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Roughening phenomena at the Au(110) surface
Roughening U. Romahn and
phenomena
‘, H. Zimmermann
W. Schommers
Received
1 October
at the Au( 110) surface ‘. M. Nold
‘I, A. Hoss
‘I. H. Giibel
h. P. von
Blanckenhagen
”
h
1990: accepted
for publication
3 January
1901
The temperature dependence of the structure of the Au(l10) surface has heen studied between 510 and 713 K \ample trmper,tturc hy LEED experiments, A strong broadening of the 23 eV specular reflex was found indicating that the surface hecomes rough alrcady below the order-disorder transition. At T> 7:. the peak wdth Increases much faster with temperature up to 70.1 K where ue ohserved a sudden change of the energy spectra.
1.
Introduction
The Au(l10) surface is reconstructed at low temperatures [l]. The (2 X 1 )-“missing row” structure shows an order-disorder transition with critical temperatures TL between 650 and 695 K depending on the influence of finite-size effects [2]. Whereas the surface structure and the order-disorder transition has been studied in great detail the roughening of this surface is not fully explored experimentally as yet. Model calculations predict a roughening transition at or above TL for the order-disorder transition [3]. Recently, solid-onsolid models have been applied for a description of roughening. pre-roughening and reconstruction at (llO)-surfaces [4]. Between the flat reconstructed state and the rough state a flat preroughening state is predicted for certain energetical conditions. A first indication for a high step density at the Au( 110)(2 x 1) surface has been found by an X-ray diffraction experiment [5]. The position and the width of the half-order Bragg peaks is influenced by the density of randomly distributed steps. But also the width of integral order peaks should depend on the step density under out-of-phase conditions [6]. Recently, X-ray diffraction experiments have shown that the Pt(110)(2 x 1) surface roughens above T, as derived from the observed 0039-602X ,91 ,/$03.50
1991
Elsevier Science
Puhlishcrs
H.V
oscillation of the position of half-order peaks [7]. We have studied the temperature dependence ol the structure of the Au( 110) surface in the temperature range between 510 and 713 K by LEED experiments. From the analysis of spot profiles 01 a specular reflex we found indication5 for roughening at this surface.
2. Experimental The sample crystal was oriented with respect to the ( 110) planes with an accuracy better than 0.2” averaged over the hole area of 10 mm diameter. The polished (110) -crystal surface was cleaned hb Ar ’ sputtering (l-2 keV) and subsequent annealing up to 850 K. The cleanliness of the surface wa:, proved by AES. Finally the contamination wax below the detection limit ( I lq of a monolayer). After annealing of the crystal above 800 K for several hours Ca contamination was detected. The origin of the Ca at the surface is not clear :I> \et. In order to prevent any influence of this (‘a on-the results the cleaning procedure was repeated before every measurement of a few hours duration. The AES peak ratio Ca (291 eV)/Au(69 eV) nab with consideration of the different re1atiL.e aensitivity less than 0.001 before and after measurements.
U. Romahn
et al. / Roughening
The reconstructed and long-range ordered surface was obtained after additional sputtering during 10 min, and 5-10 h annealing at 506 K. The diffraction intensities were measured by a Video-LEED system with a reverse view optics.
3. Results and discussion Spot profiles (r(Q)) and energy spectra (I(E)) of specular Bragg peaks have been analyzed as a function of temperature. Let us first characterize the initial state of the Au(ll0) surface. In fig. 1 a typical diffraction pattern of the reconstructed structure taken at
-0.5
-1.0
-0.5
phenomena
at the Au(l IO) surface
657
room temperature (a), and the pattern of the structure as obtained after heating the sample up to 730 K for more than 30 min, and cooling down to room temperature (b) are shown. Pattern (b) contains additional peaks besides the superstructure peaks. The pattern (in fig. lb) shows that the reconstructed surface structure is not reproduced, i.e. the order-disorder transition is not reversible. This holds also for different annealing and cooling procedures. Also during these experiments the sensitivity-corrected AES peak ratio Ca(291 eV)/Au(69 ev) was below I’?&. A generation of facets has been observed by earlier LEED experiments (e.g. ref. [S]) and by scanning tunneling microscopy [9]. We observed that the surface be-
0
Qrwo,IRLUl
Qr,oo~ IRLUI
0.5
0.5
1.0
Fig. 1. Diffraction pattern for E = 45 eV at room temperature: incidence angle 8, = 15O. (a) taken at 300 K after sputtering and annealing at 506 K. (b) after heating the crystal to Tr TC and cooling down to room temperature. One unit mesh corresponds to 0.018 reciprocal lattice units (RLU = tv/a, a = 0.408 nm).
