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Applied Surface Science 349 (2015) 123–128

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

odification of mechanical properties of Si crystal irradiated byr-beam

iaowei Guoa, Sadao Momotab,∗, Noriko Nittac, Takaharu Yamaguchib, Noriyuki Satob,ideto Tokajib

Department of Intelligent Mechanical Systems Engineering, Kochi University of Technology, Tosayamada, Kami-city, Kochi 782-8502, JapanSchool of Environmental Science and Engineering, Kochi University of Technology, Tosayamada, Kami-city, Kochi 782-8502, JapanCenter for Nanotechnology, Research Institute, Kochi University of Technology, Tosayamada, Kami-city, Kochi, 782-8502, Japan

r t i c l e i n f o

rticle history:eceived 17 February 2015eceived in revised form 25 April 2015ccepted 25 April 2015vailable online 5 May 2015

a b s t r a c t

The application of ion-beam irradiation in fabrication of structures with micro-/nanometer scale hasachieved striking improvement. However, an inevitable damage results in the change of mechanicalproperties in irradiated materials. To investigate the relation between mechanical properties and ion-irradiation damages, nanoindentation was performed on crystalline silicon irradiated by Kr-beam withan energy of 240 keV. Modified Young’s modulus and nanohardness, provided from the indentation, indi-

eywords:oung’s modulusardnessrystalline silicon

on-beam irradiation

cated fluence dependence. Stopping and range of ions in matter (SRIM) calculation, transmission electronmicroscopy (TEM) observation, and Rutherford backscattering-channeling (RBS-C) measurement wereutilized to understand the irradiation effect on mechanical properties. In addition, the longitudinal sizeof the phase transition region induced by indentation was firstly evaluated based on the depth profile ofmodified nanohardness.

© 2015 Elsevier B.V. All rights reserved.

. Introduction

The increasing importance of ion-beam irradiation effect onaterials has drawn many attentions in the field of micro-

nanofabrication because of its potential applications. For example,he tailoring or modifying effect induced by ion-beam irradiations used to fabricate surface structure [1,2] or to modify lateral sizef structure [3–7]. The effect on tailoring materials or modifyingurface can be conveniently controlled by means of irradiationarameters, such as ion energy and fluence. Ion-beam technology,s a development of fabrication method, promotes its applica-ion in the fabrication of microelectromechanical systems (MEMS).n general, ion-beam irradiation induces definite modifications of

echanical properties, which are a part of important require-ents for MEMS, such as, resonant sensor. Su et al. [8] reported

hat the hardness of poly-crystalline silicon irradiated by various

ons was affected by the formed bond because of the chemicalffects induced by ions. In addition, the ‘size effect’ on mechan-cal properties of micro-structure is also important. Unchanged

∗ Corresponding author.E-mail address: momota.sadao@kochi-tech.ac.jp (S. Momota).

ttp://dx.doi.org/10.1016/j.apsusc.2015.04.193169-4332/© 2015 Elsevier B.V. All rights reserved.

Young’s modulus of crystal silicon nano-cantilever was found ata minimum thickness of 255 nm by Namazu et al. [9]. Virwaniet al. [10] extended the minimum thickness of unchanged elasticmodulus to 193 nm. However, a significant decrease in the elas-tic modulus of silicon cantilevers was reported by Li et al. [11]when their thickness was reduced from 170 to 12 nm. Therefore,a block material irradiated with chemically inactive ions is avail-able to investigate the irradiation effect on mechanical propertiesof sub-microstructures.

At present, although many researches related to mechani-cal properties of silicon materials have been reported [11–14],the depth profile of mechanical properties modified by ion-beamirradiation was rarely reported. In this paper, the mechanicalproperties were investigated on crystalline silicon (c-Si) samplesirradiated by Kr-beam with an energy of 240 keV and fluence of1 and 5 × 1014 ions/cm2, with which a lateral deformation wasobviously observed in ref. [7]. In order to achieve the depth profileof an irradiation effect, the continuous measurement of mechanicalproperties was performed by means of nanoindentation. Depth dis-

tribution of defects was supplied from stopping and range of ions inmatter (SRIM) calculation, transmission electron microscopy (TEM)observation, and Rutherford backscattering-channeling (RBS-C) toinvestigate modification of mechanical properties.

