- Email: [email protected]
The characterization of the Te(4 × 2) overlayer on a Mo(110) surface
Nuclear Instruments and Methods in Physics Research 848 (1990) 311-314 North-Holland
THE CHARACTERIZATION P. MAHAVADI
OF THE Te(4 x 2) OVERLAYER
ON A Mo(ll0)
SURFACE
and E. BAUER
Technische ~n~versit~t Clausthat, ~~baitzstrasse
R. GHRAYEB
311
4, D-3392 C~a~thai-Zel~e~el~
FRG
and M. HOU
UniversitP Libre de Bruxelles, Campus de la Plaine, CP234, Bd du Triomphe, B-1050 Brussels, Belgium
Quantitative low-energy ion scattering (LEIS) with alkali ions is used in combination with computer simulation in order to characterize the short-range order of a (4X2) tellurium overlayer on a molybdenum substrate. The simulations are performed in the binary-collision approximation with the computer code MARLOWE. The method requires the simultaneous determination of the model potential for the simulations and the overlayer structure. The Te(4 X 2)/Mo(llO) superstructure allows two physically realistic configurations, assuming no second-layer reconstruction. In the first, the Te atoms sit in bridge positions and in the second, they sit in hollow sites. It is shown that the former can only be consistent with the simulation if the tellurium atoms are so close to the molybdenum surface that the Te-Mo distance should be close to 20% smaller than the sum of the metallic and covalent radii of molybdenum and tellurium. It is suggested that the hollow-site model is more reasonable and that the method allows a quite accurate determination of the interlayer spacing.
1. Introduction Structure and coverage of tellurium deposited on a molybdenum substrate can be monitored by low-energy electron diffraction (LEED) and Auger electron spectroscopy, as described in detail elsewhere [l]. LEED reveals that, as the coverage increases, tellurium forms well-ordered layers at well-defined submonolayer coverages. They are characterized by 4 X 2, 5 X 2 and 7 X 2 LEED patterns successively [l-3]. These LEED patterns are compatible with several geometrical configurations [1,2]. The purpose of the present work is to find the best consistent model by low-energy ion scattering (LEIS) combined with computer simulations.
2. Experimental conditions and simulation method Tellurium deposition is made in UHV conditions with a calibrated evaporator. In order to prevent neutraIization, the LEIS measurements are carried out with potassium ions whose ionization potential is smaher than the measured work function of the Te(4 X 2)/ Mo(ll0) surface. The experimental setup [1,4,5] allows to produce a well-collimated and mass-filtered beam of potassium ions. The energy may be selected in the range from 50 to 1400 eV. The sample can be rotated about an axis normal to its surface and tilted with respect to the incident direction. A conventional 127“ electrostatic analyser can be moved around the specimen. In this manner, a wide range of scattering conditions can be 0168-583X/90/$03.50 (North-HoIiand)
0 Elsevier Science Publishers B.V.
used, allowing to find the most suitable ones for comparison with computer simulations. The simulations are performed in the binary-collision approximation with the MARLOWE program [6,7]. The trajectories of potassium ions with an incident energy of 600 eV are calculated as sequences of binary collisions governed by the Moliere central potential [8] with both the Te and the MO surface atoms. Quasisimult~eous collisions are treated according to the approximate procedure described in ref. 191. A reasonable treatment of such events is necessary, in particular when several collisions with large impact parameters lead to substantial deviation of the incident trajectories, which is often the case in the present work. The choice of the model potential is somewhat arbitrary. The Moliere potential has, however, the advantage to be only dependent on a single parameter, the screening length. This was already estimated by LEIS for molybdenum by simulation with the impact area method [lo] within an excellent (18) accuracy. This method is extended in the present work in order to simulate the structures of LEIS spectra of the Te/Mo system. Details will be given in ref. [ll]. The identification of the features in LEIS spectra from overlayered surfaces is much more complex than from elemental ones. This is due to the multiplicity of the scattering mechanisms which contribute to the same peaks in the energy dist~but.ions. Simple kinematic estimates are not longer sufficient. Because of this complexity, not all the features are used for comparison of the experimental data with the simulation; a selection is V. SURFACE PHENOMENA
312
P. Mahaoadi et al. / Characterization
0.0 0.0 0000000 0.0 0.0 0000000 (al
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[email protected]
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of Te(4 x 2) overlayer
made in order to prevent possible interpretation ambiguities. The K+-Te potential could not be determined with the same procedure as in ref. [lo] because of the unknown atomic arrangement of the Te(4 X 2)/Mo(llO) structure. As a consequence, this potential needs to be estimated simultaneously with the determination of the Te distribution on MO. Basically, the Te(4 x 2)/Mo(llO) superstructure allows two physically realistic configurations assuming no second-layer reconstruction. They are sketched in fig. 1. In fig. la, the Te atoms sit in a bridge position (BP). In figs. lb and I& they are located in a hollow site (HS). Two equivalent 4 X 2 structures are possible. Hence, they must be considered as coexisting.
