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A model system for scandate cathodes
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aol~eo surface science ELSEVIER
Applied Surface Science 111 (1997) 35-41
A model system for scandate cathodes P.M. Zagwijn a,m,J.W.M. Frenken a, * ,2, U. van Slooten b, P.A. Duine b a FOM-lnstitute for Atomic- and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands b Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands Received 24 June 1996; revised 6 September 1996; accepted 1 October 1996
Abstract In cathode-ray tubes, high current densities at relatively low operating temperatures can be achieved with dispenser cathodes containing scandium oxide. The surface structure and precise emission mechanism of the scandate cathode are not known. The surface layers of such a cathode consist of the four elements Sc, Ba, O, and W. We have studied Sc, Ba, and O ovedayers and various combinations of these elements deposited on the W(001) surface, under ultrahigh vacuum conditions. We have performed in situ measurements of the work function changes accompanying the surface treatments, and correlate these with the observed surface structures. High resolution medium-energy ion scattering was used to determine the compositional depth distributions and the geometrical structures at this model cathode surface. Auger electron spectroscopy peak height ratios show that W(001), with all three overlayer elements present, can be considered representative for real scandate cathodes. The observed ultra-low work function of the W(001)/Sc,Ba,O model system explains the high current densities on real scandate cathodes. PACS: 68.35.Bs; 73.30.+ y; 85.10.Mc
1. Introduction Scandate dispenser cathodes are promising electron sources for high brilliance cathode tubes. A scandate dispenser cathode consists of a porous mixed matrix of W and S c 2 0 3 impregnated with a 4:1:1 mixture of BaO, CaO, and A1203. The scandate cathode working mechanism is not understood since the precise nature of the active surface layer - -
* Corresponding author. Tel.: +31-20-6081234; fax: +31-206684106; e-mail: [email protected]. I Present address: ASM Europe B.V., Process Development, IMEC, Kapeldreef 75, B-3001 Leuven, Belgium. z Present address: University of Leiden, Kamerlingh Onnes Laboratory, P.O. Box 9506, 2300 RA Leiden, The Netherlands.
atomic composition, structure and chemical state - is not known. The high current densities obtainable in various applications with these cathodes, e.g., above 100 A / c m 2 at 1000°C [1], suggest that the work function o f a W surface covered with the S c - B a - O complex is significantly below that o f a conventional dispenser cathode, with only Ba and O on the surface. In the past, several attempts have been made to correlate the very high emission levels achieved by dispenser cathodes with the surface atomic structure. However, even for regular dispenser cathodes, i.e., without Sc, there is at present no consensus on the details of the emissive layer [2-4]. Most authors agree on the presence of a monatomic layer of barium and oxygen but disagree on the actual ad-
0169-4332/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 ( 9 6 ) 0 0 7 5 0 - 7
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P.M. Zagwijn et al. / Applied Surface Science I11 (1997)35 41
sorption geometry of the Ba and O atoms. Nothing is known about the surface atomic structure of the scandate cathode. One of the main reasons lies in the difficulty to perform detailed measurements on the surface of a real cathode. Therefore, one has to switch to a model system, e.g., the surface of a W crystal with Sc, Ba and O overlayers, mimicking a real cathode. In this paper, we present a combination of work function measurements and medium-energy ion scattering (MEIS) experiments on the W(001) surface with various overlayers of Sc, Ba and O. The results show that high-temperature treatments lead to a stable S c - B a - O mixture of a well defined composition and structure on top of an almost unmodified W substrate. The accompanying work function change directly accounts for the high electron emission of real scandate cathodes. This paper is organized as follows. After a brief description of the experimental procedures in Section 2, we discuss the work function and MEIS results in Section 3.1 and Section 3.2. In Section 3.3, we compare Auger electron spectra on our model system with those from a real cathode in order to show that the single-crystal system is representative for real scandate cathodes. We conclude the paper with a brief discussion.
annealing procedure was repeated several times until O and C coverages determined by AES were below 1% of a monolayer (ML) [1 monolayer is defined as the number of atoms in one W(001) plane; 1.00 x 1015 atoms/cm2 ]. Low energy electron diffraction (LEED), typically at 90 eV, showed a sharp (1 X 1) pattern, corresponding to a non-reconstructed bulk-like surface.
2.2. Preparation of the Ba, Sc, and 0 overlayers Ba was deposited in situ at a deposition rate of 1.2 M L / m i n from a home-made dispenser source, that was thoroughly outgassed prior to deposition in order to remove the oxides. During deposition, the pressure in the vacuum chamber did not exceed 1 x 10 - 9 mbar. Ordered Ba overlayer structures were prepared by adsorbing a thick Ba layer (typically 5 - 2 0 ML) at room temperature on the clean W(001) crystal. The layer was then annealed at 1100 K for 1 rain [5]. Using LEED, a sharp c(2 X 2) pattern was observed. Sc was deposited at a rate of 0.40 M L / m i n from a home-made ScH 2 dispenser source. Even after extensive outgassing prior to deposition, the Sc source still contaminated the surface slightly with C and O. The surface was exposed at room temperature to 02 at a constant pressure of 5 X 10 - 9 mbar.
