Low-temperature growth and Raman scattering study of. vertically aligned ZnO nanowires on Si substrate

Transcription

1 Low-temperature growth and Raman scattering study of vertically aligned ZnO nanowires on Si substrate Ye Zhang, Hongbo Jia, Dapeng Yu a), Rongming Wang, Xuhui Luo School of Physics, National Key Laboratory of Mesoscopic Physics, and Electron Microscopy Laboratory, Peking University, Beijing , People s Republic of China Cheoljin Lee Department of Nanotechnology, Hanyang University, Seoul , Korea 1) Author to whom correspondence should be addressed; electronic mail: yudp@pku.edu.cn 1

2 Abstract High-density ZnO nanowires were successfully aligned onto Au catalyzed Si substrate through a simple low-temperature physical vapor deposition method. SEM observations, XRD, and PL spectra show that ZnO nanowires were single crystal with hexagonal wurzite structure. All the results inferred from SEM, XRD rocking curve, and Raman data for investigated samples confirm that ZnO nanowires are well-aligned and c-axis oriented. Raman spectra also indicated that ZnO nanowires on Si are under bi-axial compressive stress. Since it take the advantage of low cost, easy growth position (due to the selective deposition trait of the Au layer), potential for scale-up production, and ability to integrate with Si substrate, this technique looks a potential future for fabricating ZnO NW array-based optoelectronic devices. PACS: 7840Fy, 8280Ch, 7855-m, 8540Ux, 7866Jg 2

3 Significant advances, in the areas of wide bandgap semiconductor nanowire (NW) growth and fabrication of NWs with well-defined configuration, have been achieved over recent few years. 1 Yang s group pioneered the study of optically pumped UV lasing from epitaxially grown ZnO NW arrays on sapphire substrate (880 C). 2 Kim et al. reported that a single-crystal GaN nanorod array could be grown on sapphire substrate via hydride vapor phase epitaxy (HVPE) technique. 3 Nevertheless, sapphire is a kind of nonconductor and also relatively expensive. This may be a serious limitation to the application of NW array in optoelectric devices. In the pursuit of next generation of semiconductor NW-based optoelectronic nanodevice, it will be highly desirable if well-ordered NWs can be directly aligned on conductive and cheap substrate, such as Si wafer. Unfortunately, their large differences in thermal expansion coefficients and lattice constants cause a rather large stress between ZnO (GaN) and Si wafer. Moreover, the stress between ZnO (GaN) and Si will be aggravated at high temperature. Many previous efforts preparing well-orient ZnO NW array on Si wafer met an unsatisfactory quality because of the large stress between ZnO and Si at high growth temperature as well as by reason of their chemical dissimilarities. 4 We believe high-quality ZnO NW array can be achieved by an appropriate choice of nucleation catalyst and growth temperature. In previous paper, we reported the field emission properties from well-aligned ZnO NWs grown on Si wafer at low temperature of 450 C. 5 A sol-gel method was used to deposit Co nanoparticles on Si wafer as metal catalyst to initiate the VLS growth of ZnO NW array. However, this wet-chemistry method may cause pollution problem to semiconductor samples and also be incompatible with conventional semiconductor fabrication technique. In the letter, we report that well-aligned ZnO NW arrays were fabricated on Au-coated Si wafer via vaporizing metal Zn powder at low growth temperature of 500 C. The Raman scattering characteristics of the as-grown c-axis-oriented ZnO NW array were also studied at length. Synthesis of ZnO NWs array was carried out in a conventional furnace with a horizontal alumina tube. In a typical process, a 2 nm Au layer was thermally evaporated onto a Si (100) wafer to serve as catalyst. The Au-coated Si substrate was 3

