Ion-induced emission from cobalt single crystal under polymorphic transformation

Nuclear Instruments and Methods in Physics Research B33 (1988) 538-542 North-Holland, Amsterdam

538

ION-INDUCED EMISSION FROM COBALT SINGLE CRYSTAL UNDER POLYMORPHIC TRANSFORMATION V.A. ABRAMENKO,

A.A. ANDREEV

and V.E. YURASOVA

Department of Physics, h4oscow State Uniuersify, Moscow 117234, USSR

H.A. MOTAWEH Department of Physics, Faculty of Science, Tanta University, Tanta, Egypt

Scattering processes in Co single crystal under keV Nef, Ar+ and Kr+ bombardment are studied in case of hcp-to-fee polymorphic transformation. The yields of scattered Ne + ions and photons emitted from scattered Ne” (Ne I tine, X = 584 nm) are shown to decrease sharply under the hcp-to-fee polymorphic transition in Co (n-j3 transition). A correlation has been found among the angular dependences of the yields of scattered Ne + ions and of photons emitted by the scattered Ne atoms. Change in Co+ emission by keV Kr+ bombardment at the transformation of Co has been found to depend on the Kr+ ion energy and the orientation of the incident Kr+ on the Co(OOO1) surface.

1. Introduction Phase transitions of the first and second kinds in metals affect in practice all the secondary-emission processes [l]. The simultaneous measurement of secondary ions, photons and scattered ions under magnetic phase transition in Ni (second-kind transition) was reported by us in ref. [2] (see also [3-51). The present work is the first to carry out a simultaneous study of the effects of the polymorphic phase transition in monocrystalline cobalt on ion scattering and on ion-photon emission from the scattered excited-state atoms of a primary Ne+ or Art beam.

2. Ion scattering and ion-photon 2. I. Experimental

emission

technique

The experiments were performed with a multichannel installation which permitted simultaneous measurements of ion-photon emission (IPE), secondary-ion emission (SIE), ion electron emission (IEE) , and ion scattering from the same bombarded zone under identical experimental conditions. The main units of the installation were: the operation chamber with a system of differential outpumping, a duoplasmatron-type ion source with a system for focusing the primary ion beam, two identical spherical electrostatic analyzers (one of them is combined with a time of flight mass separator), a unit to hold the sample permitting heating and rotation in two mutually-per0168-583X/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

pendicular directions. The axes of the ion source, the energy analyzers, the mass separator and the normal to the sample surface were coplanar. The angular and temperature dependences of the scattered ion yield were studied simultaneously while analyzing the photon yield from the scattered particles of the primary beam. The photon yield was measured in a direction perpendicular to the primary beam in the plane of the sample. The experiments were carried out at the residual gas pressure P = 5 x lo-’ Torr in the operation chamber. The primary beam current density was 200 pA/cm2. To purify the specimen surface the specimen was heated at 500°C with simultaneous Ar+ ion beam bombardment for 1.6-2 h. As a result the Ar+ ion single scattering peak increased substantially (the peakto-background ratio is 8-10). This means that the concentration of oxygen-containing compounds and carbon film decrease substantially after the purification. The energy spectrum of scattered ions was recorded simultaneously during heating and cooling of the specimen. The temperature dependence of the single scattering peak was measured in a pulse counting mode. At a fixed temperature the signal accumulation time was 10 s. 2.2. Temperature

dependence

Fig. 1 shows the temperature dependence of the single-scattering peak of 7 keV Ar+ ions incident along the normal to the a-Co (0001) surface and scattered at an angle y= 135’. The dependence was obtained for the sample under cooling. From the figure it can be seen

539

V.A. Abramenko et al. / Ion-induced emission from cobalt 2.3. Angular dependence

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6200~W~‘*(~,, .z . 2 160-

0011",,,,l,,,,,,~,,

,

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~‘20,--,. TEMPERATURE

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Fig. 1. Temperature dependence of the yield of singly-scattered Ar+ ions incident along the surface normal onto the (111) or (0001)Co face at E,, = 7 keV in the case of scattering at y = 135 o (obtained for cooling).

that the scattered ion yield increases by 20% from 375 to 340°C. The same behaviour is exhibited by the temperature dependence of photon emission from the scattered Ne atoms and Ne+ ions (fig. 2). Under (Y--, /3 transition of Co the (0001) face on the surface turns into a (111) face. Packing of the closest-packed atomic layers is modified and the width of the [ill] channel becomes narrower compared with that of the [OOOl]. It was expected, therefore, that the scattering of normally incident ions on the (111) face must exceed the scattering from the (0001) face. However the opposite experimental situation was observed. This observation is probably related to the fact that the hcp-fee transition in Co results in an increased density of electron states near the Fermi level [6] and, hence, in a higher probability of neutralization and de-excitation [7].

