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Si interface by embedded metallic nanoparticles
Materials Today Physics 4 (2018) 58e63
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Enhancement of confined femto-ablation at SiO2/Si interface by embedded metallic nanoparticles Z.U. Rehman a, 1, Le T. Na b, C.L. Tan c, M. Irfan d, 1, A. Qayyum a, K.A. Janulewicz e, *, 1 a
Physics Division, Pakistan Institute of Nuclear Science and Technology, Nilore, Islamabad, 45650, Pakistan Department of Physics and Photon Science, Gwangju Institute of Science and Technology, Gwangju, 61005, Republic of Korea c Photonics Research Centre, University of Malaya, Lembah Pantai, 50603, Kuala Lumpur, Malaysia d Department of Electrical Engineering, Khwaja Fareed University of Engineering and Information Technology, Rahim Yar Khan, 64200, Pakistan e Institute of Optoelectronics, Military University of Technology, 00-908, Warsaw, Poland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 January 2018 Received in revised form 4 March 2018 Accepted 7 March 2018 Available online 22 March 2018
Influence of doping an SiO2/Si interface with metallic nanoparticles (MNPs) on confined laser ablation and resulting structural properties of the crystalline silicon (c-Si) substrate was investigated by irradiating the composed interface with a single, tightly focused femtosecond laser pulse. Confinement ablation regime was enforced by a 10 mmethick SiO2 layer capping the c-Si substrate. A mixture of gold (Au) and silver (Ag) nanoparticles was placed at the interface to take advantage of the presumed plasmon-induced enhancement of the incident field strength in a broad spectral range. The nanoplasmonic effect is visualised by numerical simulations utilising the mathematical apparatus of the finite-difference time-domain (FDTD) method. The structural transformations at the site of the laserinduced damage were investigated dominantly by the scanning (SEM) and high-resolution transmission (HRTEM) electron microscopes. A comparative analysis of the irradiation effects in the targets containing different combinations of the interface composing elements revealed clear and strong influence of the confinement and doping on the irradiation result. Character of the observed transformations (among others the crystal twinning) suggests dominant role of increased pressure in the process through the locally generated shock waves. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Confined ablation Metallic nanoparticles Dielectric interface Pressure-induced phase transformation
1. Introduction Microexplosion within the transparent dielectrics initiated by tightly focused ultra-short laser pulses proved to be a source of extreme thermodynamic conditions leading to unprecedented material transformations [1e3]. For the sake of clarity, similar, although noticeably weaker effect, one can also obtain by surface multiple-pulse irradiation of the materials [4,5]. It is obvious that confined ablation, i.e. ablation into a dense medium instead of vacuum, changes conditions of the material ejection from a surface. Frequently, the confined ablation was investigated in water plaxing the role of the dense medium [6]. In the meantime, it has been
* Corresponding author. E-mail address: [email protected] (K.A. Janulewicz). 1 The work was done when with the Department of Physics and Photon Science and the Center for Relativistic Laser Science, Institute of Basic Science, both at the Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea. https://doi.org/10.1016/j.mtphys.2018.03.002 2542-5293/© 2018 Elsevier Ltd. All rights reserved.
shown that capping a substrate surface with e.g. a layer of the oxidised material (if possible) delivers noticeably more favourable conditions for this kind of ablation while keeping still technological simplicity [7]. The advantage of the confined regime relies primarily on increase in the pressure initiated by a high density of the deposited energy and reaction of the solid material surrounding the breakdown site. In such a geometry both the interface and the subinterface area would be strongly influenced. Irreversible modification of the material occurs after exceeding the damage threshold of the sample by the intensity of irradiation. Femtosecond laser pulses tightly focused by a high-numerical aperture (NA) optics used to lead to the absorbed energy density in excess of the strength of the most of the materials [8]. A void surrounded by a shell of compressed, i.e. densified, material used to be formed as a result of the confined micro-explosion triggered in the focal area. A hot dense matter at pressures exceeding 1 TPa and temperatures of more than 105 K were reported in the tabletop experiments, exceeding on the sub-micron scale the limits of high energy density
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and mimicking the conditions existing in the cores of stars and planets. Thus, studies of the confined micro-explosion open new research and application fields including astrophysical relevance [9], creation of exotic materials [10] and processing procedures for nanophotonics [11]. The big advantage of the confined microexplosion relies on keeping the laser-affected material in a limited space. Thus, it is readily available for the post-mortem investigation. It is natural that the material composition can also be used, independently of the energy delivery procedure, to intensify and control the lasermatter interaction. As a combination of metals and dielectrics on the microscale leads, under influence of optical wave, to the nanoplasmonic effects [12], it is reasonable to expect some field enhancement in the vicinity of the metallic NPs scattered over the interface and embedded in the capping SiO2 layer. Material doping with nanoparticles has a very long history but, in the laser-matter interaction context, presence of MNPs was in the past considered dominantly as a damage precursor [19]. In this paper we suggest a simple extension of the existing methods supporting efficient transformation of the laser energy into exotic physical effects. First, the nanoparticles placed at an interface increase energy absorption, and then the confinement regime of a strong laser ablation strengthens the hydrodynamic effects accompanied by shock wave generation. Our main goal was to reveal what indeed happens below a doped interface under irradiation of the composite structure with a limited power and to find signatures of achieving the extreme thermodynamic conditions and the rare states of the matter. 2. Experimental set-up The experimental set-up was in its irradiating part exactly the same as that previously described in detail in Ref. [3], while the interface geometry and its composition constituted the element of novelty. Laser pulses with a central wavelength of 800 nm and a duration of 40 fs were tightly focused (measured waist radius equal to 0.62 mm) with a high-numerical aperture microscope objective (MO, NA ¼ 1.25) utilising an oil-immersion (n ¼ 1.515) layer (see Fig. 1a). The SiO2/Si interface, buried under a 10 mm-