2. Experimental 2.3. Work function measurements 2.1. Crystal preparation The W crystal was cut by spark erosion to dimensions of 5 × 5 × 1 mm 3. The (001) surface was mechanically polished. After chemical etching, it was cleaned in an ultrahigh vacuum (UHV) chamber with a base pressure P = 5 X 10- ~~ mbar. The surface layers were removed by sputtering with 1 keV Ar ÷ ions at an incident angle of 45 °. The remaining contaminants on the surface were O and C. These were measured with Auger electron spectroscopy (AES), by use of a cylindrical mirror analyzer equipped with a channeltron detector operating in single-electron counting mode. The C was removed by annealing the crystal to 1500 K in an oxygen ambient ( P = I × 10 7 mbar) for 30 minutes. Then the surface was flashed to 2500 K in UHV. This
The work function 4' of the W surface was measured in situ by a contact potential method using a Kelvin probe 3 [6]. This method employs the AC current signal resulting from the variation of the capacitance between the W surface and an oscillating reference electrode. The resolution and run-to-run reproducibility of the measured work functions amounted to 10 and 100 meV, respectively. The work functions were measured relative to that of the clean W(001) surface, which has a work function 4' = 4.63 eV [7].
3 Besocke Delta Phi Gmbh, Postfach 2243, D-5170 Jiilich, Germany.
P.M. Zagwijn et al. /Applied Surface Science 111 (1997) 35-41 2.4. Medium-energy ion scattering
ally losing energy to the electrons in the solid ('electronic stopping'). By use of tabulated stopping powers the energy loss can be converted into the scattering depth in the crystal. The central detection direction of the analyser was along the [111] axis. Ions backscattered from the second (or deeper) layers are blocked on their way out by other atoms, which gives rise to minima in the scattering intensity as a function of scattering angle. The atomic positions in the surface were determined from the angles at which these minima occur [8].
High resolution medium-energy ion scattering, in conjunction with ion shadowing and blocking effects [8], was used to determine the atomic structure, composition and depth distribution of the grown layers and the W substrate. The backscattered ions were detected as a function of their final energy and scattering angle. By use of an electrostatic toroidal analyser equipped with a two-dimensional position sensitive detector, the low mass elements Sc and O could be detected within relatively short dataacquisition times [9]. The W crystal was aligned with the incident ion beam of 100 keV He + along the [112] direction with an accuracy of 0.1 °. Flashing the surface to 2500 K did not change the alignment of the crystal within this accuracy. Due to the shadowing effect in this scattering geometry, the hitting probabilities of the subsurface atoms are greatly reduced. For an energy of 100 keV, virtually all backscattered ions that reach the detector have undergone a single hard collision with an atom in the solid. Before and after the backscattering event the ions travel along essentially straight paths, continu-
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3.1. Work function In Fig. 1, the work function is shown as a function of Ba coverage on the W(001) surface. The open circles are for Ba c(2 × 2) structures prepared as described in Section 2.2. The filled circles represent measurements for which the Ba was deposited at room temperature. Using MEIS, the Ba coverages were determined with an accuracy of 0.01 ML [8].
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38
P.M. Zagwijn et al. /Applied Surface Science 111 (1997)35-41
Final Ba coverages were measured by MEIS to be 0.23-0.45 ML (1 ML is defined as the number of atoms in a W(001) plane), depending on the initial excess Ba coverage, annealing time and temperature. In previous studies of the Ba/W(001) system, e.g., Ref. [10], the Ba coverage was inferred from the c(2 X 2) symmetry of the LEED pattern, to be 0.5 ML. Our MEIS coverage measurements on this as well as several other overlayer systems [8] demonstrate that sharp LEED patterns often form easily well before completion of the adsorbate overlayer. Fig. 1 shows that the work function first decreases as the Ba coverage increases, until a minimum of ~b = 1.39 eV in the curve is reached at 0.26 ML. This minimum is ca. 1.1 eV lower than earlier published results [10]. After every deposition we found no C and O contamination by AES (detection limit 0.01 ML for O and C), and LEED showed sharp c(2 X 2) patterns. The more or less linear decrease in th for low coverages reflects the dipole moment formed by the positive charges on the adatoms and the screening charge of the substrate [11]. As the coverage increases, the interaction between these individual charge distributions leads to a reduction of the average adatom-substrate dipole moment, and as a result ~b first levels off and then increases again. Finally, as the coverage increases beyond several monolayers, a value is reached close to the bulk work function of the adsorbate. The work function of bulk Ba [7,12] is indicated in the insert of Fig. 1 by the dashed line. When oxygen was adsorbed on W(001) precovered with 0.27 ML Ba, the work function decreased by an additional 0.12 eV at a dose of 0.4 L (1 L = I X 10 -6 Torr s). This is consistent with the result from previous studies of this system with MEIS and other techniques [13], that locate the O atoms below or between the Ba atoms, where they should be expected to increase the surface dipole moment. This work function of 1.27 eV is much lower than the value of 1.9 eV that is needed to explain the performance of a regular dispenser cathode [14]. As the O 2 dose increased, the work function increased again by 0.20 eV at 2 L. Between 2 and 3 L the work function was constant. For larger doses the work function increased to a saturation value of q~ = 2.1 eV at and above 12 L. The change in work function during adsorption of oxygen was also measured on clean W(001). The
