Thermal annealing of Ag implanted silicon: Relationship between structural and optical properties

Low energy Ag ions were implanted into silicon and annealed at different
 temperatures in order to generate plasmonic active silicon hybrids. It was
 found that as the ion fluence of irradiation was increased, a monotonic
 decrease in the absorption spectra in the ultraviolet region occurs, due to
 amorphization and macrostructuring of the Si surface. At the same time, the
 optical spectra are characterized by a strong band after implantation
 presenting the contribution of the surface plasmon resonance (SPR) of Ag
 nanoparticles. After heat treatment at 500 and 600?C, the SPR peak shifts to
 lower wavelengths, as compared to as implanted samples, whereas the plasmon
 position shifts to higher wavelengths for annealing at 700?C. This
 observation can be explained by either an out-diffusion of Ag or by stress
 relaxation and recrystallization of silicon.


Introduction
Over the last decades metallic nanoparticles have been the subject of many experiments because of their large technological applicability. Due to high surface-to-volume ratio metallic nanoparticles can exhibit physical and chemical properties which are dramatically different from those of bulk materials, making them a good choice for various applications, such as photonics [1], data storage [2], sensing and imaging [3], catalysis [4], optical devices [5], etc. Among them, noble metal nanoparticles are of special interest, because they support a phenomenon known as a surface plasmon resonance (SPR). The SPR is a result of interaction of the metal nanostructures with light due to the collective oscillation of conduction electrons on the metal surface, occurring when excited by light at specific wavelengths [6][7][8]. In recent years a great deal of interest has been focused on the synthesis of transitional metal (e.g. Ag, Cu, Fe, Au) nanostructures at the surface to sub-surface regions of Si and SiO 2 matrices for possible application in technological devices with enhanced electronic and optical properties [9][10][11][12][13]. In all of these applications, the size, shape and distribution of the metallic nanoparticles in the silicon matrix play a key role [14,15]. Ion implantation followed by thermal annealing is one of the most suitable methods for the synthesis of buried metal nanoparticles [16]. The implanted low energy metal ions initially amorphizes the Si substrates while being distributed at shallow depths near the substrate surface. When subjected to thermal annealing, the implanted ions may agglomerate to larger particles of different sizes at various depths depending on the ion fluence and annealing temperature. The optical properties of such systems are determined not only by the metallic nanoparticles but also by the structure of the host material itself. Therefore, it is important to understand how the silicon structure is affected by ion irradiation, as well as by the annealing temperature and how these changes influence the optical absorption at the surface plasmon resonance.
The present study aimed to synthesize Ag nanoparticles in the monocrystalline silicon matrix by using ion-implantation process. A series of experiments were conducted as a function of the ion fluence and subsequent annealing temperature. The shift in the SPR peak position of silver nanoparticles was investigated and the correlation with the structural changes was discussed.

Materials and Experimental Procedures
Commercial p-type monocrystalline (100) Si wafers thickness of 550 µm were implanted by 75 keV energy single charged 107 Ag ions to the fluences of 1×10 13 , 1×10 14 , 1×10 15 and 1×10 16 ions/cm 2 . The irradiation energy was chosen according to simulations of the implantation profiles, calculated with the software package SRIM-2008 [17]; the average ion implanted depth was calculated to be 44 nm with straggling of 13 nm. All implantations were performed at room temperature and at 7º off-normal direction to avoid ion channeling. The average ion current density during implantations was kept constant at approximately 1 μA/cm 2 , the chamber pressure was always in the low region of 10 -6 mbar. Subsequent to the implantations the samples were annealed in vacuum (10 -5 mbar) at 500, 600 and 700ºC for 2 h. These temperatures turned out to be the most suitable compared to the eutectic temperature of Ag-Si system at 830ºC [18].
