Semi-quantitative study on the Staebler-Wronski effect of hydrogenated amorphous silicon films prepared with HW-ECR-CVD system

The method of numerical simulation is used to fit the relationship between the photoconductivity in films and the illumination time. The generation and process rule of kinds of different charged defect states during illumination are revealed. It is found surprisingly that the initial photoconductivity determines directly the total account of photoconductivity degradation of sample.

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.

Surface plasmon enhanced photoluminescence in amorphous silicon carbide films by adjusting Ag island film sizes

Ag island films with different sizes are deposited on hydrogenated amorphous silicon carbide （a-SiC：H） films, and the influences of Ag island films on the optical properties of the tx-SiC：H films are investigated. Atomic force microscope images show that Ag nanoislands are formed after Ag coating, and the size of the Ag islands increases with increasing Ag deposition time. The extinction spectra indicate that two resonance absorption peaks which correspond to out-of-plane and in-plane surface plasmon modes of the Ag island films are obtained, and the resonance peak shifts toward longer wavelength with increasing Ag island size. The photoluminescence （PL） enhancement or quenching depends on the size of Ag islands, and PL enhancement by 1.6 times on the main PL band is obtained when the sputtering time is 10 min. Analyses show that the influence of surface plasmons on the PL of a-SiC：H is determined by the competition between the scattering and absorption of Ag islands, and PL enhancement is obtained when scattering is the main interaction between the Ag islands and incident light.

Smooth Surface Morphology of Hydrogenated Amorphous Silicon Film Prepared by Plasma Enhanced Chemical Vapor Deposition

Influence of the parameters of plasma enhanced chemical vapor deposition （PECVD） on the surface morphology of hydrogenated amorphous silicon （α-Si：H） film was investigated. The root-mean-square （RMS） roughness of the film was measured by atomic force microscope （AFM） and the relevant results were analyzed using the surface smoothing mechanism of film deposition. It is shown that an α-Si：H film with smooth surface morphology can be obtained by increasing the PH3/N2 gas flow rate for 10% in a high frequency （HF） mode. For high power, however, the surface morphology of the film will deteriorate when the Sill4 gas flow rate increases. Furthermore, optimized parameters of PECVD for growing the film with smooth surface were obtained to be Sill4：25 sccm （standard cubic centimeters per minute）, At： 275 sccm, 10%PH3/N2：2 sccm, HF power： 15 W, pressure： 0.9 Torr and temperature： 350℃. In addition, for in thick fihn deposition on silicon substrate, a N20 and NH3 preprocessing method is proposed to suppress the formation of gas bubbles.

Spatially-Resolved Crystallization of Amorphous Silicon Films on the Glass Substrate by Multi-beam Laser Interference

Laser interference induced crystallization of amorphous silicon （a-Si） on the glass substrate was performed using a Q-switched Nd：YAG （yttrium aluminum garnet） laser. White light interferometer （WLI） and atomic force microscope （AFM） were used to characterize the morphology of the structured films, while X-ray diffraction （XRD）, combined with the AFM, was used to analyse the crystalline structure of the film. The experimental results show that the laser energy density above a certain threshold, in the range of 400-500 mJ/cm2,triggers the patterned crystallizations which take the form similar to the laser intensity distribution. For the patterned crystallization under multipulse exposure, a definite polycrystalline structure with individual phases was observed by XRD. The difference in feature form, e.g., deepened craters or heightened lines, is related to the laser energy density relative to the threshold of evaporation of the material.

Interface states study of intrinsic amorphous silicon for crystalline silicon surface passivation in HIT solar cell

Intrinsic hydrogenated amorphous silicon（a-Si：H） film is deposited on n-type crystalline silicon（c-Si） wafer by hotwire chemical vapor deposition（HWCVD） to analyze the amorphous/crystalline heterointerface passivation properties.The minority carrier lifetime of symmetric heterostructure is measured by using Sinton Consulting WCT-120 lifetime tester system,and a simple method of determining the interface state density（D_（it）） from lifetime measurement is proposed.The interface state density（D_（it）） measurement is also performed by using deep-level transient spectroscopy（DLTS） to prove the validity of the simple method.The microstructures and hydrogen bonding configurations of a-Si：H films with different hydrogen dilutions are investigated by using spectroscopic ellipsometry（SE） and Fourier transform infrared spectroscopy（FTIR） respectively.Lower values of interface state density（D_（it）） are obtained by using a-Si：H film with more uniform,compact microstructures and fewer bulk defects on crystalline silicon deposited by HWCVD.

