J.-J. Wu

Field emission from quasi-aligned SiCN nanorods

We report on the preparation and field emission properties of quasi-aligned silicon carbon nitride (SiCN) nanorods. The SiCN nanorods are formed by using a two-stage growth method wherein the first stage involves formation of a buffer layer containing high density of nanocrystals by electron cyclotron resonance plasma enhanced chemical vapor deposition and the second stage involves using microwave plasma enhanced chemical vapor deposition for high growth rate along a preferred orientation. It should be noted that growth of the SiCN nanorods is self-mediated without the addition of any metal catalyst. Scanning electron microscopy shows that the SiCN nanorods are six-side-rod-shaped single crystals of about 1–1.5 μm in length and about 20–50 nm in diameter. Energy dispersive x-ray spectrometry shows that the nanorod contains about 26 at. % of Si, 50 at. % of C, and 24 at. % of N. Characteristic current–voltage measurements indicate a low turn-on field of 10 V/μm. Field emission current density in excess of ...

Crystalline SiCN: a hard material rivals to cubic BN

Abstract Growth and mechanical properties of SiCN materials prepared by microwave plasma enhanced chemical vapor deposition (CVD) as well as electron cyclotron resonance plasma CVD are reported. Large (several tens of microns), well-faceted ternary SiCN crystals were grown by microwave plasma-enhanced chemical vapor deposition, whereas amorphous SiCN films were deposited by ECR-CVD. The ternary crystalline compound (C; Si)xNy exhibits a hexagonal structure and consists of a network wherein the Si and C are substitutional elements. While the N content of the crystalline compound is about 50 at.%, the extent of Si substitution varies and can be as low as 10 at.%. The amorphous SiCN films contain only about 30 at.% N. Nano-indentation studies were employed to investigate the mechanical properties of the SiCN materials. From the load versus displacement curves, we estimated the hardness and the effective modulus of the SiCN crystals to be around 30 and 321.7 GPa, respectively. The corresponding values for the amorphous SiCN were around 22 and 164.4 GPa, respectively. These values are well above most reported values for CN films.

Growth of highly transparent nanocrystalline diamond films and a spectroscopic study of the growth

A series of nanocrystalline diamond films with grain size ranged from 4 nm to a few hundreds of nanometers were grown by microwave plasma enhanced chemical vapor deposition. Effects of the substrate pretreatment and the methane fraction in the source gas on the microstructure, surface roughness, and optical transmittance of the resultant films were studied. Specifically, comparison was made between two different sizes, 4 nm and 0.1 μm, of the diamond powder used for substrate pretreatment. Interestingly, the films grown on substrates scratched with coarser powder (0.1 μm) can be smoother and more transparent than those on substrates scratched with finer powder (4 nm), despite of the similarity in the grain size of these two types of films prepared at high methane fractions. It is also demonstrated that the major factor that controls the optical transparency is the surface roughness irrespective of the grain size as long as the sp2-bonded carbon in the film is avoided. In situ optical emission spectroscopy...

Deposition of silicon carbon nitride films by ion beam sputtering

Abstract Silicon carbon nitride films have been successfully synthesized at a temperature below 100°C from an adenine(C5N5H5)-silicon-mixed target sputtered by Ar ion beam. The effects of Ar ion sputtering voltage, area ratio of Si to adenine in the mixed target and nitrogen atom addition during deposition on the film growth are investigated. XPS, XRD, and ellipsometry were employed to characterize the composition, chemical bonding, structure, and optical property of the films. The growth characteristic and film properties of the silicon carbon nitride films are also compared with those of the carbon nitride films deposited from an adenine target to elucidate the effect of silicon incorporation. The silicon content of the resultant films increased significantly as the area ratio of Si to adenine increased, whereas a higher Ar ion voltage led to a lower level of silicon incorporation, presumably due to differential sputtering yield of Si and adenine. XPS chemical state analysis revealed multiple bonding structures for every element in the SiCN films, of which possible implications are given. XRD studies showed that only amorphous films for Si-rich SiCN were obtained, while the films with low Si incorporation and deposited at high Ar ion beam voltage contained nanocrystallites. Furthermore, the refractive index of the SiCN films increased with increasing silicon content. The appearance of the nanocrystalline structure in the films led to a reduction in the refractive index.

Comparative studies on field emission properties of carbon-based materials

Abstract Comparative studies on field emission properties of various carbon-related materials including diamond, amorphous carbon, SiCN films, SiCN nanorods, and carbon nanotubes are presented. While diamond is well known for its potential use in cold cathodes, the emission current of a hydrogen-treated diamond film is relatively small compared to that of amorphous carbon or diamond-like-carbon film. Meanwhile, carbon nanotubes have demonstrated large emission currents at much lower threshold voltages, showing their potential for applications. However, the emission from carbon nanotubes is subjected to significant decay under long-term operation. The emission current and long-term stability can be improved using a new SiCN compound with nanorod morphology. The effective barrier height and the field enhancement factor derived from the Fowler–Nordheim equation are discussed in this paper.

