Large-area, periodic Si nanopillar arrays (NPAs) with the periodicity of 100 nm and the diameter of 60 nm were fabricated by metal-assisted chemical etching with anodic aluminum oxide as a patterning mask. The 100-nm-periodicity NPAs serve an antireflection function especially at the wavelengths of 200~400 nm, where the reflectance is decreased to be almost tenth of the value of the polished Si (from 62.9% to 7.9%). These NPAs show very low reflectance for broadband wavelengths and omnidirectional light incidence, attributed to the small periodicity and the stepped refractive index of NPA layers. The experimental results are confirmed by theoretical calculations. Raman scattering intensity was also found to be significantly increased with Si NPAs. The introduction of this industrial-scale self-assembly methodology for light harvesting greatly advances the development of Si-based optical devices.
© 2011 OSA
Si is a popular material for optical devices, such as solar cells (SCs) and photodetectors (PDs) owing to its monolithic integratibility with low-cost complementary metal-oxide-semiconductor technology. However, the low quantum efficiency particularly in UV region (<400 nm) imposes a crucial limitation in SCs, and PDs applications in astronomical detection, atmospheric and space remote sensing . For example, the portion of UV light effectively converted by Si SCs is less than 30% . To achieve the high efficiency of Si-based optical devices through the concept of full spectrum utilization, the optical losses by reflection from the surface of devices should be suppressed in the whole spectrum ranges from UV to IR regions.
Conventionally, single or multilayer interference structures as an antireflection (AR) coating working only in a small spectral range at near normal incidence have been used , and appropriate materials for these thin films improving AR performance particularly in the UV region are limited . An alternative to these thin films is subwavelength nanostructured surfaces that provide an intermediate index profile to enable broadband and omnidirectional AR characteristics [5–11]. For optimizing the efficiency of optical devices, the behavior of light trapping in nanostructures has been investigated intensively [12–14]. Beside the remarkable optical properties, the surfaces tailored with subwavelength structures exhibit improved mechanical strength, thermal stability and durability as compared to the thin-film counterpart.
Inspired by the corneas of nocturnal insects, various fabrications of biomimetic subwavelength surfaces have been explored to demonstrate impressively enhanced AR performances . Achieving a satisfactory subwavelength surface of PDs or SCs requires a nanofabrication with the precise position, size and length of control, which can be scaled up, employed at room temperature, and yield a conformal AR structure to eliminate the reflection over a broad wavelength and a field of view [9, 16–20]. For example, for SC applications, the nanopillars with radial junctions can be obtained by the formation of the emitter layer at opposite type substrate via doping . The junction depth of each nanopillar should be the same for optimizing the photocarrier collection, showing the importance of the ordered nanopillar arrays (NPAs) with identical geometric features. To consider the AR property of the nanostructure, it is found that surface reflection can be effectively suppressed by increasing the aspect ratio deff/Λ, where deff is the effective thickness and Λ is the periodicity of a nanostructured layer [16–18]. For example, a large Λ (>500 nm) of the nanostructured surface is less effective in suppressing undesired Fresnel reflection at the wavelengths below 500 nm [9, 19, 22]. To overcome this problem, thick nanostructures (i.e., large deff) could be used to obtain broadband AR performance, but would increase parasitic resistance and disturb carrier collection decreasing the device efficiency [23, 24]. It is a key challenge to fabricate a well-controlled thin nanostructure to suppress broadband reflection but still facilitate effective charge-carrier transport. Therefore, to produce short but high-aspect-ratio Si NPAs with excellent AR characteristics, a small Λ of Si NPAs is required.
In this study, we fabricated close-packed Si NPAs with 100 nm in Λ and 60 nm in diameter using an anodic aluminum oxide (AAO) template for surface structuring combined with metal-assisted chemical etching. The 100-nm-periodicity Si NPAs broadbandly eliminate the Fresnel reflection at the angles of incidence (AOI’s) up to 60°. The interaction between the incident light and the Si NPAs is realized through the simulation based on the finite-difference time domains (FDTD) analysis. Through rigorous coupled-wave analysis (RCWA), we demonstrated that as the Λ is small, the light absorption can be significantly improved in short wavelength regions due to the grating effect, confirming our experimental results. Enhanced Raman scattering also demonstrated the AR ability of Si NPAs. An AAO template combined with metal-assisted chemical etching might be a promising surface structuring method for efficient light harvesting for next-generation Si optical devices.
