NANO LETTERS
Self-Organized Growth of Si/Silica/ Er2Si2O7 Core−Shell Nanowire Heterostructures and their Luminescence
2005 Vol. 5, No. 12 2432-2437
Heon-Jin Choi,*,† Jung H. Shin,*,‡ Kiseok Suh,‡ Han-Kyu Seong,†,§ Hee-Chul Han,† and Jung-Chul Lee§ School of AdVanced Materials Science and Engineering, Yonsei UniVersity, Seoul 120-749, Korea, Department of Physics, Korea AdVanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon, Korea, and DiVision of Materials Science and Technology, Korea Institute of Science and Technology (KIST), Seoul 136-791, Korea Received August 23, 2005; Revised Manuscript Received October 4, 2005
ABSTRACT Self-organized Si−Er heterostructure nanowires showed promising 1.54 µm Er3+ optical activity. Si nanowires of about 120-nm diameter were grown vertically on Si substrates by the vapor−liquid−solid mechanism in an Si−Er−Cl−H2 system using an Au catalyst. Meanwhile, a singlecrystalline Er2Si2O7 shell sandwiched between nanometer-thin amorphous silica shells was self-organized on the surface of Si nanowires. The nanometer-thin heterostructure shells make it possible to observe a carrier-mediated 1.53 µm Er3+ photoluminescence spectrum consisting of a series of very sharp peaks. The Er3+ spectrum and intensity showed absolutely no change as the temperature was increased from 25 to 300 K. The luminescence lifetime at room temperature was found to be 70 µs. The self-organized Si nanowires show great potential as the material basis for developing an Si-based Er light source.
Monolithic integration of photonics with silicon as the material basis has been the subject of active research in recent years.1 A key part of such integrated photonics that is yet to be developed is an efficient light source. Of the many different approaches,2-5 using Er as an optical dopant has attracted particular attention because Er can be excited via carriers6 to obtain a light emitter at the wavelength of 1.5 µm7,8 that is both technologically important and transparent to Si, enabling integration with other Si-based photonic and electronic applications.9-12 However, the solubility and luminescence efficiency of Er in Si is quite low,13 and theoretical and experimental evidences suggest that it may be fundamentally limited.14,15 Overcoming these limitations requires nanometer-scale design of the material composition and structure.16-24 Recently, we have reported on successful optical activation of Si nanowires by surface-coating with Er-doped silica, suggesting that one-dimensional Si nano* J. H. Shin and H. J. Choi contributed equally to the manuscript. Send correspondences to either
[email protected] or
[email protected]. † School of Advanced Materials Science and Engineering, Yonsei University. ‡ Department of Physics, Korea Advanced Institute of Science and Technology. § Division of Materials Science and Technology, Korea Institute of Science and Technology. 10.1021/nl051684h CCC: $30.25 Published on Web 11/11/2005
© 2005 American Chemical Society
structures have potential as a new material platform for Sibased photonics.25 Here we report on bottom-up selfassembled growth of a dense array of nanowire heterostructures that consist of a single-crystalline Si core surrounded by nanometer-thin shells of amorphous silica and single-crystalline Er2Si2O7, forming an ideal Er-Si nanostructure that provides an unprecedented combination of high Er concentration, narrow emission peak, high luminescence efficiency, and the possibility of electrical pumping in a structure that forms a dense array of high-quality optical cavity naturally.26 The Si nanowire array was prepared using a chemical vapor transport process by employing silicon tetrachloride (SiCl4, Alfa, 99.999%) as the silicon source on Si (111) substrates. A 2-nm layer of Au was ion-sputtered on the substrate, which was then placed in the region of uniform temperature in an inner quartz tube reactor. Erbium trichloride (ErCl3, Aldrich, 99.9%) powder placed in quartz susceptors was also inserted into the center of a quartz tube at intervals of 1 in. Carrier gas transfers the source precursor through a bubbler to the quartz reactor, and H2 is then introduced into the system at a flow rate of 5 sccm. H2 (100 sccm) and Ar (100 sccm) gas were used as
Figure 1. Typical SEM image of Si nanowires grown on substrates by the chloride transport process. The top view in the inset shows that the nanowires were well-aligned vertically.
