Nuclear Instruments and Methods in Physics Research B15 (1986) lOlLlO

North-Holland, Amsterdam

101

STOPPING CROSS SECTIONS OF He+ IONS IN THE METALLIC GLASS Fe,,B,,

KULDEEP and Animesh K. JAIN

Nucleur Ph_vslcs Dwrsion, Bhahhu Aromic Reseurc h Centre, Bomhu,y 400 085, Indru

The stopping cross sections, C(E), of He + ions in the metallic glass Fe,>B,, have been measured by Rutherford backscattertng

spectrometry (RBS) at incident energies of 1.6-3.4 MeV. The samples in three different physical states - (a) as-quenched. (b) relaxed,

and (c) crystallized - have been investigated at each incident energy for a scattering angle of 165”. The stopping cross sections were

determined from the heights of the Fe edge in the RBS spectra. A thin film of gold ( - 120 A thick) was deposited on the samples to

provide an in-built calibration for the total incident He’ charge and detector solid angle. The results obtained are found to be in

agreement with Bragg’s rule for the relaxed glass, while values for the as-quenched and crystalline states were lower by about 10% and 20%. respectively.

1. Introduction

Stopping cross sections of He’ ions in various materials constitute vital data for depth profiling by Rutherford backscattering spectrometry (RBS). For most of the elements, such data exist and are compiled by Ziegler [l]. In the case of multielemental targets, one commonly employs Bragg’s rule of linear additivity [2] to obtain the stopping cross sections. However, signifi- cant deviations from this rule exist depending upon the physical [3] or chemical state [4] of the target. Numer- ous experiments have been carried out to study the vahdity of this law and a compilation of such studies has recently been published by Thwaites (51. Metallic glasses provide an ideal material to investigate the de- pendence of stopping cross sections on the physical state, since they exist in different physical states ranging from amorphous to crystalline depending upon anneal treatments given. Furthermore, experimental data on stopping of He+ ions in metallic glasses are also particu- larly useful as RBS recently emerged as a powerful tool to investigate solid state diffusion in these glasses. Typi- cal diffusion distances in metallic glasses are often quite small - both due to smaller diffusion coefficients as well as to inherent restrictions on the temperature and anneal times that can be used. This fact also accentuates the need for experimental stopping data to obtain accu- rate depth conversion in samples that have undergone different anneal treatments. In this paper, we present our measurement of He+ stopping cross sections as a function of energy in Fe,,B,, alloy in three different states - (a) as-quenched (amorphous), (b) amorphous, but relaxed by an anneal treatment. and (c) crystallized.

2. Experimental

Samples of Fe,,B,, metallic glass of suitable length (- I5 mm) were cut from a 3 mm wide, 50 pm thick ribbon. Some of the samples were relaxed by giving

them a short anneal treatment of 5 min at a temperature of 4OO”C, which is sufficiently below the crystallization temperature (7; = 445’C). Crystallized samples were obtained by a 130 min anneal at a temperature of 530°C which is much higher than the crystallization temperature, T,, as well as the glass transition tempera- ture (T, = 467°C) for this glass. All annealings were carried out in vacuum at a pressure of about 1 X 10d5 Torr. The samples were then electropolished in a solu- tion containing 90% methanol and 10% perchloric acid to produce a mirror finish. A thin film of gold (- 120 A thick) was deposited on these polished samples by vacuum evaporation to provide an in-built calibration for the accumulated charge and detector solid angle in the RBS spectra. The thickness of - 120 A was care- fully optimized to give reasonable statistics in the Au yield, at the same time keeping the energy loss in the film at a negligible level (around a few keV). The gold film was also deposited simultaneously on several well- polished 99.999% pure aluminium substrates kept in the immediate vicinity of the metallic glass samples. The arrangement of the samples during vacuum evaporation of gold is schematically shown in fig. 1. Two aluminium substrates (- 25 mm x 25 mm) separated by a distance of - 5 mm were kept over a circular mask of - 20 mm diameter aperture. as shown in the figure. One metallic glass sample was placed across a pair of such aluminium substrates as shown in fig. 1. The aluminium substrates

0168-583X/86/$03.50 ,D Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

III. STOPPING POWERS

102 Kuldeep, A. K. Jain / Stopping cross sections of He + ions in Fe,, B,,y

Evoporation source

Fig. 1. The schematic arrangement of samples during vacuum

deposition of gold. The gold-deposited areas on different sam-

ples are shown in the insets. The gold thicknesses on aluminium

samples were measured by RBS at regions marked by a dot.

thus serve as a mask for metallic glass sample to expose an area of - 5 mm X 3 mm. The gold-deposited regions on each of the samples are shown schematically as insets in fig. 1.

