NANO LETTERS
Erbium Surface-Enriched Silicon Nanowires
2002 Vol. 2, No. 11 1303-1305
Zhaoyu Wang and Jeffery L. Coffer* Department of Chemistry, Texas Christian UniVersity, Ft. Worth, Texas 76129 Received August 27, 2002
ABSTRACT The synthesis of silicon nanowires containing erbium in a discrete layered structure has been achieved. These wires are prepared by the pyrolysis of silane and a volatile erbium complex on a gold catalyst surface, with the relative size of the wire and sample crystallinity affected by the timing in which erbium is introduced into the reactant stream. Products were characterized by field emission scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and energy-dispersive X-ray analysis. Given their ability to demonstrate the desired near-infrared luminescence at 1540 nm, the synthesis of these erbium surface-enriched Si nanostructures provides a synthetic strategy for new types of optically active layered platforms for studies in fundamental photophysics, carrier transport, and nanophotonics.
Nanoscale silicon (Si), in the form of zero dimensional dots1 or one-dimensional (1-D) wires,2 has drawn extensive attention recently due to fundamental size dependent carrier transport and/or optical confinement effects. For the latter, such phenomena have long-term implications with regard to optoelectronic devices, where it is useful to induce light emission from Si via ultrasmall dimensions or from extrinsically fluorescent dopant centers such as Er3+.3 As a consequence, we have been investigating the synthesis and characterization of small (3-25 nm) nanocrystals of Si doped with Er3+, since its (4I13/2)f (4I15/2) luminescent transition at 1.54 µm lies at a transmission maximum for silica-based wave guides.4 In addition to these zero-dimensional dots, it is also important to fabricate one-dimensional nanoscale wires composed of these elements for potential value in both electrical and optical applications. The former is of merit given the known ability of erbium-rich silicon phases to act as efficient ohmic contacts on Si with relatively low energy Schottky barrier height(s).5 For the latter, one can envision (with the proper wire width) the propagation of light through a continuous Si network possessing the proper refractive index gradient as a consequence of the erbium preferentially enriched at the wire surface. Hence in this letter we describe two complementary approaches to the fabrication of Si nanowires possessing distinctive layered structures with Sirich cores and Er-rich surface layers. To achieve this target, our synthetic strategies were strongly influenced by our previous approaches to Er-doped Si nanocrystals (with the rare earth centers in either randomly dispersed or surface-enriched varieties)4 coupled with the demonstrated utility of vapor-liquid-solid (VLS)-type routes for Si nanowire nucleation and growth.6 One straight* Corresponding author. E-mail:
[email protected] 10.1021/nl025771a CCC: $22.00 Published on Web 09/21/2002
© 2002 American Chemical Society
forward combination is to initially grow the silicon nanowires on Au islands in the presence of dilute silane in He, followed by pyrolysis of a volatile Er(tmhd)3 complex. In a typical reaction, a 5 × 15 mm piece of CZ Si containing thermally evaporated Au islands was placed in an alumina boat which, in turn, is positioned inside a quartz tube reactor with an active oven length of 6 cm. After annealing in He (600 °C for ∼ 2 h, resulting in Au catalyst features 400-800 nm in size), SiH4 is then introduced into the system (0.1% in UHP grade He) at a flow rate of 40 sccm, further diluted with additional He (3000 sccm), and the sample substrate is heated at a temperature of 600 °C for 2 min. In a separate step, erbium incorporation is achieved by passing He through a bubbler (heated to ∼144 °C) containing Er(tmhd)3, which was introduced into the pyrolysis oven. The oven temperature was maintained at about 600 °C for 1 h during Er incorporation. The resultant product was annealed in a vacuum at 900 °C. Scanning electron microscopy (SEM) images (Figure 1) of the reaction products reveal numerous interwoven nanowires with diameters of 90-250 nm and lengths up to 10 µm. Energy-dispersive X-ray measurements (EDX) confirmed the presence of erbium in these structures, and subsequent analysis by field emission SEM suggested appreciable roughness on the surface of the wires. To understand the structure in more detail, wires were detached from the substrate by dispersion in 2-propanol and analyzed by transmission electron microscopy (TEM). Such studies reveal that the wires have a core-shell structure with erbium enriched at the surface. Figure 2 shows a typical wire with a diameter of ∼225 nm. In terms of quantitative elemental analyses, three distinct areas are found from the EDX spectra (Supporting Information). The center of the wire is clearly
Figure 1. SEM image of an array of erbium surface-enriched Si nanowires (JEOL JSM-6320F). Scale bar is 1 µm.
Figure 3. Room-temperature photoluminescence (PL) spectrum of Er surface-enriched Si nanowires after a vacuum anneal, demonstrating the near-IR emission near 1540 nm (λex ) 488 nm).
Figure 2. A typical TEM image of a surface Er enriched Si wire (JEOL JEM-3010). Inset: SAED pattern from the center of the wire. EDX analysis for the marked three areas are presented in the text.
