Wenge Ding

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...

Dependence of the photoluminescence from silicon nanostructures on the size of silicon nanoparticles

The size dependence of photoluminescence (PL) from nanostructure semiconductors is examined. Considering the dependence of PL on both the silicon nanoparticles (Si NPs) sizes and their dispersion, we incorporated quantum confinement effects along with the effects of localized surface states to obtain an analytical expression for the PL spectra of silicon nanostructures. In order to obtain an insight into the effects of various parameters influencing the PL spectral profile in silicon nanostructures, we computed the PL spectra using relevant numbers in the expression. The computer-simulated results show (i) a marked deviation of PL spectrum from the normal distribution at higher energies due to the increase in oscillator strength with the decreasing mean Si NP size, (ii) The peak position redshifts and the peak intensity reduces with an increase in the standard deviation, and (iii) the luminescence peak blueshifts as the mean Si NP size decreases. To test our model, the Si NPs embedded in silicon nitride films were prepared by helicon wave plasma-enhanced chemical vapor deposition (HWP-CVD) technique using the H2 diluted SiH4 and N2 as reactant gas sources. The simulated PL spectra fit the experimental one rather nicely. And our results can explain the reported experimental observations on the luminescence from Si NPs.

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.

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.

Defect states of hydrogenated nanocrystalline silicon studied by surface photovoltage spectroscopy

Surface photovoltage spectroscopy (SPS) is used to determine the defect states of the nc-Si:H films which are prepared by using helicon wave plasma-enhanced chemical vapor deposition technique(HWP-CVD) under different substrate temperature. The films exhibit a 3-phase model, which suggests that nc-Si:H consists of crystalline grains surrounded by grain boundaries and embedded in an amorphous matrix. SPS measurement indicated that the nc-Si:H films have additional two types of additional defect states besides the occupied Si dangling bond states D0/Di and the empty Si dangling states D+ in a-Si, which is attributed to the interface defect states between the nanocrystalline Si grains and the amorphous matrix. The relative SPS intensity of these two kinds of defect states in samples increases with the temperature, which may be interpreted as a result of the bonded hydrogen release at the surface of nanocrystalline Si grains and in the amorphou matrix while increasing substrate temperature.

Structural and optical properties of doped silicon nanocrystals: first-principles calculation

Structural and optical properties of the B doped, P doped and B-P codoped silicon nanocrystals have been investigated using first-principles calculations. It was found that the codoped system tends to reduce structure distortion around B/P impurities compared with B/P single doped systems and shows a decreased energy band gap compared with undoped system due to there being electronic compensating effect. In addition, the spatial behaviors of density of states indicated that codoping possesses a tendency of confining the electrons and holes around the B/P impurities, which suggests the possibility of increasing electron emission transition rates between donor and acceptor. Moreover, the dielectric functions calculation demonstrated that the optical absorption of codoped silicon nanocrystals have the characteristic of the energy band gap being redshifted with respect to the undoped case together with peak appearing at lower energy side.

Influence of the size dispersion on the emission spectra of the hydrogen passivated Si nanoparticles in SiNx

The influence of the size dispersion on the emission spectra of the hydrogen passivated silicon nanopaticles (Si NPs) in silicon nitride thin films, which are grown by helicon wave plasma-enhanced chemical vapor deposition (HWP-CVD) technique with SiH4, H2 and Ar-diluted 6.25% N2 as the reactant gas sources, have been investigated. Transmission electron microscopy examination shows that Si NPs are isolated from the others by silicon nitride barriers and the sizes of Si NPs in silicon nitride matrix are distributed within a wide range. The luminescent due to quantum confinement effect of Si NPs embedded in the SiNx matrix was guaranteed by adding H2 into film deposition process. The emission correlated to the defects such as excess silicon voids and nonradiative recombination centers have been suppressed and a strong PL with a wide band was observed. The photoluminescence (PL) spectra were investigated under different excitation energy (Eex). It is shown that both the PL peak energy and their full width at half maximum (FWHM) show a decrease trend with decreasing the Eex, and no obvious PL can be observed when the Eex lower than a certain Eex. And at high excitation energy contribution of the smaller particles in size becomes remarkable larger, thus it may be extremely difficult to correlate the PL spectra with the mean size of the Si NPs. These results are explained by quantum confinement effect model taking into account there existing a size distribution of Si NPs in the SiNx matrix.

Influence of hydrogen dilution on bonding configurations and optical absorption properties of a-Si/SiNx composite films

Infrared absorption and optical transmission combined with the optical absorption model have been used to study the influence of hydrogen dilution on bonding configurations and optical absorption properties of nano-sized amorphous silicon embedded in silicon nitride thin films (a-Si /SiNx). The amount of bonded hydrogen was investigated by the N-H and Si-H infrared absorption bands. The optical band gap and sub-gap absorption coefficient were obtained by optical absorption spectra. It is shown that the film deposited at 20 sccm hydrogen flow rate, which contains the highest bonded hydrogen content, has the maximum optical band gap and the lowest density of defects. Furthermore, the optical band gap and mean size of a-Si nano-grains in SiNx matrix were obtained through simulating the optical absorption data using an optical absorption model. The simulated optical band gap was in good agreement with the experiment results. These results suggest that appropriate hydrogen dilution in the precursor is beneficial to improving the microstructure and optical properties of the a-Si /SiNx composite films.

Substrate bias effect on preparation of nanocrystalline silicon carbide thin films in helicon wave plasma chemical vapor deposition

Silicon carbide thin films are prepared by helicon wave plasma enhanced chemical vapor deposition (HW-PECVD) using a gas mixture of silane, methane, and hydrogen at a constant gas flow ratio under varying negative DC bias voltage. The structural and optical properties of the deposited films are investigated using Fourier transform infrared spectra (FTIR), ultraviolet-visible (UV-VIS) transmission spectra, and scanning electron microscopy (SEM). It is found that by applying the moderate bias on the substrates to accelerate the energetic ions, nanocrystalline silicon carbide can be deposited at lower onset temperature than without bias, and the crystalline grain size of the films is smaller and more uniform. The mechanism about the enhancing effect of the bias is discussed on the performance of positive ions in the plasma.

Comparative analysis of photoluminescence in nano-sized amorphous and crystalline silicon embedded in silicon nitride thin film

Nano-sized amorphous silicon embedded in SiNx films are prepared by helicon wave plasma-enhanced chemical vapor deposition technique (HWP-CVD), and Si nanocrystals embedded in SiNx films is obtained after furnace annealing (FA) in nitrogen ambient. The structure and optical properties of nano-sized amorphous and crystalline silicon embedded in silicon nitride (SiNx) thin film is comparatively analyzed. Raman scattering measurement shows that, apart from appearance of a new peak at about 496-510cm-1 corresponding to Si nanocrystals scattering, the relative scattering intensity of the two peaks located at 150 cm-1 and at 480cm-1 increases after high temperature annealing, indicating that the microstructures of the annealed films becomes more disordered. Meanwhile, the photothermal deflection spectra (PDS) show the optical absorption coefficient of annealed samples in the band gap increases in about one order of magnitude, indicating that more gap states exist in the annealed samples. After further annealing in forming gas and comparing the PL results of both as-deposited and thermal treatment thin film, it is found that red shift of the main peak of PL spectra is correlated with the enlarge of the silicon size. The role of interface states between silicon clusters and SiNx matrix influence the PL behavior is discussed.