⇦Chinese Physics Letters, 2017, Vol. 34, No. 2, Article code 025202 Growth of Single-Crystalline Silicon Nanocone Arrays by Plasma Sputtering Reaction Deposition * Zhi-Cheng Wu(巫志城), Lei-Lei Guan(关雷雷), Hui Li(李惠), Jia-Da Wu(吴嘉达), Jian Sun(孙剑), Ning Xu(许宁)** Affiliations Department of Optical Science and Engineering, Fudan University, Shanghai 200433 Received 27 October 2016 *Supported by the National Basic Research Program of China under Grant No 2012CB934303, the Natural Science Foundation of Shanghai under Grant No 15ZR1403300 and the National Natural Science Foundation of China under Grant No 11275051.
**Corresponding author. Email: ningxu@fudan.edu.cn Citation Text: Wu Z C, Guan L L, Li H, Wu J D and Sun J et al 2017 Chin. Phys. Lett. 34 025202 Abstract Vertically aligned single-crystalline silicon nanocone (Si-NC) arrays are grown on nickel-coated silicon (100) substrates by a novel method i.e., abnormal glow-discharge plasma sputtering reaction deposition. The experimental results show that the inlet CH$_{4}$/(N$_{2}$+H$_{2}$) ratio has great effects on the morphology of the grown Si-NC arrays. The characterization of the morphology, crystalline structure and composition of the grown Si-NCs indicates that the Si-NCs are grown epitaxially in the vapor–liquid–solid mode. The analyses of optical emission spectra further reveal that the inlet methane can promote the growth of Si-NCs by raising the plasma temperature and enhancing the ion-sputtering. The understanding of the growth mechanism of the Si-NC arrays will be helpful for fabrication of required Si-NC arrays. DOI:10.1088/0256-307X/34/2/025202 PACS:52.80.Tn, 81.15.Cd, 82.33.Xj © 2017 Chinese Physics Society Article Text Since silicon nanocone (Si-NC) arrays (also known as black silicon) were fabricated by femtosecond pulsed laser etching of silicon wafers by Her et al. in 1998,[1] they have attracted much attention in the field of photoelectric sensors and novel solar cells for their geometric-dependent antireflectance.[2-9] The reflectance of Si-NC arrays can be only about 10% over the wavelength range from 350 to 2000 nm.[5,10] The power conversion efficiency of hybrid Si-NC/polymer solar cells can attain 9.62% (quite high among hybrid Si/polymer solar cells), increasing by 48.7% from the planar to nanocone solar cell.[5] The Si-NCs also gained plentiful interest for their potential applications in field-emission devices and scanning probe tips.[11-13] So far, several methods, such as reactivation etching,[5,6] chemical etching,[8,11] ion-sputtering,[10] and microwave plasma chemical vapor deposition,[14,15] have been used to fabricate the Si-NC arrays for the possible applications. In this study, vertically aligned single-crystalline Si-NC arrays were fabricated by a novel way, i.e., the abnormal glow-discharge plasma sputtering reaction deposition (PSRD) method. This method combines highly dense plasma environment with proper bias sputtering. In addition, we are concerned that the flow of methane has an important influence on the growth of Si-NCs. This phenomenon had also occurred in other reports using the plasma-assisted chemical vapor deposition for preparation of black silicon.[15,16] Therefore, the essential role of methane in the process of the Si-NC growth was analyzed by measuring the emission spectrum of the abnormal-glow discharge plasma and by calculating the electron temperatures in the plasmas with different methane flows through the double-line method. There are two steps in preparation of the Si-NC arrays by PSRD. First, 100-nm-thick Ni films (as catalyst) were deposited on scratched Si (100) substrates by pulsed laser deposition. The wavelength, pulse energy, repetition of the used Nd:YAG laser were 532 nm, 50 mJ and 10 Hz, respectively. Secondly, the Si-NC arrays were fabricated on the above Ni-coated substrates by PSRD. In this step, the Ni-coated substrates (as cathode) were placed just below the tapered tungsten tip (as anode).