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Adsorption of tungsten on stepped tungsten surfaces studied by work function measurement
Surface Science 0 North-Holland
53 (1975) 351-358 Publishing Company
ADSORPTION OF TUNGSTEN ON STEPPED TUNGSTEN SURFACES STUDIED BY WORK FUNCTION MEASUREMENT
K. BESOCKE and H. WAGNER Institu t fiir Grenzfliichenforschung und Vakrrumphysik der Kernforschungsanlage Jiilich Gmbtf, 517 Jiilich, Germany
Work function measurements have been performed during the deposition of W on the (1lO)W plane and several stepped W surfaces with (110) terraces and different terrace width. For each sample the work function decreases with growing coverage. The total work function drop diminishes strongly with decreasing terrace width. The results are interpreted in terms of a reduced nucleation process on stepped surfaces as compared to the flat (110) plane. The step edges act as sinks for the deposited adatoms and cause in their proximity a “dead” zone for nuclei formation. Details of the work function change with coverage are discussed in terms of an edge roughening effect on stepped
surfaces.
1. Introduction Work function changes of single crystal surfaces due to some kind of surface disorder have been observed on several occasions. In general, the work function decreases upon disordering especially for densely packed low index planes. Producing a state of disorder by argon ion bombardment, Farnsworth and Madden [l] found a work function reduction of a (100) nickel surface. The evaporation of atoms represents another way of producing disordered surface structures. In preparing Al films by autoepitaxy work function reductions were recorded for the (111) and (100) surface of Al [2]. Changes in the emission current of W field emitter tips due to surface disorder have been observed by Miiller [3] and Plummer and Rhodin [4]. In a previous paper [S] we reported work function changes of the (110) tungsten surface obtained by a defined deposition of W atoms. From this investigation we concluded that a single adatom on the (11O)W plane gives rise to an electric dipole moment of about 1 D. From the variation of the work function with increasing adatom coverage and the dependence on the substrate temperature it could further be inferred that the deposited atoms combine to islands with a number per surface area given by the substrate temperature and an average size given by the deposited amount. The latter conclusion results from the observation that the work function change at a given substrate temperature follows a square root dependence on the adatom coverage 0. The square root dependence was rationalized in terms of an electric dipole moment associated with an atom in a ledge position and the fact 351
that the total circumference length of the growing islands is proportional to JO. The dipole moment of an atom in a ledge position amounts to about 0.3 D. This value is understood as an average over various possible edge structures. The previous results [5] just summarized show that work function measurements give information on the nucleation and the starting growth process. It is interesting to observe how these processes are influenced by the presence of steps on surfaces. Surfaces exhibiting an orientation close to the orientation of a low index crystal plane consist of terraces of the low index plane linked by steps of often monoatomic height and are therefore very suitable for this kind of investigation. These “stepped” surfaces can be prepared with a terrace width given by the inclination angle of the surface normal to the low index plane and a defined step orientation likewise given by the inclination direction. LEED studies have verified the existence of regular step arrays on metal [6] and semiconductor [7] surfaces as well. The following investigation reports work function measurements performed during the deposition of tungsten on stepped tungsten surfaces with (110) terraces and of several terrace widths.
