FIM INVESTIGATION OF ION-IMPLANTED Cu3Au ALLOY

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FIM INVESTIGATION OF ION-IMPLANTED Cu3Au ALLOY

V. Ivchenko, N. Syutkin, A. Bunkin

To cite this version:

V. Ivchenko, N. Syutkin, A. Bunkin. FIM INVESTIGATION OF ION-IMPLANTED Cu3Au ALLOY.

Journal de Physique Colloques, 1988, 49 (C6), pp.C6-379-C6-383. �10.1051/jphyscol:1988665�. �jpa-

00228162�

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JOURNAL DE PHYSIQUE

Colloque C6, suppl6ment au n0ll, Tome 49, novembre 1988

FIM INVESTIGATION OF ION-IMPLANTED Cu,Au ALLOY

V.A. IVCHENKO, N.N. S W T K I N , A.Yu. BUNKIN

Institute of Electrophysics, Ural Division of U.S.S.R. Academy of Sciences, Sverdlovsk 620219, U.S.S.R.

Abstract

-

FIN technique has been applied to investigation of structural state of an ordered alloy implanted by A r + and I?+ ions. An ordering-disordering phase transition has been observed in the ordered solid solution Cu Au after irradiation with ionstenergies 15-20 keV and doses 1

o ~ ~ - I o ~ ~ ~ ~ ~ - ~ .

A neahsurface region which has remained ordered was found to contain a lot of radiation defects such as dislocations and segregations of copper atoms. The size of the radiation-disordered region has been measured in the process of controlled removal of atomic layers from the surface of specimens irradiated in different crys- tallographic directions. The depth of the modified zone is shown to exceed 250 nm. The nature of the structural defects is discussed.

I INTRODUCTION

Ion implantation is a promising technique for modification of surface properties of different materials. The experimental study of the interaction of charged particles with matter, of the type of radiation-induced defects and their distribution in the surface layers of solids is therefore an urgent task of radiation materials science. The phase transitions in alloys stimulated by implantation are of special interest in this connection.

FII technique is shown to be an effective method for investigation of structure changes induced by irradiation of ordered alloys. The applica- tion of this technique is stimulated by the following advantages: visualiza- tion of pure surface in atomic scale at cryogenic temperatures and the possi- bility of analysis of crystal lattice defects in the bulk as a result of controlled evaporation of surface atomic layers by highly intensive electric field.

In the present investigation the interaction of Ar+ and N+ ion beams with a well-ordered alloy Cu3Au has been studied. The change of phase state is revealedas aresult of irradiation with ion energies of 15-20 keV and doses of 1 016-1 018 c ~ I - ~ . Firstly, the irradiation caused the disordering of the solid solution; secondly, it stimulated the formation of crystal structure defects in the region where the ordered structure has remained.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988665

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JOURNAL DE PHYSIQUE

I1 EXPERIMENTAL

ATI atomically smooth crystal surface, prepared preliminarily in si- tu by field evaporation has been irradiated. The size of thermal domains with a high degree of long-rang order exceeded 500 rim. The tip radius of the investigated specimen did not exceed 30-50 nm in most cases, hence, the probability of observation of antiphase domain boundaries (BPB) has been ve- small. The implantation of specimens with axes parallel to the crystallogra- phic direction < I l l >

,

has been carried out in the direction close to

<112>

,

the implantation of specimens with axes parallel to the <001>

d i r e c t i o n - i n < O O I > d i r e c t i o n . After irradiation the specimens were placed in- to FIM again. The analysis.of structure state in the bulk has been carried out in the process to controlled removal of surface atomic layers and surface image registration by a still camera. The depth of the region examined exceeded 500 nm in some cases.

I11 RESULTS AND DISCUSSION

It must be noted that only an ordered Cu3Au crystal has a regular ring ion image created by gold atoms only /I/. The surface of a disordered alloy gives a structureless contrast on the microscope screen corresponding to the equal probability for atoms of components to occupy the solid solution crystal lattice sites. The active irradiation zone is therefore revea- led by its structureless contrast against a background of a regular ring image given by a part of a crystal which has remained ordered.

The results of observations show that the surfaces of specimens ir- radiated with a dose of 1018 c'2m are highly damaged by the ions implanted.

Such specimens are in most cases not suitable for further FIP investigation for they are usually destroyed by electric field at the first stage of surface formation.

The typical micrograph of the surface of a specimen irradiated in the 4 12, direction with a dose 10 c16 m'2 is given in Pig. I. The image is found to consist of three zones: the first one is an ordered zone identified by the regular ring image given by gold a t o m of L12 -type ordered Cu Au al-

3 loy. The second zone is disordered one, located in the opposite part of the ion image which is identified by its structureless contrast. Finally, between these two zones a transitional region is seen which is a mixture of two phases:= ordered pbase and a disordered one. The size of radiation

-

disordered zone in the case of irradiation by 15 keV Ari ions with a dose of 1016 cm'2 has been found to be about 35 nm; in the case of irradiation by N+

ions with the same energy and dose this value has been found to be

-

¹⁶^nm.

