Photo-electronic image devices : proceedings of the sixth symposium held at Imperial College, London, September 9-13, 1974

Chapter One Proximity Focused Image Intensifier with GaAs Photocathode B. R. HOLEMAN, P. C. CONDER and J. D. SKINGSLEY S.E.R.L, Baldock, Herts., England Introduction Efficient photoemission from GaAs was first reported by Scheer and van Laar in 1965. Its potential for use in photosensitive devices was realised by many groups of workers. While there are many publications 011 the theory and practical results obtained with specimens in the laboratory and on photomultipliers, little has been reported on imaging devices. This paper will describe an investigation into the technology of incorporating a GaAs photocathode with an active area 18 mm in diameter into an image intensifier. It will also present and discuss the initial results obtained. The tube described in this paper is intended to be a test vehicle for this type of photocathode and will be used to evaluate the performance of GaAs in comparison with S-20 and S-25 photocathodes in image intensifiers. Design of Processing System and Tube Conventional photocathodes such as the 5.20 and Y.25 are prepared by a process of evaporation and chemical treatment and result in a polycrystalline film that can be formed on flat or curved surfaces. A GaAs photocathode on the other hand is grown by epitaxial techniques on a suitable flat single crystal substrate. A flat photocathode gives good resolution on the axis of an inverting image intensifier, but the resolution falls off rapidly towards the edge of the field of view. Adequate electron optical performance in inverting image intensifiers therefore requires a curved photocathode. An alternative is to construct R proximity focused image intensifier. Such a device has a uniform resolution capability over the whole of the field of view, and in addition should have no distortion. It is not practical. However, to activate either conventional or GaAs photocathodes through the narrow gap of 1 to 2 mm between photocathode and phosphor which is necessary to obtain satisfactory resolution at practical operating voltages of B to 10 kV. Thus some means of opening the gap during processing is required. One solution is to use a vacuum assembly technique as has been described by Dolizy et al. and further developed by Garfield, in which an indium seal is used to complete the tube vacuum envelope after activation of the photocathode. An alternative solution is to use a flexible tube body as described by Hughes et al. The method described in this paper is to make use of the vacuum assembly technique due to Garfield and to develop it specifically for processing semiconductor photocathodes. The tube and the apparatus were designed from the outset for the particular processing requirements of GaAs. The most important of these are: (i) the photocathode is a thin disc of single crystal material; (ii) the photocathode requires a vacuum heat clean at approximately 6000C immediately prior to activation; (iii) activation is by surface treatment with caesium and oxygen; (iv) a working vacuum of [10.sup.-7] Pa (~ [10.sup.-9] Torr) or better is required; (v) hydrocarbon and oxygen partial pressures must be below detectable limits. Figure 1 shows the design of the proximity tube and the processing chamber. The tube includes a photocathode support cup, a component that is not required when an S.20 photocathode is used. This support cup has only limited heat conduction to the body of the tube thereby allowing the required heat clean temperature to be attained by radiant heating. The processing chamber incorporates bellows 80 that the two halves of the tube can be separated for activation of the photocathode. After activation, the two halves of the tube can be sealed together and the tube extracted from the processing chamber. Care was taken in selecting materials and constructional techniques to obtain the best possible vacuum performance. For instance the chamber has only internal welds, or high temperature brazed joints, and its internal surface area is kept to a minimum by making the chamber small and by using the seamless bellows a8 the inner vacuum wall. The phosphor can be viewed through the hollow thrust tube. A number of ports provide line of sight viewing of the photocathode at an angle of 450. Two of these are used as inlets for caesium and oxygen respectively while the others provide optical viewing of the photocathode. Past