The use of a biomolecular target for crystalline carbon nitride film deposition by Ar ion-beam sputtering without any other source of nitrogen

Carbon nitride films have been synthesized by argon ion-beam sputtering from a biomolecular compound target, 8-aza-6-aminopurine (C4N6H4). The compound has a six-membered ring structure similar to that existing in the hypothetical β-C3N4. Except for the target material, no other source of nitrogen was used during sputtering deposition. It was found that crystalline carbon nitride with high N/C atomic composition ratios of 0.43–0.56 can be formed even at room temperature. The infrared spectra of the film exhibit two peaks at 1383 and 1643 cm−1, corresponding to C–N and C=N stretching modes, respectively. No detectable peak at 2200 cm−1 (C≡N stretching mode) is observed. Both x-ray diffraction and transmission electron microscopy show a very strong broad peak at 3.2 A, comparable to the d spacing of the (110) orientation in the β-C3N4 structure. However, it is suggested that the film contains a nanocrystalline phase with a crystal structure yet to be determined.

Field emission properties of two-layer structured SiCN films

Abstract The electron emission characteristics of two-layer structured silicon carbon nitride (SiCN) films, which were composed of amorphous and nanocrystalline phases, were studied. Rutherford backscattering spectroscopy (RBS) was used to determine the composition of the SiCN film. The ratio (Si;C)/N of the SiCN film was kept at approximately 0.75, which is identical to that of Si 3 N 4 film. High resolution X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to investigate the bonding structures of the SiCN films. In comparison with silicon nitride films, the turn-on voltage (for an emission current of 0.01 mA/cm 2 ) of the SiCN films was lower and the emission current densities of the SiCN significantly enhanced. The promising emission properties of the SiCN film could be due to the unique two-layer structure wherein nanocrystalline SiCN was grown on top of the amorphous interlayer with sp 2 CN bond in the SiCN film.

Bonding characterization and nano-indentation study of the amorphous SiCxNy films with and without hydrogen incorporation

Abstract The hardness and effective modulus of hydrogen-containing and hydrogen-free amorphous SiC x N y films were studied by nano-indentation. Amorphous SiC x N y films with and without hydrogen were deposited by electron cyclotron resonance plasma chemical vapor deposition (ECR-CVD) using a SiH 4 –CH 3 NH 2 –N 2 –H 2 gas mixture and hydrogen-free ion-beam sputtering deposition (IBSD), respectively. Fourier-transform infrared spectroscopy (FTIR) studies were used to investigate the bonding states of the SiC x N y materials. SiH, CH and NH bonds were detected by FTIR in ECR-CVD, but not in IBSD, films. The incorporation of hydrogen led to a reduction in both the hardness and modulus of the amorphous SiC x N y films. From nano-indentation measurements, the hardness and effective modulus of the IBSD coated, hydrogen-free amorphous SiC x N y films were 27–30 and 211–258 GPa, respectively. The corresponding values for the ECR-CVD coated, hydrogen-containing amorphous SiC x N y were 22–26 and 115–144 GPa, respectively.

Piezoreflectance study of silicon carbon nitride nanorods

Detailed piezoreflectance (PzR) measurements of quasialigned silicon carbon nitride nanorods in the temperature range between 15 and 400 K were performed. The direct band-to-band transition energies Egd at various temperatures were determined accurately through line shape fit of the experimental PzR spectra. The temperature dependence of Egd is analyzed by the Varshni equation [Y. P. Varshni, Physica, (Amsterdam) 34, 149 (1967)] and an expression containing the Bose–Einstein occupation factor for phonons proposed by L. Vina, S. Logothetidis, and M. Cardona [Phys. Rev. B 30, 1979 (1984)]. The parameters that describe the temperature dependence of Egd are evaluated and discussed.

X-ray absorption of Si–C–N thin films: A comparison between crystalline and amorphous phases

X-ray absorption near edge structure (XANES) spectra of crystalline (c)- and amorphous (a)-Si–C–N thin films were measured at the C, N, and Si K edge using the fluorescence and sample drain current modes. A sharp peak similar to the C 1s core exciton in chemical vapor deposition diamond is observed, which can be assigned to the transition from the C 1s to sp3 hybridized states in c-Si–C–N. The C K edge XANES spectrum of a-Si–C–N contains a relatively large 1s→π* peak, implying that carbon atoms in the a-Si–C–N film are bonded largely in graphite-like sp2 configurations. A shift of the a-Si–C–N π* peak towards the lower energy by ∼0.3 eV relative to that of c-Si–C–N is observed, which can be attributed to a higher degree of disorder-induced localization of excited electrons. Both a- and c-Si–C–N N K-edge XANES spectra resemble that of α-Si3N4. The Si K-edge absorption spectra of the Si–C–N thin films indicate a proportional combination of local Si–N and Si–C bonds. The increase of the binding energies of e...