2. Experimental The sample preparation and the LEED characterization of the respective surfaces have been described in detail elsewhere [6]. Tungsten surfaces with orientations close to the (750), (650) and (40 37 1) planes have been prepared all showing terraces of (110) orientations but different step densities. The respective terrace widths as determined by LEED are 12.6 + 0.1, 23.3 + 0.5 and 5 1.6 f 1.5 .% The investigation was carried out in a bakeable all metal UHV system pumped by ion getter pumps and capable of pressures in the 10~-11 Torr region. The system was equipped with conventional 4 grid LEED-Auger optics. The cleaning procedure as described in ref. [6] led to surfaces from which sharp LEED patterns with a low background intensity were obtained and showing no traces of any contamination as checked by AES. Tungsten was deposited on the sample from a high-purity (99.999%) filament source using a flash evaporation technique. The W filament was degassed for several hours at 2000°C. The Auger spectra taken after the successive evaporation steps did not show any contamination within the detection limits of AES. The deposition rate was calibrated with a quartz microbalance of high sensitivity. A deposition of0.02 monolayer of tungsten caused a frequency shift of 1.8 Hz at a resolution of 0.1 Hz. After each evaporation step the work function change was measured by the change of the contact potential difference (CPD) between the sample and the 2nd LEED grid. For this, the 2nd derivative of the secondary electron current was taken as function of ihe voltage between the sample and the LEED grid. The derivative shows a pronounced peak at a voltage close to the CPD. The accuracy of the CPD measurement was about +o.02 eV.
K. Bcsocke, H Wagner /Adsorption
of W on stepped W surfaces
353
3. Results
Tungsten was evaporated on stepped W surfaces characterized by their terrace width of 52, 23 and 12.6 8, respectively, at room temperature. Fig. 1 shows the work function change obtained for the three stepped surfaces as function of the amount of deposited W. The amount is given in equivalent monolayers of the (11O)W plane, i.e. 0 = 1 corresponds to 1.42 X 1015 atoms/cm2. The corresponding work function change obtained for the flat (I lO)W surface taken from previous results [5] is also included in fig. 1 for comparison. All surfaces show a decreasing work function with growing coverage. The total work function drop decreases with decreasing terrace width. With decreasing terrace width the work function levels off at ever smaller coverages. For each surface the work function could be brought back to its starting value by just raising the substrate temperature to about 700°C. The finding that the W deposition causes a work function change which is getting smaller the narrower the terraces are is complemented by the following LEED observations. The diffraction spots become somewhat blurred and the background intensity increases upon increasing coverage for the W(110) plane. This effect is greatly reduced for the stepped surfaces. Virtually no changes of the LEED intensities are seen for the 12.6 a terrace surface.
4. Discussion The work function change caused by the W deposition on the flat (11O)W surface could be understood [5] in terms of dipole moments associated with single adatoms and, for increasing coverage, of dipole moments associated with atoms in ledge positions of growing islands. One reason for inferring an island formation
Fig. 1. Work function change A@ versus coverage 0 of deposited W for the W(110) stepped W surfaces indicated by their respective terrace width A.
plane and
resulted from the dependence of the work function change on the square root of the coverage 8. Another reason followed from the observation that the work function change with increasing coverage was strongly dependent on the substrate temperature. This result could be understood by considering that at higher substrate temperatures less islands will be formed after the first few evaporation steps because of the larger mobilities of the single adatoms and the correspcndingly smaller nuclei density. The observation that the work function reduction of stepped surfaces becomes smaller with increasing step density shows that, for a given coverage, fewer and probably also smaller islands exist on the stepped surfaces than on the flat (110) surface. An obvious reason for this behaviour results certainly from the presence of the step edges acting as sinks for the deposited adatoms so that less adsorbed atoms are available on the terraces for participating in a nuclei formation and a following island growth. That is to say that the already existing steps hinder an additional growth of further edges and that therefore the work function change is more and more reduced for an increasing step density. The step edges already present at the stepped surfaces cause in their vicinity a “dead” zone for nuclei formation as in the case of the flat (1 IO) plane where a further nucleation process close to the growing islands does no longer occur. That this explanation applies may be deduced by considering the diffusion of the deposited adatoms to the step edges. The solution of the diffusion problem gives also an estimate on the extension of the “dead” zone in relation to the respective terrace widths. The initial and the boundary conditions are: On a terrace of width A bounded by an ascending and a decending step a certain adatom concentration C, is uniformly deposited at the time t = 0. Subsequently the adatom concentration C(x, t) follows Fick’s second law ac(x, t) _ Tit-
a*C(x, t)
-D --~__ ad
(1)
