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The growth of Fe overlayers on Ag(100)
Surface Science ! 19 (1982) L287-L291 North-Holland Publishing Company
L287
SURFACE SCIENCE LETTERS THE GROWTH OF Fe OVERLAYERS ON Ag(100) G.C. SMITH, H.A. PADMORE and C. NORRIS Department of Physics, University of Leicester, Leicester LEI 7RH, UK Received 4 February 1982; accepted for publication 29 April 1982
Using LEED and Auger spectroscopy the growth of Fe overlayers on Ag(100) was observed to follow an epitaxial layer-by-layer mode for three monolayers, after which islanding occurred. The large (0.9 eV) work function change seen on deposition is not consistent with the prediction of either the uniform positive background model or the virtual level model of adsorbate-induced work function change.
Ultra-thin transition metal overlayer-substrate combinations have frequently been observed to follow the Frank-van der Merwe model of pseudomorphic epitaxial layer-by-layer growth [1-4]. In this model growth proceeds subject to the conditions (i) that the first monolayer has a similar atomic configuration to a plane of the natural overlayer lattice and (ii) that the lattice mismatch between the overlayer and the substrate is less than 9%, or 14%, if the temperature can be reduced sufficiently to inhibit thermal activation of dislocation formation [5]. Overlayer-substrate combinations involving materials with bulk interatomic spacings differing by amounts greater than these critical values produce overlayers in which the strain is large, and layer-by-layer growth cannot be sustained. Typically growth may then follow the StranskiKrastanov model where monolayer growth is followed by island formation such as is seen in the cases of Pb/Cu(100) [6], Ag/Mo(100), Si(lll) and W(110) [7], and Au/Mo(110) at elevated temperatures [8]. In the present work the deposition of Fe films on an Ag(100) substrate maintained at room temperature was observed to follow a form of the Stranski-Krastanov growth mode. This resulted in the formation of fcc Fe at this temperature over a limited range of coverage. For bulk Fe the fcc phase, with interatomic spacing 2.57 A, is observed only between 900 and 1400°C, below 900°C bulk Fe has a bcc crystal structure with an interatomic spacing of 2.48 ,~. Ag is fcc with an interatomic spacing of 2.89 ,k,, which corresponds to a 14% lattice mismatch over bcc Fe and 11% over fcc Fe in the case of (100) oriented films. All data were recorded with a VG ADES 400 angle resolving photoelectron spectrometer equipped with 4-grid display optics for LEED and AES, noble 0039-6028/82/0000-0000/$02.75 © 1982 North-Holland
G.C. Smith et aL / Growth of Fe overlayers on Ag(lO0)
L288
gas discharge source, and an angle resolving hemispherical photoelectron analyser. The Ag(100) substrate was prepared from an oriented spark-cut slice by standard polishing procedures. Fe was deposited from a Knudsen cell vapour source enclosed in a water-cooled jacket and provided with a shutter to enable interruption of the vapour beam. During deposition the chamber pressure rose from a base value of < 5 × 10-1~ to - 2 × 10-~0 mbar. Due to the uncertainty in the detection of surface carbon on Ag substrates by AES [9] angle resolved photoelectron spectroscopy (ARPES) at 21.2 eV (HeI radiation) was used as an additional check of surface cleanliness. After repeated cycles of Ar-ion bombardment and annealing a crystal surface was obtained which gave a well-defined LEED pattern with no impurity Auger emission lines and with photoelectron spectra in good agreement with those recorded by Roloff and Nedermeyer [10]. The work function, ~, was determined from the width of the normal emission photoelectron spectrum; the value obtained for the clean surface, 4.64 eV, is in agreement with the measurements of Dwedydari ~ind Mee [11]. The cross-section for photoionisation of 3d electrons at 21.2 eV (HeI) is an order of magnitude below that for 4d electrons [12]. The photoemission was thus dominated by the substrate signal with features in the overlayer emission being difficult to discern, and the spectra are not presented here.
f~
t2
t3 Fe (47eV)
J /
J
} ,/
-.
