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Field evaporation and field desorption from HTSC single-crystal surfaces
Progress in Sudace Science, Vol. 42, pp. 131-142 Printed in the U.S.A. All rights reserved.
0079-6816/93 $24.00 + .00 Copyright © 1993 Pergamon Press Ltd.
FIELD EVAPORATION AND FIELD DESORPTION FROM HTSC SINGLE-CRYSTAL SURFACES VLADIMIR N. SHREDNIK A.F. Ioffe Physico-Technical Institute Russian Academy of Sciences St. Petersburg
Abstract
The problems of field evaporation (FE) and field desorption (FD) from surface of oxide high-temperature superconductors (HTSC) of 123, 2212 and 2223 type are discussed. Wide-angle atom probe, usual time-of-flight atom probe and field ionization magnet massspectrometer have been used in these experiments. The main purpose of the investigation was to search for effects connected with phase transition. Therefore, the experiments were carried out over a wide range of temperatures: from T of solid nitrogen to room T. Composition of pure, contaminated and deep corroded surface and typical species of FE and FD spectra have been studied. The bond energies between some atoms (or clusters) and the surface have been estimated on the basis of experimental results and FE theory. From these estimations, and from discovering of correlated ion pairs, it was concluded that the redistribution of local bonds should be taken into consideration in this case. The increase of FE rate was observed at cryogenic temperatures. Various versions are discussed to explain these effects. Some of them are connected with phase transition. A detailed study of FD of water protonized clusters is performed and the catalytic activity of HTSC crystal surfaces is discussed. Contents 1. 2. 3. 4. 5. 6. 7.
Introduction Experimental Examples of typical atom-probe spectra Estimation of bond energies in HTSC single crystals Temperature dependence of field evaporation rate Field desorption of water from HTSC single-crystal surfaces Conclusion Acknowledgements References Abbreviations
FD FE HTSC
Field desorption Field emission High-temperature superconductors 131
132
V.N. Shrednik
1. I n t r o d u c t i o n The situation that has emerged in field evaporation (FE) and field desorption (FD) from surfaces of high temperature superconductors (HTSC) reflects an unexpected turn, typical of progress in science. Indeed, was it possible 7 years ago to anticipate that specialists of field emission methods would study such complicated and unfit experimental objects such as 4-5-component oxide materials? Let us remember that previously these scientists studied, in the first place metals, sometimes metal alloys, but only a few conducting compounds (usually binary) [1]. However, the great interest aroused by new HTSC has served as an impetus to study them. Field ion microscopy and atom-probe experiments ensure the atomic scale of experiment. The problem of obtaining atomically clean and regular crystal surfaces was solved usually by field evaporation and field desorption. We shall try to summarize, in this paper, the experimental results (obtained mainly at our laboratory) concerning FE and FD from the surfaces of HTSC materials and to indicate some possible future investigations. 2. E x p e r i m e n t a l The very complicated composition of the compounds being considered means that field emission microscopy, by itself, is inadequate for the investigation of FE and FD from their surfaces. In our case, adequate to the aim of the investigation was a combination of massspectroscopy with field ion microscopy. Two kinds of time-of-flight atom probe were used in this study: wide-angle atom probe with the resolution M / A M atom probe with M / A M
,.~ 15 - 20, and probe-hole
,~ 70, as well as a magnetic field ionization mass-spectrometer
with M / A M ~ 200. Samples were prepared by Melmed's method [2]. They looked like small sharp crystals glued by conducting epoxy to the top of a tungsten tip. All the samples had the "c"-axis parallel to the tip axis. Numerous HTSC materials were investigated, but the most successful experiments were carried out with 1:2:3 single crystals on the base of Y, Tb, Eu, S m and Gd as well as Bi 2112 and 2223. These compounds were studied in a wide temperature (T) range: from room T down to T of liquid or solid nitrogen. Atom-probe experiments were accompanied by observing field ion images showed strip atomic structure [3,4]. Details of the electrical and registration circuits were described in the original papers referred to below. 3. E x a m p l e s of typical a t o m - p r o b e s p e c t r a Figure 1 shows the four spectra of FE from EuBa2Cu307-= single crystals at the temperature of liquid nitrogen (T = 82 - 85K, i.e. T < Tc ~ 90K) obtained by a wide-angle
Evaporation and Desorption From HTSC Crystals
133
atom probe [5]. Each spectrum is the sum of hundreds of elementary spectra (an elementary spectrum is a result of one evaporation pulse) taking into account the intensity and time-of-flight error A r of every component. Values A r are not negligible (20-60 ns). The resolution calculated from these spectra is lower than the real one. Therefore, these spectra are sometimes accompanied by line spectra drawn with the assumption that A r = -4-5 ns. Line spectra enable us to remove the error of appearance of an incorrect peak resulting from the summation of two very close peaks. In spite of low M / A M
value, the use of the
wide-angle probe is very effective in the FE study of multicomponent materials, because it allows large ion statistics to be kept without visible sample blunting, i.e., at constant field F during the experiment. The spectrum in Fig. la demonstrates a typical set of ions evaporated from the clean surface of a HTSC (123) sample. It reveals various metal and metal-oxide ions. The next spectrum (Fig. lb) was obtained after sample corrosion at a residual gas pressure of 10-4 Torr over a few months. It corresponds to a contaminated surface and displays only one metal ion Ba ++. Evaporation of many atomic layers (see Fig. lc) did not result in a complete surface cleaning. Peaks connected with the corrosion (H +, C ++, probably O +) remained in the spectrum, but a small Cu ++ (or O +) peak appeared. Ba ++ and E u 2 0 +++ peaks are distinguished clearly. It should be mentioned that the Eu +++ ion is absent. Then an occasional break off of a small part of the sample took place. This break off in high vacuum opened the clean (rather very clean) surface of the crystal. The spectrum (Fig. ld) obtained at the same voltage revealed practically only one peak of ions: Eu +++. These four spectra are formed by particles removed from the surface at a constant (and approximately the same) field F, and only a small part of a monoatomic layer could be removed. Therefore, these four spectra (Fig. 1) demonstrate what kinds of ions left the surface after various treatments, and what atoms made up the surface before the treatments. Intensive field evaporation, after the observation of field-ion images at 12 - 12.5 kV, formed the surface characterized by the spectrum Fig. la. As a result of this treatment, only the most strongly bound atoms remain at the surface. This is the end form of field evaporation of (123) HTSC crystal. Authors of [6] have made a computer simulation of the ion patterns of YBa2Cu3Or-z crystal and have compared these patterns with real images. They concluded that a field-ion image is formed by Y atoms. Spectra Fig. la show that, besides Eu (instead of Y), there are also Cu and O atoms on the surface, possibly not imaged. The crystal surface (corresponding to Fig.
