Fabrication of Crystalline Semiconductor Nanowires by Vapor-liquid-solid Glancing Angle Deposition (VLS- GLAD) Technique.

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1 Fabrication of Crystalline Semiconductor Nanowires by Vapor-liquid-solid Glancing Angle Deposition (VLS- GLAD) Technique. Journal: 2011 MRS Spring Meeting Manuscript ID: Manuscript Type: Symposium EE Date Submitted by the Author: 15-Apr-2011 Complete List of Authors: Alagoz, Arif; University of Arkansas at Little Rock, Applied Science Karabacak, Tansel; University of Arkansas at Little Rock, Applied Science Keywords: nanostructure, Ge, crystal growth

2 Page 1 of 6 Fabrication of Crystalline Semiconductor Nanowires by Vapor-Liquid-Solid Glancing Angle Deposition (VLS-GLAD) Technique Arif S. Alagoz and Tansel Karabacak Department of Applied Science, University of Arkansas at Little Rock, Little Rock, AR 72204, U.S.A. ABSTRACT Vapor-liquid-solid (VLS) method has become one of the few and most powerful bottomup single crystal nanowire growth techniques in nanotechnology due to its easy scalability from micro to nano feature sizes, high throughput, relatively low cost, and its applicability to various semiconductor materials. On the other hand, control of growth direction and crystal orientation of nanowires, which determine their electrical, optical, and mechanical properties, stand as major issues in VLS technique. In this study, we demonstrate new vapor-liquid-solid glancing angle deposition (VLS-GLAD) fabrication approach to make well-ordered arrays of crystalline semiconductor nanowires with controlled geometry. VLS-GLAD is a physical vapor deposition based method and utilizes selective deposition of source atoms onto metal catalyst nanoislands placed on a crystal wafer. VLS-GLAD incorporates an obliquely incident flux of source atoms and selective deposition on catalyst islands is achieved by shadowing effect. This geometrical process combined with conventional VLS growth mechanism leads to growth of crystalline semiconductor nanowire arrays tilted towards the incoming flux. In this study, we report morphological and structural properties of tilted single crystal germanium nanowire arrays fabricated by using conventional thermal evaporation system. In addition to the tilted geometry, by introducing substrate rotation, nanowires with various morphologies including helical, zigzag, or vertical shapes can be fabricated. Engineering crystalline nanowire morphology by using VLS-GLAD have the potential of enabling control of optical, electrical, and mechanical properties of these nanostructures leading to the development of novel 3D nano-devices. INTRODUCTION Nanotechnology is a rapidly expanding interdisciplinary field, promising novel devices for broad range of applications. Quantum effects and surface to volume ratio of nanostructures are strongly size dependent, and redefine material properties at nanoscale. Therefore enabling material fabrication and characterization techniques are critical for the development of nanodevices. Over the past decades, a vast amount of research work focused on controlled fabrication and characterization of 0D nanodots [1], 1D nanowires [2] and 2D layers [3]. On the other hand, fabrication of well controlled 3D nanostructures in specific shape, size, and crystal structure is a still a big challenge. About a half century ago, Wagner and Ellis demonstrated homoepitaxial growth of crystal silicon wires (or whiskers) by chemical vapor deposition (CVD) of silicon atoms trough small droplets of metal catalyst [4-6]. They showed that at elevated temperatures metal-si eutectic formation was followed by the growth of crystalline silicon wires and called this mechanism vapor-liquid-solid (VLS) growth. As an alternative approach to this CVD based VLS method, physical vapor deposition (PVD) based VLS techniques have also been developed [7].