124 X. Guo et al. / Applied Surface Science 349 (2015) 123–128

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ig. 1. Load–displacement curves of c-Si irradiated with Kr-beam at a fluence of (a)

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. Material and methods

.1. Preparation of sample and ion beam irradiation

Czochralski-grown (Cz-Si) p-type Si 〈1 0 0〉 samples with sizef 1.5 × 1.5 cm2 and thickness of 0.5 mm were used in the presenttudy. In order to remove the oxidized layer on the sample surface,ll of the samples were sequentially washed in acetone, ultra-pureater, 18% buffered hydrofluoric acid (BHF), and ultra-pure water.

In order to modify the mechanical properties of c-Si, Kr2+ beamsith an energy of 240 keV were implanted at room temperatureith an incident angle of 5◦ so as to avoid any channeling effect.r2+ beams were prepared by a 10-GHz NANOGAN, which is anCR ion source installed at Kochi University of Technology [15].he fluence of the Kr2+ beam was 1 × 1014and 5 × 1014 ions/cm2.

.2. Evaluation of irradiation effect

The samples prepared according to the procedure described inhe previous section were measured by Nano Indenter XP/DCMAgilent Technologies) equipped with a Berkovich diamond inden-er to obtain the mechanical properties. The continuous stiffness

easurement (CSM) mode was employed at room temperature and

he principle of continuous measurement of hardness and Young’s

odulus was described in ref. [16]. During the penetration, thendenter vibrates with an amplitude of 2 nm at a frequency of 45 Hz.

constant strain rate of 0.05 s−1 was applied during loading with a

Fig. 2. (a) Young’s modulus and (b) hardness of c-Si b

14 ions/cm2 and (b) 5 × 1014 ions/cm2. Insets show the corresponding pop-in events

10 s hold period at peak load. Ten indentations were made in eachsample with a maximum depth of 500 nm.

To evaluate damage induced by ion-beam irradiation, a SRIMcalculation, a TEM (JEM-2100F, JEOL), and a RBS-C were applied toprovide the evolution of the amorphous layer and defect distribu-tion.

3. Results and discussion

3.1. Modification of mechanical properties induced by Kr-beamirradiation

Fig. 1 shows the load/unload curves obtained by means of inden-tation for c-Si irradiated with Kr-beam with fluence of (a) 1 × 1014

and (b) 5 × 1014 ions/cm2. The obvious pop-out events are iden-tified in unloading process, as shown in Fig. 1. In load curves,pop-in events are identified at a displacement about 100 nm forthe curve of 1 × 1014 ions/cm2 and about 140 nm for the curve of5 × 1014 ions/cm2 in the insets, respectively. The events of pop-inand pop-out indicated the phase transition in the loading pro-cess and unloading process, respectively. The pop-in events wereexpected to be associated with discontinuity of mechanical prop-erties.

In this study, the Young’s modulus and hardness were mainlycharacterized as the mechanical properties of silicon materials.The results with the average of 10 indentations with the standarddeviation error bars are shown in Fig. 2, in which both Young’s

efore and after irradiation of 240 keV Kr-beam.

X. Guo et al. / Applied Surface Science 349 (2015) 123–128 125

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and 5 × 1014 ions/cm2 are 162 and 210 nm, respectively.TEM image shown in Fig. 5a indicates that the thickness of the

amorphous layer formed in c-Si surface reaches 199 nm after Kr-beam irradiation with the fluence of 1 × 1014 ions/cm2. When the

Fig. 3. Changes in (a) Young’s modulus and (b) hardness of c-Si

odulus and hardness modified by ion-beam irradiation decreaseith fluence. The Young’s modulus of c-Si before Kr-beam irra-iation is 176.1 GPa, which is close to that was reported for c-Si,60–180 GPa [17,18]. After Kr-beam irradiation of 1 × 1014 and

× 1014 ions/cm2, the mean Young’s modulus, which were definedn a depth range of 50–60 nm, were 163.4 and 158.9 GPa, respec-ively. For the case of hardness, the similar behavior of fluenceependence is found in Fig. 2b. The hardness of c-Si before ion-beam

rradiation is 13.0 GPa. After Kr-beam irradiation, the mean hard-ess defined in the same depth range of Young’s modulus are11.8nd 10.8 GPa for the fluence of 1 × 1014 and 5 × 1014 ions/cm2,espectively. The change in Young’s modulus and hardness can beasily recognized by the difference between curves in the typicalepth profile shown in Fig. 2.

