Effect of Systematic Control of Pd Thickness and Annealing Temperature on the Fabrication and Evolution of Palladium Nanostructures on Si (111) via the Solid State Dewetting
© The Author(s). 2017
Received: 27 February 2017
Accepted: 11 May 2017
Published: 19 May 2017
Si-based optoelectronic devices embedded with metallic nanoparticles (NPs) have demonstrated the NP shape, size, spacing, and crystallinity dependent on light absorption and emission induced by the localized surface plasmon resonance. In this work, we demonstrate various sizes and configurations of palladium (Pd) nanostructures on Si (111) by the systematic thermal annealing with the variation of Pd thickness and annealing temperature. The evolution of Pd nanostructures are systematically controlled by the dewetting of thin film by means of the surface diffusion in conjunction with the surface and interface energy minimization and Volmer-Weber growth model. Depending on the control of deposition amount ranging between 0.5 and 100 nm at various annealing temperatures, four distinctive regimes of Pd nanostructures are demonstrated: (i) small pits and grain formation, (ii) nucleation and growth of NPs, (iii) lateral evolution of NPs, and (iv) merged nanostructures. In addition, by the control of annealing between 300 and 800 °C, the Pd nanostructures show the evolution of small pits and grains, isolated NPs, and finally, Pd NP-assisted nanohole formation along with the Si decomposition and Pd-Si inter-diffusion. The Raman analysis showed the discrepancies on phonon modes of Si (111) such that the decreased peak intensity with left shift after the fabrication of Pd nanostructures. Furthermore, the UV-VIS-NIR reflectance spectra revealed the existence of surface morphology dependent on absorption, scattering, and reflectance properties.
The fabrication of metallic NPs on semiconductors have attracted considerable research interests for various optoelectronic devices. Metallic NPs can demonstrate increased light absorption and emission owing to the localized surface plasmon resonance (LSPR) [1–3], and their optical properties can efficiently be tuned by the control of shape, size, and density of NPs [4–6]. Silicon is one of the most abundant semiconductor materials widely utilized for photovoltaic solar cells and photodetectors [7–10], and the performance of Si-based optoelectronic devices can be significantly improved by the integration of metallic NPs [11–16]. For instance, self-assembled aluminum (Al) NPs fabricated on Si can enhance the efficiency of solar cells owing to the LSPR exhibited by Al NPs . Similarly, Si-based LEDs can produce a superior light emission via the excitation of surface plasmon resonance by silver (Ag) NPs . Furthermore, systematically controlled orientation of Ag NPs grown on Si (100) and Si (111) can enhance PL intensity , and 3-D morphology of gold (Au) NPs on Si (111) demonstrates an increased light emission as compared to that of the 2-D flat layers . Likewise, palladium (Pd) NPs, though not yet widely investigated, also have interesting plasmonic properties and thus applied in photo-catalysts, hydrogen-related sensing, and adsorption applications [17–19]. Taking these into account, the adaptation of Pd nanostructures on Si can offer promising opportunities for the Si-based applications; however, the systematic study on the fabrication of Pd NPs on Si (111) and its morphological and optical characterization is still quite deficient. In this paper, we demonstrate the systematical evolution of Pd nanostructures on Si (111) by the control of Pd deposition amount and annealing temperature. Depending upon the various deposition amount, the evolution of Pd nanostructures from small grains, NPs, elongated NPs, and merged nanostructures are evolved based on the thermal diffusion, Volmer-Weber growth model, and surface and interface energy minimization mechanism. On the other hand, the annealing temperature variation shows the distinctive evolution of Pd nanostructures such as tiny pits and grains, randomly distributed isolated NPs, and finally, Pd NP-assisted nanohole formation on Si (111). The effect of surface morphology evolution of Pd nanostructures on optical properties and Si lattice vibration modes are revealed by the reflectance and Raman spectra measurement.
Si (111) Substrate Preparation and Nanostructure Fabrication
In this work, the evolution of various configurations, sizes, and densities of Pd nanostructures was investigated on Si (111) (Silicon Materials Inc., Belarus) by the systematic control of Pd deposition amount at various annealing temperatures and annealing duration. Initially, the Si (111) substrates were diced into 1 × 1 μm2 by a machine saw. Then, the Si (111) were mounted on a holder for the degassing process at 750 °C for 30 min under 1 × 10−4 Torr in a pulsed laser deposition (PLD) system in order to clean the surface. Additional file 1: Figure S1 shows the Raman spectra of bare Si (111) in which the transverse acoustical (TA) mode peaks are observed at around 299.14 and 616.63 cm−1 and the transverse optic (TO) peaks are at 519.41, 950.40, and 973.83 cm−1. The clean Si (111) substrates after the degassing are now ready for the Pd deposition. In an ion coater chamber, the Pd films were deposited on Si (111) by the sputtering at a growth rate of 0.05 nm/s at the 3 mA ionization current under the vacuum of 1 × 10−1 Torr. Various thicknesses of Pd films between 0.5 and 100 nm were deposited by the systematic control of deposition time (20 s ~ 1 nm). Additional file 1: Figures S2 and S3 shows the surface morphologies of various Pd depositions and the cross-sectional line profiles, RMS roughness (Rq), surface area ratio (SAR), and height distribution histograms which show the gradual increase in the height distribution with the thickness. After the deposition of Pd on Si (111), the samples were introduced to the PLD chamber for the nanostructure fabrication. The annealing temperature was systematically increased to acquire the target temperature at a ramping rate of 4 °C/s under 1 × 10−4 Torr. The series of samples for deposition amount variation was annealed at three distinctive temperatures (450, 575, or 700 °C) for the constant annealing duration of 450 s. For the investigation of annealing temperature on evolution of Pd nanostructures, the annealing temperature was systematically varied between 300 and 800 °C, whereas deposition amount and annealing duration were set for 5 nm and 450 s respectively. The overall annealing process was controlled by the computer-operated recipe program in order to maintain the consistency. The temperature was immediately quenched down to the room temperature after the completion of each growth.