For the gold film to serve as an accurate calibration for incident He+ charge and detector solid angle, it is necessary to know the precise thickness of the film on each sample. Although the source-to-substrate distance was kept sufficiently large (- 15 cm), small thickness variations across the sample cannot be ruled out. In order to minimise systematic errors on this account in our experiment, we have followed the following proce- dure. We measured the thickness of gold by RBS on all the aluminium samples at places about 1-2 mm away from their edges (shown by a dot in the insets in fig. 1). Thus measurements on a pair of such aluminium sub- strates gave the gold thickness immediately on either side of the deposited area of the corresponding metallic glass sample. Since the deposited area on metallic glass samples is quite small (about 5 mm across), the gold thickness near the centre of this area can be estimated with good accuracy by the average of gold thicknesses measured on corresponding pairs of aluminium sub- strates. The experimentally measured thicknesses on a pair of aluminium substrates agreed within < + 3%, which is comparable to the overall accuracy of an absolute thickness determination by RBS. This also confirmed good uniformity of deposition and the valid- ity of our approach for thickness estimation on metallic glass samples.

The stopping cross sections of He+ ions in different samples were estimated from the Fe heights in the RBS

spectra. The detailed analytical procedure is described in sect. 3. The RBS spectra were recorded employing a well-collimated 0.5 mm diameter He+ beam from a 5.5 MeV Van de Graaff accelerator. The beam was incident normal to the sample and the backscattered ions were detected at a laboratory scattering angle of 165” using a surface barrier detector and the usual electronics. The incident beam energy was varied from 1.6 to 3.4 MeV, in steps of 200 keV. The total charge collected at each incident energy was adjusted to keep the height (num- ber of counts per channel) of the Fe edge at - 104, and thus maintain a uniformly good statistics.

3. Formalism for data analysis

The observed height of the Fe edge can be expressed

by

H,,= Q~~F~(E,)~SE/[~(E,)]~~. (1)

Here Q is the number of incident ions, D is the solid angle subtended by the detector, o,,(E,) is the dif- ferential scattering cross section for laboratory scattering angle 0 for Fe at energy E,. which is the energy of He+ ions immediately before scattering at the film/substrate interface, m is the atomic fraction of Fe in the alloy Fe,,B,, (i.e. 0.82 in this case), and SE is the energy width of a channel. The quantity [e( E)]FCG used in eq. (1) is the “stopping cross section factor” (see e.g. p. 79 of ref. [6]). The superscript MG refers to stopping in the alloy. Since the film thickness of gold is small, the energy loss of incident He+ ions in the inward path is negligible (- 8 keV) even at the lowest incident energy of 1.6 MeV. Thus E, can be safely replaced by the incident energy E, in eq. (1). Since direct accurate measurement of Qs2 is often difficult, we have de- termined this quantity from the area under the gold peak in the RBS spectra taken at each energy. Using a standard expression for the total number of counts, A, under the gold peak and eliminating Qti from eq. (1). we get for the stopping cross section factor,

ir(E

0 >J""= (A/H,,)(Z,,/Z,,)2(mSE)

Fe Nt (2)

where Nt is the number of Au atoms per unit area and the scattering cross section is assumed proportional to Zf_,,. The Nr used in eq. (2) has been obtained accu- rately by taking the average of the Au thicknesses measured by a standard RBS procedure on the pair of Al samples which are kept in the immediate vicinity of the metallic glass sample as described in sect. 2. The stopping cross section factor [ c( E,)]z~cG was determined

for all three types of samples at each energy using eq. (2). The stopping cross section, E(E) at an “average energy” E is finally obtained from the measured

[E( E )]?” values using the following relations based on 0 l-r ref. [6],

where

(3)

(4)

It should be noted from eqs. (3) and (4) that measure- ment of the stopping cross section factor at energy Is;;, does not yield the stopping cross section r(E) at the same energy E,, but at an average energy E. This is because of the fact that the stopping cross section factor

contains stopping at both E. as well as K,,E,. As

shown in ref. [6], e(E) values obtained using eqs. (3) and (4) take care of the energy dependence of the stopping cross section up to second order.

