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A comparison between FIM and LEED studies of surface reconstruction
SURFACE SCIENCE 21 (1970) 401-412 © North-Holland Publishing Co.
A COMPARISON
BETWEEN
FIM AND LEED STUDIES OF SURFACE RECONSTRUCTION
KLAUS D. RENDULIC Department of Physics, The Pennsylvania State University, University Park, Pa., U.S.A. Received 10 February 1970 Surface reconstruction of oxygen-covered tungsten is studied with the help of field ion microscopy. Superstructures are seen on the tungsten (100), (110) and (112) planes. Several areas of the hemispherical sample are found to develop facets of the (112) and (I 10) type. The observed structures are compared with those derived by other investigators using LEED. A discussion of the potential and limitations of field ion microscopy in the area of surface rearrangement is given.
1. Introduction It has been k n o w n for some time that surfaces are not a static assembly o f a t o m s but that there is even at r o o m t e m p e r a t u r e certain m o b i l i t y that allows the a t o m s to change sites. In recent years L E E D has shown t h a t during the a d s o r p t i o n o f gases on metal surfaces often a r e c o n s t r u c t i o n o f these surfaces occurs1,2). A t low gas coverage the r e c o n s t r u c t i o n usually shows up as a s u p e r s t r u c t u r e ; if the exposure is increased, faceting a n d severe c o r r o s i o n can result. The investigations o f these surface reconstructions have a l m o s t exclusively been d o n e by electron diffraction. Field ion m i c r o s c o p y ( F I M ) 3) with its r e s o l u t i o n o f 2 to 3 A and the high imaging c o n t r a s t should be able to directly show these surface changes. Surprisingly e n o u g h no effort has been m a d e so far by field ion microscopists to study superlattices under similar c o n d i t i o n s as observed by L E E D . Only recently Miiller a) has noticed that the F I M is indeed c a p a b l e o f showing superstructures as on the cube planes o f W, Ir and Pt. However, these were only casual o b s e r v a t i o n s in which no a t t e m p t was m a d e to define the c o n d i t i o n s u n d e r which the structures a p p e a r e d . It therefore seems desirable to investigate in detail whether the F I M gives the same results as L E E D a n d if it is possible to e m p l o y field ion m i c r o s c o p y as an i n d e p e n d e n t m e t h o d to study surface superlattices and o t h e r surface reconstructions. In view o f an open e n d e d earlier discussion ~) it will be neces401
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sary to establish the merits and limitations of the FIM in this connection. Possible discrepancies between the two methods will have to be explained and interpreted. For image interpretation in field ion microscopy it is important to know under which conditions surface rearrangement occurs. In experiments during which it is difficult to prevent contamination by residual gases, such as imaging of organic molecules, surface rearrangement may be a disturbing factor. The present study will be concentrated on several cases of superstructures and the more extensive reconstruction in the form of faceting. The system chosen was oxygen-covered tungsten because in this case ample LEED data are available for comparison.
2. Experimental In the F I M the hemispherical end of a metal needle (radius of the apex area is about 500 A to 1500 A) is imaged on a screen'S). Resolution and magnification of the microscope depend on the tip radius. The imaged area usually contains several complete sets (characteristic triangles) of crystal planes which can be inspected at the same time. The experiments on tungsten were performed in a bakeable glass microscope, the base pressure being about 10- 1o Torr. The oxygen used was produced by thermal decomposition of KMnO4. In all cases helium was used as imaging gas, the tip temperature being 78 °K. For cleaning the tungsten sample (etched from 0.01 cm diameter wire) was first outgassed at 1000°C for about 20 min and then repeatedly flashed for a couple of seconds to about 2000:'C. In an experimental run the sample was first prepared by field evaporation, a reference photograph was taken, and then the imaging field switched off. The tip was then heated to a certain temperature and the oxygen was introduced into the microscope. The temperature of the tip was determined with a pyrometer. After termination of the heat treatment the tip was cooled again and imaged. Step by step field evaporation was performed to search for changes going beyond the top metal layer.
3. Superstructures With the described experimental procedure superstructures on oxygencovered tungsten could be created. The (211) plane of tungsten consists of atom rows in the [lIT] direction separated by a distance of 4.47 A. The atom-atom distance in these rows is 2.74 A which is only rarely resolved in the micrograph. The original (211) plane therefore is imaged as a set of
403
FIM AND LEED STUDIES OF SURFACE RECONSTRUCTION
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
¢/,
O
i. q~
(a)
(b)
(c)
Fig. 1. A (2 x 1) structure on a tungsten (112) plane (fig. la) obtained by heating the oxygen covered tungsten to 750°C for 2 min. Fig. ib shows a schematic drawing of the arrangement of the tungsten atoms. The white discs are imaged tungsten atoms. The circles indicate dark atom sites in the micrograph. An undisturbed (112) plane as it is normally seen in the field ion microscope can be seen in fig. lc.
stripes in the [111] direction (fig. lc). A f t e r heating in oxygen (short e x p o s u r e o f 10 -6 T o r r . s e c ) at 750°C for 2 min, each o t h e r a t o m in the densest rows has d i s a p p e a r e d . The m i c r o g r a p h o f figure l a shows the new (2 × I) structure. In fig. l b a schematic d r a w i n g indicates the a r r a n g e m e n t o f the atoms. The white discs are a t o m s imaged in the m i c r o g r a p h whereas the circles represent d a r k a t o m sites. A n u n d i s t u r b e d (I 12) plane can be seen in fig. Ic. In the case where the r e c o n s t r u c t i o n starts from one p o i n t in the plane, part o f the plane can show a superstructure p a r t o f it can still be u n d i s t u r b e d . In fig. 2 such an e x a m p l e can be seen. O n l y the a t o m s left o f the d a r k b o u n d a r y show the (2 x I) structure. The (2 x 1) structure on the tungsten (112) plane has been observed with L E E D before~). The cube plane also showed s o m e reconstruction. A f t e r heating to 1100°C for l min (oxygen e x p o s u r e 1 0 - 6 T o r r - s e c ) a ( 2 × 2 ) structure could be
0 t •
0
~
00 OOeO0 0 ~" @QeO
O000eO0 0 0 0 0 . ~ O 0 0 0 Q O 0 0 0 0 0
(a)
(b)
Fig. 2. A partially reconstructed (112) plane. The (2 × 1) pattern (left) and the undisturbed structure (right) are separated by a boundary. Fig. 2b shows the arrangement of atoms on both sides of the boundary.