work function increased strongly, at first linearly with a slope of 0.90 e V / L . A break in this slope occurred at an exposure of 0.66 ___0.13 L and a work function of 5.22 + 0.05 eV was measured. A second break in the curve was found at 2.3 + 0.3 L and ~b= 5.93 + 0.10 eV. These values are close to the ones measured by Bauer et al. [15]. At 6.0 L the work function levelled off at 6.30 eV. An independent determination with MEIS of the O-coverage for this dose yielded (1.1 + 0.1)X 1015 0 atoms/cm 2, corresponding to 1.1 + O.l ML on the W(O01) surface. The curve of ~b as a function of Sc coverage on the W(001) surface, measured during deposition at room temperature, is similar to the one in Fig. 1. A minimum of ~b = 1.88 eV is reached at a coverage of 1.0 ML. This minimum is again much lower (ca. 1.4 eV) than results published by others [16], possibly due to contamination with O from our Sc source [17]. For higher coverages, e.g., 10 ML, the value of & approached 3.33 eV, which is expected for bulk Sc. Exposing W(001) precovered with 1.0 ML of Sc to 02 at room temperature caused the work function to drop slightly, by 0.05 eV, at l. 1 L. After exposure of the surface to 6 L, the work function had increased again to ~b = 4.2 eV. In Fig. 2, the work function vs. 02 exposure is given for a structure of 1.0 ML of Sc and 1.0 ML of Ba on W(001). The final ~ = 1.18 eV that is reached is virtually the same ( + 0.1 eV) as the ~b found after
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Fig. 3. MEIS energy spectra measured in a doubly aligned geometry for 100 keV He + ions backscattered from W(001) at different stages of the preparation of a S c - B a - O ovedayer. Solid curve: clean W(001). Dashed-dotted curve: After deposition of 1 ML Sc and heating to 1430 K. Dashed curve: Final S c - B a - O structure (see text). The backscattering yield is normalised to the 'random height' of W [8]. The arrows indicate the energy expected for ions backscattered from W, Ba and Sc atoms located at the surface.
heating the crystal to 1430 K for 1 min. The effect of oxygen is qualitatively the same as in the case of pure Ba (0.27 ML) or Sc (1.0 ML) on the surface. The value of 4~ = 1.18 eV is 0.30 eV lower than the effective work function calculated by Manenschijn et al. [14] to fit the high current densities of typical scandate cathodes and precisely in the range of the minimum values of 1.13-1.22 eV found on optimized scandate cathodes [ 18,19].
39
curve. The W peak was again shifted to lower energies. The W and Ba peaks measured in this scattering geometry are not well separated (see decomposition of the combined W + Ba peak in Fig. 3). From the peak shape, and from spectra measured at higher scattering angles where the W and Ba peak are well separated, we derive that 1.0 ML of Ba was on the surface and the amount of visible W was still 1.0 ML. The Sc peak was shifted by the same amount to lower energies, indicating that all of the Sc was buried under the Ba. Exposing the surface to oxygen did not change the size and shape of the final spectrum compared to the dashed spectrum of Fig. 3. The oxygen signal could not be detected by MEIS, which means that the amount of adsorbed oxygen was either too small for detection (<< 0.5 ML), or that the oxygen was distributed over the thickness of the S c - B a film. From the AES spectra we estimate the oxygen coverage to be 1.4-t-0.3 ML. Heating this structure to 1430 K did not change the measured MEIS and AES spectra. The ratio of Sc:Ba was measured by MEIS as 1:1. We found that precisely the same S c - B a - O complex is formed when more than 1 ML of Ba is deposited first on the clean W surface, followed by deposition of more than 1 ML of Sc, exposure of the surface to several L of oxygen at room temperature, and annealing to 1430 K. From the angular spectra in Fig. 4 the changes in
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Fig. 3 shows high resolution MEIS spectra of different preparation stages of the S c - B a - O overlayer. The solid curve is the W-peak, prior to any deposition. Its area corresponds to 1.1 visible ML of W atoms. After deposition of Sc and annealing to 1430 K the dashed-dotted spectrum of Fig. 3 was measured. The Sc peak corresponds to 1.0 ML. The W peak was shifted to lower energies. There was no W signal left at the highest energy. This shows that the Sc formed a closed two-dimensional film. After deposition of several monolayers of Ba and annealing to 1430 K to evaporate the excess amount of material, the spectrum was given by the dashed
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P.M. Zagwijn et al. / Applied Surface Science 111 (1997) 35-41
40
lateral structure and amount of disorder of the surface can be determined. The solid curve was obtained on clean W(001). It corresponds to a bulk-like surface structure without significant relaxations. LEED showed a 1 × 1 pattern. The 1 × 1 pattern remained visible during different preparation stages of the S c - B a - O complex but degraded in quality after each additional step. The dotted curve in Fig. 4 represents the angular spectrum for 0.27 ML Ba (with a c(2 × 2) LEED pattern) on W(001). The height of Ba derived from the reduction in intensity below 90 ° is 1.50 + 0.05 ,~ above the first layer of W atoms 4 [13]. This is somewhat lower than the distance of 1.58 A between W(001) planes. The angular spectrum for W(001) with 1 ML of Sc on top is given by the dashed-dotted curve. From the reduction in W-yield above 90 °, the height of the Sc atom is deduced: 1.91 ___0.05 A. The angular spectrum for the final mixture of Sc, Ba and O hardly changes (dashed curve) with respect to the previous preparation stage. The overlayer and the first layers of the W crystal underneath are still well-ordered. In summary, none of the overlayer structures leads to significant changes in the W-yield. This shows that the W merely serves as the substrate and does not take part in the surface film. The amounts of Sc, Ba and O and their structure and depth distribution in the final mixture do not depend on the details of the preparation, e.g., order of depositions, deposited amounts, temperature history.
