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Nuclear Instruments and Methods in Physics Research B 266 (2008) 1759–1763

NIMBBeam Interactions

with Materials & Atoms

Swift heavy ions induced mixing in metal/semiconductor system

Sarvesh Kumar a,b,*, R.S. Chauhan b, D.C. Agarwal b, Manvendra Kumar c, A. Tripathi d,W. Bolse e, D.K. Avasthi d

a Department of ASH (Physics), C. I. T. M., Aravali Hills, Sector-43, Faridabad 121 001, Indiab Department of Physics, R.B.S. College, Agra 282 002, India

c Department of Physics, University of Allahabad, Allahabad 211 002, Indiad Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India

e Institut fuer Strahlenphysik, Allmandring 3, 70569 Stuttgart, Germany

Received 24 September 2007; received in revised form 20 January 2008Available online 13 February 2008

Abstract

We have investigated the ion beam mixing in the metal/semiconductor (Cu/Ge) system under swift heavy ion irradiation. For thisstudy the samples have been prepared by electron gun evaporation in ultrahigh vacuum deposition system. The irradiations were per-formed at room temperature (RT) using 100 MeV Ag ions, 120 MeV and 140 MeV Au ions and at liquid nitrogen temperature (LT)using 120 MeV, 350 MeV Au ions with fluences ranging from 1 � 1013 to 2.7 � 1014 ions/cm2. Characterizations of these samples havebeen performed using Rutherford backscattering spectroscopy (RBS) and atomic force microscopy (AFM). On analyzing the RBS data,we find that mixing occurs at the interface and it increases with the fluence, electronic energy loss and irradiation temperature. The mix-ing in this case is due to interdiffusion across the interface during a transient melt phase according to the thermal spike model.� 2008 Elsevier B.V. All rights reserved.

PACS: 61.80.Jh; 68.55.Nq

Keywords: Ion beam mixing; Swift heavy ions

1. Introduction

Ion beam mixing (IBM) is an effective tool to synthesizemetastable and/or stable phases using thin films of metalsand semiconductors [1,2]. A metal–semiconductor systemis a good choice to study swift heavy ions induced mixingfrom the fundamental ion–solid interaction point of viewas well as their wide applications. High-energy heavy ionswith velocities comparable to or higher than the orbitalelectron velocity are referred to as swift heavy ions (SHI).At such energies (P a few tens of keV/nucleon) SHI losetheir energy in the target mainly via inelastic collisionsleading to the excitation of the target electrons. The elec-

0168-583X/$ - see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.nimb.2008.01.068

* Corresponding author. Address: Department of Physics, R.B.S.College, Agra 282 002, India. Tel./fax: +91 562 2520075.

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

tronic energy deposition has been found to induce modifi-cations in various target materials, which include creationof point defects, latent track formation, sputtering andintermixing at the interfaces. While defect and track forma-tion in insulators upon SHI irradiation have been studiedquite extensively, only in recent years such effects have beenobserved in metals also. Damage creation (in metals) andamorphisation (in insulators) occurs only when the elec-tronic energy loss (Se) exceeds a certain threshold value,which depends on the material. While insulators usuallyare very sensitive to SHI irradiation, very high electronicenergy deposition is needed to create defects in metals [3].However, such modifications are not straight forwardand depend a lot on the material properties, depositedenergy density, etc. In most cases, materials modificationby SHI irradiation exhibits threshold behavior in termsof electronic energy loss (Se) beyond which the defect

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Si/Cu/Ge/Al Pristine3E131E14

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Fig. 1. RBS spectra of Al/Ge/Cu film on Si(100) substrate as-depositedand after being irradiated with 100 MeV Ag ions to the fluences of3 � 1013 and 1 � 1014 ions/cm2 at RT.

1760 S. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1759–1763

production efficiency increases to a great extent due to for-mation of latent ion track. However some materials,termed as Se insensitive materials, show negligible or nodefect production behavior due to Se. The defect produc-tion and atomic motion induced by SHI in insulators[4,5], semiconductors [6] and some metals [7,8] has stimu-lated great interest to achieve SHI induced mixing at inter-faces to produce novel materials and phases. In the case oflayered structures, such energy deposition may lead toatomic intermixing across the interfaces (ion beam mixing).Avasthi et al. [9] have demonstrated atomic mixing in theFe/Si and the CuO/glass system due to the irradiation with230 MeV Au ions and 210 MeV I ions, respectively andsuggested that mixing increases with the increase in elec-tronic energy loss. Kraft et al. [10] has shown that mixingat the interface of ZnO/SiO2 is due to interdiffusion duringtransient melt phase. Bolse [11,12] group showed that mix-ing takes place when both the ion beam has Se above Se