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observed (fig. 3). In the micrograph there are several extraordinarily bright image dots that do not fit the (2 x 2) pattern ; these apparently are tungsten atoms that have been removed from their sites but still remained on the surface. Other micrographs obtained under similar conditions sometimes exhibited a mixture o f a (2 x 2) and a (2 x I) pattern. This indicates that the
~ Q 6 4 B 0 0 1 0 0 6 t O Q * ~ q U 0 0
(a)
(b)
Fig. 3. Reconstruction of a tungsten (100) plane into a (2 × 2) structure (fig. 3a). Schematic drawing of the arrangement of atoms in a (2 × 2) pattern is seen in fig. 3b. The oxygen covered tip was heated to 1100'C for I rain.
(2 x 2) pattern is preceded by a (2 × 1) pattern. Both of these reconstruction patterns have been observed with LEED7). Only for the tungsten (110) plane on which r o o m temperature rearrangement was to be expected it was difficult to observe the changes derived from LEEDS,9). In some of the micrographs indications o f ( 2 x 1) and (2 x 2) patterns could be observed but only the edges of the crystal planes were resolved. (In field ion microscopy of tungsten the imaging contrast on the (110) plane is always poor.) To exclude possible artifacts control experiments in vacuum were performed. At a temperature around 750~C the change observed consisted in the appearance of a number of bright spots randomly distributed over the surface. This seems to be the influence o f the background gas causing the displacement o f a few tungsten atoms. A r o u n d 1000°C some migration of tungsten atoms could be seen causing a flattening of the {112} and {110} areas. In no case a reconstruction similar to the one obtained in the presence of oxygen was observed. In all the cases in which reconstruction occurred the observed pattern agreed with the ones reported by L E E D . It is not possible to tell from the micrographs alone if the dark sites observed are empty atom sites or are occupied by oxygen atomsl°). Mass spectroscopy with the atom-probe F I M 11) should be able to remove this ambiguity. The L E E D results have shown that metal surfaces when reconstructed
FIM AND LEED STUDIES OF SURFACE RECONSTRUCTION
'4-05
under the influence of an adsorbate show a very regular structure. The question remains why so many ion micrographs exhibit an apparently totally random structure after a gas-surface interaction. The analysis shows that there is usually more order in these " r a n d o m patterns" than one recognizes at the first view. Two main effects seem to be responsible for that. First there is some long range order in the reconstructed areas but the order is not always 100~o. One can construct models of crystal planes in which a certain percentage of the atoms is displaced at random from their original sites. A plane that is only 50~ ordered has to be inspected quite carefully to be recognized as ordered while a diffraction pattern would clearly show the periodicity. In most cases it seems to be necessary that at least 759/o of the atoms in a plane have long range order to exhibit some regularity in the ion micrograph. The second factor that has to be taken into account is the local image brightness. As a consequence of the gas-adsorption the electron orbitals on the metal surface responsible for the local imaging contrast are changed 12). Atoms that are otherwise equivalent may have different brightness and thus enhance the random character of the pattern. To check some of the " r a n d o m patterns" observed after gas-surface interaction for regularities, the autocorrelation function of the pattern was constructed. Let us consider a transparency of an ion micrograph. The two dimensional transmission function of the pattern is T(x, y) having a maximum at the location of each image dot, We now build the autocorrelation function ~b(~, r/) by integration over the area A of the transparency:
j ' ( T(x, :) T(x +
y+
d, dy
.4
The function will only have an appreciable value where both T(x, y) and T ( x + ~, y+r/) have high values. If we have a large number of atoms each of them having a neighbor atom at a location ~', r/' away from the original atom, the function ¢ will have a maximum at ~', ~/'. With the knowledge of the function ¢(~, r/) we are able to recognize hidden periodicity in the micrographs. There is a simple optical method la) to produce the autocorrelation function ¢ of a picture. A large transparency (coordinates of two particular points are x l y l ; x'D,'l) and a smaller transparency (x2Y2; x'2)"~) of one and the same micrograph are lined up in front of a screen. If the first slide is illuminated from the back, light can reach the screen only after passing through both transparencies. Light falling through identical image points ( x l y l ; x2Y2) converges towards one point on the screen, whereas rays going via x'ly' I ; x2y 2 or x x y I ; x'2y'z produce the described autocorrelation function (for experimental details see ref. 13). In fig. 4 the autocorrelation function of a tungsten surface exposed to nitrogen was constructed. A small area
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IC D.RENDUL1C
Fig. 4. Autocorrelograph (right) of an apparent random field ion micrograph (left).
between the (112) plane and the (110) plane was analyzed. The crystallographic orientations are indicated in the figure. One can see that there is a maximum in the [001] direction as well as in the direction of the [111] zone (which runs diagonally through the picture). The connecting line between the central spot and each maximum of~b represents the distance and direction of the separation between a large number of atom pairs. Only if there is some spatial and directional short range order between a large number of atom pairs the function ~b will show a maximum. To express this distance on an absolute scale a micrograph of the undisturbed surface has to be obtained, from which the local magnification can be measured. The original micrograph and the autocorrelation function can then be printed on the same scale and distances can be compared directly. The analysis has to be confined to a rather small area of the micrograph ; only in a small area a zone line will appear as an approximately straight line. As the analysis has shown there exists a large degree of order in so called " r a n d o m " micrographs, only for reasons discussed above the order is not apparent immediately.
