Photocathodes-contemporary state and trends

Vacuum/volume36/numbers 7-9/pages 515 to 521/1986

0042-207X/86S3.00+ .00 Pergamon Journals Ltd

Printed in Great Britain

Photocathodes

c o n t e m p o r a r y state and trends

M J e d l i E k a a n d P K u l h & n e k , TESLA-Vacuum Engineering, U Vizerky 2308 164 O0 Praha 6, Czechoslovakia

The basic physical aspects of photoelectric emission from photocathodes and problems connected with the excitation, transport and escape of photoelectrons into vacuum are discussed. Further, the optical properties of the photoelectric emitter, conditions for an effective photoemission and the influence of electron affinity are mentioned. Some characteristics and physical models are presented for photoelectric emission from different types of photocathodes: the silver-oxygen-caesium photocathode, alkafi antimonide photocathodes, negativeelectron-affinity photocathodes and transferred-electron photocathodes. The significant features of modern photocathodes for application in various electron tubes are stressed.

1. Introduction

The process of photoelectric emission of electrons from a semiconductor into vacuum involves the following three steps: (i) absorption of incident photons and transfer of their energy to electrons (excitation of photoelectrons); (ii) movement of photoelectrons from the place of their excitation to the emissive photocathode-vacuum interface; (iii) passing of the photoelectrons through the interface into the vacuum. For making efficient photocathodes, the highest obtainable efficiency is required in each of the three stages. In an ideal photocathode, each incident photon should excite an electron capable of emission to provide a quantum yield of 100%. In reality, the quantum yield never exceeds 50% and even such maximum values are obtainable only in a narrow wavelength band. The quantum yield falls off towards longer and shorter wavelengths as a result of unavoidable losses in all three of the above-mentioned processes. To ensure maximum exploitation of the incident radiation, the photocathode should have a high absorption coefficient since the losses due to radiation passing through the cathode can be reduced only by increasing the layer thickness. This, however, leads to increased losses of excited photoelectrons on their way to the emitter-vacuum interface. Low optical reflectivity of the photocathode is another important condition for the efficient transfer of energy from the impinging photon to the photoelectron. The transport of photoelectrons from the place of their creation to the emission surface represents the second step of the photoemission process. If the quantum energy of the incident radiation is less than 2-3 times the width of the semiconductor gap (by < 2 - 3 Eg), the mean free path will be some tenths of a nanometer and the energy losses will be several electron-volts. The dominant loss mechanism of 'hot' electrons in ultra-pure semiconducting monocrystals is inelastic collisions with the crystal lattice. In polycrystalline materials, additional losses occur due to scattering from lattice defects, grain boundaries, impurity centres, etc. The energy losses accompanying the movement of excited photoelectrons towards the emission surface influence to a great

extent the probability of their penetration through the cathodevacuum interface. An electron which has gained sufficient energy for emission from the semiconductor into the vacuum as a result of interaction with a photon, phonon or another electron, has still to overcome the potential barrier at the emitter-vacuum interface. In semiconductors, the magnitude of this barrier is determined by the value of the electron affinity, i.e. the energy difference between the bottom of the conduction band and the energy level of the vacuum. Under normal conditions, the electron affinity of the emissive material is positive and the potential barrier will be the factor limiting the emission of those electrons which have insufficient kinetic energy (Figure l(a)). The photoelectrons with energy exceeding the value of the electron affinity Ea, have a large emission probability. This, however, is valid without exception only for photoelectrons, whose trajectories are normal to the cathode-vacuum interface. For electrons approaching the interface at a large angle to the normal, the probability of emission is less. This is mostly true in

vOC

I

0

(a)

E¢- ~ c (b)

Figure 1. Energy band model of semiconducting photocathode. Ev--top of valence band; Ep--Ferrni level; Ec--bottom of conduction band; Eg--bandgap; E=-----electronaffinity; E,,c-----energy level of vacuum; hv-~luantum energy of incident radiation. (a) Positive electron affinity. (b) Negative electron affinity. 515