The structural and compositional characterization of Ag-implanted Si samples was done by Rutherford backscattering spectrometry (RBS) and transmission electron microscopy (TEM). RBS measurements were performed using a 1 MeV 4 He ++ ion beam, generated by Tandetron accelerator. The backscattering spectra were taken at normal beam incidence using Si surface barrier detector, positioned at the angle of 170º to the beam. Silver concentration profiles were extracted from the raw RBS spectra, by using WiNDF computer package [19]. TEM and HRTEM analyses were performed on the FEI Talos F200X electron microscope operating at 200 kV. By using a nanoprobe the samples were also analyzed in scanning transmission (STEM) mode with energy dispersive X-ray analysis (EDS) elemental profiling and element color mapping. Optical measurements were done by HORIBA-Jobin Yvon UVISEL iHR320 ellipsometer with monochromator wavelength range from 260 to 2066 nm (energy range 0.6 to 4.8 eV). The spectra were taken in reflection mode, at the angle of incidence of 70º and with the light spot of 1 mm in diameter. Analysis of the obtained spectra was performed by fitting the experimental data, via commercial software package DeltaPsi2 [20]. Fig. 1a shows RBS spectra taken from a Si sample implanted with 75 keV Ag ions to an ion fluence of 1×10 15 ions/cm 2 directly after implantation as well as from samples annealed at 500, 600 and 700ºC. The signals arising from Ag are well separated in the spectra (see the enlarged section given in the inset of Fig. 1a), while due to a lower mass, the backscattering yield arising from the Si substrate start at lower energy than the yield from Ag. The RBS spectra were fitted and the silver depth profiles were extracted and displayed in Fig. 1b. It can be seen that Ag is located in the near-surface region of Si in the as-implanted sample. A maximum silver concentration of about 0.5 at.% can be found in a depth of approximately 20 nm. After annealing at 500ºC the maximum silver concentration decreased to ~0.25 at.%. In particular, the whole Ag distribution is modified after annealing, the profile being much wider as compared to the as-implanted sample. Further increasing of the annealing temperature to 600 or 700ºC results in a shift of Ag concentration maximum towards the surface. Obviously, at these elevated temperatures the diffusion of silver towards the surface and its evaporation occurs, resulting in a profile truncation. A similar behavior was already reported by other authors for silver implanted layers annealed at temperatures above 500ºC [21].  Fig. 2 (a-d) show low magnification bright-field TEM images of the Si samples implanted to 1×10 16 Ag/cm 2 and samples annealed for 2h at 500, 600 and 700ºC after irradiation. From the TEM image in Fig. 2a, one notes that nanoparticles are formed after the implantation process. The nanoparticles are distributed below the surface of the Si substrate, at depths of around 10-40 nm from the surface of the sample. This is consistent with the depth profiles of implanted Ag atoms, obtained by RBS analysis (see Fig. 1b), according to which most of the implanted 75 keV Ag ions are situated within 40 nm from the silicon surface. From the TEM micrograph one also observes the presence of an amorphous surface layer, which is due to damage caused by ion implantation, registered earlier for implantation of crystalline Si with silver ions [22]. The image was used for the determination of the average size of the Ag nanoparticles and the particles size distribution histogram is given in the inset. From the histogram one observes that the particles are small in size, most of them being smaller than 4 nm in diameter. When the samples were annealed at 500ºC (b) or 600ºC (c) similar results were obtained, with somewhat larger average particle sizes formed at 600 ºC. But, with further increase of the annealing temperature up to 700ºC (d) the size of nanoparticles changes dramatically; the particles are much larger as compared to the asimplanted sample, with maximum particles sizes up to 25 nm in diameter. Closer analysis with higher magnifications of the Ag nanoparticles region for the sample implanted to 1×10 16 Ag/cm 2 and annealed at 700 ºC are presented in Fig. 3. In Fig. 3a one can see that the particles are nearly spherical and they are formed with various sizes. Additionally, we observe that at this annealing temperature the particles are distributed in a very narrow region, at depths of around 10-20 nm below the silicon surface. Both, growth