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The influence of ion bombardment on the composition and atomic structure of the CoNi3(1 0 0) surface layers
Vacuum 66 (2002) 115–121
The influence of ion bombardment on the composition and atomic structure of the CoNi3(1 0 0) surface layers M.A. Vasylyev*, I.N. Makeyeva, A.G. Blashchuk, S.D. Gorodetsky Institute of Metal Physics NAS of Ukraine, 36, Vernadsky Blvd., 03680, Kiev 03680-142, Ukraine Received 1 September 2001; received in revised form 22 January 2002; accepted 22 January 2002
Abstract The low-energy electron diffraction method with a number of original techniques has been employed to observe and study a new radiation-stimulated reconstruction transition on the CoNi3 (1 0 0) single crystal surface. Low-energy bombardment by Ar+ ions (600 eV) leads to a change of the diffraction pattern symmetry corresponding to a surface structure change f.c.c.-h.c.p. To elucidate the causes of the effect observed the changes of the surface composition and the concentration profile under the action of ion bombardment were studied. It has been found that first atomic layer of the sample is fully enriched in cobalt, and this, precisely, leads to the formation of the h.c.p. phase on the surface. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ion bombardment; LEED; Reconstruction transition; Alloy; Surface; Surface segregation
1. Introduction Ion bombardment of the solid surface is widely used for controlled modification of the surface as well as providing the probe for surface characterization. The surface properties are significantly affected due to interaction of the ion beam. First of all the ion beam causes structure disordering by generation of the different kind defects [1]. In the case of alloys structure disorder is accompanied by changes of elemental composition in the surface and near-surface region. One of the reasons of this effect is preferential sputtering of the alloy surface components, with different sputtering yields. At the same time a change of the sub-surface region structure can be observed for the case of alloys of
*Corresponding author.
elements with very close sputtering yields. Moreover, ion bombardment often causes the formation of a non-monotonic concentration profile when segregated element has a concentration minimum at the near-surface region [2]. The behaviour of the phase transitions kinetics in the surface layers and in bulk solids can be quite different. The temperature dependence of atomic and magnetic ordering of surface layers differs from the temperature dependence of these processes in bulk alloys. Some phase transitions (for example the surface reconstruction), can occur only in surface layers [3]. Studies of phase transitions at the surfaces of alloys are of special interest because the thermal heating or ion bombardment can result in a significant change of the elemental composition of sub-surface layers and this can stimulate phase transformations. The phase transitions in the bulk
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 1 7 8 - 1
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M.A. Vasylyev et al. / Vacuum 66 (2002) 115–121
of the alloy can influence also the composition of its surface [4]. The main method used to study the surface phase transitions related to the change of atomic structure is the low-energy electron diffraction (LEED). It allows one to perform non-destructive layer-by-layer structure analysis of several surface atomic layers. In order to analyse the elemental surface composition by Auger electron spectroscopy (AES) or ionization spectroscopy (IS), a variant of the spectroscopy of electron energy losses, is usually used. In the second case the change in energy of primary electrons results in the change of probing depth. This, in turn, allows us to perform non-destructive layer-by-layer analysis of elemental composition using the method for restoring of true form of the concentration profile [5]. The purpose of the present work is a layer-bylayer study of the processes of radiation-stimulated segregation at the CoNi3(1 0 0) single crystal surface and its influence on the atomic structure of the surface layers. CoNi3 was used for our studies because CoNi alloys are widely used as catalysts and in magnetic recording. In addition they are convenient for a study of the radiation-stimulated segregation and the intercoupling of structural state and element composition of surface layers. This is because the values of the sputtering yields of Co and Ni are close [6] the change of the surface structure owing to selection sputtering may be neglected.