Room Temperature Growth of Hydrogenated Amorphous Silicon Films by Dielectric Barrier Discharge Enhanced CVD

Hydrogenated amorphous silicon （a-Si： H） films were deposited on Si （100） and glass substrates by dielectric barrier discharge enhanced chemical vapour deposition （DBD-CVD） in （SiH4＋H2） atmosphere at room temperature. Results of the thickness measurement, SEM （scanning electron microscope）, Raman, and FTIR （Fourier transform infrared spectroscopy） show that with the increase in the applied peak voltage, the deposition rate and network order of the films increase, and the hydrogen bonding configurations mainly in di-hydrogen （Si-H2） and poly hydrogen （SiH2）n are introduced into the films. The UV-visible transmission spectra show that with the decrease in SiH4/ （SiHn＋H2） the thin films＇ band gap shifts from 1.92 eV to 2.17 eV. These experimental results are in agreement with the theoretic analysis of the DBD discharge. The deposition of a-Si： H films by the DBD-CVD method as reported here for the first time is attractive because it allows fast deposition of a-Si： H films on large-area low-melting-point substrates and requires only a low cost of production without additional heating or pumping equipment.

Significant“smaller is softer”in amorphous silicon via irradiation-me diate d surface modification

“Smaller is softer”is a reverse size dependence of strength,defying the“smaller is stronger”tenet.It usually results from surface-mediated displacive or diffusive deformation and is mainly found in some ultra-small-scale(below tens of nanometers)metallic materials.Here,making use of the surface modifi-cation via ion beam irradiation,we bring the“smaller is softer”into being in a covalently-bonded,hard,and brittle material-amorphous Si(a-Si)at a much larger size regime(<∼500 nm).It is manifested as the transition from the quasi-brittle failure to the homogeneous plastic deformation as well as the de-creasing yield stress with sample volume reduction at the submicron-scale regime.An analytical model of hard core/superplastic shell has been proposed to explain the artificially-controllable size-dependent softening.This surface engineering pathway via ion irradiation is not only of particular interest to tai-lor the strength and deformation behaviors in small-sized a-Si or other covalently-bonded amorphous solids but also of practical relevance to the utility of a-Si in microelectronics and microelectromechanical systems.

Toward an Optimum Design of an Amorphous Silicon Photovoltaic/Thermal System:Simulation and Experiments

Amorphous silicon photovoltaic/thermal(a-Si-PV/T)technology is promising due to the low power temperature coefficient,thin-film property,thermal annealing effect of the solar cells,and high conversion efficiency in summer.The design of a-Si-PV/T system is influenced by a number of thermodynamic,structural,and external parameters.Parametric analysis is useful for a good design of the system.A dynamic distributed parameter model is built and verified in this paper.Outdoor tests are carried out.The impacts of operating temperature,mass flow rate,cover ratio of solar cells,heat transfer area,and frame shadow ratio on its performance are theoretically and experimentally investigated.The results indicate that seven or eight copper tubes are suitable to achieve a high overall efficiency of the a-Si-PV/T system.The frame and tilt angle shall avoid a shadow ratio of more than 8.3%during operation.The difference between power outputs at operating temperatures of 35℃and55℃in the first month is about 0.21%while it drops to less than 0.1%in the twelfth month.Compared with conventional PVT systems,the a-Si-PV/T system benefits from a higher design temperature with a minor efficiency decrement.

Properties of boron doped ZnO films prepared by reactive sputtering method: Application to amorphous silicon thin film solar cells

Reactive sputtered boron-doped zinc oxide(BZO) film was deposited from argon,hydrogen and boron gas mixture.The reactive sputtering technique provides us the flexibility of changing the boron concentration in the produced films by using the same intrinsic zinc oxide target.Textured surface was obtained in the as-deposited films.The surface morphology and the opto-electronic properties of the films can be controlled by simply varying the gas concentration ratio.By varying the gas concentration ratio,the best obtained resistivity ~6.51×10^-4Ω-cm,mobility ~19.05 cm^2 V^-1 s^-1 and sheet resistance ~7.23Ω/□ were obtained.At lower wavelength of light,the response of the deposited films improves with the increase of boron in the gas mixture and the overall transmission in the wavelength region 350-1100 nm of all the films are>85 %.We also fabricated amorphous silicon(a-Si) thin film solar cell on the best obtained BZO layers.The overall efficiency of the a-Si solar cell is 8.14 %,found on optimized BZO layer.

