Proximity Focused Image Intensifier with GaAs Photocathode

Proximity Focused Image Intensifier with GaAs Photocathode B. 13. HOIACMAN, 1'. C'. CONDkX arid J. D. SKINUSLEY S . E .R .L.,Baltlock, H e r t ~ . E'nglaitd ,

INTRO 1)FCTION

Kfficient photoemission fi*oornGaAs was first reported by Scheer and van Laar in 19Ii5.' Its potential for use in photosensitive devices was realised by rnany groups of worlsers. While there are many publications 011 the theory and practical results obtained with specimens in the laboratory and on Z)hotomultipliers, little has been reported on imaging devices. This paper will describe an investigation into the technology of incorporating a GaAa photocathode with an active area 18 nini in diameter into an image intensifier. It will also present and ditscuss 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 Dhe performance of GaAs in comparison with S-20 and S-25 photocathodes in image intensifiers. DESIGN O F PROCESSING k$YSTEM AND TCJBE

C'onventional 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 t o construct R proximity focused image intensifier. Such a device has a uniform resolution cal)ahilitp over the whole of the field of view, and in addition should have no distortion. It is not practical. however, t o activate either conventional or CaAH photocathodes tlirougli the narrow gap of I

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B. R. HOLEMAN, P. C. CONDER ANT) J. D . SKIKGSLEY

1 to 2 rnm between photocathode and phosphor which is necessary to obtain satisfactory resolution a t practical operating voltages of B to 1 0 kV. Thus some means of opening the gap during processing is required. One solution is t o use a vacuum assembly technique as has been desin which an cribed by Dolizy et uZ.2 and further developed by Ck~rfield,~ indium seal is used t o 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 ~ 1 . ~ The method described in this paper is to make use of the vacuum assembly technique due to Garfieldt 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 &As. 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 a t alq)rosiniataly 600°C immediately prior to act>ivation; (iii) activation is by surface treatnient with caesium and oxygen; (iv) a working vacuum of lo-' Pa ( 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 coml)onent that is not required when an S.20 photocathode is used. This Rupport cup has only limited heat conduction t o 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 t o obtain the best possible vacuum 1)erformance. 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 a t an angle of 45". 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 N

PROSIMITY FOCUSED INTENSIFIER WITH

GaAs CATHODE

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U. H. IIOIJIMAN, P. C. CONDER A N D J .

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SKINOSLEY

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 pper. The pumping system uses a conventional UHV ion pump with a titanium sublimation pump and sorption forepump. To reduce pumpdown and bakeout times the system is let up to atmospheric pressure using dry nitrogen.

MATERIALCHARACTERISATION AND ACTIVATION The photocathode material is liquid-phase epitaxial &As 0.5 to 1.5 ym thick, grown on liquid-phase epitaxial Ga,All-,As 20 pni thick, which is itself grown on a substrate of Gal? 500 ym thick and 20 mm in

diameter. This structure has been described by Allenfion et aL5 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) memurement of layer thickness using a n optical technique;6 (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 980 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. I n this instance the GaAs is thin enough t o transmit sonie 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 l W 7 Pa after an overnight bake and after firing the titanium sublimation pump. The photocathode is heat cleaned using a Conray quartz iodine 1amp.t The temperature is monitored by a remote infrared radiation pyrometer operating in the wavelength range 1.5 to 2.5 pm. I n order to prevent

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Model 71 Conray lamp, Argus Eng. Co., Hopewell, N.J., U.S.A.

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B. It. IIOLRMAN, P. C'. CONIIEH ANJ) J. D. S K I N G S I 9 Y

direct radiation from thc heating lanip Rffecting the pyronieter rcnding a water tilter is interposed between the lamp and photocathode to restrict the wavelength of the radiation t o less than 1.4 pm. The caesiiim source used is an SAES channel.? Direct caesitation can contaminate the photocathode due to impurities generated by the caesiuin channel. The me of a glass U-bend between the caesiuni channel and the photocathode reduces the risk. After normal outgassing of the clianriel, the caesiuni generated is trapped in the U-bend by cooling this witti an icc bath. Gaseous impurities are not condensed and are pumped away. In addition partial closure of the photocathode to pliosplior 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 liydmulic ram. For the type of seal used (cold indium) a. satisfactory seal is obtained with a force of 1%kN (-1.8 x l O 3 kg).

R II:SUI,TS Processing of GaAs photocathodes in this system was firstt evaluated using reflection photocathodes (liquid-phase epitaxial CaAs on a GaAs substrate). This material gives consistent results in resenrch processing Hystenis. Reflection sensitivities of 1000 to 1100 PA lm-1 were attained, not only using the ion pumped system already described, but also using a mercury diffusion pumpedsystem with two cold traps. However, processing using a n 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 t o 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 traiisniission structure. Nevertheless a number of photocathodes have been activated to photosensitivities between 300 and 400 PA Im -l with fair visual uniformity. Figure 3 shows a picture taken with such a photocathode while the tube was still on the puinp. The behaviour of tubes after seal off has been variable. Some are suspected of having small vacuum leaks, and their sensitivity decreases

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S.A.E.S. Gettors SpA., Via Gallsrate 216, 1-20151 Milano, Italy.

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Time after seal off (days) E’ltr. 4

l’hotocathocli~soiisiti\ritp us

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(if tinlo uftor seal-off.

