Infrared spectra of silicon nanowires

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Infrared spectra of silicon nanowires Junjie Niu a, ⁎ , Deren Yang b , Jian Sha b,c , Jian Nong Wang a , Ming Li b a School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, People's Republic of China b State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, People's Republic of China c Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China Received 8 November 2005; accepted 7 June 2006 Available online 28 June 2006 Abstract Infrared (IR) spectra of the silicon nanowires (SiNWs) with oxide layer are analyzed by introducing the disorder-induced mechanical coupling between the optically active oxygen asymmetric stretch (AS) and inactive oxygen asymmetric stretch (I-AS) modes in terms of the transverse-optic (TO) and longitudinal-optic (LO) vibrational modes. The shapes of the IR spectra are similar to that of the reported SiO 2 , indicating that the SiNWs possess an oxide layer outside. The TO frequencies of coupled AS and I-AS are experimentally observed as peak at approximately 1085 cm − 1 and its shoulder of 1200 cm − 1 , respectively. The other TO absorption peaks of ∼ 468 cm − 1 , ∼ 480 cm − 1 , and ∼ 808 cm − 1 are also observed. Furthermore, the intensity of the AS-mode TO band centered at ∼ 1085 cm − 1 decreases while those of silicon lattice absorption peaks are enhanced with the crystalline quality increased. © 2006 Elsevier B.V. All rights reserved. Keywords: Infrared spectra; Nanowires; Silicon 1. Introduction Silicon nanowires (SiNWs), as an excellent candidate material for smart nano-devices, have raised extensive interests [1–5].In general, the oxide layer as the sheath of SiNWs is generated during fabrication [6]. Because the oxide layer would undertake electronic interaction in future nano-devices especially in MOS, understanding of the structural and compositional characteristics of oxide sheath on SiNWs becomes more significant. In fact, IR absorption spectra in amorphous SiO 2 (a-SiO 2 ) or in silicon wafer with oxide layer have been detailedly studied for many years [7– 11]. Three obvious IR absorption peaks of 460 cm − 1 ,810cm − 1 , and 1070 cm − 1 in a-SiO 2 have been confirmed. The 1070 cm − 1 peak was much stronger when the disorder degree of a-SiO 2 was improved [10]. In the analysis of the a-SiO 2 on silicon wafer, Kirk detailedly explained the IR spectra by the disorder-induced mode [8]. ForSiNWs,IRabsorptionofSiO 2 on SiNWs has been report- ed [12,13]. Hu et al. showed an enhanced absorption of SiO 2 /Si nanowires around 1130 cm − 1 and 1160 cm − 1 (LO mode) com- pared with that of SiO 2 nano-particles [12]. Sun et al. studied the stabilities and reactivity of hydrogen-terminated surfaces of SiNWs by FTIR spectroscopy [13]. In this letter, we discussed the TO frequencies of ∼ 468 cm − 1 , ∼ 480 cm − 1 , ∼ 808 cm − 1 , ∼ 1085 cm − 1 ,and∼ 1200 cm − 1 in IR absorption spectra of SiNWs with oxide layer using the disorder-induced mode indicated by Kirk [8]. The IR data demonstrated that SiNWs were covered with an oxide layer and undertook a similar phenomenon with SiO 2 . Moreover, the main AS TO band of ∼ 1085 cm − 1 was weakened while those of silicon lattice were strengthened with an increase in crystalline quality. 2. Experimental procedure SiNWs were prepared by methods of chemical vapor depo- sition (CVD) and evaporation methods as reported previously [3,14]. Sample-I was synthesized by deposition of silane in a CVD system at ∼ 620 °C [3]. Sample-II was grown on an Al 2 O 3 substrate at the position of lower temperature zone by the evaporation of SiO particles (purity: 99.99%) [14]. Sample-III was grown on a p-type (111) silicon wafer with a resistivity of ∼ 0.001 Ω cm at the position of higher temperature in the same system of Sample-II. The diameter of those SiNWs varied from Materials Letters 61 (2007) 894– 896 www.elsevier.com/locate/matlet ⁎ Corresponding author. Tel.: +86 21 62932050. E-mail address: jjniu@sjtu.edu.cn (J. Niu). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.06.017 ∼ 30 nm to ∼ 100 nm and the length was about several micro- meters. The thickness of the oxide layer on a single crystal SiNW core was about several nanometers. The original oxide layer thickness decreased from sample-I to sample-III, which have been confirmed by the previous work [3]. The morphology and structure of SiNWs were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electric diffraction (SAED), and X-ray diffraction (XRD), respectively. The IR absorption measurements were conducted in a Bruker IFS 66v/S Fourie r transform infrared spectrometer (FTIR) with an absorption mode. SiNWs on Al 2 O 3 templates were mashed and mixed with high-purity KBr to make a flat pellet for measurements. The data were processed in a vacuum environment of 4 mBar. The spectral resolution was ∼ 1cm − 1 and the signal-to-noise ratio was 10,000:1 pp in 5 s in range of 400 cm − 1 – 4000 cm − 1 . The spectra with subtracted background were used for analysis. 3. Results and discussion Fig. 1 shows a typical SEM image of the as-grown SiNWs on an Al 2 O 3 template. The as-synthesized SiNWs with a uniform diameter and high purity are observed. In addition, SAED and XRD data indicated that the crystal nature is improved from sample-I to sample- III with increased temperature [3,14]. These data are not shown because the improvement of crystal quality with increasing temperature is easily understood [15]. The TO bands of sample-I and sample-II are presented in Fig. 2. The IR spectrum of the float-zone (FZ) silicon with free oxygen in the bottom part of Fig. 2 is used to be a standard reference of silicon lattice absorption. The peak of ∼ 1085 cm − 1 with high intensity is observed in both samples. This peak is characterized as Si–O AS mode in which the adjacent O atoms execute the AS motion in phase with each other. From the data in Table 1, it is clearly seen that the integrated area and peak width decreased from sample-I to sample- II. This can be explained from the analysis of oxide layer on SiNWs grown under different conditions. The crystalline quality of sample-II is better than that of sample-I because of the improvement of tempera- ture. Thus, the enhanced crystallinity induced the reducing of amor- phous oxide layer. Furthermore, the diameter of sample-II (∼ 30 nm) is smaller than that of sample-I (∼ 60 nm). Therefore, the oxide layer area in sample-II is less than that in sample-I. The less oxide layer induces that active asymmetric stretching oxygen decreases and then contrib- utes to the reducing of ∼ 1085 cm − 1 . The improvement of crystalline quality can also be indicated from the change of silicon lattice absorp- tion peaks. Compared with the IR spectra of sample-I, the absorption bands of silicon lattice in sample-I, such as 611 cm − 1 , 738 cm − 1 , and 891 cm − 1 , are obviously improved. This demonstrates that the crystal nature of sample-II is better than that of sample-I. On the shoulder of ∼ 1085 cm − 1 of sample-I and sample-II, there is a weak peak centered at ∼ 1200 cm − 1 . This vibrational behavior of the TO band is regarded as the contribution of I-AS mode in which adjacent O atoms execute the AS motion 180° out of phase with each other [16,17]. Although the peaks of 1107 and 1224 cm − 1 related to interstitial and precipitate oxygen similar to the bulk might have contribution on peaks of around ∼ 1085 cm − 1 and ∼ 1200 cm − 1 , they are blanketed and the value is so small that could be ignored here. Furthermore, the enhancement of ∼ 1130 cm − 1 is not found in our experiments as the previous report [12]. It is well-known that LO modes cannot be observed in normal- incidence infrared absorption spectra because the infrared wave cannot interact directly with longitudinal phonons except for the oblique- Fig. 1. SEM image of SiNWs on an Al 2 O 3 template. Fig. 2. IR absorption spectra of sample-I and sample-II. Sample-I was fabricated in a CVD process at 620 °C, while sample-II was fabricated in a thermal evaporation process at 1100 °C. Table 1 Integrated area and peak width of 1085 cm − 1 Sample name Integrated area Peak width (cm − 1 ) Sample-I ∼ 28.2 ∼ 293.4 Sample-II ∼ 9.7 ∼ 169.7 Sample-III ∼ 3.9 ∼ 38.7 Fig. 3. TO bands of IR absorption spectra for the SiNWs growth on silicon wafer at 1100 °C. The bottom is TO bands of the SiNWs with silicon lattice peaks subtracted. 