4. Faceting With the increase of gas exposure and prolonged heating of a metal surface the reconstruction in many cases goes beyond the first atom layer and the surface geometry is changed profoundly. In the case of oxygen on tungsten faceting of the surface occurs. During this process new crystal planes develop which have a certain angle with the original plane. It is believed that the faceted gas-covered surface has a lower surface free energy than the original gas-covered surface14).
FIM AND LEED STUDIES OF SURFACE RECONSTRUCTION
407
In our case the original surface consists of a nearly hemispherical tungsten single crystal with a complete set of crystal planes. Fig. 5 shows the pattern obtained after heating the tungsten tip in oxygen (10 -6 Torr) at 800°C for 4 min. There is a pattern of dark lines covering the whole surface. The dark lines represent recessed areas. Since the field ion microscope has only a very
Fig. 5.
Hemispherical tungsten crystal showing faceting after heat treatment in oxygen at 800°C.
small "depth of focus" areas which are only recessed by a few ~lngstr6ms will not be imaged anymore. In some areas the dark bands represent the edge of a step with a height of several atom layers, in other areas the dark band is the pattern of a valley created by two inclined facet planes. In other words each dark line is a circle (or part of the circle) at which a facet plane intersects the crystal sphere. To identify a facet plane we choose a low index plane on the crystal sphere and cut the sphere with planes parallel to that particular low index plane. If the circles produced by the intersection of these planes with the sphere coincide with the observed dark lines, the facets are made up of crystal planes with the same index as the chosen low index plane. It is difficult to show all facets on the tungsten surface in one photo-
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graph. By changing the imaging voltage above and below best image voltage the contrast in different crystal areas changes and the facets in one area will be more pronounced while they disappear in an other part of the image. In fig. 6 all the recessed areas are marked for easier recognition. For the reasons mentioned before the drawing was deduced from two micrographs,
Fig. 6. Same micrograph as in fig. 5. The facets and surface steps are marked by white lines.
the micrograph of fig. 5 and a micrograph of the same surface taken at a higher voltage. In the micrographs all the facets observed by LEED are present15). Besides this there are some more facets which have not been described so far. The cube plane is faceted into {110} planes. For the (001) plane the facets are made up by the (011), (011) and the (101), (101) planes. In the ion micrograph these facets are represented by two sets of dark bands having a 90 ° angle with each other. One can easily see that the one set of bands continues into the (013) and (012) planes which means that these planes are faceted into a roof-like structure consisting of (101) and (]01) planes. The area of the (111) plane exhibits two kinds of facets. The first type consists of three {112} planes: The (112), (121) and the (211) plane. These
FIM A N D LEED S T U D I E S O F S U R F A C E R E C O N S T R U C T I O N
409
facets can be seen as dark bands around the original {112} planes extending over the (111) plane. This last type of facets has been observed before by field ion microscopy16). Another type of facets in the (111) area consists of {I 10} planes: the (110), (011) and (101) plane. This second type is only seen occasionally. The ion image of this type shows a pyramid-like structure on the (111) plane. Finally the (112) plane exhibits facets made up of two {110} planes, the (011) and the (101) plane building a roof structure. The hemispherical tungsten crystal investigated provides us with all possible crystal orientations. This gives an excellent overall picture of the distribution of the facets on the surface. Take for example the [100] zone between the (011) and (001) plane. In the micrograph one can see this area is covered by facets of (101) and (i01) planes. In fact in some micrographs one can choose one particular "valley" built by the two planes and follow it over the (001) plane towards the (011) plane. That means every plane in that area [e.g. (016), (013), (012), (047) and so on] taken separately will facet into (101) and (101) planes. In addition we see on the (001) plane a second set of facets made from (011) and (0il) planes so that the cube plane and planes in the vicinity will exhibit facets constructed by four {110} planes. In fig. 5 one can see that all planes between (111) and (114) (e.g. (334), (223), (335), (113)) exhibit facets of (101) and (011) planes running parallel to the [110] zone. In addition the surrounding of the (112) plane shows facets of the {112} type. We can therefore say that all planes between (1 ! 1) and (114) will exhibit both types of facets, the {I 10} and the {112} type. Besides the described facet planes there are some more steps built by low index planes almost perpendicular to the surface. The height of these steps is only one to three atom layers. It is therefore more appropriate to label these steps as a surface rearrangement to adapt local curvature of the crystal rather than to look at them as facets. The area between the (111) and (01 i) plane [e.g. the (122), (133) and (144) planes] exhibits dark lines along the [110] zone line. This indicates that there are steps almost perpendicular to the surface built by the (0Ti) and (011) planes. As we follow these steps starting from the (011) plane towards the (111) area one can see that the dark lines continue as (112) facets building circles around each {I 12} plane. Another step structure is sometimes observed near the (112), (334) and (223) planes. Again the steps are almost perpendicular to the surface. They are made up by (TTI) planes. There seems to be a relationship between these steps and the superlattice observed on the (112) plane. The (112) plane can be constructed by ( i l l ) planes perpendicular to the surface of the (112) plane. By removal of the edge of each other ( l i l ) plane we get the observed (2 × 1) superlattice. In the same way the described
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steps can be constructed. Removal of the edge of one or more (T/I) planes from the curved area between (112) and (334) will create steps parallel to the [111 ] zone. The results obtained for the faceting of the tungsten (I00), (112) and (111) plane agree perfectly with the results obtained by LEED15). For higher index planes no LEED results are available, it seems that for the observation of faceting the field ion microscope gives excellent results. Especially with respect to the size of a facet one can make precise measurements. For example, the size of the facets on the (112) plane is observed by LEED as five atom spacings wide. This distance can be measured very nicely in the micrographs. 5. Conclusion
