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Study of silicon surface implanted by silver ions
Vacuum 159 (2019) 353–357
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Short communication
Study of silicon surface implanted by silver ions a,∗
a
a
A.L. Stepanov , V.I. Nuzhdin , V.F. Valeev , V.V. Vorobev a b
a,b
, A.M. Rogov
a,b
a,b
, Y.N. Osin
T
Kazan Physical-Technical Institute, Russian Academy of Sciences, 420029, Kazan, Russia Kazan Federal University, 420008, Kazan, Russia
ARTICLE INFO
ABSTRACT
Keywords: Porous silicon Ion implantation Silver nanoparticles
Ag+-ion implantation of single-crystal c-Si at low-energy (E = 30 keV) high-doses (D = 1.25⋅1015–1.5⋅1017 ion/ cm2) and current density (J = 2, 8, 15 μA/cm2) was carried out. The changes of Si surface morphology after ion implantation were studied by scanning electron and atomic force microscopy. The near surface area of samples was also analyzed by diffraction of the backscattered electrons and energy-dispersive X-ray microanalysis. At the lowest implantation doses of c-Si amorphization of near-surface layer was observed. Ag nanoparticles were synthesized and uniformly distributed over the near Si surface when the threshold dose of 3.1⋅1015 ion/cm2 is exceeded. At a dose of more than 1017 ion/cm2, the formation of a surface porous Si structure was detected. Ag nanoparticle size distribution function becomes bimodal and the largest particles were localized along Si-pore walls.
1. Introduction The search for new composite materials and effective technologies for the creation on their basis various semiconductor devices, including Si nanostructures, for their applications in modern industrial microelectronics are relevant at the present time. For such purpose in some technical cases porous Si (PSi) is used. Generally, PSi is mainly obtained by electrochemical etching of single-crystal c-Si in hydrofluoric acid solutions [1]. On the other side, in addition to the chemical approach, another physical technology for the formation of porous semiconductor layers is also tested applying implantation of Si by various types of ions in vacuum [2]. PSi is of interest for use in the field of micro-, nano- and optoelectronics [3], in particular, for the creation of solar cells [4], biosensorics [5] and so on. At present the fabrication process of PSi by high-dose implantation of Si using inert gas ions is quite well studied. The solubility of inert gas atoms in semiconductors is relatively small. Therefore, starting with some threshold doses of implantation (concentration of gas ions), the formation of nanoscale pores inside of solid is explained due to the filling of a local volume of material with gettered molecules or bubbles of implanted gas ions [2]. The formation of pores in Si was early observed during implantation by such ions as H+ [6], He+ [7], Ne+ [8], Ar+ [9] and Kr+ [8,10]. A distinctive feature of the work [10] is that when using low-energy Kr+-ion implantation, the open pores were formed on irradiated Si surface, but not inside of the Si volume only. The authors of the work [11] demonstrated an interesting method for
∗
manufacturing a porous layer in Si by implantation with Sb+ semimetal ions followed by a two-stage thermal annealing, resulting in the sequential synthesis of Sb nanoparticles, following with those nanoparticle melting. As a consequence, a free volume-pores in Si instead of Sb nanoparticles was created. In addition, in practice, different techniques for the creation of composite materials based on PSi, containing noble metal nanoparticles, for example, Au [12], Pt [13] and Ag [14], are actively searching and testing. The task of this activity is to increase the efficiency of the optical properties of PSi. Collective excitation of conduction electrons in metal nanoparticles during interaction with an electromagnetic light wave causes resonant amplification of the local field (surface plasmon resonance), which leads to stimulation of optical and nonlinear optical effects in composite materials with nanoparticles [15,16]. In particular, the presence of Ag nanoparticles in the Si matrix could contributed to an increase in the efficiency of the functioning of solar cells, due to a change in their absorptivity at plasmon resonance frequencies of nanoparticles [17,18], as well as electron photogeneration arising in nanoparticles and diffusing through the Schottky barrier into Si at the metal/semiconductor interface [19]. In addition, the plasmon field of the Ag nanoparticle deposited on PSi leads to an increase of the photoluminescence of semiconductor quantum dots [20] and to the enhancement of surface Raman scattering of organic compounds [21]. Earlier, it was suggested to use low-energy high-dose Ag+-ion implantation of c-Si to form PSi layer simultaneously with synthesis of Ag
Corresponding author. E-mail address: [email protected] (A.L. Stepanov).
https://doi.org/10.1016/j.vacuum.2018.10.060 Received 19 September 2018; Received in revised form 20 October 2018; Accepted 22 October 2018 Available online 25 October 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved.
Vacuum 159 (2019) 353–357
A.L. Stepanov et al.
