C. H. A. Huan

Investigation of annealing effects on indium tin oxide thin films by electron energy loss spectroscopy

Abstract Indium tin oxide (ITO) thin films have been deposited on substrates by the thermal evaporation method, followed by annealing in air. Post annealing effects on the composition and quality of ITO thin films were characterized by electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Spectroscopic and optical measurements showed that the low transparency of an as-deposited ITO film was mainly caused by diffusing species which probably consisted of reduced metallic indium particles and defects in the film formed during the preparation. EELS results revealed that these metallic indium particles could be removed by an annealing process leading to uniformly transparent conducting film. This work demonstrates that EELS is a sensitive method to diagnose the presence of the metallic indium in ITO films.

Indium tin oxide films prepared by radio frequency magnetron sputtering method at a low processing temperature

Abstract Low temperature deposition of thin films of indium tin oxide (ITO) prepared by radio frequency magnetron sputtering method have been developed. This involved introducing hydrogen into the sputtering gas mixture during the film preparation. The surface morphology of the ITO films was sensitive to the hydrogen partial pressures used in the gas mixture. ITO films with a root mean square roughness of 1.13 nm over an area of 300×300 nm were obtained at the hydrogen partial pressure of 7.0×10 −6 torr. The presence of hydrogen gas in the sputtering processes was also shown to increase the number of charge carriers in the ITO films. At optimal deposition conditions, thin films of ITO with a resistivity of 4.66×10 −4 Ω cm and an optical transmission of over 86% in the visible spectrum range were achieved. The ITO films fabricated by this method do not require substrate heating during the film preparation or any additional post-deposition annealing treatment.

Effect of hydrogen partial pressure on optoelectronic properties of indium tin oxide thin films deposited by radio frequency magnetron sputtering method

In this study, indium tin oxide (ITO) films were made from an oxidized target with In2O3 and SnO2 in a weight proportion of 9:1 using the radio frequency magnetron sputtering method. Hydrogen was added to the gas mixture during the preparation of the ITO films. In order to study the effect of hydrogen partial pressure on the structural and optoelectronic properties of the ITO films, we have varied the hydrogen partial pressure in the gas mixture over the range 0–1.6×10−5 Torr and kept the substrate temperature constant at 300 °C during film growth. The x-ray diffraction patterns of ITO films prepared at different hydrogen partial pressures show that the films have (111) and (100) preferred orientations. Hall effect measurements reveal that the addition of hydrogen in the sputtering gas mixture shows an increase in the number of charged carriers in the ITO films. However the carrier mobility did not increase considerably. At optimal conditions, ITO films with resistivity of 2.7×10−4 Ω cm and transparency o...

Synthesis of large area aligned carbon nanotube arrays from C2H2–H2 mixture by rf plasma-enhanced chemical vapor deposition

Large area aligned carbon nanotube (CNT) arrays have been successfully synthesized from C2H2 and H2 mixture by rf plasma-enhanced chemical vapor deposition (without hot filament) on iron-coated silicon substrates. H2 plasma (not H2 gas) was confirmed to play the role of reducing iron oxide to metallic iron and promoting the formation of evenly separated particles, as well as being the primary factor in synthesizing aligned CNTs. The addition of H2 gas with no plasma during the growth resulted in randomly oriented CNTs. Meanwhile, without the addition of H2, the C2H2 plasma resulted in the growth of very fine worm-like carbon fibers. Using substrates with a thicker catalyst layer (>90 nm) reduced the CNT density significantly.

Synergism between Cu and Zn sites in Cu/Zn catalysts for methanol synthesis

Abstract Three Cu-based catalysts prepared using RF plasma-sputtering (Cu/ZnO and Cu/Al2O3) and coprecipitation (Cu/Zn/Al oxide) methods were employed for the investigation of the synergism between Cu and Zn sites. The static secondary ion mass spectrometry (SIMS) experiments indicate that, on both Cu/ZnO/Al2O3 and Cu/ZnO catalysts, CO is adsorbed at Cu sites (SIMS peaks appear at 91 and 93 amu), and H is bound to ZnO sites (67, 69, 70, 71 and 81, 83, 84, 85 amu) when the catalyst surfaces are exposed to H2, CO and CO2. Methoxy, the intermediate species of methanol synthesis, is found to be bound at Zn sites (95, 97, 98, 99 amu). CO and CO2 are found to induce Zn migration from sub-layers to the topmost layer while H2 is heterolyzed easier due to the presence of Cu. The coexistence of ZnO with Cu could enhance the capability of Cu to adsorb CO species and itself to adsorb H2 species. Al2O3 shows no synergetic effect with Cu in this experiment. No CO and H2 are detected on the Cu/Al2O3 catalyst exposed to syngas. The synergetic effect between Cu and Zn in the course of methanol synthesis was discussed.

Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube films

By using femtosecond laser pulses at a wavelength range from 720 to 780nm, we have observed absorptive and refractive nonlinearities in a film of multiwalled carbon nanotubes grown mainly along the direction perpendicular to the surface of quartz substrate. The z-scans show that both absorptive and refractive nonlinearities are of negative and cubic nature in the laser irradiance range from a few to 300GW∕cm2. The magnitude of the third-order nonlinear susceptibility, χ(3), is of an order of 10−11esu. The degenerate pump–probe measurement reveals a relaxation time of ∼2ps.

Photoluminescence studies of SiC nanocrystals embedded in a SiO2 matrix

Abstract The dependence of the photoluminescence (PL) from SiC nanocrystals embedded in a SiO 2 matrix on annealing is presented. Blue-green PL has been observed at room temperature from annealed SiC–SiO 2 composite films. The intensity of the single emission band at 460 nm (2.7 eV) shows a strong dependence on the annealing temperature. The combination of high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) transmission spectra and PL results suggest that SiC nanocrystals have been incorporated into the SiO 2 matrix and O-deficient defects were formed. The origin of luminescence is attributed to the creation of defects in silicon oxide.

Structural and optical properties of a-Si:H/nc-Si:H thin films grown from Ar–H2–SiH4 mixture by plasma-enhanced chemical vapor deposition

Abstract The structural and optical properties of hydrogenated amorphous silicon (a-Si:H)/nanocrystalline silicon (nc-Si:H) thin films, grown from high argon/hydrogen diluted silane by plasma-enhanced chemical vapor deposition (PE-CVD) using high rf power, were investigated. The growth rate decreased with increasing hydrogen flow rate and decreasing rf power, and increased slightly with increasing substrate temperature. For all samples deposited without hydrogen addition, SiH stretching was predominant as detected by infrared spectroscopy, whereas with hydrogen addition, the SiH intensity became lower than that of SiH2 stretching. Moreover, the hydrogen content was found to decrease with increasing hydrogen flow rate and substrate temperature. Raman scattering clearly showed the nanocrystallites formed in the amorphous silicon matrix in the thin films deposited with hydrogen addition, and these films had higher optical band gaps. There was no direct relation between the hydrogen content and the optical band gap or edge width parameter due to the heterogeneity of the grown thin films. However, IR and Raman scattering spectra were found to be closely related to the microstructure of the grown thin films, and may provide clear information on the quality or heterogeneity of the films deposited using high rf power and high argon/hydrogen dilution. In addition, the smooth surface morphology and adherent stable structure of the grown thin films make them suitable for application in electronic devices.

Ground-state properties of cubic C-BN solid solutions

The ab initio calculations of the ground-state properties of cubic C-BN solid solutions are presented for the first time to our knowledge. The Lowdin perturbation technique has been used in the ab initio LMTO-ASA method to calculate the total energy, the equilibrium lattice constant, the bulk modulus and its pressure derivative and the formation energy of mixed crystals of cubic BN and diamond. The calculated results indicate that the solid solution between C (diamond) and cubic BN is nonideal, with the equilibrium lattice constants larger than the predicted values of ideal mixing (Vegards law) and the positive energies of formation. The bulk moduli of BN-rich (x > 0.5) are lower than those of diamond, the value of ideal mixing and even cubic BN. This anomalous behaviour is consistent with the nonideal expansion of the equilibrium lattice constant. The calculated results are in good agreement with the recent experimental measurements by Knittle et al 1995 Phys. Rev. B 51 12149.

Multiphase structure of hydrogenated amorphous silicon carbide thin films

Abstract The structural and optical properties of hydrogenated amorphous silicon carbon (a-Si 1− x C x :H) thin films, grown from argon diluted silane, ethylene, and hydrogen mixture by plasma-enhanced chemical vapor deposition (PE-CVD) technique, were studied. Variable flow rates and other growth conditions were applied. A variety of techniques, including X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, Raman scattering (RS), atomic force microscopy (AFM), high-resolution transmission electron microscopy (HR-TEM), UV-VIS-NIR spectroscopy, and photoluminescence (PL) were used to characterize the grown materials. The results confirmed the multiphase structure of the grown a-Si 1− x C x :H thin films: Si–C network, carbon-like and silicon-like clusters coexisting. The room temperature (RT) PL shows a different result from the previous reports. It is suggested that both graphite-like phase and a-Si:H-like phase are light-emitting grains. The two types of grains and Si–C network are the origin of the PL in hydrogenated amorphous silicon carbide material.