2. Experimental The measurements were performed using the combined LEED-AES ultrahigh-vacuum spectrometer [7], developed at the Institute of Metal Physics of the National Academy of Sciences of the Ukraine. The spectrometer has been equipped with a four-grid quasi-spherical energy analyser and by a spot photometer. A special system for heating sample allowed its temperature to vary within the range 300–1200 K and to maintain its value with the accuracy of 72 K. In order to study the influence of ion bombardment on the physical–chemical surface condition,
an electron-impact ion source was used. The ion beam was formed and focused by the electrostatic ion-optical system. Spectrally pure argon was used as the working gas, which was previously purified using a sublimation filter. The surface studied was bombarded by an Ar+ ion beam with a diameter of 3 mm and with a current density of 3 mA/cm2 and energy of 600 eV. We have studied the surface of a CoNi3 single crystal oriented in parallel to (1 0 0) face. According to Hansen et al. [8] the alloy being studied has an f.c.c. crystal lattice over the whole temperature range. Previously, the sample was polished mechanically and electrolytically, and then it was cleaned by the etching for several hours by Ar+ ions in ultrahigh vacuum at a temperature of 873 K. The process of cleaning was taken to be finished when the carbon and oxygen peaks vanished in the AES spectra and the sulphur peak (150 eV) was much smaller than the nickel peak (102 eV). Such a surface gave a clear and high contrast LEED pattern at room temperature corresponding to the (1 0 0) face without any additional spots. Low-energy Auger transitions are most sensitive to the processes of surface segregation [9]. Therefore we have used, for the analysis of the surface chemical composition, the spectra corresponding to MVV transitions (Co (52 eV), Ni (61 eV)). The Co and Ni concentrations at the sample surface were determined on the basis of Auger spectra using the technique described in the work of Godowski et al. [10]. The concentration profiles in the sub-surface region of CoNi3(1 0 0) have been studied using ionisation spectroscopy. The cobalt spectrum (DE Co ¼ 62 eV) and the nickel spectrum (DE Ni ¼ 68 eV) were recorded in dN=dE mode using a computer at ten values of the energy of primary electron beam E0;j ; which was varied from 300 up to 1300 eV. In order to increase the precision of measurements, the intensity of each peak was averaged over the number of measurements, large enough so that the root-mean-square error does not exceed 1%. The spectra intensities were measured using their ‘peak-to-peak’ amplitudes. Then, the spectra were normalised with respect to corresponding intensities of the peaks of elastically scattered electrons. The recovery of the
M.A. Vasylyev et al. / Vacuum 66 (2002) 115–121
true concentration profile was carried out using the experimental data. The relative value (determined in the experiment) of the normalised intensities of the ionization lines of A components of the sample and the standard, RA ; can be written as follows: RAj ¼
N X
Pij XAi ;
ð1Þ
i¼1
where Pði; jÞ is the weight matrix element for the layer with numbers i ¼ 1; 2; y; N and primary electron energy E0;j (j ¼ 1; 2; y; M); XA ðiÞ ¼ CAðbÞ; CAðiÞ is the concentration of component A for the layer with number i; CAðbÞ is the bulk concentration of the alloy. In this case, the probability difference between the ionisation cross-sections of the sample atoms and standard atoms, i.e. the influence of the matrix effects, was not taken into account. However, this difference is insignificant for the Co–Ni system studied in the present work. As a decay of the primary beam is taken into account in RA ðjÞ; the expression Z ymax zi Pij ¼ exp lj cos y ymin Dzi 1 exp dy; ð2Þ lj cos y is used to calculate the weight matrix elements Pi;j ; when recording the spectra within the spatial angle of quasi-spherical energy analyser. Here ymin ¼ ( and ymax ¼ 70 A ( (they are determined by the 4A geometry of the energy analyser); zi is the coordinate of the beginning of the layer i; Dzi ¼ zi þ 1 Lj is the effective inelastic mean free path, calculated according to the expression: lðE0;j ÞlðE0;j DEA Þcos y Lj ¼ ; ð3Þ lðE0;j Þ þ lðE0;j DEA Þcos y where 1194 þ 0:429E 1=2 ; E2 was taken from Ref. [9] The procedure of the recovery of the concentration profile is described in detail in Ref. [5]. The surface structure was studied using LEED method supplemented by the techniques for
lðEÞ ¼
117
photometry and modulation of the intensity of diffraction spots [4].
3. Results and discussion Under bombardment of the surface of CoNi3(1 0 0) single crystal by Ar+ ions with above mentioned parameters a change of diffraction pattern symmetry has been determined. Before the bombardment the LEED pattern corresponded to the bulk-like perfect crystalline structure of the (1 0 0) face of an f.c.c. lattice (Fig. 1a, I). The intensity of the initial pattern decreases under the influence of ion bombardment. At bombardment dose of B5 1014 ion/cm2 the additional spots arise, forming the pattern with hexagonal symmetry, displaced by B31 in the (1 0 0) direction of the initial structure. The intensities of both patterns become approximately equal to each other at the dose of B1015 ion/cm2 (Fig. 1a, II). An estimation of the interatomic distances on the basis of the diffraction pattern allows us to conclude that the new phase arises at the surface, which is typical for clean cobalt (the (1 0 0 0) face of an h.c.p. lattice). Further ion bombardment of the surface results in the disappearance of the spots of f.c.c. structure and in the formation at the surface of single-phase h.c.p. region at the doses exceeding 1016 ion/cm2 (Fig. 1a, III). The LEED method was used to study the features of the new phase formation. For this purpose, the dependence of spatial profiles on the dose of ion bombardment was studied for the intensities of (0 0) spots corresponding to both phases. The profiles were recorded in the form of a quasi-three-dimensional plot of the spots using the photometry device [7]. The dose dependencies of h:c:p: f:c:c: the intensity of spots I00 ðDÞ and I00 ðDÞ are shown in Fig. 1b. The initial segment of the curve f:c:c: I00 ðDÞ is characterised by an exponential decay of the intensity in accordance with the ratio f:c:c: I00 ðDÞ ¼ I0 expð2aDÞ; and this allowed us to % determine the average area of the damage caused by one incident ion: a% ¼ 1:2 1015 cm2 : Further increase of the bombardment dose results in the h:c:p: increase of intensity of both spots: I00 ðDÞ increases abruptly reaching saturation at
M.A. Vasylyev et al. / Vacuum 66 (2002) 115–121
118
(a)
I
II
-1 -2
III
(b)
I 00 [a.u.]
