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Structural and optical characteristics of silicon nanowires fabricated by wet chemical etching
Chemical Physics Letters 511 (2011) 106–109
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Structural and optical characteristics of silicon nanowires fabricated by wet chemical etching Meiguang Zhu, Xuejiao Chen, Zhiliang Wang, Yun Chen, Dianfei Ma, Hui Peng, Jian Zhang ⇑ Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China
a r t i c l e
i n f o
Article history: Received 2 January 2011 In final form 7 June 2011 Available online 13 June 2011
a b s t r a c t Array-ordered silicon nanowires (SiNWs) were fabricated directly on p-Si substrate by wet chemical etching. The as-prepared SiNWs apparently were composed of a single-crystalline Si core embedded in an amorphous SiO2 shell (5 nm). Raman spectra indicated that the surface of as-prepared SiNWs contained a collection of smaller Si crystalline nanograins. The characteristic peaks induced by Si nanograins were observed in the Raman and photoluminescence (PL) spectra due to the quantum confinement effect. The study revealed that the array-ordered SiNWs would have a great potential of application in nanoscale electric and optoelectronic devices by controlling the fabrication processes. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Silicon nanowires (SiNWs) are considered to be the promising building block for the next generation’s nanometer-scale optical nanodevices due to their interesting size- and shape-dependent properties [1,2]. It is believed that silicon nanowires with quantum wires diameter can emit visible light [3,4]. So far lots of attention has been paid to understand the photoluminescence (PL) mechanism of silicon nanowires. Several models have been proposed to explain the PL behavior of SiNWs [3,5–8]. In the most popular theory suggested by Canham [3], light emission of SiNWs is attributed to the quantum confinement effect. In addition, the surface states or surface related species are also reported to be responsible for the luminescence processes [7]. Since the band-gap energy will be expanded due to the quantized levels and the significant modulation of the usual electronic band structure in the band gaps, quantum confinement theory leads to the expectations that the visible PL is possible and the radiative efficiency at room temperature can be enhanced. The most pressing issue at present is to setup the relationship between the luminescent properties and the structural characteristics of Si nanostructures. The wet-etching method has attracted intensive research efforts due to its simple experimental condition and efficiency to prepare nanostructured material [9–12]. In addition, silicon nanowires can be integrated with other micro/nano-devices. Therefore, these characteristics would lead to cheap SiNWs-based optoelectronics. However, in fact, the fabricated SiNWs by wet chemical etching had a large diameter distribution, ranging from dozens to hundreds nanometers [11,12]. The diameters of most nanowires are larger ⇑ Corresponding author. Fax: +86 21 54345119. E-mail address: [email protected] (J. Zhang). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.06.013
than the critical dimension in which the quantum confinement effect occurs. In most cases, SiNWs are not formed with the perfect lattice on the whole during the HF etching process [8]. Therefore, it becomes more important to understand the structural characteristics of the large scale SiNWs with diameters in the dozens of nms range and their light emission mechanism. To the best of our knowledge, works related to this topic are seldom reported. In the present work, a simple, low cost wet chemical etching method was used to fabricate the large-scaled SiNWs. The structural characterizations of SiNWs were examined by X-ray diffraction (XRD), transmission electron microscope (TEM) and high resolution transmission electron microscopy (HRTEM). The optical properties of SiNWs were studied by Raman scattering spectroscopy and photoluminescence spectroscopy at room temperature. The light emission mechanism was investigated by analyzing the PL spectra of SiNWs oxidized by rapid thermal annealing at 1000 °C in O2 for 0, 2, 5, 10 and 15 min, respectively.
2. Experimental 2.1. Silicon nanowires fabrication Double-side polished p-type silicon-wafers (with <1 0 0> orientation and 0.1–10 X cm resistivity) were used as the substrate for the preparation of SiNWs. Before etching, the silicon wafers were cleaned carefully via standard RCA process. Then the cleaned silicon wafers were placed into a Teflon etching container which contained a mixture of 35 mM AgNO3 and 15 mM HF. The silicon wafers were etched for 60 min at room temperature under 1 atm. After etching, the samples were taken out and washed with concentrated HNO3 to remove surface byproduct Ag nanodendrites.
