Wanbing Lu

Structural and optical properties of hydrogenated amorphous silicon carbide films by helicon wave plasma-enhanced chemical vapour deposition

Hydrogenated amorphous silicon carbide (a-Si1−xCx : H) films with different carbon concentrations have been deposited using the helicon wave plasma-enhanced chemical vapour deposition technique under the condition of strong hydrogen dilution. The a-Si1−xCx:H films with carbon content x up to 0.64 have been deposited. Their structural and optical properties are investigated using Fourier transform infrared spectroscopy, Raman scattering, ultraviolet–visible transmittance spectroscopy and x-ray photoelectron spectroscopy. The deposition rate, optical band gap and B factor related to structural disorder are found to monotonically change in the investigated range with methane–silane gas flow ratios. It is found that the deposited films exist with the structure of Si-like clusters and Si–C networks when silicon content is high, while they consist mainly of C-like clusters and Si–C networks for carbon-rich samples. A large optical band gap is obtained in high carbon concentration samples, which is attributed to the high density characteristic of helicon wave plasmas and the strong hydrogen dilution condition.

Low temperature deposition of hydrogenated nanocrystalline SiC films by helicon wave plasma enhanced chemical vapor deposition

Hydrogenated nanocrystalline silicon carbide (nc-SiC:H) films have been deposited by using helicon wave plasma enhanced chemical vapor deposition technique at low substrate temperature. The influences of radio frequency (rf) power and substrate temperature on the properties of the deposited nc-SiC:H films were investigated. It is found that hydrogenated amorphous SiC films were fabricated at a low rf power, while the nc-SiC:H films with a microstructure of SiC nanocrystals embedded in amorphous counterpart could be deposited when the rf power is 400 W or more. The plasma transition from the capacitive dominated discharge to the helicon wave discharge with high plasma intensity influences the film microstructure and surface morphology. The analysis of the films deposited at various substrate temperatures reveals that the onset of SiC crystallization occurs at the substrate temperature as low as 150 °C. The low temperature deposition of nc-SiC:H films enables the fabrication of silicon-based thin-film solar...

Observation of bubble-involving spontaneous gas dissolution in superheated Al alloy melt

We present a direct visualization of spontaneous gas dissolution in Al-7.7 mass% Ca eutectic alloy melt during superheating using high-brilliance synchrotron X-ray imaging. A bubble-involving gas dissolution process was observed, which can be understood within the framework of adsorption-diffusion-dissolution mechanism. The heterogenous nucleation and combined effect of hydrogen diffusivity and solubility results in the growth of individual bubbles in a stochastic way with Gaussian distribution. This also applies to the behavior of group bubbles in early stage, while which in final stage can be treated as reverse Ostwald ripening dominated by Lifshitz-Slyozov-Wagner diffusion mechanism when pure diffusive condition is satisfied.

Spectral characteristics of surface-plasmon?enhanced photoluminescence in nanocrystalline SiC films

Multilayer structures of nanocrystalline (nc-) SiC/silicon nitride spacer/Ag island films were designed by varying the spacer thickness, and the spectral characteristics of surface-plasmons (SPs)–enhanced photoluminescence (PL) in nc-SiC films have been investigated. The optical transmission spectra show that there are two SPs resonant optical absorption bands in the out-of-plane and in-plane modes for the Ag island film on the sample surface. While PL quenching occurred for the sample with the thinnest spacer, the maximum PL enhancement for nc-SiC is achieved when the thickness of the spacer is suitable, suggesting that the SP enhancement can dominate over nonradiative energy dissipation by varying the spacer thickness. In the case of PL enhancement, the PL excitation spectrum shows an enhancement peak at the resonant wavelength of the out-of-plane mode of SPs, indicating that the excitation enhancement in nc-SiC films occurs due to the incident-light resonant coupling with out-of-plane SPs. Whereas, the increased PL decay rate is observed in the temporal PL spectrum, implying that the SPs scattering enhancement in the nc-SiC film is induced by in-plane SP resonant coupling. In the case of PL quenching, although an enhancement factor less than 1 is observed in the PL excitation spectrum, an increased light emission decay rate is also revealed in the temporal PL spectrum, which indicates that the nonradiative energy dissipation of light emission in the Ag island film is the main coupling mechanism when the spacer is too thin.

Enhanced photon-generated carrier extraction from Si nanostructure under additional infrared light irradiation

Recombination and trapping effect in quantum dots are large barriers to efficient photon-generated carrier extraction. In this paper, Infrared (IR)-assisted carrier extraction in a Si/SiO2 multiple quantum well is demonstrated. Operated at reverse bias, enhanced photoresponse from 300 to 700 nm is observed. External quantum efficiency nearly 200% is obtained when both visible light and IR are added. The enhancement is attributed to potential modulation by photo-illumination. A theoretical model including three processes is presented to explain this conclusion. The secondary light source IR could excite trapped carriers from the defects at the Si/SiO2 interface, improving extraction efficiency.

