436 Nuclear Instruments and Methods in Physics Research B2 (1984) 436-439

North-Holland. Amsterdam

TEMPERATURE EFFECTS IN LOW-ENERGY ION SCATTERING FROM COPPER

G. ENGELMANN and E. TAGLAUER

Max - Planck - Institut fiir Plasmaphysik, EURA TOM- Association, D - 8046 Garching/Miinchen, Fed. Rep. Germany

Scattering of Ne+ and Na+ ions from a clean Cu(ll0) surface was investigated in the temperature range between 100 K and 600

K. In various limited ranges of the geometrical scattering parameters. i.e. angle of incidence, scattering angle and azimuthal

orientation of the scattering plane, large temperature effects in the energy distributions of the scattered ions are observed. An attempt

is made to explain these effects by variations in the multiple scattering contributions. Accordingly, both increase and decrease of the

scattered ion intensity can be observed, depending on the scattering processes involved.

1. Introduction

Low-energy ion scattering is known to be a very sensitive method for determining masses and positions

of atoms in the topmost layer of solid surfaces [l]. The sensitivity to the geometrical arrangement of surface atoms poses the question about the influence of atomic thermal motion on ion scattering. In this ion energy

range of typically 1 keV, phonon excitation can hardly be detected. In single scattering events energy shifts of the order of ( EionE,,om)‘/2 are to be expected requiring an energy resolution AE/E - 10W3, which is not acces- sible in usual ion scattering experiments. But thermal motion also results in a quasi-static disordering of the surface lattice and multiple scattering processes depend strongly on the surface geometry [2]. The influence of thermal motion on multiple scattering has been shown by computer simulations [3-j] assuming independent vibrations of the surface atoms. There are also analyti- cal [6] and numerical [7] calculations which take corre- lated vibrations into account. d;n the experimental side there exist only few results [3,5] in a limited temperature range.

In this paper we report first results for 1 keV Ne+ and Na+ scattering from a Cu(ll0) surface in the tem- perature range from 100 K to 600 K, i.e. from 0.3To,,,

to 2 =&bye- Ne+ and Na+ were chosen for the following reasons: the various trajectories in multiple scattering can be attributed to particular scattering classes [8,9]. These are more pronounced in the energy spectra if alkali ions are used for projectiles, as was shown for Li+ [lo] and K+ [11,12], because neutralization is less im- portant. Heavier ions give better separation of the spec- tral features. The comparison between Ne+ and Na+ yields information on the neutralization processes [13].

2. Experimental

Measurements were performed in the apparatus SORBAS which is described elsewhere [14]. The copper single crystal was attached to a recently installed mani- pulator which provides rotation around two axes in and perpendicular to the target surface, respectively. The sample can be cooled with liquid nitrogen by means of a channel system in the manipulator and it can also be heated by means of electron bombardment on its back. The temperature was measured using a chromel-alumel

thermocouple. The mechanically polished (110) face of the crystal

was cleaned further by Ne+ ion beam sputtering and annealing at a maximum temperature of 650 K. The procedure was repeated until no impurities on the surface could be detected by He+ ISS spectra.

The measurements were done with a primary beam of 1000 eV Ne+ ions of about 3 nA and a diameter of 1 mm. For an angle of incidence 4 = 15” the impact area on the target was 2 3 mm2, which corresponds to a particle flux density of < 6.2 X 10” cm-* s-l. The fluence for one energy spectrum is therefore typically 3 x 1014 cmm2. For 1000 eV Na+ ions the target current was reduced to 2 to 3 pA, corresponding to a fluence of 5 3 X 10” cmp2 per energy spectrum. This did not cause any detectable contamination.

The height of scattering peaks was measured for different temperatures between 90 K and 600 K in the following way. After conditioning of the crystal as de- scribed above, the target was heated up to about 600 K and the energy spectra were measured while the temper- ature was falling at a rate of about 0.1 K/s. Cooling down from room temperature was continued by pump- ing liquid nitrogen through the channels in the manipu- lator. The uncertainty in the temperature indicated in the figures is produced by the temperature drop of the

0168-583X/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

G. Engelmonn, E. Taglauer / Temperature effects in ion scattering from Cu 437

1keV NO +Cu IllOl @I

Fig. 1. Energy spectra of Ne+ (a, b) and Na+ ions (c, d) backscattered from a Cu(ll0) surface. 8 is the scattering angle in the laboratory system, # is the angle of incidence relative to the surface and the azimuth angle ‘p = 0 corresponds to scattering along the [llO] direction. S stands for the energy of single scattering

target during the time needed to record the scattering peak.