M. Zhu et al. / Chemical Physics Letters 511 (2011) 106–109
Then the samples were rinsed with DI water and dried with nitrogen carefully for further study. The main factors which can influence the SiNWs formation were AgNO3 concentration, HF concentration and the etching time, which affected not only the formation of SiNWs but also the surface morphology of the etched silicon wafers. The effects of these factors on the formation of SiNWs can be found in our previous paper in details [13]. According to our previous work, if the experiment parameters were fixed, SiNWs with the same structural characterizations can be nicely reproduced. 2.2. Silicon nanowires characterization The structural and morphological properties were investigated by transmission electron microscope (TEM) and high resolution transmission electron microscopy (HRTEM) by using JEM-2100 (JEOL). The crystalline structures of silicon nanowires were analyzed by X-ray diffraction (XRD) using CuKa radiation (D/MAX2550 V, Rigaku Co.). Raman spectra were obtained by using a micro-Raman spectrometer (Jobin–Yvon LabRAM HR 800 UV) with a 632.8 nm He–Ne laser as the excitation light at room temperature. PL spectra were recorded with the same spectrometer, but the excitation light used is a He–Cd laser with a wavelength of 325 nm and the excitation power of 30 mW at room temperature. 3. Results and discussion 3.1. Structural characteristics Figure 1 showed the TEM image of single SiNW. As can be seen from Figure 1a, the fabricated nanowire with a diameter of 50 nm was relatively homogeneous and smooth. The average length of asprepared SiNWs was approximately 60 lm and the diameter ranged from 40 to 100 nm. Figure 1b was the HRTEM image of SiNWs which implied the SiNWs had a core/shell structure. The result of the selected area electrical diffraction (SAED) analysis further confirmed that the as-prepared single SiNW was formed by a SiNW core encapsulated by a SiO2 shell. The thickness of SiO2 shell was about 5 nm, which was due to the oxidation of Si in air. 3.2. Optical characteristics The Raman spectroscopy had been used to characterize the ptype SiNWs and the p-type Si wafers used for SiNWs growth (denoted as the Si substrates) for comparison purpose. It is found that the laser power influenced the Raman results remarkably. The effect of laser heating (homogeneous or not) on Raman spectroscopy should be considered [14,15]. Figure 2 showed the SiNWs Raman spectrum evolution as a function of the excitation power with an excitation wavelength of 632.8 nm.
107
As shown in Figure 2a, no peak shift was found when the power increased from 0.03 to 0.3 mW. When the power reached 0.75 mW, the peak shifted to the lower frequencies and broadened. When the power further increased to 1.5 mW, the Raman spectra were changed significantly, possibly due to the sample radiation damages occured during the high-energy irradiation [16,17]. While, the experimental results of p-Si substrates illustrated that the peak position was almost independent of the applied laser power, just as shown in Figure 2b. So in this study, the laser power applied was in the range from 0.03 to 0.3 mW in order to eliminate its influence on the silicon nanowires. Figure 3 presented the Raman spectra of the SiNWs and the p-Si substrates obtained with the excitation power of 0.3 mW. For the p-Si substrates, a Raman peak at 520.7 cm 1 can be seen clearly, as shown in Figure 3a. This is due to the scattering of the first-order transverse optical phonon (TO) of crystalline silicon. In addition, two broad peaks at 300 and 945 cm 1 were also found, which are from the scattering of two transverse acoustic (2TA) phonons and two transverse optical (2TO) phonons, respectively [18]. The Raman spectrum of SiNWs in Figure 3a showed the prominent Raman peak at 508.4 cm 1. Similar to that of p-Si substrates, two broad peaks at 290 and 928 cm 1 were also visible. Besides these, another two new peaks appeared at 422 and 610 cm 1. Compared with p-Si substrates, the TO peak in SiNWs shifted from 520.7 to 508.4 cm 1 and significantly broadened. The intensities increased and the peak became asymmetric, as shown in Figure 3b. For 2TO and 2TA modes, the increased intensities were also observed for the SiNWs samples. These results are in agreement with those reported by Li et al. [19] and Sui et al. [20], in which SiNWs with dimension of 50 nm and porous silicon with a porosity of 80% and thickness of 100 lm, were investigated, respectively, and confirmed that there existed a collection of smaller crystalline grains with typical dimension of a few nms. The typical Raman spectra for SiNWs can be ascribed to the quantum confinement effect of Si nano-crystalline. The diameter of fabricated SiNWs in this study was larger than the critical dimension in which the quantum confinement effect occurs. However, SiNWs formed by HF etching were not formed by the perfect lattice and had a high density of microstructural defects, such as point defects, dislocations, voids, surface protrusions and irregularities [2]. These defects will break the central silicon region into the smaller nanograins. Therefore, in our case the fabricated SiNWs would also be consisted of a collection of smaller crystalline nanograins due to the presence of abundant structure defects. The effect of the surface oxide layer was also investigated to understand the PL mechanism of SiNWs. The as-prepared SiNWs was treated by dipping into 5% aqueous hydrofluoric acid solution (HF) for 10 s to remove the surface silicon oxide layer. After HF immersion, the SiNWs surface was passivated by hydrogen to form the hydrogen (H)-terminated Si surface, which was particularly
Figure 1. TEM image of single Si nanowire: (a) typical TEM image, (b) HRTEM image (the insets are the corresponding SAED pattern of the part framed in panel).
