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Ion scattering spectroscopy studies of barium and oxygen on tungsten and tungsten-based dispenser cathodes
312
Applied
Surface Science 24 (1985) 372-390 North-Holland, Amsterdam
ION SCA-ITERING SPECTROSCOPY STUDIES OF BARIUM AND OXYGEN ON TUNGSTEN AND TUNGSTEN-BASED DISPENSER CATHODES C.R.K.
MARRIAN,
A. SHIH
Code 6832. Naval Research Laboratory,
Received
25 May 1984; accepted
and G.A. HAAS Washington, DC 20375, USA
for publication
15 March
1985
In order to use Ion Scattering Spectroscopy (ISS) for studies of tungsten dispenser cathodes, the relevant ISS sensitivities must be measured. Calibrations have been made using a polycrystalline tungsten ribbon with controlled coverages of oxygen, barium and combinations thereof. Auger Electron Spectroscopy (AES) was used to monitor these controlled surfaces and the escape depths of the tungsten Auger electrons in barium and oxygen have been measured. The absolute ISS sensitivities of all three elements were found to be strongly dependent on the barium coverage of the tungsten surface. This effect has been attributed to the lowering of the work function of the tungsten surface caused by the barium adsorption. However, the relative ISS sensitivities of the three elements are not affected in this way when both barium and oxygen (or oxygen alone) are present on the tungsten surface. ISS spectra of such surfaces have been analyzed quantitatively and found to be in reasonable agreement with AES measurements. The analysis has also been applied to ISS spectra of uncoated tungsten matrix dispenser cathodes in an active state and following exposure to oxygen. Compared to AES, these spectra indicate less oxygen on the active cathode surfaces as a result of the oxygen (associated with barium) not contributing to the oxygen ISS signal. Comparisons of the spectra from the active and oxygen poisoned cathodes suggest that oxygen adsorbed during the oxygen exposure sits on the topmost barium layer whereas the oxygen on the active cathode surface does not.
1. Introduction Ion Scattering Spectroscopy (ISS) is known as a powerful technique for the investigation of the topmost atomic layer on a surface. Many detailed descriptions of the technique have been published (for example ref. [l]). In addition, the problems of quantitative ISS have been extensively discussed (for example refs. [2-41). Tungsten based dispenser cathodes are widely believed to have near monolayer coverage of barium and oxygen at the surface and thus appear to be a type of surface for which ISS would provide interesting results. However, the relevant sensitivity factors are not known and, in any case, need to be measured in the actual spectrometer being used. Thus it has proved necessary to study the type of surface found on a dispenser cathode under
0169-4332/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
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C.R. K. Marrian et al. / ISS studies of Ba and 0 on W caihodes
373
controlled conditions. In this paper ISS results from the following three surfaces will be discussed: oxygen on tungsten, barium on tungsten and barium on tungsten which had previously been exposed to oxygen. These studies have allowed the evaluation of the relevant sensitivity factors which have been applied to the analysis of results from a dispenser cathode. The oxygen on tungsten system has been studied with ISS [3,4]. In ref. [3] the increase of the oxygen ISS peak and the decrease of the ISS tungsten peak were not found to vary linearly with the coverage of oxygen on a single crystal tungsten substrate. These effects were attributed to surface rearrangement as indicated by LEED [2]. In contrast in studies of oxygen adsorption on polycrystalline tungsten, linear behaviour of the ISS peaks was observed [4]. The authors are unaware of any ISS studies of barium on tungsten. As a dispenser cathode is a polycrystalline surface, it was felt that studies of a polycrystalline tungsten ribbon would be the most realistic way to evaluate the necessary sensitivity factors. In order to calibrate the surfaces being studied with ISS, Auger Electron Spectroscopy (AES) has been used. A section of the paper is devoted to this aspect of the work and the results of measurements of the escape depths of tungsten Auger electrons in barium and oxygen are presented.
2. Experimental conditions 2. I. Vacuum The experiments described here were performed in a stainless steel UHV system equipped with a 3M ISS spectrometer and a PHI 10 KV Auger system. Total pressure measurements were made with a Perkin-Elmer digital ion gauge. The 3M ISS system incorporates a Cylindrical Mirror Analyser (CMA) for energy analysis of the scattered ions and a coaxial ion gun. As the ISS system requires that that the entire chamber be backfilled with the noble gases being used, a 170 l/s turbo-molecular pump has been used to allow the chamber to be backfilled dynamically. The turbo pump is attached to the system via a 6 inch gate valve and 6 inch port. Prior to running any ISS measurements the entire system was baked overnight and the turbe pump was run continuously for at least a week while the pump and gate valve were baked. After this procedure the base pressure in the system with the ion pump connected was below 3 X lo-” Torr under which the conditions the barium evaporations and Auger measurements were made. With the ion pump valved off and the gate valve to the turbo pump fully open, the base pressure was less than 1.0 X 1O-9 Torr. Estimates from Residual Gas Analyser (RGA) measurements indicated that ambient contained approximately 60% H,O, 15% H,, 15% mass 28 (CO, N,) and 5% CH,. Under these conditions the chamber was backfilled with the
374
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et al. / ISS studies
of Ba and 0 on W cathodes
noble gases for the 1% measurements and oxygen for the oxygen exposures. The background pressure, while the chamber was backfilled at 5 X 1O-5 Torr of noble gas, was also monitored with the RCA and estimated to be about lo-’ Torr. The main components of the background were CH,, C2H6, mass 28 (probably N,), H, and H,O. Of these gases, water vapor (- 20% of the ambient) was considered to be the most important in terms of possible contamination of the sample during the ISS measurements. 2.2. Spectroscopy The times to set-up and record the ISS spectra were minimized as much as possible. Typically a spectra took 10 s to record following a few seconds in which the noble gas pressures were adjusted. Contamination during the ISS measurements was monitored with Auger and the ISS measurements themselves. No evidence of carbon, nitrogen or neon pickup was apparent and only a small oxygen pickup on the barium films was observed. The level of oxygen was such that it could not be detected by ISS and was estimated to be less than 5% of a monolayer. As an added precaution the samples were heated immediately prior to each ISS measurement to remove any physisorbed hydrogen. Temperatures were chosen so as not to change the surface concentration of oxygen and/or barium. The ion beam was rastered over the surface and the ion current chosen so that the effects of sputtering were less than 5% of the peak amplitudes. Thus extreme care was taken to ensure that each ISS spectrum reflected an uncontaminated and unsputtered surface. Further details of the experimental conditions, under which the ISS spectra were recorded, are provided in table 1. There are several advantages in recording an ISS spectrum with as low a primary beam energy as possible. For example, as the primary energy is lowered, the surface sensitivity is enhanced, the multiple scattering background in the spectra is reduced and sputtering of the surface decreases. However, the neutralization of the incident ions increases as their velocity decreases, see for
Table 1 1% experimental 3M spectrometer, Ion beam rastered Primary ion beam Typical operating
conditions
CMA with coaxial ion gun over 4 mm* energy: 500 V pressures: 1 x lo-’ Torr *‘Ne 4x lo-’ Torr 4He Beam current: 23 nA *’Ne 67nA 4He Scattering angle: 136’ Incident angle: 90“
C.R.K. Marrianet al. / ISS studies of Ba and 0 on W cathodes
375
example [5]. 500 V was selected as the lowest beam voltage at which an acceptable signal-to-noise ratio could be obtained in a time during which contamination and sputtering could be considered as negligible. The decrease in background is readily apparent by comparing spectra in ref. [6], recorded with 1 kV incident ions, and the ISS spectra in this paper. All the ISS spectra described here were taken with the sample within 200 K of room temperature, with the chamber backfilled with a combination of isotopically pure research grade helium 4 and neon 20. As the dispenser cathodes or the tungsten ribbon were heated to temperatures at which appreciable electron emission occurs, the scattered ion yield fell until no peaks were detectable. This was attributed to increased neutralization of the scattered and incident ions by the space charge of electrons around the sample. It was considered undesirable to apply a bias potential to the sample to remove the space charge as this would result in distortion of the optics of the CMA. Thus it is not possible to study dispenser cathodes with ISS at typical cathode operating temperatures. Compared to the conditions described previously [6], the efficiency of the Auger spectrum recording (in terms of signal-to-noise ratio per unit time of spectrum recording) has been greatly increased by using an IBM PC to multiplex the pass energy of the Cylindrical Mirror Analyser in the Auger Spectrometer. Spectra are recorded by averaging many (typically 800) separate multiplexed Auger electron energy scans. A polycrystalline tungsten ribbon has been used in these experiments. It was cleaned by direct heating so that the AES and ISS spectra indicated no impurities. Research grade oxygen was admitted dynamically into the chamber for the oxygen exposures. The barium was deposited from a barium evaporator using the common double distillation technique. .In both cases there was no detectable impurity in the depositions. Monolayers of oxygen and barium were deposited onto the tungsten ribbon using well documented techniques. An exposure to 10 L of oxygen followed by annealing at 1400 K results in a tungsten surface being covered with a monolayer of oxygen [7]. Monolayer coverage of barium can be achieved by carefully heating the tungsten ribbon when it is covered with more than a monolayer of barium (see for example refs.