experience of activating GaAs in glass test cells had indicated that reliable activation could be obtained with radiant heating of the photocathode, temperature monitoring using an optical pyrometer, oxygen admission by leak valve and caesiation by distillation from a channel. These techniques are used in the activation of the proximity tube described in this paper. The pumping system uses a conventional UHV ion pump with a titanium sublimation pump and sorption forepump. To reduce pump-down and bakeout times the system is let up to atmospheric pressure using dry nitrogen. Material Characterisation and Activation The photocathode material is liquid-phase epitaxial GaAs 0.5 to 1.5 5m thick, grown on liquid-phase epitaxial [Ga.sub.x][Al.sub.1-x]As 20 5m thick, which is itself grown on a substrate of GaP 500 5m thick and 20 mm in diameter. This structure has been described by Allenson et al. These photocathodes are inspected before use by a variety of non-destructive techniques, the most important being: (i) visual inspection of surface quality using reflected light; (ii) measurement of layer thickness using an optical technique; (iii) visual inspection of layer uniformity using transmitted light of various wavelengths together with an image converter where necessary. This includes wavelengths of 550 nm for detecting pinholes, 780 nm or white light for determining GaAs layer thickness, uniformity and 950 nm for assessing optical quality. Figure 2 shows some typical results of this inspection technique: Fig 2(a) is a dark field reflection picture in white light and shows some surface imperfections, which are mainly point and line defects with a region around the edge with a higher defect density. Figure 2(b) shows a white light transmission picture. In this instance the GaAs is thin enough to transmit some visible light so that this picture reveals any variation in the thickness of the GaAs layer. Before being placed in the processing system all components go through a rigorous cleaning schedule which, with the exception of the photocathode, includes a vacuum outgas. The pumping system used gives a working pressure of [10.sup.-7] Pa after an overnight bake and after firing the titanium sublimation pump. The photocathode is heat cleaned using a Conray quartz iodine lamp. The temperature is monitored by a remote infrared radiation pyrometer operating in the wavelength range 1.5 to 2.5 5m. In order to prevent direct radiation from the heating lamp affecting the pyrometer reading a water tilter is interposed between the lamp and photocathode to restrict the wavelength of the radiation to less than 1.4 5m. The caesium source used is an SAES channel. Direct caesitation can contaminate the photocathode due to impurities generated by the caesium channel. The me of a glass U-bend between the caesium channel and the photocathode reduces the risk. After normal outgassing of the channel, the caesium generated is trapped in the U-bend by cooling this with an ice bath. Gaseous impurities are not condensed and are pumped away. In addition partial closure of the photocathode to phosphor gap in this operation screens the photocathode from these contaminants, The photocathode is then given its final heat clean and the subsequent caesiation is controlled by gentle warming of the IT-bend. Activation is by cyclic application of caesium and oxygen until the white light sensitivity reaches a maximum. After activation the two halves of the tube are sealed together using an external removable hydraulic ram. For the type of seal used (cold indium) a satisfactory seal is obtained with a force of 1% kN (~1.8 x [10.sup.3] kg). Results Processing of GaAs photocathodes in this system was first evaluated using reflection photocathodes (liquid-phase epitaxial GaAs on a GaAs substrate). This material gives consistent results in research processing systems. Reflection sensitivities of 1000 to 1100 5A [lm.sup.-1] were attained, not only using the ion pumped system already described, but also using a mercury diffusion pumped system with two cold traps. However, processing using an ion pumped vacuum system was easier and more consistent so this method has been used subsequently. For transmission photocathodes, photosensitivities are again comparable with those attained in research processing systems. A transmission photocathode is more sensitive to parameters such as layer thickness and electron diffusion length than a reflection photocathode, and in addition these parameters are more difficult to control in the growth of a transmission structure. Nevertheless a number of photocathodes have been activated to photosensitivities between 300 and 400 5A [lm.sup.