INFLUENCE OF SLOW-ELECTRON IMPACT UPON GASES ADSORBED ON TUNGSTEN, INVESTIGATED BY,MEANS OF A FIELD ELECTRON MICROSCOPE

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1 R533 Philips Res. Repts 20, , 1965, NFLUENCE OF SLOW-ELECTRON MPACT UPON GASES ADSORBED ON TUNGSTEN, NVESTGATED BY,MEANS OF A FELD ELECTRON MCROSCOPE by W. ERMRCH 1. ntroduetion Abstract The influence of slow-electron impact is investigated by means of a field electron microscope (FEM). An electron gun mounted perpendicular to a tungsten tip in the FEM supplies an electron beam of energies between 5 and 300 ev with beam-current densities up to 0 1 A/cm 2 The gases adsorbed and bombarded are H2, N2, CO,C02, CH4, and Xe. Work-function changes with time are different at different temperatures, under constant bombarding conditions, indicating different adsorption states with unequal cross-sections for electron-impact desorption. n some cases also an electron-bombardement-induced dissociation, followed by a surface reaction, can be detected. With the apparatus used it is possible to find threshold values for some ofthe effects being studied. Since a long time it is known that particles in the gas phase may be dissociated, ionized or excited by electron impact. nvestigations concerning the effect of electron bombardment on salts, metal oxides and gases adsorbed on metal surfaces have been performed later on by a number of authors. Jacobs 1) used the contamination of an oxide cathode as a measure of oxygen being released from various surface oxides on account of electron bombardment. Plumlee and Smith 2), working with a mass spectrometer, studied the decomposition of metal oxides by electron impact. A value of 10-6 to 10-8 was found for the proportion of positive oxygen ions being released to the number of bombarding electrons. A radioactive-tracer method applied to the decomposition of barium oxide and strontium oxide by Yoshida a.o. 3) revealed yields of 10-7 to 10-5 desorbed oxygen atoms per bombarding electron. Operating with a mass spectrometer, Moore 4) adsorbed CO on tungsten and molybdenum surfaces. On bombardment with electrons he found 10-4 to 10-5 positive oxygen ions being released per impinging electron. A quantitative measurement of desorbed neutrals was not possible in his apparatus. However, he detected an electronimpact-induced dissociation of the adsorbed CO. Young 5) also used a mass spectrometer to measure the number of positive oxygen ions liberated from oxidized metal surfaces by electron impact. n all the above investigations polycrystalline targets were used and the purity of the surface could hardly be defined. Besides this, the targets often were heated to appreciable temperatures by the electron bombardment and because of this we believe the results to be more qualitative.

2 NFLUENCE OF SLOW-ELECTRON MPACT UPON GASES ADSORBED ON TUNGSTEN 95 To what.account the measurements with Bayard-Alpert ionization gauges may be influenced by electron-impact desorption (ED) has been investigated and discussed by several authors 6-9). Ehrlich and Hudda 10) found some evidence of ED in the field ion microscope. Gas adsorbed on the surface was desorbed by the electrons from the image-forming gas. A method to determine.between absorbed and adsorded gases by means of ED has been described by Degras, Peterman and Schramm 11). Redhead 12) measured the energy of ions from ED using a retarding-field analyzer. He detected several adsorption phases, ion yields laying between 10-8 to 10-5 per electron. n all the above-cited measurements adsorption phases could not be chosen arbitrarily, as the targets could not be cooled below room temperature. On account of the polycrystalline targets, structure - dependent effects of different crystal planes could not be investigated; a control of surface purity was not possible either. The field electron microscope (FEM), invented by Müller 13) in 1937,has shown up to be a very valuable tool to study phenomena concerning adsorption and absorption, dissociation effects in. adsorbed gas layers and exchange processes between different adsorbed gas phases. A direct control of all surface phenomena resulting in changes of the work function is well possible with this instrument. Thus we have developed a FEM tube, in which a target that can be cooled may be bombarded with a well-defined electron beam. ndependently a similar system was used by Menzei and Gomer 14) to study ED effects of CO, H2, 02 and Ba adsorbed on tungsten. We shall now discuss which sort of information may be drawn from experiments with such a system. The field electron current density j as a function of the field strength Fand the work function cp is given by the Fowler-Nordheim 15) equation: af2 j = - exp (~b CP 3/2 v/f), CPt 2 (1) where a and b are constants while v and t are slowly varying functions of F and cp which for practical purposes may be replaced by 1. As can be seen from eq. (1), at constant Fthe current density is very sensitive to changes in CP. For this reason the FEM pattern of a single crystal tip shows large variations in brightness which result from the different work functions of the various crystal planes. All phenomena occurring on the surface which are related to,changes of cp can be followed qualitatively by viewing the FEM screen and quantitatively by measuring the current from the total emitter or separate crystal planes. The work function is then determined from the slope of the Fowler-Nordheim plot in the form log (jff2) against /F. Even more information J?1aybe gained from the findings of Menzei and Gomer 16), Van Oostrom 17), and Holscher 18) who describe a relationship between surface