I -0.30
-0.20
0
-0.10
0.10
0.20
0.30
Ott [ RLU I
came completely ordered after short sputtering (-- 2 min. 1 kV Ar+) at 506 K. It seems by ion impacts during sputtering the activation energy for the formation of the perfect (2 x 1) structure is provided. On the other hand, the phase transition is reversible if the surface is not clean. Obviously,
in this case the complete reconstruction is induced by impurities. In ref. Cl.01 it has been shown that by surface contamination also q can be reduced. The 23 eV specular peak broadens with increasing temperature (fig. 2). At high temperature it. can he described by a Lorentzian as expected by
F
T
60 ; 40 ‘? 20 H 0 500
540
580
620
660
700
T IKI Fig. .i. Twtperaturr dependence of the FWHM (a) and the integral intenart\: I (h).
1
659
U. Romahn et al. / Roughening phenomena at the Au{1 IO) surface
16 12 ;
8
z
4
$ : ti 2 .rl
I B
!!!
T = 713 K
0 6 T = 664 K
6
4 2 0
5
15
25
35
45
65
E 1 eV I Fig. 4. Low energy part of the I(E) spectra for the specular reflex at two characteristic temperatures. 8, = 13”.
the theory [ll]. The peak width starts to increase already below T, (fig. 3a) and reaches a maximum at 703 K. A narrowing of the specular peak as observed above 703 K was reported in the literature for the Pb(ll1) surface and indicates a smoothing of the surface [12]. The integral intensity of the peaks as shown for three temperatures in fig. 2 decreases monotonously with increasing temperature (fig. 3b). The Z(E) spectra show a qualitative change in a narrow temperature range at T > 703 K (fig. 4). With exception of the 13 eV peak, which is according to the large penetration depth of the 13 eV electrons due to diffraction by the bulk, the spectra become diffuse. The diffuse intensity distribution could reflect the Fourier-transform of the height-height correlation function due to a disordered overlayer. We may conclude that the observed peak broadening and change of the energy spectra is qualitatively understandable by the development of pre-roughening, roughening and smoothing of the Au(ll0) surface with increasing temperature. The roughening and the smoothing is not fully reversible. A more detailed study of the temperature dependence of the Au(ll0) surface structure, in particular of the time constants of the roughening phenomena, is going on.
Note added in proof Due to a wrong calibration atures,
the sample temper-
T, given in the paper have to be corrected
by the relation:
1.06T - 11.8.
References 111W. Moritz and D. Wolf, Surf. Sci. 88 (1979) L29. PI D.E. Clark. W.N. Unertl and P.H. Kleban,
Phys. Rev. B 34 (1986) 4379. [31 J. Villain and L. Vilfan, Surf. Sci. 190 (1986) 4379; A.C. Levi and M. Touzani, Surf. Sci. 218 (1989) 223. [41 M. den Nijs, Phys. Rev. Lett. 64 (1990) 435. [51 I.K. Robinson, Y. Kuk and L.C. Feldman, Phys. Rev. B 29 (1984) 4762. [61 P. Fenter and T.M. Lu, Surf. Sci. 154 (1985) 15. [71 I.K. Robinson, E. Vlieg and K. Kern, Phys. Rev. Lett. 63 (1989) 2578. and D. Wolf, Surf. Sci. 77 181 W. Moritz, H. Jagodzinski (1978) 2491. 191 G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Surf. Sci. 131 (1983) L384. [lOI E.G. McRae, T.M. Buck, R.A. Malic and G.H. Wheatley. Phys. Rev. B 36 (1987) 2341. [111 M. Henzler, Appl. Surf. Sci. 11 (1982) 450. [121 H.-N. Yang, T.-M. Lu and G.C. Wang, Phys. Rev. Lett. 62 (1989) 2148.