895J. Niu et al. / Materials Letters 61 (2007) 894–896 incidence absorption spectra by using p-polarized light, according to the Berreman effect [18]. Therefore, in our measurements, there are no LO peaks to appear, such as 1160 cm − 1 , 1256 cm − 1 , etc. It is interesting to note that our TO AS and TO I-AS absorption peaks of ∼ 1085 cm − 1 and 1200 cm − 1 are the same as the results of Kirk [8]. That means that the behavior of oxide layer on SiNWs is similar to that of oxide layer on silicon wafer. Therefore, it demonstrates that the as-received SiNWs contain a quantity of oxide layer on the surface. Additionally, the peak of ∼ 1632 cm − 1 in Fig. 2 might be related to Al 2 O 3 template (Al–O stretch) which is used as a substrate or interference of KBr prepared during the measurements. One thing that must be mentioned is the reproducibility and comparison of FTIR spectra used in experiments. A number of samples with different ratios of SiNWs and KBr are pro- cessed and display a similar repeated result. For comparison, the purity of SiNWs is very high and both samples contained almost the same backgrounds including template, KBr, and other impurities. Therefore, the FTIR spectra could be used as a relative comparison to study the structure of oxide layer on SiNWs. The other TO bands of ∼ 480 cm − 1 and ∼ 808 cm − 1 are also found in sample-I and sample-II (Fig. 2). Compared with sample-I, the intensity of peaks of ∼ 480 cm − 1 and ∼ 808 cm − 1 in sample-II is decreased which is similar to the ∼ 1085 cm − 1 . The decrease is equally due to the reducing of oxide layer thickness. The low-frequency TO band of ∼ 480 cm − 1 can be described as a vibrational mode of rocking of O atom about an ax is through the two Si atoms (R). While the middle-frequency TO band of ∼ 808 cm − 1 can be characterized by symmetrical stretching of the O atom along a line bisecting the axis formed by the two Si atoms in silicon oxide (SS). As for the high-frequency TO band of ∼ 1085 cm − 1 ,theOatom moves back and forth along a line parallel to the axis through the two Si atoms. Although the LO bands have not been observed, it is still considered that the TO–LO frequency pairs coupled with AS and I-AS mode will take a compositive effect on IR absorption bands of around ∼ 480 cm − 1 , ∼ 808 cm − 1 , and 1085 cm − 1 , respectively. The corre- sponding vibrational modes of the main TO absorption peaks in SiNWs are marked in the Figs. 2 and 3, which are similar to those in silica [8]. Fig. 3 shows the IR absorption spectra of SiNWs on a silicon wafer fabricated by evaporation of SiO at 1100 °C. Similar to the above results, the AS TO band of ∼ 1085 cm − 1 is the strongest related to Si– O, and the integrated area and peak width decreased sharply according to those of sample-I and sample-II which can be seen in Table 1. Differently, the low-frequency TO band shifts down to the position of ∼ 468 cm − 1 , which is the same as the previous result [12]. This should be caused by the different rocking distance of O atom about an axis through the two Si atoms induced by different growth processes. 4. Conclusion To summarize, the IR absorption spectra of SiNWs with oxide layer are analyzed and indicate that the as-received SiNWs contain a quantity of oxide layer. The three major TO bands of ∼ 480 cm − 1 , ∼ 808 cm − 1 ,and∼ 1085 cm − 1 are ob- served and analyzed by introducing disorder-induced mechanical coupling between the optically active oxygen asymmetric stretch and inactive oxygen asymmetric stretch modes in terms of the transverse-optic and longitudinal-optic vibrational modes. Acknowledgements This work was supported by the National Natural Science Foundation of Chi na, key project of the Education Ministry of China, and Youth Teacher Fund of Shanghai Jiaotong University (A2306B). We would like to thank Instrumental Analysis Center of Shanghai Jiaotong University, for their great help in the measurements. References [1] D. Ma, C.S. Lee, F.C.K. Au, S.Y. Tong, S.T. Lee, Science 299 (2003) 1874. [2] Y. Cui, C.M. Lieber, Science 291 (2001) 851. [3] J.J. Niu, J. Sha, X.Y. Ma, J. Xu, D.R. Yang, Chem. Phys. Lett. 367 (2003) 528. [4] M.J. Konstantinovic, S. Bersier, X. Wang, M. Hayne, P. Lievens, R.E. Silverans, V.V. Moshchalkov, Phys. 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