Fabrication of porous silicon nanowires by MACE method in HF/H2O2/AgNO3 system at room temperature
© Li et al.; licensee Springer. 2014
Received: 12 December 2013
Accepted: 17 February 2014
Published: 30 April 2014
In this paper, the moderately and lightly doped porous silicon nanowires (PSiNWs) were fabricated by the ‘one-pot procedure’ metal-assisted chemical etching (MACE) method in the HF/H2O2/AgNO3 system at room temperature. The effects of H2O2 concentration on the nanostructure of silicon nanowires (SiNWs) were investigated. The experimental results indicate that porous structure can be introduced by the addition of H2O2 and the pore structure could be controlled by adjusting the concentration of H2O2. The H2O2 species replaces Ag+ as the oxidant and the Ag nanoparticles work as catalyst during the etching. And the concentration of H2O2 influences the nucleation and motility of Ag particles, which leads to formation of different porous structure within the nanowires. A mechanism based on the lateral etching which is catalyzed by Ag particles under the motivation by H2O2 reduction is proposed to explain the PSiNWs formation.
Silicon nanowires (SiNWs) have widely attracted attention due to their unique physical and chemical properties and potential applications in optoelectronics , thermoelectrics [2, 3], energy conversion and storage [4–6], and biomedicine [7, 8]. Numerous methods have been developed to fabricate SiNWs including bottom-up or top-down technologies, such as vapor-liquid–solid growth [9, 10], solid–liquid–solid growth [11, 12], reactive ion etching , or metal-assisted chemical etching (MACE) . Compared with the other techniques, the MACE is a simple and low-cost method offering better structure controllability of silicon nanowire such as diameter, length, orientation, morphology and porosity, which, therefore, has attracted increasingly research interests in the past decade [5, 14, 15]. In principle, the MACE process includes two successive steps, the nucleation of metal catalysts and anisotropic etching, which are classified as the one-step and two-step MACE, respectively . In the one-step MACE (1-MACE), the two processes take place in an etching solution containing HF and metal salts. In the two-step MACE (2-MACE), metal catalysts are firstly deposited on the wafer surface, and the subsequent anisotropic etching occurs in the HF/oxidant (oxidant = H2O2[17, 18], Fe(NO3)3[19, 20] or KMnO4, etc.) solution.
Recently, the fabrications of one-dimensional silicon nanowires with porous structure using the MACE method have been given more wide attention. The emerging mesoporous silicon nanowires (MPSiNWs) open a new door to develop the wide applications derived from the enhanced surface areas and quantum confinement effect . The doped type and concentration, fabrication methods and etching temperature have an important effect on the morphology of silicon nanowire. Yang et al.  have reported that the MPSiNWs were fabricated by 1-MACE with highly doped p-type silicon at temperature of 25°C to 50°C. To et al.  reported that the MPSiNWs were also obtained by etching highly doped n-type silicon with the 1-MACE method. In addition, the 2-MACE was also often reported to fabricate PSiNWs [24–27]. In general, it has been found that the roughness of silicon nanowires is increased with increasing doped level and H2O2 concentration [24, 28]. For both MACE, the lightly doped silicon wafers are often difficult to obtain PSiNWs [22–27].
In the present work, the H2O2 oxidant was introduced into HF/AgNO3 etching solution for fabricating PSiNWs, which might be called ‘one-pot procedure’ MACE, it is practicable method for fabricating PSiNWs, even for lightly doped ones. The effect of doped level on nanostructure of SiNWs was studied. Meanwhile, the effects of H2O2 concentration on nanostructure of lightly doped SiNWs were also investigated. According to the experiment results, a model was proposed to describe the pore formation process.
The moderately and lightly doped p-type Si(100) wafers with resistivity of 0.01 ~ 0.09 and 10 ~ 20 Ωcm were respectively selected as the starting wafer. Prior to etching, the wafers were cut into 1 × 1 cm2, and then were cleaned by ultrasonication in acetone, ethanol, and deionized water, respectively. The clean silicon wafers were immersed into dilute HF solution to remove the native oxide layers and result in a hydrogen-terminated surface. The etching process was carried out by fixing the cleaned wafers in a plastic beaker which held the etchant solution containing 4.6 mol/L HF, 0.02 mol/L AgNO3, and H2O2 with different concentrations (0, 0.03, 0.1, 0.4, 0.8 mol/L). The etching was operated for 60 min under ambient temperature in the dark room. After etching, the samples were immediately dipped into 50 wt.% HNO3 to dissolve the as-generated Ag dendrites. Finally, the wafers were thoroughly rinsed with deionized water and dried by N2 blowing.
The physical morphology of SiNWs was characterized by scanning electron microscopy (SEM; QUANTA200, FEI, Hillsboro, OR, USA) and transmission electron microscopy (TEM; JEM-2100, JEOL, Akishima-shi, Japan). The crystallinity was studied by selected-area electron diffraction (SAED, integrated with JEM-2100 TEM). For the TEM, high-resolution TEM (HRTEM), and SAED analyses, SiNWs were scratched off from the substrates and spread into ethanol and then salvaged with copper grids. The characterizations were performed under the voltage of 200 kV.
Results and discussion
The lightly doped wafer was also selected as the starting material besides medially doped silicon substrate. The H2O2 plays an important role in fabricating SiNWs through the 2-MACE process, which affects not only the etching rate, but also the morphology, nanostructure, and orientation of SiNWs [24, 25, 30, 31]. Thus, in the HF/AgNO3/H2O2 system, the effect of H2O2 concentration on the nanostructure of lightly doped SiNWs was carefully studied in this part.
The SiNW length and etching rate evolution vs. H2O2 concentration were summarized, the etching rates were calculated according to the formula R = ∆m/dSiSt. The quantity of dissolved silicon (mass loss, ∆m) is obtained by weighting the silicon wafer before and after the etching, the density of silicon (dSi) is 2.33 g/cm3, the area of the wafer (S) is 1 × 1 cm2, and etching time (t) is 60 min; the results were shown in Figure 3H. A nonmonotonic trend in SiNW length evolution with increasing H2O2 concentration is observed, and which belies the monotonic increasing etching rate. It is caused by the increasing top lateral etching with increasing H2O2 concentration.
This work has demonstrated a simple MACE method for successfully fabricating lightly doped porous silicon nanowires at room temperature. The effects of H2O2 concentration on nanostructure of moderately and lightly doped SiNWs were investigated. The results indicate that the concentration of H2O2 influences the nucleation and motility of Ag particles, which leads different porous structure within the nanowires. In the HF/AgNO3/H2O2 etching system, the H2O2 species replaces Ag+ as the oxidant and the Ag nanoparticles work as catalyst during the etching. A mechanism based on the lateral etching which is catalyzed by Ag particles with the motivation of H2O2 reduction is proposed to explain the formation of PSiNWs. The simple etching system not only synthesizes large-scale moderately doped single crystalline PSiNWs, but can also fabricate lightly doped ones, which can open up exciting opportunities in a wide range of applications. For example, the vertically aligned nanowires with a high surface area can be exploited as a high-capacity electrode for supercapacitors. The deep quantum confinement effect and biodegradability feature of the porous silicon nanowires may enable interesting applications in optoelectronics and drug delivery.
Financial supports of this work from the Specialized Research Fund for the Doctoral Program of Higher Education of China (20135314110001) and the Program for Innovative Research Team in University of Ministry of Education of China (IRT1250) were gratefully acknowledged.
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