Optical spectroscopy of silicon nanowires

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

Optical spectroscopy of silicon nanowires Jifa Qi a, * , John M. White a , Angela M. Belcher a , Yasuaki Masumoto b a Department of Chemistry and Biochemistry, University of Texas, Welch 4.212, Austin, TX 78712, USA b Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan Received 16 December 2002; in final form 13 March 2003 Abstract Silicon nanowires (SiNWs) were prepared by laser ablation at high temperature and studied by electron microscopy and optical spectroscopy. As-synthesized SiNWs are found orderly aligned on the silica substrates, exhibiting uniform shape with a silicon crystalline core and an amorphous silicon oxide sheath. Asymmetrically broadened Raman spectral peaks downshifted from 520 cm À1 were observed, which related to the confinement effects of optical phonon by nanowire boundaries. The SiNWs showed strong photoluminescence (PL) bands peaked at 455 and 525 nm, which quenches rapidly with an increase in temperature and may arise from the defects surrounding the silicon nanowire crystalline core. Ó 2003 Elsevier Science B.V. All rights reserved. Si nanowires (SiNWs) are expected to exhibit potentially useful electrical, optical, mechanical, and chemical properties due to their small di- mensions, unique shapes, and high surface-to- volume ratio. The recent progress in large-scale production of uniform and thin SiNWs has at- tracted investigation interests of the quantum confinement properties and potential applications of SiNWs [1–6]. Several researchers have reported photoluminescence (PL) from the SiNWs of as- grown and oxidized samples at room temperature [5,6]. They ascribed the observed red, green and blue PL peaks at 816, 470 and 420 nm to the quantum confinement effects and the recombina- tion emissions from the defect centers, respec- tively. Since the studies of PL behavior at low temperature, and the temperature dependence are important to understand the mechanism of the luminescence, we will report in this Letter the optical spectroscopic properties of the SiNWs synthesized by laser ablation at high temperature, including the Raman scattering behavior and the temperature dependence of PL. The synthesis of the SiNWs was carried out by laser ablation of a mixed target of silicon powder (99.999 wt%) and nanosized iron powder (99.9 wt%). Typical molar ratio for Si to Fe was 0.95– 0.05. A quartz tube was mounted inside a high- temperature 50 cm tube furnace. The target was placed in the center of the quartz tube, which was evacuated by a mechanical rotary pump to a pressure of 1 Pa. High-purity argon was then Chemical Physics Letters 372 (2003) 763–766 www.elsevier.com/locate/cplett * Corresponding author. Present address: Department of Material Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., #16-244, Cambridge, MA 02139, USA. Fax: +6173243300. E-mail address: jifa@mit.edu (J. Qi). 0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00504-9 passed through the quartz tube at a flow rate of 50–100 standard cubic centimeters per second (sccm). A pulsed XeCl excimer laser (308 nm, pulse repetition 10 Hz, energy 170 mJ; Lambda Physik product) was used to ablate the target for 3 h while the furnace temperature was kept at 1200 °C. The product was collected from the silica tube wall. A Hitachi H9000 and a JEOL 2010 transmission electron microscopes (TEM) working at 200 kV were used to characterize the products. Fig. 1 shows the typical electron microscopic image of the morphology of the SiNWs. It was observed that the product exemplified high purity with a uniform diameter 20 nm and consisted of most of the SiNWs aligning on the substrate. Fig. 2 shows a high resolution TEM (HRTEM) image of a single SiNW with a diameter of about 18 nm. The (1 1 1) lattice fringes with the interplanar spacing of 0.31 nm and the corresponding selected area electron diffraction (SAED) patterns show the SiNW consisted of a crystalline Si structure. Ad- ditionally, there is a thin amorphous silicon oxide layer (about 3 nm) sheathing the crystalline core of the SiNW. The sheath of SiO 2 is determined by the X-ray photon emission spectral measurements. The optical properties of SiNWs depend on their nanosized crystallites and the surrounding oxide layers. Raman