59
thick layer of SiO2 was placed precisely in the focal plane of the irradiating flux. The cross-sectional view of the radiationuntreated (pristine) SiO2/Si interface, doped with Au/Ag NPs is shown in Fig. 1b. In this view the nanoparticles quite regular in the top view can appear slightly distorted. It is very likely that some of them has been deformed during the cut performed with the focused ion beam (FIB) technique. Such a technique delivers additional energy to the particles of the ion beam way. Here, to illustrate the MNPs in the side view and at high magnification, we selected image with MNPs more distant from the cut plane. As a consequence, the nanoparticles are imperfectly visible but conserved their regular shape. The mix of MNPs embedded in a host dielectric material was used to extend spectral range of high absorption through generation of the relevant surface plasmons. Usually, plasmon resonances are spectrally narrow and the composition was used to assuage the influence of the fixed irradiation wavelength. Fig. 2 illustrates influence of the specific components on the effective absorptance/reflectance spectrum. The presented curves have been calculated according to the formula A ¼ 1 R T using the results of the recorded spectra (300e1600 nm) of the reflected and transmitted signals. Importantly, the applied in the measurement commercial spectrophotometer (Cary 5000, Varian, Palo Alto, CA, USA) was equipped with an integrating sphere and allowed for estimate of the absolute reflectance of the sample. The directly measured spectra were registered without the SiO2 layer. The sample was placed on a computer-controlled nano-positioning stage. An imaging system working in the mode of the reflected light microscopy was installed for in situ inspection of the breakdown site. The structure of Au/Ag NPs mixture was synthesized on a monocrystalline silicon (c-Si) substrate by the standard techniques based mainly on the temperature treatment and leading either to individual paraboloidal MNPs or metallic clusters. The dewetting process caused obtained mix of MNPs to be dominated by elongated (close to a paraboloidal shape) elements approximated in numerical modelling by a hemispherical form. Covering the structured interface with a 10 mm-thick layer of SiO2 by ebeam evaporation was the final step of the sample preparation. The mean diameters of Au and Ag NPs were 100.5 nm and
Fig. 1. a) Optical arrangement of the sample irradiation, b) cross-sectional HRTEM image of the interface of pristine SiO2/Si with embedded Au/Ag NPs.
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Fig. 2. a) Absorptance spectra of the c-Si substrate covered with different compositions of MNPs, the inset shows distribution of Au/Ag NPs on the substrate, Au NPs can be distinguished from Ag NPs by their brightness and larger size (the scale bar represents 500 nm); the absorptance spectra were calculated from the transmittance and reflectance measurements (A ¼ 1-R-T); b) Reflectance spectra of c-Si substrate coated with a mixture of Au/Ag NPs.
60.8 nm, respectively. The mean distance between the Au particles was 201 nm and between Ag 70 nm, while the mean Au/Ag separation was estimated to be equal to 105 nm. As a consequence, there were on average 149 Au-NPs and 544 Ag-NPs present in the monitored field of dimensions 245 nm 169 nm. The effect of the MNPs presence is clearly seen in the spectra of the absorbed/transmitted energy (Fig. 2). Increase in absorptance by at least 40% within a broad spectral range was recorded in the case of c-Si substrates covered with Au/Ag NPs, when compared to the bare c-Si wafers. Deposited MNPs also lowered reflectance of c-Si material acting as a sort of a broadband anti-reflection structure (Fig. 2).
3. Plasmonic effect Anticipation of nano-plasmonic effects in the vicinity of the MNPs embedded in silica resulted in the attempt to visualise the scale of the near-field enhancement in the vicinity of the particle contact to the c-Si substrate. The finite-difference time domain (FDTD) method, commonly applied in solving such problems, has been used in modelling [13]. The modelled system consisted of Au/ Ag NPs ordered on a silicon substrate according to the aforementioned experimentally estimated distances and was embedded in a transparent SiO2 layer. A linearly polarised incident wave at a wavelength of 800 nm irradiated the system normally to the SiO2/Si interface (Fig. 3). Strength of the electric field in the focus of the laser beam was calculated with an arbitrarily chosen reference value of 1 V/m ascribed to the incident beam. Hence, the resulting enhancement of the electric field in the MNP vicinity was automatically normalized to the incident electric field. The results of modelling are shown (only in the proximity of a single nanoparticle) in Fig. 3. The enhancement maximum is localised in the direct neighbourhood of the contact point between the MNPs and the silicon substrate. The effect used to be attributed to the multiple scattering and reflection of the incident electromagnetic field in the space between the particles and the substrate [14,16]. In addition, irradiation of the system by EM field induces electrical polarization of the particle and that, in turn, induces a mirror-image of the particle charge on the substrate. The presence of this charge on the substrate and the evanescent character of the scattered field are the direct reasons for localization of the enhancement maximum.