3. The results
Fig. 1. The two Te structures compatible with the 4 x 2 LEED pattern. The MO atoms of a (110) surface are represented by open circles, the Te atoms are represented by shaded circles. (a) The Te atoms sit in the bridge position; (b) the Te atoms sit in hollow sites; (c) the Te atoms sit in the other set of possible hollow sites. Both possibilities in (b) and (c) are equally probable.
The possibility of the BP structure is tested by comparison of the relative peak heights in the LEIS spectra taken in two different experimental situations. These are shown in figs. 2 and 3. Assuming the BP structure, the peaks labelled B and C in fig. 2b could be clearly identified by the simulation to be essentially associated with single scattering from MO and Te, respectively. Several processes were found to contribute to the peak centered between 200 and 300 eV in fig. 3b whose relative importance is calculated with an automatic procedure [ll]. The ratios of the two single scattering peaks in fig. 2b to the single MO peak from a clean Mo(ll0) surface (fig. 2a) are used for comparison with simulation, all other experimental parameters being kept constant. The same comparison with simulation is repeated for the ratio of the major peak intensity in fig. 3b (peak E) with the single MO scattering peak from a pure substrate in fig. 3a (peak D). In such a way, the
B
30 -
A
0 "0
200 SCATTERING
w)O ENERGYWI
0
200 SCATTERING
400 ENERGY
IeVl
Fig. 2. Typical experimental LEIS spectra of scattered K+ ions. (a) 600 eV K + ions are incident on a clean (110) molybdenum surface with a kinetic energy of 600 eV at an angle of 40° from the surface plane. The plane of incidence is parallel to the (001) direction. The scattering angle is 80 O. (b) Same incidence conditions as in (a), but the MO surface is covered by a (4 X 2) Te overlayer. The peaks labeled A, B and C are those to which the text and fig. 4 refer.
313
K*-Te/MoillO) ,” 3 8 & ii? 5 z
~~~TTER~~~
SWTERIMG
ENERGY lek’)
E%ERttY l&f
Fig- 3. (a) 600 eV K’ ions are incident on a clean (110) mo~y~enum surface with a kinetic energy of 600 eV at an angle of 75 0 from the surface plane. The plane of incidence is parallel to a (IlO) direction. The scattering angle is lOSo. (b) Same incidence conditions as in (a), but the MO surface is covered by a (4 X 2) Te overlayer. The peaks labeled D and E are those to which the text and fig. 4 refer.
differences between the spectra in figs. 2a and 2b, as well as between figs. 3a and 3b, are only due to the Te overlayer with which the measurements were performed (figs. 2b and 3b). The information about the Te arrangement can thus be discussed on the basis of LEIS spectra
obtained with suitable scattering conditions, with and without a Te overlayer. The values of the relative peak intensity ratios obtained by simulation for the BP model depend on two parameters: the distance between the Te atoms and the molybdenum surface and the screening length in the Moliere potential for the K+-Te interaction. It is thus possibfe, for each v&e of the Te-MO surface distance, to find a screening length which leads to the same intensity ratio as in the experiment with identical scattering conditions for the clean No(ll0) and the Te(4 X 2)/Mo(llO) Furfaces. This is done for the situations described in figs. 2 and 3. and the results are given in fig. 4. The three dependences of the interlayer distance on the screening length obtained in this manner intersect and give a unique solution for the BP model: a distance between the tellurium layer and the MO surface of 1.855 A and a screening length for the Moliere potential of 0.0935 A. For both parameters, the uncertainty is less than 0.1%.
