Formation mechanism of incubation layers in the initial stage of microcrystalline silicon growth by PECVD

The incubation layers in microcrystalline silicon films (\muc-Si:H) are studied in detail. The incubation layers in \muc-Si:H films are investigated by bifacial Raman spectra, and the results indicate that either decreasing silane concentration (SC) or increasing plasma power can reduce the thickness of incubation layer. The analysis of the in-situ diagnosis by plasma optical emission spectrum (OES) shows that the emission intensities of the SiH*(412\,nm) and H_\al (656nm) lines are time-dependent, thus SiH*/H_\al ratio is of temporal evolution. The variation of SiH*/H_\al ratio can indicate the variation in relative concentration of precursor and atomic hydrogen in the plasma. And the atomic hydrogen plays a crucial role in the formation of \muc-Si:H; thus, with the plasma excited, the temporal-evolution SiH*/H_\al ratio has a great influence on the formation of an incubation layer in the initial growth stage. The fact that decreasing the SC or increasing the plasma power can decrease the SiH*/H_\al ratio is used to explain why the thickness of incubation layer can reduce with decreasing the SC or increasing the plasma power.     

Influence of the deposition parameters on the transition region of hydrogenated silicon films growth

Hydrogenated microcrystalline and amorphous silicon thin films were prepared by very high frequency plasmaenhanced chemical vapour deposition （VHF PECVD） by using a mixture of silane and hydrogen as source gas. The influence of deposition parameters on the transition region of hydrogenated silicon films growth was investigated by varying the silane concentration （SC）, plasma power （Pw）, working pressure （P）, and substrate temperature （Ts）. Results suggest that SC and Ts are the most critical factors that affect the film structure transition from microcrystalline to amorphous phase. A narrow region in the range of SC and Ts, in which the rapid phase transition takes place, was identified. It was found that at lower P or higher Pw, the transition region is shifted to larger SC. In addition, the dark conductivity and photoconductivity decrease with SC and show sharp changes in the transition region. It proposed that the transition process and the transition region are determined by the competition between the etching effect of atomic hydrogen and the growth of amorphous phase.     

Mechanical strains in pecvd SiN_x:H films for nanophotonic application

Hydrogenated amorphous silicon nitride films(Si N x:H) are deposited at low temperature by high-frequency plasmaenhanced chemical vapor deposition(HF PECVD). The main effort is to investigate the roles of plasma frequency and plasma power density in determining the film properties particularly in stress. Information about chemical bonds in the films is obtained by Fourier transform infrared spectroscopy(FTIR). The stresses in the Si N x:H film are determined from substrate curvature measurements. It is shown that plasma frequency plays an important role in controlling the stresses in Si N x:H films. For silicon nitride layers grown at plasma frequency 40.68 MHz initial tensile stresses are observed to be in a range of 400 MPa–700 MPa. Measurements of the intrinsic stresses of silicon nitride films show that the stress quantity is sufficient for film applications in strained silicon photonics.     

Study on stability of hydrogenated amorphous silicon films

Hydrogenated amorphous silicon （a-Si：H） films with high and same order of magnitude photosensitivity （-10^5） but different stability were prepared by using microwave electron cyclotron resonance chemical vapour deposition system under the different deposition conditions. It was proposed that there was no direct correlation between the photosensitivity and the hydrogen content （CH） as well as H-Si bonding configurations, but for the stability, they were the critical factors. The experimental results indicated that higher substrate temperature, hydrogen dilution ratio and lower deposition rate played an important role in improving the microstructure of a-Si：H films. We used hydrogen elimination model to explain our experimental results.     

Effect of substrate temperature and pressure on properties of microcrystalline silicon films

In this paper intrinsic microcrystalline silicon films have been prepared by very high frequency plasma enhanced chemical vapour deposition (VHF-PECVD) with different substrate temperature and pressure. The film properties were investigated by using Raman spectra, x-ray diffraction, scanning electron microscope (SEM), and optical transmittance measurements, as well as dark conductivity. Raman results indicate that increase of substrate temperature improves the microcrystallinity of the film. The crystallinity is improved when the pressure increases from 50Pa to 80Pa and the structure transits from microcrystalline to amorphous silicon for pressure higher than 80Pa. SEM reveals the effect of substrate temperature and pressure on surface morphology.     

