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Enhanced Stoke Raman scattering of large-area vertically aligned Si nanorod surface etched by metal-particle-catalytic is investigated. By enlarging the surface area with lengthening Si nanorods, the linear enhancement on Stoke Raman scattering intensity at 520 cm−1 is modeled to show well correlation with increasing quantity of surface Si dangling bonds. With Si nanorod length increasing from 0.19 to 2.73 μm, the Raman peaks of the as-etched and oxidized samples gradually shift from −4 cm−1 and from −4.5 cm−1 associated with their linewidth broadening from 3 to 9 cm−1 and from 7 to 18 cm−1, respectively. The peak intensity of Raman scattering signal from Si nanorod could be enhanced with the increase of interaction area as the number of phonon mode directly corresponds to the tetrahedrally coordinated Si vibrations in the bulk crystal lattice. The asymmetric linewidth broadening and corresponding Raman peak shift is affected by the strained Si nanorod surface caused by etching and the crystal quality. Fourier transform infrared spectroscopy corroborates the dependency between nanorod length and Si-O-Si stretching mode absorption (at 1097 cm−1) on oxidized Si nanorod surface, elucidating the increased transformation of surface dangling bonds to Si-O-Si bonds for passivating Si nanorods and attenuating Stoke Raman scattering after oxidation.
©2011 Optical Society of America
1. Introduction
Semiconductor nano-structures including nanorods, nanowires and nanopillars have played a dominant role in some emerging photonic devices due to their unique optical or optoelectronic properties, such as the ultra-low reflectance on nanoroughened surface, the high optical absorption due to nanostructure enhanced Haze and the quantum confinement effect induced bandgap detuning [1–4], etc. In particular, the nanoparticle or nanoshell has been addressed as one of the best structures to induce surface Raman scattering for biosensing applications [5]. The one-dimension Si nanorods with high aspect ratio have also been successfully synthesized by versatile fabrication methods and were integrated with versatile Si optoelectronic devices including sensors, waveguides and solar cell [6–10]. The light amplification from Si nanorod sample has the potential to be used for light emitting diode, the Si-based light source can be contributed to the on-chip optical interconnect integration [11]. Increasing the spontaneous scattering efficiency of the material would enlarge the Raman gain for an appropriate pump intensity. In Si nanorod sample, the Raman scattering efficiency is larger than the bulk Si, the Raman gain is thus higher than the bulk Si. Consequently, the Raman amplifier is expected by using the nano-structure material [12,13]. During the production of Si nanorods, the defects, dangling bonds and surface states are concurrently generated to degrade the surface transmission/absorption properties and carrier transport dynamics. Few investigations are involved to correlate these characteristics of such roughened surface. Although the Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR) play important roles on studying these nanostructures, most reports were concentrated on the peak wavenumber shift and linewidth broadening of Raman signal [14,15]. Kitahara et al. have employed Raman scattering spectroscopy to characterize dangling bond defects in poly-Si via the analyses on the linewidth and intensity of Si-Si transverse-optical-phonon (at 520 cm−1) and Si-H related local-vibration mode (at 2000 cm−1) [16]. The broadened linewidth of the Si-Si optical-phonon mode and the enlarging intensity of the Si-H related local-vibration mode are correlated to the increasing dangling bonds quantity. The optical properties for Si nanorods including enhanced Raman scattering have yet not been completely realized.
In this work, the enhanced Raman scattering effect of the large-area vertically aligned Si nanorod with controlled length and diameter obtained by metal-particle-catalytic wet etching process is investigated. By controlling the etching time, the linear function of rod length can be precisely detuned to change the nanorod roughened surface area and the quantity of surface Si dangling bonds. To confirm, the FTIR is employed to detect the dependency between nanorod length and the quantity of Si-O-Si stretching mode absorption (at 1097 cm−1) on the oxidized Si nanorod surface. The effect of surface dangling bond density on the enhancement of the Stoke Raman scattering intensity at 520 cm−1 is characterized. By passivating the surface Si dangling bonds of the Si nanorods with high-temperature annealing induced oxidation, the dramatic attenuation on Stoke Raman scattering intensity is observed.
2. Experiment Setup
In experiment, a metal-particle-catalytic wet-etching in HF/AgNO3 aqueous solution at 50°C is utilized to fabricate large-area vertically aligned Si nanorod on (100)-oriented p-type Si substrate. Four chemical reactions are involved to the whole process [10]: (1) 2H+ + 2e-→H2, (2) Ag+ + e-→Ag, (3) Si + 2F-→SiF2 + 2e-, (4) Si + 2F- + 2H+→SiF2. The Si substrate is initially deposited by thin silver film and the silver atoms self-aggregate with random distribution under rapid thermal annealing process. The silver dots function as the etching pores to facilitate the Si nanorod formation during wet-etching. Afterwards, a HNO3 solution is used to clean the wrapped silver film on the rods and the Si nanorod covered Si substrate.