M Jedli6ka and P Kulh6nek. Photocathodes-~contemporary state and trends

cases where the energy of photoelectrons is close to the magnitude of the electron affinity. In certain strongly p-type doped semiconductors it is possible to reduce the electron affinity to zero or even to a negative value, by bending the energy bands near the surface (such cathodes are called NEA (negative electron affinity) photocathodes). This is achieved by applying sophisticated fabrication techniques involving the alternate deposition of thin films of electro-positive and electro-negative materials. By this method, a considerably higher emission probability is obtained for electrons with energies approaching the bottom of the conduction band E+; these electrons would not be emitted in the case of a positive electron affinity (Figure l(b)). According to Lif~ic et al ~, multiple reflections of the photoelectrons occur at the emitter-vacuum interface as well as multiple transition through the space charge region where they can interact with phonons and lose part of their energy. Thus, the probability of photoelectrons escaping from NEA photocathodes rises with increasing negative affinity. Figure 2 shows the emission probability of photoelectrons against the value of the electron affinity for some photocathodes of AraB v compounds. The emission probability is less than 10 2

alkali antimonides) with positive electron affinity is of the order of several tens of nanometers. With NEA-photocathodes, the diffusion length and also the emission depth is 1 ~tm and more. Therefore, the thickness of these emitters is comparable to the diffusion length of electrons. Metals are characterized by a high optical reflectivity (80-100%). Photoelectrons created in metals are especially susceptible to transport-losses because of the high probability of collisions with free electrons. This is the reason, why metals are not suitable as high efficiency photoelectric emitters. For similar reasons, amorphous semiconductors are not suitable materials. Very good results have been obtained with polycrystalline alkali antimonides. These materials fulfil the above-mentioned requirement for efficient photoemissive cathodes. Monocrystalline AraBv semiconducting layers with negative electron affinity are very suitable from the viewpoint of minimum lattice scattering losses and high emission probability of thermalized photoelectrons. Details of some types of conventional photocathodes and photocathodes under development are the subject of the following sections. 2. The silver-oxygen-caesium

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Figure 2. Emission probability P of a photoelectron as a function of the electron affinity E, for GaAs and GalnAs NEA-photocathodes 1. for zero electron affinity and rises abruptly as the electron affinity further decreases. The requirements placed on highly efficient photoemissive materials can thus be summarized as follows: (i) the absorption coefficient should be as high as possible in the whole wavelength region under consideration; (ii) electron energy transport losses should be minimized; (iii) the electron affinity should be as small as possible if it is positive and as high as possible if it is negative and (iv) the thermionic work function should be as high as possible and should sharply rise with increasing ambient temperature. An optimum value of the photocathode thickness exists for each composition of the cathode material, which is a compromise between maximum exploitation of the incident photons and minimum losses of the excited photoelectrons during their passage to the emission surface. The optimal thickness of polycrystalline photocathodes (e.g. of 516

(S-l) photocathode

The A g - O - C s photocathode has been used in science and industry from the beginning of the 1930s. Its composition is rather an exception from the above-mentioned principles. Although the maximum quantum yield in the visible region is only about 10- 2 electrons photon-1, this type of photocathode still finds applications for its sensitivity in the near ir region, e.g. in image converters. The Ag-O~Cs layer is also used as a component part of composite types of cathodes, which will be discussed further. Figure 3 shows the typical spectral response curve with two pronounced maxima and a threshold wavelength of about 1.2/~m. The average luminous sensitivity is about 35 # A l m - 1, exceptionally up to 70/~A l m - t. The dark current density at 20°C is rather high about 10 -13 10 - 1 1 A c m 2. The Borzjak-Sommer model of this cathode presumes that the photoelectric emission at wavelengths longer than 300 nm is based on electron emission from metallic Ag islands which are embedded in semiconducting Cs20. XPS has also shown that the S-1 photocathode is not a homogeneous layer but comprises metallic Ag particles embedded in Cs20. The surface region contains a different Cs oxide (Cs1103--1 nm) which lowers the potential barrier. The sensitivity in the visible and near ir

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Figure 3. Spectral response of a silver-oxygen-caesium (S-1) photocathode. s--spectral sensitivity, ).--wavelength of radiation.