4 put downward on an alumina boat loaded with Zn powder (purity: %). The vertical distance between Zn source and Si substrate was ~5 mm. Then, the alumina boat was transferred into the center of tube furnace. Afterwards, the chamber was heated up to 500 C at a rate of 20 C/min under a constant flow of Ar (99.9%) of 200 sccm and kept for 5h. After cooling, a white layer was found deposited on Au-coated Si wafer. The as-deposited products were characterized by scanning electron microscopy (SEM) [Amray FEG-1910] and X-ray diffraction (XRD) [X pert MRD-Philips diffractometer]. To investigate the crystal orientation, XRD rocking curve was taken. The PL spectra were excited with a 325 nm line of a He Cd laser and measured at room temperature. Raman scattering was performed in the near backscattering geometry using nm line of an Ar + laser with a power of 20 mw. The scattered radiation was analyzed with a double monochromator and detected with liquid nitrogen cooled charge coupled device. In order to identify the crystal phase of the deposited materials on Si substrate, samples were analyzed by X-ray diffraction. The XRD spectrum (Fig. 1(a)) reveals that the as-synthesized products are hexagonal wurzite ZnO with lattice constants of a = 3.24 Å and c = 5.20 Å. SEM images show that high density nanowires are vertically aligned to the Si substrate (Fig. 2). The whole surface of Au-coated Si is deposited by ZnO NW arrays which present film-like morphology as visualized by low SEM magnification images (Fig. 2(a)). The nanowires have diameters of nm and length of several micrometers. It is illustrated in the inset of figure 2(a) that some terminals of ZnO NWs are hexagonal faceted. These hexagonal faceted terminals imply that the one-dimensional (1-D) growth of wurzite ZnO follows c-axial direction. Figure 1(b) demonstrates that the FWHM of the X-ray rocking curve of the (002) peak is as narrow as 0.28 º. The narrow FWHM of XRD rocking curve provides statistical evidence that ZnO NWs are indeed vertically oriented on Si wafer preferentially with (002) plane parallel to Si substrate, viz. NWs possessing a c-axial 1-D growth direction. The O element of ZnO crystals must origin from residual oxygen in the 4

5 apparatus. The 1-D growth of ZnO nanowires can be explained by a well-known vapor-liquid-solid (VLS) growth mechanism. 6 Low growth temperature is indispensable to acquiring well-aligned ZnO NWs on Si wafer. Stress seems to be minimized at low growth temperature (500 C). Experimental results demonstrate that only randomly distributed ZnO NWs were formed on Si substrates at growth temperature 550 C. Space group of hexagonal wurzite ZnO belongs to C 4 6ν with two formula units per primitive cell. According to group theory, single crystal ZnO has eight sets of optical phonon modes at Γ point of brillouin zone classified as A 1 + E 1 + 2E 2 modes (Raman active), 2B 1 modes (Raman silent), and A 1 + E 1 modes (infrared-active). Moreover, the A 1 and E 1 modes split into longitudinal (LO) and transverse optical (TO) components. Figure 3 shows that Raman spectra of ZnO NW array exhibit only E 2 and A 1 (LO) modes at 438 and 581 cm -1, respectively. The absence of TO modes in Raman spectra could be attributed to special angle between the wave vector of photons and c axis direction of wurzite ZnO crystals in the near backscattering geometry employed in our measurement. In the measurement, Raman spectrum was recorded in the backscattering geometry with incident light exactly perpendicular to the surface of ZnO NW array, viz. incident light parallel to the axis of ZnO nanowire. In this configuration, only E 2 and A 1 (LO) modes are allowed, and meanwhile the A 1 (TO) and E 1 (TO) modes are forbidden according to the Raman selection rules. Thus, the absence of TO modes further confirms that ZnO NW array on Si substrate is highly c-axis oriented. Anyhow, all the results inferred from SEM, XRD rocking curve and Raman data for investigated ZnO NW arrays agree with each other. In order to estimate the value of stress between ZnO nanowire and Si substrate from Raman data, we use a model based on the result of Frederic Decremps and Julio Pellicer-Porres. 7 They pointed out that the frequency of E 2 mode of wurzite ZnO shifts to higher value under biaxial compressive stress within c-oriented ZnO epilayers by ω (cm -1 ) = 4.4 σ (GPa). 7 In our case, the frequency of E 2 mode observed in as-synthesized ZnO NW array is 1 cm -1 higher than that value observed in 5