nanoparticles inward (Ag:PSi) [22,23] similar as it was done with porous Ge [24]. The purpose of this work is to perform a detail study a Ag nanoparticle formation, the surface morphology and structure of Ag:PSi layers manufactured in a wide range of implantation doses and current densities using modern electronic scanning and atomic force microscopy methods. 2. Materials and methods Single-crystal c-Si wafers with thickness of 400 μm and a crystallographic orientation (100) were used as substrates. Implantation was carried out by Ag+ ions with an energy of E = 30 keV at an irradiation dose D in the range from 6.24⋅1013 to 1.5⋅1017 ion/cm2 and current densities J = 2, 8 and 15 μA/cm2 using the ion accelerator ILU-3 at room temperature of the irradiated substrates. The vacuum level in an ion accelerate chamber during ion implantation was 10−5 Torr. Observation of morphology of the sample surface and energy-dispersive X-ray (EDX) spectral microanalysis of implanted Si was carried out with with a high-resolution scanning electron microscopes (SEM) SU 8000 (Hitachi) and Merlin Zeiss equipped by Aztec X-Max spectrometer (Oxford Instruments). Structure of surface sample layer was characterized by electron backscattered diffraction (EBSD) patterns using HKL NordLys detector (Oxford Instruments). The surface topology of the samples was also analised with atomic-force microscope (AFM) Dimension FastScan (Bruker) in the Quantative Nanomechanical Mapping mode using Bruker ScanAsyst Air probes. 3. Results and discussion The calculation of Ag-ion depth distribution in Si after high-dose ion implantation taking into account surface spattering of the wafers predicted that the Ag impurity is distributed in the top layer of the sample with a thickness of about 40 nm, which was estimated in the work [22]. The results on a change in the crystal structure of Si implanted layer at the low implantation D from 6.24⋅1013 to 1.3⋅1014 ion/cm2 and a fixed values of E = 30 keV and J = 2 μA/cm2 are presented in Fig. 1. For the virgin c-Si wafer the distinct Kikuchi lines in the EBSD image are observed (Fig. 1 a). Automatic identification of this EBSD pattern with the Aztec 2.1 computer program shows the orientation (100) of the cubic syngony of the unit с-Si cell with the crystallographic parameters a = b = c = 5.43 Å and α = β = γ = 90̊. When the c-Si substrate was implanted with Ag+ ions at a low value of D = 6.24⋅1013 ion/cm2, a superposition of diffraction from c-Si substrate (in the form of slightly blurred Kikuchi bands) and a partly amorphized near-surface implanted layer of α-Si (in the form of duplex rings) are presented in the EBSD image (Fig. 1 b). Surface semiconductor layer containing collections αSi fragments formed as a result of the introduction of individual Ag ions as it was also evidenced by ellipsometric measurements and modeling of similar samples [25]. In the case of ion implantation with value higher than D = 1.3⋅1014 ion/cm2, in the EBSD image (Fig. 1 c) only diffuse α-Si rings are presented, which corresponds to the completely amorphized Si surface implanted layer. Thus from EBSD measurements it is possible to conclude about a gradual partial amorphization of the near-surface region of Si with an increase of the implantation dose. Moreover when D reaches a value of the order of 1.3⋅1014 ion/cm2, a complete amorphization of the implanted Si layer with a thickness of ∼40 nm occurs [22]. By ellipsometry study [25], it was shown that for D = 6.24 × 1013 ion/cm2, the filling of a-Si in the implanted layer is ∼90%, and for D = 1.3⋅1014 ion/cm2 - 100%. It should be noted that the morphology of the Si surface for samples implanted at D from 6.24⋅1013 to 1.3⋅1014 ion/cm2 and J = 2 μA/cm2 observed by the SEM images remains unchanged, smooth, similar to virgin c-Si substrate, as this is shown in the example for the sample implanted at rather low D = 1.25⋅1015 ion/cm2 (Fig. 2a).
Fig. 1. EBSD image for a virgin c-Si (a) and Si implanted by Ag+ ions at different D: (b) 6.24⋅1013 and (c) 1.3⋅1014 ion/cm.2.
As it was mentioned in the introduction, a principal possibility for synthesis of Ag nanoparticles by low-energy high-dose (1017 ion/cm2) implantation in Si was demonstrated earlier in the works [22]. To determine the threshold D value at which the nucleation of metal nanoparticles occurs during Ag+-ion implantation into Si, series of SEM observations for samples prepared for different values of D from 1.25⋅1015 to 1.5⋅1017 ion/cm2 were performed at constant conditions: E = 30 keV and J = 8 μA/cm2. Fig. 2 a shows that Ag+-ion implantation of Si with the lowest D = 1.25⋅1015 ion/cm2 does not effect on surface morphology of substrate. In the EDX spectrum of this sample additionally to the signal from the Si substrate some characteristic peaks located between 2.5 and
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Fig. 2. SEM image of Si implanted by Ag+ at constant E = 30 keV and J = 8 μA/cm2 with different D: (a) 1.25⋅1015, (b) 3.1⋅1015, (c) 2.5⋅1016, (d) 5.0⋅1016 ion/cm2.
the size of the nanoparticles (3 and 5 nm) and their density (39⋅109 and 60⋅109 N/cm2) increase, and the average distance between them decreases until 32 and 28 nm. Thus, it is obvious that as the ion D increases, the Ag ions entering Si in a larger quantity diffuse to the already formed small Ag (nuclei) nanoparticles, leading to an increase in Ag particle sizes and an increase in their surface density. In Fig. 3 SEM images of the Si surface formed by ion implantation at different J = 2, 8 and 15 μA/cm2 for a fixed values D = 1.5 ⋅ 1017 ion/ cm2 and E = 30 keV are shown. Bright spots on a dark background of the Si substrate, as described above, demonstrate ion-synthesized Ag nanoparticles. These Ag nanoparticles could be divided into two groups (bimodal distribution): smaller than 15 nm and larger ones. For each sample, the inserts in Fig. 3 demonstrate histograms of the nanoparticle size distribution. In the case of J = 8 μA/cm2 (Fig. 3 b), the smaller nanoparticles have an