3 96 w. ERMRCH coverage with adsorbed particles and the pre-exponential in eq. (1). term that appears 2. Experimental ED apparatus Figure 1 shows schematically our experimental set-up. The proper FE;M consists of the tip S from which electrons are drawn to the anode B, which consists of a tin-oxide layer on the glass screen, giving a highly enlarged pattern of the current-density distribution on the surface of the fluorescent screen F, as soon as a sufficient voltage is applied between Sand B. Perpendicular to the tip axis an electron gun K is mounted which is fixed by the sinter-glass bars to the beam-analyzing system A. The magnetic field H is parallel to the electron beam from the gun and by slightly changing the direction of H it is possible to feed the electron beam into the openings ofthe analyzer. The analyzer allows to determine the beam current density, the contact potential between the cathode ofthe gun and a polycryställine-tungsten target (which is necessary to define the energy ofthe bombarding electrons), and the energy distribution ofthe electrons. By means of the adjustable bellow V the point S can be brought properly into the electron beam. The bulb E is made of borosilicate glass. The tip can be cooled by filling the cold finger C with a temperature bath, e.g., liquid nitrogen. Fig. 1. FEM system. E = glass envelope, B = conducting layer of tin dioxide, F = willemite fluorescent screen, S = tungsten tip, G = sinter-glass bars, fixing the electron gun K to beam analyzer A, C = cold finger, and V = adjustable bellow system by means of which S can be manipulated into the electron beam.

4 NFLUENCE OF SLOW-ELECTRON MPACT UPON GASES ADSORBED ON TUNGSTEN 97 Tip températures are calculated from the resistance of the hairpin to which the tip is spot-welded. Because all changes of the work function with time which result from readsorption of residual gases have to be minimized, pressures of residual gases must be low and we therefore have used an ultra-high-vacuum system which is shown schematically in fig. 2. With this system pressures in the order of torr can be reached and maintained over long periods. After the introduetion of gases at about 10-6 torr so as to adsorb gas on the tip, less than 30 minutes are sufficient to reestablish the ultimate pressure in the system. All our ED experiments and work-function determinations have been undertaken at pressures below torr. Energies of bombarding electrons can be varied between about 5 and 300 ev. The dependence of the beam current density on the electron energy is given in fig. 3; current densities up to 0'1 A/cm 2 can be reached. From measurements in " VP r l i K2 GoV '1; ; ûöo--r--+_...; 00-_'''''''''';,1 H2 H3- DV- Fig. 2. Ultra-high-vacuum 0 L '" FE'" Ji set-up. VP = fore pump, DP = Hg-diffusion pump, Ki and K2 = cold traps, Hl"H2 and H3 = greased stop cocks, DV = throttle valve, G = gas reservoir, MV = bakeable metal valve, M = magnetron gauge, T = titanium Penning pump, JM = Alpert-type ionization gauge, and FEM = field electron microscope. The part of the system surrounded by the broken line is bakeable at temperatures up to 400 oe.. Beam-current density as a function 'of electron energy : _ sa U[V} Fig. 3. Current density of electron gun as a function of electron energy.

