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Chemisorption of lead on germanium (111): LEED intensity measurements
Applications of Surface Science 17 (1983) 131—135 North-Holland Publishing Company
131
COMMUNICATION CHEMISORPTION OF LEAD ON GERMANIUM (III): LEED INTENSITY MEASUREMENTS G. LE LAY
*
and J.J. METOIS
Centre de Recherche sur les Mécanismes de Ia Croissance Cristalline, CNRS, Campus the Luminy, Case 913, F- 13288 Marseille Cedex 09, France
Received 1 June 1983; accepted for publication 19 July 1983
LEED observations and AES studies of the Pb/Ge(l II) system reveal three distinct superstructures with the same X ~ unit mesh. Two of these are two-dimensional phases corresponding to lead overlayers respectively completed at 9 = 1/3 and 9 = I; the third is a new reconstruction of the germanium surface induced by a very small number of lead atoms. We also show that the kinetics of desorption of an adsorbate can be accurately studied by LEED intensity measurements.
In a recent paper [1] we have carried out a detailed chemical, structural and morphological study of the Pb/Ge(l 11) system using AES, LEED and SEM. We could show that at RT this system exhibits the Stranski—Krastanov growth mode, the two-dimensional (2D) adsorbed layer being completed at monolayer coverage (0 1 defined in substrate units, i.e. 7.2 l0’~atoms cm2). This 2D layer displays a Ge(1 1 l)%/~X R 300 (in short Vi) LEED superstructure pattern. The ~ superstructure is the most frequent one and is commonly observed on Si(lll) and Ge(lll) with almost all deposited metals. However we had noticed a special behaviour of this superstructure in the case of Pb/Ge( 111). In this letter we will briefly recall these peculiarities and give additional experimental evidence from LEED intensity measurements. (i) As long as 0 ~ 1/3, we observe at RT a very stable ~/i structure which persists until all lead is evaporated in the desorption temperature range (T 350°C) (ii) Beyond 9 1/3 and up to 8 1, while the intensity of the (2/3, 2/3) spots increases with the coverage, the intensity of the (1/3, 1/3) spots strongly decreases in such a manner that at 8 1 these spots are hardly observed (fig. 1, curve a). This situation exists up to T 280°C. But in a narrow range of temperature around 280°C(thus markedly below the desorption temperatures) =
—
=
=
=
*
Also UER de Physique, University Aix—Marseille I, Marseille, France.
0378-5963/83/0000—0000/$03.Oo
©
1983 North-Holland
132
G. Le Lay, J.J. Métois
/
Chemisorption of Pb on Ge(111)
we observed a reversible transition to a 1 X 1 structure. (iii) If we evaporate the metal onto the germanium substrate beyond this transition temperature, we first observe third order superstructure spots with 0(0)
~(2/32i3)
p(2/32/3)
/ 2 U,
1r 1l I 0
1
~
(ii3 1/3) (i~3i/3)~
1/3
2/3
I
Fig. I. LEED intensity variations (optical density measurements) of superstructure spots versus coverage V’i and ~ superstructures: (a) RT, 41 V; (b) 320°C, 40 V (beyond the transition temperature).
