Silver Structures at the Percolation Threshold, Prepared by Laser Annealing

ISSN 1063-7834, Physics of the Solid State, 2017, Vol. 59, No. 3, pp. 601 606. Pleiades Publishing, Ltd., 2017. Original Russian Text I.A. Gladskikh, V.A. Polishchuk, T.A. Vartanyan, 2017, published in Fizika Tverdogo Tela, 2017, Vol. 59, No. 3, pp. 582 587. LOW-DIMENSIONAL SYSTEMS Silver Structures at the Percolation Threshold, Prepared by Laser Annealing I. A. Gladskikh*, V. A. Polishchuk, and T. A. Vartanyan St. Petersburg National Research University of Information Technologies, Mechanics and Optics, Kronverkskii pr. 49, St. Petersburg, 197101 Russia *e-mail: 138020@mail.ru Received June 22, 2016; in final form, July 27, 2016 Abstract The electrical, optical, and structural properties of silver nanostructures at the percolation threshold, which were prepared from a conductive film by laser treatment, have been investigated experimentally. It has been found that the threshold voltage applied to the silver film leads to an abrupt change in its electrical resistance. At high voltages, there is a region with a negative differential resistance. These changes in the electrical conductivity under the influence of the applied voltage have been explained by small structural changes in the film. DOI: 10.1134/S106378341703012X 1. INTRODUCTION Electrical properties of metallic films have been studied for quite a long time and are of undoubted interest for the applications in microelectronics. In particular, the electrical conductivity of films consisting of individual nanoparticles is very small and depends on the distance between the particles and on the substrate material. Upon transition to percolation structures, the electrical conductivity abruptly increases and becomes close to the conductivity of the bulk metal. The electrical properties of metallic structures at the percolation threshold are in the intermediate region, and the electrical conductivity of these structures can be controlled by means of the switching between the high-resistance and conductive states with the use of the applied voltage. Such controllable changes in the electrical conductivity are found in many materials and actively investigated [1 4] for the use as memory devices. Nanoparticles of noble metals also exhibit unique optical properties associated with the excitation of localized surface plasmons. The resonant electromagnetic radiation is significantly enhanced and localized in the vicinity of nanoparticles, which has been widely used to enhance the optical response from the extremely small amount of a substance (single-molecule surface-enhanced Raman spectroscopy (SERS)) [5, 6]. In metallic films near the percolation threshold, which possess plasmonic properties, the enhancement of the incident field can reach 10 6 [7, 8]. The preparation of metallic structures at the percolation threshold through the deposition of metal vapors onto a substrate in a vacuum is impossible for several reasons. First, the electrical resistance of the film during the deposition gradually decreases, and the percolation transition in this case is almost indistinguishable [9]. Second, the prepared structures are nonequilibrium and continue to change after the completion of the deposition, which leads to large difficult-to-control changes in the electrical conductivity of the film [10]. Previously, we proposed a method for the preparation of structures at the percolation threshold by heat treatment of the films deposited to a thickness at which they become conductive. Heating of conductive films accelerates the diffusion of atoms forming the surface nanostructure and leads to the formation of individual particles and the breaking of bridges connecting them [11 13]. In this case, the dependence of the electrical resistance of the film on the annealing time exhibits a distinct transition of the film from the conductive structure to the high-resistance structure in the form of a sharp decrease in the electrical conductivity of the film. Among the disadvantages of this method for producing metallic films at the percolation threshold is precisely the sharpness of this transition, because, in view of the thermal inertia, the heating is impossible to quickly stop. A significantly higher controllability is provided by a new method proposed in this study for the preparation of metallic films at the percolation threshold. As is known, under the action of pulsed laser radiation with an energy density of a few tens of millijoules per square centimeter, silver nanoparticles are heated so much that change their shape [14]. The main advantage of laser annealing over thermal annealing consists 601

