SURFACE A N D INTERFACE ANALYSIS, VOL. 8, 67-69

Effect of Temperature During Ion Sputtering on the Surface Segregation Rate of Antimony in an Iron-Antimony Alloy at Higher Temperatures

M. Oku, K. Hirokawa, H. Kimura and S. Suzuki The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai, Japan

The surface segregation of antimony in an iron-0.23 at% antimony alloy was studied by XPS. The segregation rate in the temperature range between 800 and 900 K depends on the temperature during sputtering with argon ion of kinetic energy of 1 keV. The sputtering at room temperature or 473 K gives higher values of the segregation rate than those at 673 K. Both cases give the activation energy of 170 kJmol-' for the surface segregation rate. The segregation of antimony is not observed after the sample is heated at lo00 K.

INTRODUCTION

Surface segregation of atoms in alloys have been exten- sively investigated by XPS and AES in ultra high vacuum. In equilibrium (or steady) state, the composi- tion of a surface component depends on the other com- ponent elements, temperature, and crystallographic orientation of the surface. The segregation rate may be influenced by these and by the diffusion rate. Some authors have discussed low energy ion radiation- enhanced segregation and diffusion related to the com- positional distribution at the surface and subsurface of the alloy during ion sputtering.'-4 In those cases, the creation and annihilation of crystallographic defects coexist. Moreover, thermodynamic factors such as Gibbsian adsorption and preferential sputtering should be taken into account. The more simple studies have been carried out after the ion The pre- treatments for the study are classified into two groups: One involves the measurement of surface segregation rate at a high temperature after ion sputtering at a lower temperature. In the other sputtering and segregation steps are carried out at the same temperature.

The pre-treatment of this report is the former. The surface segregation rate of antimony in an iron-anti- mony alloy was measured at high temperatures after ion sputtering at room temperature or 473 K and 673 K.

EXPERIMENTAL

An iron-0.23 at% antimony alloy was prepared from an electrolytic iron and antimony of 99.9% purity by vacuum melting. The alloy was rolled to 0.5 mm in thickness. In order to remove residual impurities of carbon and nitrogen, the alloy was heated at 1073 K for 12 h in wet hydrogen and 12 h in dry hydrogen. The polycrystalline sample with the grain size of more than 1 mm in diameter was cut into 6 x 20 mm2 plate and chemically polished by hydrofluoric acid and hydrogen- peroxide solution.

XPS was performed with an AEI ES 200 electron spectrometer. The base pressure of the spectrometer was

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1 x Pa. The sample was mounted on a high tem- perature prove whose details were described in a previous paper." A P t - P t a R h thermocouple was spot- welded on the sample surface near the measured area. The temperature was controlled within 1 3 K.

The surface of the alloy was initially sputtered by argon with 1 kV, 4 x Pa of pressure at room temperature. After heating to induce the segre- gation of antimony, the sample was sputtered to obtain the non-segregated surface at desired temperatures. Although antimony signals were not observed at the sputtered surface, an 0 1s peak with binding energy of 531 5eV with a peak height ratio of 0 ls/Fe 2P3,, of 0.01 was found. The concentration of oxygen is 0.04 monolayers which is calculated by the Ebel-Hirokawa method.I3 After sputtering, the sample was heated to segregate antimony at desired temperatures.

A m-,. 6 x

RESULTS

After the argon ion sputtering at 473 K or room tem- perature, the sample was heated at 853 and 873 K. The segregation rates are shown in Fig. 1. The Sb 3d5,, XPS intensity of the segregated antimony is normalized by the value of the maximum segregated surface. The intensity of the segregated antimony did not depend on the heating temperature including the cases in Fig. 2. The concentration is 0.5 monoatomic 1aye1-s.'~

Another pre-treatment was the sputtering at 673 K. Firstly the sample temperature was held at 673 K for 30 min after ion sputtering at 673 K. At that time, the antimony signals were not observed. After that, the sample was heated at 833, 873 and 903 K. The other heating route was that the sample was immediately heated at the higher temperatures after the sputtering. Both cases gave same segregation rate within experi- mental error as shown in Fig. 2. The segregation rate in Fig. 2 is clearly lower than those in Fig. 1.

The other pre-treatment was the sample heating at 1OOOK for 2h. The surface showed no XPS signals except iron. Then the temperature was reduced to the range between 800 and 900 K, but no antimony segrega- tion was observed, even after 3 h. Later, the sample was

Received 6 January 1985 Accepted 19 October 1985

68 M. OKU, K. HIROKAWA, H. KIMURA AND S. SUZUKI

. .... . . c ' 873 K . 0 .

K I * 0, . . * ' c

v) . 053 K

3 D 0.5 . . a

m v) . I

0 I I I I 0 50 100 150 200

Time ( min )

Figure 1. Sb 3d,,, XPS intensity versus time at several different heating temperatures following argon ion sputtering at room tem- perature. The intensities are normalized to that of the maximum segregated surface.

re-sputtered at room temperature and 673 K for 2h, and they showed the same segregation rate as Figs. 1 and 2, respectively.

Some models have been proposed to explain the kinetics of the segregation p r ~ c e s s , ~ ' ~ ' ~ * ' ~ in which the time dependence in the early stage of segregation is parabolic. However, the present results do not show such a behaviour. Rather, the time dependence in the present early stage is good fitted to the solution of a diffusion equation, (l)." Equation (1) implies that the segregated amount is equal to that total quantity of a solute through the interface between a segregated layer and a bulk in the initial stage of the segregation. The segregated concentration is

where a is an enrichment factor of a solute (the ratio of surface to bulk concentration of the solute), d a thickness of a segregated layer, t time and D the diffusion

K .. /. .