2. Experimental section
Annealed high-purity (99.99%) aluminum foil was electropolished in a mixture of HClO4 and C2H5OH (volume ratio = 1:5) until the root-mean-square surface roughness of a typical 10 μm × 10 µm area was ca. 1 nm. The foil was then anodized in 0.3 M oxalic acid at 1 °C at a constant voltage of 40 V for 3 min using two-step anodization process to obtain AAO substrates with nanochannel arrays of self-organized honeycomb structure . After anodization, the nanochannels were pre-opened using a 6 wt% solution of H3PO4 at 36 °C to obtain AAO substrates with arrays of nanopores. Subsequently, a thick protecting layer of nail polish was coated on the top of AAO substrate for maintaining mechanically robust during the removal of aluminum and the barrier layer. Then, the underlying aluminum was removed in a mixture of CuCl2 and HCl, and the remaining barrier layer in the bottom of AAO was dissolved in H3PO4. After removing the nail polish by acetone, the AAO film as a patterned mask was transferred to the single crystalline p-type (001) Si substrate with ρ = 8-12 Ω-cm. SF6/O2 plasma was applied to etch Si substrate through the AAO pores using SAMCO RIE-10NR to form small etched holes on the surface of the substrate. The AAO mask was removed in H3PO4 before a 30-nm-thick Ag layer was deposited on the patterned-Si substrate using RF sputtering at a power of 50 W and chamber pressure lower than 2 × 10−6 Torr. The metal-assisted wet etching was carried out by immersing the Ag-patterned Si substrate in the solution of HF/H2O2 for 160 seconds to obtain the hexagonal NPA structures.
The reflectance measurement of the Si NPAs over the wavelength regions from 200 to 850 nm was performed by a JASCO V-670 UV-VIS-IR spectrometer with an integrating sphere. The integrating sphere collects all light reflected by the samples and measures the Rtotal. The coherent reflectance of a collimated incident light beam (Rspec) was determined by collecting the specularly reflected cone of light within an acceptance angle of 5°. The omnidirectional property of the antireflective Si NPAs was characterized by measuring the reflectance at the AOI from 5° to 80° with the fixed incident wavelength of 250 nm.
The theoretical calculations based on RCWA and FDTD were employed to simulate the reflectance spectra and |E| distribution of polished Si and periodic Si NPAs, respectively. The Raman spectroscopy was obtained by a micro-Raman Jobin Yvon T64000 system equipped with a coherent VerdiV10 532 nm laser as the excitation source. The Raman signals were detected with the back illuminated UV enhanced CCD detector.
3. Results and discussion
Figure 1(a) illustrates the flowchart of the experimental process. First, the AAO membrane was placed on a Si substrate, which was cleaned by standard RCA process. The self-assembled AAO membranes used as the templates can be easily formed with a large area of controlled pore diameters and Λ’s [26–28]. The fabrication of AAO membranes is described in Experimental Section. After the RIE treatment to etch the unmasked Si (through the pores of AAO membrane), the close-packed hexagonal pattern was transferred to the Si substrate. Subsequently, the AAO membrane was removed by H3PO4, and then a 30-nm-thick Ag layer was deposited by sputtering for the following chemical etching process. Finally, the Si NPAs were obtained by immersing the Ag-patterned Si substrate in the etching solution of HF/H2O2 for 160 seconds. Figures 1(b)-1(e) are SEM images of the corresponding experimental process. Figure 1(b) shows AAO membrane/Si substrate with uniform holes after the RIE treatment, indicating that the positions and diameters of the periodic holes match those of AAO membrane pores. The SEM image of the patterned substrate with the 30-nm-thick Ag layer is shown in Fig. 1(c). In the sputtering process, Ag was not only deposited on the substrate surface (Ag network), but also aggregated in the holes etched by the RIE treatment (Ag nanoparticles), as shown in the inset of Fig. 1(c). When the Ag-coated substrate was immersedin the HF/H2O2 solution, the chemical etching took place. Upon the adhesion to Si, Ag attracts the electrons from Si due to its strong electronegativity. The electron transfer from Si to Ag induces the etching effect, which involves Ag-induced local oxidation and subsequent dissolution of the oxidized Si by HF [20, 29, 30]. Such electron transfer is greatly related to the Schottky-barrier height (SBH) at the Ag/Si interface, and the SBH is found to be increased with reduced Ag/Si contact areas . Since the contact areas of the Ag nanoparticles deposited in the Si holes are smaller than those of the film-like Ag network deposited on the substrate surface, the Ag nanoparticles (compared with the film-like Ag) are of high SBH with Si. The