scattering spectra of SiNWs were recorded on a Jasco NRS-1800 Raman spectrometer in the back-scattering configuration, using a 514.5 nm line of Ar þ laser at normal in- cidence. The Raman scattered light was dispersed by a triple-monochromator and recorded by a li- quid nitrogen-cooled CCD detector. To avoid the laser-induced thermal effects, the exciting laser power was kept below 0.1 mW on the sample. Fig. 3 shows the Raman scattering spectra of the SiNWs and crystal silicon. A very sharp and Fig. 1. A typical SEM image of the morphology of aligned silicon nanowires of uniform diameter distribution on a silica substrate. Fig. 2. A HRTEM image of a single SiNW. Two-dimensional (1 1 1) lattice fringes can be seen, and a thin amorphous oxide layer surrounds the crystalline core. The inset shows the cor- responding SAED pattern. Fig. 3. Raman spectra at room temperature of a single crys- talline silicon (thick solid line), SiNWs (dotted line) and the theoretical fitting results (thin solid line). The power output of laser at samples is about 70 W/cm 2 . 764 J. Qi et al. / Chemical Physics Letters 372 (2003) 763–766 intense Raman line at 520 cm À1 with the full width at half maximum (FWHM) of 4:7cm À1 was wit- nessed in the Raman spectrum of crystal Si. This peak corresponds to the degenerate zone-center optical phonon mode of crystal Si. All SiNW samples exhibit similar Raman spectral peaks red- shifted from 520 cm À1 and a small shoulder at 495 cm À1 . The main peak near 520 cm À1 corre- sponds to the first-order optical phonon of crys- talline Si. The small broad peak at 495 cm À1 was attributed to the amorphous silicon that covers SiNWs or distributed on the silica substrate, which has a Raman structure between 400 and 550 cm À1 peaked at 480 cm À1 [7]. Asymmetrically broadened and frequency downshifted line shapes are usually observed in nanocrystalline and related to phonon confinement effects by nanocrystallite boundaries, hence the average crystallite size was estimated according to the strong phonon confinement model [8]. The Raman intensity profile can be written as IðxÞ¼ Z 1 0 expðÀqD=2pÞ 2 dq 3 ðx À xðqÞÞ 2 þðC 0 =2Þ 2 ; ð1Þ where xðqÞ is the phonon dispersion function, q is the normalized phonon momentum, C 0 is the natural phonon linewidth of crystalline silicon, and D is the size of crystal. Eq. (1) was used to fit the Raman spectra. The continuous line in Fig. 3 shows the best-fit result, and the average crystal size of nanowires D ¼ 11:3 nm was obtained, as shown in Fig. 3. The good agreement in spectral features between experimental and calculated Ra- man spectra indicates that the identification of the Raman peak of SiNW is correct. However, the diameters determined by Raman scattering mea- surements were smaller than that obtained from SEM and TEM observation. The reasons for this disparity are considered below. First, only the crystallite contributes to the main Raman scat- tering peaks, our nanowires are capped by the amorphous oxide layer and amorphous silicon, their contributions to the Raman spectra were not calculated. Second, the existence of defects and stresses in SiNWs can have a profound influence on the Raman spectra of SiNWs. The PL measurements have been performed by an experimental setup consisting of the excitation source of a He–Cd laser (325 nm) and a 27.5 cm monochromator equipped with a liquid nitrogen cooled CCD detector. In order to investigate the PL as a function of temperature, samples were placed in a temperature-variable cryostat. The PL spectra were measured at temperatures ranging from 10 to 300 K. Fig. 4 shows the PL spectra of SiNWs at different temperatures. Two strong emission bands in the green and blue regions re- vealed peaking at 455 and 525 nm at low temper- ature, respectively. The band that peaked at 455 nm is close to the observation results on the silica nanowires [5] that was ascribed to originate from the oxygen vacancies, while the band peaked at 525 nm is close to the results reported for as-grown silicon nanowires [6]. According to theoretical prediction, visible light emission due to size con- finement can occur only when the mean size of the Si crystalline is less than that of free exciton of silicon. Therefore, these two peaks are not con- sidered to be due to the quantum confinement ef- fect. Instead, Yu et al.