/
Ag (3S6eV)
10t
1S
l 20
2iS
3z0
J
3S
{I
T
95
-~
---z
100
Evop0rati0n time (minufes) Fig. 1. Plot of Fe (4"7 eV) and Ag (356 eV) Auger line intensities versus exposure time of Ag(100) substrate to Fe evaporation source.
G.C. Smith et al. / Growth of Fe overlayers on Ag(lO0)
L289
The peak-to-peak heights of the differentiated Fe (47 eV) and Ag (356 eV) M N N Auger lines were periodically monitored during deposition and Auger Signal versus time (AS-t) plots were drawn. Fig. 1 shows a typical plot with three straight line sections visible in the substrate signal decay and breaks occurring at times tl, 12 and t 3. After time 13 a slow monotonic decrease of substrate signal and increase of overlayer signal was observed. The Fe 47 eV peak is superposed on a steeply sloping secondary background which distorts the measured peak intensity at low coverages. It was therefore not possible to measure reliably the Fe signal until after time t I, but from then on it mirrored the Ag signal behaviour with changes in gradient discernable at times t 2 and t 3. During deposition the substrate p(1 × 1) LEED pattern persisted with clear, distinct spots until time t 2. Between 12 and t 3 a slight deterioration in the pattern was noted with a gradual rise in background intensity. After t 3 the p(1 × l) pattern rapidly became indistinct and disappeared. We therefore propose that Fe adsorption on Ag(100) at room temperature proceeds via a layer-by-layer mode for the first three layers, with the breaks in the AS-t plot (t~, t 2 and t3) indicating completion of the first, second and third monolayers respectively. It can be seen from the AS-t plot that the time required for the third monolayer to form was slightly longer than that for the first and second monolayers. If we assume no change in sticking probability and a constant deposition rate this indicates denser packing in the third layer, corresponding to the reduced ordering observed in the LEED pattern between t i m e s t 2 and t 3. The first two monolayers are in registry with the substrate but strain due to the 11% lattice expansion over that of bulk fcc Fe results in some relaxation in the third layer. This breakdown of periodicity causes subsequent depositions to follow disordered island growth. Thus growth appears to follow Frank-van der Merwe theory for three monolayers after which islanding occurs, as in the Stranski-Krastanov model. The two epitaxial Fe layers plus the strained third layer correspond to fcc Fe with a lattice expansion of I 1% over the high temperature bulk fcc allotrope. The work function of the surface was monitored during deposition and is shown plotted against coverage in fig. 2. There is initially little variation from the clean substrate value of approximately 4.6 eV but after 0.3 monolayer coverage a rapid rise was observed, reaching a peak of 5.5 eV at one monolayer. As deposition proceeded the work function fell and at three monolayers the value of 4.8 eV is close to the values of between 4.6 and 4.8 eV reported in the literature for bulk Fe [13-15]. Both the uniform positive background model of adsorbate-induced work function changes [16] and the virtual level model [17,18] have been applied to alkali metal adsorption systems with some success. The theories predict charge transfer from overlayer to substrate resulting in an initial lowering of work function at low coverages with the bulk overlayer work function being reached at one monolayer coverage. This has been confirmed experimentally in a
L290
A
c::
g
G.C. Smith et aL / Growth of Fe overlayers on Ag(lO0)
56
/
54,
/ ¢
,- 5.2
/ /
50
\
/ I
\
/
I
o
1
1
......