The Fig. 1(a) and 1(b) illustrate the nanorod length and diameter as a function of wet-etching time with six inset SEM pictures showing the corresponding morphologies of the etched Si nanorod samples. During the preparation of Si nanorods via the metallic-nanodot-catalyst assistant electrochemical etching procedure, the Si nanorod length is formed as a nonlinear function of the etching time. By increasing the etching time from 2 to 20 min, the rod length exponentially grows up from 0.19 to 2.73 μm with corresponding error bars ranged between 1.5 and 5% of the nanorod length. The comparison between linear- and exponential-function fittings indicates that the nonlinear etching response is caused by additional reaction time required to mutually aggregate Ag and Si before Si nano-nanorod formation [17]. In contrast, the nanorod diameter slightly broadens with approximate value of 50 nm. After Raman scattering diagnosis of the as-etched Si nanorod samples, a furnace annealing procedure in flowing forming gas environment was introduced at 1050°C for 30 min. Such a high-temperature annealing would passivate the dangling bonds on Si nanorod surface due to dry oxidation in forming gas, which increases numerous Si-O related bonds and leads to the degradation of Si-Si optical-phonon mode intensity (at 520 cm−1). For Raman scattering spectral analysis, a CW Nd:YAG laser with pump intensity of 0.8 mW/μm2 at wavelength of 532 nm is utilized as the light source, and a photomultiplier tube is employed behind a monochromator to collect weak Raman signal for improving detection sensitivity.
Fig. 1 The (a) nanorod length and (b) nanorod diameter as a function of etching time. Inset: the cross-section (upper) and top (lower) SEM images of three selected nanorod samples.
3. Results and Discussions
As a result, the observed Raman peaks around 520 cm−1 shown in Fig. 2 is correlated with the first-order transverse optical (TO) phonon mode of Si sample. We found that the Si-Si optical-phonon mode intensity of as-etched nanorod almost exponentially increases with the etching time, which can be presented as a function of the rod length and nanorod surface area. In the previous investigation, the surface-enhanced Raman scattering is demonstrated from the metallic nanostructures and the roughened metal surface due to the electromagnetic and chemical enhancement [18,19]. The Raman scattering enhancement can be also generated from the roughen Si substrate, however, different mechanism is utilized to describe the phenomenon. In general, the Raman scattering intensity is proportional to the equilibrium population and inelastic scattering cross-section of phonons in this mode. As the number of phonon mode directly corresponds to the tetrahedrally coordinated Si vibrations in the bulk crystal lattice, the peak intensity of Raman scattering signal from Si nanorod could be enhanced with the increase of interaction area. In particular, the Si nanorod surface possesses a dramatically different light absorption and scattering property from bulk Si substrate which could affect the inelastic scattering cross-section of phonon generated in Si. With increasing surface area, it is thus possible to create a more effective influence on the surface Raman scattering intensity of Si nanorods than that of bulk Si wafer.
As evidence, the Raman scattering signal of Si substrate greatly enhances by etching out the nanorod structure at its surface, whereas the Raman peak intensity show a slightly saturating trend with lengthening nanorods. By setting the Raman intensity of bulk Si as unitary count, the Fig. 3 shows that the Raman scattering intensity enhancement factor (related to that of bulk Si) of the as-etched Si nanorod samples are increased from 7.1 (40.5 counts/5.7 counts) to 41.9(238.8 counts/5.7 counts) as the Si nanorod length enlarges from 0.19 μm to 2.73 μm. In opposite, the Raman scattering signal of the Si nanorod samples after high-temperature annealing at atmosphere environment diminishes its peak intensity due to the heavily oxidized surface of the Si nanorod samples. The surface Raman scattering spectra of the oxidized Si nanorod samples are demonstrated in Fig. 2(b). After heavily oxidizing the Si nanorods by annealing these samples in atmosphere at 1050°C for 30 min, the Raman scattering signal dramatically degrades with its enhancement factor decaying from 41.9 to 3.5 due to the complete passivation of surface dangling bonds by oxygen. Similar diminishing phenomenon of Si-Si dependent Raman signal can also be obtained by using hydrogen or oxygen atoms to passivate the Si surface [20,21]. In principle, the oxidation condition used in this work is expected to convert approximately 10-60 nm thick Si nanorod surface into SiO2 (based on the online Silicon Thermal Oxide Thickness Calculator developed by the Stanford University). Therefore, a significant drop on the intensity of Raman scattered signal is expectable with the complete oxidation of Si nanorod surface. After such surface oxidation procedure, the Raman scattering enhancement factor of the Si nanorods with different lengths significantly attenuates to range between 1.3 and 3.5.