increasing intensity up to 0 1/3, the position of the maximum; beyond 0 1/3 these spots vanish leaving the 1 x 1 pattern (fig. 1, curve b). (iv) Moreover, we have not noticed any change of the spot size during these observations, nor of the general behaviour of the LEED features in the narrow range of incident energy investigated (30—50 eV). We thus observe two kinds of LEED patterns corresponding to the same x ~/i unit mesh but in fact to two different surface phases. The first phase (which we denote by Vi) is very stable and is completed at 8 1/3, as fig. 1 testifies. The second one, denoted by ~/i is completed at 9 1, as Auger adsorption curves ascertain (see ref. [1]). Studying by LEED the Pb/Si(l 11) system, Estrup and Morrison [2] had also observed two kinds of ~/i patterns with (0 < 1/3) or without (8 1)1/3, 1/3 spots, but as no detailed annealing experiments were performed, the question whether a reversible phase transition also occurs in this case is still open. The Ge(111)l x I Pb structure following the ~/~* ~ 1 X 1 transition at 280°C is observed during almost the whole desorption process, which we =
=
=
‘~,
=
=
—
—
G. Le Lay, J.J. Métois
/
133
Chemisorption of Pb on Ge(111)
followed by AES at constant temperature (isothermal desorption kinetics measurements), the desorption isotherms plotted in the coverage range 1 > 9> 0.1 showed a linear decrease of the lead Auger intensity (peak-to-peak height of the Pb 94 eV Auger line) versus time. Below about 9 0.1, overlap with substrates peaks makes it difficult to determine accurately peak heights at these low coverages. We thus performed more sensitive desorption experiments, now plotting the integrated intensity of the substrate integer spots versus time at constant desorption temperature. One isotherm recorded in this manner is shown in fig. 2, in comparison with the same isotherm obtained as usually with AES. We note a linear increase of the LEED intensity of the substrate integer spots versus desorption time, i.e., versus decreasing instantaneous coverage (the Auger amplitude yields the scaling factor r 8 indicated on the abscissa) until 0 — 0.1. This is in perfect agreement with the previous Auger data which also indicated zero-order kinetics. The increase of the LEED intensity when the lead coverage decreases is not surprising as the thermal vibrations of the adsorbed lead atoms are more important than those of the substrate germanium atoms, as is testified by the comparison of the Debye temperatures of lead (88 K) and germanium (290 K). The linearity of the variation of this intensity is —
~
I
0(0) 0:1
8.23
8.13
(b)
&-1/10
0~0
T=350°C
—
/
z
/
LU
\ LU
/
—
m —
a)
— s.~
—4
/
\.
.
(lii
,/A~(2I3~2!3)
~
*.~_,
0
10
20
30
40
mn
Fig. 2. Desorption isotherm T = 350°C: (a) LEED intensity measurements (40 V) (dotted line), average intensity of the substrate (1,1) integer spots and intensity variations of the ~ superstructure spots. (b) Auger intensity measurements (solid line).
134
G. Le Lay, J.J. Métois
/
Chemisorption of Ph on Ge(111)
also proof that the 1 )< 1 LEED pattern is that of a phase desorbing in a 2D island fashion. This point merits further comments: LEED intensity measurements are sometime used to plot equilibrium adsorption isotherms in physisorption systems like, for example, the adsorption at low temperatures of rare gases on graphite [3] or metal surfaces [4]; in the case of monolayer overgrowth, the LEED intensity is a direct measure of the coverage [5]. To our knowledge this fact has been used to clarify the growth mode of some strong chemisorption systems (i.e. H2/W(l00) [6], 02 and H/Rh(l 11) [7]), but not in the case of a metal deposit like Pb/Ge(l 11) (bonding energy 2 eV). Our measurements thus prove that the LEED intensity measurements are a solid alternative to the Auger intensity ones. This is especially interesting when the Auger peaks of both substrate and deposit overlap. Typically, in the case of adsorption of lead on silicon, a couple for which the major Auger lines are very close (Si LVV 92 eV, Pb N00 94 eV), this method should be very promissing. In the case of Pb/Ge(l 11) the marked advantage of this method appears at low coverage. At 9 0.1 we notice a strong increase of the LEED intensities of the substrate integer spots which reach a plateau at 8 0 when all lead atoms have desorbed. Correlatively, one observes the appearance of weak thirdThis order 1 unit mesh. is the LEED patterns corresponding to a newappears x V’different from the aspots newin superstructure, denoted by V’It which previous ones (V1 or V1*) from the point of view of (i) its obtention, (ii) the coverage below 0.1 monolayer where it is observed, and (iii) the remarkable point that its strongest intensity is obtained at very low coverage, 9 0.03, as shown in fig. 2 (let us mention that no foreign impurity is detected in the Auger spectrum). The observation of a V’1~ superstructure at such a low coverage is quite unusual: it cannot be assigned to an ordered top surface array of Pb atoms; it has to be a Ge substrate structure induced by the presence of a very small quantity of lead. A similar situation was found once for silicon substrates [8]: high lifetime Si(1 11) samples annealed for several minutes between 850 and 950°C under UHV displayed a %/1 x V1 surface structure which was considered as an impurity stabilized structure at very low coverage (carbon or metallic impurities below detection limit being responsible for this stabilization). To conclude, we emphasize the fact that LEED intensity measurements in a case of chemisorption are under some circumstances a valuable method that can replace Auger intensity measurements, especially when deposit and substrate peaks overlap. Typically, we have shown in the case of Pb/Ge( 111) that desorption isotherms can be plotted with greater sensitivity by the former method. We have also shown that the ~/1x %/1 LEED patterns observed with this system correspond in fact to three different superstructures, the first and second being due to an ordered overlayer of lead atoms respectively completed —
=
=
:
G. Le Lay, J.J. Métois
/
Chemisorplion of Pb on Ge(1 11)
135
at 9 1/3 and 8 1, and the third being an induced germanium superstructure stabilized by a very small number of lead atoms. =
=
Note added in proof Analogous observations by RHEED [9] have recently been published (after submission of our manuscript). In many respects they corroborate our LEED observations, but the interpretation of the author is somewhat different from ours. We will discuss this in more detail in a forthcoming paper.