lb) was covered by contaminants after corrosion. These
contaminants seemed to be produced by the action of H20, CO and CH4. The spectrum displays products of dissociation of these molecules (H +, C ++, O+). Only Ba ++, and maybe EuO ++, are observed, but other metallic ions are absent. The surface of EuBa2Cu3OT_~
134
V.N. Shrednik
300
300
200,
200
W o~
$. op4
J.O0 '
100
c,~c.~ I
÷ ÷ 4"411 ¢
o
l)
200
400
600
800
0
1000
200
400
600
800
~.000
time-of-flight (ns)
time-of-flight (ns)
FJg.lb
]L~.la
200
30 EU ÷'~
I
20 [00
IIt
io
°~1
i 0
~+
:o-.I ] % o?l I
0 0
200
400
600
800
time-of-flight (ns) Fig.lc
1000
0
200
400
600
800
tO00
time-of-flight (ns) Fig.ld
Figure 1: Wide-angle atom-probe spectra of EuBa2Cu307_= obtained under the following conditions: vacuum better than 10 -9 Torr; liquid nitrogen cooling of the sample. Resulting voltage U is the sum of base steady-state voltage U= and pulse voltage AU : U = U= ÷ AU. a) surface reliably cleaned by field evaporation: U = U= + / k U = 10 + 2 kV; b) corroded surface after long exposure in residual gases (H20, CH4, N2, CO) 10 -4 Torr; U=10÷2=12kV; c) after removing many (,,, 150) monoatomic layers by field evaporation from the surface corresponding to Fig. lb. The tip is blunted and requires higher U for the same operation: U = 13 ÷ 2 = 15 kV; d) really clean surface after break-off in high vacuum; U = 13 + 2 = 15 kV.
Evaporation and Oesorption From HTSC Crystals
remains contaminated (Fig.
135
lc), even after removing many monoatomic layers and tip
blunting. The spectrum now displays a small amount of Cu ++ ions and a remarkable peak of Ba ++. It is interesting that E u leaves the surface as the stoichiometrical oxide E u 2 0 +++. Some ions from contaminants are still observed. Therefore, the action of active residual gases over a long time at room T leads to corrosion of the crystal to a depth of more than 100 monoatomic layers. Break off in vacuum opens the surface, with the most strongly bound atoms, and displays E u +++ ions in the spectrum. Figure 2 shows the spectrum of a Gd (123) crystal obtained by probe-hole atom probe with a reliable resolution of all the species [7]. The sample, after preliminary treatment, should have the surface cleaned by FE. The spectrum displays a typical set of metal and oxide ions. The variety of the oxide and low charged ions should be mentioned. However, only Gd reveals oxide ions. Nevertheless, C u O ++ and C u O + were observed simultaneously with E u O +++ and E u O ++, but at the room temperature and by the same atom probe.
/
(3)
Cu ÷
E. 0 0
O
4 (D
G d +++
[ 4
0
0
0
(D
--': '-' '2'
Ba ~
I
0
i 0
+÷ o;
CdO +
+
I
I
+÷ aa÷
C,
L
.......... C, 0 0 C, ,:, ,--,
J ............. L .......
0
4
C,
$ ............ • .........
0
~: ,-,
I.- .......
J- .........
I 2:0
I...........
L . . . . . .
i ~. ,--,
I
............. [ ........
2
,-, 0
Mass-to-charge
Figure 2: Probe-hole atom-probe spectrum of GdBa~Cu3Or_~: (To = 95 K) obtained at the liquid nitrogen cooling of the sample in vacuum 10-7 Torr; U = 7 + 3 = 10 kV.
4. Estimation of bond energies in HTSC single crystals The theory of field evaporation [8] developed for metals [9,10] permits one to determine the bond energy A of an atom leaving the surface. In this case, the ionization energy I . (where
V.N. Shrednik
136
n is the charge of the ion), the work function ~b, and the atomic and ion polarizabilities a~ and al should be known and evaporation field F, should be measured. The theory [10] gives the following equation for these quantities: = A + ~ I , - n . ¢ + 1/2(c~ - ai)- F~
(1)
n
where e is the elementary charge. Usually the last term in expression (1) can be discarded. Thus:
.~= V / ~ n 3 e 3 - y~l,~ +nc~
(2)
n
Some difficulties, both of a technical and principal nature, arise in the case of HTSC materials. As regards the first, the necessity of the measurement of ¢ and Fe can be still overcome in only a few cases. For instance, we have measured ~band Fe of the sample corresponding to the spectrum given in Fig. la [11,12]. Moreover, we took into consideration the contribution of the surface dipole field of the oxide materials to determine the best image field in the case of nitrogen being the image gas. In this way, we obtained ¢ = 4.1 eV for the EuBa2Cu307_~ single crystal. Thus, one prinicipal difficulty, i.e., the essential difference between the local field structure at the oxide HTSC surface and metal one, has been partly eliminated. After ¢ and F~ have been determined, it is possible to calculate •, using the well-known tabulated values of I~. Our calculation [12] showed that only for Ba ++ and Eu ++ were the values of (4.7 eV and 3.6 eV, respectively) comparable with the bond energies in the oxides: BaO and E u O (5.8 eV and 5.7 eV [13]). It is quite reasonable to assume that on leaving the surface, the Ba (or Eu) atom cuts the bond with the oxygen. In these oxides, both atoms have the valency of 2. Thus, the comparison of £ for these two charged ions seems to be correct. The calculation for Eu +++, Cu +, Cu ++, 0 + ions resulted in absolutely improbable ), values; i.e., too small or just negative. At the same time, estimated values of ,k for
BaO + and EuO + (1.95 eV and 3.45 eV) seemed to be not too improbable. Some difficulties arising with ~ = - 6 . 5 eV for Eu +++ can be easily removed by the natural assumption of post-ionization of Eu ++. The conclusion that follows from these calculations was that only the electropositive metals Ba and Eu on the surface of a superconductor (the temperature in the experiments was always T < To) could be considered as typical adsorbate on a smooth metal surface. As regards Cu and O atoms forming the base of a cuprate lattice, a simple theoretical model of a monocomponent metal is not satisfactory in this case. The model should be modified by taking into account (1) polar bonds in the lattice and (2) change of the local energetic situation after removing an atom from its site. Too low estimated values of/k mean that the corresponding ions appear at an unreasonably low value of F, that removes the Ba and Eu ions. In other words, it means that Cu and O atoms are removed together with Ba and Eu atoms, because they (Cu and O) can no longer stay in the lattice after the change in the
Evaporation and Desorption From HTSC Crystals
137
local energetic situation has occurred. Besides, it can be energetically favorable to release the atoms as cluster ions, such as E u O +. In the same way, the field desorption of some chemically active adsorbates can be explained along the same lines. Removing of some adsorbed atom can essentially change the local bond energies of the remaining lattice (or adsorbate) atoms. Consequently, one can expect to register correlated (not occasional) pairs or triplets of some ions as a result of one pulse of evaporation.