PA. 3. Experimental results 3.1. Calibration measurements AES has been used to characterize the controlled surfaces which have been studied with ISS. The conventional technique for such a calibration involves the measurement of the Auger sensitivity factors and Auger electron escape
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes
316
/ 2.5
I
I
571B
Y
I
,
I
I
_L
W,736
II
22
I
1
24
(ARB.UNITS)
Fig. 1. Plot of the amplitude of the 170 eV tungsten AES peak versus the 1736 eV peak for different spots on the cleaned polycrystalline tungsten ribbon.
depths. However, this can lead to problems as is indicated in fig. 1 which shows a plot of the amplitudes of the tungsten Auger peaks at 170 and 1736 eV measured at different spots on the cleaned polycrystalline tungsten ribbon. The scatter in the points is greater than indicated by the error bars which reflect the accuracy of the Auger peak height measurements. Although the spectrometer does not have a facility for operating with a constant beam current, the scatter cannot be attributed to primary beam current fluctuations because the ratio of the peaks varies together with the amplitudes. Clearly this makes it difficult to measure a sensitivity factor for tungsten that would not be relevant to only a single crystallite. Auger electrons have an escape depth which is a function of their energy and the material through which they are escaping. Thus the amplitude of a substrate peak can be used to measure the coverage of an adsorbate if the escape depth of the Auger electrons (in the adsorbate) is known. In practice it is more convenient to consider the ratio between two peaks as this eliminates any effects caused by variation in the primary beam. Due to the difference in energy, the escape depth of the 1736 eV tungsten Auger electrons is about 3.3 times greater than that of the 170 eV electrons. The ratio of the two tungsten peaks can be considered as having an escape depth given by: I/W&l - l/&L where h,,, and X,736 are the escape depths Auger electrons respectively.
of the 170 and 1736 eV tungsten
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes Table 2 Escape depths of tungsten
auger electrons
in monolayers
Oxygen
W,,O/W1736
170 eV 340 eV 1136 eV
317
Barium
x
SD
h
SD
12.7 9.1 NM 32
0.9 1.2 NM 6
3.9 3.0 3.8 13.2
0.5 0.3 0.7 2.7
X = escape Depth;
SD = standard
deviation;
NM = not measured.
Thus the ratio of the two tungsten peaks provides a technique, which does not require any sensitivity factors, for measuring the thickness of an adsorbate. The escape depths of the tungsten Auger electrons and the effective escape depth of the 170 to 1736 eV peak ratio have been measured in oxygen and barium and are tabulated in table 2. Clearly there is a difference in the escape depths in the two different materials. Furthermore whereas the escape depths measured in oxygen seem to scale with the square root of the electron energy [lo], those measured in barium do not. The standard deviation values reflect the variation in escape depth measurements and include contributions both from the errors in peak height measurements and the differing packing densities on the various crystallites of tungsten. The values of the escape depths have been used to calculate the percentage transmission of the tungsten Auger electrons through a monolayer of oxygen and a monolayer of barium, table 3. For example, a monolayer of oxygen will reduce the amplitude of the tungsten 170 eV Auger peak by 10% whereas a monolayer of barium will cause a 29% reduction. An alternate method of measurement of coverage with Auger spectroscopy without the use of sensitivity factors is the break point method [11,12]. It has a significant advantage in that it does not require values for the Auger electron escape depths. However problems arise with oxygen on tungsten because the sticking coefficient changes with coverage and the attenuation of the oxygen Auger signal by oxygen itself is low. Thus there may be cases where the
Table 3 C Transmission
KdW,73, 170 eV 340 eV 1736 eV
of a monolayer Oxygen
Barium
92 90 _
7-l 71 77 93
97
378
C. R. K. Marrian et al. / ISS studies
of Ba and 0 OR W cathodes
Ba +W (He4)
I
8a(Nez0)
‘Z/E0 Fig. 2. 1% spectrum from a tungsten surface containing with the corresponding target atoms and incident ions.
method described above is preferable are known or can be measured.
oxygen and barium.
if accurate
Each peak is labeled
values of the relevant
escape
3.2. ISS peak height analysis An ISS spectrum typical of those described here is shown in fig. 2. The spectra shows a surface containing oxygen, barium and tungsten examined using a primary beam containing both helium 4 and neon 20 ions. The four peaks apparent in the spectra are close to the energies predicted from the simple “billiard ball” scattering model [l] for the target and incident ions indicated on the figure. As neon is heavier than oxygen, there will be no peak corresponding to neon scattering from oxygen. Therefore the combination of the two noble gases is required to distinguish the three elements being studied. The oxygen and tungsten ISS peaks have been studied for different coverages of oxygen on the tungsten ribbon. The results are summarized in fig. 3, where a linear increase of the oxygen peak height with coverage is apparent. However, at low coverages there is a large scatter in the tungsten peak size although at larger coverages a linear decrease in peak size with coverage is apparent. Annealing the tungsten ribbon by heating (without desorbing any oxygen) resulted in spectra with larger tungsten peak heights which fell closer to the line on fig. 3. It is felt that this change reflects different surface atomic
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes
0
OXYGEN Fig.
1 .o
0.5
0 3. Normalized
379
COVERAGE,MONOLAYERS
oxygen
and tungsten
peak heights
as a function
of oxygen coverage.
arrangements [3] and illustrates the extreme sensitivity of the ISS technique to shadowing of a substrate by an adsorbate. The scatter in the values of the oxygen peak height at monolayer coverage are due to fluctuations in the primary beam ion flux which is a problem with our ISS spectrometer. However such linear behaviour of peak heights with coverage is not evident in fig. 4 in which results from the barium on tungsten system are presented. The barium peak height increases from zero, goes through a maximum and disappears, together with the tungsten peak, into the noise level at coverages above 0.5 monolayers. The unresolved high energy peaks due to helium scattering from barium and tungsten showed the same dramatic decrease in amplitude. A similar, although not so pronounced, effect was observed in ISS studies of beryllium on tungsten [3]. It does not seem to be possible that his phenomenon is the results of contamination of the sample during the ISS measurements because, as noted above, the level of possible contaminants was too low and no evidence of the necessary level of contamination was found. Furthermore, it is not possible to produce a reasonable explanation for the observed behaviour of the ISS peaks based on helium (the only element present at a sufficient level that would not be detected by either ISS or AES) somehow masking the sample surface. Presumably the scattered ion yield decreasing is related to the lowering of the work function resulting from the tungsten being covered by barium [13]. In carefully controlled Secondary Ion Mass Spectroscopy (SIMS) of barium on tungsten, a similar trend in the Ba+ ion yield was observed [14]. However, the
C. R. K. Marrian et al. / ISS studies
380
BARIUM’COVERAGE,
Fig. 4. Normalized
of Ba and 0 on W cathodes
MONOLAYERS
barium and tungsten peak heights as a function of barium coverage.
detailed mechanisms of the neutralization processes involved may be different because of the greater electron affinity of the noble gas ions (> 20 eV) compared to Ba+ (5.2 ev). Nonetheless, it appears that the neutralization of the noble gas ions increases as the work function of the surface drops below about 4 eV. Thus the absolute sensitivities of the ISS peaks are affected by the surface work function and consequently cannot be measured. 3.3. IS.9 peak height ratios As shown above, the evaluation of absolute ISS sensitivity factors for barium, tungsten and oxygen is not possible. However, if the scattered ion yield, as a function of coverage, is the same for the ions scattered from barium and those scattered from tungsten, a measurement of the relative sensitivity factors should be possible. If one considers the ISS signals from an adsorbate A on a substrate B, the A signal and the B signal will be proportional to the terms given below. ISS, = e,s,r(
t9,),
IS% = [(I - &)S,
+ B~,J,]W,),
where 0, is the fractional coverage of A, S, is the sensitivity for A. The corresponding quantities for B are represented by the subscript B. /? represents the fraction of the B ISS signal which is transmitted through the adsorbate, A. r(i9,) describes the variation of the scattered ion yield with the coverage of A.
