Ordered arrays of nanoporous silicon nanopillars and silicon nanopillars with nanoporous shells
© Wang et al.; licensee Springer. 2013
Received: 5 September 2012
Accepted: 4 December 2012
Published: 21 January 2013
The fabrication of ordered arrays of nanoporous Si nanopillars with and without nanoporous base and ordered arrays of Si nanopillars with nanoporous shells are presented. The fabrication route is using a combination of substrate conformal imprint lithography and metal-assisted chemical etching. The metal-assisted chemical etching is performed in solutions with different [HF]/[H2O2 + HF] ratios. Both pore formation and polishing (marked by the vertical etching of the nanopillars) are observed in highly doped and lightly doped Si during metal-assisted chemical etching. Pore formation is more active in the highly doped Si, while the transition from polishing to pore formation is more obvious in the lightly doped Si. The etching rate is clearly higher in the highly doped Si. Oxidation occurs on the sidewalls of the pillars by etching in solutions with small [HF]/[H2O2 + HF] ratios, leading to thinning, bending, and bonding of pillars.
KeywordsNanoporous Si Pillars Nanowires Metal-assisted chemical etching Nanoimprint lithography
Nanostructured Si is drawing a great deal of interest due to its potential applications in nanoscale electronics[1, 2], optoelectronics, thermoelectrics, photovoltaics, biosensors, nanocapacitor arrays, and as electrodes in Li-ion batteries. It is well known that porous Si can be produced by anodic (electrochemical) etching in HF aqueous solution or stain etching in HNO3/HF solution[9, 10]. Recently, metal-assisted chemical etching (MaCE) as a simple and low-cost method to fabricate Si nanowires and nanoporous Si has attracted increasing attention[11–14]. In this process, Si wafer coated with a noble metal is etched in a solution consisting of HF and an oxidant (e.g., H2O2 or AgNO3) to form the nanostructures. Nanoparticles or thin films of noble metals (e.g., Au, Ag, or Pt) are used to catalyze the etching. Two-level nanoscaled porous Si nanowires were even synthesized with highly doped Si using MaCE, and Ag nanoparticles acted as catalyst[15–17]. Zigzag Si nanowires were fabricated with (111)-oriented Si by MaCE (with Ag nanoparticles as catalyst). These zigzag Si nanowires were even fabricated with (100)-oriented Si by a two-step MaCE (with Au film as catalyst). In general, the structural properties and morphologies of the nanostructured Si produced by MaCE are affected by three main factors: (1) the properties of the deposited noble metals, including the type and form of the metal, and its deposition method; (2) the properties of the Si wafer, including the doping type and level and the crystallographic orientation; and (3) the properties of the etchant, including etchant composition, concentration, and temperature.
By combining MaCE with nanolithography, many ordered nanostructures were fabricated. For example, ordered arrays of Si nanowires and nanopillars were fabricated using a combination of laser interference lithography or nanosphere lithography and MaCE[20–22]. An Au/Ag bi-layer metal mesh with an array of holes, prepared from an anodic aluminum oxide membrane, was used to fabricate Si nanowires by MaCE. In this paper, the fabrication of ordered arrays of nanoporous Si nanopillars, ordered arrays of nanoporous Si nanopillars with nanoporous base, and Si nanopillars with nanoporous shells using a combination of substrate conformal imprint lithography (SCIL) and MaCE (with Au film as catalyst) is presented. The mechanisms of MaCE are systematically investigated, and the fabricated nanoporous pillars should have the potential for applications in sensors and optoelectronics.
List of solutions with different molar ratios, λ , used for the etching solutions
The highly doped Si was etched for 10 min in solutions with different values of the molar ratio λ, and the formed nanopillars are shown in Figure3. Relatively long nanopillars and a thin nanoporous base layer were observed after etching in the λ1, λ2, and λ3 solutions, while shorter nanopillars and a thick homogenous nanoporous base layer with a thickness of 4.3 μm below the pillars were observed after etching in the λ4 solution. The nanoporosity of the nanopillars etched in the λ1, λ2, and λ4 solutions becomes obvious in the cracked pillars (Additional file1: Figure S2). After 10-min etching in the λ1 and λ2 solutions (Additional file1: Figure S2a,b), it was also observed that the nanoporous base layer below the pillars is thicker than that directly below the Au film. The nanopillars are strongly bent and bonded together at the top after etching in the λ1 solution (Figure3a). The bonded nanopillars at the top can be clearly seen in the magnified SEM image (Additional file1: Figure S3). In addition, the thickness of these nanopillars is about 50% smaller at the top compared to the bottom of the pillars. The bonded and bent nanopillars were also observed after etching in the λ2 solution (Figure3b), but they are less bent than those after etching in the λ1 solution. The nanopillars etched in the λ1 solution were bonded as bundles, while the nanopillars etched in the λ2 solution were bonded in rows (Additional file1: Figure S4a,b). The same thickness is seen both at the top and bottom of the nanopillars etched in the λ2 solution. Long isolated nanopillars without bending were observed after etching in the λ3 solution (Figure3c). The dependence of the bonding and bending phenomena on the value λ is more clearly seen in the tilted SEM images (Additional file1: Figure S4).