phenomena
‘, H. Zimmermann
W. Schommers
Received
1 October
at the Au( 110) surface ‘. M. Nold
‘I, A. Hoss
‘I. H. Giibel
h. P. von
Blanckenhagen
”
h
1990: accepted
for publication
3 January
1901
The temperature dependence of the structure of the Au(l10) surface has heen studied between 510 and 713 K \ample trmper,tturc hy LEED experiments, A strong broadening of the 23 eV specular reflex was found indicating that the surface hecomes rough alrcady below the order-disorder transition. At T> 7:. the peak wdth Increases much faster with temperature up to 70.1 K where ue ohserved a sudden change of the energy spectra.
1.
Introduction
The Au(l10) surface is reconstructed at low temperatures [l]. The (2 X 1 )-“missing row” structure shows an order-disorder transition with critical temperatures TL between 650 and 695 K depending on the influence of finite-size effects [2]. Whereas the surface structure and the order-disorder transition has been studied in great detail the roughening of this surface is not fully explored experimentally as yet. Model calculations predict a roughening transition at or above TL for the order-disorder transition [3]. Recently, solid-onsolid models have been applied for a description of roughening. pre-roughening and reconstruction at (llO)-surfaces [4]. Between the flat reconstructed state and the rough state a flat preroughening state is predicted for certain energetical conditions. A first indication for a high step density at the Au( 110)(2 x 1) surface has been found by an X-ray diffraction experiment [5]. The position and the width of the half-order Bragg peaks is influenced by the density of randomly distributed steps. But also the width of integral order peaks should depend on the step density under out-of-phase conditions [6]. Recently, X-ray diffraction experiments have shown that the Pt(110)(2 x 1) surface roughens above T, as derived from the observed 0039-602X ,91 ,/$03.50
1991
Elsevier Science
Puhlishcrs
H.V
oscillation of the position of half-order peaks [7]. We have studied the temperature dependence ol the structure of the Au( 110) surface in the temperature range between 510 and 713 K by LEED experiments. From the analysis of spot profiles 01 a specular reflex we found indication5 for roughening at this surface.
2. Experimental The sample crystal was oriented with respect to the ( 110) planes with an accuracy better than 0.2” averaged over the hole area of 10 mm diameter. The polished (110) -crystal surface was cleaned hb Ar ’ sputtering (l-2 keV) and subsequent annealing up to 850 K. The cleanliness of the surface wa:, proved by AES. Finally the contamination wax below the detection limit ( I lq of a monolayer). After annealing of the crystal above 800 K for several hours Ca contamination was detected. The origin of the Ca at the surface is not clear :I> \et. In order to prevent any influence of this (‘a on-the results the cleaning procedure was repeated before every measurement of a few hours duration. The AES peak ratio Ca (291 eV)/Au(69 eV) nab with consideration of the different re1atiL.e aensitivity less than 0.001 before and after measurements.
U. Romahn
et al. / Roughening
The reconstructed and long-range ordered surface was obtained after additional sputtering during 10 min, and 5-10 h annealing at 506 K. The diffraction intensities were measured by a Video-LEED system with a reverse view optics.
3. Results and discussion Spot profiles (r(Q)) and energy spectra (I(E)) of specular Bragg peaks have been analyzed as a function of temperature. Let us first characterize the initial state of the Au(ll0) surface. In fig. 1 a typical diffraction pattern of the reconstructed structure taken at
-0.5
-1.0
-0.5
phenomena
at the Au(l IO) surface
657
room temperature (a), and the pattern of the structure as obtained after heating the sample up to 730 K for more than 30 min, and cooling down to room temperature (b) are shown. Pattern (b) contains additional peaks besides the superstructure peaks. The pattern (in fig. lb) shows that the reconstructed surface structure is not reproduced, i.e. the order-disorder transition is not reversible. This holds also for different annealing and cooling procedures. Also during these experiments the sensitivity-corrected AES peak ratio Ca(291 eV)/Au(69 ev) was below I’?&. A generation of facets has been observed by earlier LEED experiments (e.g. ref. [S]) and by scanning tunneling microscopy [9]. We observed that the surface be-
0
Qrwo,IRLUl
Qr,oo~ IRLUI
0.5
0.5
1.0
Fig. 1. Diffraction pattern for E = 45 eV at room temperature: incidence angle 8, = 15O. (a) taken at 300 K after sputtering and annealing at 506 K. (b) after heating the crystal to Tr TC and cooling down to room temperature. One unit mesh corresponds to 0.018 reciprocal lattice units (RLU = tv/a, a = 0.408 nm).