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M. Zhu et al. / Chemical Physics Letters 511 (2011) 106–109
Figure 2. Raman spectra of the (a) SiNWs and (b) p-Si wafers recorded under an excitation wavelength of 632.8 nm with different laser power applied.
Figure 3. (a) Comparison of Raman spectra in the range of 200–1100 cm 1 of SiNWs and p-Si wafers measured under an excitation laser wavelength of 325 nm with a power of 0.3 mW. (b) Partial enlarged comparison of Raman spectra in the range of 480–530 cm 1.
resistant to air oxidation [21–23]. For example, a nearly perfect Hterminated Si(111) surface is reported to be stable for several days in air [23]. The HF-treated SiNWs samples were carefully kept under nitrogen atomosphere. Figure 4 gave the corresponding X-ray
Si(400)
Intensity (a.u)
Si(200)
SiO2
1 2
0
20
40
60
80
100
2Theta(deg) Figure 4. X-ray diffraction (XRD) spectra of: (1) the as-prepared SiNWs and (2) the HF-treated SiNWs.
Figure 5. PL spectra of (a) the as-prepared SiNWs and (b) the HF-treated SiNWs.
diffraction (XRD) spectra of SiNWs before and after HF treatment. In the XRD spectrum of HF-treated SiNWs, the broad SiO2 peak centered at about 23° vanished. It illustrated that SiO2 no longer existed in SiNWs after the HF treatment. Figure 5a presented the PL spectrum of the as-prepared SiNWs. Two broad PL peaks were clearly distinguishable at 525 and 424 nm, respectively. Figure 5b showed the PL spectrum of HF-treated SiNWs. In comparison with the PL spectra of the as-prepared SiNWs, the corresponding peak position and intensity remained almost unchanged, indicating silicon oxide layer had little effect on the emission properties of SiNWs. Therefore, we proposed that these two PL peaks for the as-prepared SiNWs sample should be attributed to the Si nanograins existed in the SiNWs. To further confirm the relationship between these two PL peak and the nanograins, rapid thermal annealing (RTA) was used to oxidize the as-prepared SiNWs at 1000 °C in O2 for 0, 2, 5, 10 and 15 min, respectively. Figure 6 showed the corresponding PL results. A novel blue shift behavior can be observed from the PL spectra as the oxidation time increased. When the oxidation time were set as 2, 5, 10 and 15 min, the peak originally located at 525 nm shifted to 521, 518, 513 and 494 nm, respectively. Besides the blue shift, the intensity of this peak increased with the increase of the oxidation time, especially for the SiNWs sample treated for 15 min. Meanwhile, the peak located at 424 nm gradually vanished as the oxidation time increased. The PL behavior of SiNWs could be explained from the following aspects: (i) The blue shift was due to the band-gap increase. The size of nanograins decreased during the oxidation process. Therefore, the smaller sizes of nanograins
M. Zhu et al. / Chemical Physics Letters 511 (2011) 106–109
109
Si crystalline nanograins. The PL mechanism of SiNWs was investigated by the rapid thermal annealing of as-prepared SiNWs for different time. A blue shift of the emission peak originally located at 525 nm was observed after the thermal annealing. It was mainly due to the decrease in the size of nanograins caused by the oxidation. We suggested that the quantum confinement (QC) effect was responsible for the observed Raman and photoluminescence spectra of the as-prepared SiNWs. Acknowledgements We greatly appreciate the financial supports of the National Natural Science Foundation of China (Grant Nos. 60672002, 61076070, 60976004) and Innovation Program of Shanghai Municipal Education Commission (Grant No. 09ZZ46). This work is also supported by Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT). References [1] [2] [3] [4] [5] [6] [7] [8] [9] Figure 6. PL spectra of the SiNWs oxidized by rapid thermal annealing at 1000 °C in O2 for 0, 2, 5, 10 and 15 min, respectively.