[17] Before the discharge, the CH$_{4}$, N$_{2}$ and H$_{2}$ mixed gas [N$_{2}$:H$_{2}$=1:10, CH$_{4}$/(N$_{2}$+H$_{2}$)=1/5–1/40] was introduced into the discharge chamber. The abnormal glow discharge lasted 45 min for all the samples with the discharge voltage and current of 350 V and 180 mA, respectively. The morphology of all the samples was examined by field emission scanning electron microscopy (FESEM, Hitachi S-4800). Transmission electron microscopy (TEM, JEOL2010) was performed on a single Si-NC and a thin slice cut from a selected Si-NC by focused ion beam (FIB, Helios Nanolab 600i). The crystalline structures and composition of the Si-NCs were characterized by selected area electron diffraction (SAED), micro area electron diffraction (MAED) and energy-dispersive spectroscopy (EDX, Oxford ENCA-6498) fitted within the JEOL2010. The optical emission spectra (OES) from the abnormal glow discharge plasmas were measured by a spectrometer (Acton Reseglowh, Spectra Pro 500i) and detected by a gated intensified charge-coupled device (Andor Technology, iStar DH720). The FESEM images of the nanocone arrays grown at the CH$_{4}$/(N$_{2}$+H$_{2}$) ratios of 1/5–1/40 are shown in Fig. 1. By comparison, it is obvious that the CH$_{4}$/(N$_{2}$+H$_{2}$) ratio has great effects on the morphology of the grown nanocones. Vertically aligned nanocones with perfect cone shapes were grown on the substrate at the CH$_{4}$/(N$_{2}$+H$_{2}$) ratios of 1/10 and 1/20 (Figs. 1(b) and 1(c)). The heights and conical angles of the grown nanocones are about 2–3 μm and 20$^{\circ}$–22$^{\circ}$, respectively. As the CH$_{4}$/(N$_{2}$+H$_{2}$) ratio is 1/5 or 1/40, the appearance of the grown nanocones deteriorates and their sizes also decrease, which implies that the morphology and size of the grown nanocones are strongly impacted by the methane flow.
cpl-34-2-025202-fig1.png
Fig. 1. FESEM images of the nanocone arrays grown at different CH$_{4}$/(N$_{2}$+H$_{2}$) ratios of (a) 1/5, (b) 1/10, (c) 1/20 and (d)1/40.
cpl-34-2-025202-fig2.png
Fig. 2. (a) FESEM image of the nanocone grown at the CH$_{4}$/(N$_{2}$+H$_{2}$) ratio of 1/10, (b) TEM morphology, (c) SAED patterns, and (d) EDX analytical histograms of a slice cut from the selected nanocone by FIB.
To further determine the composition and structure of the grown nanocones, the nanocones grown at the CH$_{4}$/(N$_{2}$+H$_{2}$) ratio of 1/10 were characterized by TEM (Fig. 2). The FIB technique was used on the TEM sample preparation. In the FIB process, a layer of Pt was first deposited on the nanocones to protect them, then a high-current ion beam was applied to cut a selected Pt-protected nanocone gradually thin into a 10-nm-thick slice (Fig. 2(b)). Figures 2(c) and 2(d) show the SAED pattern and the EDX analysis result of the selected nanocone. It could be obtained from Figs. 2(c) and 2(d) that the grown nanocone is of single-crystalline cubic silicon (c-Si) structure along the [100] axis and its silicon atom percentage reaches about 92%. The agreement of the crystal orientations of the grown Si-NC and Si (100) substrate indicates that the Si-NCs were grown epitaxially from the substrate.
cpl-34-2-025202-fig3.png
Fig. 3. TEM morphology of an intact nanocone scraped from the sample prepared at the CH$_{4}$/(N$_{2}$+H$_{2}$) ratio of 1/10 and the corresponding MAED patterns and EDX analytical histograms at different places along the cone axis.
cpl-34-2-025202-fig4.png
Fig. 4. (a) OES of the abnormal glow discharge plasma with different CH$_{4}$/(N$_{2}$+H$_{2}$) ratios, (b) the measured intensities of H$_{\gamma}$ and H$_{\alpha}$ peaks, and (c) electronic temperature trend chart.