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B. R . HOLEMAN, P. C. CONDEB AND J . D. SKINGSLEY

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 t o volume is such that the desorption of only 1 monolayer Tom). A number of will result in a pressure rise of about 1 Pa (tubes have shown good stability on shelf life test in the laboratory and

lo3

lo4 Normolised field parameter

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m-1)

FIG.5. Measured limiting rexolntioii of the proxiinity focused system as a function of tho parameter Vlcl-l.

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 Vt d-l. This parameter could only be varied significantly for sample 1, and the data on the other samples is

PROXIMITY FOCUSED INTENSIFIEB WITH

G & hCATHODE

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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. Tho absolute values of resolution are lower than expected and the line drawn on Fig. 5 corresponds to a mean emission energy parallel t o the photocathode plane of 74 meV. This high energy is at present unexplained but inay well be related to the surface topography of the photocathode. I n any proximity tube the high electric field required can result in local field einission from any raised features on the photoctithode. These tubes are no exception and the onset of field emission limits t h e operatting voltage of most tubes made to date. This is essentially a cleanliness problem and must be overcome in order to obtain competitive tnbe performance. CON C‘I, [’S I ONS

The vacuum assembly technique for the construction of proximity focused image tubes can give reliable Tjrocessing 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 c.om1)onent but this system is expected t o be adequiite for the cvdnation of GsAx iix ;I photocattiode in image intensifiers. ~ “ l r ~ o \ ~ ~ , ~ r ) c : n i ~ ~ ~ s

It is u plcasurc, to record our upprccicitroii of tlic part played by thc other mornhers of the tube proecwing team at S.E.R.L., together with the g r w th of the photocathock inateriiil by Mr. M. C. ltowltrrid and Mr. P. E. Oliver and the ttssessiiicnt of this material by Mr. M. B. Allenson and I h C. H. A. Syms. We also nrknowledge tho uscfiil disciissions with mombers of the Light Conversion Division tit E.b:.V. Cii. Ltil, Chdmifortl, m t l Miilltwtl Itewnrch Ld~oratories,Salford, Surrtrl .

h!XERbXCES 1. Bch(+.r, J. .J. nncl vaii Lam, J., Solid State Contmu,i. 3, 189 (19G.5). 2. T)olizy, P. arid Legoiix, R.,Itr “Adv. E.E.P.” Vol. 28A. p. 367 (1969). 3. Garfield, B., E.E.V. Po. Ltd. Cllicbnsford, Privilte Corninunict~tion. 4. Hughes, F.R., Snvngr. E. D. and Thoman. D. L.,J.EZectrott. Muter. 3,9 (1974). 5 . Allenson, M. H,, Kitig, P. 0.R . , R(wIiiiicl. M. C’., Steward, (1.J . arid Syms, U. H. A., J . Phy.~.D.5, L80 (1972). ti. A l l ~ l ~ s M. o ~ B., ~ . to be pL1blidiod.

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R . R. HOLEMAN, P. C. CONDER AND J. D. SKINGISLEY

DISCTJSSION J. D. MCGEE: (1) TR an antircfloction coating uscd on the aluminiuin backing of the screen? (2) Is the lifo of the photocathocle affectJedby operation at full voltage and uorinal light input? 13. R. HOLEWAN: At present mi antireflcct>ion coating on the aluminium backing of the scrcen is not used although samples of screens wit,h this coating have been ohtaiiiccl rmd are currently bcing tested. It, is not yet clear whetJher this antiI'QfleCtiOn coat'ing is desirddo as hhr value of the OTF a t low spatial frequericins has still to be rneaxured on these tubes. Tubes which are st.ablc on life tost (for cxampln the one shoum it1 Fig. 4) are also stablr during opnration for short periocls of time (10 to 20 min) during tosh on an optical bench, and are also st,ablcduring a t,ost lasting 72 h at an input illirrnination of 50 t o 100 mlx, iu each erne with full working voltage applicttl. G. T. REYNOLDS: ( 1 ) 'Have you nicasurctl t.hc spcct,ral respoim fitnct~innof your tubes? (2) What, is the dark riirrent, prcfcmbly in electrons cin-2 scc-I. H. IC. HOLEMAN: The spoctwl rwpoiisc of tlicso tubcs hm not, boon incnwred directly. H ( J W O V ident'ical C~ pliotocathodes liavc beon processed in UHV chambers at SI.:itL nncl tlieso show the cspected spectral response for t,his structnrc, RS is shown in Ref 5. The dark curreiit, in a tube tias not been meusured. as i t would be a difficult if not iinp(issIkbh?expcrirnent in a proximity diodr at the levels of dark current oxpcctod. T11cr l w k oiirrent of 11 GILASphotocathodo has l)rcn separat,ely dettvmined nritl is 10-15 A cm-2 (G x lO3 electrons cIn-~2sec-1) or less. M. JPI)LICKA: Wlmt can yoii say abont intufacia1 racornbiimhion velocit,y? 13. 11. HOLEMAN: 'rho int,crface recombination velocity at thc (J~,AI~-,As/GLLAs iiit,crrfttoo is low crioitgh in tliis structure (Jess than 1 0 5 cin2 sec-1) for i t not to affect tlie spect,ral response or w1iit.o light sensitivity.