induced the increment of the band-gap directly. (ii) The intensity increasing was attributed to the quantum confinement effects which enhances the oscillator strength of the direct optical transition in Si crystallites. The number of nanograins in which the quantum confinement occurs increased and lead to the increased PL signal. On the other hand, some of SiNWs with core diameter reaching quantum confinement condition can be generated during the oxidation treatment processes, resulting in the enhancement of the PL intensity in oxidation-treated sample. (iii) The disappearance of the peak at 424 nm should be ascribed to the complete oxidization of the corresponding Si nanograins during the oxidation treatment processes. 4. Conclusions The large-scaled array-ordered SiNWs with average diameter of 70 nm were fabricated by wet chemical etching method. The results of Raman and photoluminescence (PL) spectra indicated that the fabricated SiNWs were consisted of a collection of smaller
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
D. Ma, C.S. Lee, F.C.K. Au, S.Y. Tong, S.T. Lee, Science 299 (2003) 1874. R. Gupta, Q. Xiong, C.K. Adu, U.J. Kim, P.C. Eklund, Nano Lett. 3 (2003) 627. L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. G.D. Sanders, Yia-Chung Chang, Phys. Rev. B 45 (1992). Y. Kanemitsu, T. Ogawa, K. Shiraishi, K. Takeda, Phys. Rev. B 48 (1993) 4883. P.C. Searson, J.M. Macaulay, S.M. Prokes, J. Electrochem. Soc. 139 (1992) 3373. D.P. Yu, Q.L. Hang, Y. Ding, H.Z. Zhang, Z.G. Bai, J.J. Wang, et al., Appl. Phys. Lett. 73 (1998) 3076. A.I. Hochbaum, D. Gargas, Y.J. Hwang, P.D. Yang, Nano Lett. 9 (2009) 3550. E.A. Dalchiele, F. Martín, D. Leinen, R.E. Marotti, J.R. Ramos-Barrado, Thin Solid Films 518 (2010) 1804. Teng Qiu, Paul K. Chu, Mater. Sci. Eng. R 61 (2008) 59. Dinesh Kumar, Sanjay K. Srivastava, P.K. Singh, M. Husain, Vikram Kumar, Sol. Energy Mater. Sol. Cells 95 (2011) 215. Xianzhong Sun, Ran Tao, Linhan Lin, Zhengcao Li, Zhengjun Zhang, Jiayou Feng, Appl. Surf. Sci. 257 (2011) 3861. L.J. Wan, W.L. Gong, K.W. Jiang, H.L. Li, B.R. Tao, J. Zhang, Appl. Surf. Sci. 254 (2008) 4899. S. Hofmann, C. Ducati, R.J. Neill, S. Piscanec, A.C. Ferrari, J. Geng, et al., J. Appl. Phys. 94 (2003) 6005. S. Piscanec, A.C. Ferrari, M. Cantoroa, S. Hofmanna, J.A. Zapienb, Y. Lifshitz, et al., Mater. Sci. Eng. C. 23 (2003) 931. U. Ritter, P. Scharff, C. Siegmund, O.P. Dmytrenko, N.P. Kulish, Yu.I. Prylutskyy, et al., Carbon 44 (2006) 2694. Armida Torreggiani, Maurizio Tamba, Immacolata Manco, Maria Rosaria, Faraone-Mennella, et al., J. Mol. Struct. 744-747 (2005) 767. S.L. Zhang, X. Wang, K.S. Ho, J.J. Li, P. Diao, S.M. Cai, J. Appl. Phys. 76 (1994) 3061. B.B. Li, D.P. Yu, S.L. Zhang, Phys. Rev. B 59 (1999) 1645. Z. Sui, P.P. Leong, I.P. Herman, G.S. Higasni, X. Temkin, Appl. Phys. Lett. 60 (1992) 2085. E. Yablonovitch, D.L. Allara, C.C. Chang, T. Gmitter, T.B. Bright, Phys. Rev. Lett. 57 (1986) 249. G.S. Higashi, Y.J. Chabal, G.W. Trucks, K. Raghavachari, S.B. Christman, Appl. Phys. Lett. 56 (1990) 656. Y. Kim, M. Lieber, J. Am. Chem. Soc. 113 (1991) 2333.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Structural and optical characteristics of silicon nanowires fabricated by wet chemical etching Meiguang Zhu, Xuejiao Chen, Zhiliang Wang, Yun Chen, Dianfei Ma, Hui Peng, Jian Zhang ⇑ Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China
a r t i c l e
i n f o
Article history: Received 2 January 2011 In final form 7 June 2011 Available online 13 June 2011