Auger electron spectroscopy data were taken at all stages of preparation. In Fig. 5a, the spectrum for the final mixture of Sc, Ba and O on the surface is given. Note that the surface is not free of C contamination originating from the Sc source. Comparing this spectrum to the spectrum taken in a different setup on a real scandate cathode (Fig. 5b), one sees that the shape and ratios of the main peaks of Sc, Ba and O are very similar. This implies that indeed the
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Fig. 5. Auger electron spectra, obtained with 3 keV primary electrons, of (a) W(001)/Sc,Ba,O model systemand (b) of a real scandate cathode prepared by Philips Research Laboratories. The contamination with P originates from the manufacturingprocess. prepared model system is representative for a real scandate cathode. 4. Discussion and conclusions
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In the geometric structure of the W(001)/Sc,Ba,O model system, W only serves as an inert support of the S c - B a - O overlayer system. We find fixed ratios of the concentrations of the elements in the overlayer compound. These ratios depend neither on the temperature nor on the order in which the structure is prepared. There is precisely 1 ML of Sc on the W(001) substrate, precisely 1 ML of Ba on top of that, and approximately 1.5 ML of O, which is probably distributed over the two metal monolayers. The work function of our scandate model system amounts to ~b= 1.18 eV. This value compares favourably with the work function found on optimized real scandate cathodes and accounts easily for the high current densities emitted by these cathodes.
P.M. Zagwijn et al. / Applied Surface Science 111 (1997)35-41
Interestingly, the minimum work function of 1.27 eV that we obtain on the W(001)/Ba,O system without Sc is only slightly higher than that on the scandate system. However, we find that the BaO ovedayer has no preference for the lowest-workfunction coverage of 0.26 ML, so that this system usually has a work function well above 1.27 eV. Based on these observations, we speculate that the Ba dispenser cathodes suffer from the fact that the local BaO coverage is almost nowhere optimal and that the primary role of Sc is to generate a strong preference for a well-defined oxide structure, close to or precisely in the work-function optimum.
Acknowledgements We are grateful to R.H.J. Fastenau and LA.J.M. Deckers for valuable discussions. We thank R. Koper of the Surface Preparation Laboratory at the FOMInstitute AMOLF for preparing the W(001) single crystal. This work was part of the research program of the Foundation for Fundamental Research on Matter (FOM) and was made possible by financial support from the Netherlands Organization for Scientific Research (NWO) and the Philips Research Laboratories in Eindhoven, The Netherlands.
References [1] J.A.J.M. Deckers, A. Manenschijn and P.A.M. van der Heide, Conf. record of the Tri-service/NASA Cathode Workshop, Cleveland, OH (1994) 53.
41
[2] D. Norman, R.A. Tuck, H.B. Skinner, P.J. Wadsworth, T.M. Gardiner, I.W. Owen, C.H. Richardson and G. Thornton, Phys. Rev. Lett. 58 (1987) 519. [3] G. Haas, A. Shih and C.R.K. Marian, Appl. Surf. Sci. 16 (1983) 139. [4] A. Shih, C. Hor, W. Elam and J. Kirkland, Phys. Rev. B 44 (1991) 5818. [5] A. Shih, G.A. Haas and C.R.K. Marrian, Appl. Surf. Sci. 16 (1983) 93. [6] W. Thomas (Lord Kelvin), Phil. Mag. 46 (1898) 82. [7] E. Wimmer, A.J. Freeman, J.R. Hiskes and A.M. Karo, Phys. Rev. B 28 (1983) 3074. [8] J.F. van der Veen, Surf. Sci. Rep. 5 (1985) 199. [9] P.M. Zagwijn, A.M. Molenbroek, J. Vrijmoeth, G.J. Ruwiel, R.M. Uiterlinden, J. ter Horst, J. ter Beek and J.W.M. Frenken, Nucl. Instr. Meth. Phys. Res. B 94 (1994) 137. [10] A. Lamouri and I.L. Krainsky, Surf. Sci. 278 (1992) 286. [l l] R.W. Gurney, Phys. Rev. 47 (1935) 479. [12] V.K. Medvedev and T.P. Smerka, Sov. Phys. Sol. State 15 (1973) 507. [13] U. van Slooten, A.M. Molenbroek, J.W.M. Frenken, J.A.J.M. Deckers and A. Manenschijn, Conf. record of the Triservice/NASA cathode workshop, Cleveland, Ohio, USA (1994). [14] A. Manenschijn, J.A.J.M. Deckers, Th.H. Weekers and P.A.M. van der Heide, Conf. Record of the Triservice/NASA Cathode Workshop, Greenbelt, Maryland, USA (1992) p. 67. [15] E. Bauer, H. Poppa and Y. Viswanath, Surf. Sci. 58 (1976) 517. [16] A. Lamouri, I.L. Krainsky, A.G. Petukhov, W.R.L. Lambrecht and B. Segall, Phys. Rev. B 51 (1995) 1803. [17] O.K. Kultashev and A.P. Makarov, Bull. Acad. Sci. USSR Phys. Set. 35 (1971) 321. [18] S. Yamamoto, I. Watanabe, S. Sasaki and T. Yaguchi, Surf. Sci. 266 0992) 100. [19] G. G~irtner, P. Geittner, H. Lydtin and A. Ritz, Appl. Surf. Sci. these proceedings.