M Jedli6ka and P Kulh~nek: Photocathodes--contemporary state and trends

spectrum can be explained by resonant optical absorption on Ag islands2. Raman spectroscopy has demonstrated the presence of Csl ~O3 in S-1 cathodes but Cs20 was not detected 4. To explain this result, Bates 5 assumes that the Ag occlusions which are covered by C s l l O 3, are embedded in Cs20; this structure corresponds with the results of Raman spectroscopy as well as with UPS. The absorption spectrum for dispersed Ag particles, which has been recently calculated, strongly resembles the typical S-1 spectral response curve. The model explaining the emission process in S-1 cathodes on the basis of donor levels (formed by Ag particles) in Cs20 has a weakness in not being able to explain why only a small part of the Ag present in the layer causes donor levels, while the remaining silver does not participate in the photoemission. Furthermore the maximum quantum yield of 10- 2 electrons photon- ~ is too large for a process arising from emission from impurity levels only 3.

that S-20 photocathodes with good emission parameters have on top of their Na2KSb layer another thin film composed of K2CsSb 13. Similar conclusions can be drawn from an X-ray diffraction experiment ~4 and from an analysis of XPS spectra 15. In this model, the heterojunction Na2KSb-K2CsSb results in a pronounced band bending at the photocathode surface which, in turn, loads to a reduced value of the effective electron affinity of the SbNa2K layer and to a shift of the threshold towards longer wavelengths (Figure 4). The K2CsSb layer must be thin ( ~ 3 nm) Evoc --

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6

3. Alkali antimonide photocathodes

d(nm)

Compounds composed of alkali metals and Sb form semiconductors with excellent photoemissive properties. Their chemical composition can be described by the general formula M3Sb where M is the alkali metal or a combination of several alkali metals. Alkali antimonide cathodes are usually manufactured in the form of thin transparent polycrystalline films, the individual crystallites being of a cubic or hexagonal structure. A quantum chemical topological method can be used for the determination of ionization potentials, charge densities, excitation energies and band structures for the bulk or surface regions. From this we can deduce what effect the individual alkali metals or their combination will have on the basic parameters of the cathode 6. An empirical 'pseudo-potential' method has been used for calculations of the band structure of some alkali antimonidesv. The most important properties of some alkali antimonide photocathodes are summarized in Table 1. At present, the most frequently used type of the alkali antimonide cathode is the multi-alkali Cs-Na K-Sb (S-20) photocathode discovered by Sommer in 1955. Up to recently, all papers regarding the emission mechanism of this cathode assumed that its structure is based on a layer of Na2KSb covered by a monolayer of Cs 1L~2. Investigations using HEED showed

Figure 4. Multi-alkali photocathode as a heterojunction composed of Na2KSb and K2CsSb15. E--energy; d--photocathode thickness. Other symbols are as in Figure l. in order to leave the emission probability unimpaired 15. The heterojunction represents an intrinsic semiconductor, possibly an n-type semiconductor, but does not exclude the possibility of ionized Cs atoms being present on the surface of the emitter. AES shows, that there is a dipole layer of Sb-Cs on the surface of S-20 photocathodes; Cs is present only in a surface film of 0.5-1 nm thick which corresponds to from 1 to 3 atomic layers. These investigations indicated no increase of K concentration near the surface of the photocathode 16. The properties of the multi-alkali photocathode are markedly influenced by the grain sizes and grain boundary barriers. Wu Quan-De and Liu Li-Bin studied theoretically how to achieve optimal properties of such cathodes 24. Their results show that, in the case of a 'quasi-monocrystal film', a luminous sensitivity of 1400/~A lm- t is obtainable. This value is as high as the sensitivity of a good NEA-photocathode and twice that of the best contemporary multi-alkali photocathode. Sommer 1~ prepared his first S-20 photocathodes in the

Table 1. Main properties of alkali antimonides

Gap (eV)

Electron affinity (eV)

Thermionic emission at room temperature (A cm- z)

P P, I, (N) P P

1.6 1.0 1.0 1.4

0.45 1.1 1.0 0.25

10-16-10 is 10-18 10-17 10- is 10-14

30-35

P

1.0

0.55

10-16

10~150

P

1.0-1.1

0.24

10-15

Cathode

Quantum yield (%)