602 GLADSKIKH et al. in the fact that individual laser pulses produce relatively small changes in the film, and the percolation transition can be determined more accurately. An important difference of the laser annealing from the thermal annealing is in the selective effect exerted by the laser radiation on the particles, for which the plasmon absorption band is close to the laser radiation wavelength. At a sufficiently high radiation energy, the particles change their shape, which results in a shift of the plasmon resonance. Therefore, with this method, we can accurately adjust the optical properties of the film by varying the radiation wavelength, which is impossible during thermal annealing, when all the particles in the film are heated. Among the other advantages of laser annealing is also the possibility to stop the annealing almost instantly, because, in contrast to thermal annealing, the laser annealing leads to a local heating of the film, as well as the possibility to irradiate the desired part of the film. The second is particularly important taking into account that the electrodes can have a significant influence on the morphology of the film. [15] 2. SAMPLE PREPARATION, EXPERIMENTAL TECHNIQUE, AND RESULTS Silver adhesive electrodes were applied to the surface of the quartz substrate. The distance between the electrodes was 12 mm, and the electrode width was 4 mm. A silver film with a thickness of 100 Å was evaporated in gap between the electrodes at room temperature of the substrate in a PVD 75 vacuum system (Kurt J. Lesker Company) by physical vapor deposition of silver from the gas phase in vacuum. The deposition rate was controlled with a quartz microbalance and amounted to 0.8 Å/s. As was shown in previous studies, these deposition conditions provided the formation of conductive films with an electrical resistance from a few tens of ohms to a few kilohms. The micrograph of the prepared film is shown in Fig. 1. It can be seen from this figure that nanoparticles are interconnected into a network and form a single infinite cluster. The greater part of the substrate surface is filled with silver with the coverage factor f = 0.85. The electrical resistance of the film was 100 Ω. This means that there is a continuous metallic path between the electrodes. At the same time, the electrical resistance of the film is significantly higher than the resistance of bulk silver due to scattering of electrons by inhomogeneities of the film. For the preparation of the structure at the percolation threshold, the film was irradiated with a pulsed Nd : YAG laser at wavelengths of 1064 nm (12.1 mj/cm 2 ) and 532 nm (7.1 mj/cm 2 ). The duration of a single pulse was 10 ns, and the pulse repetition rate was 10 Hz. The irradiation was performed far from the electrodes. The area of the irradiated region of the film was 16 mm 2. 200 nm Fig. 1. Scanning electron microscopy image of a 100-Åthick silver film on the surface of the quartz substrate. The dependences of the electrical resistance at a voltage of 0.1 V on the irradiation time are shown in Fig. 2. The change in the electrical resistance upon laser annealing is similar to the change in the resistance upon thermal annealing [12]. It can be seen from Fig. 2 that the dependences have two sections. The first section corresponds to a sufficiently smooth increase in the electrical resistance from ~100 Ω to ~10 5 Ω for 370 and 540 s under the irradiation by the first harmonic and the second harmonic, respectively. The increase in the electrical resistance in this section is caused by the thinning of narrow parts of nanoparticles and by the decrease in their concentration. Then, there is a sharp increase in the electrical resistance by six to eight orders of magnitude the transition of the film from the conductive state to the highresistance due to the breaking of all the bridges; i.e., the structure of the film transforms from the infinite cluster into individual nanoparticles. If the irradiation is stopped almost immediately after this transition, the distance between the particles will be very small. After the irradiation, we measured the current voltage characteristics of the film (Fig. 3). Regardless of the radiation wavelength at which the laser annealing was performed, the current voltage characteristics behave almost