To extract irradiation effect on mechanical properties,hanges in Young’s modulus (�Young’s modulus) and hard-ess (�Hardness) from those of unirradiated c-Si were calculateds shown in Fig. 3. Here, negative value indicates a decreaseompared with unirradiated samples. Fig. 3a shows behavior ofYoung’s modulus and fitting results with exponential function.

n the case of lower fluence of 1 × 1014 ions/cm2, �Young’s mod-lus gradually decreased from 26 to 0 GPa with depth increasingrom 0 to ∼180 nm. In the case of higher fluence of 5 × 1014

ons/cm2, �Young’s modulus gradually decreased from 30 to GPa with depth increasing from 0 to ∼240 nm. Fig. 3b showshe similar behavior with that of Young’s modulus. In the casef lower irradiation fluence, �Hardness decreased from 2.7 to

GPa with depth increasing from 0 to 270 nm. In case of higheruence, �Hardness decreased from 5.2 to 0.5 GPa with depth

ncreasing from 0 to 300 nm. Therefore, the depth of modifiedayer for mechanical properties increased with the fluence. Andhe change in mechanical properties was promoted for higheruence.

By comparing the depth profiles between �Young’s modulusnd �Hardness, the depth of modified layer for hardness was largerhan that for Young’s modulus under the same irradiation conditionlthough they both decreased with depth. This result revealed thathe hardness was more sensitive to damage induced by ion-beamrradiation than Young’s modulus. In addition, a dip was observedn the depth profile of �Hardness at 105 nm for the curve of

× 1014 ions/cm2 and at 145 nm for the curve of 5 × 1014 ions/cm2. noticeable irregular decrease in hardness change was found in

he depth range of 60–105 nm for the curve of 1 × 1014 ions/cm2

nd 95–145 nm for the curve of 5 × 1014 ions/cm2.

ed by irradiation of Kr-beam. Solid lines indicate fitted results.

3.2. Evolution of amorphous layer induced by ion-beamirradiation

Raman spectrum, shown in our previous paper [7], indicatedthat the amorphous fraction increased with irradiation fluence andthe continuous amorphous layer was expected to be formed. There-fore, phase change was related to changes in mechanical properties.To evaluate the effect of the phase change, SRIM calculation, TEM,and RBS-C were utilized to investigate the inner change and theresults were shown in Figs. 4–6, respectively.

To confirm the thickness of the amorphous layer, the behav-ior of implanted Kr-ions into silicon was simulated by using SRIMprogram. According to previous studies [19], a threshold value forcomplete amorphization of c-Si is 5.0 × 1023 eV/cm3. It means thatthe minimum energy/ion/Å required to cause full amorphizationare 50 eV/ion/Å for the fluence of 1 × 1014 ions/cm2 and 10 eV/ion/Åfor the fluence of 5 × 1014 ions/cm2. Fig. 4 indicates that the esti-mated thicknesses of an amorphous layer for the fluence of 1 × 1014

Fig. 4. Energy of recoiled Si calculated by SRIM. The figure indicates that entirethicknesses of amorphization at fluence of 1 × 1014 and 5 × 1014 ions/cm2 are 162and 210 nm, respectively.

126 X. Guo et al. / Applied Surface Science 349 (2015) 123–128

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ig. 5. Cross-sectional TEM images of Si irradiated by a Kr-beam with fluences ofa) 1 × 1014 and (b) 5 × 1014 ions/cm2.

uence reaches to 5 × 1014 ions/cm2, the corresponding thicknessf the amorphous layer increases further to 240 nm, as shown inig. 5b.

The results obtained from SRIM calculation and TEM observationndicated that the thickness of the amorphous layer formed in c-i surface increased with increasing fluence of Kr-beam. However,he thickness calculated by SRIM was relatively thinner than thatbserved from TEM images. The difference between the thicknessf SRIM calculation and that of TEM observation might originaterom the expansion effect of Si induced by ion-beam irradiation1,2] and the diffusion effect of vacancies along the steep gradient ofhe distribution of radiation-produced point defects into the regionf larger depths [20]. However, these effects were ignored in SRIMalculation.