References [I] [2] [3] [4] [5] [6] [7] [8] [9]
J.J. Métois and G. Le Lay, Surface Sci. 133 (1983) 422. P.J. Estrup and J. Morrison, Surface Sci. 2 (1964) 465. J. Suzanne, G. Albinet and M. Bienfait, J. Crystal Growth 13/14 (1972) 164. J. Unguris, L.W. Bruch, ER. Moog and M.B. Webb, Surface Sci. 109 (1981) 522. E.D. Williams and M.H. Weinberg, Surface Sci. 109 (1981) 574. P.J. Estrup and JR. Anderson, J. Chem. Phys. 45 (1966) 2254. J.T. Yates, P.A. Thiel and W.H. Weinberg, Surface Sci. 82 (1979) 45. RN. Thomas and M.H. Francombe, Surface Sci. 25 (1971) 357. T. Ichikawa, Solid State Commun. 46 (1983) 827.
131
COMMUNICATION CHEMISORPTION OF LEAD ON GERMANIUM (III): LEED INTENSITY MEASUREMENTS G. LE LAY
*
and J.J. METOIS
Centre de Recherche sur les Mécanismes de Ia Croissance Cristalline, CNRS, Campus the Luminy, Case 913, F- 13288 Marseille Cedex 09, France
Received 1 June 1983; accepted for publication 19 July 1983
LEED observations and AES studies of the Pb/Ge(l II) system reveal three distinct superstructures with the same X ~ unit mesh. Two of these are two-dimensional phases corresponding to lead overlayers respectively completed at 9 = 1/3 and 9 = I; the third is a new reconstruction of the germanium surface induced by a very small number of lead atoms. We also show that the kinetics of desorption of an adsorbate can be accurately studied by LEED intensity measurements.
In a recent paper [1] we have carried out a detailed chemical, structural and morphological study of the Pb/Ge(l 11) system using AES, LEED and SEM. We could show that at RT this system exhibits the Stranski—Krastanov growth mode, the two-dimensional (2D) adsorbed layer being completed at monolayer coverage (0 1 defined in substrate units, i.e. 7.2 l0’~atoms cm2). This 2D layer displays a Ge(1 1 l)%/~X R 300 (in short Vi) LEED superstructure pattern. The ~ superstructure is the most frequent one and is commonly observed on Si(lll) and Ge(lll) with almost all deposited metals. However we had noticed a special behaviour of this superstructure in the case of Pb/Ge( 111). In this letter we will briefly recall these peculiarities and give additional experimental evidence from LEED intensity measurements. (i) As long as 0 ~ 1/3, we observe at RT a very stable ~/i structure which persists until all lead is evaporated in the desorption temperature range (T 350°C) (ii) Beyond 9 1/3 and up to 8 1, while the intensity of the (2/3, 2/3) spots increases with the coverage, the intensity of the (1/3, 1/3) spots strongly decreases in such a manner that at 8 1 these spots are hardly observed (fig. 1, curve a). This situation exists up to T 280°C. But in a narrow range of temperature around 280°C(thus markedly below the desorption temperatures) =
—
=
=
=
*
Also UER de Physique, University Aix—Marseille I, Marseille, France.