3 Page 2 of 6 Meantime, VLS method has become one of the few and most powerful bottom-up single crystal nanowire growth techniques in nanotechnology due to its easy scalability from micro to nano feature sizes, high throughput, relatively low cost and its applicability to various semiconductor materials [2]. Individual VLS grown nanowires have been studied for a wide range of device applications including p-n junction diodes [8], light emitting diodes (LEDs) [9], photodetectors [10], field effect transistor (FETs) [11], logic gates [12], sensors [13], nanogenerators [8], and solar cells [14-16]. On the other hand, control of growth direction and crystal orientation, which determine their electrical, optical, and mechanical properties, stand as major issues in VLS technique at wafer scale fabrication [17]. Glancing angle deposition (GLAD) technique [18], also called oblique angle deposition [19], provides the capability of fabrication 3D nanostructure arrays [20]. GLAD technique relies on the shadowing effect, where atoms depositing obliquely on to the substrate may not lower height points on due to the neighboring taller structures. Taller points capture most of the flux and and therefore shadowed shorter ones cannot grow. During GLAD, nanowire arrays can be grown on specific locations by sample patterning using conventional micro- and nanolithography techniques [20]. Controlling deposition parameters allows synthesis of three dimensional nanorods [18]. On the other hand, conventional GLAD is not an effective method to fabricate single crystal non-metallic or semiconductor nanowires, and typically results in polycrystalline [21] or amorphous [22] semiconductor or non-metallic nanostructures. Yet, selfcatalyst indium thin oxide [23] and catalyst assistant carbon nanowire [24] growth has been achieved to some extent during GLAD. In this work, we report a new vapor-liquid-solid glancing angle deposition (VLS-GLAD) approach for the growth of single crystal semiconductor nanowire arrays with a controlled geometry. Different than conventional CVD or PVD based VLS methods, VLS-GLAD can allow the control of nanowire growth due to the anisotropy in the direction catalyst island is fed, leading to the arrays of tilted nanowires. Engineering crystalline nanowire morphology by using VLS-GLAD have the potential of enabling control of optical, electrical, and mechanical properties of these nanostructures leading to the development of novel 3D nano-devices. EXPERIMENT As a first step, well-ordered gold catalyst nanoislands on silicon (100) wafer are fabricated by nanosphere lithography (NSL) [25] technique. 490 nm diameter polystyrene (PS) nanospheres are self-assembled into double layer hexagonal closely packed (hcp) by using convectional self-assembly method [26]. Next, nanospheres are etched under oxygen plasma, in order to increase the gaps in between them. These nanospheres are used as a shadow mask for 100 nm thin film Au evaporation and are lifted-off by immersing into toluene for 15 minutes. Next, samples are rinsed with deionized water and dried with nitrogen gas. Following the well-ordered gold nanoisland fabrication, sample was heated up to 400 o C, and a flux of evaporated germanium atoms was directed to gold nanoislands at a deposition angle of 85 o. Both gold and germanium evaporation were performed by using conventional thermal evaporation technique at the base pressure of about mbar and working pressure of mbar. Deposition rate of gold and germanium were measured by quartz crystal microbalance. Finally, gold catalyst on top of germanium nanowires is removed by using Transene gold etchant TFA.

4 Page 3 of 6 JEOL 7000F scanning electron microscope (SEM) is used to investigate morphology of Ge nanowires and JEOL JEM 2100F transmission electron microscope (TEM) is used to analyze crystal structure of these nanowires. DISCUSSION We fabricated well-ordered gold catalyst nanoislands by using nanosphere lithography technique. Figure 1a shows top view scanning electron microscope image of hexagonally closed packed double layer polystyrene nanospheres of 490 nm diameter. In order to increase the gaps between the nanospheres, they were etched in oxygen plasma and their diameter was decreased as shown in figure 1b. Etched nanospheres were used as a shadow mask for 100 nm thin film gold deposition. Depositing thin film gold layer on top of nanospheres filled the gaps and after nanosphere lift-off hexagonal patterned gold catalyst nanoislands were obtained as shown in figure 1c. (a) (b) (c) Figure 1 Top view scanning electron microscope image of hexagonally closed packed double layer polystyrene nanospheres of 490 nm diameter (a), nanospheres after oxygen etching and gold nanoisland after 100 nm thin film gold deposition and nanosphere lift-off. Scale bar is 500 nm. Following patterned catalyst nanoisland fabrication on crystalline silicon wafer, sample was heated up to 400 o C, and a flux of evaporated germanium atoms was directed to gold nanoislands at a deposition angle of θ = 85 o. At high deposition angles with respect to substrate normal, incoming atoms selectively deposit on catalyst islands due to shadowing effect, forms eutectic, supersaturate the liquid island, and deposit to the bottom crystalline surface to initiate the hetero-epitaxial nanowire growth. As the deposition progresses, other atoms follow the similar process making the nanowire longer and longer but still keeping the catalyst metal island on the wire tip. In figure 2 scanning electron microscope images reveal that all nanowires are affected from the oblique incidence flux of germanium atoms and formed tilted nanowires along the flux direction. Gold catalyst can be seen on top of each nanowire. Different than conventional CVD or PVD based VLS methods, anisotropy in the direction catalyst island is fed leads to the arrays of tilted nanowires. Nanowire growth is not uniform and shorter nanowires are shadowed by taller nanowires during growth.

5 Page 4 of 6 (a) (b) Figure 2 Side (a) and top (b) view scanning electron microscope images of tilted crystalline germanium nanowires fabricated by vapor-liquid-solid glancing angle deposition (VLS-GLAD) technique. Obliquely incident flux of germanium is along the direction of the arrow. Scale bars are 500 nm. On the other hand, crystalline silicon wafer without any gold catalyst does not show similar nanowire growth behavior at the same deposition conditions, instead porous thin film germanium with tilt towards the flux direction is obtained as shown in figure 3. (a) Figure 3 Side (a) and top (b) view scanning electron microscope images of tilted germanium nanowires fabricated on silicon wafer without gold catalyst. Obliquely incident flux of germanium is along the direction of the arrow. Scale bars are 500 nm. Figure 4 shows high resolution transmission electron microscope image of individual germanium nanowire. Nanowire is single crystalline and grown along (111) direction. (b)