A charge transfer is required for the dissolution of Si, and hole (h+) injection is an important charge transfer process by MaCE. The electrochemical potential of H2O2 is much more positive than the valence band of Si, and hole injection from H2O2 into the valence band is energetically possible. However, the etching rate of H2O2/HF solution is very low (<10 nm/h), and the noble metal acts as catalyst for the hole injection and thereby improves the etching rate dramatically. Holes are generated at the Au surface by the cathode reaction and injected into the valence band of Si. Normally, the Si electronic bands will equilibrate by contacting the Si surface to the liquid solution and forming an energetic barrier to hinder the charge transfer across the Si/solution interface. Charge transfer is much easier at the metal/solution interface and the metal/semiconductor interface than at the semiconductor/solution interface. Besides, the work function of Au is close to the Femi level of p-type Si, and this also facilitates a charge transfer due to the quasi-ohmic Au/p-Si contact with low barrier.
By the anodic (or electrochemical) etching of Si in a HF-containing solution, electropolishing can be regarded as a reaction limited by the diffusion of HF, and electrochemical pore formation as a reaction limited by the charge supply from the electrode. The transition from the charge-supply-limited reaction to HF-diffusion-limited reaction is characterized by the critical current density Jps, and electropolishing requires high current densities in excess of Jps. In this work, the observations of polishing (marked as vertical etching of nanopillars or vertical movement of the Au film front) at the Au film front and pore formation in the formed nanopillars, underneath the Au film and on the metal-off back side of the Si, indicate that charge transfer took place at these sites (interface between the Au film and Si and interface between the Si and solution). In other words, the Au film serves as cathode, and the Si underneath the Au film, the Si pillars, and the back side of the Si wafers can be regarded as anodes. Charge transfer with the highest current density obviously takes place at the Au film front where the holes are generated.
At the Au film front, both polishing and pore formation occurred almost simultaneously for the highly doped Si. Maybe pore formation underneath the pillars is occurring even before polishing (Figure2d,f and Additional file1: Figure S2a,b). It is supposed that dopants serve as nucleation sites for pore formation, and the higher doping level leads to a larger thermodynamic driving force for pore formation in the p-type Si. The charge supply (hole injection) is dependent on the concentration of H2O2 by MaCE, as shown in Equation 1. In the λ1, λ2, and λ3 solutions with relative higher charge supply, only a thin porous base layer is observed (Figure2f and Additional file1: Figure S2a,b), and the polishing effect is very strong (indicated by the long pillar length as seen Figure8b). The thickness of the thin porous base layer is not homogenous, and a thicker layer was generally observed underneath the pillars, where the local current density is smaller than that directly under the Au film. As the molar ratio λ increases to 0.92 (λ4) with small H2O2 concentration, thick porous base layers (Figure3d) under the Au film front were observed in the highly doped Si. The current density at the Au film front is reduced by the limited charge supply, and thereby, the polishing is depressed and the formation of pores under the Au film front becomes more active. This is also confirmed by the smaller pillar length compared with pillars etched in the λ1, λ2, and λ3 solutions (as seen in Figure8b). A thick porous base layer was also observed under the Au film front after 3-min etching in the λ3 solution (Figure2a), while the thickness of the porous base layer is reduced with increasing etching time (Figure2d,f). The polishing effect becomes stronger after the first 3-min etching (Figure8a). The initial weak polishing effect and the active formation of a thick porous base layer are probably due to the depressed charge transfer by the native SiO2 layer and solid by-products of RIE between the Au film and the Si.