J--
2
J - -
3
Fe coverage (mono~oyers) Fig. 2. Work function of the Fe/Ag(100) system plotted against Fe coverage. number of cases [19-21]. As the electronegativity of Fe is only slightly less than that of Ag (1.8 and 1.9 respectively) [22] similar but less dramatic behaviour may be expected in the present work but this is clearly not the case. The initial flat region of the curve up to ¼ monolayer indicates that charge transfer does not play an important role. According to the Lang model [16], at one monolayer coverage the work function of the bulk overlayer material is obtained, so the present results predict a value of 5.5 eV for fcc Fe with a lattice constant of 2.89 A. As the coverage is increased the work function remained approximately constant up to two monolayers. Then as the third, distorted, monolayer was built up relaxation of the lattice commenced and the work function dropped. In this region there is increasing overlap between the electronic wave functions as the interatomic spacing reduces which leads to a higher electron density ~ but the work function is found to decrease, in contrast to the prediction of the theoretical models. It should also be noted that an increase in surface roughness at this coverage due to the distorted third monolayer could cause the same effect [23]. It is suggested that an improved model of work function change including the effects of s - d hybridisation is required before results on transition metal overlayer systems, such as presented here, can be reliably interpreted. Finally we point out that the work function at a particular coverage was found to be an extremely sensitive measure of the quality of the overlayers.Reductions of ~> 0.1 eV due to contamination by the residual atmosphere of the vacuum chamber were detected on layers one hour old. In this work care was taken to obtain good vacuum conditions and this was found to be essential; on
G.C. Smith et al. / Growth of Fe overlayers on Ag(lO0)
L291
repeating the experiments at 10 - 9 m b a r pressure a smoothly varying structureless A S - t plot was recorded. We gratefully acknowledge the Science a n d E n g i n e e r i n g Research C o u n c i l for s u p p o r t of this work a n d for provision of a research associateship for one of us (H.A.P.). W e also t h a n k the staff of the S E R C ' s D a r e s b u r y Laboratory, Dr. R. J o r d a n of the University of B i r m i n g h a m Centre for Materials Science for the p r e p a r a t i o n of the Ag(100) crystal, a n d J.S.G. Taylor of Leicester U n i v e r s i t y D e p a r t m e n t of Physics for technical assistance.
References [1] [2] [3] [4]
W. Schlenk and E. Bauer, Surface Sci. 93 (1980) 9. T. Kanaji, Y. Taki and T. Urano, Japan. J. Appl. Phys. 19 (1980) 371. J.M. Guglielmacci and M. Gillet, Thin Solid Films 68 (1980) 407. I. Gonzalez, R. Miranda, M. Salmeron, J.A. Verges and F. Yndurian, in: Proc. 3rd European Conf. on Surface Science, Cannes, 1980. [5] F.C. Frank and J.H. van der Merwe, Proc. Roy. Soc. (London) A198 (1949) 205, 216:: [6] A. Scpulveda and G.E. Rhead, Surface Sci. 66 (1977) 436. [7] J.A. Venables, P. Akhter and J. Derrien, in: Proc. 3rd European Conf. on Surface Science, Cannes, 1980. [8] B. Gruzza and E. Gillet, Thin Solid Films 68 (1980) 345. [9] S.R. Kelemen and I.E. Waehs, Surface Sci. 97 (1980) L370. [10] H.F. Roloff and H. Neddermeyer, Solid State Commun. 21 (1977) 561. [11] A.W. Dwedydari and C.H.B. Mee, Phys. Status Solidi (a) 27 (1975) 223. [12] D.J. Kennedy and S.T. Manson, Phys. Rev. A5 (1972) 227. [13] K. Ueda and R. Shimizu, Japan. J. Appl. Phys. 11 (1972) 916. [14] H. Kobayashi and S. Kato, Surface Sci. 18 (1969) 341. [15] A. Cardwell, Phys. Rev. 92 (1953) 554. [16] N.D. Lang, Phys. Rev. B4 (1971) 4234. [17] J.P. Muscat and D.M. Newns, J. Phys. C7 (1974) 2630. [18] D.M. Newns, Phys. Rev. 178 (1969) 1123. [19] S. Anderson and W. Jostell, Surface Sci. 46 (1974) 625. [20] G. Brod~n and H.P. Bonzel, Surface SCi. 19 (1970) 403. [21] R.L. Gerlach and T.N. Rhodin, Surface Sci. 19 (1970) 403. [22] L. Pauling, The Nature of the Chemical Bond, 3rd ed. (CorneUUniversity Press, Ithaca, NY,
1960). [23] N.D. Lang and W. Kohn, Phys. Rev. B3 (1971) 1215.