a b s t r a c t Array-ordered silicon nanowires (SiNWs) were fabricated directly on p-Si substrate by wet chemical etching. The as-prepared SiNWs apparently were composed of a single-crystalline Si core embedded in an amorphous SiO2 shell (5 nm). Raman spectra indicated that the surface of as-prepared SiNWs contained a collection of smaller Si crystalline nanograins. The characteristic peaks induced by Si nanograins were observed in the Raman and photoluminescence (PL) spectra due to the quantum confinement effect. The study revealed that the array-ordered SiNWs would have a great potential of application in nanoscale electric and optoelectronic devices by controlling the fabrication processes. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Silicon nanowires (SiNWs) are considered to be the promising building block for the next generation’s nanometer-scale optical nanodevices due to their interesting size- and shape-dependent properties [1,2]. It is believed that silicon nanowires with quantum wires diameter can emit visible light [3,4]. So far lots of attention has been paid to understand the photoluminescence (PL) mechanism of silicon nanowires. Several models have been proposed to explain the PL behavior of SiNWs [3,5–8]. In the most popular theory suggested by Canham [3], light emission of SiNWs is attributed to the quantum confinement effect. In addition, the surface states or surface related species are also reported to be responsible for the luminescence processes [7]. Since the band-gap energy will be expanded due to the quantized levels and the significant modulation of the usual electronic band structure in the band gaps, quantum confinement theory leads to the expectations that the visible PL is possible and the radiative efficiency at room temperature can be enhanced. The most pressing issue at present is to setup the relationship between the luminescent properties and the structural characteristics of Si nanostructures. The wet-etching method has attracted intensive research efforts due to its simple experimental condition and efficiency to prepare nanostructured material [9–12]. In addition, silicon nanowires can be integrated with other micro/nano-devices. Therefore, these characteristics would lead to cheap SiNWs-based optoelectronics. However, in fact, the fabricated SiNWs by wet chemical etching had a large diameter distribution, ranging from dozens to hundreds nanometers [11,12]. The diameters of most nanowires are larger ⇑ Corresponding author. Fax: +86 21 54345119. E-mail address: [email protected] (J. Zhang). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.06.013
than the critical dimension in which the quantum confinement effect occurs. In most cases, SiNWs are not formed with the perfect lattice on the whole during the HF etching process [8]. Therefore, it becomes more important to understand the structural characteristics of the large scale SiNWs with diameters in the dozens of nms range and their light emission mechanism. To the best of our knowledge, works related to this topic are seldom reported. In the present work, a simple, low cost wet chemical etching method was used to fabricate the large-scaled SiNWs. The structural characterizations of SiNWs were examined by X-ray diffraction (XRD), transmission electron microscope (TEM) and high resolution transmission electron microscopy (HRTEM). The optical properties of SiNWs were studied by Raman scattering spectroscopy and photoluminescence spectroscopy at room temperature. The light emission mechanism was investigated by analyzing the PL spectra of SiNWs oxidized by rapid thermal annealing at 1000 °C in O2 for 0, 2, 5, 10 and 15 min, respectively.
2. Experimental 2.1. Silicon nanowires fabrication Double-side polished p-type silicon-wafers (with <1 0 0> orientation and 0.1–10 X cm resistivity) were used as the substrate for the preparation of SiNWs. Before etching, the silicon wafers were cleaned carefully via standard RCA process. Then the cleaned silicon wafers were placed into a Teflon etching container which contained a mixture of 35 mM AgNO3 and 15 mM HF. The silicon wafers were etched for 60 min at room temperature under 1 atm. After etching, the samples were taken out and washed with concentrated HNO3 to remove surface byproduct Ag nanodendrites.