Indeed, a wide-angle atom probe
very often detects pairs (or more numerous groups) of ions in one elementary spectrum (obtained from one pulse). Special analysis of such pairs in the FE spectrum of Tb (123) showed [4] Tb +++ and Cu ++ (or O +) correlated pairs, while contaminants desorbed from Tb (123) revealed a large number of correlated pairs of C ++ and O + probably originating
from CO or C02 dissociation.
Ba ++ always leaves the E u (123) corroded surface (Fig.
lb,c) together with some light ion originating from the contamination. These correlations (as seen from examples given above), after more detailed and extensive analysis, together with a reasonable ~ estimation, should supply significant data, which will be useful for the creation of an adequate FE and FD theory of multicomponent surfaces and the HTSC surface particularly. Experiments in search of such correlations and measurements of ,k are of special interest, when they are carried out near To. Phase transitions at T~ can cause changes of some interatomic bond energies. The next section gives some evidence supporting this idea. 5. T e m p e r a t u r e d e p e n d e n c e of field e v a p o r a t i o n r a t e The rate of field evaporation, registered by peaks of ions originating from a HTSC lattice, increases with transition to a lower T, in contrast to the classical FE theory. It has been observed many times (not less than 8) in wide-angle atom-probe experiments with Tb and E u (123) [4] at the transition from room T to liquid nitrogen T and with B i (2223) at the
transition from liquid nitrogen T to solid nitrogen T. In all the cases, the experiments were carried out with the same sample and at comparable statistics. The effect was more or less prominent. Sometimes, not only the rate of FE changed, but the set of evaporating ions also changed. Figure 3 shows an example of such behavior in the B i (2223) spectrum [14]. Similar phenomena have been observed for some light ions from adsorbed layers. The increase of the O + peak (reliably separated from Cu ++ now) with the cooling of the S m (123) sample to T near Tc has been proved by means of magnetic field ionization mass-spectrometry [15]. The origination of the O + ion from the lattice has been specially established. Some explanations, both general and particular, can be given for these effects: 1. Change of the field penetration at the phase transition to the superconductive state. Generally, there is some sensitivity of F to the sample conductivity. In this case, even
138
V.N. Shrednik
300 ,,t.a • f,,,~
°lit
200 gS
I
100
~
I
0 0
200
400
600
800
1000
Lime-of-flight (ns) Figure 3: Wide-angle atom-probe integral and line spectra of Bi2Sr2Ca2Cu30y at solid nitrogen cooling of the sample (solid lines) and at liquid nitrogen cooling (dotted line); U = 8.5 + 1.5 = 10 kV. a small increase in F should cause a remarkable effect, because of the exponential F dependence of FE [4]. 2. Change of the interatomic bond energy on lowering T to near or below Tc [4,14].
3. Dependence of the condensation coefficient of gases or vapors on T, which explains the behavior of some FD peaks [4]. 4. Field etching.
Influence of adsorbed gas particles on FE. This phenomenon is well
known for metals [10,16]. 5. Redistribution of vibrational energy between
Cu and O atoms in the lattice [15].
6. Some radio engineering hypotheses are also possible. They are connected with the passing of a nanosecond pulse through an electrical circuit. Some conditions can change in this case: (a) presence of liquid nitrogen in cold finger changes capacity and (b) transition at T > T~ into semiconducting state, for example, changes resistance of the circuit. Discussing these ideas, from the last one, we can say that statement (6-a) is hardly real. It was proved twice: with great variation of liquid nitrogen level and with the variation of T at the same level - liquid nitrogen and liquid nitrogen with chunks of solid nitrogen. The statement (6-b) is doubtful, because of the small change in the resistance. (HTSC sample is very small.)
Evaporation and Desorption From HTSC Crystals
139
Statement (5) is rather particular, but is quite reasonable. Unfortunately, in [15] only one peak was studied. The behavior of the remaining peaks was unknown. Besides field etching was used in this experiment. Generally, the idea of etching (4) is interesting as such, in spite of the fact that it seems to have no correlation with To. However, it is hardly valid in cases of ultrahigh vacuum (often as high as 10 -9 Torr) and is doubtful in the case of cooling from liquid nitrogen T to solid nitrogen T. Idea (2) is very interesting and important. It is connected with the phase transition, but not always with the one at To. Finally, the idea (1), connected with the transition at To, is of great interest. Though there is some criticism, based on the constancy of the carrier concentration at the phase transition. However, there is some concern. If a sample at T > Tc becomes a semiconductor, or bad conductor, will it influence the field penetration? Generally, the conception of an equal local field near the different metal surface atoms at the same positions is based on the idea of an ideal metal, but modern experiments [17] have shown that this is not the case. They have shown the importance of chemical and electrical individuality of the atom. Therefore, it is quite reasonable to expect a great difference in the field evaporation probability at a surface in the normal state from that in a superconducting state. Thus, the possibility of using FE from a HTSC material as an indication of a phase transition at T~ remains quite feasible. Irrespective of physical aspects, it is significant from a technical point of view. As regards the first, it is clear that any reliable FE experiments that pass through Tc are very interesting physically. In this case, the change of any of the following parameters is important: peak intensities, appearance or disappearance of some peaks, correlated pairs (or more complex correlated associations) of ions, mean height of certain peaks in the elementary spectra. 6. Field d e s o r p t i o n of w a t e r f r o m H T S C s i n g l e - c r y s t a l surfaces FD from HTSC materials has been occasionally observed in many experiments by both kinds of atom probe, and involve FD of different species originating from the residual gas adsorbed layer. However, a systematic detailed study of FD of water from Bi 2212 and
Gd and Srn single crystals has been carried out, using a probe-hole atom probe [7]. Two observations can be considered as the main results of this work.