L287
SURFACE SCIENCE LETTERS THE GROWTH OF Fe OVERLAYERS ON Ag(100) G.C. SMITH, H.A. PADMORE and C. NORRIS Department of Physics, University of Leicester, Leicester LEI 7RH, UK Received 4 February 1982; accepted for publication 29 April 1982
Using LEED and Auger spectroscopy the growth of Fe overlayers on Ag(100) was observed to follow an epitaxial layer-by-layer mode for three monolayers, after which islanding occurred. The large (0.9 eV) work function change seen on deposition is not consistent with the prediction of either the uniform positive background model or the virtual level model of adsorbate-induced work function change.
Ultra-thin transition metal overlayer-substrate combinations have frequently been observed to follow the Frank-van der Merwe model of pseudomorphic epitaxial layer-by-layer growth [1-4]. In this model growth proceeds subject to the conditions (i) that the first monolayer has a similar atomic configuration to a plane of the natural overlayer lattice and (ii) that the lattice mismatch between the overlayer and the substrate is less than 9%, or 14%, if the temperature can be reduced sufficiently to inhibit thermal activation of dislocation formation [5]. Overlayer-substrate combinations involving materials with bulk interatomic spacings differing by amounts greater than these critical values produce overlayers in which the strain is large, and layer-by-layer growth cannot be sustained. Typically growth may then follow the StranskiKrastanov model where monolayer growth is followed by island formation such as is seen in the cases of Pb/Cu(100) [6], Ag/Mo(100), Si(lll) and W(110) [7], and Au/Mo(110) at elevated temperatures [8]. In the present work the deposition of Fe films on an Ag(100) substrate maintained at room temperature was observed to follow a form of the Stranski-Krastanov growth mode. This resulted in the formation of fcc Fe at this temperature over a limited range of coverage. For bulk Fe the fcc phase, with interatomic spacing 2.57 A, is observed only between 900 and 1400°C, below 900°C bulk Fe has a bcc crystal structure with an interatomic spacing of 2.48 ,~. Ag is fcc with an interatomic spacing of 2.89 ,k,, which corresponds to a 14% lattice mismatch over bcc Fe and 11% over fcc Fe in the case of (100) oriented films. All data were recorded with a VG ADES 400 angle resolving photoelectron spectrometer equipped with 4-grid display optics for LEED and AES, noble 0039-6028/82/0000-0000/$02.75 © 1982 North-Holland
G.C. Smith et aL / Growth of Fe overlayers on Ag(lO0)
L288
gas discharge source, and an angle resolving hemispherical photoelectron analyser. The Ag(100) substrate was prepared from an oriented spark-cut slice by standard polishing procedures. Fe was deposited from a Knudsen cell vapour source enclosed in a water-cooled jacket and provided with a shutter to enable interruption of the vapour beam. During deposition the chamber pressure rose from a base value of < 5 × 10-1~ to - 2 × 10-~0 mbar. Due to the uncertainty in the detection of surface carbon on Ag substrates by AES [9] angle resolved photoelectron spectroscopy (ARPES) at 21.2 eV (HeI radiation) was used as an additional check of surface cleanliness. After repeated cycles of Ar-ion bombardment and annealing a crystal surface was obtained which gave a well-defined LEED pattern with no impurity Auger emission lines and with photoelectron spectra in good agreement with those recorded by Roloff and Nedermeyer [10]. The work function, ~, was determined from the width of the normal emission photoelectron spectrum; the value obtained for the clean surface, 4.64 eV, is in agreement with the measurements of Dwedydari ~ind Mee [11]. The cross-section for photoionisation of 3d electrons at 21.2 eV (HeI) is an order of magnitude below that for 4d electrons [12]. The photoemission was thus dominated by the substrate signal with features in the overlayer emission being difficult to discern, and the spectra are not presented here.
f~
t2
t3 Fe (47eV)
J /
J
} ,/
-.