average diameter of ∼10 nm. A characteristic feature of the histograms for samples formed at J = 2 and 15 μA/cm2 (Fig. 3 a and c) is the presence of the dominant nanoparticles with average size of ∼7 nm, which are uniformly distributed over the area of surfaces. The concentration of nanoparticles of a given size per unit area is higher for a sample obtained at a higher value of J, while the size distribution (the width of the histogram at half its height) is narrower. For J = 2 μA/cm2, a relatively large Ag nanoparticles with average size ∼30 nm over the surface are also observed (Fig. 3 a). In the case of J = 8 μA/cm2 (Fig. 3 b), larger nanoparticles of ∼40 nm are distributed around circles with an average diameter of 250 nm. As it will be discussed later, circles on the surface of implanted Si are formed by open pores, along the perimeter of which larger nanoparticles are concentrated. For J = 15 μA/cm2, more larger nanoparticles of ∼60 nm are observed in the SEM image (Fig. 4 c) and their arrangement no longer forms closed circles, but to some extent resembles chains. As discussed and demonstrated experimentally earlier [27], Ag+-ion implantation with D > 1. 5⋅1017 ion/cm2 leads to effective sputtering of the irradiated Si surface and so the observed morphology of Ag:PSi samples does not change a lot. In order to evaluate the morphological changes of the Si surface during the formation of metal nanoparticles by high-dose implantation with Ag+ ions, additional AFM, SEM, and TEM studies were performed for a sample irradiated at D = 1.5⋅1017 ion/cm2 and J = 8 μA/cm2. The AFM images of this sample (Fig. 4 a, b) show that the irradiated surface is porous. The profile of the surface for Ag:PSi, measured along the
3.5 keV corresponding to Ag are also observed. According to the calculated data, the thickness of the Ag+-implanted Si layer is of the order of 40 nm [22], which is certainly less than the penetration depth of probed electrons (∼1 μm) during EDX measurements. Therefore, it could be concluded that all implanted Ag species were analyzed by EDX. The absence of visible morphological uncertainties on the surface of this sample after ion implantation of Si up to D = 1.25⋅1015 ion/cm2 such as metal nanoparticles, suggests that the Ag in the near-surface Si layer is in the atomic state. In the SEM image (Fig. 2 b) of implanted sample fabricated at higher value D = 3.1⋅1015 ion/cm2, in contrast to the smaller one (Fig. 2 a), bright round spots with an average diameter ∼2 nm, uniformly distributed on the gray background of the Si surface are detected. It should be note that in the case of SEM observations, the images are formed due to the collision of electrons with the surface of the analyzing material and their reflection with some energy losses in the collecting detector. Thus, the color tone on the SEM image is determined primarily by the density of the material surface. In same time it is also known that Ag does not form chemical compounds with Si. Therefore, it is obvious that the sample shown in Fig. 2 b demonstrates the presence in its nearsurface layer of two phases with different densities and corresponding color tonality, namely the gray background for less dense Si, and bright spots that should be referred to synthesized Ag nanoparticles. The absence of extraneous chemical elements in this sample is confirmed by EDX analysis. The formation of Ag nanoparticles at a given D value is explained by the accumulation of Ag atoms in amounts exceeding a solubility limit in Si > 1016 cm−3 similar as it were experimentally observed for metal-ion implantation of oxide semiconductors [26,27]. In the event of saturation, the Ag atoms, diffusing along the near-surface volume of Si, segregate into metal nanoparticles, in the same way as, for example, in supersaturated solutions. The average distance between the nanoparticles estimated by the SEM image (Fig. 2 b) is ∼37 nm, and their density over the surface of the sample is 25⋅109 N/ cm2, where N is the number of particles. Thus, value D = 3.1⋅1015 ion/ cm2 could be considered as the critical dose - threshold (at E = 30 keV and J = 8 μA/cm2) at which Ag nanoparticles resistant to accidental destruction are generated and formed in implanted Si matrix. For samples implanted at higher doses D = 2.5⋅1016 and D = 5⋅1016 ion/cm2, the SEM images in Fig. 2 c and d, respectively, also have light spots of Ag nanoparticles on a gray Si background. With increasing D, 355
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Fig. 4. AFM image of Si implanted by Ag+ at E = 30 keV, D = 1.5⋅1017 ion/ cm2 and J = 8 μA/cm2.
(∼7 nm) are uniformly distributed over all surface areas, which is an agreement with the SEM images obtained on a microscope in the geometry “plan-view” in Fig. 3. Fig. 5 b shows the same region of the sample as in Fig. 5 a, but registered in the detection mode of backscattered electrons using an energy filter. Such technique allows to remove for constructing SEM image low-energy electrons backscattered from the Si phase of the Ag:PSi sample. In this case a detection of high-energy electrons from heavy Ag atoms on the SEM detector is only realized. As was shown by EDX analysis, two chemical elements (Ag and Si) are present in this sample, and so the image in Fig. 5 b confirms the formation of Ag nanoparticles on the surface. It should be noted that, because of the lower resolution in Fig. 5 b, in comparison with the detector of secondaryelectrons (Fig. 5 a), mainly largest Ag nanoparticles are clearly visible by selected SEM detection. According to the plan of the future study, it is assuming to realise new experiments on the mechanical properties, in particular, to determine the nanohardness and elasticity modulus of the formed composite materials based on Si implanted with Ag+ ions, using atom-force microscopy and the methods suggested in the works [28–30].
Fig. 3. SEM image of Si implanted by Ag+ at constant E = 30 keV and D = 1.5⋅1017 ion/cm2 with different J: (a) 2, (b) 8 and (c) 15 μA/cm2.