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aol~eo surface science ELSEVIER
Applied Surface Science 111 (1997) 35-41
A model system for scandate cathodes P.M. Zagwijn a,m,J.W.M. Frenken a, * ,2, U. van Slooten b, P.A. Duine b a FOM-lnstitute for Atomic- and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands b Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands Received 24 June 1996; revised 6 September 1996; accepted 1 October 1996
Abstract In cathode-ray tubes, high current densities at relatively low operating temperatures can be achieved with dispenser cathodes containing scandium oxide. The surface structure and precise emission mechanism of the scandate cathode are not known. The surface layers of such a cathode consist of the four elements Sc, Ba, O, and W. We have studied Sc, Ba, and O ovedayers and various combinations of these elements deposited on the W(001) surface, under ultrahigh vacuum conditions. We have performed in situ measurements of the work function changes accompanying the surface treatments, and correlate these with the observed surface structures. High resolution medium-energy ion scattering was used to determine the compositional depth distributions and the geometrical structures at this model cathode surface. Auger electron spectroscopy peak height ratios show that W(001), with all three overlayer elements present, can be considered representative for real scandate cathodes. The observed ultra-low work function of the W(001)/Sc,Ba,O model system explains the high current densities on real scandate cathodes. PACS: 68.35.Bs; 73.30.+ y; 85.10.Mc
1. Introduction Scandate dispenser cathodes are promising electron sources for high brilliance cathode tubes. A scandate dispenser cathode consists of a porous mixed matrix of W and S c 2 0 3 impregnated with a 4:1:1 mixture of BaO, CaO, and A1203. The scandate cathode working mechanism is not understood since the precise nature of the active surface layer - -
* Corresponding author. Tel.: +31-20-6081234; fax: +31-206684106; e-mail: [email protected]. I Present address: ASM Europe B.V., Process Development, IMEC, Kapeldreef 75, B-3001 Leuven, Belgium. z Present address: University of Leiden, Kamerlingh Onnes Laboratory, P.O. Box 9506, 2300 RA Leiden, The Netherlands.
atomic composition, structure and chemical state - is not known. The high current densities obtainable in various applications with these cathodes, e.g., above 100 A / c m 2 at 1000°C [1], suggest that the work function o f a W surface covered with the S c - B a - O complex is significantly below that o f a conventional dispenser cathode, with only Ba and O on the surface. In the past, several attempts have been made to correlate the very high emission levels achieved by dispenser cathodes with the surface atomic structure. However, even for regular dispenser cathodes, i.e., without Sc, there is at present no consensus on the details of the emissive layer [2-4]. Most authors agree on the presence of a monatomic layer of barium and oxygen but disagree on the actual ad-
0169-4332/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 ( 9 6 ) 0 0 7 5 0 - 7
36
P.M. Zagwijn et al. / Applied Surface Science I11 (1997)35 41
sorption geometry of the Ba and O atoms. Nothing is known about the surface atomic structure of the scandate cathode. One of the main reasons lies in the difficulty to perform detailed measurements on the surface of a real cathode. Therefore, one has to switch to a model system, e.g., the surface of a W crystal with Sc, Ba and O overlayers, mimicking a real cathode. In this paper, we present a combination of work function measurements and medium-energy ion scattering (MEIS) experiments on the W(001) surface with various overlayers of Sc, Ba and O. The results show that high-temperature treatments lead to a stable S c - B a - O mixture of a well defined composition and structure on top of an almost unmodified W substrate. The accompanying work function change directly accounts for the high electron emission of real scandate cathodes. This paper is organized as follows. After a brief description of the experimental procedures in Section 2, we discuss the work function and MEIS results in Section 3.1 and Section 3.2. In Section 3.3, we compare Auger electron spectra on our model system with those from a real cathode in order to show that the single-crystal system is representative for real scandate cathodes. We conclude the paper with a brief discussion.
annealing procedure was repeated several times until O and C coverages determined by AES were below 1% of a monolayer (ML) [1 monolayer is defined as the number of atoms in one W(001) plane; 1.00 x 1015 atoms/cm2 ]. Low energy electron diffraction (LEED), typically at 90 eV, showed a sharp (1 X 1) pattern, corresponding to a non-reconstructed bulk-like surface.
2.2. Preparation of the Ba, Sc, and 0 overlayers Ba was deposited in situ at a deposition rate of 1.2 M L / m i n from a home-made dispenser source, that was thoroughly outgassed prior to deposition in order to remove the oxides. During deposition, the pressure in the vacuum chamber did not exceed 1 x 10 - 9 mbar. Ordered Ba overlayer structures were prepared by adsorbing a thick Ba layer (typically 5 - 2 0 ML) at room temperature on the clean W(001) crystal. The layer was then annealed at 1100 K for 1 rain [5]. Using LEED, a sharp c(2 X 2) pattern was observed. Sc was deposited at a rate of 0.40 M L / m i n from a home-made ScH 2 dispenser source. Even after extensive outgassing prior to deposition, the Sc source still contaminated the surface slightly with C and O. The surface was exposed at room temperature to 02 at a constant pressure of 5 X 10 - 9 mbar.
