GB2081967A - Intagliated photocathode - Google Patents

Abstract

Crystalline photocathodes utilising negative electron affinity, typically activated GaAs, cannot be laid down on the curved substrates used in image inverting intensifier tubes since the crystal structure cannot be maintained across such a substrate. To overcome this, where a plano- concave fibre optic input window is used, the cores 6 and cladding 7 of the fibres on the concave surface are intagliated 10, 11 by selective etching, the crystalline photoemitter 9 being laid down by a vapour phase growth technique and separately recrystallised by annealing in each intagliation. <IMAGE>

Description

SPECIFICATION Intagliated photocathode This invention relates to electron emissive photocathodes and more particularly to photocathodes comprising a crystalline photoemissive material on the inside surface of a fibre optic window of an evacuated electron imaging tube.

Photocathodes comprising a crystalline layer of a semiconductor material having the electron emitting surface activated to a state of negative electron affinity are well known in the art and are reviewed in an article by Martinelli and Fisher in Procedings of the IEEE Vol. 62 No. 10 1974 entitled "The Application of Semiconductors with Negative Electron Affinity Surfaces to Electron Emission Devices". On page 1351 of this article, the problem of incorporating such photocathodes in imaging tubes having curved photocathode surfaces is mentioned and it is concluded that growing thin single crystal photocathodes on large area curved transparent substrates is impractical at present.

It is an object of the present invention to provide negative electron affinity photocathodes on curved fibre optic surfaces. It is another object of the invention to provide such photocathodes on extended area planar fibre optic surfaces.

The invention provides a photocathode comprising a crystalline photoemissive material on one surface of a fibre optic window comprising a fused bundle of optical fibres each of which comprises a core surrounded by a cladding, said surface being transverse to the fibre lengths and being intagliated so that the end faces of the cores of the fibres lie in a first surface and the end faces of the cladding of the fibres lie in a second surface separated from and substantially parallel to the first surface, wherein the crystalline photoemissive material on the end of each fibre has a crystal orientation relative to the fibre end face which is substantially the same on all fibre ends across the photocathode.

The intagliations provide sites for the nucleation of the crystalline material. At least two possibilities exist for the intagliation of the surface of a fibre optic window. In one possibility, the first surface lies above the second surface so that the intagliations comprise pedestals of the core material, the crystal orientation being relative to the core end face. In another possibility, the second surface lies above the first surface so that the intagliations comprise cavities the bottoms of which comprise the core end faces, the crystal orientation being relative to the core end face.

The crystalline material may be deposited on the intagliations from a suspension of ground crystallites or may be deposited from a molecular beam in vacuum or by another vapour phase growth technique. In each case, laser, electron beam or thermal annealing may be employed to induce separate crystal growth on each intagliation.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 shows an axial section of a typical fibre optic window to which the invention may be applied, Figure 2, a and b show detail sections of a fibre optic window, with photocathode, at the axis and near the periphery of the window respectively, showing a first form of intagliation, and Figure 3, a and b show corresponding detail sections, showing a second form of intagli ation.

Fig. 1 shows an axial section of an input fibre optic window 1 for use in an image tube having an electron optical system of the image-inverting diode type. Such an intensifier tube is described in British Patent Specification No. 1,1 75,597. The window consists of a fused bundle of fibres 2, each of which has a high refractive index core surrounded by a low refractive index cladding. The fibres are parallel to one another across the diameter D of the window and are normal to the flat input face 3. The output face 4 is spherical, having a radius of curvature R, and is concentric with the spherical surface of the tube output phosphor screen. An optical image focused on face 3 is dissected by the fibres and transferred without further resolution loss to the output face 4 carrying the photocathode 5.Near the axis of the tube, the faces 3 and 4 are parallel to one another and hence the fibres terminate on face 4 substantially normally. Off-axis, however, the fibres terminate at an angle to face 4 which increases with the distance of the fibre from the axis. Figs. 2a and 2b show axial and off-axis fibres respectively terminating on the output face 4. Typically, D = 50 mm and R = 34 mm, whence the maximum off-axis angle may be close to 45 .

Using conventional photocathodes, for example semiconductors formed by the reaction of alkaline earth metals with antimony, the output face 4 may be an optically polished spherical surface since the efficiency of such photocathodes is not dependent on the crystal structure of the semiconductor remaining defect free and constant with respect to the output face all across the window. However, if a photocathode utilizing negative electron affinity is to be employed, the crystal structure of the semiconductor must be much more defect free and the electron emitting surface should be of constant crystal orientation all across the window. This latter condition is clearly impossible on a curved surface.This problem and others connected with growing semiconductor crystal photocathodes on fibre optic windows, for example from Ill-V materi als, are mentioned on page 1351 of a review article by Martinelli and Fisher in Proceedings of the IEEE Vol. 62 No. 10 entitled "The Application of Semiconductors with Negative Electron Affinity Surfaces to Electron Emission Devices''.