Such an enhancement on surface Raman scattering intensity of the as-etched Si nanorod sample is mainly attributed to the increment of the sample surface area after a simulation on the proportionality between the Raman peak intensity and the total surface area of the Si nanorod sample. To calculate the increment on surface area by etching Si nanorods on the smooth Si substrate (with area of A0), we assume the surface area of the as-etched Si substrate as A1, the uniform Si nanorod length and diameter of h and d, respectively. The total surface area of the as-etched sample is enlarged by increasing the side walls of Si nanorods and written as A1 = A0 + Nπdh, where N is the quantity of Si nanorods. Without passivation, a Si atom at substrate surface has two dangling bonds and the quantity of dangling bonds is proportional to the surface area. Thus, the increasing ratio of surface dangling bonds as compared to the Si substrate is linearly proportional to the increasing ratio of surface area given by A1/A0 = 1 + (Nπdh)/A0. The geometrical parameters of nanorods are employed to quantitatively fit the Raman intensity of Si nanorods (normalized to that of Si). Within an effective area A0 of 25π μm2 of the micro-Raman system with 10-μm illuminating spot, the denser nanorods with similar diameter of 50 nm but shorter length are obtained in shorter etching time due to the smaller aggregated size of Ag nanodots [20]. The nanorod quantity N of Si nanorods per 100 μm2 reduces from 1.85 × 104 to 0.76 × 104 with increasing etching time (from 2 to 20 min), such that the modified factor A1/A0 = 1 + (Nπdh)/A0 can coincidently fit the experimental data of Raman intensity enhancement, the Raman scattering enhancement factor as a function of rod length for the as-etched and the oxidized Si nanorod samples with the simulation curve obtained by the aforementioned formula of total surface area increasing ratio are demonstrated in Fig. 3. Consequently, the Raman intensity linearly enhances with increasing Si nanorod length as well as enlarging effective surface area, which is linearly proportional to the number of surface dangling bonds with maximum surface area density up to 5 × 1013 cm−2 [22].
According to the Raman spectra of the as-etched Si nanorods and the surface oxidized Si nanorod samples illustrated in Fig. 2, it is also observed that both the Raman peaks of the as-etched samples and the oxidized samples gradually shift from 520 to 516 cm−1 and from 520 to 515.5 cm−1, respectively. As shown in Fig. 4 , both the optical-phonon linewidth (Δτ) of the Raman scattering signal of the as-etched and oxidized Si nanorod samples are broadened from 3 to 9 cm−1 and from 7 to 18 cm−1 with increasing nanorod length from 0.19 to 2.73 μm, respectively. Previously, the optical-phonon linewidth (defined as the full-width-at-half-maximum, FWHM, of the Raman scattering peak) broadens with increasing dangling bonds, and the termination of such dangling bonds via hydrogenation was demonstrated to correlate the Si-H related local-vibration mode intensity with the dangling bond density [16]. Typically, both the peak wavenumber shift and the optical-phonon linewidth of the Raman scattered signals obtained from bulk and nanorod Si samples can be affected by temperature, crystallinity, or strain. Indeed, a significant and asymmetric broadening of the Si-Si optical phonon linewidth and corresponding Raman peak shift have been shown to arise from laser-induced local heating for the deposited Si nanowire sample with its diameter distribution ranging from 20 to 100 nm under the laser illuminating intensity of ~1 mW/μm2 (on the sample surface) [23]. To rule out the effect of temperature effect, we have carefully maintained the identical analysis time for each sample. In addition, the laser intensity is precisely controlled at <0.8 mW/μm2 to avoid any overheating on Si nanorod surface.