At the ascending step the adatom concentration C(0, t) is zero for t > 0. The detending step is assumed to act as a reflecting boundary [8]. The adatom diffusion coefficient D has been measured [9,10] for the W(l IO) plane and takes a value [lo] of 3.3 X lo--l7 cm* sec.-’ at 40°C. No provision has been made in ey. (1) to account for a nucleation process and an island growth on the terrace plane so that only the random walk process to the step edge causes the adatom concentration to deplete on the terrace plane. Fig. 7_ illustrates the solution of the diffusion problem for the three differently stepped surfaces. It may be seen that the adatom concentration is greatly reduced especially in a region near the ascending step edge and that the adatom fraction remaining on the terrace after the time t I shows a very strong dependence on the terrace width A. Only 30% diffuse to the step edge on the 52 a terrace whereas 76% and even 97% reach the step edge for the 23 a and the 12.6 a terrace, respectively. Because the average distance between the deposited adatoms due to a single deposition flash is larger than 11 ,& we may expect that on
K. Besockc, il. Wagner /Adsorption
Normalized
of W OHstepped W surfaces
355
dlstonce
Fig. 2. Calculated adatom concentration as function of the normalized distance x/A from the step edge. Initial and boundary conditions are given in the text. D = 3.3 X lo-l7 cm2s-I, ‘1 = 10 min.
the 12.6 A terrace surface virtually no additional island growth will take place and consequently no substantial reduction of the work function should happen which is indeed the case. Compared with the flat (1lO)W plane the 52 a terrace surface should show a reduced island growth leading to a reduced work function change which should even much more apply to the 23 A terrace surface. The fact that more of the deposited atoms are being attached just to the available step edges the smaller the terraces are might also be regarded as a reduced disordering effect on the respective surfaces. This is in accord with the LEED observation that the sharpness of the diffraction spots is much less reduced for the stepped surfaces than for the flat (110) plane. In this sense one might talk of a self healing action of stepped surfaces [ 111. The model considerations just outlined do in fact describe the main features of the observed results. Before discussing some details of the experimental findings we want to comment on the absolute value of the work function of stepped surfaces. As may be expected from ref. [S] and the results given above, stepped surfaces should exhibit a lower work function than the corr-esponding low index plane. This expectation has been confirmed by work function measurements on vicinal tungsten surfaces [ 12,131. Based on our model assumption concerning dipole moments associated with atoms in ledge positions one would expect that the work function of stepped surfaces decreases with increasing step density, i.e. with the inclination angle to the low index plane. Preliminary measurements indicate that the work function of the 12.6 a terrace surface is about 0.5 eV lower than the work function of the (110) surface. This however means that the “kind of roughness” achieved for the (110) plane by W deposition at room temperature leads to about the same work function on an absolute scale as the stepped surface structure of the 12.6 A terrace surface. The latter can hardly be altered anymore by a further W deposition at least at a temperature around and above room temperature.
Going back to some details of the experimental results shown in fig. 1 we may examine whether the characteristic feature pertaining to an island formation may also be found for the stepped surfaces i.e. the linear dependence of the work function change on the square root of the coverage 8. Fig. 3 shows the work function change A@ obtained for the 52 A terrace surface as function ofdo. A linear portion is indeed observed and that within the same coverage region as reported earlier [5] for the flat (1 10) plane. As a remarkable distinction, however, may be pointed out that the extrapolation of the linear portion does not go through zero as was observed for the (110) plane. We may infer from this finding that not the entire work function reduction is due to newly formed islands but that another contribution of about 0.1 eV as seen by the ordinate segment also effects the work function change. The following argumentation which certainly ought to be reconfirmed by further investigations may explain this experimental result and leads simultaneously to a somewhat different understanding of the work function change observed for- the 23 A terrace plane. One may conceive that the deposited adatoms which are leaving the terraces are being attached to the step edges in some kind of statistical disorder. That is to say that the step edge does not maintain its initial structure but rather becomes irregular or “roughened”. This effect will certainly increase the dipole moment per unit length of the step edge and thereby decrease the work function of the surface. We attempt therefore to rationalize the ordinate segment of fig. 3 in terms of an “edge roughening” effect. This explanation leads us now to examine the details of the work function change observed for the 23 A terrace surface. Three features may be stressed in this context: (1) The work function change levels off already at a relatively low coverage of about 0 = 0.15. (2) No linear portion of the plot A$ versus 40 can be discerned within the coverage range as observed for the (110) and 52 A terrace surface. (3) The total work function change amounts to about twice the value of the work function reduction of the 52 A terrace surface which was attributed to an edge roughening.
