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Fabrication and Optical Properties of Erbium-Doped Silicon-Rich Silicon Oxide Nanofibers XiTian Zhang,† Zhuang Liu,† ChingChi Wong,† SuiKong Hark,*,† Ning Ke,‡ and SaiPeng Wong‡ Departments of Physics and Electronic Engineering, The Chinese UniVersity of Hong Kong, Shatin, Hong Kong ReceiVed: August 28, 2006; In Final Form: NoVember 28, 2006
Synthesis of Er-doped silicon-rich silicon oxide (SRSO) nanofibers bundled into yarns was successfully achieved. Photoluminescence (PL) studies of the yarns detected a strong and sharp band at 1.530 µm at room temperature, which is attributed to Er ions excited by energy transfer from photogenerated carriers in Si nanoclusters (nc-Si). The effective absorption cross-section of the first excited state of the Er ions is estimated to be 1.86 × 10-16 cm-2, higher by several orders of magnitude than that in an Er-doped SiO2 host, due to the presence of nc-Si that serves as a sensitizer in the SRSO nanofibers. In order to investigate the luminescence mechanism of the Er ions, PL spectra excited by incident light of different wavelengths were measured. We found that both the peak position and line shape of the Er ion emissions are unaffected by the incident wavelength. The results indicate that the Er ion luminescence in the SRSO nanofibers is completely dominated by ions located at sites that are strongly coupled to the carriers in the nc-Si. We also found that the intensity of luminescence decreases by less than 30% as the temperature is raised from 10 K to room temperature, indicating that thermal quenching is significantly reduced.
Introduction Erbium-doped Si-based materials have been the focus of intensive research recently, because of their potential applications in optical communications. So far, great progresses have been made in the syntheses of these materials in the form of thin films,1,2 especially Er-doped silicon-rich silicon oxide (SRSO) thin films, which consist of Si nanoclusters (nc-Si) embedded in a SiO2 host.3,4 On the other hand, with the rapid developments in nanoscience and nanotechnology, the syntheses of one-dimensional (1D) semiconductor nanostructures have become a focus of research.5-7 As is well-known, when the diameter of the 1D wires is decreased to nanometers, phonon confinement effects, that is, a reduction of exciton-acousticphonon interactions, result in an enhanced excitation efficiency. Recent results confirmed that Si nanowires sheathed by Er-doped SiO2 combine the advantages of both materials to achieve a high carrier-mediated excitation efficiency and Er ion luminescent efficiency simultaneously, compared to Er-doped SiO2 thin films.5 So far, incorporation of rare earth ions into nanowires and studies of their luminescence are still rare,5,8,9 and the synthesis of Er ion-doped 1D SRSO nanostructures has not been reported. In this paper, we report the successful synthesis of SRSO nanofibers with a Si content of 47 at. % by chemical vapor deposition. The Er ions were incorporated into the nanofibers by ion implantation. An intense and sharp photoluminescence (PL) from the Er ions was observed after the implanted nanofibers were thermally annealed. The luminescence mechanism of Er ions in the SRSO fiber host is discussed with regard to its wavelength and temperature dependence. Experimental Section The SRSO nanofibers were synthesized in a high-temperature tube furnace. The Si substrate was cleaned by ultrasonication *Corresponding author: e-mail
[email protected]. † Department of Physics. ‡ Department of Electronic Engineering.
in 1,1,1-trichloroethane for 30 min and etched in a solution of H2SO4/H2O2/H2O (3:1:1) to remove the surface oxide layer. It was finally rinsed with deionized water and dried. A thin film of Au was sputtered on the substrate to act as a catalyst. A Ga/Ga2O3 (99.99%, Aldrich) mixture was loaded into one end of a quartz boat and the Au-coated Si substrate was placed on the other end, downstream from the mixture. The quartz boat was placed at the center of a precleaned quartz tube that was inserted into a horizontal tube furnace. The furnace was heated at a rate of 40 °C/min to a preset temperature (1150 °C) under a flow of 100 mL/min Ar gas. When the temperature reached 900 °C, the Ar flow was replaced by NH3 at the same flow rate. The furnace temperature and pressure were maintained at 1150 °C and 80 Torr, respectively, for 200 min. Finally, the furnace was naturally cooled down to room temperature. Whitecolored products of the synthesis were found covering the entire substrate. The as-synthesized products were characterized by scanning electron microscopy (SEM; LEO, 1450 VP) and transmission electron microscopy (TEM; Philips CM120), both linked to an energy-dispersive X-ray (EDX, Oxford Link II) spectrometer. Er ion implantation was performed with a metal vapor vacuum arc ion source implanter operating in a pulse mode. PL spectra of the synthesized products were measured using laser lines from Ar+ and He-Cd lasers as the excitation sources, a grating monochromator, a liquid nitrogen-cooled Ge detector, and a lock-in amplifier. Results and Discussion Figure 1a is a typical SEM image of the as-synthesized nanofibers, which are bundled into yarns. The length of the yarns is typically 100 µm or more and the width is about 10-20 µm. As an example, a more complete view of the yarns is shown in Figure 1b. The figure also reveals the branching of the nanofibers within. The branches further split into subbranches, resulting in an increased volume of the more or less aligned nanofibers. The diameter of an individual SRSO nanofiber is about 10 nm or less as seen by TEM (not shown). The growth
10.1021/jp0655569 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/01/2007
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Figure 2. PL spectrum of Er ions in 1D Er-doped SRSO taken at room temperature.