On the other hand, the Raman peak shift Δω was proved to be inversely proportional to the surface lattice distortion of Δa = ananorod–aSi [24], where ananorod and aSi represent the lattice constant of Si nanorod and Si wafer, respectively. Therefore, a red shift of Raman peak could be occurred if there is a slightly lattice expansion of Si nanorod surface caused by the strain induced during the HF aqueous solution etching process. A similar result of the red-shifted Raman peak was also found in a strained porous Si sample etched by HF aqueous solution [25], in which the lattice expansion and dislocation of porous Si structure induced by instantaneous growth of native oxide on porous Si surface are demonstrated by using double-crystal x-ray diffractometry [26]. The FTIR analysis of Si-O-Si stretching mode dependent absorption is employed to elucidate the correlation between surface dangling bonds and Raman scattering. The relative change in FTIR absorption peak intensity of the as-etched and oxidized Si nanorod samples with different lengths are shown in Fig. 5 . The Fig. 5(a) clearly shows the O–Si–O bending mode at wavenumber of 808 cm−1, Si-O asymmetric stretching mode at wavenumber of 1082 cm−1 and the asymmetric stretching (TO) mode of 1200 cm−1 [27]. The greatest peaks appear around 1082 cm−1 after heavy oxidation indicates that the surface dangling bonds tend to form asymmetric stretching mode with oxygen, which is mainly attributed to the large interface between Si nanorod core and SiO2 outer shell. Alternatively, the other mechanisms were also considered in previous reports [28], such as the strong surface tension of Si nanorod to distort and shorten the Si-O bond length, and the increasing energy gap between excited state and ground state of Si-O vibration absorption in SiO2/Si nanorods. According to Fig. 6(a) , the Si-O-Si mode related FTIR peak wave-number shifts from 1010 cm−1 to 1097 cm−1 with its spectral linewidth broadened to reveal the formation of the standard SiO2 due to the saturation of oxygen atoms accumulated on Si nanorod surface. The relative absorption intensity around 1082 cm−1 grows up by 30 times with Si nanorod length increasing from 0.19 to 2.73 μm on Si substrates, which indicates the reduction of dangling bonds due to the formation of complicated SiOx phases with altered O/Si composition ratios on Si nanorod surface.
Fig. 6 The wavenumber (black square dots) and intensity (blue square dots) of Si-O-Si stretching mode absorption versus nanorod length for (a) as-etched and (b) oxidized Si nanorod
The FTIR results further confirm that the longer Si nanorod contributes larger surface area to generate more surface dangling bonds for enhancing Stoke Raman scattering. The FTIR results of the Si nanorod samples after annealing at 1050°C for 30 min are shown in Fig. 5(b). Obviously, the absorption intensity of O-Si-O bending mode at 808 cm−1 and Si-O-Si asymmetry stretching mode at 1082 cm−1 are observed. The peak wavenumber of stretching mode absorption slightly shifts from 1090 cm−1 to 1120 cm−1, and the relative absorption intensity increases from 0.33 to 0.71 as the nanorod lengthens from 0.19 to 2.73 μm (illustrated in Fig. 6(b)). These results indicate that all the heavily oxidized Si nanorods form standard SiO2 on surface, and two neighboring Si atoms easily break up their covalent bond to bind with the interstitial oxygen atoms [29]. The absorption enhances with increasing Si nanorod length as more dangling bonds on the Si nanorod surface are transformed to Si-O-Si related modes. The strongest absorption peaks appear above 1200 cm−1, which results from that the large degree disorder of structure would enhance the absorption of the main stretching mode at high-energy side for amorphous-SiO2. The blue shift of the absorption peaks after annealing is caused by increasing oxygen content [30]. The comparing on FTIR results of the as-etched and oxidized Si nanorod samples reveals that the enhanced absorption after annealing originates from the diminishing surface dangling bonds, hence the Raman scattering intensity also decrease accordingly after heavily oxidation.