Threshold S e n s i t i v i t y wavelength Thickness Polarity of (#Alm- 1) (nm) (nm) conduction

Sb42s (S-11)* Sb-K-Cst Sb-Na K (S-24)t:~ Sb-Rb~?s

15 30 30 20

580 660 600 730

25 25 35 30-35 30

S~Na K42s S-20 S-25

30

60 100 50~50 80 (max 130) 150-250

870

20-25

250~00

950

* Sensitivity can be doubled by an additional oxidation (oxidation produces a surface film of Cs1103 (ref 8). t Extremely small dark current. :~Only type of photocathode which does not contain any caesium; thermionic work function rises with temperature ( ~ 5 meV (°C) 1) and therefore the dark current density is low even at increased temperatures9. 517

M Jedli#ka and P Kulh~nek. Photocathodes--contemporary state and trends

following manner: an Sb layer is activated by K at 160°C, the temperature is then raised to 220°C and Na vapour is admitted, the temperature is reduced to 160°C and the photocathode is exposed to an alternating influence of K and Sb vapours. This process is continued as long as the sensitivity shows an increasing tendency. After this a similar exposure to Sb and Cs in alternating sequences completes the procedure. Numerous modifications of the production process for S-20 photocathodes have been tried and computers have been used for process-control purposes ~7; various activation temperatures have been used, the alkali metals have been applied in different sequences, Sb and alkali vapours have been deposited simultaneously (i.e. no first Sb layer was used). These variations of the production technology lead to photocathodes with a variety of properties. Figure 5 shows some of the spectral response curves, which can be attained in S-20 photocathodes by selecting appropriate production technologies. I00

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,~25

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(G)

I°I

these purposes, a photocathode transfer method is used. The photocathode is produced on a substrate separately from the tube, so that technological conditions can be adjusted solely from the viewpoint of reaching the optimal photocathode properties. The fully treated photocathode is transferred in vacuo to the tube, which has been previously processed by vacuum baking; ultrahigh vacuum conditions must be maintained during the transfer process and the tube should be sealed off without further processing~8. This transfer process produces photocathodes, which are, as a rule, rather unstable during shelf-life and operation 19. Generally, their sensitivity tends to decrease. During the shelf-life of a photocathode which is in an 'alkalifree' vacuum environment, caesium is very slowly evaporated from the emission surface. By means of a Langmuir-Taylor ionization gauge it was found that, at room temperature, the rate of loss of caesium was ~ 5 × l04 atoms cm-2 s (ref25). This evaporation is too slow and cannot be used to explain the sensitivity decrease. A more probable reason of the instability is the influence of residual gases, which react with the photocathode. The concentration of the detrimental gases ( 0 2, CO 2, CO, H20, etc.) is higher in tubes with transferred photocathodes than in tubes with cathodes prepared in situ. During operation, the concentrations of these gases are even higher because of electron and ion stimulated desorption from other parts of the tube. In addition, the photocathode surface is damaged by ion bombardment. Figure 6 shows the dependence of the sensitivity of S-20 photocathode on the time interval, during which the photocathode is exposed to O 2 at 10 -7 Pa. An analogous curve is given for

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Figure 5. Spectra] response curves of various multi-alkali photocathodes. s--spectral sensitivity; 2 wavelength of radiation. Multi-alkali photocathodes with enhanced sensitivity in the near ir have been designed 'S-25' (Figure 5(c)). Their production technology is basically the same as for S-20 photocathodes, but increased attention must be paid to prevent possible contamination. Technological circumstances have to be maintained within narrower limits and reduced rates of deposition are used. This leads primarily to layers with larger crystallites and lower impurity concentrations. Thus, the emission depth is increased and thicker photocathode layers can be used. As a rule, the photocathode is manufactured directly on a part of the interior surface of the tube envelope by means of a vacuum deposition process. This of necessity leads to the presence of alkali vapours in the tube, which may adversely affect other tube parameters such as noise, parasitic electron emission, or leakage currents, or it can lead to undesirable chemical reactions with sensitive components like fluorescent screens and channel electron multipliers. In image devices with proximity focussing it is practically impossible to use standard production methods. For 518

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0

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30

45 t (min)

60

75

Figure 6. Luminous sensitivity G of S-20 and S-11 photocathodes as a function of time t of cathode exposure to oxygen at a pressure of 10 7 Pa.

an SbCs 3 cathode. With the transfer technique, utmost care must be taken to perform meticulous cleaning, baking, outgassing and evacuation of all components and subsystems entering the final product. A getter system with a high pumping speed which needs no priming and works without delay must be used 19.

