Order-disorder phase transition of the Cu3Au(100) surface studied by ToF-ion scattering

Nuclear Instruments and Methods in Physics Research B I I8 (I 996) 467-472

Beam Interactions with Materials&Atoms

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Order-disorder phase transition of the Cu, Au( 100) surface studied by ToF-ion scattering L. Houssiau

* , P. Bertrand

L’niversitCCathdique de Lnuoain, PCPM, I plrrce Croix du Sud. B-1348 Louuuin-lu-Neuoe. Belgium Abstract Cu,Au( 100) surface has been investigated with ToF-ISS at different temperatures below and above the bulk order-disorder transition temperature (T, = 663 K). Grazing angle azimuthal scans on the surface reveal the crystallinity of the first monolayer. The polar scans measured by 2 keV Ne ion scattering aligned along the major orientations, i.e. ( 100) and ( I IO), show that the surface is terminated at room temperature by an ordered and rippled CuAu layer. The (100) rows are alternatively pure Cu rows and pure Au rows while the ( 110) rows are made by an alternated sequence of Cu and Au atoms. When increasing the temperature close to T,, the Au focusing peak at low incidence, along the (1 IO) rows, broadens in a first time owing to thermal vibrations, then shifts rapidly to higher incident angles when we approach the transition. This effect is interpreted as the result of the order-disorder transition at the surface, which modifies the ordered atomic sequence constituting the rows. By plotting the Au scattering yield vs. temperature at fixed azimuthal and incident angles, a strong decrease of the signal near the transition is observed. However, the transition is not abrupt, indicating possibly a second-order phase transition at the first monolayer, in opposition to the first-order bulk transition,

1. Introduction Cu,Au is a material which exhibits an order-disorder phase transition into the bulk at T, = 663 K. This bulk transition is first-order. The transition at the surface has been intensively studied in recent years in order to determine whether it is of a different type than in the bulk, as predicted theoretically [l-4]. AI1 the surface order studies were achieved by diffraction studies, in which the superlattices spots associated with the long-range order (LRO) vanish during the transition. Most of the analyses were made by LEED [5-7] and its variants (spin polarised LEED [8,9]), but several people used X-rays diffraction [lo,1 11 and X-ray photoelectron diffraction (XPD) [12]. All these techniques indicated that the transition is certainly not first order in the sense that the surface LRO starts to diminish continuously well below T, and disappears at T,. Segregation studies were also made, mainly by Buck et al. [13], who studied the composition of the two first monolayers of Cu,Au(lOO) with LEIS. They proved that Au concentration remains constant before T, and does not change abruptly after T,, due to a segregation of Au at the surface. They showed that the surface, which can have

two possible terminations, one pure Cu layer or one mixed CuAu layer, is of the second type: it contains 50% Au and 50% Cu, at room temperature. This has been confirmed later on by other works [9,14]. Our results give the first approach of the surface order-disorder transition by a non-diffractive technique: ToF-ISS. This technique is widely used for surface structure analyses [IS], because the scattering intensities are highly dependent on the azimuthal and incident angles. It is then possible to determine directly atomic locations, surface periodicity, surface reconstruction and/or relarations. In this work we show that the emergence of disorder at the surface, which does not involve lattice changes or surface composition modification [13], leads nevertheless to strong variations in the scattering intensity. In a first time, the structure analysis has been performed at room temperature and then the sample has been heated at different temperatures below and above T,. It is shown how the order-disorder transition modifies the atomic sequences in atomic rows, then changing the double scattering conditions.

2. Experimental

* Corresponding author. Tel. +32 10 473582, fax +32 IO 473452, e-mail: [email protected].

The experiments were Et-formed in an ultra-high vacuum chamber with a base pressure of 6 X IO-” Torr,

0168-583X/%/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved SSDIOl68-583X(95)01476-4

VII. SURFACES/INTERFACES

L. Houssiau, P. Bertrand/ Nucl. Instr. and Meth. in Phys. Res. B I I8 (19961467-472

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due to the good thermal conductivity of the sample and of the sample-holder.