2.3.1. Ion scattering In studying the angular distribution of ions for Ne+ ion incidence on a Co single crystal surface in the case of the hpc and fee modification, the scattering angle y was fixed. The primary beam incidence angle 6 (measured from the surface normal) was varied from 0” to 40”, and the azimuthal observation angle 4 was varied within 60 “. The measurements were made at three values of the ion scattering angle y = 115 “, 125 o and 135O. From fig. 3 it can be seen that the anisotropy of the angular dependence of ion scattering under ion beam incidence in the (1120) plane is most remarkable at the scattering angle y = 115O (the maximum at 6 = 18 o for y = 115 “). The following two circumstances are responsible for such a situation. First, the fraction of singly-scattered ions, for which the angular dependence anisotropy does not hold, increases with the scattering angle. Second, anisotropy of primary beam scattering arising from the changes in single crystal transparency as a function of incidence angle is superimposed on the anisotropy of the spatial distribution of scattered ions (at a fixed primary-beam ion scattering angle y the observation angle also varies with changing the incidence angle S). The highest value of the maximum at the incidence angle S = 18” for the scattering angle y = 115O, as compared with y = 125 o and 135 O, is probably due to the particles from the shadow region formed by the [4403] close-packing direction which lies in the (1120) plane and has an angle of 55O with the normal to the (0001) surface.

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6402A

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400 TEMPERATURE

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1

Fig. 2. Temperature dependence of the yield of photons emitted from Ne atoms and from Ne + ions scattered in their excited states. The angle 6 = 20° (curve 1) and 6 =12O (curve 2) for incidence in (1120) plane.

INCIDENCE

ANGLE(*)

Fig. 3. The dependence of Ne+ ion scattering on the incidence. angle at the scattering angles y = 115O, 125“ and 135“ incidence in (1120) plane. VII. SPUTTERING/SIMS

540

V.A. Abramenko et al. / Ion-induced emissionfrom cobalt NC?’

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(a)

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20 &(‘I

30

bf’)

(b)

fi transition in a Co single crystal on dependence of the yield of photons scattered in their excited state. The are the same as in fig. 4.

40

(b)

Fig. 4. Effect of the a -+ # transition in Co single crystal on the incident-ion-anguI~ dependence of ion scattering. (a) Incidence in (1120) or in (011) planes, (b) incidence in (10x0) or (112) planes. Curve 1: for (0001) Co plane bombarding (T = 320° C); Curves 2: for (111) Co plane bombarding (T= 490°C); the scattering angle y = 115O.

Under polymorphic transformation from LYto 8, the angular distribution of scattered ions changes, namely, (1) the width of the main minimum at 8 = 0 o becomes narrower to 15” because the [ill] channel is less open than [OOOl], and (2) the second ~~rnurn in the fee Co phase near S = 32O appears oniy when the primary beam is incident in the (112) plane, i.e. near [153], (see curves 1 and 2 in fig. 4). When the incident ions are deflected by 30 o from the (112X i.e. when the incidence plane coincides with the (017) plane, the ~~rnurn shifts towards larger angles (the incidence along the [Oli] channel). It must also be noted that the ratio of the highest yield of scattered-particles (at 8 = 10’ for the fee phase and 6 = Ho for hpc phase) to their yield under normal incidence of primary particles changes from 1.75 to 1.45. Because the [OOOl] channel is wider than the [ill] channel, the minimum is deeper for the [OOOl] than for the [ill] direction. 2.3.2. Photon emission from the scattered atoms of the primary beam Fig. 5 shows the incident-ion-angular dependences of photon emission (NeI, 5852 A) from the Ne atoms scattered from the (0001) and (111) Co faces. It can be seen that as the primary ion incidence angle increases

the NeI 5852 A line emission intensity increases initially and reaches a maximum at about 6 = 15 ” ), whereupon it remains constant up to 6 = 25*. As the incidence angle increases further, the NeI 5852 A intensity rises abruptly. On the whole, the polymorphic transition does not affect the characteristic feature of the angular dependence of photon yield from scattered projectiles. The only exception is the shift of the angle of incidence 6 where the yield maximum is observed. The same was also observed for the yield of scattered ions.

Ne+ co(ooo1)

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6

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Fig. 6. The dependences of the intensity of ion-photon emission from sputtered Co atoms (CoI) and from scattered Ne atoms (NeI) on the energy of the primary Ne” ion beam.

541

V.A. Abramenko et al. / Ion-induced emission from cobalt 2.4. Energy dependence of ion-photon scattered and sputtered atoms

emission

from =: ‘C

Fig. 6 shows the results of the measurement of the effect of primary Ne+ ions with energy EO bombarding the Co (0001) face on the yield of photons emitted from sputtered Co atoms (X = 3412, 3453, 3464, 3507, 3513, 3530 A curve 1) and by reflected Ne atoms (X = 5400 A curve 2). From the figure it can be seen that photon emission from sputtered Co atoms increases steadily with primary ion energy and shows an almost 50% increase in the 4-7 keV range, while the photon emission from reflected Ne atoms remains constant in this energy range. The increase of Co1 yield with E,, is probably due to the increase of the Co single crystal sputtering yield. As for the behaviour of NeI yield as a function of E,, it indicates that in the given energy range the probability of Ne+ ion neutralization accompanied by the production of excited states is independent of the energy E,.