A sharp temperature
threshold of water FD rate at low T (liquid nitrogen cooling). This threshold was rather higher than T~, but may be very close to it.
Secondly, mass-spectra obtained at low T
showed peaks of protonized polymer cluster ions H+.n(H20).
Figure 4 displays such a
spectrum for n from 1 to 6. This phenomenon is similar to that observed previously on metal substrates [10]. However, in contrast to the case of metals, in our experiment the
140
V.N. Shrednik
most intensive peak has n = 2. The peak height relation changed depending on F . This experiment revealed a high catalytic activity of the oxide HTSC crystal surface similar to t h a t of some metals. However, the peculiarities of how these effects differ from the ones for metals is more interesting. Another i m p o r t a n t aspect of this work is connected with the strong chemical action of water on the surfaces of these HTSC oxides. From a technical point of view, it was the first work to show the favorable application of the probe-hole a t o m probe to the s t u d y of adsorption and catalytic phenomena on HTSC surface. 2 ~D
~ 0
I 2 E'
~c~ @ C) (D C, :~: g:=
O (D 0
,I ,2,
II
' ..... ~ Ih,h. , !t t, , ~ i i , l . l , l l l ~
O O O C, O ,",
i E, 2 C,
I C., 4 ,-.:,
I I ,Z, ~. ,-t
i I ,Z, :-=: O
i.l
I ,Z, O
1
......
I ~ ,Z,
i 1 4 E~
Mass-to-charge Figure 4: Probe-hole a t o m - p r o b e spectrum of protonized water clusters:
H+.n(H20) n
Bi:Sr2CalCu~Oz showing
field desorption of
= 1, ..., 6. Values of n are displayed at correspond-
ing peaks. Liquid nitrogen cooling of the sample; U = 11 + 3 = 14 kV.
7. C o n c l u s i o n One can state that, for several years, a lot of experimental d a t a were accumulated in the field of F E and FD from the surfaces of HTSC materials and conventional experimental techniques were applied to study these new materials. At the same time, it became clear that the initial period of principle investigations, where more problems were created than solved in this field, has now passed. One has a definite feeling of the insufficiency of our theoretical understanding, and realizes the necessity to work on the foundations of new theoretical ideas. To do this, more fundamental experiments should be carried out. Anyway, it seems that
Evaporation and Desorption From HTSC Crystals
141
some approaches to them have been indicated by the results of the previous work discussed here. The field evaporation of multicomponent compounds with polar interatomic bonds is important and interesting as such, even irrespective of the phase transition at To. Moreover, this field will be physically significant, if it reveals the dependencies connected with the transition at Tc and can shed some light on the phenomena concerned with the HTSC lattice accompanying this transition. This research has been carried out as part of the HTSC Project No. 91097. Acknowledgements I wish to express my sincere thanks to Drs. O.L. Golubev, Yu.A. Vlasov, and M.V. Loginov, and to Mrs. E.L. Kontorovich, and T.I. Sudakova for their kind assistance in preparing this paper and for their technical assistance. References
1. E.W. Muller and T.T. Tsong, Field Ion Microscopy, Field Ionization and Field Evaporation, Prog. Surf. Sci., 4, 1 (1973). 2. A.J. Melmed, J. Phys. Colloq., 49, 67 (1988). 3. Yu.A. Vlasov, O.L. Golubev, N.A. Samokhvalov, N.N. Syutkin, E.F. Talantsev, N.M. Chebotaev, and V.N. Shrednik, Sov. Tech. Phys. Lett., 15,647 (1989). 4. Yu.A. Vlasov, O.L. Golubev, N.N. Syutkin, E.F. Talantsev, and V.N. Shrednik, Sov. Phys. Tech. Phys., 35, 1208 (1990). 5. Yu.A. Vlasov, O.L. Golubev, E.L. Kontorovich, and V.N. Shrednik, Pisma v Zhur. Tech. Fiz., 18, 1 (1992). 6. G.A. Mesyats, N.N. Syutkin, V.A. Ivchenko, E.F. Talantsev, Zhur. Tech. Fiz., 14, 1504 (1988). 7. M.V. Loginov, O.G. Saveljev, V.N. Shrednik, Zhur. Tech. Fiz., 63, in press 1993. 8. E.W. Muller, Advan. Electron. and Electron Phys., 13, 83 (1960). 9. D.G. Brandon, in "Field Ion Microscopy", Ed. by J.J. Hren and S. Ranganathan, Plenum Press, New York, 1968, section 3. 10. E.W. Muller and T.T. Tsong, Field Ion Microscopy. American Elsevier Publishing Co., New York, 1969.
Principles and Applications.
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11. Yu.A. Vlasov, O.L. Golubev, N.A. Samokhvalov, N.N. Syutkin, E.F. Talantsev, N.M. Chebotaev, and V.N. Shrednik, Sov. Tech. Phys. Lett., 15,977 (1989). 12. Yu.A. Vlasov. O.L. Golubev, E.L. Kontorovich, and V.N. Shrednik, Pisma v Zhur. Tech. Fiz., 18, l l (1992). 13. In "Energee razriva himicheskih svjazei, potenciali ionizacii i srodstvo k electronu", Moscow, "Nauka", 1974, p. 105. 14. Yu.A. Vlasov, O.L. Golubev, E.L. Kontorovich, and V.N. Shrednik, Pisma v Zhur. Tech. Fiz., 17, 5 (1991). 15. N.M. Blashenkov, G.Ya. Lavrentjev, V.N. Shrednik, Pisma v Zhur. Tech. Fiz. 17, 30 (1991). 16. D.G. Brandon, in "Field Ion Microscopy", Ed. by J.J. Hren and S. Ranganathan, Plenum Press, New York, 1968, section 4. 17. Y. Suchorski, W.A. Schmidt, J.H. Block and H.J. Kreuzer, See portion of Seminar Proceedings published in Vacuum. 18. S. Jaenicke, A. Ciszewski, J. Dosselmann, W. Drachsel, J.H. Block and D. Menzel, Desorption Induced by Electronic Transition - DIET III. V.13. Ed. by R.H. Stulen and M.L. Knotec., Springer, Ser. in Surf. Sci., Springer-Verlag, Berlin, Heidelberg, 1988, p. 236-241.