Because the slice cut by FIB just comes from a cross section of the Si-NC (deviating from its central axis), the structure and composition information of the whole nanocone could not be obtained from such a slice. To obtain such information, the intact Si-NCs were scraped from the substrate by a steel knife for TEM observation. MAED (the diameter of analytical electron beam is only a few nanometers) and EDX were conducted to analyze the structures and composition of a selected Si-NC. Figure 3 gives the MAED patterns and EDX analysis results at different places of the Si-NC body. It could be found in Fig. 3 that the nanocone body from the bottom to the middle is of single-crystalline c-Si structure with the [100] orientation while an unidentified mixed-crystal-bitmap appears on its top. Only silicon is the main element at the bottom and middle of the nanocone with the atomic percentage of about 95%, while both nickel and silicon are the main elements at its top with the atomic percentages of 43% and 48%, respectively. Much nickel existence in the tip indicates the typical vapor–liquid–solid (VLS) growth mode of the Si-NC.[18] The Si-NC arrays should be grown epitaxially in the VLS mode. The growth process could be divided into three stages: (1) the catalyst melt and fragment into separated spheres under the heating of the discharge plasma. (2) The Si-NC infants are formed as the silicon atoms produced by the sputtering of nitrogen and hydrogen ions on the silicon substrate attain the catalyst spheres. (3) The Si-NC infants grow larger and larger and their cone-angles become sharper and sharper under the bombardment of the ions in the discharge plasma. It could be found from Fig. 1 that the inlet of methane has significant influences on the growth of the Si-NC arrays. In this method, the Si-NCs could not grow out without methane inlet. To obtain the role of methane in the growth of the Si-NCs, OES measurements were conducted to analyze the contents of the glow-discharge plasma. The OES spectra in the range of 430–660 nm at different CH$_{4}$/(N$_{2}$+H$_{2}$) ratios of 1/5, 1/10, 1/20 and 1/40 are displayed in Fig. 4(a). The peaks located at 656.0, 487.5, 436.4 and 471.8 nm are the main excitation peaks, which are attributed to H$_{\alpha}$, H$_{\beta}$, H$_{\gamma}$ and C$^{2+}$, respectively. It has been known that plasma temperature is an important influencing factor in the VLS mode. Thus the electron temperatures of the plasma with different CH$_{4}$/(N$_{2}$+H$_{2}$) ratios were calculated with the double-line method.[19] In the method, the intensity relationship of the two spectral lines of the same ion or atom can be expressed by $$\begin{align} \frac{I_1 }{I_2 }=\frac{A_1 g_1 \lambda _2 }{A_2 g_2 \lambda _1 }\exp \Big[\frac{E_1 -E_2}{kT_{\rm e} }\Big],~~ \tag {1} \end{align} $$ where $I$, $A$, $g$, $\lambda$, $E$ and $T_{\rm e}$ are the radiation intensity of spectral line, transition probability, statistical weight of relevant energy level, wavelength, excitation energy and electron temperature, respectively. Here the two atom peaks of H$_{\gamma}$ and H$_{\alpha}$ are chosen as the standby lines to calculate the electron temperature. For H$_{\gamma}$ and H$_{\alpha}$, $\lambda_{1}$, $\lambda_{2}$, $E_{1}$, $E_{2}$ $g_{1}$, $A_{1}$, $g_{2}$, and $A_{2}$ are 436.4 nm, 656.0 nm, 2.84 eV, 1.89 eV, $9\times10^{4}$, $4.9484\times10^{6}$, $5\times10^{5}$ and $5.3877\times10^{7}$, respectively. Figure 4(b) gives their intensities ($I_{1}$ and $I_{2}$) varying with CH$_{4}$/(N$_{2}$+H$_{2}$) ratios. It could be found from Fig. 4(c) that the calculated electron temperature rises with the increase of the inlet methane. This tendency may be explained such that the methane in the discharge plasma can be ionized into multiple carbon particles (C$^{+}$, CH$_{n-}$, etc.), hydrogen ions and electrons, thus increasing the collisions of various particles among the plasma and elevating the electron temperature of the plasma. The introduction of methane will increase the amount of hydrogen ions in the discharge plasma, which can increase the ion-sputtering to the silicon substrate. The electron temperature rise may also have an impact on the growth of the Si-NCs, in that the VLS growth is sensitive to the environmental temperature. It is considered that too low inlet methane (for example CH$_{4}$/(N$_{2}$+H$_{2}$) of 1/40 in Fig. 1(d)) will lead to the lack of the sputtered silicon atoms for the growth of the Si-NCs and too high inlet methane (for example CH$_{4}$/(N$_{2}$+H$_{2}$) of 1/5 in Fig. 1(a)) will lead to too strong ion-bombardment on the growing nanocones, which is also not conducive to the growth of the Si-NCs. Therefore, the perfect Si-NCs can only grow at proper CH$_{4}$/(N$_{2}$+H$_{2}$) ratio (for example, CH$_{4}$/(N$_{2}$+H$_{2}$) of 1/20–1/10 in Figs. 1(b) and 1(c)). In conclusion, the vertically aligned Si-NC arrays with perfect shapes and smooth surfaces have been fabricated successfully by the PSRD method. The Si-NCs are grown epitaxially in the VLS mode. The OES analysis illustrates that the inlet methane can promote the growth of Si-NCs by raising the plasma temperature and by enhancing the ion-sputtering. The understanding of the growth mechanism of Si-NC arrays will be helpful for fabrication of required Si-NC arrays so as to make full use of this kind of material. 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