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Electron stimulated desorption of oxygen ions from tungsten carbide
654
Surface
ELECTRON TUNGSTEN .. H. STORI, Inslitut
STIMULATED CARBIDE P. BRAUN
DESORPTION
and R. GOMER
Science 141 (1984) 654-664 North-Holland, Amsterdam
OF OXYGEN
IONS FROM
*
fiir Algemeine Physik, Technische Uniuersitiir Wien, Karlsplarr 13, A- 1040 Wien, Austria
Received
16 December
1983; accepted
for publication
2 March
1984
Results on the adsorption and electron stimulated desorption of oxygen from tungsten carbide (WC) are presented and discussed. Desorbed ions were analyzed by a quadrupole mass analyzer (QMA) or a cylindrical mirror analyzer (CMA), the CMA also being used for in situ Auger electron spectroscopy (AES). The absolute total disappearance cross section of the O+ ESD signal could be determined and was found to increase with decreasing initial coverage. The cross sections at 2 keV primary energy are 2 x 1O-‘8 cm2 at 0 = 0.05 and 9 x lo-l9 cm2 at 0 = 1. From the comparison of the O+ ESD signal and the simultaneously recorded 0-KLL AES signal it could be shown that at least two different adsorption states are present at the surface and that there is also electron induced conversion to a state with low ESD cross section. The dependence of the ESD yield on the primary electron energy, determined in the range from 80 eV to 2.5 keV, shows a maximum at about 300 eV.
1. Introduction Electron stimulated desorption can provide information on basic surface processes [l-3] and is also of potential importance as an unwanted process in low energy electron diffraction, Auger measurements, and in the interaction of electrons with containment surfaces in fusion reactors. To date ESD on only a few metal and oxide surfaces has been studied. The aim of this work was to perform exploratory experiments on refractory carbides, which are important in projected fusion reactors as containment surfaces. Tungsten carbide was selected because a sufficiently clean surface of nearly stoichiometric WC can be readily obtained.
2. Experimental The experiments were performed in a slightly modified commercial surface analysis system. The pumping system consisted of an ion pump, a Ti sublima* Fulbright Chicago,
Professor 1981. Permanent Chicago, Illinois 60637, USA.
address:
The
James
Franck
0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Institute,
University
of
H. Stijri
et al. / ESB of oxygenionsfrom WC
655
tion pump, and an attached turbomolecular pump separated by an automatically controlled all metal valve. It reached a base pressure of 2 X 10-r’ mbar after bakeout at 2OO“C routinely. The turbomolecular pump is used during bakeout and for admission of pure gases. A residual gas analyzer situated in the experimental chamber permitted determination of the composition of the gases admitted. During static backfilling of the system with oxygen high concentrations of CO and H,O were observed. These originate from chemical reactions of 0, with the baked stainless steel walls. As a remedy oxygen was flushed through the system at a high flow rate, determined by the available pumping speed. This reduced the CO contamination to a few percent and the H,O contamination to unmeasurable levels. The original equipment contained a double pass CMA with a coaxial electron gun capable of providing electrons with an energy adjustable from 80 eV to 10 keV. A beam current in excess of 10 PA is available at energies above 1 keV. Below 1 keV the beam current gradually decreases to 100 nA at 80 eV. Since the focussed beam has a diameter of about 10e3 cm, rastering was used to obtain a homogeneous and sufficiently low current density at the sample from the fringes of surface. The angle of incidence was 45 ‘. Ions originating the irradiated area (a few mm’) were suppressed electronically by gating the detector in phase with the raster generator. Ions emitted from the sample were analyzed by a quadrupole mass filter equipped with a single ion counter. The entrance optics of the quadrupole accepts ions from a solid angle of approximately 0.01 sterad, centered at an angle of 50’ off the surface normal. The CMA was either used for simultaneous Auger analysis of the sample surface during desorption studies or as an auxiliary ion detector. As an ion detector the CMA offers much higher acceptance than the quadrupole, integrating over angles between 3O and 87 o with respect to the surface normal because the sample is tilted at 45O to the axis of the CMA. Also, the energy dist~bution of the emitted ions can be recorded using the CMA. Its major disadvantages as an ion detector are that it is not mass selective and that the gating technique used to suppress fringe