direction of the segment, as marked in Fig. 4 b is shown in Fig. 4 c. According to quantitative measurements, the average depth of the open pore considered is ∼45 nm, their diameter is ∼250 nm and a porous wall thickness is 60–80 nm. The SEM image of the transverse crack at the edge of specially broken sample (formed at D = 1.5⋅1017 ion/cm2 and J = 8 μA/cm2) in the registration mode of secondary electron detection is presented in Fig. 5 a. The dark area on the lower part of the SEM image of the sample corresponds to the virgin c-Si substrate. The upper part of the figure shows the surface of the Ag:PSi viewed at an angle. It is possible to recognize some morphological features on the sample surface consisting on open pores walls fabricated during ion implantation. Additionally to that, larger Ag nanoparticles with size of ∼40 nm are located along the perimeter walls of the pores. At the same time, small nanoparticles
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structures, which was demonstrated with the example of an Ag:PSi composite material formed by ion implantation. Acknowledgments This work was supported by the Russian Science Foundation, project No. 17-12-01176. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.vacuum.2018.10.060. References [1] Z. Huang, N. Ceyer, P. Werner, J. de Boor, U. Gösele, Adv. Mater. 23 (2011) 285–308. [2] V.V. Kozlovskii, V.A. Kozlov, V.N. Lomasov, Semiconductors 34 (2000) 123–140 2000. [3] V. Torres-Costa, R.J. Martín-Palma, J. Mater. Sci. 45 (2010) 2823–2838. [4] Y.-C. Tsao, T. Sondergaard, K. Kristensen, R. Rizzoli, K. Pedersen, T.G. Pedersen, Appl. Phys. A 120 (2015) 417–425. [5] V.S.-Y. Lin, K. Motesharei, K.-P.S. Dancil, M.J. Sailor, M.R. Ghadiri, Science 278 (1997) 840–843. [6] G.F. Cerofolini, L. Meda, R. Balboni, F. Corni, S. Frabboni, G. Ottaviani, R. Tonini, M. Anderle, R. Canteri, Phys. Rev. B 46 (1992) 2061–2070. [7] H.J. Stein, S.M. Myers, D.M. Follstaedt, J. Appl. Phys. 73 (1993) 2755–2764. [8] M. Wittmer, J. Roth, P. Revesz, J.M. Mayer, J. Appl. Phys. 49 (1978) 5207–5212. [9] P. Revesz, M. Wittmer, J. Roth, J.M. Mayer, J. Appl. Phys. 49 (1978) 5199–5206. [10] M.F. Galyautdinov, N.V. Kurbatova, E.Y. Buinova, E.I. Shtyrkov, A.A. Bukharaev, Semiconductors 31 (1997) 970–973. [11] P.K. Sadovskii, A.R. Chelyadinskii, V.B. Odzhaev, M.I. Tarasik, A.S. Turtsevich, Y.B. Vasiliev, Phys. Solid State 55 (2013) 1156–1158. [12] T.S. Amran, M.R. Hashim, N.K. Al-Obaidi, H. Yazid, R. Adnan, Nanoscale Res. Lett. 8 (2013) 35-1 – 35-6. [13] M. Wang, X. Wang, S. Ghoshal, Micro & Nano Lett. 8 (2013) 465–469. [14] Y. Wang, Y.P. Liu, H.L. Liang, Z.X. Mei, X.L. Du, Phys. Chem. Chem. Phys. 12 (2013) 2345–2350. [15] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin, 1995. [16] R.A. Ganeev, A.I. Ryasnyansky, A.L. Stepanov, T. Usmanov, Phys. Stat. Sol. A 238 (2003) R5–R7. [17] A. Polman, Science 322 (2008) 868–869. [18] H.A. Atwater, A. Polman, Nat. Mater. 9 (2010) 205–213. [19] M.W. Knight, H. Sobhani, P. Nordlander, N.J. Halas, Science 332 (2011) 702–704. [20] Y.G. Galyametdinov, R.R. Shamilov, A.L. Stepanov, Rus. Chem. Bullet. Intern. Edit. 65 (2016) 2773–2775. [21] C. Novara, S.D. Marta, A. Virga, A. Lamberti, A. Angelini, A. Chiado, F. Geobaldo, V. Sergo, A. Bonifacio, F. Giorgis, J. Phys. Chem. C 120 (2016) 16946–16953. [22] A.L. Stepanov, A.A. Trifonov, Y.N. Osin, V.F. Valeev, V.I. Nuzhdin, Optoelectr. Adv. Mater. Rapid Comm. 7 (2013) 692–697. [23] R.I. Batalov, V.I. Nuzhdin, V.F. Valeev, V.V. Vorobev, Y.N. Osin, G.D. Ivlev, A.L. Stepanov, J. Phys. D Appl. Phys. 52 (2018) 015109-1 - 015109-5. [24] A.L. Stepanov, V.I. Nuzhdin, V.F. Valeev, A.M. Rogov, V.V. Vorobev, Y.N. Osin, Vacuum 152 (2018) 200–204. [25] V.V. Bazarov, V.I. Nuzhdin, V.F. Valeev, A.L. Stepanov, Vacuum 148 (2018) 254–257. [26] A. Cetin, R. Kibar, M. Ayvaciki, N. Can, C. Buchal, P.D. Townsend, A.L. Stepanov, T. Karali, S. Selvei, Nucl. Instrum. Methods Phys. Res. B 249 (2006) 474–477. [27] A.L. Stepanov, Ion Implantation Synthesis and Optics of Metal Nanoparticles, Lambert. Acad. Publ., Mauritius, 2018. [28] A. Shypylenko, A.V. Pshyk, B. Grzekowiak, K. Medjanik, B. Peplinska, K. Oyoshi, A. Pogrebnjak, S. Jurga, E. Coy, Mater. Des. 110 (2016) 821–829. [29] A.L. Stepanov, Rev. Adv. Mater. Sci. 30 (2012) 150–165. [30] A.D. Pogrebnjak, I.V. Yakushchenko, O.V. Bondar, V.M. Beresnev, K. Oyoshi, O.M. Ivasishin, H. Amekura, Y. Takeda, M. Opielek, C. Kozak, J. Alloy. Comp. 679 (2016) 155–163.
Fig. 5. SEM cross-section images of Ag:PSi formed by implantation with Ag+ ions at E = 30 keV, D = 1.5⋅1017 ion/cm2 and J = 8 μA/cm2 in the detection modes such as of secondary electrons (a) and backscattered electrons using the energy filter (b).