Fig. 3. Work function
change
A@ versusJO for the 52 A terrace
surface
K. Besocke, H. Wagner / Adsorpiion
of W on stepped W surfaces
351
These features may be understood by considering mainly a roughening of the step edges. (1) Because of the considerable diffusion of the deposited adatoms to the step edges one would expect that rather more atoms have to be deposited on the 23 a terraces to reach the necessary adatom concentration [ 141 for nuclei formation and subsequent island growth than on the flat (110) plane or the 52 .& terrace surface. It seems, however, plausible that a coverage of no more than 19= 0.15 is sufficient to produce a maximum of possible edge roughness. (2) The missing of a linear portion of the A@ versus d(I plot points at the absence of an island growth on the terrace. (3) If we attribute a work function change of about 0.1 eV to the roughening of the step edges of the 52 a terrace surface we might as well explain the main part of the work function change of 0.2 eV of the 23 ,J%terrace plane by the same effect. This is because the latter surface exhibits about twice as many steps per surface area than the former one. If the interpretation of the work function change of the 52 A and especially the 23 a terrace surface is correct one should also expect a substantial work function decrease of about 0.3~-0.4 eV of the 12.6 a terrace surface by the same argumentation. This has not been observed. However, one might reasonably argue that for very small terrace widths additional effects might become important which either reduce a further build up of dipole moments at the step edges or even counteract a roughening. A depolarizing action of the neighbouring step edges may prevent a further build up of dipole moments. Bkalely and Schwoebel [ 151 have shown that a repulsive interaction force exists between steps. This force varies inversely at least with the square of the step separation. Because of the repulsive nature of this interaction the step edges might tend to stay as smooth as possible. Effects of this kind which are operative at high step densities might explain the absence of a substantial work function reduction of the 12.6.8 terrace surface.
Acknowledgement Stimulating discussions with Prof. G. Comsa, Prof. Dr. E.A. Niekisch and Prof. H. Ibach are greatefully acknowledged. We thank Mr. S. Berger for technical assistance during the measurements.
References [ I] [ 21 [3] [4] [5]
H.E. Farnsworth and H.H. Madden. J. Appl. Phys. 32 (I 96 1) 1933 R.M. Eastman and C.H.B. Mee, J. Phys. F3 (1973) 1738. E.W. Miiller, 2. Physik 126 (1949) 642. E.W. Plummer and T.N. Rhodin, J. Chcm. Phys. 49 (1968) 3479. K. Bcsocke and H. Wagner, Phys. Rev. B8 (1973) 4597.
358 [6] [7] [ 81 [9] [lo] [ 111
K. Besocke, II. Wagner / Adsorption of W 011stepped W surfaces
K. Bcsocke and II. Wagner, to be published and further references therein. M. Henzler, Surface Sci. 19 (1970) 159. D.W. Bassett and D.R. Tice, Surface Sci. 40 (1973) 499. G.E. Ehrlich and F.G. Hudda, J. Chcm. Phys. 44 (1966) 1039. D.W. Bassett and M.J. Parsley, J. Phys. D3 (1970) 707. J. Perdcreau and G.E. Rhcad, in: Colloque International sur la Structure et lcs Prop&t& des Surfaces dcs Solides (CNRS, Paris, 1969) p. 37. [ 121 B. Krahl-Urban and 1-I. Wagner, Vakuumtechnik 23 (1974) 84. 1131 W. Korner, Phps. Status Solidi (a) 22 (1974) 523. [ 141 M. Pannov, E. Michailov and G. Gcorgicv, Kristall und Technik 8 (1973) 49. [ 151 J.M. Blakely and R.L. Schwoebel, Surface Sci. 26 (1971) 321.
