Synthesis and Photoluminescence Properties of Porous Silicon Nanowire Arrays
© The Author(s) 2010
Received: 27 May 2010
Accepted: 26 July 2010
Published: 5 August 2010
Herein, we prepare vertical and single crystalline porous silicon nanowires (SiNWs) via a two-step metal-assisted electroless etching method. The porosity of the nanowires is restricted by etchant concentration, etching time and doping lever of the silicon wafer. The diffusion of silver ions could lead to the nucleation of silver nanoparticles on the nanowires and open new etching ways. Like porous silicon (PS), these porous nanowires also show excellent photoluminescence (PL) properties. The PL intensity increases with porosity, with an enhancement of about 100 times observed in our condition experiments. A “red-shift” of the PL peak is also found. Further studies prove that the PL spectrum should be decomposed into two elementary PL bands. The peak at 850 nm is the emission of the localized excitation in the nanoporous structure, while the 750-nm peak should be attributed to the surface-oxidized nanostructure. It could be confirmed from the Fourier transform infrared spectroscopy analyses. These porous SiNW arrays may be useful as the nanoscale optoelectronic devices.
Silicon with nanoscale has received much attention due to its potential applications on electronics, photonics, nanoscale sensors and renewable energy. Several silicon nanostructures, such as porous silicon (PS), silicon nanowires (SiNWs) and silicon nanocrystals, were proposed over the past decade. Due to their unique one-dimensional physical properties, SiNWs were explored for field effect transistors [1–4], chemical or biological sensors [5–9], battery electrodes [10, 11] and photovoltaics [12–14]. However, the application of silicon is still greatly restricted due to its indirect energy band gap, especially in the field of optically active material and optoelectronics. Silicon nanocrystals [15, 16] and PS [17, 18] are thought to be possible candidate systems in solving this physical inability and act as effective light emitters. PS is typically prepared by applying a voltage bias to a silicon substrate immersed in the ethanol and hydrofluoric acid mixture. The metal-assisted chemical etching process was also used to prepare PS  and SiNWs [20–24] as well. Few attempts were focused on the luminescence of SiNWs [25–29]. Recently, it is found that this method can be used to synthesize a new silicon nanostructure named Porous SiNWs [30, 31], which could combine the physical feature of SiNWs and PS. It is also possible to gain a large area uniform array controllable and repeatable. It is expected this could open a new opportunity for the silicon based optoelectronics and photoelectrochemical devices.
In this work, we synthesized porous SiNWs with different parameters, including the etchant concentration, etching time and post-treatment. The variable morphology of the SiNWs is present, and the etching mechanism is discussed. The photoluminescence (PL) properties dependent on the processing parameters are also investigated here.
SiNW arrays were prepared by Ag-assisted chemical etching of n-Si (100) wafers with the resistivity of about 0.02 Ω cm. The samples were firstly washed with acetone and deionized water and then immersed into H2SO4 and H2O2 solution in a volume ration of 3:1 to remove the organic contaminants on the surface. The thin oxide layer formed on the surface was then dissolved in a 5% HF solution. This treated wafer was transferred into an Ag deposition solution containing 4.8 M HF and 0.005 M AgNO3 for 1 min at room temperature. The Ag nanoparticles (AgNPs) coated samples were sufficiently rinsed with deionized water to remove extra silver ions and then soaked into an etchant bath. The HF concentration of the etching solutions is 4.8 M, while the H2O2 concentrations vary from 0.1 to 0.5 M. The etching times are 30, 60, 90, 120 and 180 min, respectively. The Ag metal was dissolved with nitric acid. Then, each sample was divided into two parts, one of which was immersed into 5% HF solution to remove the oxide layer induced by the nitric acid. Finally, the wafers were cleaned with water and dried under N2 flow.
The SiNW arrays were characterized by scanning electron microscopy (SEM) using JEOL JSM-6460LV, Thermally-Assisted Field Emission SEM (LEO 1530) and TEM (JEOL-200CX). The local atomic environments and bonding configurations in the samples were examined by Fourier transform infrared spectroscopy (FTIR) using Nicolet 6700. The PL measurements were conducted using an X Y triple spectrograph equipped with a liquid N2-cooled CCD camera. A 514.5-nm line Ar+ laser was employed to excite the luminescence with a spot size of about 5 μm in diameter and excitation power of 0.1 mW. All PL spectra were taken at room temperature.
Results and Discussion
The increase in H2O2 concentration could enhance the potential for the etching process, which indicates that the etching reaction is more thermodynamically favored and the etching could be accelerated. Therefore, the length of the nanowires is not only time dependent, but also relies on the oxidant concentration. Figure 3b shows that the length of SiNWs etched for 3 h is a bit lower than expected. This could be attributed to the serious conglomeration of the SiNWs.
Furthermore, we study the elementary PL intensity of the HF treated samples with different processing parameters. As is shown in Fig. 7, for the samples with lower porosity, the peak at 750 nm is stronger than the one at 850 nm. When the porosity increases, both the PL intensities increase. However, the emission intensity from the local nanoporous structure enhances more quickly and takes up the leading place. This is more obvious in Fig. 7c, the intensity of the 850-nm PL peak is twice as high as the peak at 750 nm for the 3-h-etched sample. This explains why the PL peaks of the HF-treated samples seem to “red-shift” with longer etching times or higher H2O2 concentrations.
In summary, we carried out electroless etching on the highly doped n-type silicon (100) wafers to synthesize the porous SiNW arrays. We found that longer etching time or higher H2O2 concentration could facilitate the diffusion and nucleation of Ag+ ions and effectively enhance the porosity of the nanowires. The PL intensity could be effectively enhanced by the increased porosity. Further studies including the decomposition of the PL spectrum and the FTIR analysis confirm that the surface of the HF-treated porous SiNWs are composed of Si–H x and Si–O bonds, corresponding to the peaks at 850 and 750 nm, respectively. The emission intensity from the local porous structure quickly enhances with the porosity and takes up the leading place of the PL spectrum, resulting in the “red-shift” observed. These porous SiNWs combine the physical properties of SiNWs and PS and could lead to opportunities for new generation of nanoscale optoelectronic devices.
This work was supported by Tsinghua National Laboratory for Information Science and Technology (TNList) Cross-discipline Foundation.
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