5 98 W. ERMRCH 2 3 _ 4,U[V] Fig. 4. Current-voltage characteristics in the retarding-field region. the retarding-field region which are shown in fig. 4 the contact potential between the cathode of the gun and a polycrystalline-tungsten target can be derived; thus the true average electron energy is known. From the slope of the straight line in fig. 4 it is possible to calculate the electron temperature. The deviation between measured cathode temperature and calculated electron temperature is a characteristic property of the type of oxide cathode being used. We shall now try to estimate the times that are needed for BD experiments with our apparatus, taking into account the following general formula: dn. -- = nelun(t)-n(p)s, dt where dnjdt is the change in the number of particles adsorbed on the surface per sec and cmê; nel is the number of electrons impinging per cm 2 and sec; u is the BD cross-section of particles adsorbed in cmê; N(t) is the number of particles adsorbed at time t per cm 2 surface area; N(p) is the number of particles impinging per cm 2 surface area at pressure p during 1 sec; s is the sticking probability. The expression dealing with the readsorption from the gas phase can be neglected in' our considerations, because the pressure in the system is so low that no remarkable contribution from adsorption is to be expected as long as (2)

6 NFLUENCE OF SLOW-ELECTRON MPACT UPON GASES ADSORBED ON TUNGSTEN 99 the surface coverage is high. A change in the work function which is caused by a. variation of 10per cent of a monolayer would be just detectable by our measuring system. Beam current density should be 10-2 A/cm2 = electrons/cm-sec if for a a va1ué of about cm2 is assumed. At time t = 0 one monolayer of gas (~ 1015 particles/ems) would be adsorbed. ntegration of eq. (2) yields a bombarding time of about 3 minutes if the above values are taken into account. 3. Preliminary results The ED effect was investigated qualitatively by using the following gases: H2, CO, N2, CH4, C02 and Xe. We shall shortly describe our results with the different gases without dealing in detail with adsorption phases, binding energies, surface mobilities, etc., because there are discrepancies or no data at all in the literature. Hydrogen on tungsten On adsorption at 77 "K as well as at room temperature the work function raises. Electron bombardment lowers the work function in the bombarded regions. At 77 "K, however, this effect is much more pronounced than at room temperature. These different ED cross-sections may be attributed to different binding states. Qualitatively the same behaviour was found by Menzel and Gomer 16). Carbon monoxide on tungsten n this system the work function also is increased by asdorption at all temperatures that were used. Electron bombardment decreases the work function.at room temperature the ED effect is hardly detectable with our apparatus while at 77 "K there is strong evidence of an ED-induced work-function change. After heating the tip at temperatures above 1400 "K the typical carbon-ontungsten configuration is clearly visible indicating an electron-impact-induced dissociation of CO besides desorption. Thermal desorption does not lead to a dissociation. Menzel and Gomer 16)found different cross-sections at different temperatures; from their results they conclude that 3 different adsorption states exist. They also found an electron-impact-induced dissociation. Nitrogen on tungsten The adsorption properties of nitrogen on tungsten have been investigated lately bya number of authors 17-20).They were able to prove the existence of crystalplane-dependent adsorption phases. We shall give an interpretation of our results on the basis of FEM patterns. Figure 5a shows an FEM image of clean tungsten at 77 "K; the average work function of the total emitting surface was assumed to be 4 5 ev. After 30" adsorption of N2 at 77 "K and a pressure of 10-6 torr the pattern of fig. 5b appears. The (001) planes are no longer visible