effects is not applicable. The sample (15 mm diameter, 5 mm thick) was pressed and sintered from highly pure tungsten carbide powder. After grinding, polishing and precleaning in an ultrasonic freon bath the sample was introduced into the vacuum chamber and finally cleaned by many cycles of sputtering by 2 keV krypton ions and subsequent heating to 1500 K until no impurities exept very small traces of silicon could be detected by AES. Although quantitative analysis of compounds like WC is not sufficiently accurate, there are strong indications from transient behavior of the surface composition during sputtering with ions of varying energy [4] and after heating, that a stoichiometric surface composition is restored by heating. The sample was mounted, together with a ribbon of pure polycrystalline
H. Sfij,i et al. / ESD of oxygen ions from WC
656
tungsten and a Faraday cup, on a molybdenum sample carousel. By rotating the carousel, the sample could be positioned over an electron bombardment heater; a 200 W electron beam then impinged into the back side of the sample. During bombardment the filament of the heater was kept at ground potential to prevent stray electrons from desorbing gas from the chamber walls. The pressure during heating could be maintained below 1 x 10e9 mbar. Before each ESD measurement the sample was heated for 20 s to reach 1500 K. After a few minutes of cooling, flowing oxygen was admitted at a pressure of 1.3 x lo-’ mbar for the appropriate time. Fig. 1 shows the oxygen Auger signal versus the oxygen dose in langmuirs. These data indicate that the initial sticking coefficient s,, of 0, on polycrystalline WC is 0.35 (n ,,,/10’5), where n max is the maximum number of adsorbed 0 atoms/cm*. A rough idea of the latter can be gained by comparing the intensity of the oxygen Auger signal from saturated WC and saturated polycrystalline W. If the coverage on the latter is taken as - 1.4 x 1015 0 atoms/cm2 and if it is assumed that the Auger intensities scale from the W to the WC substrate, n,,, - 7.1014 cm-* on WC and the initial sticking coefficient is s0 - 0.18. Although these estimates are rough, they indicate that the number of adsorption sites on polycrystalline WC are comparable to those on a metal surface and that s0 is quite high, unlike that found on most compound
0 0
2
4
6
8 oxygen
10
12 dose
14
16
IL
Fig. 1. 0-KLL Auger signal versus oxygen dose in the range from 0.2 to 50 L.
semiconductors. It is also interesting that s is constant for 8 I 0.35, suggesting a mobile precursor, and excluding a Langmuir model, which might be expected for isolated, widely separated adsorption sites.
3. Results and discussion Fig. 2 shows a typical mass spectrum of desorbed positive ions. There are three clearly visible peaks at masses of 1 (H+), 16 (O+) and 19 (F+) amu, the
I
I
0
2
I,,
4
I,,
6
8
,
,
I
I,
1
,
10 12 14 16 18 20 22 24 26 28 mass number, amu
Fig. 2. Mass spectra of positive ions desorbed from WC by 2 keV electrons: (a) after heating; (b) after exposure to 10 L 4; (c) after subsequent desorption by 2 As cm-*.
658
H. Stiiri et al. / ESD of oxygen
ions from
WC
most prominent being O+. H+ and F+ remain nearly unchanged during adsorption and desorption of oxygen. Remarkably, no fluorine signal was ever observed in AES. The small background signal shown in fig. 2a is probably due to residual 0 or OH diffusing to the surface during anneal after sputtering. The O+ yield as function of the primary electron energy is shown in fig. 3. Although these data were recorded using the CMA as an ion detector and are therefore not mass selective, they represent mainly the O+ count rate as can be seen from mass spectra in fig. 2. Although fig. 2 shows mass spectra for 2 keV primary electrons, the spectra for lower primary energy are the same within the limits of error. The electron current density was less than 1 PA cme2, sufficiently low to avoid changes in the count rate caused by desorption. The curve was recorded using a virgin adsorbate layer; it could be shown, however, that the relative shape remains unchanged after considerable desorption, which reduced the ESD signal by a factor of four. The maximum cross section is observed at a primary energy of 300 eV. There is no further structure at energies greater 80 eV; the threshold is below the energy range of the electron source used. As we detect the ions desorbed only in a small solid angle and the angular distribution is unknown, we are not able to determine absolute 0+/eratios. Fig. 4 shows the decay of the O+ ESD signal during desorption. The
Fig. 3. Positive ion ESD yield versus electron energy. Data were taken using the CMA and an electron current density of approximately 600 nA cme2.