The localized strain and imperfection near Si nanorod surface inevitably leads to an optical phonon scattering with asymmetric linewidth broadening, these characteristic parameters were correlated with each other by quantum mechanics [31] to give a relationship between the inhomogeneous phonon scattering linewidth broadening (Δτ) and the strain as below [31],
where τ denotes the optical-phonon linewidth for Si nanorod, τ0 the FWHM of Raman scattering signal for bulk Si, B the phonon-strain scattering probability, c the dislocation concentration, υ the phonon-dislocation interaction, w0 the central wavenumber of Raman scattering signal for Si (520 cm−1), s the dispersion parameter for Si, g the dimensionless constant, θ = s2/r02w0τ, and r0 the radius of dislocation core. After simplification, the broadened linewidth is found to exhibit a linear proportionality with the dislocation concentration (Δτ ≈cg2r02w02/4πτ). During simulation, the aforementioned parameters are set as w0 = 520 cm−1, r0 = 0.13 nm, g = 100. The area dislocation density calculated from the experimental data are shown in Fig. 7(a) , which is slightly larger than 108-109 cm−2 found in the HF etched porous Si. Figure 7(b) demonstrates the experimental and simulated results of the spectral linewidth increment on surface Raman scattering intensity (related to that of bulk Si sample). It is seen that the simulated results correlate well with the experimental results and increase with lengthening Si nanorod, which confirm the direct contribution of the area dislocation density on Si nanorod surface to the linewidth broadening of the enhanced Raman scattering. A corroborative derivation was also disclosed by Camassel et al. [31]. Note that the enlarged dislocation core could effectively release the strain, whereas the variation of the dislocation core radius represents the stress of the crystal lattice. Since there is no variation on the radius of a dislocation core at Si nanorod surface during etching procedure, this parameter is thought to be independent from Si nanorod lengthening and optical-phonon linewidth broadening. In contrast, the phonon-strain interaction probability can be linearly enhanced with lengthening Si nanorod by enlarging the dislocation concentration with increasing surface area of Si nanorod. This is attributed to be the dominant factor to cause the linewidth broadening of Raman scattering signal. Increasing the Si nanorod length by etching would concurrently enlarge the surface area as well as the dislocation concentration, which not only enhances the peak intensity but also broadens the optical-phonon linewidth of the Raman scattering at Si nanorod surface. Since the Raman scattering intensity of the lengthened Si nanorod sample is at least one order of magnitude larger than that of bulk Si due to the enhanced roughening on Si nanorod surface, the increasing efficiency is well proportional to the increment of surface area as well as the quantity of surface dangling bonds. For potential applications using the nano-roughened Si nanorod sample as a sensor of specific gas- or bio-molecules, such a phenomenon can be utilized as a tooling parameter to check the residual density of gas- or bio-molecules that passivate the Si nanorod surface. That is, the detecting sensitivity of surface Raman scattering diagnosis can also be improved by more than one order of magnitude by using the precisely controlled Si nanorod lengthening technology.Fig. 7 (a) Calculated area dislocation density of Si nanorod surface with different lengths. (b) The simulated spectral linewidth increment of surface Raman scattering signal from Si nanorod surface
4. Conclusion
In conclusion, we analyze the evolution on surface Raman scattering of the large-area and one-dimensional Si nanorods on Si substrate formed by metal-particle-catalytic etching process in aqueous HF/AgNO3 solution. The Stoke Raman scattering intensity is enhanced with the exponentially increased length of nanorods obtained by lengthening the etching time. As the number of phonon mode directly corresponds to the tetrahedrally coordinated Si vibrations in the bulk crystal lattice, the peak intensity of Raman scattering signal from Si nanorod could be enhanced with the increase of interaction area. The correlation between the surface dangling bond and the enhanced Raman scattering is also investigated. We briefly establish a theoretical model to prove that the enhancement of Raman scattering is proportional to the surface area as well as the quantity of surface dangling bond. After heavily oxidize the Si anno-rod samples by annealing at 1050°C for 30 min, the Stoke Raman scattering dramatically attenuates by the passivation on the surface dangling bonds. Raman peaks of the as-etched samples and the oxidized samples gradually shift from 520 to 516 cm−1 and from 520 to 515.5 cm−1, the linewidth (Δτ) of the Raman scattering signal of the as-etched and oxidized Si nanorod samples are broadened from 3 to 9 cm−1 and from 7 to 18 cm−1 with increasing nanorod length from 0.19 to 2.73 μm, respectively. The asymmetric linewidth broadening and corresponding Raman peak shift is affected by the strained Si nanorod which caused during the etching process and the crystal quality. A red shift of Raman peak could be occurred if there is a slightly lattice expansion on Si nanorod surface caused by the strain induced during the etching process. The phonon-strain interaction probability can be linearly enhanced with lengthening Si nanorod by enlarging the dislocation concentration with increasing surface area of Si nanorod. This is attributed to be the dominant factor to cause the linewidth broadening Raman scattering signal. The FTIR analysis observes that the Si–O related mode absorption correlates well with the etching-time dependent variation on number of surface dangling bonds of the as-etched and oxidized Si nanorods surface. The strongest absorption peak locates around 1082 cm−1 reveals that the surface dangling bonds tend to form Si-O-Si asymmetry stretching mode on the surface of Si nanorods after heavy oxidation. The Si nanorod samples with longer nanorods and more surface dangling bonds exhibit both the larger Stoke Raman scattering intensity in as-etched condition and the higher stretching mode absorption after oxidation. These observations are in good agreement with our point of view that Si nanorod samples with long Si nanorods and dense surface dangling bonds cause a large enhancement on Stoke Raman scattering.
Acknowledgement
This work was financially supported by the National Science Council and National Taiwan University under grants NSC98-2221-E-002-023-MY3, NSC98-2623-E-002-002-ET, NSC97-2221-E-002-055, and NTU98R0062-07.
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