INSTITUTE OF PHYSICS PUBLISHING
NANOTECHNOLOGY
Nanotechnology 16 (2005) 940–943
doi:10.1088/0957-4484/16/6/053
Intense blue photoluminescence from Si-in-SiNx thin film with high-density nanoparticles Cheng Liu, Chaorong Li, Ailing Ji, Libo Ma, Yongqian Wang and Zexian Cao1 National Laboratory for Condensed Matter Physics, Institute of Physics, PO Box 603, Beijing 100080, People’s Republic of China E-mail:
[email protected]
Received 12 November 2004, in final form 18 January 2005 Published 19 April 2005 Online at stacks.iop.org/Nano/16/940 Abstract Intense photoluminescence in blue was measured in as-deposited Si-in-SiN x films prepared by plasma-enhanced chemical vapour deposition on cold substrates. The density of the silicon nanoparticles amounts to 1.4 × 1013 cm−2 as calculated from transmission electron microscope images, and the particle size can be well controlled under 2.6 nm. Rapid annealing at 500 ◦ C for two minutes roughly doubles the photoluminescence intensity, while the peak profile suffers only negligible change. In contrast, annealing at 1050 ◦ C results in a broadened spectrum red shifted by about 80 nm. External quantum efficiency for the photoluminescence was estimated to be well above 1.0% by comparing with GaN. This demonstrates that, with appropriate processing parameters, the low-temperature procedure is promising for obtaining applicable light emission from Si-in-SiN x films.
1. Introduction Being the material basis for current microelectronics, silicon is considered to play an important role in the forthcoming information techniques. Since the first discovery of strong visible photoluminescence (PL) at room temperature in porous silicon by Canham [1], considerable efforts have been devoted to the fabrication of novel silicon-based nanostructures that can emit light [2–11], aiming at applicable light-emitting devices to be implemented in optoelectronics and photonic circuits. In one of the prominent candidate systems, i.e. that comprising of silicon nanocrystals embedded in amorphous silica, notably efficient PL has been confirmed. In samples annealed at 1100 ◦ C, a stable external quantum efficiency as high as 12% was measured at λ ≈ 760 nm [2]. As noted, however, PL from such a system is generally in the red or infrared—attempts towards blue–green light emission have progressed only very slowly. In recent years, researchers turned their attention to other silicon-based materials that contain silicon nitride or carbide, which have shown exciting prospects for emitting short-wavelength visible light. Seo and co-workers reported 1 Author to whom any correspondence should be addressed.
0957-4484/05/060940+04$30.00 © 2005 IOP Publishing Ltd
intense blue–white luminescence from carbon-doped Si-rich SiOx films annealed at 950 ◦ C [3], and they attributed the PL to exciton recombination in carbon-incorporated Si nanoclusters. In our previous work [4, 5], PL of efficiencies over 1.0% throughout the visible light spectrum has been realized in Si-inSiOx films that were grown on cold substrates and subsequently annealed at 500 ◦ C, in which the silicon nanoparticles are evidently amorphous. By a similar process, PL with improved efficiency, particularly in the blue–green region, was obtained in Si-in-SiN x films since there the less oxidizing nitride matrix allows the survival of small-sized silicon particles. For blue light, taking that centred at hν = 2.6 eV for instance, the PL intensity from Si-in-SiN x films is over three times as large as that from Si-in-SiOx . Moreover, the smaller bandgap of Si3 N4 (5.3 eV) is beneficial for carrier injection in subsequent electroluminescent designs. Attaining utilizable light emission from materials that are grown and/or post-treated at reasonably low temperatures is most favourable in terms of practical implementation. Generally, it is difficult to measure any PL from as-deposited Si-in-SiOx or Si-in-SiNx films irrespective of the hard work of researchers [6–9], those rare successful cases are only
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Intense blue photoluminescence from Si-in-SiNx thin film with high-density nanoparticles
Figure 1. PL spectra for a series of samples under the following conditions: (a) as-deposited, (b) after annealing at 500 ◦ C, and (c) after annealing at 1050 ◦ C.
weak red emission [10, 11]. In the present study, Si-inSiN x films were grown by plasma enhanced chemical vapour deposition (PECVD) on cold substrates, in which a maximum particle density up to 1.4 × 1013 cm−2 was attained. Such films exhibited intense PL in the blue–green range in the asdeposited condition. The external quantum efficiency of PL for annealed samples was estimated to be well above 1.0%.