H. St&+ et al. / ESD 01 oxygen ions from WC
659
different curves correspond to initial coverages varying between ei = 1 for 50 L to 8i = 0.05 for 0.2 L exposure. These relative coverages were established from the data in fig. 1. The abscissa of fig. 4 gives the electron dose from 0 to 1.5 As cm-‘, calculated from electron current densities measured with the Faraday
i 0
t .2
I
i
,
.4
.6
.6
electron
dose ,
f
1
1
1.2.
Ascm-2
Fig. 4. O+ ESD signal versus electron dose for various 02 exposures and corresponding relative coverages as indicated. The data were recorded using the QMA and an electron energy of 2 keV; the current density was 1 mA cmm2. The background present after heating has been substracted.
H. Stijri et al. / ESD
660
ofoxygenions from WC
cup, multiplied by the exposure times. The ordinate shows the ESD O+ ion signal measured with the quadrupole mass analyzer in logarithmic representation. The small background Of signal shown in fig. 2a was subtracted. Prior to the measurement of every curve the WC sample was heated, allowed to cool down and exposed to the desired amount of oxygen. Simple desorption kinetics would yield a set of parallel straight lines in this semilogarithmic representation. As fig. 4 clearly shows, the curves are neither straight lines nor parallel. This indicates that a simple model assuming a single desorption cross section independent of coverage is not applicable here. Nevertheless an initial cross section can be calculated from the O+ ESD signal. This decreases from 2 x lO_” cm* at the lowest coverage (oxygen dose 0.2 L) to 9 x lo-l9 cm* at the highest coverage (oxygen dose 50 L) (fig. 5), suggesting that there may be different oxygen states or interactions present on the WC surface. Fig. 6 shows the data of fig. 4 plotted against the oxygen KLL Auger signal, which is presumably proportional to total oxygen coverage, even after partial desorption. The ascending curve shows the initial O+ signal versus initial
oxygen 0
1
dose
2
L
I
5
10 20 50
“E 0 f 0 x
1.5 .-: z s
I 0
I
I
I
I
1
2 1 0-KLL
3
4
5
6
signal
, arb.
units
I
Auger
Fig. 5. Total initial disappearance cross section as measured from the Of decay versus initial oxygen coverage represented by the 0-KLL Auger signal. The upper non-linear scale gives the 0, exposure required to obtain the coverage.
H. S&i
coverage
et al. / ESD of oxygen
ions
fromWC
661
9, and
the descending curves show the O+ signal versus coverage electron bombardment. A number of interesting conclusions can be &awn from fig. 6. First, the initial signal increases linearly with coverage up to ei = 0.4 but then increases more steeply, reaching a value twice
remaining
after
2
0
1
I
I
I
1
0
1
2
3
0-KLL
AES,
1
4 arb.
1
1
5
6
_I
units
Fig. 6. 0’ signal, recorded with QMA, versus Auger KLL intensity. Upper curve (ascending arrow) represents the initial O+ signal versus initial coverage (in terms of Auger signal). The curves marked with descending arrows represent the O+ signal during desorption as function of remaining 0 coverage for various initial exposures. After very large electron doses the O+ signals reach a point on the lowest curve. Also shown are the amounts of 0 desorbed, as given by the differences in initial and final signal for each exposure. The upper number corresponds to the amount desorbed as fraction of maximum 0 coverage (i.e. after 50 L exposure). The lower number expresses the amount desorbed as fraction of the initial coverage for that exposure.