C.R.K. Marrian et al. / IS.9 studies of Ba and 0 on W cathodes
Fig. 5. Plot of the ISS peak ratio y versus the ratio divided tungsten and oxygen on tungsten.
by the coverage
y/8
381
for barium
on
When considering the ratio of the ISS signals from A and B, r( 6,) cancels and some simple algebra reveals that the ratio of the two signals y is a linear function of the ratio divided by the coverage, i.e UP,% = Y(I - P) + a? where OLis S,/S,. Thus a plot of y against y/f?, should give a straight line, the slope and intercept of which will allow /I and CYto be measured. Such a plot is shown in fig. 5 for both oxygen on tungsten and barium on tungsten and indeed the data fall close to straight lines. For the oxygen case, the slope indicates that the presence of a monolayer of oxygen attenuates the signal from the underlying tungsten substrate by 90%. The intercept indicates a relative sensitivity close to 0.1. Both these values correspond to those measured from fig. 3. However, for the case of barium on tungsten the simplistic approach outlined above is not sufficient as the slope of the line is less than unity indicating a negative value of fi. Although there are many possible explanations for this, the negative value is compatible with the barium atoms shadowing a greater area of tungsten than that expected from simple geometric considerations. Fig. 6 shows a plot of the ratio of the barium and tungsten (neon) ISS peaks against the ratio divided by the fractional barium coverage for the results obtained when the tungsten surface was exposed to 0.75 L of oxygen and annealed at 1450 K prior to being covered with a fractional coverage of barium. The oxygen exposure and annealing gave an oxygen coverage of 0.5 monolayers. For this surface the amplitudes of the peaks exhibited a similar
382
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes
Fig. 6. Plot of the ratio of the barium and tungsten KS peaks versus the ratio divided by the barium coverage for’barium deposited onto a tungsten surface covered with 0.5 monolayers of oxygen.
variation with barium coverage as that shown in fig. 4 for barium on clean tungsten. At coverages greater than about 0.5 monolayers of barium all the ISS peaks vanished into the noise in the spectra. The slope of the line in fig. 6 is close to unity indicating complete screening of the underlying tungsten signal. The same conclusion can be drawn from fig. 7 which shows results when the tungsten ribbon was exposed to 10 L of oxygen and annealed at 1450 K (giving monolayer coverage) prior to the barium depositions. In this case the ISS peaks disappeared when the tungsten was covered by more than about 0.6 monolayers of barium. As shown above, the intercepts on the plots shown in figs. 5-7 permit the relative barium and tungsten ISS sensitivities to be measured. However allowance must be made for the attenuation of the tungsten signal by the oxygen when analysing figs. 6 and 7. Once this is done, there is reasonable agreement between the estimates of the relative sensitivities. Furthermore the values are close to that obtained from the intercept in fig. 5. Although this may be coincidental, the simple model proposed above should fit better at the low barium coverages which give the data points close to the intercept on fig. 5. The linearity of the points in fig. 6 and 7 indicates that, for tungsten surfaces covered by oxygen and barium, the model proposed above is reasonable and the variation with coverage of the ionic yields of all t,hree elements can be described by the same function, r(@,,). The ISS calibrations may be summarized as follows. Under the conditions outlined in table 1, the relative sensitivities of the oxygen (helium) peak and the barium (neon) peak to the tungsten (neon) peak are 0.1 and 0.4 respec-
C. R. K. Martian et al. / ISS studies of Ba and 0 on W cathodes
Fig. 7. AS fig. 6, only for barium deposited onto a tungsten surface covered by a full monolayer oxygen.
383
of
tively. Whereas barium shields the signal from the underlying substrate completely, 10% of the substrate signal is transmitted through an oxygen monolayer. The value for the transmission of a barium layer only applies when there is oxygen present on the tungsten surface. The oxygen has the effect of limiting the shielding of the tungsten substrate to that expected from simple geometric considerations. It must be emphasized that the sensitivity values are critically dependent on the spectrometer and the experimental conditions described here and are only included for completeness.
4. Quantitative interpretation of ISS spectra 4.1. Controlled surfaces The spectra taken of the two systems described in figs. 6 and 7 have been evaluated quantitatively using the factors described in the previous section. The results for barium deposited on the tungsten ribbon covered with 0.5 monolayers of oxygen are shown in fig. 8. The barium fractional area coverage is seen to increase linearly with the barium evaporation time as one would hope! The surface without barium coverage has equal tungsten and oxygen area fractions which agrees well with the 0.5 monolayer coverage indicated by
C. R. K. Mm-rim et al. / ISS studies of Ba and 0 on W cathodes
384
6
0
Ba EVAPORATION Fig. 8. Quantitative evaluation monolayers of oxygen.
TIME,
mm
of the ISS spectra
of barium
deposited
onto tungsten
covered by 0.5
AES. As the barium coverage increases both the oxygen and tungsten area fractions decrease at about the same rate. The packing density of a barium monolayer is about half that of the underlying tungsten surface [8,15] and the packing density of an oxygen monolayer [7]. Thus it would appear that each barium atom is shadowing one tungsten atom and one oxygen atom. Compared to AES, the ISS measurements indicate about 12% greater barium coverage. This discrepancy may be due to chemical effects on the sensitivity values or the shadowing being somewhat more complex than assumed above. The corresponding analysis for barium deposited onto tungsten covered by a full monolayer of oxygen is shown in fig. 9. As oxygen has a far lower sensitivity than tungsten and attenuates the tungsten signal by 90%, the signal-to-noise ratio is much worse in the spectra of the surfaces containing a full monolayer of oxygen. This is reflected in the greater scatter of the data in fig. 9 as compared to fig. 8. Again the barium area fraction increases linearly with coverage although the coverage is again slightly higher than the values indicated by AES. The presence of a few percent of tungsten at the surface is a further indication of the limits of the model proposed. However in general the ISS measurements have provided a quantitative assessment of the two systems which agree (within about 12%) with the values measured by AES. 4.2. Dispenser
cathodes
The spectrum in fig. 2 is of dispenser cathode made with an uncoated tungsten matrix impregnated with barium, calcium and aluminum oxides in the ratio 6 : 1: 1. The spectrum was obtained by superposing several spectra taken with the cathode at room temperature. However, the cathode was heated to - 1500 K between the recording of each spectra in order to overcome the sputtering and poisoning of the cathode surface. Thus a signal-to-noise ratio
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes
385
6 Ba
EVAPORATION
Fig. 9. Quantitative evaluation full monolayer of oxygen.
TIME,
min
of the ISS spectra
of barium
deposited
onto tungsten
covered
by a
four times better than that indicated in fig. 2 could be obtained even though the cathode had a work function near 2 eV and a correspondingly low scattered ion yield. The spectrum has been analysed using the measured values for the sensitivity factors to obtain the results shown in table 4. Here the analysis has been performed both in terms of area fraction and atomic percentages. Compared to AES measurements (for example ref. [6]), the ISS results indicate much less tungsten at the cathode surface. In view of the greater surface sensitivity of ISS, the ISS results provide direct evidence for the widely held view that the cathode surface is tungsten covered by fractional monolayer coverage of barium and oxygen. Again compared to AES, the ISS results indicate there is less oxygen at the cathode surface. The oxygen (associated with barium) on active dispenser cathode surfaces is considered to either to lie Table 4 ISS measurements
of uncoated
tungsten
Impregnant
Barium a)
611 532
64 71
matrix
dispenser
cathodes
Oxygen
Tungsten
24 19
12 10
35 29
18 16
% Area fraction
Atomic 611 532 ‘) Oxygen
47 55
%
which is not seen by ISS may be associated
with each barium
atom.