6
4
2 2
0
1 1014
1015
1016 D [ions/cm2 ]
10
1
σ [%]
8
(c)
6 4
2 2 0
1014
1015
1016
D [ions /cm 2]
0
(d)
∆d/d [%]
2 -2 -4 -6 -8
1
-10
1014
1015
1016
D [ions / cm2 ]
1014
1015
1016
D [ions / cm2 ]
(e)
h Ni / h Co
0 .3 0
0. 2 0
0. 1 0
0. 0 0
Fig. 1. Radiation-stimulated modifications of the structure and composition of the surface CoNi3(1 0 0): (a) schematic representation of the LEED patterns for the CoNi3(1 0 0) face at different doses of ion bombardment: I—0, II—1015, III—1016 ion/cm2; (1) the f.c.c. (1 0 0) spots, (2) the h.c.p. (1 0 0 0) spots; (b) is the dependence of the intensity of (0 0) spots for f.c.c. and h.c.p. phases on the CoNi3(1 0 0) surface on the dose of ion bombardment; E0 ¼ 128 eV; (1) is the (0 0) spot for f.c.c.(1 0 0), (2) is the (0 0) spot for h.c.p. (1 0 0 0); (c) is the dependence of half-width of the (0 0) spots of f.c.c. (1) and h.c.p. phases (2) at the CoNi3 (1 0 0) surface on the dose of ion bombardment; (d) is the dependence of the relative change of lattice constants of f.c.c. (1) and h.c.p. phases (2) at the CoNi3 (1 0 0) surface on the dose of ion bombardment; (e) is the dependence of the ratio hNi =hCo of the low-energy Auger peaks from Ni and Co on the dose of ion bombardment of at the CoNi3(1 0 0) sample.
f:c:c: DX1015 ion/cm2, while I00 ðDÞ passes through a smooth maximum at dose values of B1015 ion/ cm2, and then falls to zero. Fig. 1c shows the dependencies of the half-width of the (0 0) spots on the dose of ion bombardment for both structures. The broadening of the spot corresponding to the f.c.c. structure is conditioned by the decrease of the average size of twodimensional surface defects such as steps and facets. Simultaneously, the decrease the half-width of spots (corresponding to an h.c.p. structure) down to the value of B3% is observed at a dose of 1016 ion/cm2, which is conditioned by the instrumental broadening. Such a behaviour of the curves obtained indicates the independent formation of the diffraction pictures observed on the various systems of surface domains, the average size of which determines the half-width of the corresponding spots. The influence of ion bombardment on the relative changes of the interlayer distances in the sub-surface region was studied for the (0 0) spots of f.c.c. and h.c.p. phases using a depth dilatometry method [7]. The gist of this method is an experimental measurement of the energy shift of the Bragg peak position (DE0 ) [7] as a finction of the ion bombardment dose Then the relative change of the interlayer distances along the normal to the surface for f.c.c. and h.c.p. precipitated phases is given by Dd=d ¼ DE0 =2E0 : Fig. 1d shows the strong (up to 10–12%) compression the interlayer distances for the f.c.c. structure at the dose of bombardment 5 1014–1016 ion/cm2. The energy position of the diffraction peak corresponding to the f.c.c. structure practically does not change. In order to determine the reasons for the phenomenon observed, the changes in surface composition and in concentration profile were studied under ion bombardment. Fig. 1e, shows the dependence of the ratio hNi =hCo of Augerpeaks of Ni (61 eV) and Co (52 eV) on the dose of ion bombardment, where the intensive enrichment of the surface by Co atoms is seen. For a dose of bombardment of B5 1014 ion/cm2 the cobalt concentration was of 78%, which corresponds to the beginning of the region at the phase diagram of Co–Ni system where there is a low-temperature
M.A. Vasylyev et al. / Vacuum 66 (2002) 115–121
e-phase with an h.c.p. lattice [8]. More detailed study of the concentration profile is performed using the method of ionization spectroscopy. In order to rebuild the concentration profile, the subsurface region was divided into 10 layers parallel to the surface. The thickness of the layers was taken to be equal to the interlayer distance, ( of the CoNi(1 0 0) face. The experia=2 ¼ 1:77 A, mental values of the relative intensities of the ionization lines were used in our calculations. These values were normalised on the magnitude of a peak of the elastically scattered electrons, measured at ten different E0 values and chosen on the basis of condition of the best stability of solutions for Eq. (1). The results of the layer-by-layer recovery of the concentration profile depending on the dose of ion bombardment are shown in Fig. 2. It is seen that, apart from the enrichment of surface layers by cobalt, the dose increase results also in the depletion of this element over 2–3 deeper atomic layers, which results in a non-monotonic concentration profile. For the dose of B1016 ion/cm2, the
119
monolayer formed entirely of cobalt is formed at the sample surface. Note that the segregation features under the ion bombardment are close to the features observed under heating [11], however the surface enrichment by Co is much higher. This can be explained by the vacancy mechanism of diffusion. The ion-beam etching results in formation, in the sub-surface region, of a large number of stable non-equilibrium vacancies, and this leads to a more intensive cobalt diffusion to the sample surface from subsurface region with a large concentration of radiation damages and, therefore, to the depletion of this region by the atoms of segregating element (cobalt). Thus, we have found that the ion bombardment of the CoNi3(1 0 0) single crystal causes the f.c.c.h.c.p. polymorphic-type reconstructive phase transition at the surface due to strong enrichment of the surface by cobalt and formation of a monolayer coating of Co. This surface condition is stable and remains at room temperature for tens of hours after completion of ion bombardment. The temperature dependence of the time of isothermal annealing t; required for recovery of the equilibrium surface concentrations of alloy components is shown in Fig. 3. The experimental error does not exceed 5%. The linear dependence testifies the activation nature of the annealing process. It allows us to determine an activation energy of 0.46 eV.