662
H. Stiin et al. / ESD of oxygen ions from WC
that expected by extrapolating the linear regime. Despite this fact the disappearance cross section decreases with increasing coverage, as already indicated. Second, the amounts desorbed by electron impact can be obtained as the differences between the starting and end points of the descending curves in fig. 6. These amounts are shown as fractions of (?,,, and of 19,next to the curves in fig. 6. Both quantities decrease with decreasing 8,. The increase in initial signal with ai might be interpreted as sequential rather than random filling of 2 states, the state being filled later having higher O+ yield. Since a signal is observed even at low 8, both states would have to be 0’ yielding. This assumption fails to explain, however, why only a fraction of adsorbed 0 is desorbed by electron impact. If the increase in initial O+ signal is put aside for the moment we can try to explain the decrease in amount desorbed as Bi decreases by assuming that 2 states, one desorbable, the other with very small ESD cross section, are populated randomly during exposure. This comes much closer to explaining the decreased amounts desorbed as Bi decreases but fails to explain them quantitatively, since the fraction of 8, desorbed should now be constant, while in fact it decreases. This explanation also fails to account for the increase in initial disappearance (of desorbable 0) cross section as Bi is decreased. It is therefore necessary to postulate that in addition to 2 (or more) binding states filled initially, there is also some conversion by electron impact to a state with very low desorption cross section and to assume that the process of conversion occurs more easily at low coverage. This assumption explains why there is more desorption at high f7;, where conversion is blocked by the presence of other adsorbate. If the intrinsic conversion cross section is higher than that for desorption, it also explains why the disappearance cross section decreases with increasing Bi since this blocks the faster process. It can also explain why the 0’ yield increases with 8,, since desorption of O+ can be competitive with conversion. On the other hand conversion cannot have a cross section which is very much higher than for desorption, or no desorption would be observed for ei < 0.7, just as in the extreme case of sequential filling, with one state desorbable, the other not. The initial states postulated here cannot be distinguished from changes in the adsorption state (or states) with coverage resulting from adsorbate-adsorbate interactions. In this sense, all states not corresponding to distinctly different binding geometries are modifications of a single state. Oxygen was also introduced while the electron beam irradiated the sample to obtain data on the onset of ESD at low coverages. Fig. 7 shows the onset of the ESD signal as seen by the CMA during introduction of oxygen. This part of the experiment was performed using two different settings of the electron beam, one with 2 keV and 980 PA cm -2 as usual and one with 650 eV and 31 PA cm-‘. For comparison data were also taken on a polycrystalline tungsten sample. The two different settings did not produce major differences for either sample. As expected the rate of decay of the ESD signal after cutting off the
H. Stiiri et al. / ESD of oxygen ions from WC
0
oxygen
5 dose
10 ~ L
663
15
Fig. 7. ESD signal versus 0, exposure on W and WC. Electron energy 650 eV, current density 31 gA cm-*. The 0, pressure was adjusted to 1.3 X lo-’ mbar within less than 1 s at the start of the exposure.
oxygen supply was slower for the lower current density. The onset of the ESD signal from O/W is slightly delayed, as already described in an earlier paper for W(110) [5]. In the case of O/WC the ESD signal starts immediately, but shows a slight hump around 2 L. The 0 on WC curve of fig. 7 is very similar to results obtained by Prigge et al. [6] for O/W(lOO). The hump shown in fig. 7 corresponds to a similar structure reported in ref. [6] at the same oxygen dose. Nevertheless we feel that the arguments from ref. [6] are mainly based on adsorbate structure and cannot be easily applied to our polycrystalline WC sample. The ESD signal from the tungsten sample is roughly one order of magnitude smaller than that from the tungsten carbide sample. The total disappearance cross section for the tungsten sample is 1.4 x lo-” cm2 at an electron energy of 2 keV and 4 X lO_” cm2 at 650 eV. This agrees well with earlier data from polycrystalline tungsten [7]. The difference between cross sections from CMA and from QMA data is approximately 20%, probably because of different geometry. Sputtering the samples with 2 keV krypton ions increased the ESD yield in both cases considerably, with the cross sections staying almost constant as a function of ion dose. This suggests that imperfections on the surface play an important role in the ESD process.
664
H. Stijri et oi. / ESD
of oxygen ions from WC
Acknowledgements The authors wish to thank Professor Ettmayr for samples which were prepared in his laboratory. Professor Viehbijck greatly contributed to the work with many helpful discussions. The financial support given by the Austrian Fonds zur Fiirderung der wissenschaftlichen Forschung under project No. 4265 is gratefully acknowledged. One of us (R.G.) gratefully acknowledges support from the Fulbright Commission during his stay in Vienna.
References (11 P.A. Redhead, Can. J. Phys. 42 (1964) 886. [2] D. Menzel and R. Gomer, J. Chem. Phys. 41 (1964) 3311. [3] P.I. Feibelman, in: Desorption Induced by Electronic Transitions, Eds. N.H. Traum, IS. Tully and T.E. Madey (Springer, Berlin, 1982) p. 61. [4] H. St&i, G. Betz and P. Braun, in: Proc. Symp. on Atomic and Surface Physics. 1982, Ed. W. Lindinger. [5] C. Leung, Ch. Steinbriichel and R. Gomer, Appl. Phys. 14 (1977) 79. [6] S. Prigge, N. Niehus and E. Bauer, Surface Sci. 75 (1978) 635. [7] M. Nishijima and F.M. Popst, Phys. Rev. B2 (1970) 2368.