Using Pig.2 (irradiation parallel to the COO?> direction) one may follow structural changes in the bulk of the material as one moves away from the irradiated surface. The region of ordered material saturated with crystal structure defects lies beyond the disorderea zone and the two phase

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Fig.1 a -neon image of the surface after ~'im~lantation U -11.5 kV; b -image diagram.

[OOlI

t

radiation-

-"

V=6.5rV

i

^v.7.5nv

defect

-

saturated ordered aone (&i)

> o m

v. ⁽⁰^KV

Pig.2 Structure of specimen implanted with Ar' in the <001> direction at depths of 35,85 and 225 nm.

mixture zone; its depth is more than 200 nm. This figure exceeds the calcu- lated from /2/ "amorphousn projected ranges for Ar+ and N+ ions (

-

^{10 and}

20-25 nm, respectively) by an order of magnitude. This fact and the fact that the depth of radiation-disordered region in the case of argon implantation is twice as much as the depth of the zone disordered by nitrogen ions may be explained by channeling of ions moving in low-index directions <001>

and <112>

.

Electron stopping is characteristic for channeling ions, so the particles of higher mass penetrate deeper /2/.

The most typical defects are segregations of copper atoms, disloca-

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C6-382 JOURNAL DE PHYSIQUE

tions and antiphase boundaries. Structural defects have been identified by the disturbance of the regular ring image of low-index planes. Radiation

-

induced sessile dislocation configurations (Fig.3), dislocation pile

-

^ups

(Fig.4) have been found. Analysis of ion images shows that spiral contrast (Fig.4) does not correspond to the image of dislocations having Burgers vec- tors typical for a dissociated superdislocation. So this configuration is a complex of several split dislocations.

Fig.3 shows the ion image of one of the sessile dislocation confi- gurations formed as a result of implantation. A consecutive transition of defect planes one into another, typical for such configurations, is observed in the process of field evaporation of surface atoms. The defects are usual- ly found in the {001) and {I I l} planes i n the case of barriers Fig.3b.

Fig.3 Ion contrast of sessile dislocation configurations.

Eg.4 Ion contrast of a dislocation pile-up.

Pig.5 Ion contrast of a Cu atoms segregation.

Analysis of ion images of the two-phase mixture zone reveals dark regions (Fig.5). The latter have been interpreted as segregations of copper atoms, since these atoms are not imaged in the ion patterns of the ordered Cu Au alloy /I/. The size of the segregation Fig.5 has been found to be

3

5x20~1 nm. The existence of such defect regions confirms the phenomenon of separation of atom species which can occur as a result of charged particles

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interaction with matter.

According to the results achieved the FIM technique may be regarded as an instrument for diagnostics of the ordered alloys' surface after im- plantation. 'Phe direct investigation of near-surface layers by controlled field evaporation of atoms provides information about phase state changes in the material after irradiation. The ordered solid solutions are also metho- dically convenient objects due to the perfect regular ion image of their surface. Any phase transition, specifically the ordering-disordering tran- sition, is identified by the change of the ion contrast of the implanted part of a crystal.

On the other hand, it is possible to attain different aims in stu- dying the material structure by changing the geometry of irradiation. Thus, w i n g the technique of implantation Fig.2 it is possible to estimate the depth of the modified region. In our case under the irradiation conditions mentioned above the depth of the modified region exceeded 200-250 nm. This region contains a completely disordered zone, a transitional two-phase (or- dered-disordered) mixture zone, and finally, an ordered zone saturated with radiation defects. The study of specimens irradiated in this way has revea- led structural defects such as sessile dislocation configurations, disloca- tions arrays and segregations of copper atoms at a depth of 200-250 nm which exceeds the prodected range R of N+ and Ar' ions by an order of magnLtude.

The change of the geometry of irradiation, Fig.1, allows one to es- P timate the profile of the completely disordered region according to the cal- culation of the mean specimen radius. Each ion image, recorded in the pro- cess of field evaporation of the surface atoms layer by layer shows the boundary between the radiation-disordered zone and the part of the crystal which has remained ordered. The size of the radiation-disordered zone is a function of the ion species; it increases with the growth of ion1 mass. This is due to the channeling effect.

1. V.A. Ivchenko, N.N. Syutkin, Fix. Met. Metall.

2

(1986) 575.

2. J.W. Mayer, L. Eriksson, J.A. Davis, Ion Implantation in Semiconductors.

Academic Press, New York-London, 1970.