Effect of Additives on the Sintering of Amorphous Nano-sized Silicon Nitride Powders

Amorphous nano-sized silicon nitride powders were sintered by liquid phase sintering. The influences of the additives of Y2O3 and Al2O3 prepared by two different ways, the polyacrylamide gel method and the precipitation method, were investigated. The grain sizes of the additives prepared by the first method were finer than those of prepared by the latter method. When sintered at the same temperature, 1700 ℃, the average grain size of the silicon nitride is 0.3 um for the sample with the former additives, which is much finer than the one with the latter additives. The density of additives prepared by precipitation method is clearly lower than those of prepared by polyacrylamide gel method.

Open-circuit voltage analysis of p-i-n type amorphous silicon solar cells deposited at low temperature

This paper identifies the contributions of p-a-SiC：H layers and i-a-Si：H layers to the open circuit voltage of p-i-n type a-Si：H solar cells deposited at a low temperature of 125℃. We find that poor quality p-a-SiC：H films under regular conditions lead to a restriction of open circuit voltage although the band gap of the i-layer varies widely. A significant improvement in open circuit voltage has been obtained by using high quality p-~SiC：H films optimized at the ＂low-power regime＂ under low silane flow rates and high hydrogen dilution conditions.

Influence of using amorphous silicon stack as front heterojunction structure on performance of interdigitated back contact-heterojunction solar cell （IBC-HJ）

Interdigitated back contact-heterojunction （IBC-HJ） solar cells can have a conversion efficiency of over 25%. However, the front surface passivation and structure have a great influence on the properties of the IBC-HJ solar cell. In this paper, detailed numerical simulations have been performed to investigate the potential of front surface field （FSF） offered by stack of n-type doped and intrinsic amorphous silicon （a-Si） layers on the front surface of IBC-HJ solar cells. Simulations results clearly indicate that the electric field of FSF should be strong enough to repel minority carries and cumulate major carriers near the front surface. However, the overstrong electric field tends to drive electrons into a-Si layer, leading to severe recombination loss. The n-type doped amorphous silicon （n-a-Si） layer has been optimized in terms of doping level and thickness. The optimized intrinsic amorphous silicon （i-a-Si） layer should be as thin as possible with an energy band gap （Es） larger than 1.4 eV. In addition, the simulations concerning interface defects strongly suggest that FSF is essential when the front surface is not passivated perfectly. Without FSF, the IBC-HJ solar cells may become more sensitive to interface defect density.

Annealing effect on optical and electronic properties of silicon rich amorphous silicon-carbide films

A series of Si-rich amorphous silicon carbide （a-SiC：H） thin films were deposited in conventional plasma enhanced chemical vapor deposition system with various gas ratio R = [CH4]/[SiH4]. The microstructural, optical and electronic properties of as-deposited films were investigated in this study. It was found that optical band gap was linearly proportional to carbon content in the films and it could be controlled in a range of 1.8-2.4 eV by changing the gas ratio, R. Both dark and photo conductivities in room temperature were decreased with the increasing of carbon content in the films, and the photosensitivity reached as high as 104 for the film with the optical band gap of 1.96 eV. The as-deposited samples were subsequently annealed at the temperatures of 900℃ and 1000℃. The formation of nanocrystalline silicon （nc- Si） dots in amorphous silicon carbide （a-SiC） host matrix was shown. The dark conductivity was enhanced by five orders of magnitude after annealing compared with that of as-deposited films. The result of temperature-dependent conductivity suggested that the property of carrier transport was dominated by conduction process between the extended states. Furthermore, room temperature electroluminescence （EL） was achieved from nc-Si/SiC system and the possible mechanism of radiative recombination mechanism was discussed.