4. Results and discussion Radius of the craters created by a femtosecond laser pulse at the Si/SiO2 interface was measured with some averaging procedure by using an optical microscope and then analysed as a function of the incident energy Einc. The experimental dependence r ¼ f ðEinc Þ was n , where A,n were the fitting fitted with the expression r ¼ A,Einc parameters. This procedure gave for the case of the doped interface a value of n equal to 0.45 and a value of n ¼ 0.251 in the absence of Au/Ag NPs. The second value was very close to that derived theoretically for the pristine or doped with very small nanoparticles (d 10 nm) material and equal to 1/3. The result delivered an additional evidence that presence of MNPs at the interface increases energy deposition - the fact, also confirmed by reduction in a value of the laser damage threshold (LDT). LDT, equal to 0.25 J=cm2 in the presence of MNPs was nearly doubled in the absence of them (0.49 J=cm2 ). This scenario is in qualitative agreement with the reports on the role of metallic nano-inclusions acting as the absorption enhancing damage precursors [17e19]. Majority of this work formulated in the terms of thermodynamics and thermionics has been done for nanosecond pulses and very small nanoparticles. Here, we suggest an additional important factor, i.e. the surface plasmon resonance strengthening localization of the energy deposition, in contrast to the conventional scenario. Knowing that the power density of a radiation field expressed by the Poynting vector is proportional to jEj2 , it is justified to expect that the amount of the energy deposited in the MNP's closest neighbourhood will noticeably increase. The theoretical estimate of the energy density gain suggests a value of 2000 for jE=E0 j2 and that means we should expect an increase in the energy density by at least three orders of magnitude. A micrograph of the interaction site after focusing of 65 nJ of the incident energy contained in a single laser pulse reveals an interaction site with an empty cavity (void) and a shock wave-affected (SWA) area. The size of the void is the unequivocal evidence of the strong shock wave upload, being well in excess of the Young's moduli of both Si (YSi 179 GPa for ¡100¿ orientation [20]) and SiO2 (YSiO2 75 GPa). Cross-sectional electron micrograph of the interface region proves that the void was created exactly at the interface and did not cause any damage on the front surface of the SiO2 layer (see Fig. 4d). Formation of void and a densified SWA region at this irradiating fluence, along with evaporating the MNPs, implies a very intense interaction leaving the signatures of strong melting and boiling confirmed by the images in Fig. 5a). All these observations strengthen the argument
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Fig. 3. a) Mapping of the electric field amplitude distribution in the plane normal to the interface, in the vicinity of a gold metallic nanoparticle placed on a silicon substrate and embedded in a transparent silica matrix. The system was assumed to be irradiated with a plane electromagnetic wave of linear polarization; b) Mapping of the electric field amplitude distribution close to a hemispherical nanoparticle. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
n ) fits to the experimental data with A,n used Fig. 4. a) Dependence of crater radii vs. pulse energy in the focus, with the solid and dashed curves representing the functional (rfEinc as the fitting parameters; b) image from a scanning electron microscope (SEM) showing the void created by a femtosecond laser pulse of 65 nJ energy incident on the undoped SiO2/ c-Si interface; c) SEM image of the void created on the MNPs-doped silicon surface revealed by etching-out with the reactive ion etching (RIE); randomly positioned small white arrows point towards the remained individual MNPs; d) void on Si surface in the case when the interface doped with MNPs is covered with a 10 mmethick SiO2 layer, red arrow shows the direction of the incident laser beam. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
that presence of the metallic NPs on the substrate intensifies changes in the thermodynamic parameters (p, T) of the process.
Cross-sectional, of high-resolution cuts of the voids surrounded by the shock-wave-affected (SWA) area are shown in Fig. 5a (the
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Fig. 5. a) Cross-sectional TEM image of the void and shock-wave affected region at SiO2/Si interface with embedded MNPs when an incident energy of 65 nJ was applied; b) HRTEM image of dislocation and crystal twining taken from the marked area of the SWA region; c) Zoomed selective area electron diffraction (SAED) pattern with the twinning effect and the inset (bottom, right) showing the full image; the colour lines should help to identify the structures contributing to the twinning; d) SAED pattern of pristine crystalline silicon; e) Cross-sectional TEM image of laser-modified interface SiO2/Si without Au/Ag NPs; f) magnified selected square region, marked in the panel e) near the geometrical focus, showing the amorphous phase present in the subsurface. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
case with MNPs) and in Fig. 5e for the case without doping. The semicircular area of interest is pointed out by a thin, white line being a part of the full semicircle. In the case of the interface doped with MNPs, the confined ablation is clearly localised at the interface with conspicuous lobes of the ejected molten substrate material penetrating the covering SiO2 layer. A relatively rare phenomenon of crystal twinning was found within the SWA region, in its lower part. The crystal twinning occurs when two separate crystals share some of their lattice points in a symmetrical manner (mirror reflection). Images from bright-field electron microscopy (BFTEM) of the area marked in Fig. 5a and the relevant selected area electron diffraction (SAED) images revealed a pattern of very strong crystal twinning in the monocrystalline silicon of the substrate. Having compared this diffraction pattern with a non-irradiated (pristine) region of the c-Si (Fig. 5d) one can see the perfect proof of the mirrored crystal twins. The original and the twin structures are marked by the colour lines. According to the diffraction patterns, the parent matrix was mirrored either along the (111) plane or alternatively along the (111) plane. Lattice points and planes were identified according to the lines of the work on twinning in semiconductors [21]. The presence of the twining was also confirmed by direct BFTEM imaging of the lattice structure and is presented in Fig. 5b. Moreover, some dislocations moving from the silicon surface inwards the sample along specific crystallographic orientations were also recorded as they lead to further defect spreading. Interestingly, twinning is a local effect within SWA and this implies strong influence of the individual particles with the electric field enhancement in the near field. The lack of significant area with the amorphous phase suggests inhibition of the cooling process. Temperature of the silicon melting point is 1683 K, but this is a material with a negative Clausius-Clapeyron relation (slope dTmelt =dp < 0) and the melting point temperature decreases with