2. Experimental 2.3. Work function measurements 2.1. Crystal preparation The W crystal was cut by spark erosion to dimensions of 5 × 5 × 1 mm 3. The (001) surface was mechanically polished. After chemical etching, it was cleaned in an ultrahigh vacuum (UHV) chamber with a base pressure P = 5 X 10- ~~ mbar. The surface layers were removed by sputtering with 1 keV Ar ÷ ions at an incident angle of 45 °. The remaining contaminants on the surface were O and C. These were measured with Auger electron spectroscopy (AES), by use of a cylindrical mirror analyzer equipped with a channeltron detector operating in single-electron counting mode. The C was removed by annealing the crystal to 1500 K in an oxygen ambient ( P = I × 10 7 mbar) for 30 minutes. Then the surface was flashed to 2500 K in UHV. This
The work function 4' of the W surface was measured in situ by a contact potential method using a Kelvin probe 3 [6]. This method employs the AC current signal resulting from the variation of the capacitance between the W surface and an oscillating reference electrode. The resolution and run-to-run reproducibility of the measured work functions amounted to 10 and 100 meV, respectively. The work functions were measured relative to that of the clean W(001) surface, which has a work function 4' = 4.63 eV [7].
3 Besocke Delta Phi Gmbh, Postfach 2243, D-5170 Jiilich, Germany.
P.M. Zagwijn et al. /Applied Surface Science 111 (1997) 35-41 2.4. Medium-energy ion scattering
ally losing energy to the electrons in the solid ('electronic stopping'). By use of tabulated stopping powers the energy loss can be converted into the scattering depth in the crystal. The central detection direction of the analyser was along the [111] axis. Ions backscattered from the second (or deeper) layers are blocked on their way out by other atoms, which gives rise to minima in the scattering intensity as a function of scattering angle. The atomic positions in the surface were determined from the angles at which these minima occur [8].
High resolution medium-energy ion scattering, in conjunction with ion shadowing and blocking effects [8], was used to determine the atomic structure, composition and depth distribution of the grown layers and the W substrate. The backscattered ions were detected as a function of their final energy and scattering angle. By use of an electrostatic toroidal analyser equipped with a two-dimensional position sensitive detector, the low mass elements Sc and O could be detected within relatively short dataacquisition times [9]. The W crystal was aligned with the incident ion beam of 100 keV He + along the [112] direction with an accuracy of 0.1 °. Flashing the surface to 2500 K did not change the alignment of the crystal within this accuracy. Due to the shadowing effect in this scattering geometry, the hitting probabilities of the subsurface atoms are greatly reduced. For an energy of 100 keV, virtually all backscattered ions that reach the detector have undergone a single hard collision with an atom in the solid. Before and after the backscattering event the ions travel along essentially straight paths, continu-
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3.
Results
3.1. Work function In Fig. 1, the work function is shown as a function of Ba coverage on the W(001) surface. The open circles are for Ba c(2 × 2) structures prepared as described in Section 2.2. The filled circles represent measurements for which the Ba was deposited at room temperature. Using MEIS, the Ba coverages were determined with an accuracy of 0.01 ML [8].
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38
P.M. Zagwijn et al. /Applied Surface Science 111 (1997)35-41
Final Ba coverages were measured by MEIS to be 0.23-0.45 ML (1 ML is defined as the number of atoms in a W(001) plane), depending on the initial excess Ba coverage, annealing time and temperature. In previous studies of the Ba/W(001) system, e.g., Ref. [10], the Ba coverage was inferred from the c(2 X 2) symmetry of the LEED pattern, to be 0.5 ML. Our MEIS coverage measurements on this as well as several other overlayer systems [8] demonstrate that sharp LEED patterns often form easily well before completion of the adsorbate overlayer. Fig. 1 shows that the work function first decreases as the Ba coverage increases, until a minimum of ~b = 1.39 eV in the curve is reached at 0.26 ML. This minimum is ca. 1.1 eV lower than earlier published results [10]. After every deposition we found no C and O contamination by AES (detection limit 0.01 ML for O and C), and LEED showed sharp c(2 X 2) patterns. The more or less linear decrease in th for low coverages reflects the dipole moment formed by the positive charges on the adatoms and the screening charge of the substrate [11]. As the coverage increases, the interaction between these individual charge distributions leads to a reduction of the average adatom-substrate dipole moment, and as a result ~b first levels off and then increases again. Finally, as the coverage increases beyond several monolayers, a value is reached close to the bulk work function of the adsorbate. The work function of bulk Ba [7,12] is indicated in the insert of Fig. 1 by the dashed line. When oxygen was adsorbed on W(001) precovered with 0.27 ML Ba, the work function decreased by an additional 0.12 eV at a dose of 0.4 L (1 L = I X 10 -6 Torr s). This is consistent with the result from previous studies of this system with MEIS and other techniques [13], that locate the O atoms below or between the Ba atoms, where they should be expected to increase the surface dipole moment. This work function of 1.27 eV is much lower than the value of 1.9 eV that is needed to explain the performance of a regular dispenser cathode [14]. As the O 2 dose increased, the work function increased again by 0.20 eV at 2 L. Between 2 and 3 L the work function was constant. For larger doses the work function increased to a saturation value of q~ = 2.1 eV at and above 12 L. The change in work function during adsorption of oxygen was also measured on clean W(001). The