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes
386
EXPOSED TO 1OL 02
ACTIVE STATE
x i/4
XI
I
E/E0
/L/
i
.4
.6
I
I
1
I
I
.%
I
I
1.0
J
JI ‘--I’ .4
Ji I
.6
I
I
.6
I
‘I 1.0
Fig. 10. ISS spectra of a 532 “B” cathode in an active and a poisones state.
almost coplanar with the barium on top of the tungsten substrate [13] or underneath the barium [16]. In either case it is reasonable to assume that, considering the relative sizes of the barium and oxygen in the form they exist on a cathode surface, the barium could shadow the oxygen so that the oxygen next to the barium does not contribute to an ISS peak (in our spectrometer). Within the limits of the accuracy of the ISS calibrations, the barium coverage as indicated by ISS is in agreement with AES measurements of the cathode at 1500 K. Thus the surface of the active cathode in fig. 2 can be described as 64% barium and oxygen (which is not seen by ISS), 24% oxygen (in the form of monolayer coverage on tungsten) and 12% exposed tungsten. The spectrum shown in fig. 2 is similar to previously published ISS spectra of dispenser cathodes [6,17]. Furthermore the slight differences (mainly in the relative barium and tungsten peak heights) can be attributed to the different state of activation of the cathode [6] or the different experimental conditions. The quantitative ISS results are also compatible with recent analysis [18] of AES results from uncoated tungsten matrix dispenser cathodes such as those in ref. [19]. Fig. 10 shows spectra from a “B” cathode with a 532 impregnant in an active state and after the cathode had been exposed to 10 L of oxygen. The spectrum from the active cathode indicates a similar surface (see table 4) to the spectrum from the 611 cathode shown in fig. 2. As can be seen from fig. 10, the overall scattered ion yield from the cathode exposed to oxygen was greater than from the active cathode. Presumably this is a result of the higher work function of the cathode exposed to oxygen [20]. Compared to AES, the ISS
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes aB
+ 6 532
I
0
.02
I .04 O/(2.4
I
I
.06 .OB x Ba +W) (ISS)
387
611
I
.lO
Fig. 11. Plot of the oxygen (510 eV) to tungsten (1736 eV) AES peak ratio versus the oxygen to substrate (barium and tungsten) ISS peak ratio for uncoated tungsten matrix cathodes in active and poisoned states.
spectra indicate a greater increase in the amount of oxygen at the cathode surface following the oxygen exposure. This is illustrated in fig. 11 where the ISS oxygen to substrate (tungsten plus barium) peak ratio is plotted against the corresponding AES ratio for active and oxygen poisoned uncoated tungsten dispenser cathodes. ISS is more surface sensitive than AES, which suggests that the oxygen from the poisoning exposure is on the topmost atomic layer of the cathode. The obvious sites for the adsorbed oxygen are on top of each barium atom and on any tungsten which was exposed on the active cathode. Thus one can exploit the different sampling depths of the two spectroscopies to obtain an idea of the position of the oxygen on the cathode surface. AES indicates that poisoning an uncoated tungsten matrix dispenser cathode with 10 L of oxygen does not change the amount of barium at the cathode surface [6]. Thus the quantitative interpretation of the ISS spectrum of fig. 10 must result in the amount of barium at the surface being the same as on both the active and poisoned cathode surfaces. Interpreting the ISS spectra in terms of the surfaces described above, there is agreement in the barium area fraction provided either (a) the overlying oxygen does not attenuate the barium signal or (b) the overlying oxygen does attenuate the barium ISS signal but does not contribute to the oxygen ISS signal. When a near monolayer of barium oxide on tungsten is exposed to oxygen, interatomic Auger and work function studies [13,20] suggest a significant transfer of charge to the adsorbed oxygen. In contrast when clean tungsten is
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C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes
exposed to oxygen, the surface dipole causing the change in work function (- I eV for a monolayer of oxygen on (100) tungsten [21]) is such that the charge gained by the adsorbed oxygen cannot be as great. So it appears likely that the oxygen overlying barium will have a greater negative charge per atom than the oxygen on tungsten. It seems reasonable to expect greater neutralization of the incident positive noble gas ions by the more negatively charged oxygen so the scattered ion yield from the oxygen overlying the barium would be less. Thus the second of the two explanations (b) seems reasonable. It is interesting to note that if this is indeed the case, the oxygen overlying the barium is “seen” in the ISS spectrum by its effect on the substrate rather than the signal it contributes directly. It should be emphasized that the results of the quantitative evaluation of the dispenser cathodes are by no means unique. When there are two or more species adsorbed on a surface, there may be many imaginable surface geometries and many different chemical effects on the sensitivities. Thus ISS data can only be interpreted in terms of a specific model of the surface and can rarely be evaluated unambiguously particularly when oxygen and another element are present on the surface of a third element.
5. Conclusions
In order to achieve quantitative results from ISS, closely controlled calibration experiments in conjunction with a companion spectroscopy are necessary. The results of such calibrations are so dependent on the various parameters of the spectrometer and the experimental conditions that extreme care must be taken before using the results in other experiments. AES has been used in this work as a companion spectroscopy to determine surface coverage. A technique exploiting the variation with energy of the escape depth of the tungsten Auger electrons for measuring surface coverage has been described, together with values for the escape depths in oxygen and barium. The absolute ISS sensitivities of barium, oxygen and tungsten have been found to be strongly dependent on the work function of the surface on which they are present. As the work function decreases below - 4 eV the scattered ion yield drops dramatically. In the spectrometer used for this work, the ISS peaks could not be detected above the noise if the work function of the surface was less than 2 eV. However, the three elemental peaks seem to decrease in size at the same rate so that the relative sensitivities did not change by more than 12%. Interpretation of ISS spectra is often further complicated by shadowing effects. However, when barium and oxygen are both present on a tungsten surface the shadowing can be easily modeled. As a result this system can be studied quantitatively with ISS once the spectrometer has been calibrated. Although, it is not possible to examine dispenser cathodes at temperatures
C. R. K. Marrian et al. / ISS studies
of Ba and 0 on W cathodes
389
at which electron emission occurs, their surfaces can be regenerated easily allowing the low signal level caused by the low work function of the surface to be overcome by the superposing of several ISS spectra. Analysis of the spectra provides direct evidence in support of the tungsten at the cathode surface having fractional monolayer coverage of barium and oxygen. The spectra can be evaluated quantitatively in terms of such a surface to give the area fractions of the different types of coverage. Furthermore, the apparent differences between AES and ISS results can be reconciled once the results are interpreted in terms of the surface model and shadowing discussed here. In order to obtain information on relative atomic positions, the capability to vary the incident and scattering angles is necessary. However, with the present spectrometer, some limited success has been achieved by exploiting the different sampling depths of AES and ISS for studies of the position of oxygen on poisoned cathode surfaces. In general however, interpretation of ISS spectra from surfaces containing more than two species is complicated by the many possible surface geometries that can be envisioned. Therefore it is only really possible to interpret an ISS spectrum in terms of a specific model of the surface atomic arrangement.
Acknowledgement The authors would like assistance of Charles Hor.
to express
their
appreciation
of the
technical
References PI T.M. Buck, in: Methods of Surface Analysis,
Ed. A.W. Czanderna (Elsevier, Amsterdam, 1975) ch. 3. PI E.N. Haeussler, Surface Interface Anal. 2 (1980) 134. 131 H. Niehus and E. Bauer, Surface Sci. 47 (1975) 222. [41 H.H. Brongersma, G.C.J. van der Ligt and G. Rouweler, Philips J. Res. 36 (1981) 1. [51 W. Bless and D. Hone, Surface Sci. 72 (1978) 277. [61 C.R.K. Marrian, A. Shih and G.A. Haas, Appl. Surface Sci. 16 (1983) 1. 171 E. Bauer, H. Poppa and Y. Viswanath, Surface Sci. 58 (1976) 517. PI G.E. Moore and H.W. Allison, J. Chem. Phys. 23 (1955) 1609. [91 A. Shih, G.A. Haas and C.R.K. Marrian, Appl. Surface Sci. 16 (1983) 93. [lOI M.P. Seah and W.A. Dench, Surface Interface Anal. 1 (1979) 2. 1111 J.P. Biberian and G.A. Somojai, Appl. Surface Sci. 2 (1979) 352. WI C. Argile and G.E. Rhead, Thin Solid Films 67 (1979) 352. 1131 G.A. Haas, A. Shih and C.R.K. Martian, Appl. Surface Sci. 16 (1983) 139. 1141 B.C. Lamartine, private communication. P51 D.A. Gorodetsky and Yu P. Melnik, Surface Sci. 62 (1977) 647. MI R. Forman, Appl. Surface Sci. 2 (1979) 258. 1171 W.L. Baun, Appl. Surface Sci. 4 (1980) 291.