4. Results and discussion

The stopping cross sections, c(E). of He’ ions in the metallic glass Fe,, B,, in three different states have been determined at incident energies from 1.6 to 3.4 MeV using the method described in sect. 3. The E values at each energy were evaluated for all three types of sam- ples. The measured e values for as-quenched (filled circles), relaxed (open circles) and crystallized (squares) samples are shown as a function of energy in fig. 2. The continuous curve in fig. 2 depicts e(E) for FezZB,,

1 He+ IONS IN Fe,,& METGLASS

I

$O’ U 1 BRAGG'S RULE

l as quenched o relaxed (MIO*C,S~I~)

0 crystallized (S30°C,~30min}

2 70'

B

_'i [

9' =: 60~

ii t

$ 5oj-

% i

'; on

"OO 1.0 2.0 3.0

ENERGY ( MeV)

Fig. 3. The stopping cross sections of He+ ions in as-quenched (filled circles), relaxed (open circles), and crystallized (squares) Fe,,R,, metallic glass as a function of energy. The continuous

curve represents stopping cross sections based on Bragg’s rule.

The erFOr bar in the figure depicts an error of 15%.

calculated from Bragg’s rule by using the tabulated

stopping cross sections for Fe and B from ref. [l]. We

find that our measured values for the relaxed glass (open circles) are in good agreement with Bragg’s rule (see fig, 21, while values for the as-quenched (filled circles) and crystalline {squares) states are lower by - 10% and 20% respectively over the entire energy

range studied. There is a possibility, in principle, that the observed

differences in stopping cross sections could arise from a systematic error in estimating the gold film thickness, which is used as calibration (see eq. (2)). However, actual RBS measurements on aluminium samples show

that the film uniformity is reasonably good (- i 3%). Furthermore, our estimates of film thickness on each metallic glass sample are based on RBS measurements on corresponding pairs of symmetrically placed alun~inium samples (see fig. I). This procedure is ex-

pected to yield fairly accurate values for the film thick- ness, Thus the possibility of such large variations (up to about 20%) in stopping cross sections arising solely due to the uncertainty in film thickness can be ruled out. In addition, we have also considered the ratio of the height of the Au signal to the height of the Fe edge, which should be proportional to the stopping cross section of the substrate (i.e. FeazB,,). These ratios also show a similar trend for the three physical states considered. Absolute stopping cross sections could not. however. be deduced from these ratios as the film thicknesses were comparable to the detector resolution and the gold height did not correspond to the bulk gold height in the RBS spectra.

From the present study alone it is not possible to pinpoint the exact reason for the observed differences in stopping cross sections as a function of anneal. At the same time such a result is not surprising either, since it is well known that considerable structural relaxations

can take place in metallic glasses upon annealing. For example, reversible as well as irreversible changes in several physical properties such as Young’s modulus 171,

Curie temperature [8], electrical resistivity [9], positron lifetimes [lo], and embrittlement [II] are found to occur upon thermal relaxation. In the case of the crystallized sample, the phases that precipitate are expected to a-Fe and Fe,B since the composition of the glass Fe,,B,s is close to the eutectic composition. The crystallized sam- ple thus has an entirely different microstructure as well as chemical state of atoms, as compared to the as- quenched or relaxed glass. These features can be ex- pected to give rise to the observed differences in stop- ping cross sections. Since the stopping of He+ ions at

RiIeV energies is primarily by ineiastic collisions with the electrons, our results suggest that consequent upon structural rearrangement, significant changes in the electronic density distribution may be brought about by different anneal treatments in this glass.

III. STOPPING POWERS

104 Ku!deep, A.K. Jain / Stopping cross sections of He + ions in Fe,? B,,

The authors would like to thank SK Sharma for providing the metallic glass samples, M.J. Kansara and V.P. Salvi for assistance during RBS work, and the operation staff of the Van de Graaff laboratory, BARC, for skilfui operation of the accelerator.

References

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vol. 4 (Pergamon, New York, 1977).

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[3] S. Matteson, E.K.L. Chau and D. Powers, Phys. Rev. Al4

(1976) 169.

[4] D. Powers, H.G. Olson and R. Gowda, J. Appi. Phys. 55

(1984) 1274.

[5] D.I. Thwaites. Radiat. Res. 95 (1983) 495.

[6] W.K. Chu, J.W. Mayer and M.A. Nicolet Backscattering

Spectrometry (Academic Press, New York, 1978) p. 279.

[7] M.G. Scott and A. Kurumovic, Acta Metali. 30 (1982)

853.

[S] T. Egami, Mater. Res. Bul. 13 (1978) 575.

[9] T. Komatsu, M. Takeuchi, K. Matusita and R. Yokota, J.

Non-Cry%. Sol. 57 (1983) 129.

[IO] H.S. Chen and S.Y. Chuang, Appl. Phys. Lett. 31 (1977)

255.

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