53 (1975) 351-358 Publishing Company
ADSORPTION OF TUNGSTEN ON STEPPED TUNGSTEN SURFACES STUDIED BY WORK FUNCTION MEASUREMENT
K. BESOCKE and H. WAGNER Institu t fiir Grenzfliichenforschung und Vakrrumphysik der Kernforschungsanlage Jiilich Gmbtf, 517 Jiilich, Germany
Work function measurements have been performed during the deposition of W on the (1lO)W plane and several stepped W surfaces with (110) terraces and different terrace width. For each sample the work function decreases with growing coverage. The total work function drop diminishes strongly with decreasing terrace width. The results are interpreted in terms of a reduced nucleation process on stepped surfaces as compared to the flat (110) plane. The step edges act as sinks for the deposited adatoms and cause in their proximity a “dead” zone for nuclei formation. Details of the work function change with coverage are discussed in terms of an edge roughening effect on stepped
surfaces.
1. Introduction Work function changes of single crystal surfaces due to some kind of surface disorder have been observed on several occasions. In general, the work function decreases upon disordering especially for densely packed low index planes. Producing a state of disorder by argon ion bombardment, Farnsworth and Madden [l] found a work function reduction of a (100) nickel surface. The evaporation of atoms represents another way of producing disordered surface structures. In preparing Al films by autoepitaxy work function reductions were recorded for the (111) and (100) surface of Al [2]. Changes in the emission current of W field emitter tips due to surface disorder have been observed by Miiller [3] and Plummer and Rhodin [4]. In a previous paper [S] we reported work function changes of the (110) tungsten surface obtained by a defined deposition of W atoms. From this investigation we concluded that a single adatom on the (11O)W plane gives rise to an electric dipole moment of about 1 D. From the variation of the work function with increasing adatom coverage and the dependence on the substrate temperature it could further be inferred that the deposited atoms combine to islands with a number per surface area given by the substrate temperature and an average size given by the deposited amount. The latter conclusion results from the observation that the work function change at a given substrate temperature follows a square root dependence on the adatom coverage 0. The square root dependence was rationalized in terms of an electric dipole moment associated with an atom in a ledge position and the fact 351
that the total circumference length of the growing islands is proportional to JO. The dipole moment of an atom in a ledge position amounts to about 0.3 D. This value is understood as an average over various possible edge structures. The previous results [5] just summarized show that work function measurements give information on the nucleation and the starting growth process. It is interesting to observe how these processes are influenced by the presence of steps on surfaces. Surfaces exhibiting an orientation close to the orientation of a low index crystal plane consist of terraces of the low index plane linked by steps of often monoatomic height and are therefore very suitable for this kind of investigation. These “stepped” surfaces can be prepared with a terrace width given by the inclination angle of the surface normal to the low index plane and a defined step orientation likewise given by the inclination direction. LEED studies have verified the existence of regular step arrays on metal [6] and semiconductor [7] surfaces as well. The following investigation reports work function measurements performed during the deposition of tungsten on stepped tungsten surfaces with (110) terraces and of several terrace widths.
