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www.elsevier.com/locate/ph y se Physica E 23 (2004) 221–225 Raman spectrum of array-ordered crystalline silicon nanowires Jianxun Liu a , Junjie Niu b , Deren Yang b , Mi Yan c , Jian Sha a,b, * a Department of Physics, Zhejiang University, P.O. Box 1232, Hangzhou 310027, China b State Key Laboratory of Silicon Materials, Zhejiang University, China c Department of Materials Science and Engineering, Zhejiang University, 310027 Hangzhou, China Received 27 February 2004; accepted 10 March 2004 Available online 11 May 2004 Abstract Array-ordered single-crystal silicon nanowires were fabricated by the nanochannel-aluminal and CVD method. The average length and diameter of the nanowires is about 10 mm and 60 nm, respectively. A study of the Raman spectrum of the nanowires shows that the Raman shift to low frequency is due to the quantum confinement effect, which is discussed by using the phonon confinement model. Also we determine the peaks of the Raman spectrum to be corresponding to that of crystal silicon (c-Si). r 2004 Published by Elsevier B.V. PACS: 61.46.+w; 68.65.La; 78.30.Àj Keywords: Nanowires; Raman spectra; Phonon confinement model 1. Introduction Silicon, the most important semiconductor material, plays an dominant role in the area of microelectronics materials. Silicon-based photo- electric integration is one of the highlights of todays science research. Crystal silicon is an indirect band material, and its light emission efficiency at room temperature is very low. Recently, one-dimensional silicon nanomaterials had been prepared successfully by many methods [1–8], stimulating intensive interest in it [9–11]. Silicon nanowires (SiNWs) are particularly inter- esting because they are expected to exhibit unusual confinement effects on electrical and optical properties due to its low dimension and high surface-to-volume ratio and fulfill the technology requirements for integration. As with micro-Raman technique, which offers many advantages such as much less amount of sample and lower-power laser. Raman spectra play an increasingly important role in the study of nano-structured materials. Especially that of SiNWs reveals many interesting features: the Raman peak of SiNWs changes compared to the c-Si in position, FWHM, symmetry and the Raman peak changes with the wavelength of ARTICLE IN PRESS *Corresponding author. Department of Physics, Zhejiang University, P.O. Box 1232, Hangzhou 310027, China. Tel.: +86-0571-879516593. E-mail address: phyjsha@zju.edu.cn (J. Sha). 1386-9477/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.physe.2004.03.016 exciting laser, etc. [12–15]. In this paper, we report a Raman investigation of the SiNWs and give some explanation for the discrepancy between the SiNWs and the c-Si. 2. Experiment The nanochannel-aluminal (NCA) and CVD techniques were used for preparing the array- ordered single crystal silicon nanowires [16]. The NCA were fabricated by electrochemistry method with chromium acid [17], of which the average channel diameter is 60 nm. Then gold as a catalyst was deposited by the magnetic sputtering method on one side of the template, and nanowires were grown in the channels using CVD method. After the deposition, Raman spectra of samples (NCA with SiNWs heading out of the channel) were measured with Raman spectrometer (Nicolet Thermo) at room temperature. Excitation was done with 532-nm line of an Ar-ion laser. Raman signals were measured with a spectral resolution of 1.0 cm À1 . Then the morphology of the cross section of the sample was taken by SEM (JSM- T20, JEOL). Furthermore, the sample was dis- solved in ethanol solution which was placed dropwise on a copper grid covered with a very thin carbon film, so that the deposited nanowires could be analyzed by the TEM (Philips CM2000) equipped with energy-dispersive X-ray spectrum (EDX) facility. 3. Results and discussions Fig. 1 is a SEM image of many SiNWs that revealed the general array-ordered morphology of the SiNWs. It can also be seen from Fig. 1 that the average diameter and the length of the SiNWs are 60 nm and 10 mm, respectively. TEM image of one SiNW indicates that the nanowire has a relatively homogeneous and smooth structure, and is cov- ered with a silicon oxide sheath, as shown in Fig. 2. The selected area electrical diffraction (SAED) analysis of nanowire (Fig. 2, inset) exhibits a silicon single crystal structure. Raman spectra of the two samples that were prepared in the same condition are measured as shown in Fig. 3, in which the five typical peaks of the two samples almost coincide. In order to investigate the Raman spectra of the SiNWs, we first investigate the Raman spectra of the c-Si. There is a peak at 519.4 cm À1 in c-Si. It is the first- order optical phonon mode with the full-width at half-maximum (FWHM) at 4.5 cm À1 . It is the most intensive peak in the Raman spectra. There ARTICLE IN PRESS Fig. 1. SEM image of many SiNWs. Fig. 2. The TEM image of single SiNWs (The inset is the image of SAED that is taken from the same nanowire). J. Liu et al. / Physica E 