I -0.30
-0.20
0
-0.10
0.10
0.20
0.30
Ott [ RLU I
came completely ordered after short sputtering (-- 2 min. 1 kV Ar+) at 506 K. It seems by ion impacts during sputtering the activation energy for the formation of the perfect (2 x 1) structure is provided. On the other hand, the phase transition is reversible if the surface is not clean. Obviously,
in this case the complete reconstruction is induced by impurities. In ref. Cl.01 it has been shown that by surface contamination also q can be reduced. The 23 eV specular peak broadens with increasing temperature (fig. 2). At high temperature it. can he described by a Lorentzian as expected by
F
T
60 ; 40 ‘? 20 H 0 500
540
580
620
660
700
T IKI Fig. .i. Twtperaturr dependence of the FWHM (a) and the integral intenart\: I (h).
1
659
U. Romahn et al. / Roughening phenomena at the Au{1 IO) surface
16 12 ;
8
z
4
$ : ti 2 .rl
I B
!!!
T = 713 K
0 6 T = 664 K
6
4 2 0
5
15
25
35
45
65
E 1 eV I Fig. 4. Low energy part of the I(E) spectra for the specular reflex at two characteristic temperatures. 8, = 13”.
the theory [ll]. The peak width starts to increase already below T, (fig. 3a) and reaches a maximum at 703 K. A narrowing of the specular peak as observed above 703 K was reported in the literature for the Pb(ll1) surface and indicates a smoothing of the surface [12]. The integral intensity of the peaks as shown for three temperatures in fig. 2 decreases monotonously with increasing temperature (fig. 3b). The Z(E) spectra show a qualitative change in a narrow temperature range at T > 703 K (fig. 4). With exception of the 13 eV peak, which is according to the large penetration depth of the 13 eV electrons due to diffraction by the bulk, the spectra become diffuse. The diffuse intensity distribution could reflect the Fourier-transform of the height-height correlation function due to a disordered overlayer. We may conclude that the observed peak broadening and change of the energy spectra is qualitatively understandable by the development of pre-roughening, roughening and smoothing of the Au(ll0) surface with increasing temperature. The roughening and the smoothing is not fully reversible. A more detailed study of the temperature dependence of the Au(ll0) surface structure, in particular of the time constants of the roughening phenomena, is going on.
Note added in proof Due to a wrong calibration atures,
the sample temper-
T, given in the paper have to be corrected
by the relation:
1.06T - 11.8.
References 111W. Moritz and D. Wolf, Surf. Sci. 88 (1979) L29. PI D.E. Clark. W.N. Unertl and P.H. Kleban,
Phys. Rev. B 34 (1986) 4379. [31 J. Villain and L. Vilfan, Surf. Sci. 190 (1986) 4379; A.C. Levi and M. Touzani, Surf. Sci. 218 (1989) 223. [41 M. den Nijs, Phys. Rev. Lett. 64 (1990) 435. [51 I.K. Robinson, Y. Kuk and L.C. Feldman, Phys. Rev. B 29 (1984) 4762. [61 P. Fenter and T.M. Lu, Surf. Sci. 154 (1985) 15. [71 I.K. Robinson, E. Vlieg and K. Kern, Phys. Rev. Lett. 63 (1989) 2578. and D. Wolf, Surf. Sci. 77 181 W. Moritz, H. Jagodzinski (1978) 2491. 191 G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Surf. Sci. 131 (1983) L384. [lOI E.G. McRae, T.M. Buck, R.A. Malic and G.H. Wheatley. Phys. Rev. B 36 (1987) 2341. [111 M. Henzler, Appl. Surf. Sci. 11 (1982) 450. [121 H.-N. Yang, T.-M. Lu and G.C. Wang, Phys. Rev. Lett. 62 (1989) 2148.