In all instances where one could observe clearly resolved field ion micrographs of the tungsten surface after a reaction with oxygen the structures agreed with the ones reported by LEED investigators. This is true as well for superstructures as for faceting. It remains now to sum up the positive and negative aspects of field ion microscopy in the investigation of surface reconstruction. The field ion microscope has high enough resolution and image contrast to resolve almost all but the most densely packed crystal planes [e.g. the (011) of the bcc and the (I 11) plane of the fcc system-]. As far as superstructures are concerned, and we are dealing with multiples of the original a t o m atom distances, the densely packed planes become accessible, too. It will, of course, be difficult to investigate complex structures for which exact coordinates of a certain atom within the unit cell become important. Coordinates of atoms cannot be measured more accurately than to one half of the original a t o m - a t o m distance in a low index plane. The field ion microscope complements the LEED technique if one is interested in non-periodic features like domain boundaries, measurement of sizes and distances on the surface and deviations from the periodic structure. A great advantage is the fact that the sample in a FIM contains a complete set of crystal planes which can be treated and investigated under identical conditions. Above all, the microscope gives a direct image of the rearrangement. With the help of field electron emission 17,18) it is possible to determine very accurately the degree of gas coverage of a certain plane. This method can be made sensitive enough to monitor the adsorption of a single gas atom on a small selected area of the surface19). The temperature dependance of field emission can be used to accurately determine the temperature of the small apex area of the tip2°). With another experimental arrangement, the
FIM AND LEED STUDIES OF SURFACE RECONSTRUCTION
411
atom-probe FIM 11), m a s s spectroscopy can be performed to identify single surface atoms. There are also some limitations to the application of field ion microscopy. First of all, the surface changes cannot be observed the moment they occur. The imaging field has to be switched off; only then the sample can be heated and exposed to the reactive gas. After a renewed cooling, the changes can be observed in the ion image. There is, of course, the possibility that between the end of the treatment and the application of the full imaging voltage, some changes are introduced. In the case of oxygen covered tungsten these particular artifacts seem not to be too important. For other metals (Co, Ni) with a low field evaporation field the possibility of changes introduced during application of the imaging voltage is more real. It is therefore necessary to carefully investigate the possibility of artifacts for each metal. The most serious problem is the preparation of an initially clean sample. For many metals a perfectly clean surface at the tip cap can be produced by field evaporation. However, during the subsequent heat treatment the originally clean area is contaminated by surface migration from the tip shank. Simetimes all adsorbates can be removed by heating of the tip; in other cases the shape of the emitter is changed before the necessary temperature for evaporation is reached. In these cases a chemical clean up or ion bombardment of the tip has to be used to remove the adsorbates. The high magnification of the FIM confines us to a relatively small number of atoms. While low index planes can be enlarged by annealing of the tip, the high index planes mostly consist only of a few atoms. Finally, at the present time it is still very difficult to obtain good FIM images of some ot the lower melting metals. In the application of the FIM to the study of superstructures one encounters considerable experimental difficulties. The possibility of occurrence of artifacts especially for the less refractory metals has to be kept in mind. It is within these limitations that the field ion microscope can be used in some cases as a complementary tool for the study of surface rearrangement in addition to the well established low energy electron diffraction.
Acknowledgement I want to thank Prof. E. W. Miiller for first drawing my attention to this subject; I also appreciate his many helpful comments during the investigation. This work was supported in part by The Atomic Energy Commission under Grant N Y O - A T (30-1)-3851.
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K.D. RENDUI.IC
References 1) H. E. Farnsworth ct al., J. Appl. Phys. 29 1150 (1958). 2) L. H. Germer and A. U. Mac Rae, J. Appl. Phys. 33 2923 (1962). 3) E. W. MUller and T. T. Tsong: Field Ion Microscopy, Principles and Applications (Elsevier, New York, 1969). 4) E. W. Miiller, Abstracts, 15th Field Emission Symposium, Bonn, Germany, September 1968. 5) Discussions by N. Cabrera, D. A. Degras, H. E. Farnsworth, A. J. Melmed, E. W. Mi.iller and H. Mykura, in: Moh, eular Processes on Solid Surfaces, Battelle-Kronberg Conference, Eds. E. Drauglis, R. D. Gretz and R. I. Jaffee (McGraw-Hill, New York, 1969) p. 174. 6) C. C. Chang and L. H. Germer, Surface Sci. 8 115 (1967). 7) J. Anderson and W. E. Danforth, J. Franklin Inst. 279 160 (1965). 8) L. H. Germer, Phys. Today 17 (7) (1964) 19. 9) L. H. Germer and J. W. May, Surface Sci. 4 452 (1968). 10) E. W. Miiller, Quart. Rev. 23 (2) (1969) 177. 11) E. W. Mi.iller, J. A. Panitz and S. B. McLane, Rev. Sci. Instr. 39 (1968) 83. 12) Z. Knot and E. W. MiJller, Surface Sci. 10 (1968) 21. 13) W. Meyer-Eppler and G. Darius, in: London Syrup. hTfi~rmation Theory, 3rd, Ed. C. Cherry (Academic Press, New York, 1956). 14) A. J. W. Moore, in: Metal Surfaces: Structure, L'nergetics and Kinetics (American Society of Metals, Metals Park, Ohio, 1963). 15) J. C. Tracy and J. M. Blakely, Surface Sci. 13 (1969) 313. 16) D. W. Basset, Trans. Faraday Soc. 64 (1968) 489. 17) R. H. Good, Jr. and E. W. Mtiller, Field Emission. in: Handbuch der Physik (Encyclopedia o[Physics) Ed. S. Fliigge, Vol. 21 (Springer, Berlin, 1956). 18) R. Gomer, Field Emission and Field Ionization (Harvard University Press, Cambridge, Mass. 1961). 19) A. A. Holscher, Thesis, University of Leiden (1967). 20) M. Drechsler, Z. Physik 167 (1962) 558.