4. Negative electron affinity (NEA) photocathodes The first photoelectric NEA emitter was announced by Scheer and van Laar in 1965. Monocrystalline p-type GaAs with an acceptor concentration of 3 x 1019 cm-3 was cleaved in vacuum and activated with Cs. The luminous sensitivity obtained was higher than 500/aA lm- 1. During the last 20yr much effort has been devoted to investigations of the photoemission from AraBv compounds and

M Jedli#ka and P Kulh6nek."

Photocathodes~contemporastryateandtrends

to technological processes which could be used in the production of tubes using these photocathodes. Most of the A]I[Bv compounds crystallize in sphaleritic structures. Lattice constants and band gaps for some AraBv semiconductors are given in Figure 7. Binary compounds are defined by individual points, ternaries by lines joining appropriate points and quaternaries by areas enclosed by corresponding lines. Contemporary AnlBVphotocathodes are produced by growing an epitaxial layer on a monocrystalline substrate. The substrate must be of the same crystalline structure as the emissive layer and should have the same lattice constants in order to reduce the stress and the occurrence of undesirable energetic interface states. The commercially available A InBVmaterial for the substrate is usually GaP, GaAs or InP; InAs and InSb have gaps, which are too narrow.

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56

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Figure 7. Bandgap Eg and lattice constant a for some AraBv compounds.

Opaque GaAs photocathodes are produced by letting the epitaxial layer grow together with the appropriate dopant directly on the substrate. Indium can be used if an increased sensitivity in the long wavelength region is desired. To avoid a change of the crystal constant, the indium content increases gradually from zero up to a nominal value. Monoatomic films of Cs and 0 2 are deposited on the surface. Under optimal conditions, these photocathodes have sensitivities of above 2000 # A l m - 1. Figure 8 shows the spectral response curve of an opaque GaAs photocathode together with that of a multi-alkali S-25 photocathode. Another important type of photocathode working on similar principles uses GaAs 1_xPx deposited on a GaP substrate. This photocathode has a quantum efficiency of nearly 50% at 530 nm. One of the most satisfactory photocathodes grown on InP substrates has the following composition: Ino.ssGaoJ2Aso.23P0.77. This cathode has a quantum efficiency above 10% at 1.1 pm. Ternary and quaternary AraBv compounds can be epitaxially grown with perfectly-matched lattice constants on InP substrates. Their gaps can be up to 0.8 eV which corresponds to a longwave threshold of about 1.7 #m. However, there is no advantage to be gained from using these narrow gap materials in photocathodes for reasons, which will be discussed later. All photocathodes with a crystalline substrate are used with front illumination, i.e. with radiation incident on the same side of

the photocathode as that from which electrons are emitted. These opaque photocathodes find most frequent applications in photomultipliers. Image devices (i.e. converters, intensifiers, camera tubes) are constructed practically exclusively for transparent photocathodes, where the optical image impinges on the surface opposite to that from which the photoelectrons are emitted. Transparent or semi-transparent NEA photocathodes are usually manufactured using an 'inverted' structure, which is illustrated in Figure 9. First, an AIGaAs layer is grown on a substrate of GaAs, so that the lattice constants differ only slightly. After this, the proper photoemissive layer of GaAs or InGaAs is grown on the exposed surface. When In is used, its concentration is adjusted so that the lattice constants of the deposit will be as close as possible to those of the substrate. A further epitaxial layer of AIGaAs is deposited. Glass cement, e.g. Ca-B-AI-Si, is used for

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Figure 9. Schematicillustration of the 'inverse structure' NEA-photocathode before removal of the substrate and of the intermediate layer by selective etching2°. 519