Si-rich (96%) (area 2) and the wire clearly has a dark rim visible which is high in Er concentration. On the surface of the wire, however, there are two different regions; one relatively smoother area closer to the core with an appreciably larger erbium concentration (∼12%) (area 3), along with nodules emanating from the “stalk” that are nearly balanced in terms of amounts of erbium (53%) and silicon (47%) (area 1). Crystallographically selective area electron diffraction (SAED) measurements recorded on the nanowire exhibit a (110) diffraction pattern and clearly indicate that the doped Si NW is single-crystal in nature (inset, Figure 2). The diffraction pattern taken from the Er rich surface layer reveals that the surface layer is also crystalline. Unfortunately, the wire is apparently too thick to obtain lattice images at atomic resolution from the Si. Importantly, the room-temperature photoluminescence of these arrays has also been measured. Upon excitation at 488 nm, the anticipated Er3+ emission maximum near 1.54 µm, associated with the (4I13/2) f (4I15/2) transition, is observed (Figure 3). Detectable erbium emission at room temperature in a crystalline semiconductor is often difficult to achieve, and it is likely that dimensional suppression of the Auger processes play a role. Preliminary excitation wavelength dependence measurements (in range of 470-514 nm) indicate a rather insensitive response in terms of near-IR emission intensity. This would suggest that an ensemble of 1304
Figure 4. SEM images of Er-doped Si NW grown by concomitant co-pyrolysis of silane and Er(tmhd)3. Top: plan view image; bottom: backscattered electron image illustrating the presence of the erbium preferentially at the surface (light shading) vs the Sirich core (dark).
these wires emits with a combination of both carrier-mediated and direct excitation pathways, perhaps a consequence of the two different average structural environments detected in their microstructure. This effect is unique in a rare earth doped nanostructure, yet understandable in terms of the erbium present within the core as being accessible to energy transfer from the Si while those erbium centers at the shell of the wire are subject to direct excitation of the relevant ligand field states. This effect is currently under further investigation. The second strategy to fabricate Si wires with Er-rich wires entails the concomitant presence of silane and the Er(tmhd)3 precursor during the pyrolysis event under conditions which will exude the erbium from wire (as a consequence of the limited solubility of erbium in Si).7 For the second strategy, the same reactor design is employed as in the first approach but now in a three step reaction sequence. After a brief silane preexposure (20 sccm, 5 min), the SiH4 and Er(tmhd)3 are co-pyrolyzed at 600 °C for 25 min, followed by an extended Nano Lett., Vol. 2, No. 11, 2002
Er3+ exposure for an additional 60 min. SEM studies of this reaction product, particularly from Z-contrast imaging in backscattered mode, also show a network of uniform interwoven wires with a distinct Er rich surface (as high as 50%) and a Si rich core. The presence of the Er3+ centers during SiH4 pyrolysis clearly impacts wire growth kinetics, as these wires are much larger in terms of diameter with widths on the order of 700-800 nm. SAED measurements and micro-Raman spectroscopy indicate that the nanostructures are poorly crystalline in the Si core and amorphous in the Er-enriched shell. Preliminary micro-Raman analyses suggest that the crystallinity of such structures can be improved by high-temperature annealing at 800 °C. In summary, Si/Er core-shell nanowires were fabricated successfully via a simple vapor transport process. Structural analyses by electron microscopies and X-ray dispersive measurements reveal that the Er is enriched on the surfaces of the nanowires. After a vacuum high-temperature anneal, the characteristic near-IR PL associated with Er3+ transitions was obtained. The synthesis of these crystalline core-shell luminescent nanowires is expected to open up new possibilities in the creation of novel optoelectronic devices using these nanowires as building blocks. Acknowledgment. The authors thank the National Science Foundation (Grant DMR 98-19178) and the Robert A. Welch Foundation for financial support of this research. The expertise of Dr. Alan Nichols of the Electron Microscopy
Nano Lett., Vol. 2, No. 11, 2002
Facility of the Research Resources Center of the University of Illinois-Chicago is also gratefully acknowledged. Supporting Information Available: Details of reactor design and representative EDX spectra. This material is available via the Internet at http://pubs.acs.org. References (1) Brus, L. E. J. Phys. Chem. 1994, 98, 3575. (2) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. Cui, Y.; Duan, X.; Hu, J.; Lieber, C. M. J. Phys. Chem. B 2000, 104, 5213. Cui, Y.; Lieber, C. M. Science 2001, 291, 851. Cui, Y.; Wei, Q,; Park, H.; Lieber, C. M. Science 2001, 293, 1289. Huang, Y.; Duan, X.; Wei, Q.; Lieber, C. M. Science 2001, 291, 630. Holmes, J. D.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Science 2000, 287, 1471. Yu, J.Y.; Chuang, S.-W.; Heath, J. K. J. Phys. Chem. B 2000, 104, 11864. Wu, Y.; Fan, R.; Yang, P. Nano Lett. 2002, 2, 83. Cui, Y.; Lauhon, L. J.; Gudiksen, M. S.; Wang, J.; Lieber, C. M. Appl. Phys. Lett. 2001, 291, 851. (3) Polman, A. J. Appl. Phys. 1997, 82, 1. (4) St. John, J.; Coffer, J.; Chen, Y.; Pinizzotto, R. J. Am. Chem. Soc. 1999, 121, 1888. St. John, J.; Coffer, J.; Chen, Y.; Pinizzotto, R. Appl. Phys. Lett. 2000, 77, 1635. Senter, R.; Chen, Y.; Coffer, J.; Tessler, L. Nano Lett. 2001, 1, 383. St. John, J.; Coffer, J. J. Phys. Chem. 2001, 105, 7599. Ji, J.; Senter, R.; Coffer, J. Chem. Mater. 2001, 13, 4783. (5) Norde, N.; deSousaPires, J.; d’Heurle, F.; Pessavento, F.; Peterson, S.; Tove, P. A. Appl. Phys. Lett. 1981, 38, 865. (6) Westwater, J.; Gosain, D. P.; Tomiya, S.; Usui, S.; Ruda, H. J. Vac. Sci. Technol. B, 1997, 15, 554. (7) Polman, A.; van den Hoven, G. N.; Custer, J. S.; Shin, J. H.; Serna, R. J. Appl. Phys. 1997, 77, 1256.
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