identically. As the voltage increases from 0 to 60 V, the film is in the high-resistance state. At voltages in the range from 60 to 120 V, there is a sharp drop in the electrical resistance by six orders of magnitude. A further increase in the voltage to 200 V leads to a slight drop in the electrical resistance. As the voltage decreases, the electrical resistance remains almost unchanged down to 20 V. After the removal of the voltage, the film turns back into the high-resistance state. This dependence was repeated several times with small changes in the threshold voltage required to transform the film into the conductive state. Similar dependences of the current voltage characteristics of the samples were obtained after the heat

SILVER STRUCTURES AT THE PERCOLATION THRESHOLD 603 10 11 (a) 10 11 (b) 10 9 10 9 R, Ω 10 7 10 5 10 3 R, Ω 10 7 10 5 10 3 10 10 0 100 200 300 400 0 100 200 300 400 500 600 700 Time, s Time, s Fig. 2. Dependences of the electrical resistance of the silver film on the surface of the quartz substrate on the time of laser irradiation at wavelengths λ = (a) 1064 and (b) 532 nm. 10 4 (a) 10 4 (b) 10 6 10 8 10 10 10 12 10 6 10 8 10 10 10 12 50 100 150 200 50 100 150 200 10 3 (c) 10 5 10 7 10 9 10 11 10 13 0 50 100 150 200 Fig. 3. Current voltage characteristics of the silver films (a, b) after the laser irradiation at wavelengths λ = (a) 1064 and (b) 532 nm and (c) after thermal annealing. treatment (Fig. 3c). The character of the dependence is nearly identical to that for the samples prepared under the laser irradiation. The transition to the conductive state was sharper and occurred when the voltage of 60 V was applied to the sample. The electrical resistance in the conductive state was decreased by approximately one order of magnitude. It should be noted that, at an applied voltage of more than 200 V, the current voltage characteristic had a different character. Figure 4 shows the current voltage characteristics on a linear scale for the silver film after the laser irradiation at a wavelength of 532 nm, which were measured in the voltage range of 0 500 V. In the first run at voltages of more than 200 V, the increase in the current as a function of the

604 GLADSKIKH et al. 0.00030 0.00025 0.00020 0.00015 0.00010 0.00005 0 1st cycle 2nd cycle 3rd cycle 100 200 300 400 500 Fig. 4. Series of current voltage characteristics of the silver films after the laser irradiation at a wavelength λ = 532 nm. voltage significantly slows down, and when the voltage approaches 500 V, the current decreases with increasing voltage; i.e., there is a region with a negative differential resistance. With a decrease in the voltage, the electrical resistance corresponds to the resistance at the maximum applied voltage. In subsequent measurements, the threshold voltage required to switch the resistance increases from 75 to 150 175 V. Thus, the electrical properties of the films at the percolation threshold, which were prepared by the laser annealing, are similar to those of the previously studied films, which were produced by the heat treatment. The current voltage characteristics are also slightly different for the film regions irradiated at wavelengths of 532 and 1064 nm. However, the spectral properties of the films are radically different. The optical density spectra of the silver film after the laser annealing and heat treatment are shown in Fig. 5a. The extinction spectrum of the film before the irradiation is inhomogeneously broadened because of its complex structure. Upon heat treatment, there is a short-wavelength shift of the absorption maximum due to the change in the overall morphology of the film. As was already mentioned, the laser radiation has the selective effect on the particles, which causes permanent spectral hole burning in the optical density spectra. After the laser irradiation at a wavelength of 532 nm, the plasmon resonance is shifted toward the short-wavelength range. In this case, the spectrum is separated into two parts by a spectral dip at the radia- OD 0.7 0.6 532 nm 0.5 0.4 0.3 0.2 0.1 (a) After deposition After irradiation of 532 nm After irradiation of 1064 nm After thermal annealing 1064 nm 0 400 500 600 700 800 900 1000 1100 λ, nm (c) (b) 2 μm (d) 200 nm 200 nm Fig. 5. (a) Optical density spectra of the silver films after the deposition and after the laser and thermal treatments and (b d) scanning electron microscopy images of the silver film after the laser irradiation at wavelengths λ = (b, c) 532 and (d) 1064 nm.