high SBH prevents the electrons from transporting from Si to Ag nanoparticles, and thus slows down the oxidation rate of the Si underneath Ag nanoparticles, giving rise to low etching rate in the holes. These different etching rates by HF/H2O2 on the substrate surface and in the holes eventually lead to the formation of Si NPAs; i.e., when the etching depth of the Ag network overtook that of the Ag nanoparticles in the holes, the Si underneath the Ag nanoparticles appears with the wire-like shape, as shown in Fig. 1(d). After 160-second etching and removal of the Ag by HNO3, Si NPAs were obtained, as shown in Fig. 1(e). From the inset of Fig. 1(e), the Λ and the diameter of Si NPAs are 100 and 60 nm, respectively. The NPAs duplicate geometric features of the AAO template; i.e., the Λ and the diameter of NPAs can be controlled by AAO templates. The self-assembled AAO templates can be fabricated with controlled pore diameters ranging from 10 to 200 nm and Λ’s ranging from 50 to 420 nm by varying the electrochemical parameters, such as the voltage and the electrolyte . However, the formation of NPAs is due to the different etching rates by HF/H2O2 on the substrate surface (with Ag network) and in the holes (with Ag particles). The large pore diameters of AAO membranes cause the difficulty in nanopillar formation because of little difference in the etching rate between substrate surface (with Ag network) and holes (with Ag particles). The optimized spatial filling ratio of NPAs is 0.33, as shown in Fig. 1(e). The average length of nanopillars is 344 nm, which only depends on the etching time. However, long nanopillars would increase parasitic resistance and disturb carrier collection, which decreases the device efficiency [23, 24]. Accordingly, it is the main reason that we emphasize the importance of small Λ of NPAs.
The dependence of reflectance on the deff of nanostructured layer and the wavelength (λ) has been studied by several groups [15, 32]. Clapham et al. found that the AR effect would be pronounced as the ratio of deff/λ is comparable to or larger than 0.4 . In our case, deff is given by deff = neff*d, where d is the structural thickness of the NWA layer and neff is the effective refractive index. For the wavelength region from 200 to 850 nm, neff is among 1.01~2.23 determined by the spatial filling ratio ( = 0.33) of NWAs, estimated from the inset of Fig. 1(e), using the Bruggeman effective medium approximations [33–35]. The deff/λ ratios are among 0.41~3.84. These ratios are expected to result in the broadband AR ability. In order to confirm the AR performance, the total reflectance (Rtotal) and specular reflectance (Rspec) spectra of the Si NWAs were measured as compared with polished Si over the wavelength region from 200 to 850 nm [Fig. 2(a) and 2(b)]. As shown in Fig. 2(a), the Si NWAs exhibit much lower Rtotal than that of the polished Si, and particularly at the wavelength region from 200 to 400 nm the Rtotal of NWAs is about a tenth of the value of the polished Si (from 62.9% to 7.9%). In Fig. 2(b), the Rspec of NWAs noticeably decreases from the long wavelength to the short wavelength region. The diffuse reflectance (Rdiff), defined by Rtotal-Rspec, decreases as the wavelengths increase, as shown in Fig. 2(c). The different tendencies of Rspec and Rdiff of the NWAs can be explained using Fig. 2(d), in which the ratios of Rspec/Rtotal and Rdiff/Rtotal for Si NWAs and the polished Si are plotted as the function of wavelengths. These two samples present distinct behaviors. On the polished Si, Rtotal is dominated by Rspec (average Rspec/Rtotal = 90.4%), and the ratios remain nearly unchanged at the entire wavelength range. This is because the reflection from a mirror-like Si surface is governed by the ordinary theorem of geometrical optics. In contrast, Rdiff/Rtotal on the NPAs is more than 85.2% for the wavelengths below 500 nm, and gradually decreases with wavelengths. This phenomenon manifests the fact that light scattering occurs significantly only when the wavelength is comparable with the Λ of NPAs . The periodic NPAs can be regarded as a diffraction grating . The light impinging the NPAs proceeds with three steps: coupling with the grated surface, diffracting to several beams with different diffraction angles into the NPAs, and re-bouncing between NPAs until being absorbed. At the wavelengths comparable with the Λ, the grating reduces the zero-order reflectance (i.e., Rspec), but the light beams are redistributed to the diffracted orders , leading to the high ratios of Rdiff/Rtotal. The high diffracted orders caused by surface grating can increase optical path lengths [12, 18]. These multiple scattering light paths inside the NPAs are folded up, which effectively suppresses the reflection and enhances the absorption .