6 Page 5 of 6 Figure 4 High resolution transmission electron microscope image of single germanium nanowire fabricated by vaporliquid-solid glancing angle deposition (VLS-GLAD) technique, scale bar is 5 nm. CONCLUSIONS In conclusion, we showed a new vapor-liquid-solid (VLS) based nanowire fabrication approach to fabricate well-ordered arrays of crystalline semiconductor nanowires. This new nanowire fabrication approach enables control on feature size, shape, and alignment of nanowires. Engineering crystalline nanowire morphology by using VLS-GLAD can enable the control of optical, electrical, and mechanical properties of nanostructures leading to the development of novel 3D electronic and optoelectronic devices such as solar cells, photodetectors, and light emitting diodes. ACKNOWLEDGMENTS This work was supported by a NASA/Arkansas EPSCoR Research Infrastructure Development grant administered through the Arkansas Space Grant Consortium, and by matching funds from the University of Arkansas at Little Rock. The authors would also like to thank the UALR Nanotechnology Center staff for their assistance in SEM and TEM measurements. REFERENCES 1. A. P. Alivisatos, Science 271, 5251 (1996). 2. V. Schmidt, J. V. Wittemann, S. Senz, and U. Gosele, Adv Mater 21, (2009). 3. A. K. Geim, Science 324, 5934 (2009).

7 4. R. S. Wagner, and W. C. Ellis, Appl. Phys. Lett. 4, 5 (1964). 5. R. S. Wagner, and W. C. Ellis, Transactions of the Metallurgical Society of Aime 233, 6 (1965). 6. R. S. Wagner, and C. J. Doherty, J. Electrochem. Soc. 113, 12 (1966). 7. A. M. Morales, and C. M. Lieber, Science 279, 5348 (1998). 8. X. D. Wang, J. H. Song, J. Liu, and Z. L. Wang, Science 316, 5821 (2007). 9. X. F. Duan, Y. Huang, Y. Cui, J. F. Wang, and C. M. Lieber, Nature 409, 6816 (2001). 10. J. F. Wang, M. S. Gudiksen, X. F. Duan, Y. Cui, and C. M. Lieber, Science 293, 5534 (2001). 11. Y. Cui, and C. M. Lieber, Science 291, 5505 (2001). 12. Y. Huang, X. F. Duan, Y. Cui, L. J. Lauhon, K. H. Kim, and C. M. Lieber, Science 294, 5545 (2001). 13. Y. Cui, Q. Q. Wei, H. K. Park, and C. M. Lieber, Science 293, 5533 (2001). 14. M. Law, L. E. Greene, J. C. Johnson, R. Saykally, and P. D. Yang, Nature Materials 4, 6 (2005). 15. L. Tsakalakos, J. Balch, J. Fronheiser, B. A. Korevaar, O. Sulima, and J. Rand, Appl. Phys. Lett. 91, 23 (2007). 16. M. D. Kelzenberg, D. B. Turner-Evans, B. M. Kayes, M. A. Filler, M. C. Putnam, N. S. Lewis, and H. A. Atwater, Nano Letters 8, 2 (2008). 17. M. Willander, O. Nur, Q. X. Zhao, L. L. Yang, M. Lorenz, B. Q. Cao, J. Z. Perez, C. Czekalla, G. Zimmermann, M. Grundmann, A. Bakin, A. Behrends, M. Al-Suleiman, A. El- Shaer, A. C. Mofor, B. Postels, A. Waag, N. Boukos, A. Travlos, H. S. Kwack, J. Guinard, and D. L. S. Dang, Nanotechnology 20, 33 (2009). 18. K. Robbie, and M. J. Brett, Journal of Vacuum Science & Technology A-Vacuum Surfaces and Films 15, 3 (1997). 19. T. Karabacak, J. P. Singh, Y. P. Zhao, G. C. Wang, and T. M. Lu, Phys. Rev. B 68, 12 (2003). 20. J. P. Singh, T. Karabacak, D. X. Ye, D. L. Liu, C. Picu, T. M. Lu, and G. C. Wang, J. Vac. Sci. Technol. B 23, 5 (2005). 21. W. K. Choi, L. Li, H. G. Chew, and F. Zheng, Nanotechnology 18, 38 (2007). 22. C. Patzig, and B. Rauschenbach, Journal of Vacuum Science & Technology a 26, 4 (2008). 23. C. H. Chang, P. Yu, and C. S. Yang, Appl. Phys. Lett. 94, 5 (2009). 24. H. X. Zhang, and P. X. Feng, Journal of Physics D-Applied Physics 42, 2 (2009). 25. C. L. Haynes, and R. P. Van Duyne, J Phys Chem B 105, 24 (2001). 26. B. G. Prevo, D. M. Kuncicky, and O. D. Velev, Colloids and Surfaces A-Physicochemical and Engineering Aspects 311, 1-3 (2007). Page 6 of 6