10.5 10
ln τ
9.5 9 8.5 8 7.5 0.8
1
1.2
1.4
1.6
1.8
2
2.2
T -1, 10-3 [ K -1]
Fig. 2. The dose dependence of the concentration profile near the surface CoNi3(1 0 0) rebuilt using the ionisation spectroscopy; i is the number of monolayers.
Fig. 3. The temperature dependence of the isothermal annealing time to CoNi3(1 0 0), required for recovery of the equilibrium surface concentrations after ion bombardment with dose of 1016 ion/cm2.
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All the experimental data mentioned above enable us to assume the following qualitative model for the process of radiation-stimulated phase transition at the CoNi3(1 0 0) surface, which has been observed in our study. In the initial stages of bombardment (up to a dose of B5 1014 ion/ cm2) the generation of the point defects occurs in the surface layer, leading to exponential decay of the intensity of diffraction spots corresponding to the initial f.c.c. (1 0 0) structure (Fig. 1b), without their significant spatial broadening (Fig. 1c) and displacement in the energy scale (Fig. 1d). The increase of point defect concentration in the subsurface layer stimulates the segregation of cobalt atoms (Fig. 2). The increase of cobalt concentration shifts the content of the near-surface region of the sample to the region of the phase diagram of Co–Ni system, in which the low-temperature h.c.p. phase exists. This results in the possibility of the phase transition described. When increasing the dose of bombardment, the elementary point defects form clusters which result in the surface restriction. This effect was observed in many studies [12,13]. Segregating Co atoms come to the surface of the facets produced and form the domains with a close packing corresponding to the (1 0 0 0) face of the h.c.p. lattice. This should occur in the case when the Co–Co interatomic interaction is stronger then the Co–Ni interaction. The reason for such a condition can be the structure defects generated by ion bombardment as well as the subsurface relaxation of the parameters of chemical and magnetic interatomic interaction [14]. Also, an argument for the existence of repulsive interaction between the forming h.c.p. Co monolayer and deeper crystal layers depleted by cobalt, is the strong compression of the interlayer distances of the initial structure, which has been observed in the experiment at an ion bombardment dose of D >5 1014 ion/cm2. The increasing size of the h.c.p. domains at the sample surface resulting in the reduction of the half-width of the corresponding LEED spot (Fig. 1c), screen further generation of the defects in deeper layers. This results in an f:c:c: increase in intensity of I00 ðDÞ due to the annealing of defects produced during the initial stage of ion bombardment (Fig. 1e). The max-
imum of this intensity is reached at the dose of B1015 ion/cm2. Then intensity decay takes place, conditioned by the reduction of size of the domains of the initial structure due to an increase of the degree of the surface covering by the h.c.p. Co monolayer. At the dose of B1016 ion/cm2 practically the 100% Co concentration is reached in the surface monolayer (Fig. 2) and the process reaches dynamical equilibrium. The condition obtained is metastable, i.e. the free energy of the subsurface region is in a local minimum with respect to the number of defects. The thermodynamically equilibrium state may be restored with a high-enough activation energy eA ¼ 0:46 eV.
4. Conclusions We have firstly observed a new radiationstimulated phase transition f.c.c.-h.c.p. in the surface layer of CoNi3(1 0 0). A qualitative model to describe this transition is proposed. We have shown the phase transition caused by strong radiation-stimulated Co segregation on the surface with formation of monolayer coverage.
Acknowledgements The authors are grateful to Dr. S. Mankovsky for helpful discussions This work has been supported by the Foundation for Basic Research of Ukraine, Project No. 04.07/77.
References [1] Vasylyev MA, Blashchuk AG, Mashovets NS, Vilkova NYu. Vacuum 2000;57:71–80. [2] Allie G, Sauroz C, Villemain P. Surf Sci 1982;116:179–81. [3] Vasylyev MA. J Phys D 1997;30:3037–70. [4] Vasylyev MA, Gorodetsky SD. Surf Sci 1986;171:543–54. [5] Blashchuk AG. Met Phys Adv Tech 2001;19:1127–93. [6] Behrisch R, editor, Sputtering by particle bombardment I, Topics Applied Physics, Vol. 47, Berlin, Heidelberg, New York: Springer, 1981. [7] Vasylyev MA, Gorodetsky SD. Vacuum 1987;37:723–8. [8] Hansen M, Anderko K. Constitution of binary alloys. New York: Mc Graw Hill, 1958.