M. Zhu et al. / Chemical Physics Letters 511 (2011) 106–109
Then the samples were rinsed with DI water and dried with nitrogen carefully for further study. The main factors which can influence the SiNWs formation were AgNO3 concentration, HF concentration and the etching time, which affected not only the formation of SiNWs but also the surface morphology of the etched silicon wafers. The effects of these factors on the formation of SiNWs can be found in our previous paper in details [13]. According to our previous work, if the experiment parameters were fixed, SiNWs with the same structural characterizations can be nicely reproduced. 2.2. Silicon nanowires characterization The structural and morphological properties were investigated by transmission electron microscope (TEM) and high resolution transmission electron microscopy (HRTEM) by using JEM-2100 (JEOL). The crystalline structures of silicon nanowires were analyzed by X-ray diffraction (XRD) using CuKa radiation (D/MAX2550 V, Rigaku Co.). Raman spectra were obtained by using a micro-Raman spectrometer (Jobin–Yvon LabRAM HR 800 UV) with a 632.8 nm He–Ne laser as the excitation light at room temperature. PL spectra were recorded with the same spectrometer, but the excitation light used is a He–Cd laser with a wavelength of 325 nm and the excitation power of 30 mW at room temperature. 3. Results and discussion 3.1. Structural characteristics Figure 1 showed the TEM image of single SiNW. As can be seen from Figure 1a, the fabricated nanowire with a diameter of 50 nm was relatively homogeneous and smooth. The average length of asprepared SiNWs was approximately 60 lm and the diameter ranged from 40 to 100 nm. Figure 1b was the HRTEM image of SiNWs which implied the SiNWs had a core/shell structure. The result of the selected area electrical diffraction (SAED) analysis further confirmed that the as-prepared single SiNW was formed by a SiNW core encapsulated by a SiO2 shell. The thickness of SiO2 shell was about 5 nm, which was due to the oxidation of Si in air. 3.2. Optical characteristics The Raman spectroscopy had been used to characterize the ptype SiNWs and the p-type Si wafers used for SiNWs growth (denoted as the Si substrates) for comparison purpose. It is found that the laser power influenced the Raman results remarkably. The effect of laser heating (homogeneous or not) on Raman spectroscopy should be considered [14,15]. Figure 2 showed the SiNWs Raman spectrum evolution as a function of the excitation power with an excitation wavelength of 632.8 nm.
107
As shown in Figure 2a, no peak shift was found when the power increased from 0.03 to 0.3 mW. When the power reached 0.75 mW, the peak shifted to the lower frequencies and broadened. When the power further increased to 1.5 mW, the Raman spectra were changed significantly, possibly due to the sample radiation damages occured during the high-energy irradiation [16,17]. While, the experimental results of p-Si substrates illustrated that the peak position was almost independent of the applied laser power, just as shown in Figure 2b. So in this study, the laser power applied was in the range from 0.03 to 0.3 mW in order to eliminate its influence on the silicon nanowires. Figure 3 presented the Raman spectra of the SiNWs and the p-Si substrates obtained with the excitation power of 0.3 mW. For the p-Si substrates, a Raman peak at 520.7 cm 1 can be seen clearly, as shown in Figure 3a. This is due to the scattering of the first-order transverse optical phonon (TO) of crystalline silicon. In addition, two broad peaks at 300 and 945 cm 1 were also found, which are from the scattering of two transverse acoustic (2TA) phonons and two transverse optical (2TO) phonons, respectively [18]. The Raman spectrum of SiNWs in Figure 3a showed the prominent Raman peak at 508.4 cm 1. Similar to that of p-Si substrates, two broad peaks at 290 and 928 cm 1 were also visible. Besides these, another two new peaks appeared at 422 and 610 cm 1. Compared with p-Si substrates, the TO peak in SiNWs shifted from 520.7 to 508.4 cm 1 and significantly broadened. The intensities increased and the peak became asymmetric, as shown in Figure 3b. For 2TO and 2TA modes, the increased intensities were also observed for the SiNWs samples. These results are in agreement with those reported by Li et al. [19] and Sui et al. [20], in which SiNWs with dimension of 50 nm and porous silicon with a porosity of 80% and thickness of 100 lm, were investigated, respectively, and confirmed that there existed a collection of smaller crystalline grains with typical dimension of a few nms. The typical Raman spectra for SiNWs can be ascribed to the quantum confinement effect of Si nano-crystalline. The diameter of fabricated SiNWs in this study was larger than the critical dimension in which the quantum confinement effect occurs. However, SiNWs formed by HF etching were not formed by the perfect lattice and had a high density of microstructural defects, such as point defects, dislocations, voids, surface protrusions and irregularities [2]. These defects will break the central silicon region into the smaller nanograins. Therefore, in our case the fabricated SiNWs would also be consisted of a collection of smaller crystalline nanograins due to the presence of abundant structure defects. The effect of the surface oxide layer was also investigated to understand the PL mechanism of SiNWs. The as-prepared SiNWs was treated by dipping into 5% aqueous hydrofluoric acid solution (HF) for 10 s to remove the surface silicon oxide layer. After HF immersion, the SiNWs surface was passivated by hydrogen to form the hydrogen (H)-terminated Si surface, which was particularly
Figure 1. TEM image of single Si nanowire: (a) typical TEM image, (b) HRTEM image (the insets are the corresponding SAED pattern of the part framed in panel).