2. Experimental The sample preparation and the LEED characterization of the respective surfaces have been described in detail elsewhere [6]. Tungsten surfaces with orientations close to the (750), (650) and (40 37 1) planes have been prepared all showing terraces of (110) orientations but different step densities. The respective terrace widths as determined by LEED are 12.6 + 0.1, 23.3 + 0.5 and 5 1.6 f 1.5 .% The investigation was carried out in a bakeable all metal UHV system pumped by ion getter pumps and capable of pressures in the 10~-11 Torr region. The system was equipped with conventional 4 grid LEED-Auger optics. The cleaning procedure as described in ref. [6] led to surfaces from which sharp LEED patterns with a low background intensity were obtained and showing no traces of any contamination as checked by AES. Tungsten was deposited on the sample from a high-purity (99.999%) filament source using a flash evaporation technique. The W filament was degassed for several hours at 2000°C. The Auger spectra taken after the successive evaporation steps did not show any contamination within the detection limits of AES. The deposition rate was calibrated with a quartz microbalance of high sensitivity. A deposition of0.02 monolayer of tungsten caused a frequency shift of 1.8 Hz at a resolution of 0.1 Hz. After each evaporation step the work function change was measured by the change of the contact potential difference (CPD) between the sample and the 2nd LEED grid. For this, the 2nd derivative of the secondary electron current was taken as function of ihe voltage between the sample and the LEED grid. The derivative shows a pronounced peak at a voltage close to the CPD. The accuracy of the CPD measurement was about +o.02 eV.
K. Bcsocke, H Wagner /Adsorption
of W on stepped W surfaces
353
3. Results
Tungsten was evaporated on stepped W surfaces characterized by their terrace width of 52, 23 and 12.6 8, respectively, at room temperature. Fig. 1 shows the work function change obtained for the three stepped surfaces as function of the amount of deposited W. The amount is given in equivalent monolayers of the (11O)W plane, i.e. 0 = 1 corresponds to 1.42 X 1015 atoms/cm2. The corresponding work function change obtained for the flat (I lO)W surface taken from previous results [5] is also included in fig. 1 for comparison. All surfaces show a decreasing work function with growing coverage. The total work function drop decreases with decreasing terrace width. With decreasing terrace width the work function levels off at ever smaller coverages. For each surface the work function could be brought back to its starting value by just raising the substrate temperature to about 700°C. The finding that the W deposition causes a work function change which is getting smaller the narrower the terraces are is complemented by the following LEED observations. The diffraction spots become somewhat blurred and the background intensity increases upon increasing coverage for the W(110) plane. This effect is greatly reduced for the stepped surfaces. Virtually no changes of the LEED intensities are seen for the 12.6 a terrace surface.
4. Discussion The work function change caused by the W deposition on the flat (11O)W surface could be understood [5] in terms of dipole moments associated with single adatoms and, for increasing coverage, of dipole moments associated with atoms in ledge positions of growing islands. One reason for inferring an island formation
Fig. 1. Work function change A@ versus coverage 0 of deposited W for the W(110) stepped W surfaces indicated by their respective terrace width A.