4. Discu,ssian and conclusion
i
190
0.092
O.QY4
0.096
K-Te SEREENING LENGTH dt Fig. 4. Relation between the Mo-Te interlayer distance d and the screenmg length aK_re in the Moliere potential. Each point represents a pair of values fd, aK_Te) which provides the same intensity ratio in the simulations as in the experiments. Curve (a) represents the results for the ratios between the intensities in peaks A and B, (b) the intensity ratios in peaks C and A, (c) the intensity ratios in peaks D and E. The peaks A, 8, C, D and E are shown in figs, 2 and 3.
At the present stage of this work, simulation consistency is found for the BP model but does not exclude a consistent solution for the HS model. In other words, it cannot be excluded that a different interlayer spacing, with the “I%atoms sitting in HS positions, can also lead to realistic potential predictions. In addition, no independent criterium is avaiIable which allows the check the screening distance found. Therefore, the validity of the sdution presented in the preceding section will be examined now. Contrary to the screening length in the Moliere potential, the Te-Mo distance can be compared with other independent data. The interlayer distance of 1.855 A gives a Te-Mo distance of 2,433 &which is much smaller than the sum of the metallic and covalent radii of V. SURFACE
PHENOMENA
314
P. Mahaoadi et al. / Characterization
molybdenum and tellurium, 1.57 and 1.37 ,& respectively [12]. These two values give a minimum possible interlayer spacing between MO and Te planes in the BP model of 2.24 A. For such a distance, it is clearly seen from fig. 4 that no consistent screening length can be found. On the other hand, interatomic Te-substrate atom distances smaller than expected for the metallic and covalent radii have been found before. The Te-Cu interatomic distance in the p(2 X 2) T: layer on Cu(100) was determined to be 2.48 + 0.1 A [13]. From the metallic radius of copper (1.28 A) a Te-Cu distance of 2.65 i is expected. This difference (6%) is, however, smaller than in the Te-Mo case, where it amounts to 17%. Such a large difference is physically unrealistic and suggests that the BP model has to be discarded. It will be shown in ref. [ll] that a detailed computer analysis of the experimental data with the HS model is more successful. A selection of appropriate incidence conditions allows to discriminate between the HS and BP models. Using these, a procedure analogous to that employed in the present paper for the BP model allows one to find a consistent interlayer spacing and a screening length with the HS model. Owing to the large sensitivity of the intensity calculations to the screening distance, the estimate can be done with quite good accuracy. In conclusion, we have shown that a systematic comparison of measured relative peak intensities with intensities from computer simulations allows the simul-
of Te(4 x 2) overlayer
taneous determination of the screening length and the atomic positions for a given model. It also indicates which apparently realistic models can be excluded.
References [l] P. Mahavadi, Ph.D. Thesis, Clausthal, FRG (1989). [2] M. Stolzenberg, unpublished. [3] Ch. Park, E. Bauer and H.H. Kramer, Surf. Sci. 119 (1982) 251. [4] T. v.d. Hagen, Ph.D. thesis, Clausthal, FRG (1982). [5] E. Bauer and T.v.d. Hagen, in: Chemistry and Physics of Solid Surfaces, vol. 6, eds. R. Vanselow and R. Howe (Springer, Berlin, 1986) p. 547. [6] M.T. Robinson and I.M. Torrens, Phys. Rev. B9 (1974) 5008. [7] M.T. Robinson, in: Sputtering by Particle Bombardment, vol. 1 (Topics in Applied Physics, vol. 47) ed. R. Behrisch (Springer, Berlin, Heidelberg, New York, 1981). 181 G. MoliBre, Z. Naturforsch. 2A (1947) 133. I9j M. Hou and M.T. Robinson, Nucl. Instr. and Meth. 132 (1976) 64. M. Hou and E. Bauer, WI R. Ghrayeb, M. Purushotham, Phys. Rev. B136 (1987) 7364. Dll P. Mahavadi, E. Bauer, R. Ghrayeb and M. Hou, to be published. PropWI G.V. Samsonov, Handbook of the Physicochemical erties of the Elements (Plenum Press, New York, 1968) p. 98. 1131 A. Salwtn and J. Rundgren, Surf. Sci. 53 (1975) 523.