Characterization of Pd Nanostructures
The surface morphologies of Pd nanostructures on Si (111) were characterized using an atomic force microscope (AFM) from the Park Systems Corp. (XE-70, South Korea) under an atmospheric condition. In order to minimize the tip effect, the tips from a single batch were used, having a length of 17–21 μm and a radius curvature less than 10 nm, force constant 40 nm−1, and resonant frequency ~300 kHz. After the characterization, the AFM images were analyzed using the XEI software (Park Systems) in terms of Rq, SAR, Fourier filter transform (FFT) spectra, and cross-sectional line profiles. The elemental analyses of the corresponding samples were carried out by using an energy-dispersive X-ray spectroscope (EDS) system with the spectral mode (Thermo Fisher Noran System 7, USA). The Raman and reflectance spectra were measured by UNIRAM II system (UniNanoTech Co. Ltd, South Korea), Andor shamrock sr500 spectrograph, laser −532 nm (for Raman), and deuterium and halogen light (for reflectance). The optical characterization was performed in dark room in order to avoid the external light interference.
Results and Discussion
Meanwhile, for the identical deposition range, the temperature effect was investigated at the increased (750 °C) and decreased (450 °C) points, which are presented in the supplementary information in Additional file 1: Figures S7–S17. Overall, similar growth behavior was observed such that the size of the Pd nanostructures was increased, and density decreased accordingly with the increased deposition amount. As compared with the 575 °C set, the 700 °C set demonstrated slightly larger size of Pd nanostructures while the evolution trend was similar. The enhanced surface diffusion at 700 °C drove the Pd nanostructures to be a more stable configuration by minimizing surface energy such that the Pd NPs at 10 and 15 nm in the 700 °C set were much rounder. The evolution of surface morphology is also clearly evidenced by the cross-sectional line profiles, FFT power spectra, and the summary plots of Rq and SAR and summarized in Additional file 1: Table S4. The Raman spectra and their peak position, FWHM, and peak counts were also varied accordingly with the various morphologies of Pd nanostructures with increased deposition amount between 0.5 and 100 nm as clearly shown in Additional file 1: Figure S11 and summarized in Additional file 1: Table S5. The reflectance spectra in Additional file 1: Figure S12 also showed similar behavior in correlation with the surface morphology as described for the previous set. Meanwhile, annealing at comparatively low temperature of 450 °C resulted in the very small Pd grains on top over the Pd layer due to the limited diffusion of Pd adatoms as clearly illustrated by the AFM images, line profiles, FFT power spectra, RMS roughness, surface area ratio, EDS spectra, and Raman and reflectance spectra in Additional file 1: Figures S13–S17 and Additional file 1: Tables S4 and S5.
In this work, the fabrication of Pd nanostructures with the various configurations, sizes, and densities was successfully demonstrated with the systematic control of deposition amount and annealing temperature on the morphology of Pd nanostructures on Si (111). Upon the annealing at temperatures (575 and 700 °C), the systematic increase in deposition amount resulted in the four distinctive configurations of Pd nanostructures such as small pit and grain formation, nucleation and growth of Pd NPs, lateral evolution of Pd NPs, and finally, merged nanostructures. The evolution of various configurations of Pd nanostructures were discussed based on the thermal diffusion, Volmer-Weber growth model, and surface and interface energy minimization mechanisms. Moreover, the incremental variation of annealing temperature demonstrated the formation of tiny pits and grains at relatively low temperature (300 to 400 °C), randomly distributed Pd NPs (450 to 700 °C), and finally, Pd NP-assisted hole formation at high temperature (750 to 800 °C). Depending on the surface conditions, the Raman spectra revealed the decreased intensity and left shift of phonon modes of Si (111) as correlated to the average surface coverage of Pd nanostructures and the stress between Pd and Si. Moreover, the reflectance spectra were dependent with the size and average surface coverage of Pd nanostructures that modulates the average reflectivity, absorption, and scattering. Finally, this study can provide the simple approach of fabricating Pd nanostructures with distinctive and controllable physical structures thereby exploiting related properties for the range of catalytic- and plasmonic-related application. In addition, the Pd NP-assisted porous Si formation can be achieved for antireflective Si substrates.
The financial support from the National Research Foundation of Korea (nos. 2011-0030079 and 2016R1A1A1A05005009) and in part by the research grant of Kwangwoon University in 2017 is gratefully acknowledged.
SK, PP, MS, and JL participated in the experimental design and carried out the experiments. SK, PP, MS, QZ, ML, and JL participated in the analysis of the data. SK, PP, and JL designed the experiments and testing methods. SK and JL carried out the writing. All authors helped in drafting and read and approved the final manuscript.
The authors declare that they have no competing interests.
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