threshold for both the film and substrate. Mixing in thinfilms of Ti, Fe, W with Si by 100 MeV Au ions have beenstudied by Gupta et al. [13] where they observed that metaldamage/intermixing occurred at Se values well below thethreshold value in bulk. Som et al. [14] studied interfacemodification of Ni/Ge bilayer structure using 100 MeVAu ions irradiation where they showed that mixingincreases with irradiation temperature. At higher irradia-tion temperature, the SHI induced defects become moremobile and enhance the mixing. Srivastava et al. [15]recently demonstrated that the SHI induced mixing inFe/Si system is consequence of transient molten state inter-diffusion across the interface. In this paper, we investigatemixing in Cu/Ge interface using 100 MeV Ag ions,120 MeV and 140 MeV Au ions at RT and 120 MeV and350 MeV Au ions at LT with varying fluences.

2. Experimental

The Al (32 nm)/Ge (87 nm)/Cu (94 nm) bilayer sampleswere deposited on Si(1 00) substrate at room temperatureby electron beam evaporation in a cryo-pumped ultra highvacuum deposition system. Thin layer of Al was depositedon top to protect the Ge layer from oxidation. The deposi-tion was performed at a pressure of �2 � 10�7 Torr. The Siwafers were properly cleaned and etched chemically priorto deposition. The samples were irradiated by 100 MeVAg ions, 120 MeV Au ions and 140 MeV Au ions at roomtemperature (RT) and 120 MeV Au ions at liquid nitrogentemperature (LT) with different fluences using the 15 UDPelletron at Inter-University Accelerator Centre (IUAC),New Delhi and 350 MeV Au ions at LT at Hahn-Meit-ner-Institut (HMI), Berlin with different fluences. The irra-diation flux was kept low to avoid sample heating duringirradiation. The ranges and the energies deposited by theprojectile ions were calculated using the TRIM code [16].The ion range of ions for Cu/Ge is far deeper than theinterfaces at a depth of �13.8 lm, �14.1 lm, �14.5 lmand �27.0 lm for 100 MeV Ag ions, 120 MeV, 140 MeV

and 350 MeV Au ions, respectively. The thickness andthe composition of the pristine and irradiated samples werestudied by Rutherford backscattering spectrometry (RBS)using 1 MeV He ions at scattering angle 165� except for350 MeV Au (170�). Analyses of the RBS data were per-formed using the RUMP simulation code [17]. Atomicforce microscopy (AFM) has also been performed on pris-tine and irradiated samples to check that the observed mix-ing is not due to surface roughness, using DigitalInstrument Nanoscope IIIa system at IUAC, New Delhi.

3. Results and discussion

Figs. 1–3 show the RBS spectra of the as-deposited sam-ple and samples irradiated at room temperature (RT) atfluences 3 � 1013 and 1 � 1014 ions/cm2 by 100 MeV Agions, 6 � 1013, 1 � 1014 ions/cm2 by 120 MeV Au and5 � 1013 by 140 MeV Au ions, respectively (showing herehigher fluences only). The decrease in the slope of the loweredge of the Ge peak and front edge of Cu peak indicatethat the mixing occurred at this interface which alsoincreases with the fluence. The RBS spectra of theas-deposited sample and samples irradiated at liquidnitrogen temperature (LT) at fluence 3 � 1013, 1 � 1014,2 � 1014 ions/cm2 by 120 MeV Au ions and 2.7 � 1013,8.9 � 1013 and 2.7 � 1014 ions/cm2 by 350 MeV Au ionsalso show that mixing increases with the fluence, whichare shown in Figs. 4 and 5. The experimental data ofRBS were fitted with RUMP simulation code and Ge depthprofiles were extracted for different fluences.

The roughness of the top layer of pristine and irradiatedsamples is determined by atomic force microscopy (AFM).Fig. 6 shows the AFM image of pristine and irradiatedsamples with different energies and temperature. From

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Fig. 4. RBS spectra of Al/Ge/Cu film on Si(100) substrate as-depositedand after being irradiated with 120 MeV Au ions to the fluences of3 � 1013, 1 � 1014 and 2 � 1014 ions/cm2 at LT.

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Si/Cu/Ge/Al,Pristine2.7E138.9E132.7E14

Fig. 5. RBS spectra of Al/Ge/Cu film on Si(100) substrate as-depositedand after being irradiated with 350 MeV Au ions at fluences of 2.7 � 1013,8.9 � 1013 and 2.7 � 1014 ions/cm2 at LT.

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Si/Cu/Ge/Al Pristine5e13

Fig. 3. RBS spectra of Al/Ge/Cu film on Si(100) substrate as-depositedand after being irradiated with 140 MeV Au ions to the fluence of5 � 1013 ions/cm2 at RT.

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Fig. 2. RBS spectra of Al/Ge/Cu film on Si(100) substrate as-depositedand after being irradiated with 120 MeV Au ions to the fluences of6 � 1013 and 1 � 1014 ions/cm2 at RT.

S. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1759–1763 1761

AFM results, we observe that there is insignificant differ-ence between the roughness of pristine and irradiated sam-ples. Therefore it is safe to assume that the observedchanges at the lower energy edge of Ge and high energyedge of Cu (in RBS spectra) are not due to the surfaceroughness and are solely due to the mixing at the interface.

To study the mixing effect quantitatively, the profiles ofGe were fitted with Gaussian error functions and the vari-ances were determined. The observed increase of the vari-ance Dr2(/) = r2(/) – r2(0), (where r2(/) and r2(0) arethe variances of irradiated and as-deposited samplesrespectively and / is fluence), was taken as a measure ofthe mixing effect. The variation of difference of the vari-ances Dr2/ with fluence (/) at LT and RT are shown in

Figs. 7 and 8. The mixing rate defined as k = Dr2(/)//,was estimated. Fig. 7 shows that Dr2(/) increases linearlywith fluence (/) and the slope can be defined as mixing ratek. However at RT, the difference of the variances showsnon-linear behavior with fluence as shown in Fig. 8. In thiscase Dr2(/) can be described by the equation [11]

Dr2ð/Þ ¼ k/þ m/2 ð1Þwhere m is the reaction rate. This effect indicates that mix-ing is driven by a chemical solid-state reaction. The calcu-lated mixing rate and reaction rate as estimated from theFigs. 7 and 8 are given in Table 1.

Fig. 6. Atomic force microscopy (AFM) image of Al/Ge/Cu/Si system: (a) pristine, (b) irradiated with 120 MeV Au ions to a fluence of 2 � 1014 ions/cm2

at LT, (c) irradiated with 120 MeV Au ions to a fluence of 2 � 1014 ions/cm2 at RT and (d) irradiated with 350 MeV Au ions to a fluence of2.7 � 1014 ions/cm2 at LT.

Fig. 7. Variation of Dr2 with ion fluence. Fig. 8. Variation of Dr2 with ion fluence.

1762 S. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1759–1763

Table 1Energy, ion, irradiation temperature, electronic stopping power Se,nuclear stopping power Sn, mixing rate and reaction rate for Al/Ge/Cu/Si system

Energy(MeV)

Ion Irradiationtemperature

Se (keV/nm)for Cu/Ge

Sn (keV/nm)for Cu/Ge

k

(nm4)m

(nm6)

100 Ag RT 28.58/16.37 0.18/0.10 64.7 85.7120 Au RT 33.05/19.22 0.67/0.37 65.8 244.6140 Au RT 36.50/21.23 0.59/0.33 68.9 466.0120 Au LT 33.05/19.22 0.67/0.37 74.1 –350 Au LT 54.24/31.33 0.29/0.16 144.9 –

S. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1759–1763 1763

The Table 1 shows the mixing rate at the interface ofCu/Ge which increases with the increase in electronicenergy loss at room temperature (RT) as well as at liquidnitrogen temperature (LT). Som et al. [14] reported thatdeposited Ge would be in amorphous form. The depositedfilm of a:Ge has been reported to have a Se threshold valueof 5.3 keV/nm [18]. The value of Se in our case is higherthan its threshold and therefore, it is expected that Ge willget damaged due to Se, but Cu is insensitive material. How-ever, the energy transfer from Ge to Cu will create a thinmolten Cu layer leading to interdiffusion of Ge and Cu atthe interface. The difference of variance at LT (Fig. 7)shows linear behavior with fluence whereas for RT(Fig. 8) variation is non-linear. At RT the mixing rateincreases slowly but reaction rate increases drastically withelectronic energy loss. Non-linear behavior at RT can beexplained by considering additional mixing driven bychemical solid-state reaction as suggested by Schattatet al. [11]. In our case the chemical solid-state reaction iseffective at RT but may be insignificant at LT. This maybe due to the radiation enhanced diffusion (RED) duringirradiation at the room temperature or due to the largertrack radius in a:Ge and temperature spike duration in caseof RT as compared to that of LT.

4. Conclusions

This work demonstrates the mixing at Cu/Ge interfaceusing 100 MeV Ag ions, 120 MeV and 140 MeV Au ionsat RT and 120 MeV and 350 MeV Au ions at LT withvarying fluences. The mixing increases with the fluence,with the electronic energy loss and with the irradiation tem-perature. The study on SHI induced mixing at LT and RT

reveals that the non-linear behavior of mixing at RT is dueto the mixing driven by chemical solid-state reaction. Themixing in this case is due to interdiffusion across the inter-face during a transient melt phase as described by the ther-mal spike model.

Acknowledgements

The authors would like to thank the Pelletron group atIUAC, New Delhi. They are also thankful to DST, NewDelhi and DAAD, Bonn for the financial support.

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