A new analytical model of high voltage silicon on insulator （SOI） thin film devices

A new analytical model of high voltage silicon on insulator (SOI) thin film devices is proposed, and a formula of silicon critical electric field is derived as a function of silicon film thickness by solving a 2D Poisson equation from an effective ionization rate, with a threshold energy taken into account for electron multiplying. Unlike a conventional silicon critical electric field that is constant and independent of silicon film thickness, the proposed silicon critical electric field increases sharply with silicon film thickness decreasing especially in the case of thin films, and can come to 141V/μm at a film thickness of 0.1μm which is much larger than the normal value of about 30V/μm. From the proposed formula of silicon critical electric field, the expressions of dielectric layer electric field and vertical breakdown voltage (V_B,V) are obtained. Based on the model, an ultra thin film can be used to enhance dielectric layer electric field and so increase vertical breakdown voltage for SOI devices because of its high silicon critical electric field, and with a dielectric layer thickness of 2μm the vertical breakdown voltages reach 852 and 300V for the silicon film thicknesses of 0.1 and 5μm, respectively. In addition, a relation between dielectric layer thickness and silicon film thickness is obtained, indicating a minimum vertical breakdown voltage that should be avoided when an SOI device is designed. 2D simulated results and some experimental results are in good agreement with analytical results.     

Nanomorph Silicon Thin Films Prepared by Using an HW-MWECR CVD System

We have prepared hydrogenated nano-amorph silicon (na-Si:H) films by using a hot-wire-assisted microwave electron-cyclotron-resonance (HW-MWECR) chemical vapour deposition (CVD) system. The films are deposited in two steps: in the first 9rain, a hydrogenated amorphous silicon layer is deposited by using hydrogen-diluted silane with a concentration of SiH4/(SiH4 H2) = 20%, and then a nanocrystalline silicon (nc-Si) layer is deposited by using various highly hydrogen-diluted silane. The Raman TO-like mode peak of the films was found in the range 497-508cm^-1. When the silane concentration used for preparation of the nc-Si layer is 14.3%, the film has a large crystalline volume fraction of 65.4%, a wide optical band gap of 1.89eV and a low hydrogen content of 9.5 at.%. Moreover, the na-Si:H films rather than nc-Si possess high photosensitivity of about 10^5.     

Significant Improvement of Passivation Performance by Two-Step Preparation of Amorphous Silicon Passivation Layers in Silicon Heterojunction Solar Cells

The key feature of amorphous/crystalline silicon heterojunction solar cells is extremely low surface recombination,which is related to superior passivation on the crystalline silicon wafer surface using thin hydrogenated amorphous silicon(a-Si:H)layers,leading to a high open-circuit voltage.In this work,a two-step method of a-Si:H passivation is introduced,showing excellent interface passivation quality,and the highest effective minority carrier lifetime exceeds 4500 μs.By applying a buffer layer deposited through pure silane plasma,the risk of film epitaxial growth and plasma damage caused by hydrogen diluted silane plasma is effectively reduced.Based on this,excellent passivation is realized through the following hydrogen diluted silane plasma process with the application of high density hydrogen.In this process,hydrogen diffuses to a-Si/c-Si interface,saturating residual dangling bonds which are not passivated by the buffer layer.Employing this two-step method,a heterojunction solar cell with an area of 239 cm~2 is prepared,yielding to open-circuit voltage up to 735 mV and total-area efficiency up to 22.4%.     

Influence of ignition condition on the growth of silicon thin films using plasma enhanced chemical vapour deposition

The influences of the plasma ignition condition in plasma enhanced chemical vapour deposition (PECVD) on the interfaces and the microstructures of hydrogenated microcrystalline Si (μc-Si:H) thin films are investigated.The plasma ignition condition is modified by varying the ratio of SiH 4 to H 2 (R H).For plasma ignited with a constant gas ratio,the time-resolved optical emission spectroscopy presents a low value of the emission intensity ratio of Hα to SiH (I Hα /I SiH) at the initial stage,which leads to a thick amorphous incubation layer.For the ignition condition with a profiling R H,the higher I Hα /I SiH values are realized.By optimizing the R H modulation,a uniform crystallinity along the growth direction and a denser μc-Si:H film can be obtained.However,an excessively high I Hα /I SiH may damage the interface properties,which is indicated by capacitance-voltage (C-V) measurements.Well controlling the ignition condition is critically important for the applications of Si thin films.     