SILVER STRUCTURES AT THE PERCOLATION THRESHOLD 605 tion wavelength. Figure 5b shows the scanning electron microscopy image, in which there are regions consisting of single large particle with a diameter of 100 200 nm and the regions consisting of a labyrinthine structure. The short-wavelength maximum at a wavelength of 465 nm can be attributed to the excitation of plasmon resonance in round particles. Then, the long-wavelength tail will be responsible for the absorption of the labyrinthine structure of the film (Fig. 5c). The laser radiation at a wavelength of 1064 nm resulted in the hole burning in the extinction spectrum of the film. With this variation, the overall changes were not as significant. In the scanning electron microscopy images of the film, there are also labyrinth structures (Fig. 5d) and individual particles (the image is not presented). 3. DISCUSSION OF THE RESULTS The laser annealing, like the thermal annealing, causes a change in the structure of the film due to the enhanced diffusion of atoms under the influence of temperature. The thermal annealing results in a change of the overall morphology of the film. A deep annealing of the films similar to those studied in this work inevitably leads to the formation of individual particles, the shape of which is close to spherical [16]. For these particles, the plasmon resonance is observed at approximately 450 nm. On the other hand, the breaking of the infinite cluster with an abrupt change in the electrical conductivity of the film takes place long before the formation of particles of regular shape [11]. Therefore, the change in the optical density spectrum of the films at the percolation threshold is insignificant compared to the as-deposited films (Fig. 5a), while their structure remains labyrinth-like. Thus, upon thermal annealing, it is actually impossible to obtain the structures at the percolation threshold that exhibit different optical properties. During the laser annealing, not the film as a whole is heated, but only particles with the plasmon resonance close to the laser radiation wavelength. This effect leads to a change in their shape and, consequently, to permanent spectral hole burning at the radiation wavelength. However, the preparation of the structures at the percolation threshold required a fairly prolonged treatment at a high pulse energy. This led to a significant change in some parts of the film with the formation of individual nanoparticles of regular shape, which had the plasmon resonance in the short-wavelength region of the spectrum. It is not difficult to assume that such a character of the electrical conductivity of the film with significant changes in the electrical resistance under the action of the voltage will be determined by regions with the labyrinth structure (Figs. 5c, 5d). The observed switchings of the resistance under the action of the voltage are attributed by us to small structural changes of the film. The sizes of metallic nanoparticles in the scanning electron microscopy images are greater than 100 nm. Taking into account the millimeter-scale irradiation region and the particle size, approximately 10 4 gaps are formed. Since the annealing was stopped immediately after the sharp increase in the electrical resistance of the film, the distance between the nanoparticles will be very small (~1 nm). Therefore, despite the relatively low voltage applied to the film and a large number of gaps, the fields in these places can exceed 10 3 V/cm, which, in turn, can lead to the deformation of particles (small elongation) and, consequently, to their interlocking. This process is avalanche-like, because the formation of one bridge between the nanoparticles leads to an increase in the voltage at the other gaps. In our case, the formation of bridges and new conduction paths covers a long voltage range from 60 to 120 V. After the removal of the voltage, the film can again turn into the high-resistance state through the diffusion of atoms from the formed small bridges to massive nanoparticles. The small differences in the current voltage characteristics obtained upon thermal and laser annealings can be associated with the formation of individual particles during the laser treatment. In these films, the amount of structural elements responsible for the electrical conductivity will be somewhat smaller. This can be the reason for the higher resistance of the film as compared to the sample prepared by heat treatment. The observed saturation in the current and negative differential resistance at high voltages can be explained by the heating of the bridges. Presumably, the electrical resistance of bridges is slightly higher than the resistance of massive nanoparticles. Therefore, the voltage drop will be observed predominantly on