390 [IS] G.A.
CR. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes Haas, C.R.K.
Marrian and A. Shih, Appl.
Surface Sci. 24 (1985)
430.
[19] G. Eng, H.K.A. Kan and K.T. Luey, Appl. Surface Sci. 16 (1983) 181. [20] C.R.K. Marrian, G.A. Haas and A. Shih, Appl. Surface Sci. 16 (1983) 73. [21] E. Bauer, H. Poppa and Y. Viswanath, Surface Sci. 58 (1976) 517.
Applied
Surface Science 24 (1985) 372-390 North-Holland, Amsterdam
ION SCA-ITERING SPECTROSCOPY STUDIES OF BARIUM AND OXYGEN ON TUNGSTEN AND TUNGSTEN-BASED DISPENSER CATHODES C.R.K.
MARRIAN,
A. SHIH
Code 6832. Naval Research Laboratory,
Received
25 May 1984; accepted
and G.A. HAAS Washington, DC 20375, USA
for publication
15 March
1985
In order to use Ion Scattering Spectroscopy (ISS) for studies of tungsten dispenser cathodes, the relevant ISS sensitivities must be measured. Calibrations have been made using a polycrystalline tungsten ribbon with controlled coverages of oxygen, barium and combinations thereof. Auger Electron Spectroscopy (AES) was used to monitor these controlled surfaces and the escape depths of the tungsten Auger electrons in barium and oxygen have been measured. The absolute ISS sensitivities of all three elements were found to be strongly dependent on the barium coverage of the tungsten surface. This effect has been attributed to the lowering of the work function of the tungsten surface caused by the barium adsorption. However, the relative ISS sensitivities of the three elements are not affected in this way when both barium and oxygen (or oxygen alone) are present on the tungsten surface. ISS spectra of such surfaces have been analyzed quantitatively and found to be in reasonable agreement with AES measurements. The analysis has also been applied to ISS spectra of uncoated tungsten matrix dispenser cathodes in an active state and following exposure to oxygen. Compared to AES, these spectra indicate less oxygen on the active cathode surfaces as a result of the oxygen (associated with barium) not contributing to the oxygen ISS signal. Comparisons of the spectra from the active and oxygen poisoned cathodes suggest that oxygen adsorbed during the oxygen exposure sits on the topmost barium layer whereas the oxygen on the active cathode surface does not.
1. Introduction Ion Scattering Spectroscopy (ISS) is known as a powerful technique for the investigation of the topmost atomic layer on a surface. Many detailed descriptions of the technique have been published (for example ref. [l]). In addition, the problems of quantitative ISS have been extensively discussed (for example refs. [2-41). Tungsten based dispenser cathodes are widely believed to have near monolayer coverage of barium and oxygen at the surface and thus appear to be a type of surface for which ISS would provide interesting results. However, the relevant sensitivity factors are not known and, in any case, need to be measured in the actual spectrometer being used. Thus it has proved necessary to study the type of surface found on a dispenser cathode under
0169-4332/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
C.R. K. Marrian et al. / ISS studies of Ba and 0 on W caihodes
373
controlled conditions. In this paper ISS results from the following three surfaces will be discussed: oxygen on tungsten, barium on tungsten and barium on tungsten which had previously been exposed to oxygen. These studies have allowed the evaluation of the relevant sensitivity factors which have been applied to the analysis of results from a dispenser cathode. The oxygen on tungsten system has been studied with ISS [3,4]. In ref. [3] the increase of the oxygen ISS peak and the decrease of the ISS tungsten peak were not found to vary linearly with the coverage of oxygen on a single crystal tungsten substrate. These effects were attributed to surface rearrangement as indicated by LEED [2]. In contrast in studies of oxygen adsorption on polycrystalline tungsten, linear behaviour of the ISS peaks was observed [4]. The authors are unaware of any ISS studies of barium on tungsten. As a dispenser cathode is a polycrystalline surface, it was felt that studies of a polycrystalline tungsten ribbon would be the most realistic way to evaluate the necessary sensitivity factors. In order to calibrate the surfaces being studied with ISS, Auger Electron Spectroscopy (AES) has been used. A section of the paper is devoted to this aspect of the work and the results of measurements of the escape depths of tungsten Auger electrons in barium and oxygen are presented.
2. Experimental conditions 2. I. Vacuum The experiments described here were performed in a stainless steel UHV system equipped with a 3M ISS spectrometer and a PHI 10 KV Auger system. Total pressure measurements were made with a Perkin-Elmer digital ion gauge. The 3M ISS system incorporates a Cylindrical Mirror Analyser (CMA) for energy analysis of the scattered ions and a coaxial ion gun. As the ISS system requires that that the entire chamber be backfilled with the noble gases being used, a 170 l/s turbo-molecular pump has been used to allow the chamber to be backfilled dynamically. The turbo pump is attached to the system via a 6 inch gate valve and 6 inch port. Prior to running any ISS measurements the entire system was baked overnight and the turbe pump was run continuously for at least a week while the pump and gate valve were baked. After this procedure the base pressure in the system with the ion pump connected was below 3 X lo-” Torr under which the conditions the barium evaporations and Auger measurements were made. With the ion pump valved off and the gate valve to the turbo pump fully open, the base pressure was less than 1.0 X 1O-9 Torr. Estimates from Residual Gas Analyser (RGA) measurements indicated that ambient contained approximately 60% H,O, 15% H,, 15% mass 28 (CO, N,) and 5% CH,. Under these conditions the chamber was backfilled with the
374
C. R. K. Ma&an
et al. / ISS studies
of Ba and 0 on W cathodes
noble gases for the 1% measurements and oxygen for the oxygen exposures. The background pressure, while the chamber was backfilled at 5 X 1O-5 Torr of noble gas, was also monitored with the RCA and estimated to be about lo-’ Torr. The main components of the background were CH,, C2H6, mass 28 (probably N,), H, and H,O. Of these gases, water vapor (- 20% of the ambient) was considered to be the most important in terms of possible contamination of the sample during the ISS measurements. 2.2. Spectroscopy The times to set-up and record the ISS spectra were minimized as much as possible. Typically a spectra took 10 s to record following a few seconds in which the noble gas pressures were adjusted. Contamination during the ISS measurements was monitored with Auger and the ISS measurements themselves. No evidence of carbon, nitrogen or neon pickup was apparent and only a small oxygen pickup on the barium films was observed. The level of oxygen was such that it could not be detected by ISS and was estimated to be less than 5% of a monolayer. As an added precaution the samples were heated immediately prior to each ISS measurement to remove any physisorbed hydrogen. Temperatures were chosen so as not to change the surface concentration of oxygen and/or barium. The ion beam was rastered over the surface and the ion current chosen so that the effects of sputtering were less than 5% of the peak amplitudes. Thus extreme care was taken to ensure that each ISS spectrum reflected an uncontaminated and unsputtered surface. Further details of the experimental conditions, under which the ISS spectra were recorded, are provided in table 1. There are several advantages in recording an ISS spectrum with as low a primary beam energy as possible. For example, as the primary energy is lowered, the surface sensitivity is enhanced, the multiple scattering background in the spectra is reduced and sputtering of the surface decreases. However, the neutralization of the incident ions increases as their velocity decreases, see for
Table 1 1% experimental 3M spectrometer, Ion beam rastered Primary ion beam Typical operating
conditions
CMA with coaxial ion gun over 4 mm* energy: 500 V pressures: 1 x lo-’ Torr *‘Ne 4x lo-’ Torr 4He Beam current: 23 nA *’Ne 67nA 4He Scattering angle: 136’ Incident angle: 90“
C.R.K. Marrianet al. / ISS studies of Ba and 0 on W cathodes
375
example [5]. 500 V was selected as the lowest beam voltage at which an acceptable signal-to-noise ratio could be obtained in a time during which contamination and sputtering could be considered as negligible. The decrease in background is readily apparent by comparing spectra in ref. [6], recorded with 1 kV incident ions, and the ISS spectra in this paper. All the ISS spectra described here were taken with the sample within 200 K of room temperature, with the chamber backfilled with a combination of isotopically pure research grade helium 4 and neon 20. As the dispenser cathodes or the tungsten ribbon were heated to temperatures at which appreciable electron emission occurs, the scattered ion yield fell until no peaks were detectable. This was attributed to increased neutralization of the scattered and incident ions by the space charge of electrons around the sample. It was considered undesirable to apply a bias potential to the sample to remove the space charge as this would result in distortion of the optics of the CMA. Thus it is not possible to study dispenser cathodes with ISS at typical cathode operating temperatures. Compared to the conditions described previously [6], the efficiency of the Auger spectrum recording (in terms of signal-to-noise ratio per unit time of spectrum recording) has been greatly increased by using an IBM PC to multiplex the pass energy of the Cylindrical Mirror Analyser in the Auger Spectrometer. Spectra are recorded by averaging many (typically 800) separate multiplexed Auger electron energy scans. A polycrystalline tungsten ribbon has been used in these experiments. It was cleaned by direct heating so that the AES and ISS spectra indicated no impurities. Research grade oxygen was admitted dynamically into the chamber for the oxygen exposures. The barium was deposited from a barium evaporator using the common double distillation technique. .In both cases there was no detectable impurity in the depositions. Monolayers of oxygen and barium were deposited onto the tungsten ribbon using well documented techniques. An exposure to 10 L of oxygen followed by annealing at 1400 K results in a tungsten surface being covered with a monolayer of oxygen [7]. Monolayer coverage of barium can be achieved by carefully heating the tungsten ribbon when it is covered with more than a monolayer of barium (see for example refs.