-1] with fair visual uniformity. Figure 3 shows a picture taken with such a photocathode while the tube was still on the pump. The behaviour of tubes after seal off has been variable. Some are suspected of having small vacuum leaks, and their sensitivity decreases steadily with time. It is also possible that outgassing of tube components after seal off is a factor in this lack of stability. The ratio of internal surface area to volume is such that the desorption of only 1 monolayer will result in a pressure rise of about 1 Pa (~ [10.sup.-2] Torr). A number of tubes have shown good stability on shelf life test in the laboratory and have also been stable during operation on an optical bench. The sensitivity on shelf life test was monitored a t intervals by applying a relatively low voltage of 60 to 70 V a t a light level of 0.05 to 0.1 lx. Figure 4 shows the results of this measurement for a stable tube. The resolution of the proximity structure has been measured for a variety of values of both phosphor to cathode separation, d , and applied voltage, V . Figure 5 shows a plot of the observed limiting resolution against the usual parameter of [ V .sup.1/2] [ d .sup.-1]. This parameter could only be varied significantly for sample 1, and the data on the other samples is included to show the consistency of the results obtained. If the photoelectrons have a finite mean emission velocity parallel to the photocathode plane, and this limits the attainable resolution, then this plot will be a straight line with a slope of unity. This is seen to be the case for these photocathodes. The absolute values of resolution are lower than expected and the line drawn on Fig. 5 corresponds to a mean emission energy parallel to the photocathode plane of 74 meV. This high energy is at present unexplained but may well be related to the surface topography of the photocathode. In any proximity tube the high electric field required can result in local field emission from any raised features on the photocathode. These tubes are no exception and the onset of field emission limits the operating voltage of most tubes made to date. This is essentially a cleanliness problem and must be overcome in order to obtain competitive tube performance. Conclusions The vacuum assembly technique for the construction of proximity focused image tubes can give reliable processing of transmission GaAs photocathodes a t high photosensitivities. It is too early as yet to say whether the detailed performance and life of these tubes is acce1)table for an optical component but this system is expected to be adequate for the evaluation of GaAs as a photocathode in image intensifiers. Chapter Two Developments in Proximity Focused Diode Image Intensifiers B. R. C. GARFIELD, R. J. P. WILSON, J. H. GOODSON and D. J. BUTLER English Electric Valve Company Ltd., Chelmsford, Essex, England Introduction A proximity focused diode is an image intensifier in which a plane photocathode is situated parallel to, and closely spaced from, a plane phosphor screen. Electron imaging is effected by the direct transfer of photoelectrons across the narrow gap between the photocathode and the phosphor under the action of the applied accelerating field. No additional electron focusing is required and the hi-planar structure results in a device which is compact, operates with zero geometrical image distortion and has complete uniformity of resolution over the entire image area. The resolution of a proximity diode is proportional to [ V .sup.1/2][ d .sup.-1] where V is the applied voltage and d is the photocathode to phosphor gap. For useful resolution and gain a voltage of about 10 kV applied across a gap of less than 2 mm is necessary. These criteria impose problems with regard to tube assembly and processing which do not exist with conventional intensifiers and i t is the failure to solve these problems which has in the past prevented the development of the proximity diode into it practical device. The situation has now changed following recent developments in proximity diodes at English Electric Valve Company, particularly in the areas of phosphor screen preparation and vacuum transfer processing methods. L'rac4aal devices have been realised with performance comparable to intensifiers of the crossover focused type. Tube Construction Work on proximity diodes having 18, 25, 40 and 75 1mn useful diameters is in progress. Tubes are of rugged construction and utilise a brazed ceramic to metal envelope. Input and output faceplates (normally fibre optic) are 'Pyroceram' sealed to metal flanges, which in turn are argon arc welded to the tube envelope. All tubes are processed by the vacuum transfer method as described in the next section and are vacuum sealed by pressing an indium gasket between mating vee groove flanges. S?25 photocathodes and P.20 phosphors are standard. The phosphor screens used can withstand operation a t field stresses in excess of 10 kV[mm.sup.