M.A. Vasylyev et al. / Vacuum 66 (2002) 115–121 [9] Seah MP, Dench WA. Surf Interf Anal 1979;1:2–11. [10] Godowski P, Rudny J. Phys Stat Sol A 1987;527–36. [11] Vasylyev MA, Blashchuk AG, Hryshenko OP, Mashovets NS. Phys Low—Dim Struct 1999;11 and 12:115–24.
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[12] Verheij LK. Surf Sci 1982;114:667–72. [13] Robinson RS, Rossnagel SM. J Vac Sci Technol 1982;21:790–7. [14] Vrias T, Moran-Lopez Tl. Surf Sci 1981;103:164–70.
The influence of ion bombardment on the composition and atomic structure of the CoNi3(1 0 0) surface layers M.A. Vasylyev*, I.N. Makeyeva, A.G. Blashchuk, S.D. Gorodetsky Institute of Metal Physics NAS of Ukraine, 36, Vernadsky Blvd., 03680, Kiev 03680-142, Ukraine Received 1 September 2001; received in revised form 22 January 2002; accepted 22 January 2002
Abstract The low-energy electron diffraction method with a number of original techniques has been employed to observe and study a new radiation-stimulated reconstruction transition on the CoNi3 (1 0 0) single crystal surface. Low-energy bombardment by Ar+ ions (600 eV) leads to a change of the diffraction pattern symmetry corresponding to a surface structure change f.c.c.-h.c.p. To elucidate the causes of the effect observed the changes of the surface composition and the concentration profile under the action of ion bombardment were studied. It has been found that first atomic layer of the sample is fully enriched in cobalt, and this, precisely, leads to the formation of the h.c.p. phase on the surface. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ion bombardment; LEED; Reconstruction transition; Alloy; Surface; Surface segregation
1. Introduction Ion bombardment of the solid surface is widely used for controlled modification of the surface as well as providing the probe for surface characterization. The surface properties are significantly affected due to interaction of the ion beam. First of all the ion beam causes structure disordering by generation of the different kind defects [1]. In the case of alloys structure disorder is accompanied by changes of elemental composition in the surface and near-surface region. One of the reasons of this effect is preferential sputtering of the alloy surface components, with different sputtering yields. At the same time a change of the sub-surface region structure can be observed for the case of alloys of
*Corresponding author.
elements with very close sputtering yields. Moreover, ion bombardment often causes the formation of a non-monotonic concentration profile when segregated element has a concentration minimum at the near-surface region [2]. The behaviour of the phase transitions kinetics in the surface layers and in bulk solids can be quite different. The temperature dependence of atomic and magnetic ordering of surface layers differs from the temperature dependence of these processes in bulk alloys. Some phase transitions (for example the surface reconstruction), can occur only in surface layers [3]. Studies of phase transitions at the surfaces of alloys are of special interest because the thermal heating or ion bombardment can result in a significant change of the elemental composition of sub-surface layers and this can stimulate phase transformations. The phase transitions in the bulk
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 1 7 8 - 1
116
M.A. Vasylyev et al. / Vacuum 66 (2002) 115–121
of the alloy can influence also the composition of its surface [4]. The main method used to study the surface phase transitions related to the change of atomic structure is the low-energy electron diffraction (LEED). It allows one to perform non-destructive layer-by-layer structure analysis of several surface atomic layers. In order to analyse the elemental surface composition by Auger electron spectroscopy (AES) or ionization spectroscopy (IS), a variant of the spectroscopy of electron energy losses, is usually used. In the second case the change in energy of primary electrons results in the change of probing depth. This, in turn, allows us to perform non-destructive layer-by-layer analysis of elemental composition using the method for restoring of true form of the concentration profile [5]. The purpose of the present work is a layer-bylayer study of the processes of radiation-stimulated segregation at the CoNi3(1 0 0) single crystal surface and its influence on the atomic structure of the surface layers. CoNi3 was used for our studies because CoNi alloys are widely used as catalysts and in magnetic recording. In addition they are convenient for a study of the radiation-stimulated segregation and the intercoupling of structural state and element composition of surface layers. This is because the values of the sputtering yields of Co and Ni are close [6] the change of the surface structure owing to selection sputtering may be neglected.