diluent gases, which regulate the concentration of the mixture containing SiCl4 vapor and carrier gas. Typically, the system was heated to 800 °C and maintained for 20 min and then cooled to room temperature. Figure 1 shows a scanning electron microscopy (SEM) image of the reaction product. We observe a dense array of straight and vertically well-aligned nanowires that uniformly cover the entire substrate. The X-ray diffraction (XRD) pattern of the Si nanowires grown on the substrates confirmed growth of diamond-cubic crystalline Si without the presence of a second phase (data not shown). The average diameter of nanowires is ca. 120 nm with a standard deviation of 6 nm, which is about 20% larger than that of nanowires without Er doping. We note, however, that others have also reported that incorporation of Er during growth tends to increase the diameter of Si nanowires.23 Figure 2 shows the photoluminescence (PL) spectrum of the nanowire array measured at 25 K using the 488-nm line of an Ar laser. PL spectra were measured using either the 488- or 477-nm line of an Ar laser, a grating monochromator, a thermoelectrically cooled InGaAs diode, and by employing the standard lock-in technique. The low-temperature measurements were made using a closed-cyle cryostat. We observe a typical Er3+ luminescence centered around 1.53 µm. We note that the PL spectrum consists of a series of very sharp peaks. The spectral width of observable peaks is limited by the system resolution, which was 1.5 nm. The shape of the main peak near 1.532 µm indicates, however, that the actual spectral width of the peaks are likely to be much narrower because as many as four peaks at 1528, 1531, 1533, and 1536 nm can be identified within the main peak alone. The inset shows the PL spectrum measured using the 477-nm line of an Ar laser. The Er3+ luminescence intensity was lower with the 477-nm pump beam, necessitating a lower system resolution. However, it is clear that the spectrum is identical to that obtained with the 488-nm pump beam. We note that because the 477-nm pump beam does not coincide with any absorption band of Er3+, its sole effect is the Nano Lett., Vol. 5, No. 12, 2005
Figure 2. Photoluminescence spectra of the Si nanowire array, measured at 25 K. The positions of four identifiable peaks that make up the main peak near 1533 nm are indicated by arrows. The inset shows the room-temperature PL spectrum measured using the 477-nm line of an Ar laser. The 477-nm pump beam does not coincide with any absorption band of Er3+, and therefore can excite Er3+ only through carriers photoinjected into the Si nanowires. The Er3+ luminescence intensity was lower with the 477-nm pump beam, necessitating a lower system resolution.
photoinjection of carriers into nanowires. Therefore, the fact that we obtain the same Er3+ luminescence spectrum indicates that the Er3+ emission is from the same Er3+ centers and they can be excited via carriers injected into the nanowires. Because the carrier-mediated excitation mechanism for Er3+ is the same for photo- and electroinjected carriers6, and there is no barrier against injection of current into Si nanowires, Figure 2 indicates that electrically driven emission may also be feasible. Such sharp peaks in the luminescence spectrum are unlike anything reported from Er-doped nanowires.23-25 Because the intra-4f transition of rare earth ions are forbidden transitions that are allowed because of parity mixing by the crystal field, such sharp, well-defined peaks as seen in Figure 2 indicate that the Er3+ ions are located in well-defined sites in a crystalline environment. Indeed, the PL spectrum resembles, but does not exactly match, the PL spectrum reported by Isshiki et al. from an unidentified erbium silicate formed by a 1200 °C anneal of ErCl3-coated Si.27 The presence of such a crystalline phase is confirmed by transmission electron microscopy (TEM), as shown in Figure 3. The general morphology of the nanowires is shown in Figure 3a. It reveals that the nanowires are straight with uniform growth morphology along the growth direction. The outer sheath of the nanowires, however, consists of a trilayer with nanometer-thin shells (Figure 3a). An energy dispersive spectroscopy (EDS) analysis of the surface region indicates presence of Er, Si, and O in the surface layer, and the SAED pattern shows the existence of single-crystalline monolithic P21/c structure that corresponds to the Er2Si2O7 phase. A high-resolution TEM image of the surface region confirms the presence of a single-crystalline Er2Si2O7 shell sandwiched between nanometer-thin amorphous silica shells (Figure 3c). The whole trilayer surrounds a single-crystalline Si core with the [111] growth direction parallel to the long axis. The single-crystalline nature of the Er2Si2O7 layer and the one-step fabrication process for these nanowires imply that 2433
Figure 3. (a) TEM image of an individual Si nanowire showing the core-shell structure with a shell layer consisting of a