The study of amorphous incubation layers during the growth of microcrystalline silicon films under different deposition conditions

<正>The structural un-uniformity of μc-Si:H films prepared using a very high frequency plasma-enhanced chemical vapour deposition method has been investigated by Raman spectroscopy,spectroscopic ellipsometer and atomic force microscopy.It was found that the formation of amorphous incubation layer was caused by the back diffusion of SiH_4 and the amorphous induction of glass surface during the initial ignition process,and growth of the incubation layer can be suppressed and uniformμc-Si:H phase is generated by the application of delayed initial SiH_4 density and silane profiling methods.     

Photoluminescence of amorphous carbon films fabricated by layer—by—layer hydrogen plasma chemical annealing method

A method in which nanometre-thick film deposition was alternated with hydrogen plasma annealing (layer-by-layer method) was applied to fabricate hydrogenated amorphous carbon films in a conventional plasma-enhanced chemical vapour deposition system.It was found that the hydrogen plasma treatment could decrease the hydrogen concentration in the films and change the sp^2/sp^3 ratio to some extent by chemical etching.Blue photoluminescence was observed at room temperature,as a result of the reduction of sp^2 clusters in the films.     

Soft Magnetic Thin Films FeCoHfO for High-Frequency Noise Suppression Applications

A series of FeCoHfO films were fabricated by dc magnetron reactive sputtering at varying partial pressure of oxygen （Po2） from 0 to 11.7%, and the electrical and magnetic properties of films have been studied. It is shown that optimal Fe43.29 Co19.51Hf7.49 O29.71 films with desired properties can be obtained when the films were prepared under Po2= 5.1%. The films show superior properties of low coereivity, Hc ∽5.5 Oe, relatively high saturation magnetization, 47rMs · 18.3 kG, high anisotropy field Hk ∽ 65 Oe, and high electrical resistivity ρ∽ 2675 μΩ·cm. Permeability spectra shows that the natural ferromagnetic resonant frequency is as high as 3.1 GHz. The combined merits of the film make the films taken as an ideal candidate material for high-frequency applications such as noise suppressor. In addition, the effects of the film thickness and annealing treatment on the magnetic properties are also reported.     

Research on the optimum hydrogenated silicon thin films for application in solar cells

Hydrogenated silicon (Si:H) thin films for application in solar cells were deposited by using very high frequency plasma enhanced chemical vapour deposition (VHF PECVD) at a substrate temperature of about 170℃. The electrical, structural, and optical properties of the films were investigated. The deposited films were then applied as i-layers for p-i-n single junction solar cells. The current--voltage (I-V) characteristics of the cells were measured before and after the light soaking. The results suggest that the films deposited near the transition region have an optimum properties for application in solar cells. The cell with an i-layer prepared near the transition region shows the best stable performance.     

Formations and morphological stabilities of ultrathin CoSi₂ films

In this paper we investigate the formations and morphological stabilities of Co-silicide films using 1-8-nm thick Co layers sputter-deposited on silicon(100) substrates.These ultrathin Co-silicide films are formed via solid-state reaction of the deposited Co films with Si substrate at annealing temperatures from 450℃ to 850℃.For a Co layer with a thickness no larger than 1 nm,epitaxially aligned CoSi2 films readily grow on silicon(100) substrate and exhibit good morphological stabilities up to 600℃.For a Co layer thicker than 1 nm,polycrystalline CoSi and CoSi2 films are observed.The critical thickness below which epitaxially aligned CoSi2 film prevails is smaller than the reported critical thickness of the Ni layer for epitaxial alignment of NiSi2 on silicon(100) substrate.The larger lattice mismatch between the CoSi2 film and the silicon substrate is the root cause for the smaller critical thickness of the Co layer.     

Induced growth of high quality ZnO thin filmsby crystallized amorphous ZnO

This paper reports the induced growth of high quality ZnO thin film by crystallized amorphous ZnO. Firstly amorphous ZnO was prepared by solid-state pyrolytic reaction, then by taking crystallized amorphous ZnO as seeds (buffer layer), ZnO thin films have been grown in diethyene glycol solution of zinc acetate at 80℃. X-ray Diffraction curve indicates that the films were preferentially oriented [001] out-of-plane direction of the ZnO. Atomic force microscopy and scanning electron microscopy were used to evaluate the surface morphology of the ZnO thin film. Photoluminescence spectrum exhibits a strong ultraviolet emission while the visible emission is very weak. The results indicate that high quality ZnO thin film was obtained.     