Rspec/Rtotal of NPAs increases with the incident wavelengths, suggesting that as the incident wavelength increases, the scattering on the periodic structure diminishes , and Rspec dominates Rtotal, being close to the situation on polished Si, as shown in Fig. 2(d). Because the wavelengths are much longer than Λ ( = 100 nm) of NPAs, the light interacts with the whole Si NPA layer rather than each nanopillars. It is found that Rspec of the NPAs gradually increases with the wavelengths, indicating that the periodic NPAs become less resolved by the light with long incident wavelengths and therefore the grating effect is suppressed. Overall, the decrease in Rtotal at the long wavelength regions by NPA surfaces, shown in Fig. 2(a), should be explained by the effective medium theory (EMT) . In EMT, light strikes on the subwavelength structures as if it encounters an AR thin layer with an neff between refractive indices of air and Si, avoiding the abrupt transition of refractive index from air (n = 1) to Si (n = 3.7), and therefore effectively suppresses the reflection.
To confirm the experimental results, we simulated the optical behavior of light within the near-field regime propagating on the polished and periodic NPA surfaces with Λ = 100 nm using FDTD analysis. A plane wave was launched from z = 1 μm to the Si surface with/without the NPA structures. The grid sizes are Δx × Δy × Δz = 2 × 1 × 5 nm3 in space domain, and the time step for every calculation is 0.0029 fs. In the figures, the distribution of time-averaged electric field (|E|) in polished Si [Fig. 3(a) ] and Si NPAs [Fig. 3(b)] is calculated using λ = 250 nm. The AR abilities of the two structures can be compared by the |E| distribution above z = 1 μm, which indicates the reflection from the structure surface without the interference by the incident waves. The improved AR ability of the NPAs is clearly demonstrated by the reduced intensities at z = 1~1.5 μm in Fig. 3(b), which are significantly lower than those in the same region in Fig. 3(a). The result agrees with that presented in Fig. 2(a), and confirms that the NPAs with Λ = 100 nm efficiently eliminate the reflection even in UV region.
It is noticed that our NPAs achieve a significantly low reflectance at the short wavelengths (i.e., UV region) as compared with the results by Lin et al. , whose neff profile is similar to that in the present study due to almost identical geometric features (height = 360 nm, filling ratio = 0.35) of NPAs except the Λ ( = 100 and 500 nm). In order to investigate this discrepancy, we employed RCWA to simulate the reflectance spectra with two types of Si NPA structures, i.e., Λ = 100 and 500 nm, to gain the insight of Λ-dependent optical behavior. In the simulation, the reflectance is calculated with a fixed height = 350 ± 10 nm and filling ratio = 0.33, as shown in Fig. 4(a) , at the wavelengths from 200 to 850 nm. Figure 4(b) presents simulated Rtotal of the polished Si and the Si NPAs with Λ = 100 and 500 nm. The polished Si exhibits Rtotal>30% while Rtotal is effectively decreased with Si NPA layer over broadband regions, consistent with experimental results [Fig. 2(a)]. Moreover, Si NPAs with Λ = 100 nm reduce Rtotal more effectively than those with Λ = 500 nm in short wavelength regions, exhibiting a superior AR performance. Li et al. shows that for the fixed length but varying the Λ’s of Si nanowire arrays, the lowest reflectance over the whole photon energy spanning 1−4 eV can be reached at the nanowire array with the Λ of 100 nm, consistent with our simulation results . The impact of Λ on the AR properties of NPAs can be further differentiated from Fig. 4(c) and 4(d); i.e., Rspec of NPAs with Λ = 100 nm is much lower than that with Λ = 500 nm for the wavelengths below 400 nm. Figure 4(e) displays the ratios of Rspec/Rtotal and Rdiff/Rtotal for Si NPAs with Λ = 100 nm whose tendencies agree well with the experimental results [Fig. 2(d)]. On the other hand, the diffraction order ratios of Si NPAs with Λ = 500 nm [Fig. 4(f)] present distinct tendencies from those with Λ = 100 nm. Because the filling ratio is fixed, i.e., the neff’s of two structures are the same, the discrepancy between two Λ’s structures can be