4. Conclusions Low-energy high-dose implantation of c-Si by Ag+ ions at E = 30 keV at D from 1.25⋅1015 to 1.5⋅1017 ion/cm2 and J of 2, 8, 15 μA/cm2 is studied. It was discussed that at a value of D = 1.3⋅1014 ion/cm2, the near-surface region of Si is completely amorphized. Starting with D = 3.1⋅1015 ion/cm2, Ag nanoparticles are nucleated in the α-Si layer. With further growth of D an increase in the amount of nanoparticles is observed. At D = 1.5⋅1017 ion/cm2, the formation of open pores with a depth of ∼45 nm, a diameter of ∼250 nm and a porous wall thickness of 60–80 nm, is observed on the surface of implanted Si. In this case, the size distribution of Ag nanoparticles becomes bimodal, consisting of two fractions: small nanoparticles (5–10 nm), located uniformly throughout the sample surface area and large ones (40–50 nm) selected on the pore walls. Thus, the combined use of complementary methods of microscopy (SEM, AFM) and their variations allows detailed characterization of complex surface
357
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Short communication
Study of silicon surface implanted by silver ions a,∗
a
a
A.L. Stepanov , V.I. Nuzhdin , V.F. Valeev , V.V. Vorobev a b
a,b
, A.M. Rogov
a,b
a,b
, Y.N. Osin
T
Kazan Physical-Technical Institute, Russian Academy of Sciences, 420029, Kazan, Russia Kazan Federal University, 420008, Kazan, Russia
ARTICLE INFO
ABSTRACT
Keywords: Porous silicon Ion implantation Silver nanoparticles
Ag+-ion implantation of single-crystal c-Si at low-energy (E = 30 keV) high-doses (D = 1.25⋅1015–1.5⋅1017 ion/ cm2) and current density (J = 2, 8, 15 μA/cm2) was carried out. The changes of Si surface morphology after ion implantation were studied by scanning electron and atomic force microscopy. The near surface area of samples was also analyzed by diffraction of the backscattered electrons and energy-dispersive X-ray microanalysis. At the lowest implantation doses of c-Si amorphization of near-surface layer was observed. Ag nanoparticles were synthesized and uniformly distributed over the near Si surface when the threshold dose of 3.1⋅1015 ion/cm2 is exceeded. At a dose of more than 1017 ion/cm2, the formation of a surface porous Si structure was detected. Ag nanoparticle size distribution function becomes bimodal and the largest particles were localized along Si-pore walls.
1. Introduction The search for new composite materials and effective technologies for the creation on their basis various semiconductor devices, including Si nanostructures, for their applications in modern industrial microelectronics are relevant at the present time. For such purpose in some technical cases porous Si (PSi) is used. Generally, PSi is mainly obtained by electrochemical etching of single-crystal c-Si in hydrofluoric acid solutions [1]. On the other side, in addition to the chemical approach, another physical technology for the formation of porous semiconductor layers is also tested applying implantation of Si by various types of ions in vacuum [2]. PSi is of interest for use in the field of micro-, nano- and optoelectronics [3], in particular, for the creation of solar cells [4], biosensorics [5] and so on. At present the fabrication process of PSi by high-dose implantation of Si using inert gas ions is quite well studied. The solubility of inert gas atoms in semiconductors is relatively small. Therefore, starting with some threshold doses of implantation (concentration of gas ions), the formation of nanoscale pores inside of solid is explained due to the filling of a local volume of material with gettered molecules or bubbles of implanted gas ions [2]. The formation of pores in Si was early observed during implantation by such ions as H+ [6], He+ [7], Ne+ [8], Ar+ [9] and Kr+ [8,10]. A distinctive feature of the work [10] is that when using low-energy Kr+-ion implantation, the open pores were formed on irradiated Si surface, but not inside of the Si volume only. The authors of the work [11] demonstrated an interesting method for
∗
manufacturing a porous layer in Si by implantation with Sb+ semimetal ions followed by a two-stage thermal annealing, resulting in the sequential synthesis of Sb nanoparticles, following with those nanoparticle melting. As a consequence, a free volume-pores in Si instead of Sb nanoparticles was created. In addition, in practice, different techniques for the creation of composite materials based on PSi, containing noble metal nanoparticles, for example, Au [12], Pt [13] and Ag [14], are actively searching and testing. The task of this activity is to increase the efficiency of the optical properties of PSi. Collective excitation of conduction electrons in metal nanoparticles during interaction with an electromagnetic light wave causes resonant amplification of the local field (surface plasmon resonance), which leads to stimulation of optical and nonlinear optical effects in composite materials with nanoparticles [15,16]. In particular, the presence of Ag nanoparticles in the Si matrix could contributed to an increase in the efficiency of the functioning of solar cells, due to a change in their absorptivity at plasmon resonance frequencies of nanoparticles [17,18], as well as electron photogeneration arising in nanoparticles and diffusing through the Schottky barrier into Si at the metal/semiconductor interface [19]. In addition, the plasmon field of the Ag nanoparticle deposited on PSi leads to an increase of the photoluminescence of semiconductor quantum dots [20] and to the enhancement of surface Raman scattering of organic compounds [21]. Earlier, it was suggested to use low-energy high-dose Ag+-ion implantation of c-Si to form PSi layer simultaneously with synthesis of Ag
Corresponding author. E-mail address: [email protected] (A.L. Stepanov).
https://doi.org/10.1016/j.vacuum.2018.10.060 Received 19 September 2018; Received in revised form 20 October 2018; Accepted 22 October 2018 Available online 25 October 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved.