3. Secondary ion emission 3.1. Experimental

conditions

Residual gas pressure was 8 X 10-i’ Torr after baking the operation chamber at 200 o C for 3 to 4 h. When the ion source was operated, the pressure in the sample chamber increased up to (4-8) X 10m9 Torr. The ion beam was incident parallel to the surface normal and at angle S = 12O from the normal. Mass separation of the 20-60 eV secondary ions was performed. 3.2. Temperature

dependence

Fig. 7 shows the results of the measurements of the secondary ion emission from a Co polycrystal under

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400 TEMPERATURE

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Fig. 8. Temperature dependence of 59Co+ ion emission from the (0001) Co face under (1) normal 8 keV and (2) 4 keV Krf ion bombardments. The curves were obtained during heating: y=45o, Es = 20-60 eV, j = 30 mA/cm*.

polymorphic o - p transformation. It can be seen that the 59Co+ ion emission decreases by 30% as the specimen is cooled from 375 to 300 o C. Curve 1 in fig. 8 is the temperature dependence of 59Co+ ion emission from the (0001) Co face under normal 8 keV Kr+ ion bombardment. As the primary ion energy decreases from 8 keV to 4 keV, the 59Co+ yield does not vary under polymorphic transformation in Co (see curve 2 in fig. 8) From fig. 9 it can be seen that when the (0001) Co face was bombarded at 12” from the direction of the [OOOl]open channel the 59Co+ yield varies inversely to what was found for the [OOOl]channel bombardment and is the same as in the case of 59Co+ emission from polycrystal (see fig. 7). A discussion of these results on secondary ion emission during the a + /? transition in Co can be found in ref. [l].

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400

TEMPERATURE

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Fig. 7. Temperature dependenceof “Co+ secondary ion emission from Co polycrystal under 8 keV Kr+ bombardment at 6 = 0, j = 30 mA/cm’, where the energies of secondary 59C~f ions were 20-60 eV and the observation angle was 45 O.

L 200

300

400

TEMPERATURE

500 ( ‘Cl

Fig. 9. Temperature dependence of 59Co+ ion emission from the (0001) Co face under (1) 8 keV and (2) 4 keV Kr+ bombardment in a random direction (12” from [OOOl]) y = 35 O, j = 30 mA/cm*, E, = 20-60 eV. VII. SPUTTERING/SIMS

542

KA. Abramenko

et al, / Ion-induced emission from cobalt

4. Conclusions

(1) A jump-like decrease under a polymorphic o + p

transition in a Co single crystal ((0001) --) (111)) has been found not only in the number of scattered Nei ions but also in the number of scattered excited NeI atoms emitting the X = 5852 A photons. (2) The angular dependence of charged and neutral excited components has been found to change under Q + B transition in Co. (3) The yield of ion-photon emission from the (0001) Co face has been found to rise in the case of sputtered atoms (the Co1 line) and to remain constant in the case of scattered atoms (NeI) with varying of the bombarding Ne+ ion energy from 4 to 7 keV. (4) The secondary ion emission from a Co polycrystal during the (Y+ /3 transition decreases. (5) The variation of the Co+ secondary ion emission during the (Y-+ fi transition in a single crystal depends si~fi~tly on the primary ion energy and the direction of incidence. In the case of the incidence of 8 keV Kr + in a random direction (8 = 12 o ), the emission variation is of the same sign as for a

polycrystal. In the case of the bombardment along the [Oool] sign of the ion emission variation for polycrystals, i.e. the Co+ ion during the (Y-+ fl transition.

(0001)Co face for open channel the is opposite to that emission increases

References [I] V.E. Yurasova, Vacuum 36 (1986) [2] V.A.

609.

Abramenko, A.A. Andreev, G.A. Dubsky, M.V. Kuvakin, L.B. Shelyakin, V.E. Yurasova and H.A. Motaweh, Nucl. Instr. and Meth. B13 (1986) 609. [3] B.A. Brusilovsky, Proc. Vth All-Union Conf. on Atomic Particle Interactions with Solids, Minsk, Part I (1981) p. 131. [4] V.A. Abramenko, G.A. Dubsky, D.V. Ledyankin and B.M. Manaev, Proc. VI&h All-Union Conf. on Atomic Particle Interactions with Solids, Minsk, V. 1 (1984) p. 68. [S] V.I. Bachurin, Thesis, Moscow State Univ., Moscow (1982); Yu.A. Bandurin, V.I. Bachurin Pop, V.S. Chemych and V.E. Yurasova, Pis’ma Zh. Eksp. Tear. Fir. 3 (1982) 760. 161 S. I&da, J. Phys. Sot. Jpn 23 (1972) 369. [7] G. Blaise and G. Slodzian, J. Phys. (Paris) 34 (1974) 237. [8] V.T. Cherepin and V.E. Yurasova, Proc. Symp. on Diagnostics of Surface by Ion Beam, Uzhgorod (1977) p. 552.