2. Experimental The measurements were performed using the combined LEED-AES ultrahigh-vacuum spectrometer [7], developed at the Institute of Metal Physics of the National Academy of Sciences of the Ukraine. The spectrometer has been equipped with a four-grid quasi-spherical energy analyser and by a spot photometer. A special system for heating sample allowed its temperature to vary within the range 300–1200 K and to maintain its value with the accuracy of 72 K. In order to study the influence of ion bombardment on the physical–chemical surface condition,
an electron-impact ion source was used. The ion beam was formed and focused by the electrostatic ion-optical system. Spectrally pure argon was used as the working gas, which was previously purified using a sublimation filter. The surface studied was bombarded by an Ar+ ion beam with a diameter of 3 mm and with a current density of 3 mA/cm2 and energy of 600 eV. We have studied the surface of a CoNi3 single crystal oriented in parallel to (1 0 0) face. According to Hansen et al. [8] the alloy being studied has an f.c.c. crystal lattice over the whole temperature range. Previously, the sample was polished mechanically and electrolytically, and then it was cleaned by the etching for several hours by Ar+ ions in ultrahigh vacuum at a temperature of 873 K. The process of cleaning was taken to be finished when the carbon and oxygen peaks vanished in the AES spectra and the sulphur peak (150 eV) was much smaller than the nickel peak (102 eV). Such a surface gave a clear and high contrast LEED pattern at room temperature corresponding to the (1 0 0) face without any additional spots. Low-energy Auger transitions are most sensitive to the processes of surface segregation [9]. Therefore we have used, for the analysis of the surface chemical composition, the spectra corresponding to MVV transitions (Co (52 eV), Ni (61 eV)). The Co and Ni concentrations at the sample surface were determined on the basis of Auger spectra using the technique described in the work of Godowski et al. [10]. The concentration profiles in the sub-surface region of CoNi3(1 0 0) have been studied using ionisation spectroscopy. The cobalt spectrum (DE Co ¼ 62 eV) and the nickel spectrum (DE Ni ¼ 68 eV) were recorded in dN=dE mode using a computer at ten values of the energy of primary electron beam E0;j ; which was varied from 300 up to 1300 eV. In order to increase the precision of measurements, the intensity of each peak was averaged over the number of measurements, large enough so that the root-mean-square error does not exceed 1%. The spectra intensities were measured using their ‘peak-to-peak’ amplitudes. Then, the spectra were normalised with respect to corresponding intensities of the peaks of elastically scattered electrons. The recovery of the
M.A. Vasylyev et al. / Vacuum 66 (2002) 115–121
true concentration profile was carried out using the experimental data. The relative value (determined in the experiment) of the normalised intensities of the ionization lines of A components of the sample and the standard, RA ; can be written as follows: RAj ¼
N X
Pij XAi ;
ð1Þ
i¼1
where Pði; jÞ is the weight matrix element for the layer with numbers i ¼ 1; 2; y; N and primary electron energy E0;j (j ¼ 1; 2; y; M); XA ðiÞ ¼ CAðbÞ; CAðiÞ is the concentration of component A for the layer with number i; CAðbÞ is the bulk concentration of the alloy. In this case, the probability difference between the ionisation cross-sections of the sample atoms and standard atoms, i.e. the influence of the matrix effects, was not taken into account. However, this difference is insignificant for the Co–Ni system studied in the present work. As a decay of the primary beam is taken into account in RA ðjÞ; the expression Z ymax zi Pij ¼ exp lj cos y ymin Dzi 1 exp dy; ð2Þ lj cos y is used to calculate the weight matrix elements Pi;j ; when recording the spectra within the spatial angle of quasi-spherical energy analyser. Here ymin ¼ ( and ymax ¼ 70 A ( (they are determined by the 4A geometry of the energy analyser); zi is the coordinate of the beginning of the layer i; Dzi ¼ zi þ 1 Lj is the effective inelastic mean free path, calculated according to the expression: lðE0;j ÞlðE0;j DEA Þcos y Lj ¼ ; ð3Þ lðE0;j Þ þ lðE0;j DEA Þcos y where 1194 þ 0:429E 1=2 ; E2 was taken from Ref. [9] The procedure of the recovery of the concentration profile is described in detail in Ref. [5]. The surface structure was studied using LEED method supplemented by the techniques for
lðEÞ ¼
117
photometry and modulation of the intensity of diffraction spots [4].
3. Results and discussion Under bombardment of the surface of CoNi3(1 0 0) single crystal by Ar+ ions with above mentioned parameters a change of diffraction pattern symmetry has been determined. Before the bombardment the LEED pattern corresponded to the bulk-like perfect crystalline structure of the (1 0 0) face of an f.c.c. lattice (Fig. 1a, I). The intensity of the initial pattern decreases under the influence of ion bombardment. At bombardment dose of B5 1014 ion/cm2 the additional spots arise, forming the pattern with hexagonal symmetry, displaced by B31 in the (1 0 0) direction of the initial structure. The intensities of both patterns become approximately equal to each other at the dose of B1015 ion/cm2 (Fig. 1a, II). An estimation of the interatomic distances on the basis of the diffraction pattern allows us to conclude that the new phase arises at the surface, which is typical for clean cobalt (the (1 0 0 0) face of an h.c.p. lattice). Further ion bombardment of the surface results in the disappearance of the spots of f.c.c. structure and in the formation at the surface of single-phase h.c.p. region at the doses exceeding 1016 ion/cm2 (Fig. 1a, III). The LEED method was used to study the features of the new phase formation. For this purpose, the dependence of spatial profiles on the dose of ion bombardment was studied for the intensities of (0 0) spots corresponding to both phases. The profiles were recorded in the form of a quasi-three-dimensional plot of the spots using the photometry device [7]. The dose dependencies of h:c:p: f:c:c: the intensity of spots I00 ðDÞ and I00 ðDÞ are shown in Fig. 1b. The initial segment of the curve f:c:c: I00 ðDÞ is characterised by an exponential decay of the intensity in accordance with the ratio f:c:c: I00 ðDÞ ¼ I0 expð2aDÞ; and this allowed us to % determine the average area of the damage caused by one incident ion: a% ¼ 1:2 1015 cm2 : Further increase of the bombardment dose results in the h:c:p: increase of intensity of both spots: I00 ðDÞ increases abruptly reaching saturation at
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(a)
I
II
-1 -2
III
(b)
I 00 [a.u.]