6 bulk ZnO. 8 The higher value of E 2 mode of ZnO NW arrays in comparison with 437 cm -1 of bulk crystal demonstrates that ZnO nanowires with larger bulk lattice constants grown on Si substrate are under compressive stress. The magnitude of the stress estimated from the frequency shift of the E 2 mode is GPa. It was reported that the biaxial compressive stress within ZnO films grown on Si (100) was in the range GPa. 9 The smaller stress of ZnO NW arrays compared with that of ZnO films should be attributed to stress relaxation effect from ZnO nanowires. Figure 4 shows representative PL spectrum of ZnO NW array which consists of a strong UV peak at 3.29 ev (377.0 nm in wavelength) and a weak green band showing a broad feature in the rang ev ( 430~600 nm in wavelength). The UV emission band must be related to a near bandedge transition of ZnO, namely the recombination of free excitons through an exciton exciton collision process. 10 The green band was generally explained by the radial recombination of a photo-generated hole with the electron in a singly ionized oxygen vacancy. 11 The strong UV emission and weak green band in the PL spectra means that the ZnO NWs grown at low temperature are of good crystal quality with few oxygen vacancies. In summary, high-density vertically oriented ZnO nanowire arrays were fabricated on Si wafer via a low growth temperature physical vapor deposition method. SEM observations, XRD, and PL spectra demonstrate that ZnO NW arrays possess good crystalline character. The nanowires have diameters of nm and length of several micrometers. All the results inferred from SEM, XRD rocking curve, and Raman data for investigated samples confirm that ZnO nanowires are well-aligned and c-axis oriented. Low growth temperature is an excellent candidate for high-quality ZnO NW array production on Si substrate. Raman spectra also indicated that ZnO nanowires on Si are under bi-axial compressive stress. The estimated stress value from Raman spectra is Pa. Since it take the advantage of low cost, easy growth position (due to the selective deposition trait of the Au layer), potential for scale-up production, and ability to integrate with Si substrate, this technique looks a potential future for fabricating ZnO NW array-based optoelectronic devices. 6

7 Dr. Ye Zhang and Mr. Hongbo Jia contributed equally to the work. This work was supported by the National Natural Science Foundation of China (Grant No ), the Research Fund for the Doctoral Program of Higher Education (RFDP) of China, and Jun-Zheng Fund of Peking University, China. References 1 (a) Y. Y. Wu, H. Q. Yan, M. Huang, B. J. Messer, J. H. Song, and P. D. Yang, Chem. Eur. J. 8, 1261 (2002). (b) Y. N. Xia, P. D. Yang, Y. G. Sun, Y. Y. Wu, B. Mayers, B. Gates, Y. D. Yin, F. L. Kim, and H. Q. Yan, Adv. Mater. 15, 353 (2003). 2 M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, and P. Yang, Science 292, 1897 (2001). 3 H. M. Kim, T. W. Kang, and K. S. Chang, Adv. Mater. 15, 567 (2003). 4 M. H. Huang, Y. Y. Wu, H. Feick, N. Tran, E. Weber, and P. D. Yang, Adv. Mater. 13, 113 (2001). 5 C. J. Lee, T. J. Lee, S. C. Lyu, Y. Zhang, H. Ruh, and H. J. Lee, Appl. Phys. Lett. 81, 3648 (2002). 6 R. S. Wagner and W. C. Ellis, Appl. Phys. Lett. 4, 89 (1964). 7 F. Decremps, J. P. Porres, A. M. Saitta, J. C. Chervin, and A. Polian Phys. Rev. B 65, (2002). 8 J. M. Calleja and M. Cardona, Phys. Rev. B 16, 3753 (1977). 9 S. W. Whangbo, H. K. Jang, S. G. Kim, M. H. Cho, K. Jeong, and C. N. Whang J. Kor. Phys. Soc. 37, 456 (2000). 10 Y. C. Kong, D. P. Yu, B. Zhang, W. Fang, and S. Q. Feng, Appl. Phys. Lett. 78, 407 (2001). 11 K. Vanheusden, W. L. Warren, C. H. Seager, D. K. Tallant, J. A. Voigt, and B. E. Gnade, J. Appl. Phys. 79, 7983 (1996). 7

8 Figure Captions FIG. 1. (a) XRD spectrum of ZnO nanowire array on Si (100) substrate. (b) The X-ray rocking curve of ZnO (002). FIG. 2. SEM images of well-aligned ZnO nanowires grown on Si substrate. (a) Low magnification image; inset: magnified image of well-aligned ZnO nanowires. (b) High-density well-aligned ZnO nanowires. (c) The image of ZnO nanowire array from an oblique view angle. FIG. 3. Raman spectrum of well-aligned ZnO nanowires on Si substrate. FIG. 4. Photoluminescence of ZnO nanowires measured at room temperature. 8

9 Intensity (a. u.) (100) (002) (101) (102) (103) (a) theta (degree) Intensity (a. u.) (b) rocking curve of ZnO (002) theta (degree) FIG. 1 9

10 (a) (b) (c) FIG. 2 10

11 20 Si sub. (520 cm -1 ) Intensity (a. u.) E 2 (high, 438 cm -1 ) A 1 (LO, 581 cm -1 ) Raman shift (cm -1 ) FIG. 3 11

12 20000 Intensity (a. u.) Wavelength (A) FIG. 4 12