of particles as well as their localization in the near-surface region are due to diffusion processes promoted by heating the samples during the high-temperature annealing [23,24]. Spontaneous dissolution of small nanoparticles at larger depths and re-deposition of the dissolved species in the near-surfaces region occurs -according to Oswald ripening. A feature of the highmagnification TEM image of the sample (marked with an arrow in Fgure 3a) is a clearly visible interface at the depth of approximately 50 nm from the surface, indicating a surface layer consisting of a material with a different density. The reason for this structural change is a recovering of the crystal structure of silicon initially amorphized by silver ions. Obviously, the diffusion processes deeper into the silicon are not as effective as in the near-surface region of the substrate. As a result, a recrystallization of silicon happens only in the first 50 nm below the surface, while the rest of the damaged region still remains amorphous. The HRTEM image in Fig. 3b was taken at a depth of the maximum silver nanoparticles, showing also the surrounding silicon. It can be seen that the nanoparticles are crystalline in nature. The measured lattice spacing of 0.236 nm is in very good agreement with the theoretical value for cubic Ag [25]. This image also confirms the presence of the polycrystalline nature of the silicon below the nanoparticles and verifies the recrystallization of the silicon substrate in this region.   4a shows a low magnification STEM-HAADF image of the same Si implanted with 1×10 16 ions/cm 2 Ag ions, subsequently annealed at 700ºC. Bright regions are visible at depths of around 10-20 nm from the surface. The HAADF brightness is approximately proportional to the square of the atomic number, Z 2 , therefore the white regions in the image correspond to the heaviest element in the sample, i.e. implanted Ag metal particles, while the intensity of Si is negligible compared to that of Ag and are presented as dark areas. To determine the distribution of elements with high precision, one isolated nanoparticle (marked with a square in Fig. 4a) was further investigated with high resolution and EDS elemental mapping, as shown in Fig. 4(b-e). The yellow and red color images are associated with the Ag and Si, respectively. Also, the overlap image is shown. Based on this analysis the diameter of nanoparticle is found to be 11 nm. According to the elemental mapping, it can be seen that the silver is completely localized in the nanoparticle, with no Ag atoms distributed into the surrounding silicon. Fig. 4f presents a line profile, taken along the green line indicated in STEM-HAADF image. Red and yellow lines correspond to Si K edge and Ag L edge, respectively. The analysis confirms that the bright regions in STEM-HAADF image belong to the Ag, whereas surrounding darker region originate from Si substrate.

Optical characterization of silver implanted Si substrates
The evolution of the optical properties of the Ag-implanted and subsequently thermally treated Si samples was followed by spectroscopic ellipsometry measurements. For illustration, we present in Fig. 5 the typical (Ψ, Δ) spectra, taken from the Si samples irradiated with 1×10 13 , 1×10 14 , 1×10 15 and 1×10 16 ions/cm 2 Ag ions. In order to obtain the optical parameters from these measured values, it is necessary to create an appropriate model of the samples and fit the parameters of this model to the measured data. For instance, for the fitting of unimplanted Si a two-layer model was used consisting of crystalline silicon with a thin native oxide layer on the top of the sample. The model for the implanted and annealed Si layers was based on the amorphous Si model represented as ion-induced amorphized silicon (a-Si) or composition of a-Si and silver phases using self-consistent Maxwell-Garnett effective medium approximation [26]. The fitting results, i.e. the extinction coefficients (k) for unirradiated Si and Agimplanted sublayers, are shown in Fig. 6a. For the unimplanted Si one observes characteristic absorption bands with maxima at 278 nm and 361 nm, which are due to the interband transitions. After implantation to ion fluence of 1×10 13 ions/cm 2 the extinction coefficient almost matches the k values of Si, while the higher fluence Ag implantations (10 14 -10 16 ions/cm 2 range) lead to the substantial variation of the optical extinction. The intensity of the absorption band decreases with a maximum of 278 nm and shift to the