Highly efficient and stable photoluminescence from silicon nanowires coated with sic

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

Highly ecient and stable photoluminescence from silicon nanowires coated with SiC X.T. Zhou, R.Q. Zhang, H.Y. Peng, N.G. Shang, N. Wang, I. Bello, C.S. Lee, S.T. Lee * Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong, People's Republic of China Received 23 June 2000 Abstract A reaction of silicon nanowires (SiNW) with methane and hydrogen has been performed to produce a thin coating layer of cubic silicon carbide (b-SiC) using an ion beam deposition technique. High resolution transmission electron microscopy (HRTEM) showed that silicon oxide shells originally cladding the as-grown SiNW were removed and replaced by a thin layer of nano-sized crystals of b-SiC. This has led to stable photoluminescence (PL) observed from the SiC-coated SiNW with high eciency almost tripled as compared with that before SiC coating. Ó 2000 Elsevier Science B.V. All rights reserved. In the past decade, intensive studies on porous silicon related nano-technology have been stimu- lated by its potential applications in silicon-based optoelectronic devices [1±12]. One-dimensional silicon nanowire in the same family is considered more promising for its inherent quantum con®ne- ment eect in the other two dimensions [13]. Large-scale synthesis, which is always an essential requirement for wide applications, of silicon nanowire (SiNW) has recently been achieved rou- tinely [14,15]. However, problems of degradation [2,16,17] and low photoluminescence (PL) e- ciency [18] from the silicon nano-structures remain unsolved. The PL degradation, usually accompa- nied with a blue-shift of the PL peak position, is generally believed to originate from the chemical instability of the surfaces [2,16,17]. Upon exposure to the atmosphere, the thickness of the surface silicon oxide layer increases, and leads to increased absorption and re¯ection of the incident light. This eventually leads to degradation of the PL intensity [16,17,19]. At the same time, the oxidation process reduces the size of the core Si nano-crystallites and thus results in the blue shift [16,17]. Before large- scale application is possible, this instability prob- lem has to be solved. One possible solution is to passivate the silicon surface. Among the existing passivation methods, a nitridation is generally believed to be an advantage [20]. However, there is a technological complication involving the re- moval of the surface oxide layer in order to achieve a satisfactory nitriding eect. In addition, a dia- mond-like carbon ®lm has been used to coat po- rous silicon [21]. Nevertheless, the problem of stability has not yet been satisfactorily solved and further eort is still needed to make the promising 22 December 2000 Chemical Physics Letters 332 (2000) 215±218 www.elsevier.nl/locate/cplett * Corresponding author. Fax: +852-2784-4696. E-mail address: apannale@cityu.edu.hk (S.T. Lee). 0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 145-3 silicon nano-structures applicable for the widely desired nano-technology. Silicon carbon (SiC) is a semiconductor with various merits including a wide energy band gap, a high breakdown electric ®eld, and a high satura- tion electron drift velocity. Furthermore, it is du- rable at high temperature and possesses high thermal conductivity and chemical stability. In addition, it has good mechanical properties, such as high tensile strength and a high Young's mod- ulus which facilitate its application as a protective coating material. Such a material should be ideal for the integration with silicon in optoelectronic devices. In this Letter, we show that SiNW can be coated with b-SiC so as to integrate the various advantages of the SiC with the important nano- scaled materials in order to advance its applica- tions in photophysics and device physics. SiNW were synthesized by thermal evapora- tion from a mixture of silicon and silicon dioxide powder [14,15]. The SiNWs deposited on a silicon wafer have an average diameter of about 20 nm, as shown in Fig. 1. High resolution transmission electron microscopy (HRTEM) shows that the SiNW consists of a core of crystalline silicon clothed by a shell of silicon oxide with a thickness up to half of the wire radius (Fig. 1b). A micro- Raman spectrometer (Ranishaw 2000, wavelength of laser: 514 nm) was used to characterize the PL properties of the sample. Very weak PL intensity was obtained from the as-grown SiNW sample (Fig. 2a). The PL peak centers at around 630 nm (2.0 eV). No PL peak relating to the presence of the silicon oxide was found. The low PL intensity is likely due to the presence of the silicon oxide shell, which covers the SiNWs. The silicon oxide can absorb and re¯ect both the excitation inci- dent light as well as the emitted PL