AMORPHOUS SILICON NANO-PARTICLES IN A-SiNx:H PREPARED BY HELICON WAVE PLASMA-ENHANCED CHEMICAL VAPOUR DEPOSITION

Amorphous silicon nano-particles embedded in hydrogenated amorphous silicon nitride (a-SiNx:H) matrix have been prepared using an approach based on the deposition of Si-rich a-SiNx:H thin films by helicon wave plasma-enhanced chemical vapour deposition (HWP-CVD) technique, which has a characteristic of high plasma density at low working pressure. X-ray photoelectron spectroscopy analysis shows that the silicon atom bonds exist in the Si-Si and Si-N configurations and the amorphous silicon regions appear separately in the Si-rich a-SiNx:H films. The existence of amorphous silicon nano-particles without any post annealing in the a-SiNx:H random matrix is confirmed by the image of high-resolution transmission electron microscopy. Through infrared absorption analysis, the formation of the separated amorphous silicon nano-particles structure is closely correlated with the deposition parameters such as low working pressure and Ar dilution in the HWP-CVD process.

Preparation of silver island films with tunable surface plasmon resonance

Silver (Ag) island films have been prepared by pulsed laser ablation in vacuum using a XeCl excimer laser. The effects of the number of ablation pulses, the temperature and time of post-annealing on morphology and surface plasmons properties of the prepared Ag island films were investigated by extinction spectra and scanning electron microscopy. It is found that the films deposited with the ablation pulses of 60 or less are isolated Ag nanoparticles and the mean size of the nanoparticles increases monotonically with increasing the number of ablation pulses. Further increase of the number of ablation pulses up to 240, quasi-percolated Ag films are obtained, and for 600 pulses or more, continuous films will be formed. Extinction spectra results show that localized surface plasmons (LSPs) are supported by the Ag island films, while propagating surface plasmons are supported by the continuous Ag films. The LSPs of Ag island films consist of inplane and out-of-plane modes. By changing the ablation pulse numbers and annealing conditions, the longitudinal and transverse dimensions of Ag islands could be adjusted, and then the peak positions and peak widths of in-plane and outof- plane LSPs resonance modes could be effectively controlled.

MICROSTRUCTURAL PROPERTIES OF NC-Si/SiO2 FILMS IN SITU GROWN BY REACTIVE MAGNETRON CO-SPUTTERING

Nanocrystalline silicon embedded in silicon oxide (nc-Si/SiO2) films have been in situ grown at a low substrate temperature of 300°C by reactive magnetron co-sputtering of Si and SiO2 targets in a mixed Ar/H2 discharge. The influences of H2 flow rate (FH) on the microstructural properties of the deposited nc-Si/SiO2 films were investigated. The results of XRD and the deposition rate of nc-Si/SiO2 films show that the introduction of H2 contributes to the growth of nc-Si grains in silicon oxide matrix. With further increasing FH, the average size of nc-Si grains increases and the deposition rate of nc-Si/SiO2 films decreases gradually. Fourier transform infrared spectra analyses reveal that introduction of hydrogen contributes to the phase separation of nc-Si and SiOx in the deposited films. Moreover, the Si–O4-nSin(n = 0, 1) concentration of the deposited nc-Si/SiO2 films reduces with the increase of FH, while that of Si–O4-nSin(n = 2, 3) concentration increases. These results can be explained by that active hydrogen atoms increase the probability of reducing oxygen from precursor in the plasma and prompting oxygen desorption from the growing surface. This low-temperature procedure for preparing nc-Si/SiO2 films opens up the possibility of fabricating the silicon-based thin-film solar cells onto low-cost glass substrates using nc-Si/SiO2 films.

Plasmon-enhanced luminescence from nanocrystalline SiC films through adjusting spacer layer thickness

We report the photoluminescence enhancement of nc-SiC films by coating nanostructure Ag films and study the influences of surface plasmon on photoluminescence properties by varying spacer thickness. PL curves of the samples deposited with different thickness of α-SiNx present two PL peaks which are contributed to the interference in the films and surface plasmon resonance, respectively. The PL intensity of the sample coated with Ag film is quenched due to combination of Forster nonradiative process and coherent photonic mode reduction in nc-SiC films, while the PL intensity of the samples with inserted spacer α-SiNx is enhanced because of the surface plasmon resonance.

Influence of annealing environment on the hydrogen related bonding structure in silicon nitride thin films containing silicon nanoparticles

The Si-rich SiNx:H films have been prepared by helicon wave plasma-enhanced chemical vapor deposition (HWP-CVD) technique. Parts of the samples have been post-annealed at 800 °C in the H2, FG (10%H2 in N2), and N2 ambient, respectively. Fourier transform infrared spectroscopy (FTIR) and the optical absorption spectroscopy have been used to investigate the influence of different annealing environment on the structural and optical properties of the films. After the thermal annealing process, there is a significant increase of Si-N bonding density. Meanwhile, the band related to hydrogen (N-H and Si-H) decreased which indicates that the hydrogen is effused out of the films during the annealing treatment. The Si-sH stretching vibrations can be divided into three components by Gaussian distribution; the Si-H absorption band at different wave numbers corresponds to different configurations. The changes of the three peaks contributions decreased indicate that the configurations of the Si-H stretching vibrations band occurs restructuring in the different annealing environments. Furthermore, the investigation of the optical absorption spectroscopy suggests that the band gap Eg decreased after the thermal annealing process. The decreased optical gap should be related to the loss of hydrogen and the slightly increase in the mean size of silicon nanoparticles, which is in good agreement with that of the hydrogen bonding structure.