Figs. 2a and 2b show sections of a fibre optic window, at the axis and near the periphery of the window respectively, in which the surface 4 of the window has been intagliated, for example by differential etching of the cores 6 and the cladding 7. A layer of transparent crystalline semiconducting material 8, for example GaP, is deposited into the holes 10 and over the walls 11 to form an interface layer between the glass of the fibre optic and the layer 9 of N.E.A. activated photoemitter, for example GaAs activated with a monolayer of Cs or Cs and 02. As is known, a layer may be employed between layers 8 and 9, for example of GaxAl, xAs or Ga0.51 nO 5P, to provide a crystal matching layer.

The walls 11 between the holes 10 provide a region in which a break in the continuity of the crystal structure of both layers 8 and 9 can occur, allowing the layers in the hole to adopt a crystal orientation relative to the base of the hole 12, the fibre core end face, which is the same near the axis (Fig. 2a) as it is near the periphery of the window (Fig. 2b).

Figs. 3a and 3b show an alternative form of intagliation in which the walls 7 have been etched back to provide pedestals 13 on the ends of the cores 6. The trenches 14 around the pedestals provide the region of crystal discontinuity between the electron emissive facets 15.

The lower refractive index cladding glass approximates in composition a conventional laboratory resistance glass, typically having some 68% of SiO2 and is substantially resistance to attack by most acids. The higher refractive index core glass contains typically only 16% of SiO2 with higher percentages of Lanthanum, Tantalum and Barium oxides.

Holes 2 ,um deep can be produced in fibre optic plates having 4 5 pm square section cores and 1 ,um thick walls by boiling in a constant boiling mixture of hydrochloric acid and distilled water for 1-5 hours at atmospheric pressure. Pedestals about 1 ,um high can be produced by immersion in a 50/50 mixture of buffered hydrofluoric acid and hydrogen peroxide for + hour at room temperature.

The semiconducting crystalline material may be applied to the holes or the pedestals as a suspension of ground crystallites, less than 4 pm in size, in a dilute solution of polyvinyl alcohol (PVA) in water. The crystallites are spun down onto the fibre optic face, which is subsequently baked at 400"C in air to drive off the PVA. Laser annealing may then be employed to fuse the crystallites and to induce localized crystal growth. Typically a RUBY laser is pulsed for 25 to 30 nsec at a power density on the fibre optic surface of 0-7 Joules per cm2. For extended photocathodes the laser beam or the sample may be scanned, with the scan rate chose in relation to the beam size and the interpulse delay so that all parts of the photocathode are annealed, as described by Albert R.Tebo in the article "Laser Annealing-and Beyond" on pages 29 to 37 of the journal ''Electro-Optical Systems Design", December 1979. A helical scan pattern may be used on a circular photocathode.

Alternatively, the semiconducting material may be applied to the holes or the pedestals as a molecular beam in vacuum or by another vapour phase growth technique producing an amorphous or polycrystalline layer which is subsequently laser annealed to induce crystal growth.

The invention has been described in relation to curved photocathodes, but it is equally applicable to planar photocathodes on fibre optic windows. Extended area crystalline photocathodes are more difficult to produce since the crystal orientation must be preserved across the whole surface. Since with the intagliated photocathodes of the present invention the crystal orientation on each intagliation is established separately, this limitation on the area of planar photocathodes can be removed.

Claims (10)

1. A photocathode comprising a crystalline photoemissive material on one surface of a fibre optic window comprising a fused bundle of optical fibres each of which comprises a core surrounded by a cladding, said surface being transverse to the fibre lengths and being intagliated so that the end faces of the cores of the fibres lie in a first surface and the end surfaces of the cladding of the fibres lie in a second surface separated from and substantially parallel to the first surface, wherein the crystalline photoemissive material on the end of each fibre has a crystal orientation relative to the fibre end face which is substantially the same on all fibre ends across the photocathode.

2. A photocathode as claimed in Claim 1 wherein the electron emitting surface of said crystalline photoemissive material is activated to a state of negative electron affinity.

3. A photocathode as claimed in Claim 1 or Claim 2 wherein said first surface lies above said second surface whereby the intagliations comprise pedestals of core material, and wherein the crystal orientation is relative to the core erid faces.

4. A photocathode as claimed in Claim 1 or Claim 2 wherein said second surface lies above said first surface whereby the intagliations comprise cavities the bottoms of which comprise the core end faces, and wherein the crystal orientation is relative to the core end faces.

5. A method of preparing a photocathode as claimed in any one of the preceding claims comprising the steps of depositing the crystalline photoemissive material on the intagliations from a suspension of ground crystallites and subsequently annealing the crystallites to induce localised crystal growth.

6. A method of preparing a photocathode as claimed in any one of Claims 1 to 4 inclusive comprising the steps of depositing the photoemissive material on the intagliations by a vapour phase growth technique and subsequently annealing the material to induce localised crystal growth.

7. A photocathode substantially as described with reference to Figs. 1, 2a and 2b of the accompanying drawing.

8. A photocathode substantially as described with reference to Figs. 1, 3a and 3b of the accompanying drawing.

9. A method of preparing a photocathode substantially as described with reference to Figs. 2a and 2b of the accompanying drawing.

10. A method of preparing a photocathode substantially as described with reference to Figs. 3a and 3b of the accompanying drawing.