/
Ag (3S6eV)
10t
1S
l 20
2iS
3z0
J
3S
{I
T
95
-~
---z
100
Evop0rati0n time (minufes) Fig. 1. Plot of Fe (4"7 eV) and Ag (356 eV) Auger line intensities versus exposure time of Ag(100) substrate to Fe evaporation source.
G.C. Smith et al. / Growth of Fe overlayers on Ag(lO0)
L289
The peak-to-peak heights of the differentiated Fe (47 eV) and Ag (356 eV) M N N Auger lines were periodically monitored during deposition and Auger Signal versus time (AS-t) plots were drawn. Fig. 1 shows a typical plot with three straight line sections visible in the substrate signal decay and breaks occurring at times tl, 12 and t 3. After time 13 a slow monotonic decrease of substrate signal and increase of overlayer signal was observed. The Fe 47 eV peak is superposed on a steeply sloping secondary background which distorts the measured peak intensity at low coverages. It was therefore not possible to measure reliably the Fe signal until after time t I, but from then on it mirrored the Ag signal behaviour with changes in gradient discernable at times t 2 and t 3. During deposition the substrate p(1 × 1) LEED pattern persisted with clear, distinct spots until time t 2. Between 12 and t 3 a slight deterioration in the pattern was noted with a gradual rise in background intensity. After t 3 the p(1 × l) pattern rapidly became indistinct and disappeared. We therefore propose that Fe adsorption on Ag(100) at room temperature proceeds via a layer-by-layer mode for the first three layers, with the breaks in the AS-t plot (t~, t 2 and t3) indicating completion of the first, second and third monolayers respectively. It can be seen from the AS-t plot that the time required for the third monolayer to form was slightly longer than that for the first and second monolayers. If we assume no change in sticking probability and a constant deposition rate this indicates denser packing in the third layer, corresponding to the reduced ordering observed in the LEED pattern between t i m e s t 2 and t 3. The first two monolayers are in registry with the substrate but strain due to the 11% lattice expansion over that of bulk fcc Fe results in some relaxation in the third layer. This breakdown of periodicity causes subsequent depositions to follow disordered island growth. Thus growth appears to follow Frank-van der Merwe theory for three monolayers after which islanding occurs, as in the Stranski-Krastanov model. The two epitaxial Fe layers plus the strained third layer correspond to fcc Fe with a lattice expansion of I 1% over the high temperature bulk fcc allotrope. The work function of the surface was monitored during deposition and is shown plotted against coverage in fig. 2. There is initially little variation from the clean substrate value of approximately 4.6 eV but after 0.3 monolayer coverage a rapid rise was observed, reaching a peak of 5.5 eV at one monolayer. As deposition proceeded the work function fell and at three monolayers the value of 4.8 eV is close to the values of between 4.6 and 4.8 eV reported in the literature for bulk Fe [13-15]. Both the uniform positive background model of adsorbate-induced work function changes [16] and the virtual level model [17,18] have been applied to alkali metal adsorption systems with some success. The theories predict charge transfer from overlayer to substrate resulting in an initial lowering of work function at low coverages with the bulk overlayer work function being reached at one monolayer coverage. This has been confirmed experimentally in a
L290
A
c::
g
G.C. Smith et aL / Growth of Fe overlayers on Ag(lO0)
56
/
54,
/ ¢
,- 5.2
/ /
50
\
/ I
\
/
I
o
1
1
......