A COMPARISON
BETWEEN
FIM AND LEED STUDIES OF SURFACE RECONSTRUCTION
KLAUS D. RENDULIC Department of Physics, The Pennsylvania State University, University Park, Pa., U.S.A. Received 10 February 1970 Surface reconstruction of oxygen-covered tungsten is studied with the help of field ion microscopy. Superstructures are seen on the tungsten (100), (110) and (112) planes. Several areas of the hemispherical sample are found to develop facets of the (112) and (I 10) type. The observed structures are compared with those derived by other investigators using LEED. A discussion of the potential and limitations of field ion microscopy in the area of surface rearrangement is given.
1. Introduction It has been k n o w n for some time that surfaces are not a static assembly o f a t o m s but that there is even at r o o m t e m p e r a t u r e certain m o b i l i t y that allows the a t o m s to change sites. In recent years L E E D has shown t h a t during the a d s o r p t i o n o f gases on metal surfaces often a r e c o n s t r u c t i o n o f these surfaces occurs1,2). A t low gas coverage the r e c o n s t r u c t i o n usually shows up as a s u p e r s t r u c t u r e ; if the exposure is increased, faceting a n d severe c o r r o s i o n can result. The investigations o f these surface reconstructions have a l m o s t exclusively been d o n e by electron diffraction. Field ion m i c r o s c o p y ( F I M ) 3) with its r e s o l u t i o n o f 2 to 3 A and the high imaging c o n t r a s t should be able to directly show these surface changes. Surprisingly e n o u g h no effort has been m a d e so far by field ion microscopists to study superlattices under similar c o n d i t i o n s as observed by L E E D . Only recently Miiller a) has noticed that the F I M is indeed c a p a b l e o f showing superstructures as on the cube planes o f W, Ir and Pt. However, these were only casual o b s e r v a t i o n s in which no a t t e m p t was m a d e to define the c o n d i t i o n s u n d e r which the structures a p p e a r e d . It therefore seems desirable to investigate in detail whether the F I M gives the same results as L E E D a n d if it is possible to e m p l o y field ion m i c r o s c o p y as an i n d e p e n d e n t m e t h o d to study surface superlattices and o t h e r surface reconstructions. In view o f an open e n d e d earlier discussion ~) it will be neces401
402
K.D.RENDULIC
sary to establish the merits and limitations of the FIM in this connection. Possible discrepancies between the two methods will have to be explained and interpreted. For image interpretation in field ion microscopy it is important to know under which conditions surface rearrangement occurs. In experiments during which it is difficult to prevent contamination by residual gases, such as imaging of organic molecules, surface rearrangement may be a disturbing factor. The present study will be concentrated on several cases of superstructures and the more extensive reconstruction in the form of faceting. The system chosen was oxygen-covered tungsten because in this case ample LEED data are available for comparison.
2. Experimental In the F I M the hemispherical end of a metal needle (radius of the apex area is about 500 A to 1500 A) is imaged on a screen'S). Resolution and magnification of the microscope depend on the tip radius. The imaged area usually contains several complete sets (characteristic triangles) of crystal planes which can be inspected at the same time. The experiments on tungsten were performed in a bakeable glass microscope, the base pressure being about 10- 1o Torr. The oxygen used was produced by thermal decomposition of KMnO4. In all cases helium was used as imaging gas, the tip temperature being 78 °K. For cleaning the tungsten sample (etched from 0.01 cm diameter wire) was first outgassed at 1000°C for about 20 min and then repeatedly flashed for a couple of seconds to about 2000:'C. In an experimental run the sample was first prepared by field evaporation, a reference photograph was taken, and then the imaging field switched off. The tip was then heated to a certain temperature and the oxygen was introduced into the microscope. The temperature of the tip was determined with a pyrometer. After termination of the heat treatment the tip was cooled again and imaged. Step by step field evaporation was performed to search for changes going beyond the top metal layer.
3. Superstructures With the described experimental procedure superstructures on oxygencovered tungsten could be created. The (211) plane of tungsten consists of atom rows in the [lIT] direction separated by a distance of 4.47 A. The atom-atom distance in these rows is 2.74 A which is only rarely resolved in the micrograph. The original (211) plane therefore is imaged as a set of
403
FIM AND LEED STUDIES OF SURFACE RECONSTRUCTION
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
¢/,
O
i. q~
(a)
(b)
(c)
Fig. 1. A (2 x 1) structure on a tungsten (112) plane (fig. la) obtained by heating the oxygen covered tungsten to 750°C for 2 min. Fig. ib shows a schematic drawing of the arrangement of the tungsten atoms. The white discs are imaged tungsten atoms. The circles indicate dark atom sites in the micrograph. An undisturbed (112) plane as it is normally seen in the field ion microscope can be seen in fig. lc.
stripes in the [111] direction (fig. lc). A f t e r heating in oxygen (short e x p o s u r e o f 10 -6 T o r r . s e c ) at 750°C for 2 min, each o t h e r a t o m in the densest rows has d i s a p p e a r e d . The m i c r o g r a p h o f figure l a shows the new (2 × I) structure. In fig. l b a schematic d r a w i n g indicates the a r r a n g e m e n t o f the atoms. The white discs are a t o m s imaged in the m i c r o g r a p h whereas the circles represent d a r k a t o m sites. A n u n d i s t u r b e d (I 12) plane can be seen in fig. Ic. In the case where the r e c o n s t r u c t i o n starts from one p o i n t in the plane, part o f the plane can show a superstructure p a r t o f it can still be u n d i s t u r b e d . In fig. 2 such an e x a m p l e can be seen. O n l y the a t o m s left o f the d a r k b o u n d a r y show the (2 x I) structure. The (2 x 1) structure on the tungsten (112) plane has been observed with L E E D before~). The cube plane also showed s o m e reconstruction. A f t e r heating to 1100°C for l min (oxygen e x p o s u r e 1 0 - 6 T o r r - s e c ) a ( 2 × 2 ) structure could be
0 t •
0
~
00 OOeO0 0 ~" @QeO
O000eO0 0 0 0 0 . ~ O 0 0 0 Q O 0 0 0 0 0
(a)
(b)
Fig. 2. A partially reconstructed (112) plane. The (2 × 1) pattern (left) and the undisturbed structure (right) are separated by a boundary. Fig. 2b shows the arrangement of atoms on both sides of the boundary.