M Jedli#ka and P Kulhgnek: P h o t o c a t h o d e s - - c o n t e r n p o r a r y

state a n d trends

sealing the product to an optically flat glass plate, which is to serve as faceplate of the electron tube. Thermo-compression with a protective SiO 2 layer can be used as well. Then the crystalline GaAs substrate and the first layer of GaA1As are removed by selective etching, so that the active photosensitive GaAs (or InGaAs) layer with strong doping to a p-type conductivity will be exposed. The next step is a thorough cleaning of this layer by highvacuum baking at a temperature very close to the compound decomposition temperature. Finally Cs and O 2 are adsorbed on the surface so as to form a dipole layer, which reduces the electron affinity below the bottom of the conduction band. A similar method can be used for the preparation of semitransparent GalnAsP photocathodes where the etched-off substrate is InP. Various technological processes have been employed, including vapour phase epitaxy, liquid phase epitaxy, metal organicvapour phase epitaxy and molecular beam epitaxy. The basic requirements for materials to be used in NEA photocathodes can be summarized as follows: (i) the material should have a high optical absorption coefficient; (ii) the electron diffusion length must be comparable to the thickness of the photoemissive layer; (iii) the gap must not be less than the photoelectric work function of the activated surface; (iv) lattice constants and coefficients of thermal expansion of the component materials should mutually be matched as closely as possible.

5. Transferred-electron photocathodes As a rule, photocathodes with either positive or negative electron affinity have a longwave threshold for efficient photoemission not beyond 1.1 #m. Initially, it was optimistically assumed, that the principle of NEA could be applied to materials with a gap of 0.7 eV as well. However, it was soon found that the limiting factor was a potential barrier of ~ 1 eV arising from the CsO 2 on the surface, which blocks emission of electrons excited by photons of lower energy. Recently, photoemission excited by radiation of a wavelength longer than 1.1 #m has been attained by means of field-assisted photoemission using the 'electron transfer' mechanism. The original idea submitted by Bell, James and Moon 21 in 1974 is based on the fact, that in some AraBv semiconductors (e.g. InP, InGaAs, GaAs) an internal electric field can raise electrons from a lower to an upper valley which facilitates their emission. An Ag film 5-10 nm thick is deposited on the photocathode surface and a suitable voltage is applied to it. This creates an electric field of the order of 104 V cm- 1 in the semiconductor. A Schottky barrier is formed between the photoemitter and the Ag film. A dipole layer of CsO 2 covering the Ag film lowers the work function to about 1 eV. When a potential is applied to the Schottky barrier in the reverse direction, excited electrons are accelerated towards the surface and pass through the Ag film into the vacuum. A photocathode of this type with a longwave threshold of 2.1 #m has been made using a ternary compound Ino.TvGao.23As with a gap of 0.52 eV 22 (Figure 10). An intermediate layer of InAsl_xP x was used for matching the lattice constant of the emissive layer to that of the InP substrate. By changing the value of x stepwise a variable gap was attained. When this structure is illuminated through the substrate, only photons with an energy from 0.52 to 0.83 eV will reach the photolayer. Photons with an energy between 0.83 and 1.35 eV (from 1.5 to 0.9 #m) will be absorbed in the matching layer of InAsP and photons with an energy greater than 1.35 eV will not pass through the InP 520

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Figure 11. Comparison of the spectral response curves of different photocathodes with the spectral night sky radiance26. 2--radiation wavelength;hv--photon energy;r/~uantum yield;Lpa--spectral photon radiance.

M Jedli~ka and P Kulh~nek: Photocathodes---contemporary state and trends

of a transferred-electron cathode (composition InPIn0.53Ga0.47As ), of S-1 and S-25 photocathodes and of a GaAs N E A photocathode are compared with the spectral photon radiance of the night sky 26. U p to now, we know no other photoemitter which has, in the region between 1 and 2/~m, operating properties comparable to those of the transferredelectron photocathode.