increase in the hydrostatic pressure [22]. Therefore, it is possible to melt silicon at a temperature much lower than the ambient melting temperature, subject to using a moderate pressure (12e17 GPa). Under our irradiation conditions (F ¼ 5.37 J/cm2), and taking into account the depth of the skin layer equal to labs 23 nm and the energy absorption factor equal to 0.50 [15], the estimated values of the maximum pressure pmax and electron temperature Te in the skin depth were 2.3 TPa and 1.12 106 K, respectively. They were the driving factor behind the shock wave generated in the material. Obviously, this estimated pressure significantly exceeds the Young's modulus of silicon and easily forms the void with the surrounding densified shell. The characteristic time of the electron heat conduction has been estimated (according to the formula tcool ¼ l2abs =De , under the assumption of diffusion being the main heat transfer mechanism) as equal to 19.5 ps, with De ¼ 2.7 cm2/s [8]. As a consequence, the temperature and accompanying pressure near the energy deposition region drop about ten times across the SWA region. Under shock wave loading (shock compression), when the pressure exceeds the Hugoniot elastic limit (HEL for Si 9 GPa), a strain state is established, with both the hydrostatic (isotropic pressure component) and the deviatoric (shear) stress components being high. The induced shear stress, which is higher than the critical shear stress i.e. 3.61 106 Pa for Si ends in creation of lattice defects as dislocations and twinning, both observed in our experiment. The motion of these defects increases the local disorder of the lattice and associated with it significant heating. Under shock loading, adiabatic conditions are established in the sheared region and the temperature rises correspondingly. The phase transition occurs after reaching the lowered melting point temperature of silicon (due to pressure). Furthermore, the rapid release of the stress/ pressure wave initiates a self-quenching mechanism with a high
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cooling rate (on the nanosecond scale) of 5.9 105 K/s) preserving the disordered structure. Extreme conditions can be created also around and the interface without MNPs but only at sufficiently intense irradiation. However, the extreme high pressure and stress tend to be sustained for longer period of time in the case of confined ablation with the embedded MNPs, resulting in more durable changes in the structure. The effect of crystal twinning is frequently met in mineralogy but in the laboratory experiments is rare. Recently, it was reported, e.g. in the slow-pressure release experiments performed on germanium [24] and during irradiation of silicon surface by a high power laser (l 351 nm, pulse width 1 ns and energy of 150 J) [25]. We observed twinning effect by tightly focusing an incident energy even as low as 65 nJ, owing to MNPs doping. This is an unambiguous proof of active support by confined ablation and MNPs in reaching the extreme thermodynamic conditions. There was no evidence of twinning without MNPs at the interface irradiated by the pulse of the same energy (Fig. 5e and f). In this case, SWA region consisted of a thin amorphous layer of Si. Formation of the thin amorphous region is the result of much faster quenching that occurs during irradiation. The presence of amorphous silicon (a-Si) implies existence of the molten silicon which was supercooled at very high rate preventing material from nucleation, followed by crystal growth. It has to be clarified again that twinning was observed in both samples, i.e. with and without MNPs, the latter however, at much higher incident energy of 130 nJ. It means, there exists certain threshold energy energy density (pressure) for twining effect, above which this phenomenon will be observed. This threshold could be controlled by MNP doping.
5. Summary To summarise, we investigated structural modification induced during confined ablation at the interface between SiO2 and c-Si substrate. Embedding Au/Ag NPs at the interface strengthened the field of the optical wave in the vicinity of the individual MNPs due to the localised surface plasmons and led to increase in radiative energy absorption. The scale of the EM field enhancement on the interface was predicted by numerical simulations utilising the FDTD method. The size of the damage areas was determined as a function of the incident energy and interface composition. The result showed faster growth of the damage area with the incident energy and the lowered damage threshold value in the presence of MNPs. Moreover, high resolution, crosssectional investigation of the laser-affected region in the Si substrate revealed, for the case of the interface doped with MNPs, localised formation of the crystal twinning within the shell (SWA) region. Presence of this rare (in a laboratory) effect proves that the generated thermodynamic conditions were more favourable for existence of a novel metastable end-phase. Irradiation under the same energetic condition of the undoped interface resulted only in a thin amorphous layer of Si, presumably due to very fast quenching and limited time for nucleation of any crystalline structure. The result also implies that controlled doping of an interface with MNPs can constitute an additional degree of freedom in controlling localised energy deposition and the scale of its consequences. It can be applicable in high energy density experiments but also in laser material processing or machining.
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Funding This work was supported by the Ministry of Education, Science and Technology of Korea through Basic Science Research Program (No.R15-2008-006-03001-00), by IBS under IBS-R012-D1 and by the Gwangju Institute of Science and Technology through the Top Brand Project (TBP).