Fig. 6 shows the density distributions of defect in c-Si. Theensity was calculated from RBS-C spectrum through a procedureescribed in ref. [21]. The saturation of the defect density was asso-iated with the amorphous phase. According to the ref. [22], the fullidth at half maximum (FWHM) of defect distribution was defined

s the thickness of the amorphous layer to reduce the ambigu-ties induced by the energy straggling and system resolution. Fig. 6ndicates that thicknesses of the defined amorphous layers reach05 and 250 nm for the fluence of 1 × 1014 and 5 × 1014 ions/cm2,espectively. The results are consistent with the thicknesses ofmorphous layers observed by TEM.

.3. Defects distributions in the damaged region

Previous section has revealed that the evolution of amorphousayer on c-Si, induced by Kr-beam irradiation, has a close relation

ig. 6. Density distribution of defect in irradiated c-Si obtained from RBS-C spectra.

Fig. 7. Distributions of Kr in c-Si measured by RBS-C. Solid lines indicate the fittedresults by means of Gaussian function.

with the modification of mechanical properties. To further under-stand the difference of modified depth between Young’s modulusand hardness, the defect distribution in the damaged region shouldbe considered.

The density distribution of defects in silicon shown in Fig. 6 indi-cates that the distributions of defects covered a depth range from0 to 277 nm at a fluence of 1 × 1014 ions/cm2 and a depth rangefrom 0 to 325 nm at a fluence of 5 × 1014 ions/cm2. Distributionsof Kr in silicon, as shown in Fig. 7, are close to a Gaussian-like dis-tribution. The center and FWHM of the distribution are about 180and 120 nm, respectively. Therefore, major part of Kr-ions, ∼75%,stopped at the depth between 120 and 240 nm. These experimen-tal results indicated that the surface layer containing the radiationdefects was larger than the range of implanted Kr-ions. And thisphenomenon has been well reported in previous researches [20,23]and is ascribed to the two possible reasons, such as diffusion effectof vacancies [20] and the ion-channeling effect [23]. For the presentcase, the diffusion effect of vacancies is the more possible reason.

Fig. 8a shows the amorphous-crystal interface of Si sampleirradiated with a fluence of 1 × 1014 ions/cm2. Fig. 8c–e indicatesdiffraction patterns, which were obtained in regions assigned in

Fig. 8a. Fig. 8c indicates complete amorphization. And Fig. 8d indi-cates incomplete amorphization. Fig. 8e indicates that the regionlocated beneath the transition region keeps crystal nature. Amor-phous pockets, highlighted by white circles, are found around the

Fig. 8. TEM images around the interface between amorphous and crystalline region,irradiated with a fluence of 1 × 1014 ions/cm2. (a) Cross-sectional image in low res-olution, and (b) Cross-sectional image in high resolution. (c), (d), and (e) are Fouriertransforms of regions marked in (a).

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ransition region in Fig. 8b. Small misorientation regions, high-ighted by black circles, are found in the amorphous region in Fig. 8a.

.4. Effect of irradiation damage on mechanical properties

The experimental results indicated that irradiation damage is crucial factor to understand the modification of mechanicalroperties. Firstly, changes in mechanical properties indicated flu-nce dependence, as shown in Fig. 2. Depth of modified layer forechanical properties increased with the increasing of fluence, andechanical properties decreased with the increasing of fluence at

he same depth. In case of Young’s modulus, the depth of 180 nmt the fluence of 1 × 1014 ions/cm2 and 240 nm at the fluence of

× 1014 ions/cm2 were close to the observed thicknesses of themorphous layers, as shown in Fig. 5, of 199 nm at the fluence of