Vacuum 159 (2019) 353–357
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nanoparticles inward (Ag:PSi) [22,23] similar as it was done with porous Ge [24]. The purpose of this work is to perform a detail study a Ag nanoparticle formation, the surface morphology and structure of Ag:PSi layers manufactured in a wide range of implantation doses and current densities using modern electronic scanning and atomic force microscopy methods. 2. Materials and methods Single-crystal c-Si wafers with thickness of 400 μm and a crystallographic orientation (100) were used as substrates. Implantation was carried out by Ag+ ions with an energy of E = 30 keV at an irradiation dose D in the range from 6.24⋅1013 to 1.5⋅1017 ion/cm2 and current densities J = 2, 8 and 15 μA/cm2 using the ion accelerator ILU-3 at room temperature of the irradiated substrates. The vacuum level in an ion accelerate chamber during ion implantation was 10−5 Torr. Observation of morphology of the sample surface and energy-dispersive X-ray (EDX) spectral microanalysis of implanted Si was carried out with with a high-resolution scanning electron microscopes (SEM) SU 8000 (Hitachi) and Merlin Zeiss equipped by Aztec X-Max spectrometer (Oxford Instruments). Structure of surface sample layer was characterized by electron backscattered diffraction (EBSD) patterns using HKL NordLys detector (Oxford Instruments). The surface topology of the samples was also analised with atomic-force microscope (AFM) Dimension FastScan (Bruker) in the Quantative Nanomechanical Mapping mode using Bruker ScanAsyst Air probes. 3. Results and discussion The calculation of Ag-ion depth distribution in Si after high-dose ion implantation taking into account surface spattering of the wafers predicted that the Ag impurity is distributed in the top layer of the sample with a thickness of about 40 nm, which was estimated in the work [22]. The results on a change in the crystal structure of Si implanted layer at the low implantation D from 6.24⋅1013 to 1.3⋅1014 ion/cm2 and a fixed values of E = 30 keV and J = 2 μA/cm2 are presented in Fig. 1. For the virgin c-Si wafer the distinct Kikuchi lines in the EBSD image are observed (Fig. 1 a). Automatic identification of this EBSD pattern with the Aztec 2.1 computer program shows the orientation (100) of the cubic syngony of the unit с-Si cell with the crystallographic parameters a = b = c = 5.43 Å and α = β = γ = 90̊. When the c-Si substrate was implanted with Ag+ ions at a low value of D = 6.24⋅1013 ion/cm2, a superposition of diffraction from c-Si substrate (in the form of slightly blurred Kikuchi bands) and a partly amorphized near-surface implanted layer of α-Si (in the form of duplex rings) are presented in the EBSD image (Fig. 1 b). Surface semiconductor layer containing collections αSi fragments formed as a result of the introduction of individual Ag ions as it was also evidenced by ellipsometric measurements and modeling of similar samples [25]. In the case of ion implantation with value higher than D = 1.3⋅1014 ion/cm2, in the EBSD image (Fig. 1 c) only diffuse α-Si rings are presented, which corresponds to the completely amorphized Si surface implanted layer. Thus from EBSD measurements it is possible to conclude about a gradual partial amorphization of the near-surface region of Si with an increase of the implantation dose. Moreover when D reaches a value of the order of 1.3⋅1014 ion/cm2, a complete amorphization of the implanted Si layer with a thickness of ∼40 nm occurs [22]. By ellipsometry study [25], it was shown that for D = 6.24 × 1013 ion/cm2, the filling of a-Si in the implanted layer is ∼90%, and for D = 1.3⋅1014 ion/cm2 - 100%. It should be noted that the morphology of the Si surface for samples implanted at D from 6.24⋅1013 to 1.3⋅1014 ion/cm2 and J = 2 μA/cm2 observed by the SEM images remains unchanged, smooth, similar to virgin c-Si substrate, as this is shown in the example for the sample implanted at rather low D = 1.25⋅1015 ion/cm2 (Fig. 2a).
Fig. 1. EBSD image for a virgin c-Si (a) and Si implanted by Ag+ ions at different D: (b) 6.24⋅1013 and (c) 1.3⋅1014 ion/cm.2.
As it was mentioned in the introduction, a principal possibility for synthesis of Ag nanoparticles by low-energy high-dose (1017 ion/cm2) implantation in Si was demonstrated earlier in the works [22]. To determine the threshold D value at which the nucleation of metal nanoparticles occurs during Ag+-ion implantation into Si, series of SEM observations for samples prepared for different values of D from 1.25⋅1015 to 1.5⋅1017 ion/cm2 were performed at constant conditions: E = 30 keV and J = 8 μA/cm2. Fig. 2 a shows that Ag+-ion implantation of Si with the lowest D = 1.25⋅1015 ion/cm2 does not effect on surface morphology of substrate. In the EDX spectrum of this sample additionally to the signal from the Si substrate some characteristic peaks located between 2.5 and
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Fig. 2. SEM image of Si implanted by Ag+ at constant E = 30 keV and J = 8 μA/cm2 with different D: (a) 1.25⋅1015, (b) 3.1⋅1015, (c) 2.5⋅1016, (d) 5.0⋅1016 ion/cm2.
the size of the nanoparticles (3 and 5 nm) and their density (39⋅109 and 60⋅109 N/cm2) increase, and the average distance between them decreases until 32 and 28 nm. Thus, it is obvious that as the ion D increases, the Ag ions entering Si in a larger quantity diffuse to the already formed small Ag (nuclei) nanoparticles, leading to an increase in Ag particle sizes and an increase in their surface density. In Fig. 3 SEM images of the Si surface formed by ion implantation at different J = 2, 8 and 15 μA/cm2 for a fixed values D = 1.5 ⋅ 1017 ion/ cm2 and E = 30 keV are shown. Bright spots on a dark background of the Si substrate, as described above, demonstrate ion-synthesized Ag nanoparticles. These Ag nanoparticles could be divided into two groups (bimodal distribution): smaller than 15 nm and larger ones. For each sample, the inserts in Fig. 3 demonstrate histograms of the nanoparticle size distribution. In the case of J = 8 μA/cm2 (Fig. 3 