3. Results and discussion 3.1. The Cu, Au(100) surface structure at room temperature

-7.0

0

20

40

Azimuthal

60 8ngle

80

100

120

(de&)

Fig. I. 2 keV Ar azimuthal scan on Cu, Au( 100)at 15”incidence.

rising to 6 X 10T9 Torr with the noble gas beam on. The sample is fixed at the center of the chamber on a dual-axis manipulator, controlled by two step-by-step motors. The incident ion beam is pulsed at 50 kHz by deflection plates combined with a diaphragm. The averaged pulsed current is 30 pA, the ion pulse width is about 100 ns and the beam spot is 1 mm in diameter. The backscattered particles are collected at scattering angle 0 = 143” into a microchannel plate located at 335 mm flight distance from the sample. A channeltron is also fixed at scattering angle 0 = 22.5” in order to provide direct recoil analysis. available at The Cu,Au sample, commercially Monocrystals Company (Cleveland, Ohio), is pure and stoichiometric. It was polished mechanically with 1 p,rn alumina. Before analysis, the sample was cleaned by repeated cycles of 1 keV Ar sputtering followed by a 800 K annealing during = 1 h. The presence of residual impurities was checked with direct recoil analysis, until the C and 0 peaks are not detectable anymore. After this standard cleaning procedure, the crystallinity of the sample surface was controlled by taking azimuthal scans. These are obtained by measuring the 2 keV Ar on Au scattering peak area vs. the azimuthal angles, for a grazing incidence (typically 15”). When the beam is parallel to the major atomic rows, the scattered intensity is minimum, owing to the strong shadowing on the nearest neighbours, which are very close in packed rows. This can identify if the structure is well annealed, if it is very crystalline, and locates the major atomic rows. Fig. 1 shows such a measurement: the observed minima, repeated every 45”, an angle consistent with the surface periodicity, are easily associated with the (100) and ( 110) rows. We note that the amplitude of the variations and the good symmetry of the plot indicate an excellent annealing. The sample is heated at its rear face by a filament encapsulated in ceramics. The temperature is measured by two thermocouples clipped as close as possible to the sample: this may not give exactly the actual surface temperature, but the error should not be too large,

When considering the Cu,Au lattice, it appears that the (100) planes are alternatively CuAu layers and pure Cu layers. It results that two truncations are possible, then the first experiment to do aims at determining how the surface is terminated. This can be done by taking polar scans, i.e. plots of the backscattered intensity vs. the incident, or polar, angle. In ICISS mode, this measurement is sensitive to the atomic position at the surface [ 16,171. Fig. 2 shows a typical ToF-ISS spectrum of 2 keV Ne scattering on Cu,Au( 100). The first peak is due to scattering on Au and the second to scattering on Cu. The Au scattering peak is a sharp and intense peak, while the Cu scattering peak is a short peak above a large background, due to complex multiple collisions. To take the polar scans, the (quasi-) single scattering peaks on each element have to be evaluated. This is done easily in the Au case by taking the area of a short window centred in the first peak maximum. In the Cu case, a linear background subtraction under the peak is performed and the area above this background is considered. To gain information on the outermost layer, the low incidence (0”~40”) part of the polar plots will be analysed in details. Figs. 3a and 3b present the polar plots taken by scattering 2 keV Ne along the azimuth (100) (a) and along (110) (b). The backscattered signals issued from Au and from Cu are plotted in the same graphs. Along the (100) rows, 2 keV Ne scattering at grazing incidence really analyses the outermost layer, whatever the

IcGfJ F---

Ob

0

2

4 6 Time-of-flight (~1

8

IO

Fig. 2. Typical 2 keV Ne ToF-ISS spectrum on Cu, Au(100): the peak intensities are the grey areas.

L. Houssiau, P. Bertrand/ Nucl. Instr. and Meth. in Phys. Res. B 1 I8 (1996) 467-472

truncation is: as a strong Au signal is detected, it is evident that the surface is Au rich. It can be concluded that the surface is CuAu terminated. In Fig. 3a, the Cu signal increases before the Au signal. It is consistent with the model, because all (100) rows are either pure Cu rows or pure Au rows. All the Cu atoms are shadowed by Cu atoms and all Au atoms are shadowed by Au atoms. As the Ne on Au shadow cones (calculated in Ref. [ 181)are much wider than the Ne on Cu shadow cones, it results that the critical angle for a Cu shadowing is smaller than the one for Au shadowing. Along the (110) rows, the second monolayer becomes now visible. At the first layer, the primary ions scatter on the ( 1IO) rows, which are made by an alternated sequence of Cu and Au atoms (-Cu-Au-

! ,

35000

I I

Cu-Au-): the Au atoms are now always shadowed by Cu atoms, and vice versa. At the second pure Cu layer, all rows are pure Cu rows: Cu atoms are always shadowed by Cu atoms. However, the experiment indicates a large shift (= 3”) between the Au and Cu signal, although these signals are all due to Cu shadowing. The anomalous low critical angle of the Au signal is explained by a rippling at the first monolayer, i.e. that Au atoms are located above Cu atoms. This important effect, theoretically predicted [19,20], will be described in detail in a forthcoming paper [21]. This model proposed for the Cu,Au(lOO) surface, i.e. an Au-rich first layer and a pure Cu second layer, is the same as given by other authors, experimentally [9,13,14] and theoretically [ 19,201.