3. Results and discussion

In fig. 1 energy spectra for Ne+ and Na+ are shown for target temperatures of 290 K and 95 K (6 is the scattering angle in the laboratory system, 4 is the angle of incidence relative to the surface and the azimuthal angle cp = 0 corresponds to scattering along the [llO] direction). It can be seen from the broad peaks that the Na+ spectra show a considerable amount of multiple scattering, which in contrast to Li+ spectra [lo] extend

mainly to energies above the single scattering energy.

There are also multiple scattering contributions in the Ne+ spectra, but due to the chosen scattering geometry “single” and “double” structures are not resolved. It is most interesting to note that for the case shown in fig. 1 the Net peak intensities increase with decreasing tem- perature, whereas the Na+ intensity shows the opposite behaviour. Disregarding this effect the intensity ratios between Na+ and Ne+ are comparable to those found earlier for Li+ and He+ [lo]. This indicates that neu- tralization is important for Ne+ but not for Na+ as has to be expected.

The temperature dependence of the peak maximum for the scattering geometry of fig. 1 is shown in figs. 2 and 3 and the differences in Na+ and Ne+ scattering are clearly demonstrated. The Ne+ intensity rises signifi-

t

Fig. 2. Temperature dependence of the peak maximum for the scattering geometry of figs. la and b (Ne+).

1 keV Na - Cu (110 I

3 i 600. w i 15’. (p 1 00

0 ’ lb0 260 360 Lb0 500

TEMPERATURE 1 K)

Fig. 3. Temperature dependence of the peak maximum for the

scattering geometry of figs. lc and d (Na+). Intensities denoted

by 0 are those measured after heating, with x after sputtering.

VI. LOW ENERGY SURFACE INTERACTIONS

438 G. Engelmann, E. Taglauer / Temperature effects in ion scattering from Cu

0 0 100 200 300 LOO 500

TEMPERATURE ( K )

Fig. 4. Temperature dependence of the peak m~mum as in

fig. 2 (Ne+); scattering along the azimuthal angle ‘p = 32”

relative to the [llO] direction.

cantly at temperatures below the bulk Debye tempera- ture whereas Na+ exhibits a gradual decrease. This observation could be tentatively explained by the fol- lowing qualitative arguments. NeC scattering can be expected to arise mainly from the topmost atomic layer [10,15]. Within the model of a rigid ordered surface, particularly in the linear string model, no Ne+ scatter- ing can be expected for # = 1.5O into 6 = 60 1161. That is, the Ne+ intensity arises mainly from irregularities, such as for instance uncorrelated thermal vibrations. It has been shown [6], that correlation effects decrease with decreasing temperature. There could of course also be an increasing influence of persistent surface damage with decreasing temperature [l7]. A similar dependence was found for 9 = 97’.

Na+ scattering, in contrast to Ne’, arises mainly from multiple scattering with contributions from more than one lattice plane. Reduced vibrational amplitudes

1 keV Na - CU 1110)

J 1 60’. ‘4 = 150. q :32’

TEMPERATURE I K I

Fig. 5. Temperature dependence of the peak maximum for the scattering geometry of fig. 4; (Na+).

can give rise to the gradually decreasing intensity for this scattering geometry.

The situation changes if an azimuthal angle of ‘p = 32” is chosen (figs. 4 and 5). The distance between surface atoms along a scattering trajectory is now about 6 A and therefore “single” scattering is possible even considering a string-like model. Accordingly the scatter- ing intensity is high and almost constant throughout the observed temperature range as demonstrated in figs. 4 and 5. Also for a scattering angle of 97” only a slight increase with decreasing temperature is observed.

Further investigations using computer calculations have to be done to substantiate the given explanations. This is intended to include correlated vibrations and also static surface damage.

4. Conclusions

The following conclusions can be drawn from our

results: Scattering of Ne+ and Na* from Cu(ll0) in the temperature range between 90 K and 600 K shows drastic temperature effects for special scattering geometries, these temperature effects are different for Ne+ and Na+, indicating the different trajectories contributing to the scattered intensity, a decreasing amount of local atomic order at low temperatures may be responsible for the observed

effects.

We thank W. Englert for helpful discussions and experimental advice.

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VI. LOW ENERGY SURFACE INTERACTIONS