0378-5963/83/0000—0000/$03.Oo
©
1983 North-Holland
132
G. Le Lay, J.J. Métois
/
Chemisorption of Pb on Ge(111)
we observed a reversible transition to a 1 X 1 structure. (iii) If we evaporate the metal onto the germanium substrate beyond this transition temperature, we first observe third order superstructure spots with 0(0)
~(2/32i3)
p(2/32/3)
/ 2 U,
1r 1l I 0
1
~
(ii3 1/3) (i~3i/3)~
1/3
2/3
I
Fig. I. LEED intensity variations (optical density measurements) of superstructure spots versus coverage V’i and ~ superstructures: (a) RT, 41 V; (b) 320°C, 40 V (beyond the transition temperature).
increasing intensity up to 0 1/3, the position of the maximum; beyond 0 1/3 these spots vanish leaving the 1 x 1 pattern (fig. 1, curve b). (iv) Moreover, we have not noticed any change of the spot size during these observations, nor of the general behaviour of the LEED features in the narrow range of incident energy investigated (30—50 eV). We thus observe two kinds of LEED patterns corresponding to the same x ~/i unit mesh but in fact to two different surface phases. The first phase (which we denote by Vi) is very stable and is completed at 8 1/3, as fig. 1 testifies. The second one, denoted by ~/i is completed at 9 1, as Auger adsorption curves ascertain (see ref. [1]). Studying by LEED the Pb/Si(l 11) system, Estrup and Morrison [2] had also observed two kinds of ~/i patterns with (0 < 1/3) or without (8 1)1/3, 1/3 spots, but as no detailed annealing experiments were performed, the question whether a reversible phase transition also occurs in this case is still open. The Ge(111)l x I Pb structure following the ~/~* ~ 1 X 1 transition at 280°C is observed during almost the whole desorption process, which we =
=
=
‘~,
=
=
—
—
G. Le Lay, J.J. Métois
/
133
Chemisorption of Pb on Ge(111)
followed by AES at constant temperature (isothermal desorption kinetics measurements), the desorption isotherms plotted in the coverage range 1 > 9> 0.1 showed a linear decrease of the lead Auger intensity (peak-to-peak height of the Pb 94 eV Auger line) versus time. Below about 9 0.1, overlap with substrates peaks makes it difficult to determine accurately peak heights at these low coverages. We thus performed more sensitive desorption experiments, now plotting the integrated intensity of the substrate integer spots versus time at constant desorption temperature. One isotherm recorded in this manner is shown in fig. 2, in comparison with the same isotherm obtained as usually with AES. We note a linear increase of the LEED intensity of the substrate integer spots versus desorption time, i.e., versus decreasing instantaneous coverage (the Auger amplitude yields the scaling factor r 8 indicated on the abscissa) until 0 — 0.1. This is in perfect agreement with the previous Auger data which also indicated zero-order kinetics. The increase of the LEED intensity when the lead coverage decreases is not surprising as the thermal vibrations of the adsorbed lead atoms are more important than those of the substrate germanium atoms, as is testified by the comparison of the Debye temperatures of lead (88 K) and germanium (290 K). The linearity of the variation of this intensity is —
~
I
0(0) 0:1
8.23
8.13
(b)
&-1/10
0~0
T=350°C
—
/
z
/
LU
\ LU
/
—
m —
a)
— s.~
—4
/
\.
.
(lii
,/A~(2I3~2!3)
~
*.~_,
0
10
20
30
40
mn
Fig. 2. Desorption isotherm T = 350°C: (a) LEED intensity measurements (40 V) (dotted line), average intensity of the substrate (1,1) integer spots and intensity variations of the ~ superstructure spots. (b) Auger intensity measurements (solid line).
