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.
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.
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.
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.
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.
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.
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.
Hydrogenated amorphous silicon nitride (a-SiNx:H) thin films are deposited by helicon wave plasma chemical vapor deposition technique. The structural and photoluminescence properties of these films have been characterized by X-ray photoelectron spectroscopy (XPS), Photoluminescence (PL) and ultraviolet-visible (UV-VIS) spectroscopy. It is shown 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 films. All the PL spectra of the deposited films manifest itself as several interference peaks superposed on an energy-dependent Gaussian distributed band. The PL and absorption results of the deposited films with different nitrogen content support that the luminescence of the Si-rich a-SiNx:H films is related to the photo-excited carriers radiation process in the separated amorphous silicon potential well region, while the blue shift of PL main peaks and the enlargement of PL intensity with increase nitrogen content are ascribed to the size reduction of amorphous silicon separated regions and the enhancement of confinement effect.
Silicon-rich hydrogenated silicon nitride thin films (a-SiNx:H) characterized with amorphous silicon cluster separations are deposited by helicon wave plasma-enhanced chemical vapor deposition technique. The optical absorption properties of the deposited films are obtained and analyzed from both light transmittance and reflectance measurements. A trend of blue shift of the exponential tail absorption region is observed with increasing nitrogen content x and the optical gap Eg, the Tauc coefficient B and the Urbach parameter EU have been discussed in terms of the compositional and structural characteristics of the deposited films. It is concluded that the separation of amorphous silicon particles from the a-SiNx:H matrices leads an increasing trend of more disorder microstructure presenting as the features of large EU and small B compared with normal films, especially, the blue shift of the optical absorption edge and the widening of the optical band gap correlated with a three-dimensional quantum confinement effect of amorphous silicon nano-particles is suggested for the films with higher x.
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