PA. 3. Experimental results 3.1. Calibration measurements AES has been used to characterize the controlled surfaces which have been studied with ISS. The conventional technique for such a calibration involves the measurement of the Auger sensitivity factors and Auger electron escape
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes
316
/ 2.5
I
I
571B
Y
I
,
I
I
_L
W,736
II
22
I
1
24
(ARB.UNITS)
Fig. 1. Plot of the amplitude of the 170 eV tungsten AES peak versus the 1736 eV peak for different spots on the cleaned polycrystalline tungsten ribbon.
depths. However, this can lead to problems as is indicated in fig. 1 which shows a plot of the amplitudes of the tungsten Auger peaks at 170 and 1736 eV measured at different spots on the cleaned polycrystalline tungsten ribbon. The scatter in the points is greater than indicated by the error bars which reflect the accuracy of the Auger peak height measurements. Although the spectrometer does not have a facility for operating with a constant beam current, the scatter cannot be attributed to primary beam current fluctuations because the ratio of the peaks varies together with the amplitudes. Clearly this makes it difficult to measure a sensitivity factor for tungsten that would not be relevant to only a single crystallite. Auger electrons have an escape depth which is a function of their energy and the material through which they are escaping. Thus the amplitude of a substrate peak can be used to measure the coverage of an adsorbate if the escape depth of the Auger electrons (in the adsorbate) is known. In practice it is more convenient to consider the ratio between two peaks as this eliminates any effects caused by variation in the primary beam. Due to the difference in energy, the escape depth of the 1736 eV tungsten Auger electrons is about 3.3 times greater than that of the 170 eV electrons. The ratio of the two tungsten peaks can be considered as having an escape depth given by: I/W&l - l/&L where h,,, and X,736 are the escape depths Auger electrons respectively.
of the 170 and 1736 eV tungsten
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes Table 2 Escape depths of tungsten
auger electrons
in monolayers
Oxygen
W,,O/W1736
170 eV 340 eV 1136 eV
317
Barium
x
SD
h
SD
12.7 9.1 NM 32
0.9 1.2 NM 6
3.9 3.0 3.8 13.2
0.5 0.3 0.7 2.7
X = escape Depth;
SD = standard
deviation;
NM = not measured.
Thus the ratio of the two tungsten peaks provides a technique, which does not require any sensitivity factors, for measuring the thickness of an adsorbate. The escape depths of the tungsten Auger electrons and the effective escape depth of the 170 to 1736 eV peak ratio have been measured in oxygen and barium and are tabulated in table 2. Clearly there is a difference in the escape depths in the two different materials. Furthermore whereas the escape depths measured in oxygen seem to scale with the square root of the electron energy [lo], those measured in barium do not. The standard deviation values reflect the variation in escape depth measurements and include contributions both from the errors in peak height measurements and the differing packing densities on the various crystallites of tungsten. The values of the escape depths have been used to calculate the percentage transmission of the tungsten Auger electrons through a monolayer of oxygen and a monolayer of barium, table 3. For example, a monolayer of oxygen will reduce the amplitude of the tungsten 170 eV Auger peak by 10% whereas a monolayer of barium will cause a 29% reduction. An alternate method of measurement of coverage with Auger spectroscopy without the use of sensitivity factors is the break point method [11,12]. It has a significant advantage in that it does not require values for the Auger electron escape depths. However problems arise with oxygen on tungsten because the sticking coefficient changes with coverage and the attenuation of the oxygen Auger signal by oxygen itself is low. Thus there may be cases where the
Table 3 C Transmission
KdW,73, 170 eV 340 eV 1736 eV
of a monolayer Oxygen
Barium
92 90 _
7-l 71 77 93
97
378
C. R. K. Marrian et al. / ISS studies
of Ba and 0 OR W cathodes
Ba +W (He4)
I
8a(Nez0)
‘Z/E0 Fig. 2. 1% spectrum from a tungsten surface containing with the corresponding target atoms and incident ions.
method described above is preferable are known or can be measured.
oxygen and barium.
if accurate
Each peak is labeled
values of the relevant
escape
3.2. ISS peak height analysis An ISS spectrum typical of those described here is shown in fig. 2. The spectra shows a surface containing oxygen, barium and tungsten examined using a primary beam containing both helium 4 and neon 20 ions. The four peaks apparent in the spectra are close to the energies predicted from the simple “billiard ball” scattering model [l] for the target and incident ions indicated on the figure. As neon is heavier than oxygen, there will be no peak corresponding to neon scattering from oxygen. Therefore the combination of the two noble gases is required to distinguish the three elements being studied. The oxygen and tungsten ISS peaks have been studied for different coverages of oxygen on the tungsten ribbon. The results are summarized in fig. 3, where a linear increase of the oxygen peak height with coverage is apparent. However, at low coverages there is a large scatter in the tungsten peak size although at larger coverages a linear decrease in peak size with coverage is apparent. Annealing the tungsten ribbon by heating (without desorbing any oxygen) resulted in spectra with larger tungsten peak heights which fell closer to the line on fig. 3. It is felt that this change reflects different surface atomic
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes
0
OXYGEN Fig.