6
4
2 2
0
1 1014
1015
1016 D [ions/cm2 ]
10
1
σ [%]
8
(c)
6 4
2 2 0
1014
1015
1016
D [ions /cm 2]
0
(d)
∆d/d [%]
2 -2 -4 -6 -8
1
-10
1014
1015
1016
D [ions / cm2 ]
1014
1015
1016
D [ions / cm2 ]
(e)
h Ni / h Co
0 .3 0
0. 2 0
0. 1 0
0. 0 0
Fig. 1. Radiation-stimulated modifications of the structure and composition of the surface CoNi3(1 0 0): (a) schematic representation of the LEED patterns for the CoNi3(1 0 0) face at different doses of ion bombardment: I—0, II—1015, III—1016 ion/cm2; (1) the f.c.c. (1 0 0) spots, (2) the h.c.p. (1 0 0 0) spots; (b) is the dependence of the intensity of (0 0) spots for f.c.c. and h.c.p. phases on the CoNi3(1 0 0) surface on the dose of ion bombardment; E0 ¼ 128 eV; (1) is the (0 0) spot for f.c.c.(1 0 0), (2) is the (0 0) spot for h.c.p. (1 0 0 0); (c) is the dependence of half-width of the (0 0) spots of f.c.c. (1) and h.c.p. phases (2) at the CoNi3 (1 0 0) surface on the dose of ion bombardment; (d) is the dependence of the relative change of lattice constants of f.c.c. (1) and h.c.p. phases (2) at the CoNi3 (1 0 0) surface on the dose of ion bombardment; (e) is the dependence of the ratio hNi =hCo of the low-energy Auger peaks from Ni and Co on the dose of ion bombardment of at the CoNi3(1 0 0) sample.
f:c:c: DX1015 ion/cm2, while I00 ðDÞ passes through a smooth maximum at dose values of B1015 ion/ cm2, and then falls to zero. Fig. 1c shows the dependencies of the half-width of the (0 0) spots on the dose of ion bombardment for both structures. The broadening of the spot corresponding to the f.c.c. structure is conditioned by the decrease of the average size of twodimensional surface defects such as steps and facets. Simultaneously, the decrease the half-width of spots (corresponding to an h.c.p. structure) down to the value of B3% is observed at a dose of 1016 ion/cm2, which is conditioned by the instrumental broadening. Such a behaviour of the curves obtained indicates the independent formation of the diffraction pictures observed on the various systems of surface domains, the average size of which determines the half-width of the corresponding spots. The influence of ion bombardment on the relative changes of the interlayer distances in the sub-surface region was studied for the (0 0) spots of f.c.c. and h.c.p. phases using a depth dilatometry method [7]. The gist of this method is an experimental measurement of the energy shift of the Bragg peak position (DE0 ) [7] as a finction of the ion bombardment dose Then the relative change of the interlayer distances along the normal to the surface for f.c.c. and h.c.p. precipitated phases is given by Dd=d ¼ DE0 =2E0 : Fig. 1d shows the strong (up to 10–12%) compression the interlayer distances for the f.c.c. structure at the dose of bombardment 5 1014–1016 ion/cm2. The energy position of the diffraction peak corresponding to the f.c.c. structure practically does not change. In order to determine the reasons for the phenomenon observed, the changes in surface composition and in concentration profile were studied under ion bombardment. Fig. 1e, shows the dependence of the ratio hNi =hCo of Augerpeaks of Ni (61 eV) and Co (52 eV) on the dose of ion bombardment, where the intensive enrichment of the surface by Co atoms is seen. For a dose of bombardment of B5 1014 ion/cm2 the cobalt concentration was of 78%, which corresponds to the beginning of the region at the phase diagram of Co–Ni system where there is a low-temperature
M.A. Vasylyev et al. / Vacuum 66 (2002) 115–121
e-phase with an h.c.p. lattice [8]. More detailed study of the concentration profile is performed using the method of ionization spectroscopy. In order to rebuild the concentration profile, the subsurface region was divided into 10 layers parallel to the surface. The thickness of the layers was taken to be equal to the interlayer distance, ( of the CoNi(1 0 0) face. The experia=2 ¼ 1:77 A, mental values of the relative intensities of the ionization lines were used in our calculations. These values were normalised on the magnitude of a peak of the elastically scattered electrons, measured at ten different E0 values and chosen on the basis of condition of the best stability of solutions for Eq. (1). The results of the layer-by-layer recovery of the concentration profile depending on the dose of ion bombardment are shown in Fig. 2. It is seen that, apart from the enrichment of surface layers by cobalt, the dose increase results also in the depletion of this element over 2–3 deeper atomic layers, which results in a non-monotonic concentration profile. For the dose of B1016 ion/cm2, the
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monolayer formed entirely of cobalt is formed at the sample surface. Note that the segregation features under the ion bombardment are close to the features observed under heating [11], however the surface enrichment by Co is much higher. This can be explained by the vacancy mechanism of diffusion. The ion-beam etching results in formation, in the sub-surface region, of a large number of stable non-equilibrium vacancies, and this leads to a more intensive cobalt diffusion to the sample surface from subsurface region with a large concentration of radiation damages and, therefore, to the depletion of this region by the atoms of segregating element (cobalt). Thus, we have found that the ion bombardment of the CoNi3(1 0 0) single crystal causes the f.c.c.h.c.p. polymorphic-type reconstructive phase transition at the surface due to strong enrichment of the surface by cobalt and formation of a monolayer coating of Co. This surface condition is stable and remains at room temperature for tens of hours after completion of ion bombardment. The temperature dependence of the time of isothermal annealing t; required for recovery of the equilibrium surface concentrations of alloy components is shown in Fig. 3. The experimental error does not exceed 5%. The linear dependence testifies the activation nature of the annealing process. It allows us to determine an activation energy of 0.46 eV.