7 100 w. ERMRCH and their surroundings emit more strongly than the rest of. the < surface. The work function has decreásed to 4 3 ± 0 05 ev. Bombardment ofthis adsorbed nitrogen with 2.5:10-2 Afcm 2 ; 40-eV electrons for 1 minute leads to the pattern shown in fig. 5c (bombardment from the left-hand side). A determination ofthe work function from the total cwrellt is not reasonable in this case, because the. bombarded are!l appears.darker than the unbombarded regions. The (001) plane is now clearly visible and the bombarded area shows some granularity. After 2 minutes bombardment under the same conditions the pattern of fig. 5d is reached. Emission in the bombarded region has further decreased and the granularity is more pronounced. Some crystal-plane dependence of the ED effect can be seen as the surroundings of.the (121) planes and (031) area emit more strongly than the other bombarded regions. t was not possible to reestablish the state of fig. 5b by adsorption from the gas phase at high pressures for long times. Thermal diffusion, which according to Oguri 20) starts at about 1000 "K, was tried so as to reestablish the state offig. 5b. However, the diffusion boundary moving from the shank of the tip across the emitting area stopped abruptly when reaching the bombarded region. Temperatures of about 2100 "K were needed to clean the tip before the "clean tungsten" state of fig. 5a could be reached again. Oguri 21) has reported a complete desorption of adsorbed nitrogen at 1800 "K. From the above we conclude that the phenomena described are related to an electron-impact-induced dissociation of the adsorbed N2 which leads to the formation of a tungsten nitride on the surface which has other properties than adsorbed N2. At room temperature no influence at all of the electron bombardment upon the adsorbed layer could be detected with our. apparatus even after prolonged bombarding times. From this it is concluded that the BD cross-section of the state adsorbed at 77 "K (named y phase by Ehrlich and Hudda 19)) is much larger than that of the room-temperature state. This is supported by the fact that nitrogen adsorbed at room temperature and then cooled and bombarded at 77 "K shows no indication of an ED effect either. Methane on tungsten After adsorption ai room temperature the methane was bombarded with electrons; even after long bombarding times no ED effect was detectable. Thermal desorption led to the "clean tungsten" state without any evidence of carbon left on the surface. When the tip was.sliglitly covered with carbon, bombardment led to an increased emission of the bombarded area and after thermal desorption of CH4 the amount of carbon on the surface had increased. f adsorption and bombardment were performed at 77 OK the bombarded area appeared brighter than the rest of the surface. After thermal desorption a large amount of carbon could be found on the surface as can be seen in fig. 6.

8 NFLUENCE OF SLOW-ELECTRON MPACT UPON GASES ADSORBED ON TUNGSTEN 101 Fig. Sa. FEM pattern of clean tungsten. Fig. Sb. Nitrogen adsorbed on tungsten at 77 ok (30",10-6 torr, <1> 4 3 ev). Fig. Sc. Bombardment (from the left) of the state shown in fig. Sb (60", 40 ev, j = 2, AJcm2). Fig. Sd. Same as fig. Sc after 120" bombarding time. Fig. 6. Carbon on tungsten, generated by bombardment of CH4 adsorbed on tungsten.

9 102 W. ERMRJCH Fig. 7. Carbon dioxide adsorbed on tungsten at 77 "K (60", torr, 4>= 4 5 ev) and bombarded with electrons from the left (600",40eV,j = Afcm 2, 4>=4-45 ev). Fig. 8. Carbon dioxi adsorbed on tungsten at room temperature (300",10-7 torr) and 10-minutes bombardment with Afcm 2 40-eV electrons. Fig. 9. Carbon dioxide adsorbed on tungsten at room temperature and then heated to x, 4> = 4 65 ev. Fig. 10. State of fig. 9 bombarded with electrons (10',) = 6, Afcm 2, 40 ev, 4> = 4 45 ev). Fig. 11. Xenon adsorbed on tu ngsten at 77 K (120", torr). Fig. 12. State of fig. 11 after 60" bombardment with} = Afcm 2 at 40 ev.

10 NFLUENCE OF SLOW-ELECTRON MPACT UPON GASES ADSORBED ON TUNGSTEN 103 Carbon dioxide on tungsten A lowering of the work function was the consequence "ofelectron impact upon C02 adsorbed at 77 OK on tungsten. Figure 1shows the FEM pattern after one-minute bombardment with 0 1 Afcm2 at 40-eV electron energy. At room temperature a similar effect was found (fig. 8: 10-minutes bombardment 0 1 Afcm2 40 ev). n both cases the typical carbon-on-tungsten picture appeared after thermal desorption; without bombardment no carbon could be idbntified. Carbon dioxide after adsorption at room temperature heated to approxima- tely 1300 "K shows the image of fig. 9. After thermal treatment emission now comes mainly from the surroundings of the (111) planes. This phenomenon is attributed to 02, descending from the dissociation of carbon dioxide (C02 -+ CO + t 02). f this purely thermally generated state is now bombarded with electrons (40 ev, Afcm2), after 10 minutes the influence of the electron impact is clearly visible as can be seen from fig. 10. Emission from the bombarded area is remarkably increased now. Xenon on tungsten With the above more-atomic gases not only desorption but also dissociation was on the forehand to be expected. Xenon, however, is only physically 22,23) adsorbed and a pure desorption as a consequence of electron impact should take place. n fig. 11 xenon is adsorbed on tungsten at 77 "K (2 minutes"io- 7 torr). After l-á-minutes electron bombardment (40 ev, Afcm2) the state shown in fig. 12 was reached. The work function has increased towards the value of clean tungsten. From further experiments it is not quite clear if only a desorption has occurred. Surface diffusion by a thermal treatment of the tip was not completely similar in the bombarded and unbombarded regions. Whether the electron impact has formed a more tightly bound xenon state, has to be cleared by further experiments. t was tried to measure threshold values for the ED processes. The tip with the adsorbed gas was bombarded with different electron energies, starting with about 5 ev. After every 20-minutes bombarding time the FEM:imagè and the work function were controlled. Electron energies were determined as follows. From current-voltage measurements in the retarding-field region (fig. 4) the contact potential between the cathode of the electron gun and the target of the analyzer (fig. 1) was defined as the voltage at which a current of 10-8 A is flowing. The target consisted of freshly degassed polycrystalline tungsten, the work function ofwhich was assumed to be 4 5 ev. As thechangein the work function by adsorption is identical to the change in the contact potential-ïwork function ofthe oxide cathode unchanged) electron energies can be ~à.si1y: caiculated from the voltage between cathode and field emitter. Threshold values derived by this method "are given in table.."