× 1014 ions/cm2 and 240 nm at the fluence of 5 × 1014 ions/cm2.urthermore, the thickness of amorphous layer obtained fromRIM calculation, TEM observation, and RBS-C measurement indi-ated the similar fluence dependence to the behaviors of modifiedoung’s modulus. Therefore, it was suggested that the phase tran-ition was strongly related to the modification of the Young’sodulus. The network of amorphous silicon is considered to be

hort and medium range order [24]. Pantchev et al. [25] reportedhat the decrease in Young’s modulus was associated with thetructure change in the irradiated a-Si, such as the irradiationnduced increase in the Si Si bond angle deviation. The inducedtructural disorder in the silicon network, which are associatedith the variation of the short and intermediate range order, leaded

o a significant decrease in the mechanical stress [26]. The ion irra-iation effect on the decay in Young’s modulus is ascribed to the

nduced increase in the short-range order of the silicon networknd a lower Young’s modulus of ion-beam irradiated a-Si can beelated to the irradiation induced increase in the Si Si bond angeleviations [12]. Therefore, crystal-to-amorphous phase transition

ndicates the long-range order of Si network transforms to short-ange order of Si network. As shown in Fig. 8, lattice of crystallineilicon can be found in the crystalline region while some small mis-rientation regions, highlighted by black circles, can be found in themorphous region. It was suggested that the decrease in Young’sodulus was associated with the induced decrease in the order of

i network in the process of crystal-to-amorphous phase transition.ased on this suggestion, the decrease in Young’s modulus with the

ncreasing of fluence at the same depth was expected to originaterom the decrease in the short-range order of Si network inducedy Kr-beam irradiation.

In the case of hardness, the depths of modified layer, shown inig. 2b, were 270 and 320 nm for the fluence of 1 × 1014 and 5 × 1014

ons/cm2, respectively. These results were consistent with the RBS- measurement results of defects distribution range of 0–277 nmor the fluence of 1 × 1014 ions/cm2 and of 0–325 nm for the fluencef 5 × 1014 ions/cm2, as shown in Fig. 6. The consistent results indi-ated that the modification of hardness was associated with defectsistribution induced by Kr-beam irradiation. The measured dis-ribution range of defect also showed fluence dependence, whichas consistent with the behavior of modified layer for hardness.

n addition, Pfanner et al. [27] reported that dangling bond defectn a-Si, which corresponds to a network defect, was generated atuitable geometric distortions in the network while the danglingond in crystalline silicon was usually related to the presence of aacancy. According to ref. [12], the decrease of hardness inducedy ion implantation is related to the increase in density of danglingond, caused by displacements of atoms in the target. Therefore,

high concentration of defects, such as vacancies, in c-Si resulted

n the decrease in long-range order of Si network to form a con-inuous amorphous layer and a great number of dangling bondsere generated at the same time. According to refs. [28,29], the

ence 349 (2015) 123–128 127

density of dangling bond is about 1019–1020 cm−3 in the hydrogen-free amorphous silicon. For the relatively lower concentration ofdefects is not enough to form a continuous amorphous layer but thedefects still generate a good many of dangling bonds, whose den-sity is relatively lower than that in the amorphous region. Previousresearches [30,31] indicate that the indentation process induces aphase transition to cause plastic deformation via plastic flow, whichis promoted by high density of dangling bond defects. Based onprevious reports, the distribution of dangling bonds density wasstrongly associated with that of defects density (Fig. 6). The flu-ence dependence of modified hardness could be understood basedon the behavior of dangling bonds distribution, which correspondswith the distribution of defect. And the decrease of hardness withfluence was ascribed to the irradiation induced increase of defectdensity, which determined the dangling bond density.

Secondly, comparison between the depth profiles of �Young’smodulus and �Hardness indicated that the hardness was more sen-sitive to the damage induced by ion-beam irradiation. Based on theabove arguments, the different sensitivity can be understood as fol-lowings. The Young’s modulus was related to the order degree of Sinetwork. On the other hand, the hardness was related to the densityof dangling-bond defect in Si materials. Under the same irradiationcondition, to produce defects was easier than to induce remarkablechange in the order of Si network.