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Materials Today Physics journal homepage: https://www.journals.elsevier.com/ materials-today-physics
Enhancement of confined femto-ablation at SiO2/Si interface by embedded metallic nanoparticles Z.U. Rehman a, 1, Le T. Na b, C.L. Tan c, M. Irfan d, 1, A. Qayyum a, K.A. Janulewicz e, *, 1 a
Physics Division, Pakistan Institute of Nuclear Science and Technology, Nilore, Islamabad, 45650, Pakistan Department of Physics and Photon Science, Gwangju Institute of Science and Technology, Gwangju, 61005, Republic of Korea c Photonics Research Centre, University of Malaya, Lembah Pantai, 50603, Kuala Lumpur, Malaysia d Department of Electrical Engineering, Khwaja Fareed University of Engineering and Information Technology, Rahim Yar Khan, 64200, Pakistan e Institute of Optoelectronics, Military University of Technology, 00-908, Warsaw, Poland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 January 2018 Received in revised form 4 March 2018 Accepted 7 March 2018 Available online 22 March 2018
Influence of doping an SiO2/Si interface with metallic nanoparticles (MNPs) on confined laser ablation and resulting structural properties of the crystalline silicon (c-Si) substrate was investigated by irradiating the composed interface with a single, tightly focused femtosecond laser pulse. Confinement ablation regime was enforced by a 10 mmethick SiO2 layer capping the c-Si substrate. A mixture of gold (Au) and silver (Ag) nanoparticles was placed at the interface to take advantage of the presumed plasmon-induced enhancement of the incident field strength in a broad spectral range. The nanoplasmonic effect is visualised by numerical simulations utilising the mathematical apparatus of the finite-difference time-domain (FDTD) method. The structural transformations at the site of the laserinduced damage were investigated dominantly by the scanning (SEM) and high-resolution transmission (HRTEM) electron microscopes. A comparative analysis of the irradiation effects in the targets containing different combinations of the interface composing elements revealed clear and strong influence of the confinement and doping on the irradiation result. Character of the observed transformations (among others the crystal twinning) suggests dominant role of increased pressure in the process through the locally generated shock waves. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Confined ablation Metallic nanoparticles Dielectric interface Pressure-induced phase transformation
1. Introduction Microexplosion within the transparent dielectrics initiated by tightly focused ultra-short laser pulses proved to be a source of extreme thermodynamic conditions leading to unprecedented material transformations [1e3]. For the sake of clarity, similar, although noticeably weaker effect, one can also obtain by surface multiple-pulse irradiation of the materials [4,5]. It is obvious that confined ablation, i.e. ablation into a dense medium instead of vacuum, changes conditions of the material ejection from a surface. Frequently, the confined ablation was investigated in water plaxing the role of the dense medium [6]. In the meantime, it has been
* Corresponding author. E-mail address: [email protected] (K.A. Janulewicz). 1 The work was done when with the Department of Physics and Photon Science and the Center for Relativistic Laser Science, Institute of Basic Science, both at the Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea. https://doi.org/10.1016/j.mtphys.2018.03.002 2542-5293/© 2018 Elsevier Ltd. All rights reserved.
shown that capping a substrate surface with e.g. a layer of the oxidised material (if possible) delivers noticeably more favourable conditions for this kind of ablation while keeping still technological simplicity [7]. The advantage of the confined regime relies primarily on increase in the pressure initiated by a high density of the deposited energy and reaction of the solid material surrounding the breakdown site. In such a geometry both the interface and the subinterface area would be strongly influenced. Irreversible modification of the material occurs after exceeding the damage threshold of the sample by the intensity of irradiation. Femtosecond laser pulses tightly focused by a high-numerical aperture (NA) optics used to lead to the absorbed energy density in excess of the strength of the most of the materials [8]. A void surrounded by a shell of compressed, i.e. densified, material used to be formed as a result of the confined micro-explosion triggered in the focal area. A hot dense matter at pressures exceeding 1 TPa and temperatures of more than 105 K were reported in the tabletop experiments, exceeding on the sub-micron scale the limits of high energy density
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and mimicking the conditions existing in the cores of stars and planets. Thus, studies of the confined micro-explosion open new research and application fields including astrophysical relevance [9], creation of exotic materials [10] and processing procedures for nanophotonics [11]. The big advantage of the confined microexplosion relies on keeping the laser-affected material in a limited space. Thus, it is readily available for the post-mortem investigation. It is natural that the material composition can also be used, independently of the energy delivery procedure, to intensify and control the lasermatter interaction. As a combination of metals and dielectrics on the microscale leads, under influence of optical wave, to the nanoplasmonic effects [12], it is reasonable to expect some field enhancement in the vicinity of the metallic NPs scattered over the interface and embedded in the capping SiO2 layer. Material doping with nanoparticles has a very long history but, in the laser-matter interaction context, presence of MNPs was in the past considered dominantly as a damage precursor [19]. In this paper we suggest a simple extension of the existing methods supporting efficient transformation of the laser energy into exotic physical effects. First, the nanoparticles placed at an interface increase energy absorption, and then the confinement regime of a strong laser ablation strengthens the hydrodynamic effects accompanied by shock wave generation. Our main goal was to reveal what indeed happens below a doped interface under irradiation of the composite structure with a limited power and to find signatures of achieving the extreme thermodynamic conditions and the rare states of the matter. 2. Experimental set-up The experimental set-up was in its irradiating part exactly the same as that previously described in detail in Ref. [3], while the interface geometry and its composition constituted the element of novelty. Laser pulses with a central wavelength of 800 nm and a duration of 40 fs were tightly focused (measured waist radius equal to 0.62 mm) with a high-numerical aperture microscope objective (MO, NA ¼ 1.25) utilising an oil-immersion (n ¼ 1.515) layer (see Fig. 1a). The SiO2/Si interface, buried under a 10 mm-