6. Conclusion Almost 100 yr ago the external photo-effect was discovered by Hertz and Hallwachs (1887-1888). Ever since then both science and technology have invested great interest in this field. Fundamental work in photometry and spectroscopy is based on applications of photoemission. The beginnings as well as the development of television have been made possible by advances in the technology of photocathode production. Also in a foreseeable future photocathodes will continue to be irreplaceable in devices for the detection and transformation of low light level signals and, in other cases, where especially short response times are required. The fast response of a photocathode is significant in many problems of optical measurement. Photocathodes respond with a delay time shorter than 10- ~2 s and their response time dispersion is of the order of 10-~3 s. The theoretically-derived limit for the time constant is 10- ~4 s. In suitably constructed electron tubes, fast photon detectors find numerous applications in the investigation of transient optical phenomena, e.g. in lasers or in systems for information processing 23. F o r optical image processing it is of great importance that electrons escaping into vacuum can be accelerated, focussed and deflected and that they can be used to excite such phenomena as secondary emission, fluorescence, etc. The most significant future development may be expected in the application of modern photocathodes in combination with amplifying elements for optical image processing, where their high sensitivity, fast response and selectable spectral response curves can be used to advantage.

References 1 T M Lif~ic, A L Musatov, V L Korotkich, A D Korinfskij and V M Tur6inskij, Probl Sovrem Radiotech i Elektron, p 250, Moskva (1983). (In Russian.) 2 C W Bates, Phys Rev Letts, 47, 204 (1981). 3 A H Sommer, Photoemissive Materials, Wiley, New York (1968). 4 K U von Raben, K C Lee, R K Chang and R E Benner, J appl Phys, 55, 3907 (1984). 5 C W Bates, Appl Phys Letts, 45, 1058 (1984). 6 j Pancii', I Haslingerovfi and M Jedli6ka, Proc 1 lth lnt Syrup oflMEKO on Photon Detectors, p 133, OMIKK-Technoinform, Budapest (1985). 7 A A Mostovskii, V A Chaldyshev, G F Karavaev, A I Klimin and I N Ponomarenko, Izv Akad Nauk SSSR, Set Fiz, 38, 195 (1974). (In Russian.) 8 C W Bates, Th M van Atekum, G K Wertheim, D N E Buchanan and K E Clements, Appl Phys Letts, 38, 387 (1981). 9 M Jedli(zka and J Rau~, Proc 4th Czech Confon Electronics and Vacuum Physics, 303, Praha (1968). 10 M Jedli6ka and P Vilim, Advances in Electronics and Electron Physics, vol 22B, p 449, Academic Press, New York (1966). 11 A H Sommer, Rev Sci Instrum, 26, 725 (1955). 12 W E Spicer, Phys Rev, 112, 114 (1958). 13 A A Dowman, T H Jones and A H Beck, J Phys D: Appl Phys, 8, 69 (1975). 14 p Dolizy, O De Luca and M Deloron, Acta Electronica, 10, 265 (1977). 15 L Galan and C W Bates, J Phys D: Appl Phys, 14, 293 (1981). 16 R Holtom, G P Hopkins and P M Gundry, J Phys D: Appl Phys, 12, 1169 (1979). 1v I Berfinek, M Hra~ov~t, J Jandus, P Kulhfinek, Proc 7th Czech Confon Electronics and Vacuum Physics, (to be published). 18 A B Kostin and T A Filimonova, Itogi Nauki i Techniki-Elektronika, vol 15, p 217, VINITI, Moskva (1983). 19 j D McGee, Residual Gases in Electron Tubes, p 295, Academic Press, New York (1972). 2o j p Andr6, M Bolou, P Giuttard and E Roaux, Inst Phys Conf Ser, no 56, 412 (1981). 21 R L Bell, L W Jones and R L Moon, Appl Phys Letts, 25, 645 (1974). 22 p E Gregory, J S Escher, R R Saxana and S B Hyder, Appl Phys Letts, 36, 639 (1980). 23 H J Jfipner, Proc 11th Int Syrup of lMEKO on Photon Detectors, p 21, OMIKK-Technoinform, Budapest (1985). 24 Wu Quan-De and Liu Li-bin, Advances in Electronics and Electron Physics, vol 64B, p 373, Academic Press, New York (1985). z5 M Oliver, J Phys D: Appl Phys, 4, 962 (1971). 26 j S Escher, R L Bell, P E Gregory, S B Hyder, T J Maloney and G A Antypas, IEEE Trans Electron Devices, 27, 1244 (1980).

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