Talk,
M.M.
Maria
Afm,
Surface
ELECTRON TUNGSTEN .. H. STORI, Inslitut
STIMULATED CARBIDE P. BRAUN
DESORPTION
and R. GOMER
Science 141 (1984) 654-664 North-Holland, Amsterdam
OF OXYGEN
IONS FROM
*
fiir Algemeine Physik, Technische Uniuersitiir Wien, Karlsplarr 13, A- 1040 Wien, Austria
Received
16 December
1983; accepted
for publication
2 March
1984
Results on the adsorption and electron stimulated desorption of oxygen from tungsten carbide (WC) are presented and discussed. Desorbed ions were analyzed by a quadrupole mass analyzer (QMA) or a cylindrical mirror analyzer (CMA), the CMA also being used for in situ Auger electron spectroscopy (AES). The absolute total disappearance cross section of the O+ ESD signal could be determined and was found to increase with decreasing initial coverage. The cross sections at 2 keV primary energy are 2 x 1O-‘8 cm2 at 0 = 0.05 and 9 x lo-l9 cm2 at 0 = 1. From the comparison of the O+ ESD signal and the simultaneously recorded 0-KLL AES signal it could be shown that at least two different adsorption states are present at the surface and that there is also electron induced conversion to a state with low ESD cross section. The dependence of the ESD yield on the primary electron energy, determined in the range from 80 eV to 2.5 keV, shows a maximum at about 300 eV.
1. Introduction Electron stimulated desorption can provide information on basic surface processes [l-3] and is also of potential importance as an unwanted process in low energy electron diffraction, Auger measurements, and in the interaction of electrons with containment surfaces in fusion reactors. To date ESD on only a few metal and oxide surfaces has been studied. The aim of this work was to perform exploratory experiments on refractory carbides, which are important in projected fusion reactors as containment surfaces. Tungsten carbide was selected because a sufficiently clean surface of nearly stoichiometric WC can be readily obtained.
2. Experimental The experiments were performed in a slightly modified commercial surface analysis system. The pumping system consisted of an ion pump, a Ti sublima* Fulbright Chicago,
Professor 1981. Permanent Chicago, Illinois 60637, USA.
address:
The
James
Franck
0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Institute,
University
of
H. Stijri
et al. / ESB of oxygenionsfrom WC
655
tion pump, and an attached turbomolecular pump separated by an automatically controlled all metal valve. It reached a base pressure of 2 X 10-r’ mbar after bakeout at 2OO“C routinely. The turbomolecular pump is used during bakeout and for admission of pure gases. A residual gas analyzer situated in the experimental chamber permitted determination of the composition of the gases admitted. During static backfilling of the system with oxygen high concentrations of CO and H,O were observed. These originate from chemical reactions of 0, with the baked stainless steel walls. As a remedy oxygen was flushed through the system at a high flow rate, determined by the available pumping speed. This reduced the CO contamination to a few percent and the H,O contamination to unmeasurable levels. The original equipment contained a double pass CMA with a coaxial electron gun capable of providing electrons with an energy adjustable from 80 eV to 10 keV. A beam current in excess of 10 PA is available at energies above 1 keV. Below 1 keV the beam current gradually decreases to 100 nA at 80 eV. Since the focussed beam has a diameter of about 10e3 cm, rastering was used to obtain a homogeneous and sufficiently low current density at the sample from the fringes of surface. The angle of incidence was 45 ‘. Ions originating the irradiated area (a few mm’) were suppressed electronically by gating the detector in phase with the raster generator. Ions emitted from the sample were analyzed by a quadrupole mass filter equipped with a single ion counter. The entrance optics of the quadrupole accepts ions from a solid angle of approximately 0.01 sterad, centered at an angle of 50’ off the surface normal. The CMA was either used for simultaneous Auger analysis of the sample surface during desorption studies or as an auxiliary ion detector. As an ion detector the CMA offers much higher acceptance than the quadrupole, integrating over angles between 3O and 87 o with respect to the surface normal because the sample is tilted at 45O to the axis of the CMA. Also, the energy dist~bution of the emitted ions can be recorded using the CMA. Its major disadvantages as an ion detector are that it is not mass selective and that the gating technique used to suppress fringe effects is not applicable. The sample (15 mm diameter, 