THE CHARACTERIZATION P. MAHAVADI
OF THE Te(4 x 2) OVERLAYER
ON A Mo(ll0)
SURFACE
and E. BAUER
Technische ~n~versit~t Clausthat, ~~baitzstrasse
R. GHRAYEB
311
4, D-3392 C~a~thai-Zel~e~el~
FRG
and M. HOU
UniversitP Libre de Bruxelles, Campus de la Plaine, CP234, Bd du Triomphe, B-1050 Brussels, Belgium
Quantitative low-energy ion scattering (LEIS) with alkali ions is used in combination with computer simulation in order to characterize the short-range order of a (4X2) tellurium overlayer on a molybdenum substrate. The simulations are performed in the binary-collision approximation with the computer code MARLOWE. The method requires the simultaneous determination of the model potential for the simulations and the overlayer structure. The Te(4 X 2)/Mo(llO) superstructure allows two physically realistic configurations, assuming no second-layer reconstruction. In the first, the Te atoms sit in bridge positions and in the second, they sit in hollow sites. It is shown that the former can only be consistent with the simulation if the tellurium atoms are so close to the molybdenum surface that the Te-Mo distance should be close to 20% smaller than the sum of the metallic and covalent radii of molybdenum and tellurium. It is suggested that the hollow-site model is more reasonable and that the method allows a quite accurate determination of the interlayer spacing.
1. Introduction Structure and coverage of tellurium deposited on a molybdenum substrate can be monitored by low-energy electron diffraction (LEED) and Auger electron spectroscopy, as described in detail elsewhere [l]. LEED reveals that, as the coverage increases, tellurium forms well-ordered layers at well-defined submonolayer coverages. They are characterized by 4 X 2, 5 X 2 and 7 X 2 LEED patterns successively [l-3]. These LEED patterns are compatible with several geometrical configurations [1,2]. The purpose of the present work is to find the best consistent model by low-energy ion scattering (LEIS) combined with computer simulations.
2. Experimental conditions and simulation method Tellurium deposition is made in UHV conditions with a calibrated evaporator. In order to prevent neutraIization, the LEIS measurements are carried out with potassium ions whose ionization potential is smaher than the measured work function of the Te(4 X 2)/ Mo(ll0) surface. The experimental setup [1,4,5] allows to produce a well-collimated and mass-filtered beam of potassium ions. The energy may be selected in the range from 50 to 1400 eV. The sample can be rotated about an axis normal to its surface and tilted with respect to the incident direction. A conventional 127“ electrostatic analyser can be moved around the specimen. In this manner, a wide range of scattering conditions can be 0168-583X/90/$03.50 (North-HoIiand)
0 Elsevier Science Publishers B.V.
used, allowing to find the most suitable ones for comparison with computer simulations. The simulations are performed in the binary-collision approximation with the MARLOWE program [6,7]. The trajectories of potassium ions with an incident energy of 600 eV are calculated as sequences of binary collisions governed by the Moliere central potential [8] with both the Te and the MO surface atoms. Quasisimult~eous collisions are treated according to the approximate procedure described in ref. 191. A reasonable treatment of such events is necessary, in particular when several collisions with large impact parameters lead to substantial deviation of the incident trajectories, which is often the case in the present work. The choice of the model potential is somewhat arbitrary. The Moliere potential has, however, the advantage to be only dependent on a single parameter, the screening length. This was already estimated by LEIS for molybdenum by simulation with the impact area method [lo] within an excellent (18) accuracy. This method is extended in the present work in order to simulate the structures of LEIS spectra of the Te/Mo system. Details will be given in ref. [ll]. The identification of the features in LEIS spectra from overlayered surfaces is much more complex than from elemental ones. This is due to the multiplicity of the scattering mechanisms which contribute to the same peaks in the energy dist~but.ions. Simple kinematic estimates are not longer sufficient. Because of this complexity, not all the features are used for comparison of the experimental data with the simulation; a selection is V. SURFACE PHENOMENA
312
P. Mahaoadi et al. / Characterization
0.0 0.0 0000000 0.0 0.0 0000000 (al
0.0
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[email protected]
0.0
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0000000
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0000000
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of Te(4 x 2) overlayer
made in order to prevent possible interpretation ambiguities. The K+-Te potential could not be determined with the same procedure as in ref. [lo] because of the unknown atomic arrangement of the Te(4 X 2)/Mo(llO) structure. As a consequence, this potential needs to be estimated simultaneously with the determination of the Te distribution on MO. Basically, the Te(4 x 2)/Mo(llO) superstructure allows two physically realistic configurations assuming no second-layer reconstruction. They are sketched in fig. 1. In fig. la, the Te atoms sit in a bridge position (BP). In figs. lb and I& they are located in a hollow site (HS). Two equivalent 4 X 2 structures are possible. Hence, they must be considered as coexisting.