Electron field emission characteristics of nano-catkin carbon films deposited by electron cyclotron resonance microwave plasma chemical vapour deposition

This paper reported that the nano-catkin carbon films were prepared on Si substrates by means of electron cyclotron resonance microwave plasma chemical vapour deposition in a hydrogen and methane mixture. The surface morphology and the structure of the fabricated films were characterized by using scanning electron microscopes and Raman spectroscopy, respectively. The stable field emission properties with a low threshold field of 5V/μm corresponding to a current density of about 1μA/cm^2 and a current density of 3.2mA/cm^2 at an electric field of 10V/μm were obtained from the carbon film deposited at CH4 concentration of 8%. The mechanism that the threshold field decreased with the increase of the CH4 concentration and the high emission current appeared at the high CH4 concentration was explained by using the Fowler-Nordheim theory.     

Formations and morphological stabilities of ultrathin CoSi2 films

In this paper we investigate the formations and morphological stabilities of Co-silicide fihns using 1-8-nm thick Co layers sputter-deposited on silicon （100） substrates. These ultrathin Co-silicide films are formed via solid-state reaction of the deposited Co films with Si substrate at annealing temperatures from 450 ℃ to 850 ℃. For a Co layer with a thickness no larger than i nm, epitaxially aligned CoSi2 films readily grow on silicon （100） substrate and exhibit good morphological stabilities up to 600 ℃. For a Co layer thicker than 1 nm, polycrystalline CoSi and CoSi2 films are observed. The critical thickness below which epitaxially aligned CoSi2 film prevails is smaller than the reported critical thickness of the Ni layer for epitaxial alignment of NiSi2 on silicon （100） substrate. The larger lattice mismatch between the CoSi2 film and the silicon substrate is the root cause for the smaller critical thickness of the Co layer.     

Preparation of hydrogenated microcrystalline silicon films with hot-wire-assisted MWECR-CVD system

Intrinsic hydrogenated microcrystalline silicon (\muc-Si:H) films have been prepared by hot-wire-assisted microwave electron-cyclotron-resonance chemical vapour deposition (HW-MWECR-CVD) under different deposition conditions. Fourier-transform infrared spectra and Raman spectra were measured. Optical band gap was determined by Tauc plots, and experiments of photo-induced degradation were performed. It was observed that hydrogen dilution plays a more essential role than substrate temperature in microcrystalline transformation at low temperatures. Crystalline volume fraction and mean grain size in the films increase with the dilution ratio (R=H₂/(H₂+SiH₄)). With the rise of crystallinity in the films, the optical band gap tends to become narrower while the hydrogen content and photo-induced degradation decrease dramatically. The samples, were identified as \mu c-Si:H films, by calculating the optical band gap. It is considered that hydrogen dilution has an effect on reducing the crystallization activation energy of the material, which promotes the heterogeneous solid-state phase transition characterized by the Johnson--Mehl--Avrami (JMA) equation. The films with the needed structure can be prepared by balancing deposition and crystallization through controlling process parameters.     

PHOTO- AND ELECTRO-LUMINESCENCE FROM HYDROGENATED AMORPHOUS SILICON CARBIDE FILMS PREPARED BY USING ORGANIC CARBON SOURCE

Hydrogenated amorphous silicon carbide (a-SiC:H) films were grown by using an organic source, xylene (C₈H₁₀), instead of methane (CH₄) in a conventional plasma enhanced chemical vapor deposition system. The optical band gap of these samples was increased gradually by changing the gas ratio of C₈H₁₀ to SiH₄. The film with high optical band gap was soft and polymer-like and intense photoluminescence were obtained. Room temperature electro-luminescence was also achieved with peak energy at 2.05 eV (600 nm) for the a-SiC:H film with optical band gap of 3.2 eV.     

Thick c-BN films deposited by radio frequency magnetron sputtering in argon/nitrogen gas mixture with additional hydrogen gas

The excellent physical and chemical properties of cubic boron nitride(c-BN) film make it a promising candidate for various industry applications. However, the c-BN film thickness restricts its practical applications in many cases. Thus, it is indispensable to develop an economic, simple and environment-friend way to synthesize high-quality thick, stable c-BN films. High-cubic-content BN films are prepared on silicon(100) substrates by radio frequency(RF) magnetron sputtering from an h-BN target at low substrate temperature. Adhesions of the c-BN films are greatly improved by adding hydrogen to the argon/nitrogen gas mixture, allowing the deposition of a film up to 5-μm thick. The compositions and the microstructure morphologies of the c-BN films grown at different substrate temperatures are systematically investigated with respect to the ratio of H_2 gas content to total working gas. In addition, a primary mechanism for the deposition of thick c-BN film is proposed.