5 mm thick) was pressed and sintered from highly pure tungsten carbide powder. After grinding, polishing and precleaning in an ultrasonic freon bath the sample was introduced into the vacuum chamber and finally cleaned by many cycles of sputtering by 2 keV krypton ions and subsequent heating to 1500 K until no impurities exept very small traces of silicon could be detected by AES. Although quantitative analysis of compounds like WC is not sufficiently accurate, there are strong indications from transient behavior of the surface composition during sputtering with ions of varying energy [4] and after heating, that a stoichiometric surface composition is restored by heating. The sample was mounted, together with a ribbon of pure polycrystalline
H. Sfij,i et al. / ESD of oxygen ions from WC
656
tungsten and a Faraday cup, on a molybdenum sample carousel. By rotating the carousel, the sample could be positioned over an electron bombardment heater; a 200 W electron beam then impinged into the back side of the sample. During bombardment the filament of the heater was kept at ground potential to prevent stray electrons from desorbing gas from the chamber walls. The pressure during heating could be maintained below 1 x 10e9 mbar. Before each ESD measurement the sample was heated for 20 s to reach 1500 K. After a few minutes of cooling, flowing oxygen was admitted at a pressure of 1.3 x lo-’ mbar for the appropriate time. Fig. 1 shows the oxygen Auger signal versus the oxygen dose in langmuirs. These data indicate that the initial sticking coefficient s,, of 0, on polycrystalline WC is 0.35 (n ,,,/10’5), where n max is the maximum number of adsorbed 0 atoms/cm*. A rough idea of the latter can be gained by comparing the intensity of the oxygen Auger signal from saturated WC and saturated polycrystalline W. If the coverage on the latter is taken as - 1.4 x 1015 0 atoms/cm2 and if it is assumed that the Auger intensities scale from the W to the WC substrate, n,,, - 7.1014 cm-* on WC and the initial sticking coefficient is s0 - 0.18. Although these estimates are rough, they indicate that the number of adsorption sites on polycrystalline WC are comparable to those on a metal surface and that s0 is quite high, unlike that found on most compound
0 0
2
4
6
8 oxygen
10
12 dose
14
16
IL
Fig. 1. 0-KLL Auger signal versus oxygen dose in the range from 0.2 to 50 L.
semiconductors. It is also interesting that s is constant for 8 I 0.35, suggesting a mobile precursor, and excluding a Langmuir model, which might be expected for isolated, widely separated adsorption sites.
3. Results and discussion Fig. 2 shows a typical mass spectrum of desorbed positive ions. There are three clearly visible peaks at masses of 1 (H+), 16 (O+) and 19 (F+) amu, the
I
I
0
2
I,,
4
I,,
6
8
,
,
I
I,
1
,
10 12 14 16 18 20 22 24 26 28 mass number, amu
Fig. 2. Mass spectra of positive ions desorbed from WC by 2 keV electrons: (a) after heating; (b) after exposure to 10 L 4; (c) after subsequent desorption by 2 As cm-*.
658
H. Stiiri et al. / ESD of oxygen
ions from
WC
most prominent being O+. H+ and F+ remain nearly unchanged during adsorption and desorption of oxygen. Remarkably, no fluorine signal was ever observed in AES. The small background signal shown in fig. 2a is probably due to residual 0 or OH diffusing to the surface during anneal after sputtering. The O+ yield as function of the primary electron energy is shown in fig. 3. Although these data were recorded using the CMA as an ion detector and are therefore not mass selective, they represent mainly the O+ count rate as can be seen from mass spectra in fig. 2. Although fig. 2 shows mass spectra for 2 keV primary electrons, the spectra for lower primary energy are the same within the limits of error. The electron current density was less than 1 PA cme2, sufficiently low to avoid changes in the count rate caused by desorption. The curve was recorded using a virgin adsorbate layer; it could be shown, however, that the relative shape remains unchanged after considerable desorption, which reduced the ESD signal by a factor of four. The maximum cross section is observed at a primary energy of 300 eV. There is no further structure at energies greater 80 eV; the threshold is below the energy range of the electron source used. As we detect the ions desorbed only in a small solid angle and the angular distribution is unknown, we are not able to determine absolute 0+/eratios. Fig. 4 shows the decay of the O+ ESD signal during desorption. The
Fig. 3. Positive ion ESD yield versus electron energy. Data were taken using the CMA and an electron current density of approximately 600 nA cme2.