2. Experimental details Si-in-SiN x films were grown on Si(100) wafers in a customdesigned plasma-enhanced chemical vapour deposition setup, for which a gas mixture of SiH4 , N2 and H2 was supplied as precursor. The total flow rate was fixed at 30 sccm to maintain a work pressure of 34 Pa, while the nitrogen-to-silane flow rate ratio was tuned around 16. Stable discharge was sustained with a rf power of 35 W. No designed substrate heating was applied, thus the system temperature throughout the 2 h deposition measured below 50 ◦ C. For comparison study, some samples were post-annealed in N2 atmosphere for 2 min at 500 and 1050 ◦ C, respectively. X-ray photoemission spectroscopy (XPS, ESCA-lab mark II) operating with a Mg Kα line and energy-dispersive xray spectroscopy (SIRION, FEI) were used to examine the film chemistry, and transmission electron microscopy (TEM, Tecnai f20) was employed to evaluate the Si nanoparticles in the deposit. Room-temperature photoluminescence spectra were registered on a PTI-710 fluorescence spectrometer using the 325 nm line from a He–Cd laser for excitation.
3. Results and discussion More than 20 Si-in-SiN x films were prepared with careful adjustment of the gas precursor make-up to maintain the film composition at an overall atomic ratio [N]/[Si] around 0.75. In such as-deposited films, intense blue luminescence can be measured at room temperature. In figure 1, for a case study, the PL spectra from a sample with [N]/[Si] ≈ 0.77 are plotted. The as-deposited sample exhibits intense light emission centred at 466 nm (hν = 2.66 eV). The full width at half maximum (FWHM) is about 105 nm, corresponding
Figure 2. Si 2p spectral lines for the same series of samples referred to figure 1.
to 0.6 eV when converted to the photon energy scale. By comparing with the PL spectra from corresponding samples annealed at 500 ◦ C, curve (b), and 1050 ◦ C, curve (c), one notices that, unlike the previous observation in Si in SiOx [4], here annealing does not significantly improve the lightemission capability of the films concerned. The film annealed at 500 ◦ C displays a PL of doubled peak intensity, while the peak profile shows negligible change. By annealing at 1050 ◦ C, the peak intensity increases by a factor of only 1.3. This is, however, at the cost of remarkable peak broadening and red shift. The PL peak now shifts to 535 nm, showing a FWHM of 150 nm. The XPS and TEM results can provide valuable clues to the understanding of the PL behaviour and the effect of annealing upon the deposits. Here only the results for the samples with [N]/[Si] ≈ 0.77, referring to figure 1, are presented. From the Si 2p spectral lines in figure 2, no distinct components can be assigned to nitrided or elemental silicon. Annealing at 1050 ◦ C shifts the peak towards larger binding energy for 0.6 eV (from 101.8 to 102.4 eV) in companion with a moderate broadening (FWHM from 2.70 to 2.92 eV). The situation for the sample annealed at 500 ◦ C lies in between. This modification may have twofold origin. On one hand, annealing will rearrange the bonding configurations resulting in structural components Si–Si4−n Nn (n = 0, . . . , 4) of larger n values for the nitride matrix, thus enhanced chemical shift, as proposed by K¨archer et al [12]. On the other hand, annealing improves the particle/matrix interface. Both effects lead to a considerably increased electrical resistance of this semi-insulating material. The charging effect thus arising contributes greatly to the chemical shift of the Si 2p peak observed. Although no distinct phase separation can be confirmed by XPS measurement, TEM micrographs provide direct verification of the presence of well separated silicon nanoparticles in the film. Due to the negligibly small atomic number difference and the amorphous nature of Si in SiNx , TEM images as presented in figure 3 are generally not sharply contrasted. In order to calculate the particle density with confidence, over-focused images corresponding to figures 3(a)–(c) were taken, in which the silicon particles appear as well separated dark spots on a bright background. In 941
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Figure 3. TEM micrographs for the same series of Si-in-SiNx samples as in figures 1 and 2. Particle coarsening can be confirmed by comparing (a) and (c).