Figure 1. (a) SEM image of as-synthesized SRSO nanofibers with 47 at. % Si. (b) SEM image of a complete SRSO yarn. (c) SEM image and (d) EDX spectrum of Er-doped SRSO nanofibers after annealing.
phenomenon observed here is similar to that in silica nanowires.10-12 EDX examination of the nanofibers in the SEM confirms that they are composed of O and Si, with an O:Si ratio of about 1.2, less than that in typical SiOx films (x > 1.5).3,13,14 There is also a small amount of gallium (1.3 at. %) detected, probably from droplets on the substrates. X-ray diffraction was employed to analyze the crystallization of the nanofibers. No SiOx-related diffraction peak was observed (not shown), indicating that the nanofibers are amorphous. Er ions were incorporated into SRSO nanofibers by the metal vapor vacuum arc ion source at a dose of 5 × 1016 cm-2 and an extraction voltage of 25 keV. To activate the Er ion luminescent centers and remove defects caused by the implantation process, the implanted nanofibers were annealed at 900 °C in a N2 ambient atmosphere for 1 h. In addition, the annealing is likely to induce a growth of second phase in the SRSO nanofibers, resulting in nc-Si dispersed inside the SiO2 nanofibers. This is a general method used to form nc-Si in SiOx and has been widely adopted in SRSO thin films.13,15 The general morphology of the annealed, Er-implanted nanofibers was again examined by SEM. Figure 1c shows that the morphology does not change significantly after implantation and annealing. The EDX spectrum shown in Figure 1d identifies Er presence at about 0.72 atom %, in addition to O and Si, in the implanted nanofibers.
Figure 3. (a) Energy level diagrams of a free Er ion and an Er ion in solid. (b) Probable processes involved in the excitation of Er ion via nc-Si: process: a, creation of an electron-hole pair by absorption of a photon; b and c, nonradiative recombination of the electron-hole pair and energy transfer to the Er ion in the ground state by an Augerlike process; d, the Er ion is excited to an unspecified state; e, the excited Er ion quickly relaxes to the first excited state by a nonradiative process; f, the Er ion in first excited state directly and radiatively transits to the ground state, emitting a 1.530 µm photon.
Figure 2 shows a room-temperature PL spectrum of the Erdoped SRSO nanofibers with 47 at. % Si, excited by the 514.5 nm line of an Ar+ laser at a power of 12.7 mW. The spectral feature is typical of Er-doped materials. It consists of a main peak at 1.530 µm and a shoulder at 1.548 µm, attributed to the intra-4f Er3+ transition between the Stark-split first excited state (4I13/2) and ground state (4I15/2). The emission wavelengths are relatively insensitive to the host material, because the 4f shell is shielded from its surroundings by the filled 5s and 5p shells. In general, erbium exists in the trivalent charge state (Er3+) in most materials and has the electronic configuration of an incompletely filled 4f shell. Spin-spin and spin-orbit coupling
Er-Doped Silicon-Rich Silicon Oxide Nanofibers
Figure 4. (a) PL spectra of Er ions measured at different excitation wavelengths. (b) Intensity of the 1.530 µm PL peak as a function of the excitation wavelength. The spectra from top to bottom in panel a are excited by 442, 476, 488, and 515 nm laser lines, respectively. The solid line is a guide for the eyes.
give rise to a number of energy levels, as shown in Figure 3a. Each degenerate level is Stark-split into manifold levels, due to the presence of the host. Therefore, the transition from the first excited state (4I13/2) to the ground state (4I15/2) generally emits a photon of ∼1.54 µm wavelength but not an individual emission peak.16 The full width at half-maximum (fwhm) of the main peak is 39 nm, which is significantly broader than the main peak of Er ion PL in SRSO films with 41 at. % Si (fwhm of 8.3 nm).14 The broad PL peak probably results from the wide range of possible atomic configurations for the Er ions in the amorphous SRSO and their proximity to the surface in the smalldiameter nanofibers. In order to probe the possible erbium atomic configurations, excitation wavelength-dependent PL of the Er-doped SRSO nanofibers was measured at room temperature (Figure 4a). The power of the excitation was fixed at 3.7 mW, while the wavelengths used were 422, 476, 488, and 514.5 nm. The wavelength 488 nm falls within the 4I15/2 f 4F7/2 optical absorption band of Er ions (Figure 3a), while those at 422, 476, and 514.5 nm do not resonate with any known transitions. However, the peak position and shape of the Er ion emissions in the resulting PL spectra are unaffected by the excitation wavelength, as can be seen in Figure 4a. The results agree with the featureless photoluminescence excitation spectrum observed for Er ions in SRSO thin films and suggest to us that the Er ions are indirectly excited.14,15 The weak dependence of the PL peak intensity of Er ions in our SRSO nanofibers on the excitation wavelength is plotted in Figure 4b. As concluded in ref 14, the above-mentioned spectral characteristics indicate that (1) Er ions, located at sites strongly coupled to the carriers in the nc-Si, dominate the luminescence; (2) they are likely to belong to a single class of atomic configurations; and (3) nc-Si may be present in the Er-doped SRSO nanofibers. The presence of nc-Si in thermally annealed bulk SRSO hosts has been observed in the experiments of Franzo` et al.15 and others17 by TEM coupled with an electron energy loss spectrometer.