plane and
resulted from the dependence of the work function change on the square root of the coverage 8. Another reason followed from the observation that the work function change with increasing coverage was strongly dependent on the substrate temperature. This result could be understood by considering that at higher substrate temperatures less islands will be formed after the first few evaporation steps because of the larger mobilities of the single adatoms and the correspcndingly smaller nuclei density. The observation that the work function reduction of stepped surfaces becomes smaller with increasing step density shows that, for a given coverage, fewer and probably also smaller islands exist on the stepped surfaces than on the flat (110) surface. An obvious reason for this behaviour results certainly from the presence of the step edges acting as sinks for the deposited adatoms so that less adsorbed atoms are available on the terraces for participating in a nuclei formation and a following island growth. That is to say that the already existing steps hinder an additional growth of further edges and that therefore the work function change is more and more reduced for an increasing step density. The step edges already present at the stepped surfaces cause in their vicinity a “dead” zone for nuclei formation as in the case of the flat (1 IO) plane where a further nucleation process close to the growing islands does no longer occur. That this explanation applies may be deduced by considering the diffusion of the deposited adatoms to the step edges. The solution of the diffusion problem gives also an estimate on the extension of the “dead” zone in relation to the respective terrace widths. The initial and the boundary conditions are: On a terrace of width A bounded by an ascending and a decending step a certain adatom concentration C, is uniformly deposited at the time t = 0. Subsequently the adatom concentration C(x, t) follows Fick’s second law ac(x, t) _ Tit-
a*C(x, t)
-D --~__ ad
(1)
At the ascending step the adatom concentration C(0, t) is zero for t > 0. The detending step is assumed to act as a reflecting boundary [8]. The adatom diffusion coefficient D has been measured [9,10] for the W(l IO) plane and takes a value [lo] of 3.3 X lo--l7 cm* sec.-’ at 40°C. No provision has been made in ey. (1) to account for a nucleation process and an island growth on the terrace plane so that only the random walk process to the step edge causes the adatom concentration to deplete on the terrace plane. Fig. 7_ illustrates the solution of the diffusion problem for the three differently stepped surfaces. It may be seen that the adatom concentration is greatly reduced especially in a region near the ascending step edge and that the adatom fraction remaining on the terrace after the time t I shows a very strong dependence on the terrace width A. Only 30% diffuse to the step edge on the 52 a terrace whereas 76% and even 97% reach the step edge for the 23 a and the 12.6 a terrace, respectively. Because the average distance between the deposited adatoms due to a single deposition flash is larger than 11 ,& we may expect that on
K. Besockc, il. Wagner /Adsorption
Normalized
of W OHstepped W surfaces
355
dlstonce
Fig. 2. Calculated adatom concentration as function of the normalized distance x/A from the step edge. Initial and boundary conditions are given in the text. D = 3.3 X lo-l7 cm2s-I, ‘1 = 10 min.
the 12.6 A terrace surface virtually no additional island growth will take place and consequently no substantial reduction of the work function should happen which is indeed the case. Compared with the flat (1lO)W plane the 52 a terrace surface should show a reduced island growth leading to a reduced work function change which should even much more apply to the 23 A terrace surface. The fact that more of the deposited atoms are being attached just to the available step edges the smaller the terraces are might also be regarded as a reduced disordering effect on the respective surfaces. This is in accord with the LEED observation that the sharpness of the diffraction spots is much less reduced for the stepped surfaces than for the flat (110) plane. In this sense one might talk of a self healing action of stepped surfaces [ 111. The model considerations just outlined do in fact describe the main features of the observed results. Before discussing some details of the experimental findings we want to comment on the absolute value of the work function of stepped surfaces. As may be expected from ref. [S] and the results given above, stepped surfaces should exhibit a lower work function than the corr-esponding low index plane. This expectation has been confirmed by work function measurements on vicinal tungsten surfaces [ 12,131. Based on our model assumption concerning dipole moments associated with atoms in ledge positions one would expect that the work function of stepped surfaces decreases with increasing step density, i.e. with the inclination angle to the low index plane. Preliminary measurements indicate that the work function of the 12.6 a terrace surface is about 0.5 eV lower than the work function of the (110) surface. This however means that the “kind of roughness” achieved for the (110) plane by W deposition at room temperature leads to about the same work function on an absolute scale as the stepped surface structure of the 12.6 A terrace surface. The latter can hardly be altered anymore by a further W deposition at least at a temperature around and above room temperature.