10.5 10
ln τ
9.5 9 8.5 8 7.5 0.8
1
1.2
1.4
1.6
1.8
2
2.2
T -1, 10-3 [ K -1]
Fig. 2. The dose dependence of the concentration profile near the surface CoNi3(1 0 0) rebuilt using the ionisation spectroscopy; i is the number of monolayers.
Fig. 3. The temperature dependence of the isothermal annealing time to CoNi3(1 0 0), required for recovery of the equilibrium surface concentrations after ion bombardment with dose of 1016 ion/cm2.
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All the experimental data mentioned above enable us to assume the following qualitative model for the process of radiation-stimulated phase transition at the CoNi3(1 0 0) surface, which has been observed in our study. In the initial stages of bombardment (up to a dose of B5 1014 ion/ cm2) the generation of the point defects occurs in the surface layer, leading to exponential decay of the intensity of diffraction spots corresponding to the initial f.c.c. (1 0 0) structure (Fig. 1b), without their significant spatial broadening (Fig. 1c) and displacement in the energy scale (Fig. 1d). The increase of point defect concentration in the subsurface layer stimulates the segregation of cobalt atoms (Fig. 2). The increase of cobalt concentration shifts the content of the near-surface region of the sample to the region of the phase diagram of Co–Ni system, in which the low-temperature h.c.p. phase exists. This results in the possibility of the phase transition described. When increasing the dose of bombardment, the elementary point defects form clusters which result in the surface restriction. This effect was observed in many studies [12,13]. Segregating Co atoms come to the surface of the facets produced and form the domains with a close packing corresponding to the (1 0 0 0) face of the h.c.p. lattice. This should occur in the case when the Co–Co interatomic interaction is stronger then the Co–Ni interaction. The reason for such a condition can be the structure defects generated by ion bombardment as well as the subsurface relaxation of the parameters of chemical and magnetic interatomic interaction [14]. Also, an argument for the existence of repulsive interaction between the forming h.c.p. Co monolayer and deeper crystal layers depleted by cobalt, is the strong compression of the interlayer distances of the initial structure, which has been observed in the experiment at an ion bombardment dose of D >5 1014 ion/cm2. The increasing size of the h.c.p. domains at the sample surface resulting in the reduction of the half-width of the corresponding LEED spot (Fig. 1c), screen further generation of the defects in deeper layers. This results in an f:c:c: increase in intensity of I00 ðDÞ due to the annealing of defects produced during the initial stage of ion bombardment (Fig. 1e). The max-
imum of this intensity is reached at the dose of B1015 ion/cm2. Then intensity decay takes place, conditioned by the reduction of size of the domains of the initial structure due to an increase of the degree of the surface covering by the h.c.p. Co monolayer. At the dose of B1016 ion/cm2 practically the 100% Co concentration is reached in the surface monolayer (Fig. 2) and the process reaches dynamical equilibrium. The condition obtained is metastable, i.e. the free energy of the subsurface region is in a local minimum with respect to the number of defects. The thermodynamically equilibrium state may be restored with a high-enough activation energy eA ¼ 0:46 eV.
4. Conclusions We have firstly observed a new radiationstimulated phase transition f.c.c.-h.c.p. in the surface layer of CoNi3(1 0 0). A qualitative model to describe this transition is proposed. We have shown the phase transition caused by strong radiation-stimulated Co segregation on the surface with formation of monolayer coverage.
Acknowledgements The authors are grateful to Dr. S. Mankovsky for helpful discussions This work has been supported by the Foundation for Basic Research of Ukraine, Project No. 04.07/77.
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