J--
2
J - -
3
Fe coverage (mono~oyers) Fig. 2. Work function of the Fe/Ag(100) system plotted against Fe coverage. number of cases [19-21]. As the electronegativity of Fe is only slightly less than that of Ag (1.8 and 1.9 respectively) [22] similar but less dramatic behaviour may be expected in the present work but this is clearly not the case. The initial flat region of the curve up to ¼ monolayer indicates that charge transfer does not play an important role. According to the Lang model [16], at one monolayer coverage the work function of the bulk overlayer material is obtained, so the present results predict a value of 5.5 eV for fcc Fe with a lattice constant of 2.89 A. As the coverage is increased the work function remained approximately constant up to two monolayers. Then as the third, distorted, monolayer was built up relaxation of the lattice commenced and the work function dropped. In this region there is increasing overlap between the electronic wave functions as the interatomic spacing reduces which leads to a higher electron density ~ but the work function is found to decrease, in contrast to the prediction of the theoretical models. It should also be noted that an increase in surface roughness at this coverage due to the distorted third monolayer could cause the same effect [23]. It is suggested that an improved model of work function change including the effects of s - d hybridisation is required before results on transition metal overlayer systems, such as presented here, can be reliably interpreted. Finally we point out that the work function at a particular coverage was found to be an extremely sensitive measure of the quality of the overlayers.Reductions of ~> 0.1 eV due to contamination by the residual atmosphere of the vacuum chamber were detected on layers one hour old. In this work care was taken to obtain good vacuum conditions and this was found to be essential; on
G.C. Smith et al. / Growth of Fe overlayers on Ag(lO0)
L291
repeating the experiments at 10 - 9 m b a r pressure a smoothly varying structureless A S - t plot was recorded. We gratefully acknowledge the Science a n d E n g i n e e r i n g Research C o u n c i l for s u p p o r t of this work a n d for provision of a research associateship for one of us (H.A.P.). W e also t h a n k the staff of the S E R C ' s D a r e s b u r y Laboratory, Dr. R. J o r d a n of the University of B i r m i n g h a m Centre for Materials Science for the p r e p a r a t i o n of the Ag(100) crystal, a n d J.S.G. Taylor of Leicester U n i v e r s i t y D e p a r t m e n t of Physics for technical assistance.
References [1] [2] [3] [4]
W. Schlenk and E. Bauer, Surface Sci. 93 (1980) 9. T. Kanaji, Y. Taki and T. Urano, Japan. J. Appl. Phys. 19 (1980) 371. J.M. Guglielmacci and M. Gillet, Thin Solid Films 68 (1980) 407. I. Gonzalez, R. Miranda, M. Salmeron, J.A. Verges and F. Yndurian, in: Proc. 3rd European Conf. on Surface Science, Cannes, 1980. [5] F.C. Frank and J.H. van der Merwe, Proc. Roy. Soc. (London) A198 (1949) 205, 216:: [6] A. Scpulveda and G.E. Rhead, Surface Sci. 66 (1977) 436. [7] J.A. Venables, P. Akhter and J. Derrien, in: Proc. 3rd European Conf. on Surface Science, Cannes, 1980. [8] B. Gruzza and E. Gillet, Thin Solid Films 68 (1980) 345. [9] S.R. Kelemen and I.E. Waehs, Surface Sci. 97 (1980) L370. [10] H.F. Roloff and H. Neddermeyer, Solid State Commun. 21 (1977) 561. [11] A.W. Dwedydari and C.H.B. Mee, Phys. Status Solidi (a) 27 (1975) 223. [12] D.J. Kennedy and S.T. Manson, Phys. Rev. A5 (1972) 227. [13] K. Ueda and R. Shimizu, Japan. J. Appl. Phys. 11 (1972) 916. [14] H. Kobayashi and S. Kato, Surface Sci. 18 (1969) 341. [15] A. Cardwell, Phys. Rev. 92 (1953) 554. [16] N.D. Lang, Phys. Rev. B4 (1971) 4234. [17] J.P. Muscat and D.M. Newns, J. Phys. C7 (1974) 2630. [18] D.M. Newns, Phys. Rev. 178 (1969) 1123. [19] S. Anderson and W. Jostell, Surface Sci. 46 (1974) 625. [20] G. Brod~n and H.P. Bonzel, Surface SCi. 19 (1970) 403. [21] R.L. Gerlach and T.N. Rhodin, Surface Sci. 19 (1970) 403. [22] L. Pauling, The Nature of the Chemical Bond, 3rd ed. (CorneUUniversity Press, Ithaca, NY,
1960). [23] N.D. Lang and W. Kohn, Phys. Rev. B3 (1971) 1215.