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observed (fig. 3). In the micrograph there are several extraordinarily bright image dots that do not fit the (2 x 2) pattern ; these apparently are tungsten atoms that have been removed from their sites but still remained on the surface. Other micrographs obtained under similar conditions sometimes exhibited a mixture o f a (2 x 2) and a (2 x I) pattern. This indicates that the
~ Q 6 4 B 0 0 1 0 0 6 t O Q * ~ q U 0 0
(a)
(b)
Fig. 3. Reconstruction of a tungsten (100) plane into a (2 × 2) structure (fig. 3a). Schematic drawing of the arrangement of atoms in a (2 × 2) pattern is seen in fig. 3b. The oxygen covered tip was heated to 1100'C for I rain.
(2 x 2) pattern is preceded by a (2 × 1) pattern. Both of these reconstruction patterns have been observed with LEED7). Only for the tungsten (110) plane on which r o o m temperature rearrangement was to be expected it was difficult to observe the changes derived from LEEDS,9). In some of the micrographs indications o f ( 2 x 1) and (2 x 2) patterns could be observed but only the edges of the crystal planes were resolved. (In field ion microscopy of tungsten the imaging contrast on the (110) plane is always poor.) To exclude possible artifacts control experiments in vacuum were performed. At a temperature around 750~C the change observed consisted in the appearance of a number of bright spots randomly distributed over the surface. This seems to be the influence o f the background gas causing the displacement o f a few tungsten atoms. A r o u n d 1000°C some migration of tungsten atoms could be seen causing a flattening of the {112} and {110} areas. In no case a reconstruction similar to the one obtained in the presence of oxygen was observed. In all the cases in which reconstruction occurred the observed pattern agreed with the ones reported by L E E D . It is not possible to tell from the micrographs alone if the dark sites observed are empty atom sites or are occupied by oxygen atomsl°). Mass spectroscopy with the atom-probe F I M 11) should be able to remove this ambiguity. The L E E D results have shown that metal surfaces when reconstructed
FIM AND LEED STUDIES OF SURFACE RECONSTRUCTION
'4-05
under the influence of an adsorbate show a very regular structure. The question remains why so many ion micrographs exhibit an apparently totally random structure after a gas-surface interaction. The analysis shows that there is usually more order in these " r a n d o m patterns" than one recognizes at the first view. Two main effects seem to be responsible for that. First there is some long range order in the reconstructed areas but the order is not always 100~o. One can construct models of crystal planes in which a certain percentage of the atoms is displaced at random from their original sites. A plane that is only 50~ ordered has to be inspected quite carefully to be recognized as ordered while a diffraction pattern would clearly show the periodicity. In most cases it seems to be necessary that at least 759/o of the atoms in a plane have long range order to exhibit some regularity in the ion micrograph. The second factor that has to be taken into account is the local image brightness. As a consequence of the gas-adsorption the electron orbitals on the metal surface responsible for the local imaging contrast are changed 12). Atoms that are otherwise equivalent may have different brightness and thus enhance the random character of the pattern. To check some of the " r a n d o m patterns" observed after gas-surface interaction for regularities, the autocorrelation function of the pattern was constructed. Let us consider a transparency of an ion micrograph. The two dimensional transmission function of the pattern is T(x, y) having a maximum at the location of each image dot, We now build the autocorrelation function ~b(~, r/) by integration over the area A of the transparency:
j ' ( T(x, :) T(x +
y+
d, dy
.4
The function will only have an appreciable value where both T(x, y) and T ( x + ~, y+r/) have high values. If we have a large number of atoms each of them having a neighbor atom at a location ~', r/' away from the original atom, the function ¢ will have a maximum at ~', ~/'. With the knowledge of the function ¢(~, r/) we are able to recognize hidden periodicity in the micrographs. There is a simple optical method la) to produce the autocorrelation function ¢ of a picture. A large transparency (coordinates of two particular points are x l y l ; x'D,'l) and a smaller transparency (x2Y2; x'2)"~) of one and the same micrograph are lined up in front of a screen. If the first slide is illuminated from the back, light can reach the screen only after passing through both transparencies. Light falling through identical image points ( x l y l ; x2Y2) converges towards one point on the screen, whereas rays going via x'ly' I ; x2y 2 or x x y I ; x'2y'z produce the described autocorrelation function (for experimental details see ref. 13). In fig. 4 the autocorrelation function of a tungsten surface exposed to nitrogen was constructed. A small area
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IC D.RENDUL1C
Fig. 4. Autocorrelograph (right) of an apparent random field ion micrograph (left).
between the (112) plane and the (110) plane was analyzed. The crystallographic orientations are indicated in the figure. One can see that there is a maximum in the [001] direction as well as in the direction of the [111] zone (which runs diagonally through the picture). The connecting line between the central spot and each maximum of~b represents the distance and direction of the separation between a large number of atom pairs. Only if there is some spatial and directional short range order between a large number of atom pairs the function ~b will show a maximum. To express this distance on an absolute scale a micrograph of the undisturbed surface has to be obtained, from which the local magnification can be measured. The original micrograph and the autocorrelation function can then be printed on the same scale and distances can be compared directly. The analysis has to be confined to a rather small area of the micrograph ; only in a small area a zone line will appear as an approximately straight line. As the analysis has shown there exists a large degree of order in so called " r a n d o m " micrographs, only for reasons discussed above the order is not apparent immediately.