In addition, the dips observed in �Hardness (Fig. 3b) at 105 nmfor the curve of 1 × 1014 ions/cm2 and at 145 nm for the curve of5 × 1014 ions/cm2 were consistent with the events of pop-in in theloading-displacement curves (Fig. 1). This consistency indicated theformation of a new phase. The phase transition of a-Si inducedby the indentation process is confirmed in refs. [30,31]. Previ-ous research [32] reports silicon undergoes transformation fromambient-pressure diamond cubic structure (Si-I) to more dense�-Sn (Si-II) structure at pressures of 10–13 GPa. This semicon-ductor transformation was accompanied by a volume reductionof 22% [33]. Ruffell et al. [34] provided a model to understandthe phase transition, which proceeds beneath the tip of nano-indenter through indentation process with different loading speed.During the penetration of indenter in a-Si, this phase transitionregion moves toward deeper region. According to Fig. 5, the thick-ness of amorphous layers induced by different fluence of 1 × 1014

and 5 × 1014 ions/cm2 are 199 and 240 nm, respectively. Whenthe region of phase transition induced by indentation reachedthe boundary between the amorphous and crystalline silicon, themovement of phase transition region was confined. Thus, the extrastress was accumulated so as to compensate partly decrease ofhardness induced by Kr-beam irradiation. This compensation canbe confirmed by the irregular decrease of �Hardness (Fig. 3b) in thedepth range of 60–105 nm for the curve of 1 × 1014 ions/cm2 and of95–145 nm for the curve of 5 × 1014 ions/cm2. According to Fig. 2b,the c-Si beneath the boundary undergoes transformation at a pres-sure of 12.5–13.2 GPa, which were consistent with the reportedpressure, 10–13 GPa, from Si-I phase to Si-II phase [32]. Corre-sponding event of pop-in was observed in loading-displacementcurve because of the volume reduction [33].

Thirdly, based on the above discussion, the depth profile ofmodified mechanical properties can be regarded as a new tool todetermine the size of the phase transition region during the load-ing process. According to ref. [30], metallic Si-II phase inducedby nanoindentation is unstable at the pressure below 2 GPa. Andthe material undergoes further transformation to different phases,such as an amorphous silicon phase or polycrystalline phases (Si-III and Si-XII), during the press release. Therefore, it was littlechance to evaluate the size of metallic Si-II phase after unloading.

Based on previous discussion on the dip found in the depth pro-file of �Hardness, the irregular decrease of �Hardness observedin the depth range of 60–105 nm for the curve of 1 × 1014 ions/cm2

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Fig. 3b) was ascribed to the interaction between the region of phaseransition induced by nanoindentation and the interface betweenmorphous and crystalline layer in sample. It was revealed that theoundary of phase transition region induced by nano-indentationad reached the interface between amorphous and crystalline Sihen tip of nano indenter reached a depth of 60 nm. Considering

he thickness of amorphous layer of 199 nm in Fig. 5, the longi-udinal size of the phase transition region was about 139 nm. Inhe same way, the longitudinal size of 145 nm was estimated fromhe curve of 5 × 1014 ions/cm2. These consistent results indicated aotential application of depth profile of mechanical properties to

nvestigate the longitudinal size of phase transition region inducedy nanoindentation.

In addition, high sensitivity of Young’s modulus to the phasehange from crystalline to the amorphous can be applied toetermine the thickness of the amorphous layer by continuouseasurement of nanoindentation.

. Conclusions

In conclusions, the nanoindentation measurements have beenerformed on c-Si irradiated by a Kr-ion beam with the energy of40 keV to evaluate the effect of ion beam irradiation on mechanicalroperties. The observed results showed the modified mechanicalroperties depend on the fluence. In addition, the hardness wasore sensitive to the damage induced by ion beam irradiation than

he Young’s modulus. The fluence dependence of Young’s modu-us can be understood by the decrease in the order of Si networknduced by ion beam irradiation. On the other hand, the fluenceependence of hardness was ascribed to the induced increase ofefects caused by ion beam irradiation. The different sensitivitiesf modified mechanical properties to damage induced by ion beamrradiation were ascribed to their different mechanism of modifica-ion. In addition, a new method of determining the longitudinal sizef phase transition region induced by nanoindentation was pro-osed and the longitudinal size was firstly estimated to be about40 nm.

cknowledgements

The author would like to thank Prof. Taniwaki for providing hisaluable suggestions and Dr. D. Wang for his help. We are indebtedo all colleagues of MOMOTA-lab for helping to perform exper-ments involving ion-beam irradiation, and to the experimentalupport coming from Center for Nanotechnology, Research Insti-ute in Kochi University of Technology.

eferences

[1] S. Momota, J. Zhang, T. Toyonaga, H. Terauchi, K. Maeda, J. Taniguchi, T. Hirao, M.Furuta, T. Kawaharamura, Control of swelling height of Si crystal by irradiatingAr beam, J. Nanosci. Nanotechnol. 12 (2012) 552–556.

