Carbon-assisted growth and high visible-light optical reflectivity of amorphous silicon oxynitride nanowires
© Zhang et al; licensee Springer. 2011
Received: 7 May 2011
Accepted: 25 July 2011
Published: 25 July 2011
Large amounts of amorphous silicon oxynitride nanowires have been synthesized on silicon wafer through carbon-assisted vapor-solid growth avoiding the contamination from metallic catalysts. These nanowires have the length of up to 100 μm, with a diameter ranging from 50 to 150 nm. Around 3-nm-sized nanostructures are observed to be homogeneously distributed within a nanowire cross-section matrix. The unique configuration might determine the growth of ternary amorphous structure and its special splitting behavior. Optical properties of the nanowires have also been investigated. The obtained nanowires were attractive for their exceptional whiteness, perceived brightness, and optical brilliance. These nanowires display greatly enhanced reflection over the whole visible wavelength, with more than 80% of light reflected on most of the wavelength ranging from 400 to 700 nm and the lowest reflectivity exceeding 70%, exhibiting performance superior to that of the reported white beetle. Intense visible photoluminescence is also observed over a broad spectrum ranging from 320 to 500 nm with two shoulders centered at around 444 and 468 nm, respectively.
Silicon oxynitride (Si-O-N) materials have received considerable attention due to their special physical, chemical, and electrical properties [1–4]. Compositionally and structurally, silicon oxynitride can be regarded as the transition from silicon oxide to silicon nitride. Many of its physical properties also display a high extent of flexibility between the two extremes, changing continuously with N/O ratio . For example, the Si-O-N film possesses a large range of refractive indices spanning from 1.45 to 2.00. Moreover, the Si-O-N layers also show a high degree of optical transparency in the visible and near infrared spectral regions, which enables a variety of optical designs for integrated optics applications [6–10]. On the other hand, nanowires have intrigued considerable research enthusiasm for their unique physical properties and promising application as building blocks in nanoscale electronics and optoelectronics . Therefore, a controlled synthesis of silicon oxynitride nanowires deserves intense research attention.
However, reports on Si-O-N nanowires were so far rather rare [12–16]. The reported synthesis processes often involved the utilization of transition metals as catalysts in quartz tube furnace for pyrolysis, and sometimes inductively coupled coil was applied to obtain NH3 plasma for the nanowire growth [12–15]. These methods are unfavorable due to either the metal contamination to the resulted nanowires or the complicated equipment. Up to now, the optical properties of the Si-O-N nanowires remain largely unexplored, with only blue photoluminescence property recorded in literature [13, 16]. In this letter, we develop an inexpensive, easy, repeatable, and catalyst-free method to obtain a kind of amorphous Si-O-N nanowire showing high optical reflectivity in visible-light wavelength, and investigate its growth mechanism.
In a typical synthesis procedure, an amorphous carbon film was first sputtered on a single-crystal Si wafer (1 0 0) in spraying etching instrument (SCD050, Faraday Technology, Clayton, OH, USA). Secondly, the resulting silicon substrate was loaded to an alumina crucible boat, placed inside a quartz tube furnace. After the furnace was evacuated to 10-3 Torr, H2 (5%)/N2 mixed gas flow was kept through the tube at the rate of 2,000 sccm. The crucible was heated up to 1,200°C with a ramping rate of 15°C/min. After being maintained in 1,200°C for 4 h, the furnace was naturally cooled down to room temperature, and white products (later found to be Si-O-N nanowires) were found on the Si wafer.
Finally, the morphologies of these white products were characterized by scanning electron microscopy (SEM, Quanta 200, FEI Company, Hillsboro, OR, USA). Please check, high-resolution transmission electron microscopy (HRTEM, Tecnai 12, FEI Company, Hillsboro, OR, USA) equipped with an energy-dispersive X-ray (EDX), and high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM, Tecnai G2 F30 S-TWIN, FEI Company, Hillsboro, OR, USA). Chemical composition analysis was investigated by X-ray photoelectron spectroscopy (XPS, Shimadzu/Kratos AXIS Ultra DLD, Kratos Analytical, Chestnut Ridge, NY, USA), equipped with a standard and monochromatic source (Al Kα) operated at 150 W. The optical reflectivity of these nanowires was also studied using a Datacolor Elrepho photospectrometer (Datacolor ELREPHO, Lawrenceville, NJ, USA), and their photoluminescence (PL) measurement was conducted at room temperature using an FP-6500 with Xenon lamp line (Jasco, Essex, UK) of 258 nm as the excitation source.
Results and discussion
First, this oxide layer partly gets reduced into SiO λ (1 < λ < 2) vapor by the incoming carbon atoms from amorphous carbon film and oxidized carbon (CO), respectively, as explained with the reactions (1) and (2) . Then, the SiO λ vapor reacts with N2 and H2 gas into Si-O-N nucleation nanoislands, as shown in reaction (3). The constant reaction on the nanoisland surface would lead to the growth of nanowire arrays.
The PL spectrum of the Si-O-N nanowires on the Si wafer, taken under excitation with the 258-nm line of a Xe lamp, is presented in Figure 5b. A broad peak ranges from 380 to 500 nm with a maximum centered at 410 nm and two shoulders centered at 444 and 468 nm, respectively. The strong emission around 410 nm arises from recombination either from the conduction band to the N2 0 level or from the valence band to the N4 + level . The weak emission at 444 nm (approximately 2.8 eV) has been experimentally suggested by Noma et al. , originates from Si-N bonds in Si oxynitride. While the blue PL emission at 470 nm probably has an origin related to Si-O bonds .
In summary, large-scale ultrabrilliant white Si-O-N nanowires were synthesized through carbon-assisted growth. The unique cross-sectional nanostructure of a ternary amorphous nanowire was observed, which might open a new research horizon for growth mechanism of multicomponent nanowires. The nanowires demonstrate extraordinary optical reflectivity in visible wavelength, which will provide new applications in optoelectronic and energy areas such as backlight scattering coating in flat light panels and diffuse reflector for high-power white LED lighting.
This work is financially supported by the National Science Foundation of China (no. 90923019, 50875103, 50975114) and the Fundamental Research Funds for the Central Universities HUST#2010MS076.
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