0079-6816/93 $24.00 + .00 Copyright © 1993 Pergamon Press Ltd.
FIELD EVAPORATION AND FIELD DESORPTION FROM HTSC SINGLE-CRYSTAL SURFACES VLADIMIR N. SHREDNIK A.F. Ioffe Physico-Technical Institute Russian Academy of Sciences St. Petersburg
Abstract
The problems of field evaporation (FE) and field desorption (FD) from surface of oxide high-temperature superconductors (HTSC) of 123, 2212 and 2223 type are discussed. Wide-angle atom probe, usual time-of-flight atom probe and field ionization magnet massspectrometer have been used in these experiments. The main purpose of the investigation was to search for effects connected with phase transition. Therefore, the experiments were carried out over a wide range of temperatures: from T of solid nitrogen to room T. Composition of pure, contaminated and deep corroded surface and typical species of FE and FD spectra have been studied. The bond energies between some atoms (or clusters) and the surface have been estimated on the basis of experimental results and FE theory. From these estimations, and from discovering of correlated ion pairs, it was concluded that the redistribution of local bonds should be taken into consideration in this case. The increase of FE rate was observed at cryogenic temperatures. Various versions are discussed to explain these effects. Some of them are connected with phase transition. A detailed study of FD of water protonized clusters is performed and the catalytic activity of HTSC crystal surfaces is discussed. Contents 1. 2. 3. 4. 5. 6. 7.
Introduction Experimental Examples of typical atom-probe spectra Estimation of bond energies in HTSC single crystals Temperature dependence of field evaporation rate Field desorption of water from HTSC single-crystal surfaces Conclusion Acknowledgements References Abbreviations
FD FE HTSC
Field desorption Field emission High-temperature superconductors 131
132
V.N. Shrednik
1. I n t r o d u c t i o n The situation that has emerged in field evaporation (FE) and field desorption (FD) from surfaces of high temperature superconductors (HTSC) reflects an unexpected turn, typical of progress in science. Indeed, was it possible 7 years ago to anticipate that specialists of field emission methods would study such complicated and unfit experimental objects such as 4-5-component oxide materials? Let us remember that previously these scientists studied, in the first place metals, sometimes metal alloys, but only a few conducting compounds (usually binary) [1]. However, the great interest aroused by new HTSC has served as an impetus to study them. Field ion microscopy and atom-probe experiments ensure the atomic scale of experiment. The problem of obtaining atomically clean and regular crystal surfaces was solved usually by field evaporation and field desorption. We shall try to summarize, in this paper, the experimental results (obtained mainly at our laboratory) concerning FE and FD from the surfaces of HTSC materials and to indicate some possible future investigations. 2. E x p e r i m e n t a l The very complicated composition of the compounds being considered means that field emission microscopy, by itself, is inadequate for the investigation of FE and FD from their surfaces. In our case, adequate to the aim of the investigation was a combination of massspectroscopy with field ion microscopy. Two kinds of time-of-flight atom probe were used in this study: wide-angle atom probe with the resolution M / A M atom probe with M / A M
,.~ 15 - 20, and probe-hole
,~ 70, as well as a magnetic field ionization mass-spectrometer
with M / A M ~ 200. Samples were prepared by Melmed's method [2]. They looked like small sharp crystals glued by conducting epoxy to the top of a tungsten tip. All the samples had the "c"-axis parallel to the tip axis. Numerous HTSC materials were investigated, but the most successful experiments were carried out with 1:2:3 single crystals on the base of Y, Tb, Eu, S m and Gd as well as Bi 2112 and 2223. These compounds were studied in a wide temperature (T) range: from room T down to T of liquid or solid nitrogen. Atom-probe experiments were accompanied by observing field ion images showed strip atomic structure [3,4]. Details of the electrical and registration circuits were described in the original papers referred to below. 3. E x a m p l e s of typical a t o m - p r o b e s p e c t r a Figure 1 shows the four spectra of FE from EuBa2Cu307-= single crystals at the temperature of liquid nitrogen (T = 82 - 85K, i.e. T < Tc ~ 90K) obtained by a wide-angle
Evaporation and Desorption From HTSC Crystals
133
atom probe [5]. Each spectrum is the sum of hundreds of elementary spectra (an elementary spectrum is a result of one evaporation pulse) taking into account the intensity and time-of-flight error A r of every component. Values A r are not negligible (20-60 ns). The resolution calculated from these spectra is lower than the real one. Therefore, these spectra are sometimes accompanied by line spectra drawn with the assumption that A r = -4-5 ns. Line spectra enable us to remove the error of appearance of an incorrect peak resulting from the summation of two very close peaks. In spite of low M / A M
value, the use of the
wide-angle probe is very effective in the FE study of multicomponent materials, because it allows large ion statistics to be kept without visible sample blunting, i.e., at constant field F during the experiment. The spectrum in Fig. la demonstrates a typical set of ions evaporated from the clean surface of a HTSC (123) sample. It reveals various metal and metal-oxide ions. The next spectrum (Fig. lb) was obtained after sample corrosion at a residual gas pressure of 10-4 Torr over a few months. It corresponds to a contaminated surface and displays only one metal ion Ba ++. Evaporation of many atomic layers (see Fig. lc) did not result in a complete surface cleaning. Peaks connected with the corrosion (H +, C ++, probably O +) remained in the spectrum, but a small Cu ++ (or O +) peak appeared. Ba ++ and E u 2 0 +++ peaks are distinguished clearly. It should be mentioned that the Eu +++ ion is absent. Then an occasional break off of a small part of the sample took place. This break off in high vacuum opened the clean (rather very clean) surface of the crystal. The spectrum (Fig. ld) obtained at the same voltage revealed practically only one peak of ions: Eu +++. These four spectra are formed by particles removed from the surface at a constant (and approximately the same) field F, and only a small part of a monoatomic layer could be removed. Therefore, these four spectra (Fig. 1) demonstrate what kinds of ions left the surface after various treatments, and what atoms made up the surface before the treatments. Intensive field evaporation, after the observation of field-ion images at 12 - 12.5 kV, formed the surface characterized by the spectrum Fig. la. As a result of this treatment, only the most strongly bound atoms remain at the surface. This is the end form of field evaporation of (123) HTSC crystal. Authors of [6] have made a computer simulation of the ion patterns of YBa2Cu3Or-z crystal and have compared these patterns with real images. They concluded that a field-ion image is formed by Y atoms. Spectra Fig. la show that, besides Eu (instead of Y), there are also Cu and O atoms on the surface, possibly not imaged. The crystal surface (corresponding to Fig.