light. Al- though some kinds of silicon oxide can also give visible PL, it is dicult to obtain a suitable composition of silicon oxide required for ecient luminescence. Therefore, it is desirable to mini- mize or eliminate the silicon oxide in order to obtain silicon nano-structures with ecient PL. To address such a need, the following experiment has been done. The as-grown SiNW sample was transferred to an ion beam deposition chamber with a base vacuum of 2 Â 10 À7 Torr. A broad-beam Kaufman ion source fed with a mixture of methane (>99.9%), hydrogen (>99.999%) and argon (>99.995%) was used to deposit thin ®lms onto the Fig. 1. Typical TEM images of SiNW: (a) morphology at low magni®cation; (b) morphology at higher magni®cation. Inset shows the corresponding electron diraction pattern. Fig. 2. PL from: (a) as-grown; (b) ion beam treated silicon nanowire sample. 216 X.T. Zhou et al. / Chemical Physics Letters 332 (2000) 215±218 SiNW sample. The three gases were mixed in a ratio of CH 4 :H 2 :Ar  1:50:173. The total ¯ow rate was 2 sccm and the deposition pressure was kept at about 5 Â 10 À4 Torr. Ions from the Kaufman source were accelerated to bombard the SiNW with an accelerating voltage of 200 V. The ion dose as measured by a Faraday cup placed next to the SiNW sample was 3 Â 10 19 cm À2 . The substrate temperature measured by an infrared pyrometer was 700°C. The TEM image and selected area electron diraction (SAED) pattern as shown in Fig. 3a indicate that a cubic silicon carbide layer has been formed just outside the silicon nanowire. The HRTEM images (Fig. 3b,c) show that a few b-SiC nanoparticles contact the core of the SiNW di- rectly for some nanowires (Fig. 3b), and that a very thick outer layer composed of b-SiC nano- particles exists for the other nanowires (Fig. 3c). There is no clear evidence of the presence of the silicon oxide layer in the ion bombarded SiNW after the ion beam deposition (Figs. 2b and 3c). It would be of great importance that the previously observed silicon oxide layer covering the silicon nanowire has been removed after the coating of the silicon carbide. It is of particular interest that the undesirable oxide layer was replaced by a thin layer of stable, wide band gap semiconductor. This new shell would allow the high transmission of both the ex- citation light and the PL emission with much less loss. It is believed that during the deposition pro- cess, hydrocarbon and hydrogen ions react with silicon oxide to form carbon oxide and silicon car- bide. The carbon oxide gas was pumped out while silicon carbide remained on the silicon nanowire. Fig. 2b shows the PL spectrum of the SiC- coated SiNW sample. It can be seen that the PL intensity has been increased by about three times. The broadening of the PL peak towards higher energy is probably due to the reduction of the di- ameters of the SiNW upon ion bombardment. The PL peak is in a similar region to that of the porous silicon, showing no apparent change dierence when the dimension of the silicon nano-structure is increased from zero to one. It is well known that SiC is relatively inert to air and thus can prevent the silicon nanowire core from further oxidation. Therefore, the coated SiNW are likely to be very stable. We have mea- sured PL of the coated SiNW after prolonged exposure to a laser and exposure to the atmo- sphere (75 days). No obvious PL degradation was Fig. 3. Typical TEM images of SiNW after ion beam treat- ment: (a) morphology at lower magni®cation; (b) morphology at higher magni®cation. X.T. Zhou et al. / Chemical Physics Letters 332 (2000) 215±218 217 observed. The present results indicate that the ion beam SiC coating process may ®nd practical usage in fabricating silicon nano-devices or photo- devices. Although there have been reports on the PL from SiC nanostructures [22,23], the nano-scale SiC layer on the silicon nanowire shows no no- ticeable PL. The absence of PL relating to SiC nano-structures may be due to insucient excita- tion in our PL measurement. In conclusion, silicon carbide coating of SiNW can be achieved by ion beam deposition. The coated SiNW show enhanced performance in terms of both the intensity and stability of PL. Acknowledgements Financial support from the Research Grant Council of Hong Kong under Grant No. 9040365 is gratefully acknowledged. References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. 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[23] O. Jessensky, F. Muller, U. Gosele, Thin Solid Films 297 (1997) 224. 218 X.T. Zhou et al. / Chemical Physics Letters 332 (2000) 215±218 . Highly ecient and stable photoluminescence from silicon nanowires coated with SiC X.T. Zhou, R.Q. Zhang, H.Y. Peng,. has led to stable photoluminescence (PL) observed from the SiC -coated SiNW with high eciency almost tripled as compared with that before SiC coating. Ó 2000