as a barrier to impurities diffusing out of the glass substrate.

b) The window layer helps to reduce the back surface recombination velocity of electrons at the interface.

c) By virtue of the hetero junction between the window and the photocathode layers, the window layer reduces the probability that crystal dislocations will travel from the glass/semiconductor interface to the photocathode, thereby destroying the electronic properties of the photocathode layer.

d) The window layer can be used as an anti-reflection coating (with or without the conventional SiO2/Si;3No layer) if it has an appropriate thickness.

The present invention proposes solutions to some of the problems posed by the Varian devices. More specifically, it suggests that viable devices, with longer wavelength sensitivity, can be made from other III-V materials, and that dislocations and/or impurities in the glass substrate can be more effectively trapped therein, by the relatively simple expedient of using not single layer single composition etch stop and window layers but instead laminar layers - that is to say, layers which are themselves layered - wherein alternate laminae within a layer have different compositions.

In one aspect, therefore, this invention provides, for use in a method of making a III-V photocathode involving the formation of etch stop and./or window (high bandgap) layers, the step of forming these layers, wholly or partially, as laminar structures having alternate laminae of at least two different epitaxially grown III-V semiconductor materials.

In a second aspect, the invention provides a III-V photocathode made according to the aforesaid method and having its III-V photocathode layer mounted upon a glass support via a window (high bandgap) layer which itself is wholly or partially a laminar structure having alternate laminae of at least two different epitaxially grown III-V semiconductor materials.

Alternating III-V semiconductor material laminar structures, which are conveniently referred to hereinafter as "super lattices", are not in themselves new. They are, for example, described in a Paper entitled "Advanced Long Wavelength Optoelectronic Device Materials" given by H D Scott at the MOVPE Conference in Birmingham, England, on 19th May lob7. However, they have not until now been proposed for use in place wholly or partially of the conventional single layer III-U semiconductor material etch stop and/or window/bandgap layers in third generation photocathodes.

The invention involves the "replacement", either in whole or in part, of the conventional etch stop and/or window layers by super lattice structures, a super lattice being alternating laminae of epitaxially grown III-V semiconductor - that is to say, of a semiconductor made from a compound of at least two elements, one in Periodic Table Group III and the other in Periodic Table Group V. The Group III element may advantageously be aluminium (Al), gallium (Ga) or indium (In), while the Group V element may conveniently be nitrogen (N), phosphorus (P) or arsenic (As). Moreover, the compound may be a mixed alloy, containing three (or more) Ill-V elements - typical examples are those made from Ga, Al and As, or from In, Ga and As.

In a super lattice alternate laminae differ in III-V composition. For example, the laminae might alternate between AlAs and GaAs, or between AlGaAs and InGaAs.

The individual laminae within a super lattice may be of almost any thickness, varying from a single monatomic layer < about 7 A (0.7 nm)) up to several hundred atomic layers (say, 2000 A t200 nm)), but generally from around 5 to 50 atomic layers (about 30 to 300 A t3 to 30 nm)), and the laminae may be of different thickness. For instance, one super lattice may have all of its laminae around 15 A t 1. 5 nm) thick, while another may have one set of laminae thin and the other thick.

Moreover, in order to improve "matching" of, say, the photocathode layer to the semiconductor single crystal temporary substrate it may be desirable to have the thickness of each set of laminae change - for there to be a variable "mark/space ratio" - over the sequence of laminae, preferably in the form of an increase or decrease from one end to the other.Thus, the super lattice might start of with a thin lamina - say, 10 A (1 nm) - of the first material and a thick lamina - say, 190 A (19 nm) - of the second, and have the thickness of each change, pair by pair, in 10 A (1 nm > steps until the final pair has the original thickness reversed thus, a thick (say, 190 A (19 nm)) layer of the first material, and a thin (say, 10 A t1 nm3) lamina of the second material.

The super lattice may be constructed of as many laminae as considered desirable to achieve the required ends. There may be as few as two, but typically there will be at least two or three pairs of laminae, and possibly ten or more - even as many as 50 - in all.

The main advantages of III-V semiconductor photocathodes made in accordance with the invention are: 1) Any dislocations or impurities growing out of the temporary substrate will be held within the relevant super lattice (as is well known, impurities and dislocations are trapped at epitaxial layer boundaries). Moreover, after bonding to the glass (or other transparent support substrate) the relevant super lattice will again arrest impurities and dislocations. As a result, there is attained a higher quantum efficiency and reliability.

2) InGaAs has a different crystal lattice constant to AlAs and GaAs, and cannot normally be grown in single crystal form without a substantial grading layer 10 to 50 micrometre thick. However, the properties of a super lattice, in arresting the propagation of dislocations, enables the crystal grower to make the transition between, say, GaAs and InGaAs within perhaps 0.5 micrometre, depending upon the percentage of indium (the more efficient form of super lattice for this purpose has a variable mark/space ratio). Hence, the super lattice allows there to be made a photocathode with a longer wavelength threshold then pure GaAs, without increasing dislocations or reducing crystal quality.

3) In the conventional third generation photocathode the functions of the window layer and the anti-reflection coating are separated between different materials, generally AlGaAs and silicon nitride respectively. However, the III-V super lattice window layer used in the invention will display a different (lower) optical refractive index to the III-V photocathode material, and if the total thickness of the super lattice is chosen appropriately the window layer itself can be made to help reduce optical reflection in a significant manner. Indeed, with care it is possible to arrange the thickness of each lamina in each pair of laminae so as to minimize optical reflection from the window and maximize optical transmission into the underlying photocathode layer.