1 .o
0.5
0 3. Normalized
379
COVERAGE,MONOLAYERS
oxygen
and tungsten
peak heights
as a function
of oxygen coverage.
arrangements [3] and illustrates the extreme sensitivity of the ISS technique to shadowing of a substrate by an adsorbate. The scatter in the values of the oxygen peak height at monolayer coverage are due to fluctuations in the primary beam ion flux which is a problem with our ISS spectrometer. However such linear behaviour of peak heights with coverage is not evident in fig. 4 in which results from the barium on tungsten system are presented. The barium peak height increases from zero, goes through a maximum and disappears, together with the tungsten peak, into the noise level at coverages above 0.5 monolayers. The unresolved high energy peaks due to helium scattering from barium and tungsten showed the same dramatic decrease in amplitude. A similar, although not so pronounced, effect was observed in ISS studies of beryllium on tungsten [3]. It does not seem to be possible that his phenomenon is the results of contamination of the sample during the ISS measurements because, as noted above, the level of possible contaminants was too low and no evidence of the necessary level of contamination was found. Furthermore, it is not possible to produce a reasonable explanation for the observed behaviour of the ISS peaks based on helium (the only element present at a sufficient level that would not be detected by either ISS or AES) somehow masking the sample surface. Presumably the scattered ion yield decreasing is related to the lowering of the work function resulting from the tungsten being covered by barium [13]. In carefully controlled Secondary Ion Mass Spectroscopy (SIMS) of barium on tungsten, a similar trend in the Ba+ ion yield was observed [14]. However, the
C. R. K. Marrian et al. / ISS studies
380
BARIUM’COVERAGE,
Fig. 4. Normalized
of Ba and 0 on W cathodes
MONOLAYERS
barium and tungsten peak heights as a function of barium coverage.
detailed mechanisms of the neutralization processes involved may be different because of the greater electron affinity of the noble gas ions (> 20 eV) compared to Ba+ (5.2 ev). Nonetheless, it appears that the neutralization of the noble gas ions increases as the work function of the surface drops below about 4 eV. Thus the absolute sensitivities of the ISS peaks are affected by the surface work function and consequently cannot be measured. 3.3. IS.9 peak height ratios As shown above, the evaluation of absolute ISS sensitivity factors for barium, tungsten and oxygen is not possible. However, if the scattered ion yield, as a function of coverage, is the same for the ions scattered from barium and those scattered from tungsten, a measurement of the relative sensitivity factors should be possible. If one considers the ISS signals from an adsorbate A on a substrate B, the A signal and the B signal will be proportional to the terms given below. ISS, = e,s,r(
t9,),
IS% = [(I - &)S,
+ B~,J,]W,),
where 0, is the fractional coverage of A, S, is the sensitivity for A. The corresponding quantities for B are represented by the subscript B. /? represents the fraction of the B ISS signal which is transmitted through the adsorbate, A. r(i9,) describes the variation of the scattered ion yield with the coverage of A.
C.R.K. Marrian et al. / IS.9 studies of Ba and 0 on W cathodes
Fig. 5. Plot of the ISS peak ratio y versus the ratio divided tungsten and oxygen on tungsten.
by the coverage
y/8
381
for barium
on
When considering the ratio of the ISS signals from A and B, r( 6,) cancels and some simple algebra reveals that the ratio of the two signals y is a linear function of the ratio divided by the coverage, i.e UP,% = Y(I - P) + a? where OLis S,/S,. Thus a plot of y against y/f?, should give a straight line, the slope and intercept of which will allow /I and CYto be measured. Such a plot is shown in fig. 5 for both oxygen on tungsten and barium on tungsten and indeed the data fall close to straight lines. For the oxygen case, the slope indicates that the presence of a monolayer of oxygen attenuates the signal from the underlying tungsten substrate by 90%. The intercept indicates a relative sensitivity close to 0.1. Both these values correspond to those measured from fig. 3. However, for the case of barium on tungsten the simplistic approach outlined above is not sufficient as the slope of the line is less than unity indicating a negative value of fi. Although there are many possible explanations for this, the negative value is compatible with the barium atoms shadowing a greater area of tungsten than that expected from simple geometric considerations. Fig. 6 shows a plot of the ratio of the barium and tungsten (neon) ISS peaks against the ratio divided by the fractional barium coverage for the results obtained when the tungsten surface was exposed to 0.75 L of oxygen and annealed at 1450 K prior to being covered with a fractional coverage of barium. The oxygen exposure and annealing gave an oxygen coverage of 0.5 monolayers. For this surface the amplitudes of the peaks exhibited a similar
382
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes
Fig. 6. Plot of the ratio of the barium and tungsten KS peaks versus the ratio divided by the barium coverage for’barium deposited onto a tungsten surface covered with 0.5 monolayers of oxygen.
variation with barium coverage as that shown in fig. 4 for barium on clean tungsten. At coverages greater than about 0.5 monolayers of barium all the ISS peaks vanished into the noise in the spectra. The slope of the line in fig. 6 is close to unity indicating complete screening of the underlying tungsten signal. The same conclusion can be drawn from fig. 7 which shows results when the tungsten ribbon was exposed to 10 L of oxygen and annealed at 1450 K (giving monolayer coverage) prior to the barium depositions. In this case the ISS peaks disappeared when the tungsten was covered by more than about 0.6 monolayers of barium. As shown above, the intercepts on the plots shown in figs. 5-7 permit the relative barium and tungsten ISS sensitivities to be measured. However allowance must be made for the attenuation of the tungsten signal by the oxygen when analysing figs. 6 and 7. Once this is done, there is reasonable agreement between the estimates of the relative sensitivities. Furthermore the values are close to that obtained from the intercept in fig. 5. Although this may be coincidental, the simple model proposed above should fit better at the low barium coverages which give the data points close to the intercept on fig. 5. The linearity of the points in fig. 6 and 7 indicates that, for tungsten surfaces covered by oxygen and barium, the model proposed above is reasonable and the variation with coverage of the ionic yields of all t,hree elements can be described by the same function, r(@,,). The ISS calibrations may be summarized as follows. Under the conditions outlined in table 1, the relative sensitivities of the oxygen (helium) peak and the barium (neon) peak to the tungsten (neon) peak are 0.1 and 0.4 respec-
C. R. K. Martian et al. / ISS studies of Ba and 0 on W cathodes
Fig. 7. AS fig. 6, only for barium deposited onto a tungsten surface covered by a full monolayer oxygen.
383
of
tively. Whereas barium shields the signal from the underlying substrate completely, 10% of the substrate signal is transmitted through an oxygen monolayer. The value for the transmission of a barium layer only applies when there is oxygen present on the tungsten surface. The oxygen has the effect of limiting the shielding of the tungsten substrate to that expected from simple geometric considerations. It must be emphasized that the sensitivity values are critically dependent on the spectrometer and the experimental conditions described here and are only included for completeness.
4. Quantitative interpretation of ISS spectra 4.1. Controlled surfaces The spectra taken of the two systems described in figs. 6 and 7 have been evaluated quantitatively using the factors described in the previous section. The results for barium deposited on the tungsten ribbon covered with 0.5 monolayers of oxygen are shown in fig. 8. The barium fractional area coverage is seen to increase linearly with the barium evaporation time as one would hope! The surface without barium coverage has equal tungsten and oxygen area fractions which agrees well with the 0.5 monolayer coverage indicated by
C. R. K. Mm-rim et al. / ISS studies of Ba and 0 on W cathodes
384
6
0
Ba EVAPORATION Fig. 8. Quantitative evaluation monolayers of oxygen.
TIME,
mm
of the ISS spectra
of barium
deposited
onto tungsten
covered by 0.5
AES. As the barium coverage increases both the oxygen and tungsten area fractions decrease at about the same rate. The packing density of a barium monolayer is about half that of the underlying tungsten surface [8,15] and the packing density of an oxygen monolayer [7]. Thus it would appear that each barium atom is shadowing one tungsten atom and one oxygen atom. Compared to AES, the ISS measurements indicate about 12% greater barium coverage. This discrepancy may be due to chemical effects on the sensitivity values or the shadowing being somewhat more complex than assumed above. The corresponding analysis for barium deposited onto tungsten covered by a full monolayer of oxygen is shown in fig. 9. As oxygen has a far lower sensitivity than tungsten and attenuates the tungsten signal by 90%, the signal-to-noise ratio is much worse in the spectra of the surfaces containing a full monolayer of oxygen. This is reflected in the greater scatter of the data in fig. 9 as compared to fig. 8. Again the barium area fraction increases linearly with coverage although the coverage is again slightly higher than the values indicated by AES. The presence of a few percent of tungsten at the surface is a further indication of the limits of the model proposed. However in general the ISS measurements have provided a quantitative assessment of the two systems which agree (within about 12%) with the values measured by AES. 4.2. Dispenser
cathodes
The spectrum in fig. 2 is of dispenser cathode made with an uncoated tungsten matrix impregnated with barium, calcium and aluminum oxides in the ratio 6 : 1: 1. The spectrum was obtained by superposing several spectra taken with the cathode at room temperature. However, the cathode was heated to - 1500 K between the recording of each spectra in order to overcome the sputtering and poisoning of the cathode surface. Thus a signal-to-noise ratio
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes
385
6 Ba
EVAPORATION
Fig. 9. Quantitative evaluation full monolayer of oxygen.