H. St&+ et al. / ESD 01 oxygen ions from WC
659
different curves correspond to initial coverages varying between ei = 1 for 50 L to 8i = 0.05 for 0.2 L exposure. These relative coverages were established from the data in fig. 1. The abscissa of fig. 4 gives the electron dose from 0 to 1.5 As cm-‘, calculated from electron current densities measured with the Faraday
i 0
t .2
I
i
,
.4
.6
.6
electron
dose ,
f
1
1
1.2.
Ascm-2
Fig. 4. O+ ESD signal versus electron dose for various 02 exposures and corresponding relative coverages as indicated. The data were recorded using the QMA and an electron energy of 2 keV; the current density was 1 mA cmm2. The background present after heating has been substracted.
H. Stijri et al. / ESD
660
ofoxygenions from WC
cup, multiplied by the exposure times. The ordinate shows the ESD O+ ion signal measured with the quadrupole mass analyzer in logarithmic representation. The small background Of signal shown in fig. 2a was subtracted. Prior to the measurement of every curve the WC sample was heated, allowed to cool down and exposed to the desired amount of oxygen. Simple desorption kinetics would yield a set of parallel straight lines in this semilogarithmic representation. As fig. 4 clearly shows, the curves are neither straight lines nor parallel. This indicates that a simple model assuming a single desorption cross section independent of coverage is not applicable here. Nevertheless an initial cross section can be calculated from the O+ ESD signal. This decreases from 2 x lO_” cm* at the lowest coverage (oxygen dose 0.2 L) to 9 x lo-l9 cm* at the highest coverage (oxygen dose 50 L) (fig. 5), suggesting that there may be different oxygen states or interactions present on the WC surface. Fig. 6 shows the data of fig. 4 plotted against the oxygen KLL Auger signal, which is presumably proportional to total oxygen coverage, even after partial desorption. The ascending curve shows the initial O+ signal versus initial
oxygen 0
1
dose
2
L
I
5
10 20 50
“E 0 f 0 x
1.5 .-: z s
I 0
I
I
I
I
1
2 1 0-KLL
3
4
5
6
signal
, arb.
units
I
Auger
Fig. 5. Total initial disappearance cross section as measured from the Of decay versus initial oxygen coverage represented by the 0-KLL Auger signal. The upper non-linear scale gives the 0, exposure required to obtain the coverage.
H. S&i
coverage
et al. / ESD of oxygen
ions
fromWC
661
9, and
the descending curves show the O+ signal versus coverage electron bombardment. A number of interesting conclusions can be &awn from fig. 6. First, the initial signal increases linearly with coverage up to ei = 0.4 but then increases more steeply, reaching a value twice
remaining
after
2
0
1
I
I
I
1
0
1
2
3
0-KLL
AES,
1
4 arb.
1
1
5
6
_I
units
Fig. 6. 0’ signal, recorded with QMA, versus Auger KLL intensity. Upper curve (ascending arrow) represents the initial O+ signal versus initial coverage (in terms of Auger signal). The curves marked with descending arrows represent the O+ signal during desorption as function of remaining 0 coverage for various initial exposures. After very large electron doses the O+ signals reach a point on the lowest curve. Also shown are the amounts of 0 desorbed, as given by the differences in initial and final signal for each exposure. The upper number corresponds to the amount desorbed as fraction of maximum 0 coverage (i.e. after 50 L exposure). The lower number expresses the amount desorbed as fraction of the initial coverage for that exposure.