For example, a photocathode built with a 16 layer (8+8=16) super lattice window, wherein each of the layer pairs being 15 A t 1. 5 nm) GaAs and 15 A (1.5 nm) AlAs (see Example 3 hereinafter) shows a deep blue colour in reflection of white light, and minimal reflection in the Infra-Red at 800 nm (8000 A), corresponding to a refractive index of about 3. 1 The invention extends, of course, to any device, whenever made using a photocathode itself made in accordance with the invention. Typical such devices are image intensifiers, photomultipliers, and television camera tubes.

Examples of experimental super lattice photocathodes in accordance with the invention are now given, though by way of illustration only.

Example 1: A Gallium Arsenide Photocathode A GaAs photocathode was constructed using the super lattice technique disclosed herein. Both the etch stop and window layers were the same. Each was partially a conventional GaAlAs layer (about 60% Al) but on the photocathode side was an AlAs/GaAs super lattice each lamina of which was about 15 A thick.

A series of successful photocathodes were made this way with super lattices containing from 5 to 50 laminae.

Example 2: An Indium Gallium Arsenide Photocathode An InGaAs photocathode was constructed using the super lattice technique described herein. Both the etch stop layer and the window layer layer were super lattices of the variable mark/space ratio as described hereinbefore, and were constructed of AlAs and In0.31Ga0.9As, giving a long wavelength cutoff of 12,900 A (1290 nm).

Example 3:: A Gallium Arsenide Photocathode As in Example 1, a GaAs photocathode was constructed having a window including an AlAs/GaAs super lattice of 16 laminae (8 pairs each of AlAs and GaAs layers). Each GaAs or AlAs layer was 150 A (15 nm) thick, so giving a total thickness of 640 A (64 nm).

The produced. photocathode showed a deep blue colour in reflection of white light, and minimal reflection in the Infra-Red at 800 nm (8000 A), corresponding to a refractive index of about 3. 1 This experimental photocathode also displayed well above average photo-luminescence (which is usually taken as a reliable indicator of high photocathode efficiency).

Example 4: Another Gallium Arsenide Photocathode As a further refinement of the photocathode of Example 3, a silicon nitride anti-reflection coating was built on top of the super lattice anti-reflection coating therein described. The thicknesses of the two coatings were adjusted so that each individually was effective at individual wavelengths in the required spectral response curve, and so that the sum of the thicknesses of the two provided a XX anti-reflection coating at a third wavelength of interest.

A cathode was built with a super lattice 500 A (50 nm) thick (ten pairs in which each component was 25 A (2.5 nm) thick), giving an optical thickness of 1500 A (150 nm), and thus minimal reflection at 4X1500=6000 A (600 nm). On top of this was grown 1000 A (100 nm) of Si3NX, of refractive index 2.0, hence optical thickness 2XiOO=2000 A (200 nm) and minimal reflectivity at 4x2000=8000 A (800 nm). Simce the total optical thickness of the two coatings was 2000+1500=3500 A (350 nm), there was also a dip in reflectivity at around 4700 A (470 nm) corresponding to the M wavelength effect.

The structure showed lower reflectivity over the spectral range 4000 to 9000 A (400 to 900 nm) than is usually obtained by a simple SiaN coating on top of the conventional homogenous layer of AlGaAs normally employed by the industry.

Claims (16)

1. In a method of making a III-V photocathode involving the formation of etch stop and/or window (high bandgap) layers, the step of forming these layers, wholly or partially, as laminar structures having alternate laminae of at least two different epitaxially grown III-V semiconductor materials.

2. A process as claimed in Claim 1, in which the III-V semiconductor material is one wherein the Group III element is aluminium (Al), gallium (Ga) or indium (In), while the Group V element is nitrogen (N), phosphorus (P) or arsenic (As).

3. A process as claimed in Claim 2, in which the III-V semiconductor material is a mixed alloy, containing three (or more) III-V elements.

4. A process as claimed in Claim 3, in which the III-V semiconductor material is made from Ga, Al and As, or from In, Ga and As.

5. A process as claimed in any of the preceding Claims, in which the laminae alternate between AlAs and GaAs, or between Al GaAs and InGaAs.

6. A process as claimed in any of the preceding Claims, in which the individual laminae vary from a single monatomic layer up to several hundred atomic layers.

7. A process as claimed in any of the preceding Claims, in which the laminae in each set vary in thickness from each other and from those in the other set.

8. A process as claimed in Claim 7, in which the thickness of the laminae in each set of laminae changes over the sequence of laminae in the form of an increase or decrease from one end to the other.

9. A process as claimed in any of Claims 1 to 6, in which the thickness of the laminae in the window layer is such as to minimize optical reflection from the window and maximize optical transmission into the underlying photocathode layer.

10. A process as claimed in any of the preceding Claims, in which there are from two to fifty pairs of laminae.

11. A process as claimed in any cf the preceding Claims and substantially as described hereinbefore.

12. A III-V photocathode whenever prepared by a process as claimed in any of the preceding Claims.

13. A III-V photocathode having its III-V photocathode layer mounted upon a glass support via a window (high bandgap) layer which itself is wholly or partially a laminar structure having alternate laminae of at least two different epitaxially grown III-V semiconductor materials.

14. A photocathode as claimed in Claim 13, wherein the laminar structures are as defined in any of Claims 1 to 12.

15. A III-V photocathode as claimed in either of Claims 13 and 14 and substantially as described hereinbefore.

16. A device whenever made using a photocathode as claimed in any of Claims 12 to 15.

Amendments to the claims have been filed as follows 1. In a method of making a III-V photocathode involving the formation of etch stop and/or window (high bandgap) layers, the step of forming these layer-s, wholly or partially, as laminar structures having alternate laminae of at least two elementally-different epitaxially grown III-V semiconductor materials.