/ I I I

, 1

I , , I

,

3000

2500 25000

10000 500

5ooo 0 20 Incident

30 angle

(deg.)

0 0

10

20 Incident

Fig. 3. 2 keV Ne polar scans on Cu,Au(lOO)

469

40

30 angle

along the (100)

50

(deg.)

azimuth (a) and the (110) azimuth (b). VII. SURFACES/INTEl7FACE!S

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470

”

5

10

I5 20 25 Polar snele (deg.1

M

35

Fig. 4. 2 keV Ne polar scans along ( 110) azimuth at 300 K, 623 K and 673 K. The difference between the scans at 673 K and 623 K is also plotted.

3.2. Temperature effects at the Cu, Au(100) surface The aim of this work is not only to provide a detailed structure analysis of the surface, but also to observe the order-disorder phase transition at the outermost layer. Fig. 4 presents the polar plots taken at 300 K, 623 K and 673 K. When increasing temperature, the atomic vibrations are enhanced and the critical shadowing angles are distributed. This leads to a broadening of the focusing peak. The intensity at grazing incidence increases because the atoms become more and more visible with thermal vibrations. We would expect these plots to broaden continuously when increasing temperature, but that is not the case from 623 to 673 K, which is 10 K above c. At 673 K, the polar plot is suddenly shifted towards higher angles: this completely inverted behaviour means that a transition occurs at the surface. The difference between the two curves is also represented in Fig. 4. At grazing incidence, the intensity tends to decrease, although we would expect an increase. This decrease is maximum at 15”, where it reaches - 16%. This effect may be interpreted as the creation of disorder in the first monolayer (Fig. 5). In the ordered state, the Au atoms are always preceded by Cu atoms. This means

that, at low incidence, the collisions with Au are preceded by a small angle collision with Cu. When disorder arises at the surface, the perfect Cu-Au altemance of the rows disappears, and new Au-Au couples of atoms are created, new double collisions with Au-Au may happen. Owing to the relatively narrow shadow cones of Ne on Cu, and to the fact that Cu atoms lie slightly below Au atoms, it is clear that the scattering will be strongly reduced on Au-Au couples, because Au has large shadow cones and it lies at the same level as the neighbouring Au atom. The anomalous shift of the polar plots is then interpreted in terms of creation of Au-Au couples of atoms at the surface, due to the occurrence of disorder. 3.3. Observation of the order-disorder

transition

In our experiments, there are three degrees of freedom: the polar angle, the azimuthal angle and the temperature. The backscattered intensity is always measured as a function of one of these parameters, the two others remaining constant. This was done when taking polar and azimuthal scans with fixed temperature and, respectively, azimuthal and incident angle. Now the geometry of the experiment (azimuthal and incident angles) will be fixed and the temperature will be changed, in order to observe the effect of the phase transition. The previous results suggest the choice of a 2 keV Ne scattering along the azimuth ( 110) at 15” incidence (Fig. 6). Before 630 K, the curve has the normal evolution. Owing to the increased thermal vibrations, more and more Ne ions are allowed to backscatter towards the detector. After 630 K, this increase stops and the intensity severely falls: this is the beginning of the order-disorder phase transition. After 690 K, the transition is finished and the Au signal tends to follow the thermal increase again. When cooling the sample, the measurement indicates a hysteresis: at first the intensity falls, because thermal vibration is reduced, but it tends to increase at about 660 K. It seems however that the reverse transition

Top views

ordr?red.smrhce

Dlsordmdsurlm

Fig. 5. Ordered and disordered Cu, Au(100) surface strucNre (black circles are Au atoms and white circles are Cu atoms). Note the surface rippling in the (1 IO> cuts.

300

400

500 600 Temperature (K)

700

800

Fig. 6. 2 keV Ne scattering intensity on Au vs. temperature at 15” incidence, along azimuth (110).

L. Houssiuu,

P. Bertrand/

471

Nucl. Instr. and Meth. in Phys. Res. B I18 (1996) 467-472

is not finished and that the surface remains in a partially disordered state. This may be due to the relatively short time elapsed between two measured points, typically 5 min. This experiment has been reproduced many times, with other incident angle (lo”), other heating or cooling rates, other annealing conditions, and the effect of the transition has always been observed. It was however difficult to reproduce exactly the absolute variations, because of experimental and physical reasons. Experimentally, these measurements require a very stable beam current and very low fluctuating noise. Physically, the experiment depends heavily on the good crystallinity of the surface. We observed that the annealing after a continuous beam irradiation may lower the intensity by a factor of 2, due to the fact that annealing restores the surface from a (partially) amorphous state to a well-crystalline state. At grazing incidence, the surface defects allow backscattering, that is normally prohibited by the shadowing effect. So, the creation of defects at the surface has to be avoided as much as possible, but this is difficult to control because ion scattering generates such defects. Finally, this experiment depends on the crystal history itself, i.e. annealing times. number of cooling and heating cycles and a kind of crystal ageing due to irradiation and some slight plastic deformation caused by differential thermal expansion.