TIME,
min
of the ISS spectra
of barium
deposited
onto tungsten
covered
by a
four times better than that indicated in fig. 2 could be obtained even though the cathode had a work function near 2 eV and a correspondingly low scattered ion yield. The spectrum has been analysed using the measured values for the sensitivity factors to obtain the results shown in table 4. Here the analysis has been performed both in terms of area fraction and atomic percentages. Compared to AES measurements (for example ref. [6]), the ISS results indicate much less tungsten at the cathode surface. In view of the greater surface sensitivity of ISS, the ISS results provide direct evidence for the widely held view that the cathode surface is tungsten covered by fractional monolayer coverage of barium and oxygen. Again compared to AES, the ISS results indicate there is less oxygen at the cathode surface. The oxygen (associated with barium) on active dispenser cathode surfaces is considered to either to lie Table 4 ISS measurements
of uncoated
tungsten
Impregnant
Barium a)
611 532
64 71
matrix
dispenser
cathodes
Oxygen
Tungsten
24 19
12 10
35 29
18 16
% Area fraction
Atomic 611 532 ‘) Oxygen
47 55
%
which is not seen by ISS may be associated
with each barium
atom.
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes
386
EXPOSED TO 1OL 02
ACTIVE STATE
x i/4
XI
I
E/E0
/L/
i
.4
.6
I
I
1
I
I
.%
I
I
1.0
J
JI ‘--I’ .4
Ji I
.6
I
I
.6
I
‘I 1.0
Fig. 10. ISS spectra of a 532 “B” cathode in an active and a poisones state.
almost coplanar with the barium on top of the tungsten substrate [13] or underneath the barium [16]. In either case it is reasonable to assume that, considering the relative sizes of the barium and oxygen in the form they exist on a cathode surface, the barium could shadow the oxygen so that the oxygen next to the barium does not contribute to an ISS peak (in our spectrometer). Within the limits of the accuracy of the ISS calibrations, the barium coverage as indicated by ISS is in agreement with AES measurements of the cathode at 1500 K. Thus the surface of the active cathode in fig. 2 can be described as 64% barium and oxygen (which is not seen by ISS), 24% oxygen (in the form of monolayer coverage on tungsten) and 12% exposed tungsten. The spectrum shown in fig. 2 is similar to previously published ISS spectra of dispenser cathodes [6,17]. Furthermore the slight differences (mainly in the relative barium and tungsten peak heights) can be attributed to the different state of activation of the cathode [6] or the different experimental conditions. The quantitative ISS results are also compatible with recent analysis [18] of AES results from uncoated tungsten matrix dispenser cathodes such as those in ref. [19]. Fig. 10 shows spectra from a “B” cathode with a 532 impregnant in an active state and after the cathode had been exposed to 10 L of oxygen. The spectrum from the active cathode indicates a similar surface (see table 4) to the spectrum from the 611 cathode shown in fig. 2. As can be seen from fig. 10, the overall scattered ion yield from the cathode exposed to oxygen was greater than from the active cathode. Presumably this is a result of the higher work function of the cathode exposed to oxygen [20]. Compared to AES, the ISS
C. R. K. Marrian et al. / ISS studies of Ba and 0 on W cathodes aB
+ 6 532
I
0
.02
I .04 O/(2.4
I
I
.06 .OB x Ba +W) (ISS)
387
611
I
.lO
Fig. 11. Plot of the oxygen (510 eV) to tungsten (1736 eV) AES peak ratio versus the oxygen to substrate (barium and tungsten) ISS peak ratio for uncoated tungsten matrix cathodes in active and poisoned states.
spectra indicate a greater increase in the amount of oxygen at the cathode surface following the oxygen exposure. This is illustrated in fig. 11 where the ISS oxygen to substrate (tungsten plus barium) peak ratio is plotted against the corresponding AES ratio for active and oxygen poisoned uncoated tungsten dispenser cathodes. ISS is more surface sensitive than AES, which suggests that the oxygen from the poisoning exposure is on the topmost atomic layer of the cathode. The obvious sites for the adsorbed oxygen are on top of each barium atom and on any tungsten which was exposed on the active cathode. Thus one can exploit the different sampling depths of the two spectroscopies to obtain an idea of the position of the oxygen on the cathode surface. AES indicates that poisoning an uncoated tungsten matrix dispenser cathode with 10 L of oxygen does not change the amount of barium at the cathode surface [6]. Thus the quantitative interpretation of the ISS spectrum of fig. 10 must result in the amount of barium at the surface being the same as on both the active and poisoned cathode surfaces. Interpreting the ISS spectra in terms of the surfaces described above, there is agreement in the barium area fraction provided either (a) the overlying oxygen does not attenuate the barium signal or (b) the overlying oxygen does attenuate the barium ISS signal but does not contribute to the oxygen ISS signal. When a near monolayer of barium oxide on tungsten is exposed to oxygen, interatomic Auger and work function studies [13,20] suggest a significant transfer of charge to the adsorbed oxygen. In contrast when clean tungsten is
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exposed to oxygen, the surface dipole causing the change in work function (- I eV for a monolayer of oxygen on (100) tungsten [21]) is such that the charge gained by the adsorbed oxygen cannot be as great. So it appears likely that the oxygen overlying barium will have a greater negative charge per atom than the oxygen on tungsten. It seems reasonable to expect greater neutralization of the incident positive noble gas ions by the more negatively charged oxygen so the scattered ion yield from the oxygen overlying the barium would be less. Thus the second of the two explanations (b) seems reasonable. It is interesting to note that if this is indeed the case, the oxygen overlying the barium is “seen” in the ISS spectrum by its effect on the substrate rather than the signal it contributes directly. It should be emphasized that the results of the quantitative evaluation of the dispenser cathodes are by no means unique. When there are two or more species adsorbed on a surface, there may be many imaginable surface geometries and many different chemical effects on the sensitivities. Thus ISS data can only be interpreted in terms of a specific model of the surface and can rarely be evaluated unambiguously particularly when oxygen and another element are present on the surface of a third element.
5. Conclusions
In order to achieve quantitative results from ISS, closely controlled calibration experiments in conjunction with a companion spectroscopy are necessary. The results of such calibrations are so dependent on the various parameters of the spectrometer and the experimental conditions that extreme care must be taken before using the results in other experiments. AES has been used in this work as a companion spectroscopy to determine surface coverage. A technique exploiting the variation with energy of the escape depth of the tungsten Auger electrons for measuring surface coverage has been described, together with values for the escape depths in oxygen and barium. The absolute ISS sensitivities of barium, oxygen and tungsten have been found to be strongly dependent on the work function of the surface on which they are present. As the work function decreases below - 4 eV the scattered ion yield drops dramatically. In the spectrometer used for this work, the ISS peaks could not be detected above the noise if the work function of the surface was less than 2 eV. However, the three elemental peaks seem to decrease in size at the same rate so that the relative sensitivities did not change by more than 12%. Interpretation of ISS spectra is often further complicated by shadowing effects. However, when barium and oxygen are both present on a tungsten surface the shadowing can be easily modeled. As a result this system can be studied quantitatively with ISS once the spectrometer has been calibrated. Although, it is not possible to examine dispenser cathodes at temperatures
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at which electron emission occurs, their surfaces can be regenerated easily allowing the low signal level caused by the low work function of the surface to be overcome by the superposing of several ISS spectra. Analysis of the spectra provides direct evidence in support of the tungsten at the cathode surface having fractional monolayer coverage of barium and oxygen. The spectra can be evaluated quantitatively in terms of such a surface to give the area fractions of the different types of coverage. Furthermore, the apparent differences between AES and ISS results can be reconciled once the results are interpreted in terms of the surface model and shadowing discussed here. In order to obtain information on relative atomic positions, the capability to vary the incident and scattering angles is necessary. However, with the present spectrometer, some limited success has been achieved by exploiting the different sampling depths of AES and ISS for studies of the position of oxygen on poisoned cathode surfaces. In general however, interpretation of ISS spectra from surfaces containing more than two species is complicated by the many possible surface geometries that can be envisioned. Therefore it is only really possible to interpret an ISS spectrum in terms of a specific model of the surface atomic arrangement.
Acknowledgement The authors would like assistance of Charles Hor.
to express
their
appreciation
of the
technical
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