work function increased strongly, at first linearly with a slope of 0.90 e V / L . A break in this slope occurred at an exposure of 0.66 ___0.13 L and a work function of 5.22 + 0.05 eV was measured. A second break in the curve was found at 2.3 + 0.3 L and ~b= 5.93 + 0.10 eV. These values are close to the ones measured by Bauer et al. [15]. At 6.0 L the work function levelled off at 6.30 eV. An independent determination with MEIS of the O-coverage for this dose yielded (1.1 + 0.1)X 1015 0 atoms/cm 2, corresponding to 1.1 + O.l ML on the W(O01) surface. The curve of ~b as a function of Sc coverage on the W(001) surface, measured during deposition at room temperature, is similar to the one in Fig. 1. A minimum of ~b = 1.88 eV is reached at a coverage of 1.0 ML. This minimum is again much lower (ca. 1.4 eV) than results published by others [16], possibly due to contamination with O from our Sc source [17]. For higher coverages, e.g., 10 ML, the value of & approached 3.33 eV, which is expected for bulk Sc. Exposing W(001) precovered with 1.0 ML of Sc to 02 at room temperature caused the work function to drop slightly, by 0.05 eV, at l. 1 L. After exposure of the surface to 6 L, the work function had increased again to ~b = 4.2 eV. In Fig. 2, the work function vs. 02 exposure is given for a structure of 1.0 ML of Sc and 1.0 ML of Ba on W(001). The final ~ = 1.18 eV that is reached is virtually the same ( + 0.1 eV) as the ~b found after
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P.M. Zagwijn et al. / Applied Surface Science 111 (1997) 35-41 0.4
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Fig. 3. MEIS energy spectra measured in a doubly aligned geometry for 100 keV He + ions backscattered from W(001) at different stages of the preparation of a S c - B a - O ovedayer. Solid curve: clean W(001). Dashed-dotted curve: After deposition of 1 ML Sc and heating to 1430 K. Dashed curve: Final S c - B a - O structure (see text). The backscattering yield is normalised to the 'random height' of W [8]. The arrows indicate the energy expected for ions backscattered from W, Ba and Sc atoms located at the surface.
heating the crystal to 1430 K for 1 min. The effect of oxygen is qualitatively the same as in the case of pure Ba (0.27 ML) or Sc (1.0 ML) on the surface. The value of 4~ = 1.18 eV is 0.30 eV lower than the effective work function calculated by Manenschijn et al. [14] to fit the high current densities of typical scandate cathodes and precisely in the range of the minimum values of 1.13-1.22 eV found on optimized scandate cathodes [ 18,19].
39
curve. The W peak was again shifted to lower energies. The W and Ba peaks measured in this scattering geometry are not well separated (see decomposition of the combined W + Ba peak in Fig. 3). From the peak shape, and from spectra measured at higher scattering angles where the W and Ba peak are well separated, we derive that 1.0 ML of Ba was on the surface and the amount of visible W was still 1.0 ML. The Sc peak was shifted by the same amount to lower energies, indicating that all of the Sc was buried under the Ba. Exposing the surface to oxygen did not change the size and shape of the final spectrum compared to the dashed spectrum of Fig. 3. The oxygen signal could not be detected by MEIS, which means that the amount of adsorbed oxygen was either too small for detection (<< 0.5 ML), or that the oxygen was distributed over the thickness of the S c - B a film. From the AES spectra we estimate the oxygen coverage to be 1.4-t-0.3 ML. Heating this structure to 1430 K did not change the measured MEIS and AES spectra. The ratio of Sc:Ba was measured by MEIS as 1:1. We found that precisely the same S c - B a - O complex is formed when more than 1 ML of Ba is deposited first on the clean W surface, followed by deposition of more than 1 ML of Sc, exposure of the surface to several L of oxygen at room temperature, and annealing to 1430 K. From the angular spectra in Fig. 4 the changes in
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Fig. 3 shows high resolution MEIS spectra of different preparation stages of the S c - B a - O overlayer. The solid curve is the W-peak, prior to any deposition. Its area corresponds to 1.1 visible ML of W atoms. After deposition of Sc and annealing to 1430 K the dashed-dotted spectrum of Fig. 3 was measured. The Sc peak corresponds to 1.0 ML. The W peak was shifted to lower energies. There was no W signal left at the highest energy. This shows that the Sc formed a closed two-dimensional film. After deposition of several monolayers of Ba and annealing to 1430 K to evaporate the excess amount of material, the spectrum was given by the dashed
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P.M. Zagwijn et al. / Applied Surface Science 111 (1997) 35-41
40
lateral structure and amount of disorder of the surface can be determined. The solid curve was obtained on clean W(001). It corresponds to a bulk-like surface structure without significant relaxations. LEED showed a 1 × 1 pattern. The 1 × 1 pattern remained visible during different preparation stages of the S c - B a - O complex but degraded in quality after each additional step. The dotted curve in Fig. 4 represents the angular spectrum for 0.27 ML Ba (with a c(2 × 2) LEED pattern) on W(001). The height of Ba derived from the reduction in intensity below 90 ° is 1.50 + 0.05 ,~ above the first layer of W atoms 4 [13]. This is somewhat lower than the distance of 1.58 A between W(001) planes. The angular spectrum for W(001) with 1 ML of Sc on top is given by the dashed-dotted curve. From the reduction in W-yield above 90 °, the height of the Sc atom is deduced: 1.91 ___0.05 A. The angular spectrum for the final mixture of Sc, Ba and O hardly changes (dashed curve) with respect to the previous preparation stage. The overlayer and the first layers of the W crystal underneath are still well-ordered. In summary, none of the overlayer structures leads to significant changes in the W-yield. This shows that the W merely serves as the substrate and does not take part in the surface film. The amounts of Sc, Ba and O and their structure and depth distribution in the final mixture do not depend on the details of the preparation, e.g., order of depositions, deposited amounts, temperature history.