-1] without incurring damage to their structure. An encapsulated 40 mm proximity diode is shown in Fig. 1. Tube Processing In the past, the vacuum transfer technique has usually been considered to be a complicated and expensive method of tube processing which has been known to give tubes with poor operational life characteristics. In one well reported transfer method, the two tube sections, one incorporating the photocathode faceplate and the other the anode, are mounted in separate units which are housed in u large vacuum chamber. Such large volume systems suffer from the inherent disadvantages of being complex and expensive, and require long processing times. As such they are unsuitable for the manufacture of tubes on an economic production basis. A new transfer method in which the disadvantages of the previous methods have been overcome has been developed. This method utilises small, readily portable, transfer modules. The module used for processing 18 and 25 mm proximity diodes is shown in Fig. 2. A similar module is used for 40 mm diodes. The anode section of the proximity diode is located in the integral bellows assembly while the photocathode plate is located in the module lid. In this position the two tube sections are spaced several centimetres apart. The lid is sealed to the transfer module by a gold 'O' ring seal. After vacuum bakeout at 350 to 4000C the transfer module is sealed off from the outgassing vacuum system and removed to a photocathode processing unit. Pumping of the module a t a pressure of about [10.sup.-8] Torr is continued by means of a miniature ion pump. (Continues...) Excerpted from PHOTO-ELECTRONIC IMAGE DEVICES Volume 40A Copyright © 1976 by ACADEMIC PRESS INC. (LONDON) LTD.. Excerpted by permission. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher. Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site. Content: Editorial board Page ii Edited by Page iii Copyright page Page iv List of Contributors Pages v-xi Preface Page xiii B.L. Morgan, D. Mcmullan, R.W. Atrey Abbreviations Page xiv Proximity Focused Image Intensifier with GaAs Photocathode Original Research Article Pages 1-10 B.R. Holeman, P.C. Conder, J.D. Skingsley Developments in Proximity Focused Diode Image Intensifiers Original Research Article Pages 11-20 B.R.C. Garfield, R.J.F. Wilson, J.H. Goodson, D.J. Butler Low Noise Proximity Focused Image Intensifiers Original Research Article Pages 21-31 H. Pollehn, J. Bratton, R. Feingold A Colour Image Intensifier System for Night Vision Original Research Article Pages 33-39 H. Mulder Improvements to an Image Intensifier for a gamma-ray Scintillation Camera Original Research Article Pages 41-49 B. Driard, G. Roziere, L.F. Guyot, M. Verat Photochron II: An Image Tube for Sub-picosecond Chronography Original Research Article Pages 51-58 P.R. Bird, D.J. Bradley, W. Sibbet Grid Shuttered Image Converter Tube in Nanosecond Operating Mode Original Research Article Pages 59-67 L. Diamant The Oblique Image Converter Original Research Article Pages 69-82 C.B. Johnson, K.L. Hallam Image Intensifier Tubes with New Very Simple Electron Optics Original Research Article Pages 83-89 R. Evrard Some Applications of MicroChannel Plates to Electronic Imaging Devices Original Research Article Pages 91-102 G.R. Carruthers, J. Kervitsky, C.B. Opal A Microchannel Plate with Curved Channels: An Improvement in Gain, Relative Variance and Ion Noise for Channel Plate Tubes Original Research Article Pages 103-111 J.P. Boutot, G. Eschard, R. Polaert, V. Duchenois Space Charge in Channel Multipliers Original Research Article Pages 113-122 W. Baumgartner, B. Gilliard Impulse and Frequency Response of Channel Electron Multipliers Original Research Article Pages 123-139 K. Oba, H. Maeda Signal to Noise and Collection Efficiency Measurements in MicroChannel