lb) was covered by contaminants after corrosion. These
contaminants seemed to be produced by the action of H20, CO and CH4. The spectrum displays products of dissociation of these molecules (H +, C ++, O+). Only Ba ++, and maybe EuO ++, are observed, but other metallic ions are absent. The surface of EuBa2Cu3OT_~
134
V.N. Shrednik
300
300
200,
200
W o~
$. op4
J.O0 '
100
c,~c.~ I
÷ ÷ 4"411 ¢
o
l)
200
400
600
800
0
1000
200
400
600
800
~.000
time-of-flight (ns)
time-of-flight (ns)
FJg.lb
]L~.la
200
30 EU ÷'~
I
20 [00
IIt
io
°~1
i 0
~+
:o-.I ] % o?l I
0 0
200
400
600
800
time-of-flight (ns) Fig.lc
1000
0
200
400
600
800
tO00
time-of-flight (ns) Fig.ld
Figure 1: Wide-angle atom-probe spectra of EuBa2Cu307_= obtained under the following conditions: vacuum better than 10 -9 Torr; liquid nitrogen cooling of the sample. Resulting voltage U is the sum of base steady-state voltage U= and pulse voltage AU : U = U= ÷ AU. a) surface reliably cleaned by field evaporation: U = U= + / k U = 10 + 2 kV; b) corroded surface after long exposure in residual gases (H20, CH4, N2, CO) 10 -4 Torr; U=10÷2=12kV; c) after removing many (,,, 150) monoatomic layers by field evaporation from the surface corresponding to Fig. lb. The tip is blunted and requires higher U for the same operation: U = 13 ÷ 2 = 15 kV; d) really clean surface after break-off in high vacuum; U = 13 + 2 = 15 kV.
Evaporation and Oesorption From HTSC Crystals
remains contaminated (Fig.
135
lc), even after removing many monoatomic layers and tip
blunting. The spectrum now displays a small amount of Cu ++ ions and a remarkable peak of Ba ++. It is interesting that E u leaves the surface as the stoichiometrical oxide E u 2 0 +++. Some ions from contaminants are still observed. Therefore, the action of active residual gases over a long time at room T leads to corrosion of the crystal to a depth of more than 100 monoatomic layers. Break off in vacuum opens the surface, with the most strongly bound atoms, and displays E u +++ ions in the spectrum. Figure 2 shows the spectrum of a Gd (123) crystal obtained by probe-hole atom probe with a reliable resolution of all the species [7]. The sample, after preliminary treatment, should have the surface cleaned by FE. The spectrum displays a typical set of metal and oxide ions. The variety of the oxide and low charged ions should be mentioned. However, only Gd reveals oxide ions. Nevertheless, C u O ++ and C u O + were observed simultaneously with E u O +++ and E u O ++, but at the room temperature and by the same atom probe.
/
(3)
Cu ÷
E. 0 0
O
4 (D
G d +++
[ 4
0
0
0
(D
--': '-' '2'
Ba ~
I
0
i 0
+÷ o;
CdO +
+
I
I
+÷ aa÷
C,
L
.......... C, 0 0 C, ,:, ,--,
J ............. L .......
0
4
C,
$ ............ • .........
0
~: ,-,
I.- .......
J- .........
I 2:0
I...........
L . . . . . .
i ~. ,--,
I
............. [ ........
2
,-, 0
Mass-to-charge
Figure 2: Probe-hole atom-probe spectrum of GdBa~Cu3Or_~: (To = 95 K) obtained at the liquid nitrogen cooling of the sample in vacuum 10-7 Torr; U = 7 + 3 = 10 kV.
4. Estimation of bond energies in HTSC single crystals The theory of field evaporation [8] developed for metals [9,10] permits one to determine the bond energy A of an atom leaving the surface. In this case, the ionization energy I . (where
V.N. Shrednik
136
n is the charge of the ion), the work function ~b, and the atomic and ion polarizabilities a~ and al should be known and evaporation field F, should be measured. The theory [10] gives the following equation for these quantities: = A + ~ I , - n . ¢ + 1/2(c~ - ai)- F~
(1)
n
where e is the elementary charge. Usually the last term in expression (1) can be discarded. Thus:
.~= V / ~ n 3 e 3 - y~l,~ +nc~
(2)
n
Some difficulties, both of a technical and principal nature, arise in the case of HTSC materials. As regards the first, the necessity of the measurement of ¢ and Fe can be still overcome in only a few cases. For instance, we have measured ~band Fe of the sample corresponding to the spectrum given in Fig. la [11,12]. Moreover, we took into consideration the contribution of the surface dipole field of the oxide materials to determine the best image field in the case of nitrogen being the image gas. In this way, we obtained ¢ = 4.1 eV for the EuBa2Cu307_~ single crystal. Thus, one prinicipal difficulty, i.e., the essential difference between the local field structure at the oxide HTSC surface and metal one, has been partly eliminated. After ¢ and F~ have been determined, it is possible to calculate •, using the well-known tabulated values of I~. Our calculation [12] showed that only for Ba ++ and Eu ++ were the values of (4.7 eV and 3.6 eV, respectively) comparable with the bond energies in the oxides: BaO and E u O (5.8 eV and 5.7 eV [13]). It is quite reasonable to assume that on leaving the surface, the Ba (or Eu) atom cuts the bond with the oxygen. In these oxides, both atoms have the valency of 2. Thus, the comparison of £ for these two charged ions seems to be correct. The calculation for Eu +++, Cu +, Cu ++, 0 + ions resulted in absolutely improbable ), values; i.e., too small or just negative. At the same time, estimated values of ,k for
BaO + and EuO + (1.95 eV and 3.45 eV) seemed to be not too improbable. Some difficulties arising with ~ = - 6 . 5 eV for Eu +++ can be easily removed by the natural assumption of post-ionization of Eu ++. The conclusion that follows from these calculations was that only the electropositive metals Ba and Eu on the surface of a superconductor (the temperature in the experiments was always T < To) could be considered as typical adsorbate on a smooth metal surface. As regards Cu and O atoms forming the base of a cuprate lattice, a simple theoretical model of a monocomponent metal is not satisfactory in this case. The model should be modified by taking into account (1) polar bonds in the lattice and (2) change of the local energetic situation after removing an atom from its site. Too low estimated values of/k mean that the corresponding ions appear at an unreasonably low value of F, that removes the Ba and Eu ions. In other words, it means that Cu and O atoms are removed together with Ba and Eu atoms, because they (Cu and O) can no longer stay in the lattice after the change in the
Evaporation and Desorption From HTSC Crystals
137
local energetic situation has occurred. Besides, it can be energetically favorable to release the atoms as cluster ions, such as E u O +. In the same way, the field desorption of some chemically active adsorbates can be explained along the same lines. Removing of some adsorbed atom can essentially change the local bond energies of the remaining lattice (or adsorbate) atoms. Consequently, one can expect to register correlated (not occasional) pairs or triplets of some ions as a result of one pulse of evaporation.