Auger electron spectroscopy data were taken at all stages of preparation. In Fig. 5a, the spectrum for the final mixture of Sc, Ba and O on the surface is given. Note that the surface is not free of C contamination originating from the Sc source. Comparing this spectrum to the spectrum taken in a different setup on a real scandate cathode (Fig. 5b), one sees that the shape and ratios of the main peaks of Sc, Ba and O are very similar. This implies that indeed the
4 The Ba height of 1.50+0.05 previously,
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Fig. 5. Auger electron spectra, obtained with 3 keV primary electrons, of (a) W(001)/Sc,Ba,O model systemand (b) of a real scandate cathode prepared by Philips Research Laboratories. The contamination with P originates from the manufacturingprocess. prepared model system is representative for a real scandate cathode. 4. Discussion and conclusions
3.3. Real cathode surface
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In the geometric structure of the W(001)/Sc,Ba,O model system, W only serves as an inert support of the S c - B a - O overlayer system. We find fixed ratios of the concentrations of the elements in the overlayer compound. These ratios depend neither on the temperature nor on the order in which the structure is prepared. There is precisely 1 ML of Sc on the W(001) substrate, precisely 1 ML of Ba on top of that, and approximately 1.5 ML of O, which is probably distributed over the two metal monolayers. The work function of our scandate model system amounts to ~b= 1.18 eV. This value compares favourably with the work function found on optimized real scandate cathodes and accounts easily for the high current densities emitted by these cathodes.
P.M. Zagwijn et al. / Applied Surface Science 111 (1997)35-41
Interestingly, the minimum work function of 1.27 eV that we obtain on the W(001)/Ba,O system without Sc is only slightly higher than that on the scandate system. However, we find that the BaO ovedayer has no preference for the lowest-workfunction coverage of 0.26 ML, so that this system usually has a work function well above 1.27 eV. Based on these observations, we speculate that the Ba dispenser cathodes suffer from the fact that the local BaO coverage is almost nowhere optimal and that the primary role of Sc is to generate a strong preference for a well-defined oxide structure, close to or precisely in the work-function optimum.
Acknowledgements We are grateful to R.H.J. Fastenau and LA.J.M. Deckers for valuable discussions. We thank R. Koper of the Surface Preparation Laboratory at the FOMInstitute AMOLF for preparing the W(001) single crystal. This work was part of the research program of the Foundation for Fundamental Research on Matter (FOM) and was made possible by financial support from the Netherlands Organization for Scientific Research (NWO) and the Philips Research Laboratories in Eindhoven, The Netherlands.
References [1] J.A.J.M. Deckers, A. Manenschijn and P.A.M. van der Heide, Conf. record of the Tri-service/NASA Cathode Workshop, Cleveland, OH (1994) 53.
41
[2] D. Norman, R.A. Tuck, H.B. Skinner, P.J. Wadsworth, T.M. Gardiner, I.W. Owen, C.H. Richardson and G. Thornton, Phys. Rev. Lett. 58 (1987) 519. [3] G. Haas, A. Shih and C.R.K. Marian, Appl. Surf. Sci. 16 (1983) 139. [4] A. Shih, C. Hor, W. Elam and J. Kirkland, Phys. Rev. B 44 (1991) 5818. [5] A. Shih, G.A. Haas and C.R.K. Marrian, Appl. Surf. Sci. 16 (1983) 93. [6] W. Thomas (Lord Kelvin), Phil. Mag. 46 (1898) 82. [7] E. Wimmer, A.J. Freeman, J.R. Hiskes and A.M. Karo, Phys. Rev. B 28 (1983) 3074. [8] J.F. van der Veen, Surf. Sci. Rep. 5 (1985) 199. [9] P.M. Zagwijn, A.M. Molenbroek, J. Vrijmoeth, G.J. Ruwiel, R.M. Uiterlinden, J. ter Horst, J. ter Beek and J.W.M. Frenken, Nucl. Instr. Meth. Phys. Res. B 94 (1994) 137. [10] A. Lamouri and I.L. Krainsky, Surf. Sci. 278 (1992) 286. [l l] R.W. Gurney, Phys. Rev. 47 (1935) 479. [12] V.K. Medvedev and T.P. Smerka, Sov. Phys. Sol. State 15 (1973) 507. [13] U. van Slooten, A.M. Molenbroek, J.W.M. Frenken, J.A.J.M. Deckers and A. Manenschijn, Conf. record of the Triservice/NASA cathode workshop, Cleveland, Ohio, USA (1994). [14] A. Manenschijn, J.A.J.M. Deckers, Th.H. Weekers and P.A.M. van der Heide, Conf. Record of the Triservice/NASA Cathode Workshop, Greenbelt, Maryland, USA (1992) p. 67. [15] E. Bauer, H. Poppa and Y. Viswanath, Surf. Sci. 58 (1976) 517. [16] A. Lamouri, I.L. Krainsky, A.G. Petukhov, W.R.L. Lambrecht and B. Segall, Phys. Rev. B 51 (1995) 1803. [17] O.K. Kultashev and A.P. Makarov, Bull. Acad. Sci. USSR Phys. Set. 35 (1971) 321. [18] S. Yamamoto, I. Watanabe, S. Sasaki and T. Yaguchi, Surf. Sci. 266 0992) 100. [19] G. G~irtner, P. Geittner, H. Lydtin and A. Ritz, Appl. Surf. Sci. these proceedings.