Nuclear Instruments and Methods in Physics Research B 212 (2003) 238–241

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Mixing in Au/Si system by nuclear energy loss

Sarvesh Kumar a,*, P.K. Sahoo b, R.S. Chauhan a, D. Kabiraj c,Umesh Tiwari d, D. Varma d, D.K. Avasthi c

a Department of Physics, R.B.S. College, Agra 282 002, Indiab Department of Physics, Indian Institute of Technology, Kanpur 208 016, India

c Nuclear Science Centre, Aruna Asaf Ali Marg, New Delhi 110 067, Indiad Solid State Physics Laboratory, Timarpur, Delhi 110 054, India

Abstract

In the present work, we report the formation of silicide phases in the Au/Si system by ion beam mixing at room

temperature. The samples (58 nm Au on Si) were irradiated by 1 MeV Xe ions. The ion energy was chosen in such a way

that it deposits maximum energy at the interface. The Rutherford backscattering spectrometry measurements were done

on the pristine and irradiated samples to determine the composition of mixed region. Grazing incidence X-ray dif-

fraction measurements were performed which showed the formation of silicide phase (Au2Si, Au3Si, Au5Si and Au5Si2).

Scanning electron microscopy measurements indicated the micron size crystallites in the irradiated samples.

� 2003 Elsevier B.V. All rights reserved.

PACS: 68.55.Ln; 68.55.Nq; 68.37.Hk

Keywords: Ion beam mixing; Phase formation; Surface morphology

1. Introduction

Ion beam mixing is a technique for the forma-

tion of stable, metastable, amorphous and crys-talline phases in the bilayer and multilayer [1]. In

case of low energy (keV/nucleon) region, elastic

collisions between the ion and the target atoms

dominate the slowing down of the ion and this

nuclear energy loss induces recoil cascades. It

causes interface mixing by direct displacements

(ballistic mixing), diffusion in overlapping subcas-

cades (thermal spike mixing) and by the thermallyactivated migration of the remaining radiation

* Corresponding author. Tel./fax: +91-562-2520075.

E-mail address: guptask10@yahoo.com (S. Kumar).

0168-583X/$ - see front matter � 2003 Elsevier B.V. All rights reser

doi:10.1016/S0168-583X(03)01738-5

defects (radiation enhanced diffusion) [2–5].

Therefore nuclear energy loss is considered to be

responsible for ion beam mixing. Growth of epit-

axial gold silicide islands has been observed whenan Au film deposited on a bromine-passivated

Si(1 1 1) substrate was annealed at eutectic tem-

perature. The islands grow in the shape of equi-

lateral triangles, reflecting the symmetry of the

(1 1 1) substrate, up to a critical size beyond which

the symmetry of the structures is broken, resulting

in a shape transition from triangle to trapezoid [6].

The formation of fractal and the isolateral trian-gles is also observed when the sample is irradiated

at the eutectic temperature of Au–Si system. The

fractal growth and triangle growth could be the

effect of thermal annealing. Complete growth of

shape and size of the triangles may depend on the

ved.

Fig. 1. Rutherford backscattering spectra (RBS) of a thin (58

S. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 238–241 239

ion irradiations compared to unirradiated samples

[7]. When Au–Si samples was irradiated with 300

keV Xeþ ions at room temperature (RT), it com-

pletely mixed with Si and formed amorphouscomposition of Au5Si2 through out the mixed layer

[8,9]. The formation of crystalline Au5Si2 phase is

observed when the samples is irradiated at 150 �C.The formation of new phase occurs when the

samples is irradiated at 363 �C (the eutectic tem-

perature of Au–Si system) with 120 keV Arþ ions.

The mixture of Au and Si is only found when the

sample is irradiated by 120 keV Arþ at 250 �C [7].The present paper discusses the formation of sili-

cide phases in Au/Si system by ion beam mixing at

RT. The surface morphology due to irradiation at

RT is also discussed.

nm) Au film on Si(1 0 0) substrate after being irradiated with 1

MeV Xe10þ ions to a fluences of 1� 1015 and 5� 1015 ions/cm2

at RT.

Fig. 2. Grazing incidence X-ray diffraction spectra (GIXRD)

pattern of Au/Si system: (a) as-deposited, (b) Xe10þ ions ir-

radiated at RT to a fluence 5� 1015 ions/cm2.