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Figure 2. Raman spectra of the (a) SiNWs and (b) p-Si wafers recorded under an excitation wavelength of 632.8 nm with different laser power applied.
Figure 3. (a) Comparison of Raman spectra in the range of 200–1100 cm 1 of SiNWs and p-Si wafers measured under an excitation laser wavelength of 325 nm with a power of 0.3 mW. (b) Partial enlarged comparison of Raman spectra in the range of 480–530 cm 1.
resistant to air oxidation [21–23]. For example, a nearly perfect Hterminated Si(111) surface is reported to be stable for several days in air [23]. The HF-treated SiNWs samples were carefully kept under nitrogen atomosphere. Figure 4 gave the corresponding X-ray
Si(400)
Intensity (a.u)
Si(200)
SiO2
1 2
0
20
40
60
80
100
2Theta(deg) Figure 4. X-ray diffraction (XRD) spectra of: (1) the as-prepared SiNWs and (2) the HF-treated SiNWs.
Figure 5. PL spectra of (a) the as-prepared SiNWs and (b) the HF-treated SiNWs.
diffraction (XRD) spectra of SiNWs before and after HF treatment. In the XRD spectrum of HF-treated SiNWs, the broad SiO2 peak centered at about 23° vanished. It illustrated that SiO2 no longer existed in SiNWs after the HF treatment. Figure 5a presented the PL spectrum of the as-prepared SiNWs. Two broad PL peaks were clearly distinguishable at 525 and 424 nm, respectively. Figure 5b showed the PL spectrum of HF-treated SiNWs. In comparison with the PL spectra of the as-prepared SiNWs, the corresponding peak position and intensity remained almost unchanged, indicating silicon oxide layer had little effect on the emission properties of SiNWs. Therefore, we proposed that these two PL peaks for the as-prepared SiNWs sample should be attributed to the Si nanograins existed in the SiNWs. To further confirm the relationship between these two PL peak and the nanograins, rapid thermal annealing (RTA) was used to oxidize the as-prepared SiNWs at 1000 °C in O2 for 0, 2, 5, 10 and 15 min, respectively. Figure 6 showed the corresponding PL results. A novel blue shift behavior can be observed from the PL spectra as the oxidation time increased. When the oxidation time were set as 2, 5, 10 and 15 min, the peak originally located at 525 nm shifted to 521, 518, 513 and 494 nm, respectively. Besides the blue shift, the intensity of this peak increased with the increase of the oxidation time, especially for the SiNWs sample treated for 15 min. Meanwhile, the peak located at 424 nm gradually vanished as the oxidation time increased. The PL behavior of SiNWs could be explained from the following aspects: (i) The blue shift was due to the band-gap increase. The size of nanograins decreased during the oxidation process. Therefore, the smaller sizes of nanograins
M. Zhu et al. / Chemical Physics Letters 511 (2011) 106–109
109
Si crystalline nanograins. The PL mechanism of SiNWs was investigated by the rapid thermal annealing of as-prepared SiNWs for different time. A blue shift of the emission peak originally located at 525 nm was observed after the thermal annealing. It was mainly due to the decrease in the size of nanograins caused by the oxidation. We suggested that the quantum confinement (QC) effect was responsible for the observed Raman and photoluminescence spectra of the as-prepared SiNWs. Acknowledgements We greatly appreciate the financial supports of the National Natural Science Foundation of China (Grant Nos. 60672002, 61076070, 60976004) and Innovation Program of Shanghai Municipal Education Commission (Grant No. 09ZZ46). This work is also supported by Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT). References [1] [2] [3] [4] [5] [6] [7] [8] [9] Figure 6. PL spectra of the SiNWs oxidized by rapid thermal annealing at 1000 °C in O2 for 0, 2, 5, 10 and 15 min, respectively.