these bridges, and, hence, they will be more heated. In this case, the electrical resistance of the film with an increase in the voltage can be increased for two reasons. First, the resistance of the film can increase during the heating because of the scattering of electrons by lattice vibrations. Second, the same behavior can be associated with the enhanced diffusion of atoms during the heating of unannealed films, which leads to a thinning of the bridges. In the literature, the data on the coverage factor for the percolation threshold are significantly different from each other and lie in the range from 0.40 to 0.78 [1, 17, 18]. In our experiments, the coverage factor for the film with an equivalent thickness of 10 nm after the deposition was equal to 0.85, which is greater than the specified range. After the irradiation, the coverage factor of the surface was close to 0.6, or more precisely, f c = 0.61 and f c = 0.59 after the laser irradiation at wavelengths λ = 1064 and 532 nm, respectively, even

606 GLADSKIKH et al. though the irradiation by the first harmonic leads to the formation of larger-sized particles. 4. CONCLUSIONS In this work, we demonstrated the possibility of the preparation of silver nanostructures at the percolation threshold under the irradiation of a conductive film by high-power laser pulses. In contrast to the spectra of the films prepared by thermal annealing, the extinction spectra of the film after the laser treatment exhibit dips at the radiation wavelength. For the films produced by the laser annealing, the electrical conductivity at the percolation threshold was similar to that of the previously investigated films prepared by the thermal annealing. The current voltage characteristics of the film after the irradiation have a strong nonlinear dependence, which manifests itself in the form of abrupt changes in the electrical conductivity of the film under the influence of the applied voltage. At an applied voltage of less than 200 V, the resistance switching is quite stable in repeated measurements, and the threshold voltage amounts to 75 V. As the applied voltage increases above 200 V, the current reaches saturation. At a still higher voltage that approaches 500 V, there is a region with a negative differential resistance. The surface coverage factor for silver nanostructures at the percolation threshold is close to 0.6. Thus, the laser annealing has clear advantages over the thermal annealing for the preparation of plasmonic structures at the percolation threshold, which consist in the possibility of a fine tuning of the plasmon resonance frequency. At the same time, owing to their electrical properties, these structures can be used in the design and fabrication of memory devices based on the controlled change in the electrical conductivity or as current limiters. ACKNOWLEDGMENTS This study was supported by the Russian Foundation for Basic Research (project nos. 16-32-60028 mol_a_dk and 16-32-00165mol_a) and the Ministry of Education and Science of the Russian Federation (state task no. 2014/190). REFERENCES 1. A. Kiesow, J. E. Morris, C. Radehaus, and A. Heilmann, J. Appl. Phys. 94, 6988 (2003). 2. J. Y. Son, Y.-H. Shin, and C. S. Park, Appl. Phys. Lett. 92, 133510 (2008). 3. D. Tondelier, K. Lmimouni, and D. Vuillaume, Appl. Phys. Lett. 85, 5763 (2004). 4. A. Mehonic, S. Cueff, M. Wojdak, S. Hudziak, O. Jambois, C. Labbé, B. Garrido, R. Rizk, and A. J. Kenyon, J. Appl. Phys. 111, 074507 (2012). 5. J. N. Anker, W. P. Hall, O. Lyandres, N. C. Shah, J. Zhao, and R. P. Van Duyne, Nat. Mater. 7, 442 (2008) 6. K. A. Willets and R. P. Van Duyne, Annu. Rev. Phys. Chem. 58, 267 (2007). 7. A. Otto, J. Raman Spectrosc. 37, 937 (2006). 8. Z. Wang and L. J. Rothberg, Appl. Phys. B: Lasers Opt. 84, 289 (2006). 9. S. Wagner and A. Pundt, Phys. Rev. B: Condens. Matter 78, 155131 (2008). 10. J. Wu, Z. Wang, K. Wu, J. Zhang, C. Li, and D. Yin, Thin Solid Films 295, 315 (1997). 11. I. A. Gladskikh, N. B. Leonov, S. G. Przhibel skii, and T. A. Vartanyan. Opt. Zh. 81, 67 (2014). 12. T. A. Vartanyan, I. A. Gladskikh, N. B. Leonov, and S. G. Przhibel skii, Phys. Solid State 56 (4), 816 (2014). 13. P. V. Gladskikh, I. A. Gladskikh, N. A. Toropov, M. A. Baranov, and T. A. Vartanyan, J. Nanopart. Res. 17, 424 (2015). 14. J. Bosbach, F. Steiz, T. Vartanyan, and F. Trager, Appl. Phys. B: Lasers Opt. 73, 391 (2001). 15. R. D. Fedorovich, A. G. Naumovets, and P. M. Tomchuk, Phys. Rep. 328, 73 (2000). 16. N. B. Leonov, I. A. Gladskikh, V. A. Polishchuk, and T. A. Vartanyan, Opt. Spectrosc. 119 (3), 450 (2015). 17. K. Y. Yang, K. C. Choi, I.-S. Kang, and C. W. Ahn, Opt. Express 18, 16379 (2010). 18. E. Dobierzewska-Mozrzymas and P. Bieganski, J. Phys. F: Met. Phys. 18, 2061 (1988). Translated by O. Borovik-Romanova