4. Faceting With the increase of gas exposure and prolonged heating of a metal surface the reconstruction in many cases goes beyond the first atom layer and the surface geometry is changed profoundly. In the case of oxygen on tungsten faceting of the surface occurs. During this process new crystal planes develop which have a certain angle with the original plane. It is believed that the faceted gas-covered surface has a lower surface free energy than the original gas-covered surface14).
FIM AND LEED STUDIES OF SURFACE RECONSTRUCTION
407
In our case the original surface consists of a nearly hemispherical tungsten single crystal with a complete set of crystal planes. Fig. 5 shows the pattern obtained after heating the tungsten tip in oxygen (10 -6 Torr) at 800°C for 4 min. There is a pattern of dark lines covering the whole surface. The dark lines represent recessed areas. Since the field ion microscope has only a very
Fig. 5.
Hemispherical tungsten crystal showing faceting after heat treatment in oxygen at 800°C.
small "depth of focus" areas which are only recessed by a few ~lngstr6ms will not be imaged anymore. In some areas the dark bands represent the edge of a step with a height of several atom layers, in other areas the dark band is the pattern of a valley created by two inclined facet planes. In other words each dark line is a circle (or part of the circle) at which a facet plane intersects the crystal sphere. To identify a facet plane we choose a low index plane on the crystal sphere and cut the sphere with planes parallel to that particular low index plane. If the circles produced by the intersection of these planes with the sphere coincide with the observed dark lines, the facets are made up of crystal planes with the same index as the chosen low index plane. It is difficult to show all facets on the tungsten surface in one photo-
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graph. By changing the imaging voltage above and below best image voltage the contrast in different crystal areas changes and the facets in one area will be more pronounced while they disappear in an other part of the image. In fig. 6 all the recessed areas are marked for easier recognition. For the reasons mentioned before the drawing was deduced from two micrographs,
Fig. 6. Same micrograph as in fig. 5. The facets and surface steps are marked by white lines.
the micrograph of fig. 5 and a micrograph of the same surface taken at a higher voltage. In the micrographs all the facets observed by LEED are present15). Besides this there are some more facets which have not been described so far. The cube plane is faceted into {110} planes. For the (001) plane the facets are made up by the (011), (011) and the (101), (101) planes. In the ion micrograph these facets are represented by two sets of dark bands having a 90 ° angle with each other. One can easily see that the one set of bands continues into the (013) and (012) planes which means that these planes are faceted into a roof-like structure consisting of (101) and (]01) planes. The area of the (111) plane exhibits two kinds of facets. The first type consists of three {112} planes: The (112), (121) and the (211) plane. These
FIM A N D LEED S T U D I E S O F S U R F A C E R E C O N S T R U C T I O N
409
facets can be seen as dark bands around the original {112} planes extending over the (111) plane. This last type of facets has been observed before by field ion microscopy16). Another type of facets in the (111) area consists of {I 10} planes: the (110), (011) and (101) plane. This second type is only seen occasionally. The ion image of this type shows a pyramid-like structure on the (111) plane. Finally the (112) plane exhibits facets made up of two {110} planes, the (011) and the (101) plane building a roof structure. The hemispherical tungsten crystal investigated provides us with all possible crystal orientations. This gives an excellent overall picture of the distribution of the facets on the surface. Take for example the [100] zone between the (011) and (001) plane. In the micrograph one can see this area is covered by facets of (101) and (i01) planes. In fact in some micrographs one can choose one particular "valley" built by the two planes and follow it over the (001) plane towards the (011) plane. That means every plane in that area [e.g. (016), (013), (012), (047) and so on] taken separately will facet into (101) and (101) planes. In addition we see on the (001) plane a second set of facets made from (011) and (0il) planes so that the cube plane and planes in the vicinity will exhibit facets constructed by four {110} planes. In fig. 5 one can see that all planes between (111) and (114) (e.g. (334), (223), (335), (113)) exhibit facets of (101) and (011) planes running parallel to the [110] zone. In addition the surrounding of the (112) plane shows facets of the {112} type. We can therefore say that all planes between (1 ! 1) and (114) will exhibit both types of facets, the {I 10} and the {112} type. Besides the described facet planes there are some more steps built by low index planes almost perpendicular to the surface. The height of these steps is only one to three atom layers. It is therefore more appropriate to label these steps as a surface rearrangement to adapt local curvature of the crystal rather than to look at them as facets. The area between the (111) and (01 i) plane [e.g. the (122), (133) and (144) planes] exhibits dark lines along the [110] zone line. This indicates that there are steps almost perpendicular to the surface built by the (0Ti) and (011) planes. As we follow these steps starting from the (011) plane towards the (111) area one can see that the dark lines continue as (112) facets building circles around each {I 12} plane. Another step structure is sometimes observed near the (112), (334) and (223) planes. Again the steps are almost perpendicular to the surface. They are made up by (TTI) planes. There seems to be a relationship between these steps and the superlattice observed on the (112) plane. The (112) plane can be constructed by ( i l l ) planes perpendicular to the surface of the (112) plane. By removal of the edge of each other ( l i l ) plane we get the observed (2 × 1) superlattice. In the same way the described
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steps can be constructed. Removal of the edge of one or more (T/I) planes from the curved area between (112) and (334) will create steps parallel to the [111 ] zone. The results obtained for the faceting of the tungsten (I00), (112) and (111) plane agree perfectly with the results obtained by LEED15). For higher index planes no LEED results are available, it seems that for the observation of faceting the field ion microscope gives excellent results. Especially with respect to the size of a facet one can make precise measurements. For example, the size of the facets on the (112) plane is observed by LEED as five atom spacings wide. This distance can be measured very nicely in the micrographs. 5. Conclusion