Wafer Image Intensifies Original Research Article Pages 141-152 G. Eschard, J. Graf, R. Polaert Secondary Electron Emission and Compositional Studies on Channel Plate Glass Surfaces Original Research Article Pages 153-165 G.E. Hill Changes in Secondary Electron Yield from Reduced Lead Glasses Original Research Article Pages 167-181 A. Authinarayanan, R.W. Dudding A Small High-Precision Electrostatic Pick-up Tube Original Research Article Page 183 W.M. Van Alphend TV Camera Tube with a Gallium Arsenide Target Original Research Article Pages 185-199 H. Rougeot Integrating Ultraviolet Sensitive Camera Tube Original Research Article Pages 201-208 Y. Beauvais, M. Blamoutier A Large Diameter X-ray Sensitive Vidicon with Beryllium Window Original Research Article Pages 209-221 Y. Suzuki, K. Uchiyama, M. Ito Properties of the Detector System for the International Ultraviolet Explorer Satellite Original Research Article Pages 223-237 K.G.K. Allen, B.E. Anderson, A. Boksenberg, M.B. Oliver Photometric Statistical Performance of the SEC Target Original Research Article Pages 239-252 P. Zucchino Characteristics of an Optically Scanned SEC Device Original Research Article Pages 253-262 A. Choudry Measurements of Point Source Sensitivity of Three High Gain Camera Tubes Original Research Article Pages 263-277 E.W. Rork, M.R. St. John, P.L. Manly, K.E. Kissell An Electron Beam Readout Technique Original Research Article Pages 279-285 R.E. Rutherford Jr. High Resolution Electron Microscope Imaging with Silicon Diode Array Target Vidicons Original Research Article Pages 287-300 H. Alsberg, R.E. Hartman Pyroelectric Materials for Operation in a Hard Vacuum Pyroelectric Vidicon Original Research Article Pages 301-312 R. Watton, G.R. Jones, C. Smith Thermal Diffusion Limitations of the Resolution of a Pyroelectric Vidicon Original Research Article Pages 313-322 A.L. Harmer, W.M. Wreathall Some Properties of Evaporated and Sprayed CdSe Layers for Heterojunction Vidicon Targets Original Research Article Pages 323-333 M. Jedlicka, R. Ladman, O. Vitovský, D. Lezal, I. Srb A Uniform CdS-CdTe-As 2 Se 3 Heterojunction Target for TV Camera Tubes Original Research Article Pages 335-348 M. Nogami, S. Okamoto, H. Nishida Antimony Trisulfide Heterojunction Vidicon Structures Original Research Article Pages 349-364 C.R. Wronski, A.D. Cope Near Infrared Camera Tube Studies with an Ag 2 S Target Original Research Article Pages 365-375 H. Roehrig, P. Aceto, S. Mardix, P.M. Mcilvaine, S. Nudelman Gallium Indium Arsenide Photocathodes Original Research Article Pages 377-385 C. Piaget, R. Polaert, J.C. Richard Thermionic Emission from Negative Electron Affinity Silicon Original Research Article Pages 387-396 J.R. Howorth, R. Holtom, C.J.R. Sheppard, E.W.L. Trawny Quantum Yield of Cs 3 Sb Photocathodes as a Function of Thickness and Angle of Incidence Original Research Article Pages 397-408 W. Greschat, H. Heinrich, P. Römer Photoelectronic Device Development and Related Research at B.A.R.C. Original Research Article Pages 409-418 T.B. Bhatia, G.K. Bhide, C. Ghosh, G.N. Kelkar, M. Srinivasan, B.P. Varma, R.L. Verma A Simple Photocathode Transfer system Original Research Article Pages 419-425 C.F. Van Huyssteen Residual Gases and the Stability of Photocathodes Original Research Article Pages 427-439 D. Mcmullan, J.R. Powell The Effects of High Electric Fields on Photocathodes Original Research Article Pages 441-448 J.A. Cochrane, R.F. Thumwood Wavelength Dependent Resolution in the Far Ultraviolet for Proximity Focused Imaging from a Caesium Teluride Photocathode Original Research Article Page 449 K.G.R. Allen, B.R. Anderson, A. Boksenberg, D.G. Ross S.1 Photocathode Response Linearity and Dynamic Range with Picosecond 1.06 μm Laser Pulses Original Research Article Pages 451-462 S.W. Thomas, G.R. Tripp, L.W. Coleman The Negative Electron Affinity GaAsP Cold Cathode Silicon Vidicon Original Research Article Pages 463-472 J.R. Howorth, R.K. Surridge, I.C. Palmer Some Problems of Electron Optical System Design Using a Computer Original Research Article Pages 473-483 V. Jareà, B. Novotný A Method for Efficient Numeric Computation of Axially Symmetric Electrostatic Fields in Image Tubes Original Research Article Pages 485-491 A.G. Du Toit An Image Tube for Experimental Electron Optics Original Research Article Pages 493-505 K.F. Hartley, D. Mcmullan Asymmetrical Astigmatism of X-Ray Image Intensifies Original Research Article Pages 507-517 F.W. Lange, S. Schweda