2. Experiment

The film (58 nm) of Au was deposited on (1 0 0)

Si substrate at RT using an electron gun in the

ultrahigh vacuum deposition system. The vacuum

before deposition was 8� 10�8 Torr and during

deposition was 2� 10�7 Torr. The substrate was

cleaned and etched by the usual procedure before

deposition. The samples were irradiated by 1 MeV

Xe10þ ions to fluences of 1� 1015 and 5� 1015 ions/cm2 using the low energy ion beam facility at

Nuclear Science Centre, New Delhi in 5� 5 mm2

area. The flux was 3.1� 1010 ions cm�2 s�1 during

irradiation. The range (98.8 nm) and energy loss of

the incident ions were calculated using the SRIM

program. The Rutherford backscattering spect-

rometry (RBS) measurements on the pristine and

irradiated samples were carried at the van de Grafflaboratory at IIT, Kanpur. A surface barrier de-

tector placed at 150� in backscattering geometry

was used to detect the scattered ions (Fig. 1). RBS

spectra analysis was performed by the RUMP

simulation code [10]. Grazing incidence X-ray

diffraction (GIXRD) was carried out using CuKa

radiation in a Diffractometer (Fig. 2). Incident

angle was kept at 1� and the detector angle wasvaried from 30� to 50�. Scanning electron micros-

copy (SEM) was carried out using JSM-840 scan-

ning microscope JEOL attached with SiLi

detector. The energy of the incident electron beam

Fig. 3. Scanning electron microscopy (SEM) micrograph of

Au/Si system: (a) Xe10þ ions irradiated at RT to a fluence

1� 1015 ions/cm2, (b) irradiated at RT to a fluence 5� 1015 ions/

cm2.

240 S. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 238–241

was 20 keV and magnification was 10,000. TypicalSEM micrograph is shown in Fig. 3.

Fig. 4. Variation of nuclear energy loss (Sn) and electronic

energy loss (Se) with depth for 120 keV Ar, 300 keV Xe and

1000 keV Xe in Au.

3. Results and discussion

Fig. 1 shows the RBS spectra of the as-depos-

ited sample and samples irradiated at RT at flu-

ences of 1� 1015 and 5� 1015 ions/cm2. It is seenfrom Fig. 1 that ion irradiation induces a pro-

gressive intermixing at the Au/Si. The reduction in

the slope of the lower edge of the Au peak and

front edge of silicon indicates that the mixing oc-

curred in the Au/Si interface which increases with

the fluence. The RBS spectra show a large mixing

at fluence 5� 1015 ions/cm2. We observed that the

areal density of gold peak reduced to 3.4� 1017

and 3.1� 1017 atoms/cm2 at fluences of 1� 1015

and 5� 1015 ions/cm2, respectively, from the as-

deposited sample (having 3.5� 1017 atoms/cm2) as

shown in Fig. 1. The number of sputtered atoms

are expected to be around 14 atoms/ion as calcu-

lated from SRIM [13] program. Therefore, we can

say that the reduction in area under gold peak isdue to the sputtered atoms. Fig. 2(a) shows the

GIXRD pattern of the as-deposited sample in

which only the peaks of Au(1 1 1) and Au(2 0 0) are

seen. Fig. 2(b) shows the GIXRD of the irradiated

sample at fluence 5� 1015 ions/cm2. It indicates the

formation of gold silicide (Au2Si, Au3Si, Au5Si

and Au5Si2) and unreacted gold [11]. The forma-

tion of a phase depends on the mobility of thereacting species and the temperature during irra-

diation. In a solid state reaction and ion beam

mixing, both the reaction kinetics as well as ther-

modynamics driving forces play an active role

during phase formation [12]. The silicide phase

formation in the present work is obtained by ir-

radiation at RT and without any post-annealing,

whereas in all the previous work, the Au silicideformation is reported either by irradiation at eu-

tectic temperature [7] or by post-irradiation an-

nealing [9]. This may be due to the fact that in the

present work the nuclear energy deposition (Sn) byelastic collision is around 5.77 keV/nm, which is

optimized to be maximum at the interface as

shown in Fig. 4. Whereas in all the previous work

the Sn is lower and it is not optimized for the in-terface as shown in Fig. 4.

S. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 238–241 241

The surface morphology of the films was stud-

ied by SEM. Fig. 3(a) and (b) shows the SEM

micrograph of the irradiated at fluences of 1� 1015

and 5� 1015 ions/cm2. Fig. 3(a) shows the mixtureof Au and amorphous gold silicide phase and Fig.

3(b) shows crystalline size of gold silicide around

0.5 lm. Two figures show that with the increase of

the fluence, amorphous phase changed to crystal-

line phase. The previous work reported formation

of triangular islands in Au/Si(1 1 1) system during

thermal annealing at eutectic temperature. The

formation of islands must be likely due to thethermal annealing effect. The size of this type is-

lands as obtained by Sarker et al. [7] is around few

micron. But in the present case, the crystalline size

of gold silicide is less than 1 lm at RT.

4. Conclusions

RBS measurements shows the extent of mixing

with the fluence in the Au/Si(1 0 0) system. GI-

XRD showed the formation of gold silicide phase

at RT and also SEM indicated the micron size

crystallites in the irradiated samples. Present work

reports the crystalline Au silicide phase by irradi-

ation at RT with Sn ffi 5:77 keV/nm at the inter-

face.

Acknowledgements

We would like to thank C.P. Safvan for pro-viding low energy ion beam facility at Nuclear

Science Centre, New Delhi.

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