At the Au film front, only polishing occurred with the lightly doped Si (Figure4d,f) in the λ2 and λ3 solutions. Pore formation on the sidewalls of the pillars was followed by polishing. It can be imagined that a small amount of holes diffuses from the Au film to the outer surface of the nanopillars, leading to the formation of a nanoporous shell. The nanoporous shell is thicker at the upper side of the pillars (Figure4d,f) due to the longer time for pore formation at these positions than at the ‘fresh’ bottom. For the λ4 solution with small H2O2 concentration, the polishing effect was also suppressed (reduced pillar length in the λ4 solution as seen in Figure8b), and pore formation is active (as seen in Figure4g and7) due to the low current density. However, the thermodynamic driving force for pore formation is smaller in the lightly doped Si, and only few bundles of pores were observed (Figures4g and7). The transition from polishing to pore formation is more obvious in the lightly doped Si, while pore formation is much more active in the highly doped Si. The formation of lightly double-bent nanopillars (Figure4e) is probably due to the periodic depletion of H2O2 at the etching front (Au film front), and the corresponding periodic oscillations of the cathodic current can switch the etching directions. It is still unclear why the inhomogeneous etching occurred with lightly doped Si in the λ1 solution (Additional file1: Figures S5 and S6). However, this indicates that the current density was not homogenous over the whole Au film during etching in the λ1 solution.
The etching rate is dependent on the value of λ and reaches its maximum at λ = 0.7 for both lightly and highly doped Si (Figure8b). Chartier et al. have systematically studied the dependence of the etching rate on λ. As λ increases from small values to large values, the reaction changes from HF-concentration-controlled to H2O2-concentration-controlled, and the value of λ between 0.7 and 0.9 is optimized for high etching rate. The etching rate of highly doped Si is clearly higher than that of lightly doped Si, and this phenomenon was also observed in the work of Qu et al.. This is probably due to the higher current density in the highly doped Si. However, the etching rate in this work is clearly higher than that in the work of Qu et al., and this is because a much higher concentration of the total active chemicals in the solution (([HF] + [H2O2) / [H2O] = 1/5) was used here.
Higher oxygen coverage with higher amount of Si-O-Si bridge-bonded oxygen was observed when the H2O2 concentration is increased by the HF-H2O2 treatment on the Si surface. Nevertheless, the etching rate of naked Si (without metal coat) is smaller than 10 nm/h in HF/H2O2 solutions. The thinning or etching rate observed here is clearly higher than that value, indicating that the oxidation is a charge-transfer (or electrochemically)-aided process. The SEM image of the thinned top of the pillars (Additional file1: Figure S3) suggests that some oxides remain immediately after MaCE. This is also confirmed by the overcharge effect during SEM investigation. However, the pillar thinning or charge-transfer-aided oxidation occurs only in the solutions with high H2O2 concentrations. Pillar thinning was observed mainly at the top of the pillars because the H2O2 concentration is higher at the top than at the bottom. For the latter, most of the H2O2 is consumed for hole injection.
The pillar thinning was found to be always accompanied by pillar bonding and bending. The pillar surface will change from hydrophobic to hydrophilic when Si is oxidized. Therefore, the capillary force becomes more significant when the surface is coated with an oxide layer. Gas bubbles are formed by MaCE (as seen in Equation 2), and the liquid is disturbed locally by the gas bubbling. The surface-oxidized pillars then were bent due to capillary forces. When the top regions of some pillars come into contact, bonding occurs due to the charge-transfer-aided reaction. Both bending and bonding are so strong that fracture or cracking occurs by proceeding MaCE (Figure5). Besides that, a lower value of λ (or higher H2O2 concentration) for causing the effects of pillar thinning, bending, and bonding is required for highly doped Si. This is probably due to the higher etching rate and the corresponding higher consumption of H2O2 for highly doped Si.
In summary, the fabrication of ordered nanoporous Si nanopillar arrays with and without nanoporous base layers and ordered Si nanopillar arrays with nanoporous shells is demonstrated. Pore formation is much more active in the highly doped Si, and the transition from polishing to pore formation is much clearer in the lightly doped Si. Higher etching rates are observed in the Si with higher doping level. Pillar thinning and oxidation are only observed for etching in the solutions with small values of λ. Strong bonding and bending of the pillars occur when the surface of the pillars is oxidized. These results help in understanding the MaCE mechanisms. Furthermore, this synthesis has a potential for applications in optoelectronics, sensors, and Li-ion batteries.
DW is a staff scientist at TU Ilmenau. SD is a student at TU Ilmenau. AA is the head of the laboratory (Center for Micro- and Nanotechnologies) at TU Ilmenau. PS is a professor at TU Ilmenau. RJ is an application engineer at SÜSS MicroTec.
metal-assisted chemical etching
reactive ion etching
substrate conformal imprint lithography
scanning electron microscope.
The authors are grateful to Mrs. Manuela Breiter, Mrs. Birgitt Hartmann, Mrs. Ilona Marquardt, and Mr. Joachim Döll, all from Ilmenau University of Technology, for their help with the sample preparation. This work was partially supported by a grant (NanoBatt TNA VII-1/2012) from the state of Thuringia (TMWAT by LEG Thüringen) and co-financed by the European Union within the frame of the European Funds for Regional Development (EFRD).
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