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thick layer of SiO2 was placed precisely in the focal plane of the irradiating flux. The cross-sectional view of the radiationuntreated (pristine) SiO2/Si interface, doped with Au/Ag NPs is shown in Fig. 1b. In this view the nanoparticles quite regular in the top view can appear slightly distorted. It is very likely that some of them has been deformed during the cut performed with the focused ion beam (FIB) technique. Such a technique delivers additional energy to the particles of the ion beam way. Here, to illustrate the MNPs in the side view and at high magnification, we selected image with MNPs more distant from the cut plane. As a consequence, the nanoparticles are imperfectly visible but conserved their regular shape. The mix of MNPs embedded in a host dielectric material was used to extend spectral range of high absorption through generation of the relevant surface plasmons. Usually, plasmon resonances are spectrally narrow and the composition was used to assuage the influence of the fixed irradiation wavelength. Fig. 2 illustrates influence of the specific components on the effective absorptance/reflectance spectrum. The presented curves have been calculated according to the formula A ¼ 1 R T using the results of the recorded spectra (300e1600 nm) of the reflected and transmitted signals. Importantly, the applied in the measurement commercial spectrophotometer (Cary 5000, Varian, Palo Alto, CA, USA) was equipped with an integrating sphere and allowed for estimate of the absolute reflectance of the sample. The directly measured spectra were registered without the SiO2 layer. The sample was placed on a computer-controlled nano-positioning stage. An imaging system working in the mode of the reflected light microscopy was installed for in situ inspection of the breakdown site. The structure of Au/Ag NPs mixture was synthesized on a monocrystalline silicon (c-Si) substrate by the standard techniques based mainly on the temperature treatment and leading either to individual paraboloidal MNPs or metallic clusters. The dewetting process caused obtained mix of MNPs to be dominated by elongated (close to a paraboloidal shape) elements approximated in numerical modelling by a hemispherical form. Covering the structured interface with a 10 mm-thick layer of SiO2 by ebeam evaporation was the final step of the sample preparation. The mean diameters of Au and Ag NPs were 100.5 nm and
Fig. 1. a) Optical arrangement of the sample irradiation, b) cross-sectional HRTEM image of the interface of pristine SiO2/Si with embedded Au/Ag NPs.
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Fig. 2. a) Absorptance spectra of the c-Si substrate covered with different compositions of MNPs, the inset shows distribution of Au/Ag NPs on the substrate, Au NPs can be distinguished from Ag NPs by their brightness and larger size (the scale bar represents 500 nm); the absorptance spectra were calculated from the transmittance and reflectance measurements (A ¼ 1-R-T); b) Reflectance spectra of c-Si substrate coated with a mixture of Au/Ag NPs.
60.8 nm, respectively. The mean distance between the Au particles was 201 nm and between Ag 70 nm, while the mean Au/Ag separation was estimated to be equal to 105 nm. As a consequence, there were on average 149 Au-NPs and 544 Ag-NPs present in the monitored field of dimensions 245 nm 169 nm. The effect of the MNPs presence is clearly seen in the spectra of the absorbed/transmitted energy (Fig. 2). Increase in absorptance by at least 40% within a broad spectral range was recorded in the case of c-Si substrates covered with Au/Ag NPs, when compared to the bare c-Si wafers. Deposited MNPs also lowered reflectance of c-Si material acting as a sort of a broadband anti-reflection structure (Fig. 2).
3. Plasmonic effect Anticipation of nano-plasmonic effects in the vicinity of the MNPs embedded in silica resulted in the attempt to visualise the scale of the near-field enhancement in the vicinity of the particle contact to the c-Si substrate. The finite-difference time domain (FDTD) method, commonly applied in solving such problems, has been used in modelling [13]. The modelled system consisted of Au/ Ag NPs ordered on a silicon substrate according to the aforementioned experimentally estimated distances and was embedded in a transparent SiO2 layer. A linearly polarised incident wave at a wavelength of 800 nm irradiated the system normally to the SiO2/Si interface (Fig. 3). Strength of the electric field in the focus of the laser beam was calculated with an arbitrarily chosen reference value of 1 V/m ascribed to the incident beam. Hence, the resulting enhancement of the electric field in the MNP vicinity was automatically normalized to the incident electric field. The results of modelling are shown (only in the proximity of a single nanoparticle) in Fig. 3. The enhancement maximum is localised in the direct neighbourhood of the contact point between the MNPs and the silicon substrate. The effect used to be attributed to the multiple scattering and reflection of the incident electromagnetic field in the space between the particles and the substrate [14,16]. In addition, irradiation of the system by EM field induces electrical polarization of the particle and that, in turn, induces a mirror-image of the particle charge on the substrate. The presence of this charge on the substrate and the evanescent character of the scattered field are the direct reasons for localization of the enhancement maximum.