50 100 I50 21 Time ( min )

Figure 2. Sb 3d5,, XPS intensity (the same as Fig. 1 ) versus time at the indicated temperatures after the argon sputtering at 673 K.

t 1.2

10-1~1 I ; I

lOOO/T (K-')

Figure 3. Arrhenius plot for diffusion coefficient for antimony surface segregation: (a) calculated from Fig. 1; (b) calculated from Fig. 2.

coefficient. The thickness is set to 0.3 nm being about atomic diameter of antimony. a is 200. The amount of the segregated antimony is proportional to XPS anti- mony intensity, because the antimony concentration in the bulk is too low to be detected by XPS. The diffusion coefficients at various temperatures are obtained as shown in Fig. 3. The activation energies for the cases of two different pre-treatments coincided with each other within experimental error. The derived activation energy, 1703~ 10 kJmole-' is smaller than that for the bulk diffusion of antimony in a pure iron, 270 kJmol-' between 973 and 1173 K.I7

The present antimony concentration is too low to determine the depth distribution. However, one may consider that the depth distribution of antimony below the surface is a determining factor on the rate of the surface segregation of antimony. When iron metal is heated above 1173 K, the sputtering yield by inert gas decreases.I8 This is due to the phase transition of iron from bcc to fcc. As the temperature during the sputtering is below 700 K in this study, the sputtering ratio of antimony to iron may not change at the conditions. In the equilibrium (or steady) state, antimony segregates to the surface of the alloy. Then, if Gibbsian adsorption' played a major role in the depth profile, the concentra- tion of antimony near the surface at 673 K during the sputtering should be larger than at 473 K or room tem- perature. This would suggest that the apparent segrega- tion rate for the sample sputtered at 673 K should be larger than those for the sample at the lower tem- peratures, contrary to the present results.

Many studies have been done on high energy ion beam radiation to solids.19 The bombarding particles produce damage within the solid, creating point defects and displacement cascades in a random fashion. Although the scale of the damage by high energy ion irradiation differs from that by low ion energy ion irradi- ation, the diffusion mechanism of the components in alloy for the latter case have also been explained by crystallographic defects.'-4 Eyre and Bartlett studied the change of the visible damage in neutron irradiated iron by annealing.*' They observed that fine defect clusters remained unchanged after annealing temperature up to 570 K and annealing at about 670 K leaded to coarsening the clusters of the defects. The heating at 673 K in this

EFFECT OF TEMPERATURE DURING ION SPUTTERING 69

study may be able to induce the short range rearrange- ment of the atoms. Thus the heating at 673 K during the sputtering gives a lower diffusion rate than holding the temperature below the 473 K. Even if there were a difference of the activation energy for the diffusion in the two cases, the difference would be too small to be detected.

The solute atoms could also segregate to the surface through the grain boundary. One example is the surface segregation of sulphur in a polycrystalline molybdenum, which activation energy is a quarter that of bulk diffusion.2' The grain boundary diffusion processes for this study is negligible because the grain size of the alloy is larger than 1 mm in diameter. However, the activation energy of the present study is smaller than that of the bulk diffusion, and larger than that of the migration energy of interstitial in metals.22 The activation energy of the surface segregation of tin for an iron-tin single crystal is the same as that of the bulk diffusion." Plessis and Viljoen have reported that an iron-silicon alloy have two stages in silicon surface segregation process.' The activation energies for the initial and second stages are 170 and 480 kJmole-' respectively, whereas the activa- tion energy of the bulk diffusion of silicon is 220 kJmole.2' These and the present results indicate that the segregation rate of solutes is not always domi- nated by the diffusion in bulk. The damage formed by ion sputtering may cause the small activation energy for segregation compared to the activation energy for the bulk diffusion.

Next, we consider the effect of heating above 1000 K on the segregation of the antimony. One factor for the disappearance of the antimony segregation is the deficient zone of antimony below the surface which is

made by the evaporation of the antimony. The other factor is disappearance of the segregation sites at the surface. In general, it is impossible to obtain a LEED pattern in an iron sample sputtered at low temperature because the sputtering may produce some damage near the surface. If the sample is heated above 800 K in ultra high vacuum, the pattern can be obtained.24 This means that the heating above 800 K removes the crystallo- graphic damages and makes the ordered surface struc- ture. Moreover, Zhou et al. observed the different behavior of the segregation of solutes between heating and cooling process in a heating cycle of an iron-tin- carbon alloy sputtered at room temperat~re.~' These phenomena and the present result indicate the following: The annealing at high temperature removes damage due to the ion sputtering. The crystallographic defects remaining at the temperatures for the surface segregation are the segregation sites. The size and number of the defects depends on the annealing temperature, and the disappearance of them decreases the segregation of the solutes. Consequently, the damage produced by inert gas ion at low temperatures enhances the rate of anti- mony segregation to the iron surface.

CONCLUSION

It is shown that the surface segregation rate of antimony on an iron-antimony alloy depends on the temperature during the ion sputtering even with low energy. The apparent rate and activation energy of the segregation imply that the surface segregation is influenced by crys- tallographic defects due to the ion sputtering.

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