Modeling of current–voltage characteristics for dual-gate amorphous silicon thin-film transistors considering deep Gaussian density-of-state distribution

Accounting for the deep Gaussian and tail exponential distribution of the density of states, a physical approximation for potentials of amorphous silicon thin-film transistors using a symmetric dual gate （sDG a-Si：H TFT） has been presented. The proposed scheme provides a complete solution of the potentials at the surface and center of the layer without solving any transcendental equations. A channel current model incorporating features of gate voltage-dependent mobility and coupling factor is derived. We show the parameters required for accurately describing the current-voltage （l-V） characteristics of DG a-Si：H TFT and just how sensitively these parameters affect TFT current. Particularly, the parameters＇ dependence on the I-V characteristics with respect to the density of deep state and channel thickness has been investigated in detail. The resulting scheme and model are successively verified through comparison with numerical simulations as well as the available experimental data.

Fabrication of Fine-Grained Si_3N_4-Si_2N_2O Composites by Sintering Amorphous Nano-sized Silicon Nitride Powders

Si3N4-Si2N2O composites were fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering （ LPS ）. The Si2 N2O phase was generated by an in-situ reaction 2 Si3 N4 （ s ） ＋ 1.5 02 （ g ） = 3 Si2 N2O （ s ） ＋ N2 （ g ） . The content of Si2 N2 O phase up to 60% in the volume was obtained at a sintering temperature of 1 650℃ and reduced when the sintering temperature increased or decreased, indicating the reaction is reversible. The mass loss, relative density and average grain size increased with increasing the sintering temperature. The average grain size was less than 500 nm when the sintering temperature was below 1 700 ℃. The sintering procedure contains a complex crystallization and a phase transition ： amorphous silicon nitride→equiaxial α- Si3 N4→ equiaxial β- Si3 N4→ rod- like Si2 N2O→ needle- like β- Si3N4 . Small round-shaped β→ Si3 N4 particles were entrapped in the Si2 N2O grains and a high density of staking faults was situated in the middle of Si2 N2O grains at a sintering temperature of 1 650 ℃. The toughness inereased from 3.5 MPa·m^1/2 at 1 600 ℃ to 7.2 MPa· m^1/2 at 1 800 ℃ . The hardness was as high as 21.5 GPa （Vickers） at 1 600 ℃ .

Structural and band tail state photoluminescence properties of amorphous SiC films with different amounts of carbon

Amorphous silicon carbide films are deposited by the plasma enhanced chemical vapour deposition technique,and optical emissions from the near-infrared to the visible are obtained.The optical band gap of the films increases from 1.91 eV to 2.92 eV by increasing the carbon content,and the photoluminescence（PL） peak shifts from 1.51 eV to 2.16 eV.The band tail state PL mechanism is confirmed by analysing the optical band gap,PL intensity,the Stocks shift of the PL,and the Urbach energy of the film.The PL decay times of the samples are in the nanosecond scale,and the dependence of the PL lifetime on the emission energy also supports that the optical emission is related to the radiative recombination in the band tail state.

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.

Micromorph tandem solar cells: optimization of the microcrystalline silicon bottom cell in a single chamber system

We report on the development of single chamber deposition of microcrystalline and micromorph tandem solar cells directly onto low-cost glass substrates. The cells have pin single-junction or pin/pin double-junction structures on glass substrates coated with a transparent conductive oxide layer such as SnO2 or ZnO. By controlling boron and phosphorus contaminations, a single-junction microcrystalline silicon cell with a conversion efficiency of 7.47% is achieved with an i-layer thickness of 1.2 μm. In tandem devices, by thickness optimization of the microcrystalline silicon bottom solar cell, we obtained an initial conversion efficiency of 9.91% with an aluminum （Al） back reflector without a dielectric layer. In order to enhance the performance of the tandem solar cells, an improved light trapping structure with a ZnO/Al back reflector is used. As a result, a tandem solar cell with 11.04% of initial conversion efficiency has been obtained.

Dual-polarization multiplexing amorphous Si:H grating couplers for silicon photonic transmitters in the photonic BiCMOS backend of line

In this paper,we report on polarization combining two-dimensional grating couplers(2D GCs)on amorphous Si:H,fabricated in the backend of line of a photonic BiCMOS platform.The 2D GCs can be used as an interface of a hybrid silicon photonic coherent transmitter,which can be implemented on bulk Si wafers.The fabricated 2D GCs operate in the telecom C-band and show an experimental coupling efficiency of−5 dB with a wafer variation of±1.2 dB.Possibilities for efficiency enhancement and improved performance stability in future design generations are outlined and extension toward O-band devices is also investigated.