4. Results and discussion Radius of the craters created by a femtosecond laser pulse at the Si/SiO2 interface was measured with some averaging procedure by using an optical microscope and then analysed as a function of the incident energy Einc. The experimental dependence r ¼ f ðEinc Þ was n , where A,n were the fitting fitted with the expression r ¼ A,Einc parameters. This procedure gave for the case of the doped interface a value of n equal to 0.45 and a value of n ¼ 0.251 in the absence of Au/Ag NPs. The second value was very close to that derived theoretically for the pristine or doped with very small nanoparticles (d 10 nm) material and equal to 1/3. The result delivered an additional evidence that presence of MNPs at the interface increases energy deposition - the fact, also confirmed by reduction in a value of the laser damage threshold (LDT). LDT, equal to 0.25 J=cm2 in the presence of MNPs was nearly doubled in the absence of them (0.49 J=cm2 ). This scenario is in qualitative agreement with the reports on the role of metallic nano-inclusions acting as the absorption enhancing damage precursors [17e19]. Majority of this work formulated in the terms of thermodynamics and thermionics has been done for nanosecond pulses and very small nanoparticles. Here, we suggest an additional important factor, i.e. the surface plasmon resonance strengthening localization of the energy deposition, in contrast to the conventional scenario. Knowing that the power density of a radiation field expressed by the Poynting vector is proportional to jEj2 , it is justified to expect that the amount of the energy deposited in the MNP's closest neighbourhood will noticeably increase. The theoretical estimate of the energy density gain suggests a value of 2000 for jE=E0 j2 and that means we should expect an increase in the energy density by at least three orders of magnitude. A micrograph of the interaction site after focusing of 65 nJ of the incident energy contained in a single laser pulse reveals an interaction site with an empty cavity (void) and a shock wave-affected (SWA) area. The size of the void is the unequivocal evidence of the strong shock wave upload, being well in excess of the Young's moduli of both Si (YSi 179 GPa for ¡100¿ orientation [20]) and SiO2 (YSiO2 75 GPa). Cross-sectional electron micrograph of the interface region proves that the void was created exactly at the interface and did not cause any damage on the front surface of the SiO2 layer (see Fig. 4d). Formation of void and a densified SWA region at this irradiating fluence, along with evaporating the MNPs, implies a very intense interaction leaving the signatures of strong melting and boiling confirmed by the images in Fig. 5a). All these observations strengthen the argument
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Fig. 3. a) Mapping of the electric field amplitude distribution in the plane normal to the interface, in the vicinity of a gold metallic nanoparticle placed on a silicon substrate and embedded in a transparent silica matrix. The system was assumed to be irradiated with a plane electromagnetic wave of linear polarization; b) Mapping of the electric field amplitude distribution close to a hemispherical nanoparticle. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
n ) fits to the experimental data with A,n used Fig. 4. a) Dependence of crater radii vs. pulse energy in the focus, with the solid and dashed curves representing the functional (rfEinc as the fitting parameters; b) image from a scanning electron microscope (SEM) showing the void created by a femtosecond laser pulse of 65 nJ energy incident on the undoped SiO2/ c-Si interface; c) SEM image of the void created on the MNPs-doped silicon surface revealed by etching-out with the reactive ion etching (RIE); randomly positioned small white arrows point towards the remained individual MNPs; d) void on Si surface in the case when the interface doped with MNPs is covered with a 10 mmethick SiO2 layer, red arrow shows the direction of the incident laser beam. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
that presence of the metallic NPs on the substrate intensifies changes in the thermodynamic parameters (p, T) of the process.
Cross-sectional, of high-resolution cuts of the voids surrounded by the shock-wave-affected (SWA) area are shown in Fig. 5a (the
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Fig. 5. a) Cross-sectional TEM image of the void and shock-wave affected region at SiO2/Si interface with embedded MNPs when an incident energy of 65 nJ was applied; b) HRTEM image of dislocation and crystal twining taken from the marked area of the SWA region; c) Zoomed selective area electron diffraction (SAED) pattern with the twinning effect and the inset (bottom, right) showing the full image; the colour lines should help to identify the structures contributing to the twinning; d) SAED pattern of pristine crystalline silicon; e) Cross-sectional TEM image of laser-modified interface SiO2/Si without Au/Ag NPs; f) magnified selected square region, marked in the panel e) near the geometrical focus, showing the amorphous phase present in the subsurface. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
case with MNPs) and in Fig. 5e for the case without doping. The semicircular area of interest is pointed out by a thin, white line being a part of the full semicircle. In the case of the interface doped with MNPs, the confined ablation is clearly localised at the interface with conspicuous lobes of the ejected molten substrate material penetrating the covering SiO2 layer. A relatively rare phenomenon of crystal twinning was found within the SWA region, in its lower part. The crystal twinning occurs when two separate crystals share some of their lattice points in a symmetrical manner (mirror reflection). Images from bright-field electron microscopy (BFTEM) of the area marked in Fig. 5a and the relevant selected area electron diffraction (SAED) images revealed a pattern of very strong crystal twinning in the monocrystalline silicon of the substrate. Having compared this diffraction pattern with a non-irradiated (pristine) region of the c-Si (Fig. 5d) one can see the perfect proof of the mirrored crystal twins. The original and the twin structures are marked by the colour lines. According to the diffraction patterns, the parent matrix was mirrored either along the (111) plane or alternatively along the (111) plane. Lattice points and planes were identified according to the lines of the work on twinning in semiconductors [21]. The presence of the twining was also confirmed by direct BFTEM imaging of the lattice structure and is presented in Fig. 5b. Moreover, some dislocations moving from the silicon surface inwards the sample along specific crystallographic orientations were also recorded as they lead to further defect spreading. Interestingly, twinning is a local effect within SWA and this implies strong influence of the individual particles with the electric field enhancement in the near field. The lack of significant area with the amorphous phase suggests inhibition of the cooling process. Temperature of the silicon melting point is 1683 K, but this is a material with a negative Clausius-Clapeyron relation (slope dTmelt =dp < 0) and the melting point temperature decreases with