3. The results
Fig. 1. The two Te structures compatible with the 4 x 2 LEED pattern. The MO atoms of a (110) surface are represented by open circles, the Te atoms are represented by shaded circles. (a) The Te atoms sit in the bridge position; (b) the Te atoms sit in hollow sites; (c) the Te atoms sit in the other set of possible hollow sites. Both possibilities in (b) and (c) are equally probable.
The possibility of the BP structure is tested by comparison of the relative peak heights in the LEIS spectra taken in two different experimental situations. These are shown in figs. 2 and 3. Assuming the BP structure, the peaks labelled B and C in fig. 2b could be clearly identified by the simulation to be essentially associated with single scattering from MO and Te, respectively. Several processes were found to contribute to the peak centered between 200 and 300 eV in fig. 3b whose relative importance is calculated with an automatic procedure [ll]. The ratios of the two single scattering peaks in fig. 2b to the single MO peak from a clean Mo(ll0) surface (fig. 2a) are used for comparison with simulation, all other experimental parameters being kept constant. The same comparison with simulation is repeated for the ratio of the major peak intensity in fig. 3b (peak E) with the single MO scattering peak from a pure substrate in fig. 3a (peak D). In such a way, the
B
30 -
A
0 "0
200 SCATTERING
w)O ENERGYWI
0
200 SCATTERING
400 ENERGY
IeVl
Fig. 2. Typical experimental LEIS spectra of scattered K+ ions. (a) 600 eV K + ions are incident on a clean (110) molybdenum surface with a kinetic energy of 600 eV at an angle of 40° from the surface plane. The plane of incidence is parallel to the (001) direction. The scattering angle is 80 O. (b) Same incidence conditions as in (a), but the MO surface is covered by a (4 X 2) Te overlayer. The peaks labeled A, B and C are those to which the text and fig. 4 refer.
313
K*-Te/MoillO) ,” 3 8 & ii? 5 z
~~~TTER~~~
SWTERIMG
ENERGY lek’)
E%ERttY l&f
Fig- 3. (a) 600 eV K’ ions are incident on a clean (110) mo~y~enum surface with a kinetic energy of 600 eV at an angle of 75 0 from the surface plane. The plane of incidence is parallel to a (IlO) direction. The scattering angle is lOSo. (b) Same incidence conditions as in (a), but the MO surface is covered by a (4 X 2) Te overlayer. The peaks labeled D and E are those to which the text and fig. 4 refer.
differences between the spectra in figs. 2a and 2b, as well as between figs. 3a and 3b, are only due to the Te overlayer with which the measurements were performed (figs. 2b and 3b). The information about the Te arrangement can thus be discussed on the basis of LEIS spectra
obtained with suitable scattering conditions, with and without a Te overlayer. The values of the relative peak intensity ratios obtained by simulation for the BP model depend on two parameters: the distance between the Te atoms and the molybdenum surface and the screening length in the Moliere potential for the K+-Te interaction. It is thus possibfe, for each v&e of the Te-MO surface distance, to find a screening length which leads to the same intensity ratio as in the experiment with identical scattering conditions for the clean No(ll0) and the Te(4 X 2)/Mo(llO) Furfaces. This is done for the situations described in figs. 2 and 3. and the results are given in fig. 4. The three dependences of the interlayer distance on the screening length obtained in this manner intersect and give a unique solution for the BP model: a distance between the tellurium layer and the MO surface of 1.855 A and a screening length for the Moliere potential of 0.0935 A. For both parameters, the uncertainty is less than 0.1%.