Indeed, a wide-angle atom probe
very often detects pairs (or more numerous groups) of ions in one elementary spectrum (obtained from one pulse). Special analysis of such pairs in the FE spectrum of Tb (123) showed [4] Tb +++ and Cu ++ (or O +) correlated pairs, while contaminants desorbed from Tb (123) revealed a large number of correlated pairs of C ++ and O + probably originating
from CO or C02 dissociation.
Ba ++ always leaves the E u (123) corroded surface (Fig.
lb,c) together with some light ion originating from the contamination. These correlations (as seen from examples given above), after more detailed and extensive analysis, together with a reasonable ~ estimation, should supply significant data, which will be useful for the creation of an adequate FE and FD theory of multicomponent surfaces and the HTSC surface particularly. Experiments in search of such correlations and measurements of ,k are of special interest, when they are carried out near To. Phase transitions at T~ can cause changes of some interatomic bond energies. The next section gives some evidence supporting this idea. 5. T e m p e r a t u r e d e p e n d e n c e of field e v a p o r a t i o n r a t e The rate of field evaporation, registered by peaks of ions originating from a HTSC lattice, increases with transition to a lower T, in contrast to the classical FE theory. It has been observed many times (not less than 8) in wide-angle atom-probe experiments with Tb and E u (123) [4] at the transition from room T to liquid nitrogen T and with B i (2223) at the
transition from liquid nitrogen T to solid nitrogen T. In all the cases, the experiments were carried out with the same sample and at comparable statistics. The effect was more or less prominent. Sometimes, not only the rate of FE changed, but the set of evaporating ions also changed. Figure 3 shows an example of such behavior in the B i (2223) spectrum [14]. Similar phenomena have been observed for some light ions from adsorbed layers. The increase of the O + peak (reliably separated from Cu ++ now) with the cooling of the S m (123) sample to T near Tc has been proved by means of magnetic field ionization mass-spectrometry [15]. The origination of the O + ion from the lattice has been specially established. Some explanations, both general and particular, can be given for these effects: 1. Change of the field penetration at the phase transition to the superconductive state. Generally, there is some sensitivity of F to the sample conductivity. In this case, even
138
V.N. Shrednik
300 ,,t.a • f,,,~
°lit
200 gS
I
100
~
I
0 0
200
400
600
800
1000
Lime-of-flight (ns) Figure 3: Wide-angle atom-probe integral and line spectra of Bi2Sr2Ca2Cu30y at solid nitrogen cooling of the sample (solid lines) and at liquid nitrogen cooling (dotted line); U = 8.5 + 1.5 = 10 kV. a small increase in F should cause a remarkable effect, because of the exponential F dependence of FE [4]. 2. Change of the interatomic bond energy on lowering T to near or below Tc [4,14].
3. Dependence of the condensation coefficient of gases or vapors on T, which explains the behavior of some FD peaks [4]. 4. Field etching.
Influence of adsorbed gas particles on FE. This phenomenon is well
known for metals [10,16]. 5. Redistribution of vibrational energy between
Cu and O atoms in the lattice [15].
6. Some radio engineering hypotheses are also possible. They are connected with the passing of a nanosecond pulse through an electrical circuit. Some conditions can change in this case: (a) presence of liquid nitrogen in cold finger changes capacity and (b) transition at T > T~ into semiconducting state, for example, changes resistance of the circuit. Discussing these ideas, from the last one, we can say that statement (6-a) is hardly real. It was proved twice: with great variation of liquid nitrogen level and with the variation of T at the same level - liquid nitrogen and liquid nitrogen with chunks of solid nitrogen. The statement (6-b) is doubtful, because of the small change in the resistance. (HTSC sample is very small.)
Evaporation and Desorption From HTSC Crystals
139
Statement (5) is rather particular, but is quite reasonable. Unfortunately, in [15] only one peak was studied. The behavior of the remaining peaks was unknown. Besides field etching was used in this experiment. Generally, the idea of etching (4) is interesting as such, in spite of the fact that it seems to have no correlation with To. However, it is hardly valid in cases of ultrahigh vacuum (often as high as 10 -9 Torr) and is doubtful in the case of cooling from liquid nitrogen T to solid nitrogen T. Idea (2) is very interesting and important. It is connected with the phase transition, but not always with the one at To. Finally, the idea (1), connected with the transition at To, is of great interest. Though there is some criticism, based on the constancy of the carrier concentration at the phase transition. However, there is some concern. If a sample at T > Tc becomes a semiconductor, or bad conductor, will it influence the field penetration? Generally, the conception of an equal local field near the different metal surface atoms at the same positions is based on the idea of an ideal metal, but modern experiments [17] have shown that this is not the case. They have shown the importance of chemical and electrical individuality of the atom. Therefore, it is quite reasonable to expect a great difference in the field evaporation probability at a surface in the normal state from that in a superconducting state. Thus, the possibility of using FE from a HTSC material as an indication of a phase transition at T~ remains quite feasible. Irrespective of physical aspects, it is significant from a technical point of view. As regards the first, it is clear that any reliable FE experiments that pass through Tc are very interesting physically. In this case, the change of any of the following parameters is important: peak intensities, appearance or disappearance of some peaks, correlated pairs (or more complex correlated associations) of ions, mean height of certain peaks in the elementary spectra. 6. Field d e s o r p t i o n of w a t e r f r o m H T S C s i n g l e - c r y s t a l surfaces FD from HTSC materials has been occasionally observed in many experiments by both kinds of atom probe, and involve FD of different species originating from the residual gas adsorbed layer. However, a systematic detailed study of FD of water from Bi 2212 and
Gd and Srn single crystals has been carried out, using a probe-hole atom probe [7]. Two observations can be considered as the main results of this work.