134
G. Le Lay, J.J. Métois
/
Chemisorption of Ph on Ge(111)
also proof that the 1 )< 1 LEED pattern is that of a phase desorbing in a 2D island fashion. This point merits further comments: LEED intensity measurements are sometime used to plot equilibrium adsorption isotherms in physisorption systems like, for example, the adsorption at low temperatures of rare gases on graphite [3] or metal surfaces [4]; in the case of monolayer overgrowth, the LEED intensity is a direct measure of the coverage [5]. To our knowledge this fact has been used to clarify the growth mode of some strong chemisorption systems (i.e. H2/W(l00) [6], 02 and H/Rh(l 11) [7]), but not in the case of a metal deposit like Pb/Ge(l 11) (bonding energy 2 eV). Our measurements thus prove that the LEED intensity measurements are a solid alternative to the Auger intensity ones. This is especially interesting when the Auger peaks of both substrate and deposit overlap. Typically, in the case of adsorption of lead on silicon, a couple for which the major Auger lines are very close (Si LVV 92 eV, Pb N00 94 eV), this method should be very promissing. In the case of Pb/Ge(l 11) the marked advantage of this method appears at low coverage. At 9 0.1 we notice a strong increase of the LEED intensities of the substrate integer spots which reach a plateau at 8 0 when all lead atoms have desorbed. Correlatively, one observes the appearance of weak thirdThis order 1 unit mesh. is the LEED patterns corresponding to a newappears x V’different from the aspots newin superstructure, denoted by V’It which previous ones (V1 or V1*) from the point of view of (i) its obtention, (ii) the coverage below 0.1 monolayer where it is observed, and (iii) the remarkable point that its strongest intensity is obtained at very low coverage, 9 0.03, as shown in fig. 2 (let us mention that no foreign impurity is detected in the Auger spectrum). The observation of a V’1~ superstructure at such a low coverage is quite unusual: it cannot be assigned to an ordered top surface array of Pb atoms; it has to be a Ge substrate structure induced by the presence of a very small quantity of lead. A similar situation was found once for silicon substrates [8]: high lifetime Si(1 11) samples annealed for several minutes between 850 and 950°C under UHV displayed a %/1 x V1 surface structure which was considered as an impurity stabilized structure at very low coverage (carbon or metallic impurities below detection limit being responsible for this stabilization). To conclude, we emphasize the fact that LEED intensity measurements in a case of chemisorption are under some circumstances a valuable method that can replace Auger intensity measurements, especially when deposit and substrate peaks overlap. Typically, we have shown in the case of Pb/Ge( 111) that desorption isotherms can be plotted with greater sensitivity by the former method. We have also shown that the ~/1x %/1 LEED patterns observed with this system correspond in fact to three different superstructures, the first and second being due to an ordered overlayer of lead atoms respectively completed —
=
=
:
G. Le Lay, J.J. Métois
/
Chemisorplion of Pb on Ge(1 11)
135
at 9 1/3 and 8 1, and the third being an induced germanium superstructure stabilized by a very small number of lead atoms. =
=
Note added in proof Analogous observations by RHEED [9] have recently been published (after submission of our manuscript). In many respects they corroborate our LEED observations, but the interpretation of the author is somewhat different from ours. We will discuss this in more detail in a forthcoming paper.
References [I] [2] [3] [4] [5] [6] [7] [8] [9]
J.J. Métois and G. Le Lay, Surface Sci. 133 (1983) 422. P.J. Estrup and J. Morrison, Surface Sci. 2 (1964) 465. J. Suzanne, G. Albinet and M. Bienfait, J. Crystal Growth 13/14 (1972) 164. J. Unguris, L.W. Bruch, ER. Moog and M.B. Webb, Surface Sci. 109 (1981) 522. E.D. Williams and M.H. Weinberg, Surface Sci. 109 (1981) 574. P.J. Estrup and JR. Anderson, J. Chem. Phys. 45 (1966) 2254. J.T. Yates, P.A. Thiel and W.H. Weinberg, Surface Sci. 82 (1979) 45. RN. Thomas and M.H. Francombe, Surface Sci. 25 (1971) 357. T. Ichikawa, Solid State Commun. 46 (1983) 827.