all the samples, the silicon nanoparticles are homogeneously distributed throughout the film. Even in the film annealed at 1050 ◦ C, both the particles and matrix remain amorphous. The particle density in the as-deposited film is about 1.4 × 1013 cm−2 , four times higher than we have ever achieved in our previous samples [4, 5]. The particle size falls within 2.2–2.6 nm, a favourable condition for blue-light emission if the quantum confinement mechanism governs the PL process [13, 14]. Annealing at 500 ◦ C slightly increases the particle density up to 1.6 × 1013 cm−2 without noticeable change in the particle size distribution. In the 1050 ◦ C annealed sample, the particle size falls in the range 3.3–4.2 nm while the particle density is reduced to 5.9 × 1012 cm−2 . This is consistent with the observation of the red-shifted PL spectrum with a broadened profile. By comparing the PL features against the particle statistics, it is concluded that there is another PL enhancement mechanism by annealing in addition to the particle number variation, which can presumably be ascribed to the improved particle–matrix interface [15–17]. The PL energy here is exclusively larger than 2.4 eV, but for porous silicon of similar size it is generally below 1.8 eV. This difference is unlikely to be accounted for by the current quantum confinement model where the particle is simply handled as a quantum well. A more realistic model should treat the particle–matrix composite as a seamless whole, and incorporate more details such as bonding configuration both inside the particle and at the interface. Research along this direction has begun to provide us with some insights into the problem [18]. As a rough estimation of external quantum efficiency, PL spectra from the Si-in-SiN x sample annealed at 500 and 1050 ◦ C, respectively, are presented against that, centred at λ ≈ 520 nm, from a high-quality GaN sample grown by metallorganic chemical vapour deposition, which was prepared for fabrication of a commercial light-emitting diode (figure 4). One sees that, for both samples, the integrated intensity is roughly one-tenth of that of GaN, indicating that the external quantum efficiency of the Si-in-SiN x film is about 1.0%, when a conservative external quantum efficiency of 10% is assumed for GaN. Moreover, when exposed to a UV-light lamp operated at 6 W, the luminous Si-in-SiN x film is visible to the unaided eye, though weaker than the GaN sample. 942
Figure 4. PL spectra of the Si-in-SiNx films with [N]/[Si] ∼ 0.77 and 1050 ◦ C plotted against that from a GaN sample prepared by metallorganic chemical vapour deposition, which is used for fabrication of a light emitting diode. Inset is a photograph of GaN (green, right) and the 500 ◦ C annealed Si-in-SiNx sample (blue, left) under irradiation of a 6 W ultraviolet lamp. (This figure is in colour only in the electronic version)
Based on the above observation, annealing at high temperature such as over 1000 ◦ C deteriorates the PLfavourable features through reducing particle numbers while effecting the coarsening process. In the amorphous films with particle size below 2.6 nm that can emit blue light in the as-deposited condition, annealing can be of limited help in promoting PL efficiency without causing considerable red shift. Whatever the eventual functioning mechanism for PL, growing samples with increased particle density will be the most practical way to obtain applicable silicon-based light emitting materials. There is still much room for progress along this direction.
4. Summary In summary, intense blue PL was observed in the amorphous, as-deposited Si-in-SiN x films prepared by plasma-enhanced chemical vapour deposition, in which the particle density amounts to 1.4 × 1013 cm−2 , and the particle size is controlled below 2.6 nm. Annealing of these films at a moderate 500 ◦ C can double the PL peak intensity, but at 1050 ◦ C annealing results in a red-shifted and broadened PL profile without appreciable enhancement of intensity. An external
Intense blue photoluminescence from Si-in-SiNx thin film with high-density nanoparticles
quantum efficiency at the level of 1.0% was estimated. Further enhancement of the light emission efficiency can be expected by improving the quality of the film, in particular the particle and matrix interface. Deliberate work along this direction is underway in our laboratory.
Acknowledgments This work is supported by the National Science Foundation of China, grant Nos 60306009 and 50472070, and the National High Technology Research and Development Program of China, grant No 2003AA302170. The authors are indebted to Dr Cao Li for his kind help in PL measurement and to Professor Zhou Junming for the high-quality GaN sample.
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