0

t

450

,

500

(

-.L_iLL_-Ll

I

550

600

Temperature

Fig. 7. Approximative

650

700

750

(K)

evolution of the LRO parameter S with the

temperature.

Au-Au pairs, we can write the following tion. where 1 is the scattered intensity:

s*+ 1

=--__I 2

s=Au-cu +--

linear combina-

1 2

Au-Au

from which it is easy to derive the important formula: 3.4.

Discussion

A striking observation is that the effect starts well below T,, which proves that the surface transition is not of the first-order type. That is in good agreement with the diffraction methods, and with the theoretical predictions. By nature, the diffraction methods give information about the long range order (LRO) which is connected with the superlattice spots intensities. In our measurements, sensitive to the real atomic locations, the effect arises from double collisions, which represent a short range interaction. Our measurements are so, to some extent, sensitive to the short-range order (SRO). The most natural order parameter in our experiment is the proportion of Au atoms whose neighbour in the (110) rows is a Cu atom, i.e. the proportion of Cu-Au pairs (nCu_*“), relative to the amount of pairs involving Au atoms (the complementary parameter is the proportion of Au-Au pairs, nAu_Au). This can be linked with the classical LRO parameter S, defined as the ratio (p - r)/(l - T), where p is the probability of presence of a type A atom in a site normally occupied by A atoms in the ordered state, and r is the atomic concentration of A [22]. When LRO is perfect, S = I, and when LRO does not exist, S = 0. At a mixed CuAu surface, this ratio becomes S = ( p - 0.5)/0.5 = 2p - I. It is easy to demonstrate that nCu_Au = (S* + I)/2 and nAU_Au= (I S2)/2. If I&Au is the intensity scattered by the Cu-Au pairs and IAu_Au the (lower) intensity scattered by the

,( S) = S2[ I( 1) - I(O)] + 1(O), where I( I) [I(O)] is the intensity for a completely ordered [disordered] surface. Within this approximation, I is a quadratic function of the LRO parameter S. At this stage, a semi-quantitative estimation of S(T) can be given from our experiment. From Fig. 6, the contribution of thermal vibration is suppressed, by dividing the intensity by a linear extrapolation of the thermal effect, in order to extract the intrinsic intensity variation caused by the transition. This normalised intensity is the function I(S) calculated in our model. From I(S), it is then possible to find S, if we know the values of 1(l) and I(O), which are respectively the values before (resp. after) the transition. This conversion is shown is Fig. 7, where an approximation of S(T) is given. It appears that our results show similarities with the LEED results, because they are both functions of S2. However, there are differences, mainly the existence of a hysteresis, and the fact that the transition is not stopped at T,. These could be due to the specificities of ISS, mainly the sensitivity to the extreme surface (LEED analyses more than the first layer) and to SRO. It seems that above the order-disorder transition, LRO disappears over many interatomic distance, but that some SRO or correlation among near neighbours may persist 1121, because Cu-Au pairs are still favoured. It results that the function nCu_Au(S) is strictly true when LRO is dominant, but that it is underestimated when LRO vanishes.

VII. SURFACES/INTERFACES

L. Houssiau, P. Ber!rund/ Nucl. Instr. and Meth. in Phys. Res. B I1 8 (1996) 467-472

472 4. Summary

This work presents the first measurement of an orderdisorder transition by use of a non-diffractive technique: ToF-ISS at large scattering angle. It is shown that this method is sensitive to the creation of disorder at the outermost layer, due to the change in the atomic sequence of the rows, which modifies the double scattering conditions. So, it is basically sensitive to variations of the SRO. When LRO exists, it is also possible to link the intensity with the LRO parameter. In agreement with other techniques, we prove that this parameter decreases well below the bulk transition temperature. Care must be taken when comparing the LEED and X-ray experiments with our KS experiments, owing to the very different range of interaction, and analysed depth. We believe that ToF-KS is potentially a powerful technique for probing the SRO at the surface, then for providing a complementary information on surface order-disorder transitions, which are very different from the bulk transitions.

Acknowledgements

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This work is supported by the Belgian program on Interuniversity Poles of Attraction initiated by the Belgium State (PAI-IUAP 49). Laurent Houssiau acknowledges a grant from IRSIA-Belgium.

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