4. Discu,ssian and conclusion
i
190
0.092
O.QY4
0.096
K-Te SEREENING LENGTH dt Fig. 4. Relation between the Mo-Te interlayer distance d and the screenmg length aK_re in the Moliere potential. Each point represents a pair of values fd, aK_Te) which provides the same intensity ratio in the simulations as in the experiments. Curve (a) represents the results for the ratios between the intensities in peaks A and B, (b) the intensity ratios in peaks C and A, (c) the intensity ratios in peaks D and E. The peaks A, 8, C, D and E are shown in figs, 2 and 3.
At the present stage of this work, simulation consistency is found for the BP model but does not exclude a consistent solution for the HS model. In other words, it cannot be excluded that a different interlayer spacing, with the “I%atoms sitting in HS positions, can also lead to realistic potential predictions. In addition, no independent criterium is avaiIable which allows the check the screening distance found. Therefore, the validity of the sdution presented in the preceding section will be examined now. Contrary to the screening length in the Moliere potential, the Te-Mo distance can be compared with other independent data. The interlayer distance of 1.855 A gives a Te-Mo distance of 2,433 &which is much smaller than the sum of the metallic and covalent radii of V. SURFACE
PHENOMENA
314
P. Mahaoadi et al. / Characterization
molybdenum and tellurium, 1.57 and 1.37 ,& respectively [12]. These two values give a minimum possible interlayer spacing between MO and Te planes in the BP model of 2.24 A. For such a distance, it is clearly seen from fig. 4 that no consistent screening length can be found. On the other hand, interatomic Te-substrate atom distances smaller than expected for the metallic and covalent radii have been found before. The Te-Cu interatomic distance in the p(2 X 2) T: layer on Cu(100) was determined to be 2.48 + 0.1 A [13]. From the metallic radius of copper (1.28 A) a Te-Cu distance of 2.65 i is expected. This difference (6%) is, however, smaller than in the Te-Mo case, where it amounts to 17%. Such a large difference is physically unrealistic and suggests that the BP model has to be discarded. It will be shown in ref. [ll] that a detailed computer analysis of the experimental data with the HS model is more successful. A selection of appropriate incidence conditions allows to discriminate between the HS and BP models. Using these, a procedure analogous to that employed in the present paper for the BP model allows one to find a consistent interlayer spacing and a screening length with the HS model. Owing to the large sensitivity of the intensity calculations to the screening distance, the estimate can be done with quite good accuracy. In conclusion, we have shown that a systematic comparison of measured relative peak intensities with intensities from computer simulations allows the simul-
of Te(4 x 2) overlayer
taneous determination of the screening length and the atomic positions for a given model. It also indicates which apparently realistic models can be excluded.
References [l] P. Mahavadi, Ph.D. Thesis, Clausthal, FRG (1989). [2] M. Stolzenberg, unpublished. [3] Ch. Park, E. Bauer and H.H. Kramer, Surf. Sci. 119 (1982) 251. [4] T. v.d. Hagen, Ph.D. thesis, Clausthal, FRG (1982). [5] E. Bauer and T.v.d. Hagen, in: Chemistry and Physics of Solid Surfaces, vol. 6, eds. R. Vanselow and R. Howe (Springer, Berlin, 1986) p. 547. [6] M.T. Robinson and I.M. Torrens, Phys. Rev. B9 (1974) 5008. [7] M.T. Robinson, in: Sputtering by Particle Bombardment, vol. 1 (Topics in Applied Physics, vol. 47) ed. R. Behrisch (Springer, Berlin, Heidelberg, New York, 1981). 181 G. MoliBre, Z. Naturforsch. 2A (1947) 133. I9j M. Hou and M.T. Robinson, Nucl. Instr. and Meth. 132 (1976) 64. M. Hou and E. Bauer, WI R. Ghrayeb, M. Purushotham, Phys. Rev. B136 (1987) 7364. Dll P. Mahavadi, E. Bauer, R. Ghrayeb and M. Hou, to be published. PropWI G.V. Samsonov, Handbook of the Physicochemical erties of the Elements (Plenum Press, New York, 1968) p. 98. 1131 A. Salwtn and J. Rundgren, Surf. Sci. 53 (1975) 523.