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Figure 5. (a) PL spectra of Er ions in 1D SRSO taken at different temperatures. (b) PL intensity of the 1.530 µm peak as a function of inverse temperature. The solid line is a guide for the eyes. The spectra were excited by the 514.5 nm line of the Ar+ laser.
Figure 6. Intensity of the Er ion PL peak as a function of the excitation photon flux. Spectra were taken at room temperature and excited by the 514.5 nm line. The solid line represents the fitting to the data according to eq 1.
Therefore, we believe that the Er ion luminescence process is as follows: (i) electron-hole pairs are created in nc-Si by absorbing photons (Figure 3b, process a); (ii) the carriers recombine nonradiatively, transferring their energy to the Er ions in the ground state by an Auger-like process (processes b and c); (iii) the Er ions are excited to unspecified states (process d); (iv) Er ions in these excited states quickly relax to the first excited state (4I13/2) (process e); and (v) finally radiative transit to the ground state occurs, resulting in the emission of the 1.530 µm photons. The existence of a single class of Er ion site is further confirmed by the temperature dependence of the PL spectra shown in Figure 5a. We find that raising the temperature from 10 to 300 K broadens the peak but does not change its overall shape or position. This indicates that all the Er ions in different atomic configurations have the same temperature dependence, making it very likely that they indeed belong to the same class of sites. Reports have shown that not only will the spectra be different, but the temperature dependence of the peak intensity will also be different, if there are two or more kinds of Er ion
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centers in the SRSO host.18 Figure 5b shows the effect of temperature on the integrated PL intensity. We observe that it decreases by less than 30% as the temperature is raised from 10 to 300 K. The small temperature quenching mainly originates from Auger de-excitation processes, namely, the back transfer of energy from excited Er ions to free electrons and holes in nc-Si.19
magnitude, compared to that of Er ions in pure SiO2, because of the presence of the nc-Si serving as a sensitizer in the former.
To further investigate the luminescence mechanism, we measured the 1.530 µm peak intensity as a function of excitation photon flux at 514.5 nm; the data are shown in Figure 6. Assuming that Er ions are excited only indirectly via the nc-Si, the relationship between PL intensity and excitation power can be expressed as20,21
Supporting Information Available: TEM image showing the diameter of individual SRSO nanofibers. This material is available free of charge via the Internet at http://pubs.acs.org.
ηφσeff στφ + 1 I(Er) ∝ σeffφ 1 + στφ + 1 τ Er
(1)
d
where σ is the absorption cross-section of nc-Si, τ is the effective lifetime of nc-Si emission, φ is the photon flux, η is the quantum efficiency of the filling of the Er ion metastable state, σeff is the effective absorption cross-section of Er ions, and τdEr is the lifetime of the Er ion metastable state, which we assume to be 8.3 ms.5 The continuous line in Figure 6 represents the fit to the experimental data to eq 1 by following the procedures of Priolo et al.22 The estimated σeff of the first excited state of the Er ions is found to be ∼1.86 × 10-16 cm2, in agreement with recent findings in Er-doped SRSO films.22 It is also noteworthy that σeff in the Er-doped SRSO nanofibers is much larger than the typical values for Er ions in pure insulating hosts, which are on the order of 10-19-10-21 cm2 (with excitation at 488 nm).22 The enhancement is attributed to the presence of nc-Si serving as a sensitizer for the Er ions.9,23 Conclusions Er-doped 1D SRSO nanofibers were successfully synthesized. An intense and sharp PL peak at 1.530 µm from the Er ions was observed at room temperature. Selective wavelength excitation indicates that the Er ions were not excited directly by the photons but rather were indirectly excited via energy transfer from carrier recombinations in nc-Si. The absorption cross-section of the first excited state of the Er ions in SRSO nanofibers was effectively enhanced by several orders of
Acknowledgment. The work described in this paper was partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project 401003) and CUHK direct grants (Project Codes 2060287, 2060293, 2060305, and 2060308).
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