b), the smaller nanoparticles have an average diameter of ∼10 nm. A characteristic feature of the histograms for samples formed at J = 2 and 15 μA/cm2 (Fig. 3 a and c) is the presence of the dominant nanoparticles with average size of ∼7 nm, which are uniformly distributed over the area of surfaces. The concentration of nanoparticles of a given size per unit area is higher for a sample obtained at a higher value of J, while the size distribution (the width of the histogram at half its height) is narrower. For J = 2 μA/cm2, a relatively large Ag nanoparticles with average size ∼30 nm over the surface are also observed (Fig. 3 a). In the case of J = 8 μA/cm2 (Fig. 3 b), larger nanoparticles of ∼40 nm are distributed around circles with an average diameter of 250 nm. As it will be discussed later, circles on the surface of implanted Si are formed by open pores, along the perimeter of which larger nanoparticles are concentrated. For J = 15 μA/cm2, more larger nanoparticles of ∼60 nm are observed in the SEM image (Fig. 4 c) and their arrangement no longer forms closed circles, but to some extent resembles chains. As discussed and demonstrated experimentally earlier [27], Ag+-ion implantation with D > 1. 5⋅1017 ion/cm2 leads to effective sputtering of the irradiated Si surface and so the observed morphology of Ag:PSi samples does not change a lot. In order to evaluate the morphological changes of the Si surface during the formation of metal nanoparticles by high-dose implantation with Ag+ ions, additional AFM, SEM, and TEM studies were performed for a sample irradiated at D = 1.5⋅1017 ion/cm2 and J = 8 μA/cm2. The AFM images of this sample (Fig. 4 a, b) show that the irradiated surface is porous. The profile of the surface for Ag:PSi, measured along the
3.5 keV corresponding to Ag are also observed. According to the calculated data, the thickness of the Ag+-implanted Si layer is of the order of 40 nm [22], which is certainly less than the penetration depth of probed electrons (∼1 μm) during EDX measurements. Therefore, it could be concluded that all implanted Ag species were analyzed by EDX. The absence of visible morphological uncertainties on the surface of this sample after ion implantation of Si up to D = 1.25⋅1015 ion/cm2 such as metal nanoparticles, suggests that the Ag in the near-surface Si layer is in the atomic state. In the SEM image (Fig. 2 b) of implanted sample fabricated at higher value D = 3.1⋅1015 ion/cm2, in contrast to the smaller one (Fig. 2 a), bright round spots with an average diameter ∼2 nm, uniformly distributed on the gray background of the Si surface are detected. It should be note that in the case of SEM observations, the images are formed due to the collision of electrons with the surface of the analyzing material and their reflection with some energy losses in the collecting detector. Thus, the color tone on the SEM image is determined primarily by the density of the material surface. In same time it is also known that Ag does not form chemical compounds with Si. Therefore, it is obvious that the sample shown in Fig. 2 b demonstrates the presence in its nearsurface layer of two phases with different densities and corresponding color tonality, namely the gray background for less dense Si, and bright spots that should be referred to synthesized Ag nanoparticles. The absence of extraneous chemical elements in this sample is confirmed by EDX analysis. The formation of Ag nanoparticles at a given D value is explained by the accumulation of Ag atoms in amounts exceeding a solubility limit in Si > 1016 cm−3 similar as it were experimentally observed for metal-ion implantation of oxide semiconductors [26,27]. In the event of saturation, the Ag atoms, diffusing along the near-surface volume of Si, segregate into metal nanoparticles, in the same way as, for example, in supersaturated solutions. The average distance between the nanoparticles estimated by the SEM image (Fig. 2 b) is ∼37 nm, and their density over the surface of the sample is 25⋅109 N/ cm2, where N is the number of particles. Thus, value D = 3.1⋅1015 ion/ cm2 could be considered as the critical dose - threshold (at E = 30 keV and J = 8 μA/cm2) at which Ag nanoparticles resistant to accidental destruction are generated and formed in implanted Si matrix. For samples implanted at higher doses D = 2.5⋅1016 and D = 5⋅1016 ion/cm2, the SEM images in Fig. 2 c and d, respectively, also have light spots of Ag nanoparticles on a gray Si background. With increasing D, 355
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Fig. 4. AFM image of Si implanted by Ag+ at E = 30 keV, D = 1.5⋅1017 ion/ cm2 and J = 8 μA/cm2.
(∼7 nm) are uniformly distributed over all surface areas, which is an agreement with the SEM images obtained on a microscope in the geometry “plan-view” in Fig. 3. Fig. 5 b shows the same region of the sample as in Fig. 5 a, but registered in the detection mode of backscattered electrons using an energy filter. Such technique allows to remove for constructing SEM image low-energy electrons backscattered from the Si phase of the Ag:PSi sample. In this case a detection of high-energy electrons from heavy Ag atoms on the SEM detector is only realized. As was shown by EDX analysis, two chemical elements (Ag and Si) are present in this sample, and so the image in Fig. 5 b confirms the formation of Ag nanoparticles on the surface. It should be noted that, because of the lower resolution in Fig. 5 b, in comparison with the detector of secondaryelectrons (Fig. 5 a), mainly largest Ag nanoparticles are clearly visible by selected SEM detection. According to the plan of the future study, it is assuming to realise new experiments on the mechanical properties, in particular, to determine the nanohardness and elasticity modulus of the formed composite materials based on Si implanted with Ag+ ions, using atom-force microscopy and the methods suggested in the works [28–30].
Fig. 3. SEM image of Si implanted by Ag+ at constant E = 30 keV and D = 1.5⋅1017 ion/cm2 with different J: (a) 2, (b) 8 and (c) 15 μA/cm2.