A sharp temperature
threshold of water FD rate at low T (liquid nitrogen cooling). This threshold was rather higher than T~, but may be very close to it.
Secondly, mass-spectra obtained at low T
showed peaks of protonized polymer cluster ions H+.n(H20).
Figure 4 displays such a
spectrum for n from 1 to 6. This phenomenon is similar to that observed previously on metal substrates [10]. However, in contrast to the case of metals, in our experiment the
140
V.N. Shrednik
most intensive peak has n = 2. The peak height relation changed depending on F . This experiment revealed a high catalytic activity of the oxide HTSC crystal surface similar to t h a t of some metals. However, the peculiarities of how these effects differ from the ones for metals is more interesting. Another i m p o r t a n t aspect of this work is connected with the strong chemical action of water on the surfaces of these HTSC oxides. From a technical point of view, it was the first work to show the favorable application of the probe-hole a t o m probe to the s t u d y of adsorption and catalytic phenomena on HTSC surface. 2 ~D
~ 0
I 2 E'
~c~ @ C) (D C, :~: g:=
O (D 0
,I ,2,
II
' ..... ~ Ih,h. , !t t, , ~ i i , l . l , l l l ~
O O O C, O ,",
i E, 2 C,
I C., 4 ,-.:,
I I ,Z, ~. ,-t
i I ,Z, :-=: O
i.l
I ,Z, O
1
......
I ~ ,Z,
i 1 4 E~
Mass-to-charge Figure 4: Probe-hole a t o m - p r o b e spectrum of protonized water clusters:
H+.n(H20) n
Bi:Sr2CalCu~Oz showing
field desorption of
= 1, ..., 6. Values of n are displayed at correspond-
ing peaks. Liquid nitrogen cooling of the sample; U = 11 + 3 = 14 kV.
7. C o n c l u s i o n One can state that, for several years, a lot of experimental d a t a were accumulated in the field of F E and FD from the surfaces of HTSC materials and conventional experimental techniques were applied to study these new materials. At the same time, it became clear that the initial period of principle investigations, where more problems were created than solved in this field, has now passed. One has a definite feeling of the insufficiency of our theoretical understanding, and realizes the necessity to work on the foundations of new theoretical ideas. To do this, more fundamental experiments should be carried out. Anyway, it seems that
Evaporation and Desorption From HTSC Crystals
141
some approaches to them have been indicated by the results of the previous work discussed here. The field evaporation of multicomponent compounds with polar interatomic bonds is important and interesting as such, even irrespective of the phase transition at To. Moreover, this field will be physically significant, if it reveals the dependencies connected with the transition at Tc and can shed some light on the phenomena concerned with the HTSC lattice accompanying this transition. This research has been carried out as part of the HTSC Project No. 91097. Acknowledgements I wish to express my sincere thanks to Drs. O.L. Golubev, Yu.A. Vlasov, and M.V. Loginov, and to Mrs. E.L. Kontorovich, and T.I. Sudakova for their kind assistance in preparing this paper and for their technical assistance. References
1. E.W. Muller and T.T. Tsong, Field Ion Microscopy, Field Ionization and Field Evaporation, Prog. Surf. Sci., 4, 1 (1973). 2. A.J. Melmed, J. Phys. Colloq., 49, 67 (1988). 3. Yu.A. Vlasov, O.L. Golubev, N.A. Samokhvalov, N.N. Syutkin, E.F. Talantsev, N.M. Chebotaev, and V.N. Shrednik, Sov. Tech. Phys. Lett., 15,647 (1989). 4. Yu.A. Vlasov, O.L. Golubev, N.N. Syutkin, E.F. Talantsev, and V.N. Shrednik, Sov. Phys. Tech. Phys., 35, 1208 (1990). 5. Yu.A. Vlasov, O.L. Golubev, E.L. Kontorovich, and V.N. Shrednik, Pisma v Zhur. Tech. Fiz., 18, 1 (1992). 6. G.A. Mesyats, N.N. Syutkin, V.A. Ivchenko, E.F. Talantsev, Zhur. Tech. Fiz., 14, 1504 (1988). 7. M.V. Loginov, O.G. Saveljev, V.N. Shrednik, Zhur. Tech. Fiz., 63, in press 1993. 8. E.W. Muller, Advan. Electron. and Electron Phys., 13, 83 (1960). 9. D.G. Brandon, in "Field Ion Microscopy", Ed. by J.J. Hren and S. Ranganathan, Plenum Press, New York, 1968, section 3. 10. E.W. Muller and T.T. Tsong, Field Ion Microscopy. American Elsevier Publishing Co., New York, 1969.
Principles and Applications.
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V.N. Shrednik
11. Yu.A. Vlasov, O.L. Golubev, N.A. Samokhvalov, N.N. Syutkin, E.F. Talantsev, N.M. Chebotaev, and V.N. Shrednik, Sov. Tech. Phys. Lett., 15,977 (1989). 12. Yu.A. Vlasov. O.L. Golubev, E.L. Kontorovich, and V.N. Shrednik, Pisma v Zhur. Tech. Fiz., 18, l l (1992). 13. In "Energee razriva himicheskih svjazei, potenciali ionizacii i srodstvo k electronu", Moscow, "Nauka", 1974, p. 105. 14. Yu.A. Vlasov, O.L. Golubev, E.L. Kontorovich, and V.N. Shrednik, Pisma v Zhur. Tech. Fiz., 17, 5 (1991). 15. N.M. Blashenkov, G.Ya. Lavrentjev, V.N. Shrednik, Pisma v Zhur. Tech. Fiz. 17, 30 (1991). 16. D.G. Brandon, in "Field Ion Microscopy", Ed. by J.J. Hren and S. Ranganathan, Plenum Press, New York, 1968, section 4. 17. Y. Suchorski, W.A. Schmidt, J.H. Block and H.J. Kreuzer, See portion of Seminar Proceedings published in Vacuum. 18. S. Jaenicke, A. Ciszewski, J. Dosselmann, W. Drachsel, J.H. Block and D. Menzel, Desorption Induced by Electronic Transition - DIET III. V.13. Ed. by R.H. Stulen and M.L. Knotec., Springer, Ser. in Surf. Sci., Springer-Verlag, Berlin, Heidelberg, 1988, p. 236-241.