ascribed to the grating effect when the NPAs are regarded as a diffraction grating. The diffraction behavior of NPAs can be described using the grating equation [37, 38],Fig. 4(e) and 4(f), for the wavelengths below 400 nm, Rdiff/Rtotal ratios of Si NPAs with Λ = 100 nm maintain high values, but the Si NPAs with Λ = 500 nm show that the Rdiff/Rtotal ratios are lower than Rspec/Rtotal ratios and gradually increase with wavelengths. It means that the effect of light diffraction by Si NPAs with Λ = 100 nm is more pronounced than that with Λ = 500 nm at UV region, and thus Rspec of Si NPAs with Λ = 100 nm is much lower than that with Λ = 500 nm at this region. From Eq. (1), for a constant λ, the small Λ leads to the diffracted beams with large θm for the same diffraction order (m) . The beams transmitting with the larger θm result in the elongation of optical paths and enhance internal bounces within the NPAs, increasing the probability of absorption. On the other hand, at UV wavelength region, the improvement in the light trapping can be realized by reducing the Λ of NPAs (increasing the density of NPAs), which increases the number of reflection between adjacent nanopillars. For the NPAs with large Λ, the incident light virtually strikes on the bottom and reflects to the air, so the light cannot be effectively trapped by NPAs, causing the reflectance to be increased at UV region.
A desirable AR coating should give consideration to angle-dependent effects to suppress Fresnel reflection over a wide range of AOI’s, which is so-called omnidirectionality . In order to investigate the omnidirectional characteristics of the NPAs in the UV region, where the reflectance can be greatly reduced by our structure, the Rspec was measured with the AOI’s ranging from 5° to 80° with the wavelength fixed at 250 nm, as shown in Fig. 5 . The reflectance on the NPAs remains below 1.2% for the AOI up to 60°, exhibiting significantly improved omnidirectionality in comparison with the polished surface. The reflectance on the two samples gradually increases after AOI reaches 60°. Intuitively, when the light reaches the NPA layer’s surface at a large AOI, the portion of the light entering the NPA layer is decreased, suggesting that the probability of trapping light within the NPA layer is reduced, and therefore the reflectance is increased.
Raman scattering describes the inelastic scattering of lattice vibration, and has been widely used as an analytic spectroscopic technique . Figure 6 presents the Raman spectra of the polished Si and the Si NPAs. Two samples exhibit a peak at 520 cm−1, which caused by the first-order optical phonon mode of single-crystal Si . No frequency shift is observed with the Si characteristic Raman peak, indicating that the Si NPAs were not extensively damaged by the chemical etching process. The scattering intensity on Si NPAs is enhanced by a factor of 8 as compared with the case on polished Si. This can be ascribed to an AR effect. The Si NPAs lead to the major portion of incident laser light entering the structure, and therefore increase the absorption of light. In addition, Raman backscatter traveling toward the surfaces will also encounter the NPA layer, suggesting an additional enhancement factor for the light extraction due to a mediate neff of NPA layer. Therefore, enhanced insertion and extraction of light lead to a substantial Raman signal improvement.
In summary, periodic Si NPAs with 100 nm in Λ were fabricated by an AAO templating method combined with metal-assisted chemical etching. Their broadband AR performance eliminates the Fresnel reflection at the AOI up to 60° especially at the wavelengths below 400 nm, indicating the importance of small Λ and stepped refractive index of NPA layer, which was confirmed by RCWA and FDTD simulations. Substantial Raman signal improvement also demonstrated the AR ability of Si NPAs. The nanofabrication for broadband omnidirectional light-harvesting demonstrated here will greatly benefit the design of optical devices.
The research was supported by the National Science Council Grant No. NSC 99-2120-M-007-012, NSC 99-2112-M-002-024-MY3 and NSC 99-2622-E-002-019-CC3.
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