Going back to some details of the experimental results shown in fig. 1 we may examine whether the characteristic feature pertaining to an island formation may also be found for the stepped surfaces i.e. the linear dependence of the work function change on the square root of the coverage 8. Fig. 3 shows the work function change A@ obtained for the 52 A terrace surface as function ofdo. A linear portion is indeed observed and that within the same coverage region as reported earlier [5] for the flat (1 10) plane. As a remarkable distinction, however, may be pointed out that the extrapolation of the linear portion does not go through zero as was observed for the (110) plane. We may infer from this finding that not the entire work function reduction is due to newly formed islands but that another contribution of about 0.1 eV as seen by the ordinate segment also effects the work function change. The following argumentation which certainly ought to be reconfirmed by further investigations may explain this experimental result and leads simultaneously to a somewhat different understanding of the work function change observed for- the 23 A terrace plane. One may conceive that the deposited adatoms which are leaving the terraces are being attached to the step edges in some kind of statistical disorder. That is to say that the step edge does not maintain its initial structure but rather becomes irregular or “roughened”. This effect will certainly increase the dipole moment per unit length of the step edge and thereby decrease the work function of the surface. We attempt therefore to rationalize the ordinate segment of fig. 3 in terms of an “edge roughening” effect. This explanation leads us now to examine the details of the work function change observed for the 23 A terrace surface. Three features may be stressed in this context: (1) The work function change levels off already at a relatively low coverage of about 0 = 0.15. (2) No linear portion of the plot A$ versus 40 can be discerned within the coverage range as observed for the (110) and 52 A terrace surface. (3) The total work function change amounts to about twice the value of the work function reduction of the 52 A terrace surface which was attributed to an edge roughening.
Fig. 3. Work function
change
A@ versusJO for the 52 A terrace
surface
K. Besocke, H. Wagner / Adsorpiion
of W on stepped W surfaces
351
These features may be understood by considering mainly a roughening of the step edges. (1) Because of the considerable diffusion of the deposited adatoms to the step edges one would expect that rather more atoms have to be deposited on the 23 a terraces to reach the necessary adatom concentration [ 141 for nuclei formation and subsequent island growth than on the flat (110) plane or the 52 .& terrace surface. It seems, however, plausible that a coverage of no more than 19= 0.15 is sufficient to produce a maximum of possible edge roughness. (2) The missing of a linear portion of the A@ versus d(I plot points at the absence of an island growth on the terrace. (3) If we attribute a work function change of about 0.1 eV to the roughening of the step edges of the 52 a terrace surface we might as well explain the main part of the work function change of 0.2 eV of the 23 ,J%terrace plane by the same effect. This is because the latter surface exhibits about twice as many steps per surface area than the former one. If the interpretation of the work function change of the 52 A and especially the 23 a terrace surface is correct one should also expect a substantial work function decrease of about 0.3~-0.4 eV of the 12.6 a terrace surface by the same argumentation. This has not been observed. However, one might reasonably argue that for very small terrace widths additional effects might become important which either reduce a further build up of dipole moments at the step edges or even counteract a roughening. A depolarizing action of the neighbouring step edges may prevent a further build up of dipole moments. Bkalely and Schwoebel [ 151 have shown that a repulsive interaction force exists between steps. This force varies inversely at least with the square of the step separation. Because of the repulsive nature of this interaction the step edges might tend to stay as smooth as possible. Effects of this kind which are operative at high step densities might explain the absence of a substantial work function reduction of the 12.6.8 terrace surface.
Acknowledgement Stimulating discussions with Prof. G. Comsa, Prof. Dr. E.A. Niekisch and Prof. H. Ibach are greatefully acknowledged. We thank Mr. S. Berger for technical assistance during the measurements.
References [ I] [ 21 [3] [4] [5]
H.E. Farnsworth and H.H. Madden. J. Appl. Phys. 32 (I 96 1) 1933 R.M. Eastman and C.H.B. Mee, J. Phys. F3 (1973) 1738. E.W. Miiller, 2. Physik 126 (1949) 642. E.W. Plummer and T.N. Rhodin, J. Chcm. Phys. 49 (1968) 3479. K. Bcsocke and H. Wagner, Phys. Rev. B8 (1973) 4597.
358 [6] [7] [ 81 [9] [lo] [ 111
K. Besocke, II. Wagner / Adsorption of W 011stepped W surfaces
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