11 104 W. ERMRCH gas signum of Ll<P by adsorption TABLE signum of Ll<Pcaused by ED ED effect detectable.from ev (± 0 5 ev) 77 -x 300 ok 77 ok 300 -x 1300 ok H not measured N no effect detectable CO no effect detectable CO2 const CH not measured Xe A detailed discussion of different adsorption states, cross-sections and crystalplane-dependent phenomena is not possible from the above results due to lack of sensitivity of our work-function determination. Because of this a modified FEM tube has been constructed which will enable us to get more detailed information about phenomena on different crystal planes. For this purpose an analyzing system has been attached to the fluorescent screen, makingit possible to follow the behaviour of defined areas of the tip surface. Electrons from a. certain part of the tip are deflected magnetically or electrostatically into the opening of the analyzer, thus allowing the measurement ofthe current-voltage characteristics from the selected area separately. Also an increase of sensitivity of the work-function determination is expected from automatically recording of Fowler-Nordheim plots. t is hoped that information gained by the improved apparatus will allow a quantitative interpretation of electron-impact phenomena. Philips Zentrallaboratorium GmbH Aachen, November 1964 REFERENCES 1) H. Jaco b s, J. app!. Phys. 17, 596, ) R. H. P1um1ee and P. L. Smith, J. app!. Phys. 21, 811, ) S. Yoshida, N. Shirata,. garashi and H. Arata, J. app!. Phys, 27, 497, ) G. E. M oore, J. app!. Phys, 32, 1241, ). R. You n g, J. app!. Phys. 31, 921, ) P. A. Redhead, Vacuum 12, 267, ) T. E. Hartman, Rev. sci. nstr. 34, 1190, ) P. Marmet and J. D. Morrison, J. chem. Phys. 36, 1238, ) J. L. Ro bins, Can. J. Phys. 41, 1385, ) G. Ehrlich and F. G. Hudda, Phi!. Mag. 8, 1587, ) D. A. Degras, L. A. Peterman and A. Schramm, Trans. 9th vacuum symp., Macmillan, New York, 1962, p. 497.

12 NFLUENCE OF SLOW-ELECTRON MPACT UPON GASES ADSORBED ON TUNGSTEN ) P. A. Redhead, Vacuum 13, 253, ) E. W. Müller, Z. Physik 106, 541, 1937., 14) D. Menzei and R: Gomer, J. chem. Phys. 40, 1164, ) R. H. Fowler and L. W. Nordheim, Proc. roy. Soc. London A119, 173, ) D. Menzei and R. Gomer, 11th Annual Field Emission Symp., Cambridge, 1964; J. chem. Phys., on the press. 17) A. van Oostrom, 11th Annual Field Emission Symp., Cambridge, S) A. A. Holscher, 11th Annual Field Emission Symp., Cambridge, ) G. Ehrlich and F. Hudda, J. chem. Phys. 35, 1421, ) T. 0 guri, J. phys. Soc. Japan 19, 83, 1964.' 21) T. Oguri, J. phys. Soc. Japan 18, 1280, ) R. Gomer, Austr. J. Phys. 13, 391, ) G. Ehrlich and F. G. Hud d a, J. chem. Phys, 30, 493, 1959.