662
H. Stiin et al. / ESD of oxygen ions from WC
that expected by extrapolating the linear regime. Despite this fact the disappearance cross section decreases with increasing coverage, as already indicated. Second, the amounts desorbed by electron impact can be obtained as the differences between the starting and end points of the descending curves in fig. 6. These amounts are shown as fractions of (?,,, and of 19,next to the curves in fig. 6. Both quantities decrease with decreasing 8,. The increase in initial signal with ai might be interpreted as sequential rather than random filling of 2 states, the state being filled later having higher O+ yield. Since a signal is observed even at low 8, both states would have to be 0’ yielding. This assumption fails to explain, however, why only a fraction of adsorbed 0 is desorbed by electron impact. If the increase in initial O+ signal is put aside for the moment we can try to explain the decrease in amount desorbed as Bi decreases by assuming that 2 states, one desorbable, the other with very small ESD cross section, are populated randomly during exposure. This comes much closer to explaining the decreased amounts desorbed as Bi decreases but fails to explain them quantitatively, since the fraction of 8, desorbed should now be constant, while in fact it decreases. This explanation also fails to account for the increase in initial disappearance (of desorbable 0) cross section as Bi is decreased. It is therefore necessary to postulate that in addition to 2 (or more) binding states filled initially, there is also some conversion by electron impact to a state with very low desorption cross section and to assume that the process of conversion occurs more easily at low coverage. This assumption explains why there is more desorption at high f7;, where conversion is blocked by the presence of other adsorbate. If the intrinsic conversion cross section is higher than that for desorption, it also explains why the disappearance cross section decreases with increasing Bi since this blocks the faster process. It can also explain why the 0’ yield increases with 8,, since desorption of O+ can be competitive with conversion. On the other hand conversion cannot have a cross section which is very much higher than for desorption, or no desorption would be observed for ei < 0.7, just as in the extreme case of sequential filling, with one state desorbable, the other not. The initial states postulated here cannot be distinguished from changes in the adsorption state (or states) with coverage resulting from adsorbate-adsorbate interactions. In this sense, all states not corresponding to distinctly different binding geometries are modifications of a single state. Oxygen was also introduced while the electron beam irradiated the sample to obtain data on the onset of ESD at low coverages. Fig. 7 shows the onset of the ESD signal as seen by the CMA during introduction of oxygen. This part of the experiment was performed using two different settings of the electron beam, one with 2 keV and 980 PA cm -2 as usual and one with 650 eV and 31 PA cm-‘. For comparison data were also taken on a polycrystalline tungsten sample. The two different settings did not produce major differences for either sample. As expected the rate of decay of the ESD signal after cutting off the
H. Stiiri et al. / ESD of oxygen ions from WC
0
oxygen
5 dose
10 ~ L
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15
Fig. 7. ESD signal versus 0, exposure on W and WC. Electron energy 650 eV, current density 31 gA cm-*. The 0, pressure was adjusted to 1.3 X lo-’ mbar within less than 1 s at the start of the exposure.
oxygen supply was slower for the lower current density. The onset of the ESD signal from O/W is slightly delayed, as already described in an earlier paper for W(110) [5]. In the case of O/WC the ESD signal starts immediately, but shows a slight hump around 2 L. The 0 on WC curve of fig. 7 is very similar to results obtained by Prigge et al. [6] for O/W(lOO). The hump shown in fig. 7 corresponds to a similar structure reported in ref. [6] at the same oxygen dose. Nevertheless we feel that the arguments from ref. [6] are mainly based on adsorbate structure and cannot be easily applied to our polycrystalline WC sample. The ESD signal from the tungsten sample is roughly one order of magnitude smaller than that from the tungsten carbide sample. The total disappearance cross section for the tungsten sample is 1.4 x lo-” cm2 at an electron energy of 2 keV and 4 X lO_” cm2 at 650 eV. This agrees well with earlier data from polycrystalline tungsten [7]. The difference between cross sections from CMA and from QMA data is approximately 20%, probably because of different geometry. Sputtering the samples with 2 keV krypton ions increased the ESD yield in both cases considerably, with the cross sections staying almost constant as a function of ion dose. This suggests that imperfections on the surface play an important role in the ESD process.
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Acknowledgements The authors wish to thank Professor Ettmayr for samples which were prepared in his laboratory. Professor Viehbijck greatly contributed to the work with many helpful discussions. The financial support given by the Austrian Fonds zur Fiirderung der wissenschaftlichen Forschung under project No. 4265 is gratefully acknowledged. One of us (R.G.) gratefully acknowledges support from the Fulbright Commission during his stay in Vienna.
References (11 P.A. Redhead, Can. J. Phys. 42 (1964) 886. [2] D. Menzel and R. Gomer, J. Chem. Phys. 41 (1964) 3311. [3] P.I. Feibelman, in: Desorption Induced by Electronic Transitions, Eds. N.H. Traum, IS. Tully and T.E. Madey (Springer, Berlin, 1982) p. 61. [4] H. St&i, G. Betz and P. Braun, in: Proc. Symp. on Atomic and Surface Physics. 1982, Ed. W. Lindinger. [5] C. Leung, Ch. Steinbriichel and R. Gomer, Appl. Phys. 14 (1977) 79. [6] S. Prigge, N. Niehus and E. Bauer, Surface Sci. 75 (1978) 635. [7] M. Nishijima and F.M. Popst, Phys. Rev. B2 (1970) 2368.
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