23 (2004) 221–225222 are two broad peaks at 301.2 and 966.4 cm À1 , corresponding to the second transverse optical phonon model (2TO) and the second transverse acoustic phonon model (2TA), respectively. Be- cause of the similar Raman spectra for the two samples, we only investigate Raman spectra of sample a. In the Raman spectra of sample a, there are five peaks at 505, 927, 296, 604 and 423 cm À1 . Since Si x O y has no contribution to Raman spectrum, we neglect the influence of silicon oxide sheath of SiNWs. It can be seen that the spectrum of the sample is very similar to that of the c-Si, except for the weak peaks at 604 and 423 cm À1 . These two peaks were found in nanoparticle Si [18] and porous Si [19] and can be related to quantum confinement effect of Si. The most intensive peak in the SiNWs is found to shift to the lower frequency and to be asymmetric. The Raman spectra of microcrystalline silicon films were combined with the spectra of c-Si and the spectra of porous Si. So its Raman spectrum is asym- metric. In our experiment, the asymmetry can also be seen as the combination of the c-Si and the porous Si. The FWHM of sample a is 20 cm À1 which is increasingly larger than that of c-Si. The explanation is that the FWHM increase is evidently associated with the disorder at bound- aries of crystallites which leads to a decrease of the phonon time. We give a qualitative explanation of shift of the first-order optical phonon of SiNWs in phonon confinement model. In an ideal crystal, the correlation length is infinite, and hence the phonon eigenstates are plane waves. Therefore the usual ~ kk ¼ 0 momentum selection rule of the first-order Raman spectrum can be satisfied. As the crystallite is reduced to nanosize, the most important effect on the Raman spectra is that the crystal momen- tum conservation rules are relaxed. This allows phonons with wave vector j ~ kkj¼j ~ kk 0 j¼72p=L to participate in the first-order Raman scattering. Here ~ kk 0 is the wave vector of the incident light and L is the size of crystal. The phonon scattering is no longer limited to the center of the Brillioun zone, and the scattering near the zone center must be considered. As a result, the symmetry-forbidden modes must be observed, in addition to shift of the first-order optical phonon. The size-dependent Raman shift was investi- gated, in which Raman shift increased with the decrease of the diameter of SiNWs. In our experiment, the Raman shift of SiNWs TO compared to c-Si is 15 cm À1 , which is larger than than it should be [15]. If we only consider the diameter of the crystal in the SiNWs, the shift of 15 cm can be explained qualitatively. This means that the Raman shift actually depends on the size ARTICLE IN PRESS 296 423 505 604 927 (a) 292 416 507 600 925 (b) 5000 1 0000 1 5000 2 0000 2 5000 3 0000 3 5000 4 0000 4 5000 5 0000 5 5000 6 0000 6 5000 7 0000 Int 200 400 600 800 1000 1200 1400 1600 1800 Raman shift (cm -1 ) Fig. 3. The Raman spectrum of the SiNWs. J. Liu et al. / Physica E 23 (2004) 221–225 223 of crystals in the nanowire, and other matters do not contribute to the shift. The MCM based on quantum confinement has been successfully ap- plied to many nanoscale materials [20]. According to MCM, the theoretical first Raman spectrum can be obtained from the following equation [21]: IðoÞ¼ Z d 3 qjCð0; ~ qqÞj ½o À oð ~ qqÞ 2 þðG 0 =2Þ 2 ; ð1Þ where oð ~ kkÞ represents the phonon dispersion curve. G 0 is the natural linewidth (inversely proportional to the intrinsic phonon lifetime); Cð0; ~ qqÞ is the coefficient describing the phonon confinement at q 0 =0, which is appropriate for the first-order Raman scattering. The integration must be performed over the entire Brilloun zone. Cð0; ~ qqÞ is the Gaussian one: Cð0; ~ qqÞ¼exp À q 2 L 2 4p 2 ; ð2Þ where L is the size of the crystal. From Eq. (2) we can see that the crystal size determines the TO peak the wavelength of the exciting light. Our experiment uses an Ar-ion laser with wavelength 532.0 nm, the exciting light of Raman spectrum in the experiment above discussed is 514.5 nm. It is known that because of resonance Raman scatter- ing, the Raman peak will shift with the wavelength of exciting laser. Here the difference between the two wavelengths is small. So the first explanation is the most important. However, there are also some discrepancies. There should be amorphous Si covering the SiNW. But there is no signal of amorphous Si in the Raman spectra. The possible explanation is that the Raman peak of the amorphous Si is about 490 cm À1 and the FWHM of the sample is 20 cm À1 , so the peak may be covered by the TO peak. 4. Conclusion Array-ordered silicon nanowires with single- crystal structure were prepared by the NCA and CVD method. Their Raman spectra have been measured, in which the Raman shift could be considered. This results from the confinement effect of nanomaterials, some peaks could belong to that of c-Si, expect for the breadth and asymmetry of the peak. Also, we gave the qualitative explanation of the changes of the peak. 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