In all instances where one could observe clearly resolved field ion micrographs of the tungsten surface after a reaction with oxygen the structures agreed with the ones reported by LEED investigators. This is true as well for superstructures as for faceting. It remains now to sum up the positive and negative aspects of field ion microscopy in the investigation of surface reconstruction. The field ion microscope has high enough resolution and image contrast to resolve almost all but the most densely packed crystal planes [e.g. the (011) of the bcc and the (I 11) plane of the fcc system-]. As far as superstructures are concerned, and we are dealing with multiples of the original a t o m atom distances, the densely packed planes become accessible, too. It will, of course, be difficult to investigate complex structures for which exact coordinates of a certain atom within the unit cell become important. Coordinates of atoms cannot be measured more accurately than to one half of the original a t o m - a t o m distance in a low index plane. The field ion microscope complements the LEED technique if one is interested in non-periodic features like domain boundaries, measurement of sizes and distances on the surface and deviations from the periodic structure. A great advantage is the fact that the sample in a FIM contains a complete set of crystal planes which can be treated and investigated under identical conditions. Above all, the microscope gives a direct image of the rearrangement. With the help of field electron emission 17,18) it is possible to determine very accurately the degree of gas coverage of a certain plane. This method can be made sensitive enough to monitor the adsorption of a single gas atom on a small selected area of the surface19). The temperature dependance of field emission can be used to accurately determine the temperature of the small apex area of the tip2°). With another experimental arrangement, the
FIM AND LEED STUDIES OF SURFACE RECONSTRUCTION
411
atom-probe FIM 11), m a s s spectroscopy can be performed to identify single surface atoms. There are also some limitations to the application of field ion microscopy. First of all, the surface changes cannot be observed the moment they occur. The imaging field has to be switched off; only then the sample can be heated and exposed to the reactive gas. After a renewed cooling, the changes can be observed in the ion image. There is, of course, the possibility that between the end of the treatment and the application of the full imaging voltage, some changes are introduced. In the case of oxygen covered tungsten these particular artifacts seem not to be too important. For other metals (Co, Ni) with a low field evaporation field the possibility of changes introduced during application of the imaging voltage is more real. It is therefore necessary to carefully investigate the possibility of artifacts for each metal. The most serious problem is the preparation of an initially clean sample. For many metals a perfectly clean surface at the tip cap can be produced by field evaporation. However, during the subsequent heat treatment the originally clean area is contaminated by surface migration from the tip shank. Simetimes all adsorbates can be removed by heating of the tip; in other cases the shape of the emitter is changed before the necessary temperature for evaporation is reached. In these cases a chemical clean up or ion bombardment of the tip has to be used to remove the adsorbates. The high magnification of the FIM confines us to a relatively small number of atoms. While low index planes can be enlarged by annealing of the tip, the high index planes mostly consist only of a few atoms. Finally, at the present time it is still very difficult to obtain good FIM images of some ot the lower melting metals. In the application of the FIM to the study of superstructures one encounters considerable experimental difficulties. The possibility of occurrence of artifacts especially for the less refractory metals has to be kept in mind. It is within these limitations that the field ion microscope can be used in some cases as a complementary tool for the study of surface rearrangement in addition to the well established low energy electron diffraction.
Acknowledgement I want to thank Prof. E. W. Miiller for first drawing my attention to this subject; I also appreciate his many helpful comments during the investigation. This work was supported in part by The Atomic Energy Commission under Grant N Y O - A T (30-1)-3851.
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References 1) H. E. Farnsworth ct al., J. Appl. Phys. 29 1150 (1958). 2) L. H. Germer and A. U. Mac Rae, J. Appl. Phys. 33 2923 (1962). 3) E. W. MUller and T. T. Tsong: Field Ion Microscopy, Principles and Applications (Elsevier, New York, 1969). 4) E. W. Miiller, Abstracts, 15th Field Emission Symposium, Bonn, Germany, September 1968. 5) Discussions by N. Cabrera, D. A. Degras, H. E. Farnsworth, A. J. Melmed, E. W. Mi.iller and H. Mykura, in: Moh, eular Processes on Solid Surfaces, Battelle-Kronberg Conference, Eds. E. Drauglis, R. D. Gretz and R. I. Jaffee (McGraw-Hill, New York, 1969) p. 174. 6) C. C. Chang and L. H. Germer, Surface Sci. 8 115 (1967). 7) J. Anderson and W. E. Danforth, J. Franklin Inst. 279 160 (1965). 8) L. H. Germer, Phys. Today 17 (7) (1964) 19. 9) L. H. Germer and J. W. May, Surface Sci. 4 452 (1968). 10) E. W. Miiller, Quart. Rev. 23 (2) (1969) 177. 11) E. W. Mi.iller, J. A. Panitz and S. B. McLane, Rev. Sci. Instr. 39 (1968) 83. 12) Z. Knot and E. W. MiJller, Surface Sci. 10 (1968) 21. 13) W. Meyer-Eppler and G. Darius, in: London Syrup. hTfi~rmation Theory, 3rd, Ed. C. Cherry (Academic Press, New York, 1956). 14) A. J. W. Moore, in: Metal Surfaces: Structure, L'nergetics and Kinetics (American Society of Metals, Metals Park, Ohio, 1963). 15) J. C. Tracy and J. M. Blakely, Surface Sci. 13 (1969) 313. 16) D. W. Basset, Trans. Faraday Soc. 64 (1968) 489. 17) R. H. Good, Jr. and E. W. Mtiller, Field Emission. in: Handbuch der Physik (Encyclopedia o[Physics) Ed. S. Fliigge, Vol. 21 (Springer, Berlin, 1956). 18) R. Gomer, Field Emission and Field Ionization (Harvard University Press, Cambridge, Mass. 1961). 19) A. A. Holscher, Thesis, University of Leiden (1967). 20) M. Drechsler, Z. Physik 167 (1962) 558.