higher wavelengths, while the band at 361 nm is completely lost. These changes are attributed to the amorphization and macrostructuring of the Si surface occurring during the heavy ions irradiation, earlier obtained by several authors [27][28][29]. Furthermore, changes at the highwavelength side of the spectra are visible. In particular, the spectra are characterized by a strong band at λ > 500 nm after implantation. These results demonstrate the contribution of the surface plasmon resonance (SPR) of Ag nanoparticles, as predicted by the Mie theory [30]. The SPR peak positions vary from 829 to 1271 nm, and also the peak intensity increases continuously with increasing ion fluence. An increase in the absorption together with the shift of the SPR peak to higher wavelengths after increasing implantation fluence have been reported recently in the case of 60 or 75 keV Ag irradiated Si samples [22,26], where the shift in SPR position was associated with the formation of larger number of Ag nanoparticles as the ion fluence increases, although the average size of particles was not changed.  Fig. 6b shows the spectra of the optical constant k for the samples implanted to the highest ion fluence of 1×10 16 ions/cm 2 and after annealing at 500, 600 and 700ºC. One can observe the evolution of the surface plasmon resonance as a function of annealing temperature. Indeed, at 500ºC the SPR appear at lower wavelengths as compared to as implanted sample and continues with the similar trend for annealing at 600ºC. Then, with rising annealing temperature to 700ºC the plasmon position shifts to higher wavelengths, whereas the peak intensity decreases continuously. These changes could be explained by further consideration of the RBS results (Fig. 1b), where a decrease of the Ag concentration was observed at elevated temperatures. On the other hand, during a process such as thermal annealing of an ion irradiated samples, stress relaxation on the surface as well as inside the Si substrate will occur. Specially, the thermal annealing at 700ºC produces polycrystalline Si and larger Ag nanoparticles in the ion induced amorphized Si region (see Fig.s 2 and 3). The metal nanoparticles and polycrystalline Si acts as optically active centers [31], which highly interact with incoming light and enhances absorption of the light. However, in our case the overall decrease in the optical absorption was observed. We assumed the reason for this could be in the concurrent contribution originating from Ag-Si interdiffusion caused by the hightemperature annealing. This is consistent with the earlier findings of Gould et al. [32] who investigate the thermal stability of silver nanoparticles embedded in amorphous silicon. The authors found that interdiffusion of Ag and Si atoms are more prevalent in this case of small particles and also when surface imperfections, such as defects, are present. According to molecular dynamics simulations, the diffusion of Si atoms into the Ag nanoparticles is rather expected since the calculated Ag−Si bond found to be 0.75 eV/atom stronger than the Ag−Ag (2.95 eV) [32,33]. This result perhaps suggests that the decrease in optical response in our experiment could alternatively be caused by diffusion of Ag atoms or that a-Si layer has some degree of porosity, hosting the Ag atoms in pores.

Conclusion
Monocrystalline Si samples were implanted with 75 keV Ag ions in order to synthesize a plasmonic active Ag-Si near surface hybrid. A series of experiments were conducted as a function of ion fluence and subsequent annealing temperature. The characteristic features of absorption spectra of the Ag irradiated samples are low intensity in the ultraviolet region of the spectra (<400 nm) caused by amorphization and macrostructuring of Si surface and the presence of an SPR peak of Ag nanoparticles at high-wavelength side. The formation of Ag nanoparticles was observed by TEM and for the heat treated samples at 500 and 600ºC, the SPR peak appears at lower wavelengths as compared to as implanted sample, and then at 700ºC the plasmon position shifts to higher wavelengths. We describe this behavior as a result of two competing thermally assisted processes, such as migration of Ag atoms away from the sample due to their high mobility in silicon and the recrystallization of the silicon, initially amorphized by energetic ions. An overall decrease in optical absorption of annealed samples was probably caused by the different degree of stability of Ag nanoparticles and interdiffusion of Ag and Si atoms.