Õs explanation that the green and blue PL emissions come from the radiative recombination from the defect centers in the over- coated silicon oxide layer and the interface be- tween crystalline core and amorphous sheath layer, such as oxygen vacancies [9], seems to agree with this observation. The inset curve in Fig. 4 shows the integrated PL intensity of SiNWs as a function of tempera- Fig. 4. PL spectra of SiNWs at different temperatures. Inset is a temperature dependence of integrated PL intensity versus temperature. The solid line shows the fit results. J. Qi et al. / Chemical Physics Letters 372 (2003) 763–766 765 ture. The PL intensities decrease rapidly with an increase of temperature. The thermal quenching of the luminescence is considered to originate from the thermal ionization of electrons or holes trap- ped on the defect centers in the sheath or interface layer. On the basis of two thermally activated non- radiative recombination model, the temperature dependence of the luminescence intensity can be simply written by [10] IðT Þ¼ I 0 1 þ C A expðÀE A =kT ÞþC B expðÀE B =kT Þ ; ð2Þ E A and E B are thermal activation energies of cen- ters A and B, respectively, while C A and C B are temperature-independent factors. The fit result by using Eq. (2) presented by the solid line in the inset of Fig. 4 shows a good coincidence with the ex- periment results. The best-fit parameters are E A ¼ 20 meV and E B ¼ 104 meV, respectively. The E A value is very close to the binding energy of excitons in silicon. The related PL thermal quenching process can be considered to be a de- crease of luminescent carriers due to the thermally induced exciton ionization in the SiNW. E B may be related to the deep trap states. The related PL thermal quenching process can be considered in terms of excited carriers that are trapped by the defect levels and relaxed to the ground state through a non-radiative process causing the de- crease of luminescence with an increase in tem- perature. Thus, the excitation process of the observed luminescent centers in a SiNW is con- sidered as light-created excited carriers in the SiNW and the carriers are trapped at the defect centers and relaxed to the ground state by radia- tive and non-radiative processes. In conclusion, SiNWs were synthesized by laser ablation at high temperature. The typical SiNW exhibits a uniform shape of silicon crystallite sheathed by an amorphous silicon oxide layer. A downshifted and broadened Raman spectral peak was observed, which is related to the confinement effects of optical phonons by the nanowire boundaries. SiNWs emit green and blue light un- der ultraviolet photoexcitation. The green and blue bands are related to the radiative recombi- nation of the defect centers in the outer oxide layer of the SiNWs. The luminescence quenches rapidly with an increase of temperature. Acknowledgements The authors would like to thank the Research Center for Advanced Carbon Materials, AIST, for use of the micro-Raman spectrometer instrument. References [1] M. Morales, C.M. Lieber, Science 279 (1998) 208. [2] D.P. Yu, C.S. Lee, I. Bello, X.S. Sun, Y.H. Tang, G.W. Zhou, Z.G. Bai, Z. Zhang, S.Q. Feng, Solid State Commun. 105 (1998) 403. [3] Y.F. Zhang, Y.H. Zhang, N. Wang, D.P. Yu, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 72 (1998) 1835. [4] J. Qi, Y. Masumoto, Mater. Res. Bull. 36 (2001) 1407. [5] D.P. Yu, Q.L. Hang, Y. Ding, H.Z. Zhang, Z.G. Bai, J.J. Wang, Y.H. Zou, W. Qian, G.C. Xiong, S.Q. Feng, Appl. Phys. Lett. 73 (1998) 3076. [6] Z.G. Bai, D.P. Yu, J.J. Wang, Y.H. Zou, W. Qian, J.S. Fu, S.Q. Feng, J. Xu, L.P. You, Mater. Sci. Eng. B 72 (2000) 117. [7] Z. Iqbal, S. Vep  rrek, J. Phys. C 15 (1982) 377. [8] I.H. Campbell, P.M. Fauchet, Solid State Commun. 58 (1986) 739. [9] H. Nishikawa, T. Shiroyama, R. Nakamura, Y. Ohki, K. Nagasawa, Y. Hama, Phys. Rev. B 45 (1992) 586. [10] G. Davies, Phys. Rep. 176 (1989) 83. 766 J. Qi et al. / Chemical Physics Letters 372 (2003) 763–766 . scattering spectra of the SiNWs and crystal silicon. A very sharp and Fig. 1. A typical SEM image of the morphology of aligned silicon nanowires of uniform diameter. Optical spectroscopy of silicon nanowires Jifa Qi a, * , John M. White a , Angela M. Belcher a , Yasuaki Masumoto b a Department of Chemistry