direction of the segment, as marked in Fig. 4 b is shown in Fig. 4 c. According to quantitative measurements, the average depth of the open pore considered is ∼45 nm, their diameter is ∼250 nm and a porous wall thickness is 60–80 nm. The SEM image of the transverse crack at the edge of specially broken sample (formed at D = 1.5⋅1017 ion/cm2 and J = 8 μA/cm2) in the registration mode of secondary electron detection is presented in Fig. 5 a. The dark area on the lower part of the SEM image of the sample corresponds to the virgin c-Si substrate. The upper part of the figure shows the surface of the Ag:PSi viewed at an angle. It is possible to recognize some morphological features on the sample surface consisting on open pores walls fabricated during ion implantation. Additionally to that, larger Ag nanoparticles with size of ∼40 nm are located along the perimeter walls of the pores. At the same time, small nanoparticles
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structures, which was demonstrated with the example of an Ag:PSi composite material formed by ion implantation. Acknowledgments This work was supported by the Russian Science Foundation, project No. 17-12-01176. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.vacuum.2018.10.060. References [1] Z. Huang, N. Ceyer, P. Werner, J. de Boor, U. Gösele, Adv. Mater. 23 (2011) 285–308. [2] V.V. Kozlovskii, V.A. Kozlov, V.N. Lomasov, Semiconductors 34 (2000) 123–140 2000. [3] V. Torres-Costa, R.J. Martín-Palma, J. Mater. Sci. 45 (2010) 2823–2838. [4] Y.-C. Tsao, T. Sondergaard, K. Kristensen, R. Rizzoli, K. Pedersen, T.G. Pedersen, Appl. Phys. A 120 (2015) 417–425. [5] V.S.-Y. Lin, K. Motesharei, K.-P.S. Dancil, M.J. Sailor, M.R. Ghadiri, Science 278 (1997) 840–843. [6] G.F. Cerofolini, L. Meda, R. Balboni, F. Corni, S. Frabboni, G. Ottaviani, R. Tonini, M. Anderle, R. Canteri, Phys. Rev. B 46 (1992) 2061–2070. [7] H.J. Stein, S.M. Myers, D.M. Follstaedt, J. Appl. Phys. 73 (1993) 2755–2764. [8] M. Wittmer, J. Roth, P. Revesz, J.M. Mayer, J. Appl. Phys. 49 (1978) 5207–5212. [9] P. Revesz, M. Wittmer, J. Roth, J.M. Mayer, J. Appl. Phys. 49 (1978) 5199–5206. [10] M.F. Galyautdinov, N.V. Kurbatova, E.Y. Buinova, E.I. Shtyrkov, A.A. Bukharaev, Semiconductors 31 (1997) 970–973. [11] P.K. Sadovskii, A.R. Chelyadinskii, V.B. Odzhaev, M.I. Tarasik, A.S. Turtsevich, Y.B. Vasiliev, Phys. Solid State 55 (2013) 1156–1158. [12] T.S. Amran, M.R. Hashim, N.K. Al-Obaidi, H. Yazid, R. Adnan, Nanoscale Res. Lett. 8 (2013) 35-1 – 35-6. [13] M. Wang, X. Wang, S. Ghoshal, Micro & Nano Lett. 8 (2013) 465–469. [14] Y. Wang, Y.P. Liu, H.L. Liang, Z.X. Mei, X.L. Du, Phys. Chem. Chem. Phys. 12 (2013) 2345–2350. [15] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin, 1995. [16] R.A. Ganeev, A.I. Ryasnyansky, A.L. Stepanov, T. Usmanov, Phys. Stat. Sol. A 238 (2003) R5–R7. [17] A. Polman, Science 322 (2008) 868–869. [18] H.A. Atwater, A. Polman, Nat. Mater. 9 (2010) 205–213. [19] M.W. Knight, H. Sobhani, P. Nordlander, N.J. Halas, Science 332 (2011) 702–704. [20] Y.G. Galyametdinov, R.R. Shamilov, A.L. Stepanov, Rus. Chem. Bullet. Intern. Edit. 65 (2016) 2773–2775. [21] C. Novara, S.D. Marta, A. Virga, A. Lamberti, A. Angelini, A. Chiado, F. Geobaldo, V. Sergo, A. Bonifacio, F. Giorgis, J. Phys. Chem. C 120 (2016) 16946–16953. [22] A.L. Stepanov, A.A. Trifonov, Y.N. Osin, V.F. Valeev, V.I. Nuzhdin, Optoelectr. Adv. Mater. Rapid Comm. 7 (2013) 692–697. [23] R.I. Batalov, V.I. Nuzhdin, V.F. Valeev, V.V. Vorobev, Y.N. Osin, G.D. Ivlev, A.L. Stepanov, J. Phys. D Appl. Phys. 52 (2018) 015109-1 - 015109-5. [24] A.L. Stepanov, V.I. Nuzhdin, V.F. Valeev, A.M. Rogov, V.V. Vorobev, Y.N. Osin, Vacuum 152 (2018) 200–204. [25] V.V. Bazarov, V.I. Nuzhdin, V.F. Valeev, A.L. Stepanov, Vacuum 148 (2018) 254–257. [26] A. Cetin, R. Kibar, M. Ayvaciki, N. Can, C. Buchal, P.D. Townsend, A.L. Stepanov, T. Karali, S. Selvei, Nucl. Instrum. Methods Phys. Res. B 249 (2006) 474–477. [27] A.L. Stepanov, Ion Implantation Synthesis and Optics of Metal Nanoparticles, Lambert. Acad. Publ., Mauritius, 2018. [28] A. Shypylenko, A.V. Pshyk, B. Grzekowiak, K. Medjanik, B. Peplinska, K. Oyoshi, A. Pogrebnjak, S. Jurga, E. Coy, Mater. Des. 110 (2016) 821–829. [29] A.L. Stepanov, Rev. Adv. Mater. Sci. 30 (2012) 150–165. [30] A.D. Pogrebnjak, I.V. Yakushchenko, O.V. Bondar, V.M. Beresnev, K. Oyoshi, O.M. Ivasishin, H. Amekura, Y. Takeda, M. Opielek, C. Kozak, J. Alloy. Comp. 679 (2016) 155–163.
Fig. 5. SEM cross-section images of Ag:PSi formed by implantation with Ag+ ions at E = 30 keV, D = 1.5⋅1017 ion/cm2 and J = 8 μA/cm2 in the detection modes such as of secondary electrons (a) and backscattered electrons using the energy filter (b).
4. Conclusions Low-energy high-dose implantation of c-Si by Ag+ ions at E = 30 keV at D from 1.25⋅1015 to 1.5⋅1017 ion/cm2 and J of 2, 8, 15 μA/cm2 is studied. It was discussed that at a value of D = 1.3⋅1014 ion/cm2, the near-surface region of Si is completely amorphized. Starting with D = 3.1⋅1015 ion/cm2, Ag nanoparticles are nucleated in the α-Si layer. With further growth of D an increase in the amount of nanoparticles is observed. At D = 1.5⋅1017 ion/cm2, the formation of open pores with a depth of ∼45 nm, a diameter of ∼250 nm and a porous wall thickness of 60–80 nm, is observed on the surface of implanted Si. In this case, the size distribution of Ag nanoparticles becomes bimodal, consisting of two fractions: small nanoparticles (5–10 nm), located uniformly throughout the sample surface area and large ones (40–50 nm) selected on the pore walls. Thus, the combined use of complementary methods of microscopy (SEM, AFM) and their variations allows detailed characterization of complex surface
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