increase in the hydrostatic pressure [22]. Therefore, it is possible to melt silicon at a temperature much lower than the ambient melting temperature, subject to using a moderate pressure (12e17 GPa). Under our irradiation conditions (F ¼ 5.37 J/cm2), and taking into account the depth of the skin layer equal to labs 23 nm and the energy absorption factor equal to 0.50 [15], the estimated values of the maximum pressure pmax and electron temperature Te in the skin depth were 2.3 TPa and 1.12 106 K, respectively. They were the driving factor behind the shock wave generated in the material. Obviously, this estimated pressure significantly exceeds the Young's modulus of silicon and easily forms the void with the surrounding densified shell. The characteristic time of the electron heat conduction has been estimated (according to the formula tcool ¼ l2abs =De , under the assumption of diffusion being the main heat transfer mechanism) as equal to 19.5 ps, with De ¼ 2.7 cm2/s [8]. As a consequence, the temperature and accompanying pressure near the energy deposition region drop about ten times across the SWA region. Under shock wave loading (shock compression), when the pressure exceeds the Hugoniot elastic limit (HEL for Si 9 GPa), a strain state is established, with both the hydrostatic (isotropic pressure component) and the deviatoric (shear) stress components being high. The induced shear stress, which is higher than the critical shear stress i.e. 3.61 106 Pa for Si ends in creation of lattice defects as dislocations and twinning, both observed in our experiment. The motion of these defects increases the local disorder of the lattice and associated with it significant heating. Under shock loading, adiabatic conditions are established in the sheared region and the temperature rises correspondingly. The phase transition occurs after reaching the lowered melting point temperature of silicon (due to pressure). Furthermore, the rapid release of the stress/ pressure wave initiates a self-quenching mechanism with a high
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cooling rate (on the nanosecond scale) of 5.9 105 K/s) preserving the disordered structure. Extreme conditions can be created also around and the interface without MNPs but only at sufficiently intense irradiation. However, the extreme high pressure and stress tend to be sustained for longer period of time in the case of confined ablation with the embedded MNPs, resulting in more durable changes in the structure. The effect of crystal twinning is frequently met in mineralogy but in the laboratory experiments is rare. Recently, it was reported, e.g. in the slow-pressure release experiments performed on germanium [24] and during irradiation of silicon surface by a high power laser (l 351 nm, pulse width 1 ns and energy of 150 J) [25]. We observed twinning effect by tightly focusing an incident energy even as low as 65 nJ, owing to MNPs doping. This is an unambiguous proof of active support by confined ablation and MNPs in reaching the extreme thermodynamic conditions. There was no evidence of twinning without MNPs at the interface irradiated by the pulse of the same energy (Fig. 5e and f). In this case, SWA region consisted of a thin amorphous layer of Si. Formation of the thin amorphous region is the result of much faster quenching that occurs during irradiation. The presence of amorphous silicon (a-Si) implies existence of the molten silicon which was supercooled at very high rate preventing material from nucleation, followed by crystal growth. It has to be clarified again that twinning was observed in both samples, i.e. with and without MNPs, the latter however, at much higher incident energy of 130 nJ. It means, there exists certain threshold energy energy density (pressure) for twining effect, above which this phenomenon will be observed. This threshold could be controlled by MNP doping.
5. Summary To summarise, we investigated structural modification induced during confined ablation at the interface between SiO2 and c-Si substrate. Embedding Au/Ag NPs at the interface strengthened the field of the optical wave in the vicinity of the individual MNPs due to the localised surface plasmons and led to increase in radiative energy absorption. The scale of the EM field enhancement on the interface was predicted by numerical simulations utilising the FDTD method. The size of the damage areas was determined as a function of the incident energy and interface composition. The result showed faster growth of the damage area with the incident energy and the lowered damage threshold value in the presence of MNPs. Moreover, high resolution, crosssectional investigation of the laser-affected region in the Si substrate revealed, for the case of the interface doped with MNPs, localised formation of the crystal twinning within the shell (SWA) region. Presence of this rare (in a laboratory) effect proves that the generated thermodynamic conditions were more favourable for existence of a novel metastable end-phase. Irradiation under the same energetic condition of the undoped interface resulted only in a thin amorphous layer of Si, presumably due to very fast quenching and limited time for nucleation of any crystalline structure. The result also implies that controlled doping of an interface with MNPs can constitute an additional degree of freedom in controlling localised energy deposition and the scale of its consequences. It can be applicable in high energy density experiments but also in laser material processing or machining.
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Funding This work was supported by the Ministry of Education, Science and Technology of Korea through Basic Science Research Program (No.R15-2008-006-03001-00), by IBS under IBS-R012-D1 and by the Gwangju Institute of Science and Technology through the Top Brand Project (TBP).
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