induced the increment of the band-gap directly. (ii) The intensity increasing was attributed to the quantum confinement effects which enhances the oscillator strength of the direct optical transition in Si crystallites. The number of nanograins in which the quantum confinement occurs increased and lead to the increased PL signal. On the other hand, some of SiNWs with core diameter reaching quantum confinement condition can be generated during the oxidation treatment processes, resulting in the enhancement of the PL intensity in oxidation-treated sample. (iii) The disappearance of the peak at 424 nm should be ascribed to the complete oxidization of the corresponding Si nanograins during the oxidation treatment processes. 4. Conclusions The large-scaled array-ordered SiNWs with average diameter of 70 nm were fabricated by wet chemical etching method. The results of Raman and photoluminescence (PL) spectra indicated that the fabricated SiNWs were consisted of a collection of smaller
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
D. Ma, C.S. Lee, F.C.K. Au, S.Y. Tong, S.T. Lee, Science 299 (2003) 1874. R. Gupta, Q. Xiong, C.K. Adu, U.J. Kim, P.C. Eklund, Nano Lett. 3 (2003) 627. L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. G.D. Sanders, Yia-Chung Chang, Phys. Rev. B 45 (1992). Y. Kanemitsu, T. Ogawa, K. Shiraishi, K. Takeda, Phys. Rev. B 48 (1993) 4883. P.C. Searson, J.M. Macaulay, S.M. Prokes, J. Electrochem. Soc. 139 (1992) 3373. D.P. Yu, Q.L. Hang, Y. Ding, H.Z. Zhang, Z.G. Bai, J.J. Wang, et al., Appl. Phys. Lett. 73 (1998) 3076. A.I. Hochbaum, D. Gargas, Y.J. Hwang, P.D. Yang, Nano Lett. 9 (2009) 3550. E.A. Dalchiele, F. Martín, D. Leinen, R.E. Marotti, J.R. Ramos-Barrado, Thin Solid Films 518 (2010) 1804. Teng Qiu, Paul K. Chu, Mater. Sci. Eng. R 61 (2008) 59. Dinesh Kumar, Sanjay K. Srivastava, P.K. Singh, M. Husain, Vikram Kumar, Sol. Energy Mater. Sol. Cells 95 (2011) 215. Xianzhong Sun, Ran Tao, Linhan Lin, Zhengcao Li, Zhengjun Zhang, Jiayou Feng, Appl. Surf. Sci. 257 (2011) 3861. L.J. Wan, W.L. Gong, K.W. Jiang, H.L. Li, B.R. Tao, J. Zhang, Appl. Surf. Sci. 254 (2008) 4899. S. Hofmann, C. Ducati, R.J. Neill, S. Piscanec, A.C. Ferrari, J. Geng, et al., J. Appl. Phys. 94 (2003) 6005. S. Piscanec, A.C. Ferrari, M. Cantoroa, S. Hofmanna, J.A. Zapienb, Y. Lifshitz, et al., Mater. Sci. Eng. C. 23 (2003) 931. U. Ritter, P. Scharff, C. Siegmund, O.P. Dmytrenko, N.P. Kulish, Yu.I. Prylutskyy, et al., Carbon 44 (2006) 2694. Armida Torreggiani, Maurizio Tamba, Immacolata Manco, Maria Rosaria, Faraone-Mennella, et al., J. Mol. Struct. 744-747 (2005) 767. S.L. Zhang, X. Wang, K.S. Ho, J.J. Li, P. Diao, S.M. Cai, J. Appl. Phys. 76 (1994) 3061. B.B. Li, D.P. Yu, S.L. Zhang, Phys. Rev. B 59 (1999) 1645. Z. Sui, P.P. Leong, I.P. Herman, G.S. Higasni, X. Temkin, Appl. Phys. Lett. 60 (1992) 2085. E. Yablonovitch, D.L. Allara, C.C. Chang, T. Gmitter, T.B. Bright, Phys. Rev. Lett. 57 (1986) 249. G.S. Higashi, Y.J. Chabal, G.W. Trucks, K. Raghavachari, S.B. Christman, Appl. Phys. Lett. 56 (1990) 656. Y. Kim, M. Lieber, J. Am. Chem. Soc. 113 (1991) 2333.