[2] P.K. Giri, V. Raineri, G. Franzo, E. Rimini, Mechanism of swelling in low-energyion-irradiated silicon, Phys. Rev. B 65 (2001) 012110-1–012110-4.

[3] T. van Dillen, M.J.A. de Dood, J.J. Penninkhof, A. Polman, S. Roorda, A.M.Vredenberg, Ion beam-induced anisotropic plastic deformation of siliconmicrostructures, Appl. Phys. Lett. 84 (2004) 3591–3593.

[4] K. Otani, X. Chen, J.W. Hutchinson, J.F. Chervinsky, M.J. Aziz, Three-dimensionalmorphology evolution of SiO2 patterned films under MeV ion irradiation, J.Appl. Phys. 100 (2006) 023535-1–023535-5.

[5] M.-O. Ruault, F. Fortuna, H. Bernas, M.C. Ridgway, J.S. Williams, How nanocav-

ities in amorphous Si shrink under ion beam irradiation: an in situ study, Appl.Phys. Lett. 81 (2002) 2617–2619.

[6] T. van Dillen, E. van der Giessen, P. Onck, A. Polman, Size-dependent ion-beam-induced anisotropic plastic deformation at the nanoscale by nonhydrostaticcapillary stresses, Phys. Rev. B 74 (2006) 132103-1–132103-4.

[

[

ence 349 (2015) 123–128

[7] X. Guo, S. Momota, N. Nitta, K. Maeda, Lateral deformation of a silicon crystalsurface structure induced by low-fluence ion-beam irradiation, e-J. Surf. Sci.Nanotechnol. 13 (2015) 35–41.

[8] W.-S. Su, W. Fang, M.-S. Tsai, Tuning the mechanical properties of poly-siliconfilm by surface modification using plasma treatment, in: 2004 MRS SpringMeeting, Materials Research Society, San Francisco, 2004, pp. O8.9.1–O8.9.6.

[9] T. Namazu, Y. Isono, T. Tanaka, Evaluation of size effect on mechanicalproperties of single crystal silicon by nanoscale bending test using AFM, J.Microelectromech. Syst. 9 (2000) 450–459.

10] K.R. Virwani, A.P. Malshe, W.F. Schmidt, D.K. Sood, Young’s modulusmeasurements of silicon nanostructures using a scanning probe system:a non-destructive evaluation approach, Smart Mater. Struct. 12 (2003)1028–1032.

11] X. Li, T. Ono, Y. Wang, M. Esashi, Ultrathin single-crystalline-silicon cantileverresonators: fabrication technology and significant specimen size effect onYoung’s modulus, Appl. Phys. Lett. 83 (2003) 3081–3083.

12] P. Danesh, B. Pantchev, J. Wiezorek, B. Schmidt, Effect of keV ion irradiationon mechanical properties of hydrogenated amorphous silicon, Nucl. Instrum.Methods Phys. Res. Sect. B 268 (2010) 2660–2665.

13] Y.S. Sohn, J. Park, G. Yoon, J. Song, S.W. Jee, J.H. Lee, S. Na, T. Kwon, K. Eom,Mechanical properties of silicon nanowires, Nanoscale Res. Lett. 5 (2009)211–216.

14] X. Li, T. Ono, Y. Wang, M. Esashi, Study on ultra-thin NEMS cantilevers – highyield fabrication and size-effect on Young’s modulus of silicon, in: The FifteenthIEEE International Conference on Micro Electro Mechanical Systems, Las Vegas,2002, pp. 427–430.

15] S. Momota, Y. Nojiri, M. Saihara, A. Sakamoto, H. Hamagawa, K. Hamaguchi,First operation of ECR ion source at Kochi University of Technology, Rev. Sci.Instrum. 75 (2004) 1497–1498.

16] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness andelastic modulus using load and displacement sensing indentation experiments,J. Mater. Res. 7 (1992) 1564–1583.

17] D.M. Follstaedt, J.A. Knapp, S.M. Myers, Mechanical properties of ion-implantedamorphous silicon, J. Mater. Res. 19 (2004) 338–346.

18] B. Bhushan, X. Li, Micromechanical and tribological characterization of dopedsingle-crystal silicon and polysilicon films for microelectromechanical systemsdevices, J. Mater. Res. 12 (1997) 54–63.

19] M. Szabadi, P. Hess, A.J. Kellock, H. Coufal, J.E.E. Baglin, Elastic and mechani-cal properties of ion-implanted silicon determined by surface-acoustic-wavespectrometry, Phys. Rev. B 58 (1998) 8941–8948.

20] T. Aruga, S. Takamura, K. Nakata, Y. Ito, Depth profiles of defects inAr-ion-irradiated steels determined by a least-squares fit of S parame-ters from variable-energy positron annihilation, Appl. Surf. Sci. 85 (1995)229–238.

21] M. Nastasi, j.W. Mayer, Y. Wang, Ion Beam Analysis Fundamentals and Appli-cations, CRC Press Tay & Francis Group, Boca Raton, 2014.

22] W.-K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectrometry, AcademicPress. INC., New York, 1978.

23] A. Kinomura, R. Suzuki, T. Ohdaira, N. Oshima, K. Ito, Y. Kobayashi, Depthprofiling of defects in ion-implanted Ni and Fe by positron annihilation mea-surements, Surf. Coat. Technol. 206 (2011) 834–836.

24] B. Pantchev, P. Danesh, E. Liarokapis, B. Schmidt, J. Schmidt, D. Grambole, Effectof post-hydrogenation on the structural properties of amorphous silicon net-work, Jpn. J. Appl. Phys. 43 (2004) 454–458.

25] B. Pantchev, P. Danesh, J. Wiezorek, B. Schmidt, Nanoindentation-induced pile-up in hydrogenated amorphous silicon, J. Phys.: Conf. Ser. 253 (2010) 012054-1–012054-6.

26] B. Pantchev, P. Danesh, I. Savatinova, E. Liarokapis, B. Schmidt, D. Grambole,The effect of structural disorder on mechanical stress in a-Si:H films, J. Phys. D:Appl. Phys. 34 (2001) 2589–2592.

27] G. Pfanner, C. Freysoldt, J. Neugebauer, F. Inam, D. Drabold, K. Jarolimek, M.Zeman, Dangling-bond defect in a-Si:H: characterization of network and straineffects by first-principles calculation of the EPR parameters, Phys. Rev. B 87(2013) 125308-1–125308-7.

28] Z.E. Smith, S. Wagner, Intrinsic dangling-bond density in hydrogenated amor-phous silicon, Phys. Rev. B 32 (1985) 5510–5513.

29] S. Roorda, W. Sinke, J. Poate, D. Jacobson, S. Dierker, B. Dennis, D. Eaglesham, F.Spaepen, P. Fuoss, Structural relaxation and defect annihilation in pure amor-phous silicon, Phys. Rev. B 44 (1991) 3702–3725.

30] B. Haberl, J.E. Bradby, M.V. Swain, J.S. Williams, P. Munroe, Phase trans-formations induced in relaxed amorphous silicon by indentation at roomtemperature, Appl. Phys. Lett. 85 (2004) 5559–5561.

31] R. Ctvrtlik, J. Tomastik, V. Ranc, Nanoindentation-induced phase transformationin silicon thin films, Key Eng. Mater. 586 (2014) 112–115.

32] A. Kailer, Y.G. Gogotsi, K.G. Nickel, Phase transformations of silicon caused bycontact loading, J. Appl. Phys. 81 (1997) 3057–3063.

33] V. Domnich, Y. Gogotsi, Phase tansformations in silicon under contact loading,Rev. Adv. Mater. Sci. 3 (2002) 1–36.

34] S. Ruffell, J.E. Bradby, N. Fujisawa, J.S. Williams, Identification ofnanoindentation-induced phase changes in silicon by in situ electricalcharacterization, J. Appl. Phys. 101 (2007) 083531-1–083531-7.