The fabrication of large-scale sub-10-nm core-shell silicon nanowire arrays
© Su et al.; licensee Springer. 2013
Received: 21 June 2013
Accepted: 25 September 2013
Published: 1 October 2013
A combination of template-assisted metal catalytic etching and self-limiting oxidation has been successfully implemented to yield core-shell silicon nanowire arrays with inner diameter down to sub-10 nm. The diameter of the polystyrene spheres after reactive ion etching and the thickness of the deposited Ag film are both crucial for the removal of the polystyrene spheres. The mean diameter of the reactive ion-etched spheres, the holes on the Ag film, and the nanowires after metal catalytic etching exhibit an increasing trend during the synthesis process. Two-step dry oxidation and post-chemical etching were employed to reduce the diameter of the silicon nanowires to approximately 50 nm. A self-limiting effect was induced by further oxidation at lower temperatures (750°C ~ 850°C), and core-shell silicon nanowire arrays with controllable diameter were obtained.
KeywordsCore-shell silicon nanowire Polystyrene sphere Metal catalytic etching Self-limiting oxidation 81.07.Vb 81.16.Dn 81.16.He
Silicon is one of the most important semiconductor materials due to its crucial role in modern integrated circuit technology. However, the indirect bandgap structure restricts its future application in optoelectronics. Nowadays, silicon nanomaterials are regarded as promising candidates in various areas such as renewable energy[1–4], biological applications[5, 6], and chemical sensors[7–10]. It is also considered that silicon nanostructure, with diameter below the Bohr radius of silicon (4.3 nm), could conquer the physical disability of poor luminescence in bulk Si[11, 12]. Several silicon nanostructures, such as porous Si[13–15] and Si nanocrystals[16–18], have been widely studied in the past 20 years. However, little attention has been paid to the luminescence property of silicon nanowires (SiNWs) due to the difficulty of preparing nanowires with the diameter of several nanometers. It has been reported that vapor–liquid-solid (VLS) process is available for the achievement of nanoscale SiNWs[19, 20]. Yet, the luminescence stability is poor due to the surface termination conditions. In addition, it is difficult to avoid the creation of defects in the nanowires. Another typical method is a combination of electron beam lithography (EBL), reactive ion etching (RIE), and self-limiting thermal oxidation to fabricate sub-10-nm SiNWs[21–24]. It should be noted that this technique is expensive, and the aspect ratio is highly restricted.
In this paper, we demonstrate a technique based on a combination of template-assisted metal catalytic etching[25–28] and self-limiting oxidation to prepare large-scale core-shell SiNW arrays with an aspect ratio of more than 200:1 and the inner diameter of sub-10 nm. A careful discussion of the morphology and structure of the core-shell SiNW arrays is also included.
The diameter of the PS spheres was reduced via RIE, with an O2 flow rate of 40 sccm, pressure of 2 Pa, and applied radio frequency power of 50 W. Ag was sputtered onto the Si substrate, forming a porous Ag film as catalyzer. The PS sphere template was removed from the substrate by ultrasonication in ethanol. The porous Ag film-coated Si substrate was etched in the solution containing deionized water, HF, and H2O2 at 30°C. The concentrations of HF and H2O2 were 4.8 and 0.3 M, respectively. The retained Ag film was dissolved with nitric acid (1:1 (v/v) HNO3/H2O) for 5 min. The diameter of the as-prepared SiNWs was reduced by dry oxidation in a tube furnace at 1,050°C and post-chemical treatment to remove the oxide layer in the HF solution. At last, the SiNWs, with diameter around 50 nm, were oxidized at 800°C for 10 h. Due to the self-limiting effect, a core-shell structure with sub-10-nm single crystal SiNW was obtained.
The morphology of the SiNW arrays was analyzed using thermally assisted field-emission scanning electron microscope (FE-SEM, JEOL-JSM 7001F, Tokyo, Japan). Transmission electron microscopy (TEM, JEOL-JSM 2011) was further introduced to investigate the core-shell structure.
Results and discussion
The initial diameter of the PS spheres is also crucial for the chemical etching process. Excessive reduction of the sphere size by RIE would prevent the removal of the spheres and the metal catalytic etching. Decreasing the RIE time could avoid excessive reduction of the sphere diameter. However, the gap between the etched spheres would also be limited, leading to the size reduction of the porous Ag film. Figure 4c,d displays the morphology of the SiNW arrays employing PS spheres of 200 nm as the template. At the initial stage of the chemical etching, it is shown that the nanopillars are separated from each other. As the reaction proceeded, the slight dissolution of silver would gradually reduce the size of the porous Ag film, resulting in the increase of the nanowire dimension and, therefore, causing the root section of the nanowires to be connected. Thus, the PS spheres with initial mean diameter of 250 nm are the smallest commercially available spheres that can be used in this experiment.
As a fabrication method with so many steps, especially with the RIE step which fluctuates a lot, it is hard to obtain nanowire arrays of equal diameter for dry oxidation from every sample. This instability can be corrected by dry oxidation treatment. For each 3 cm × 3 cm silicon substrate, several 2 mm × 5 mm pieces would be cut down prior to the formal experiment to try out the proper oxidation time parameters through the abovementioned methods. Then, the tried-out parameters would be applied to the whole remaining sample. Figure 5h summarizes the dependence of the reduced diameter of the SiNWs on the oxidation time for samples with typical initial diameters.
In summary, this study illustrates a promising technique of preparing controllable single crystal SiNW arrays covering a large area. PS monolayer template was employed to prepare the nanoporous Ag film as catalyzer for the solution etching process, which would yield SiNW arrays. Two-step dry oxidation at 1,050°C reduced the nanowire diameter to around 50 nm while preventing nanowires from becoming sharp. Temperature is crucial for the self-limiting oxidation process. After oxidation at 800°C, the inner diameter of the core-shell SiNW arrays can be controlled below 10 nm within a tight tolerance. The fabrication process is easy to conduct and has good reproducibility. As the experiment was conducted top-down on single crystal silicon wafers, the SiNWs produced through this way have low defect concentration and consistent crystallography orientation. In addition, the core-shell structure guarantees their property stability in atmosphere. Since this technique combines functionality and economy, it is of high possibility to be applied to silicon-based optical devices in the future.
All authors belong to School of Materials Science and Engineering, Tsinghua University, People's Republic of China. SS is a master candidate interested in silicon-based light emission. LL is a Ph.D. candidate concentrating on semiconductor nanomaterials. ZL is an associate professor whose research fields include thin film material and nuclear material. JF is a professor working on thin film material and nanomaterials. ZZ is the school dean professor with research interest in nanostructures and SERS effect.
Electron beam lithography
Field-emission scanning electron microscope
High-resolution transmission electron microscopy
Reactive ion etching
Sodium dodecyl sulfate
Scanning electron microscopy
Transmission electron microscopy
The authors wish to thank Professor Joseph F. Chiang from the State University of New York and Professor Yiping Zhao from the University of Georgia for their kind advices and suggestions to this work. The Central Laboratory of Institute of Materials Science and Engineering, Tsinghua University and the National Center for Electron Microscopy (Beijing) are also gratefully acknowledged for supporting the analysis and characterization of the silicon nanowires in this work. The authors are grateful to the financial support by the National Basic Research Program of China (973 program, 2010CB832900 and 2010CB731600) and the National Natural Science Foundation of China (61076003 and 61176003).
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