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.

Fluorination-mitigated high-current degradation of amorphous InGaZnO thin-film transistors

As growing applications demand higher driving currents of oxide semiconductor thin-film transistors(TFTs),severe instabilities and even hard breakdown under high-current stress(HCS)become critical challenges.In this work,the triggering voltage of HCS-induced self-heating(SH)degradation is defined in the output characteristics of amorphous indium-galliumzinc oxide(a-IGZO)TFTs,and used to quantitatively evaluate the thermal generation process of channel donor defects.The fluorinated a-IGZO(a-IGZO:F)was adopted to effectively retard the triggering of the self-heating(SH)effect,and was supposed to originate from the less population of initial deep-state defects and a slower rate of thermal defect transition in a-IGZO:F.The proposed scheme noticeably enhances the high-current applications of oxide TFTs.

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.

Nanoporous SiO_x coated amorphous silicon anode material with robust mechanical behavior for high-performance rechargeable Li-ion batteries

Silicon is a promising anode material for rechargeable Li-ion battery (LIB) due to its high energy density and relatively low operating voltage. However, silicon based electrodes suffer from rapid capacity degradation during electrochemical cycling. The capacity decay is predominantly caused by (i) cracking due to large volume variations during lithium insertion/extraction and (ii) surface degradation due to excessive solid electrolyte interface (SEI) formation. In this work, we demonstrate that coating of a-Si thin film with a Li-active, nanoporous SiOx layer can result in exceptional electrochemical performance in Li-ion battery. The SiOx layer provides improved cracking resistance to the thin film and prevent the active material loss due to excessive SEI formation, benefiting the electrode cycling stability. Half-cell experiments using this anode material show an initial reversible capacity of 2173 mAh g^-1 with an excellent coulombic efficiency of 90.9%. Furthermore, the electrode shows remarkable capacity retention of ~97% after 100 cycles at C/2 charging rate. The proposed anode architecture is free from Liinactive binders and conductive additives and provides mechanical stability during the charge/discharge process.

Growth characteristics of amorphous-layer-free nanocrystalline silicon films fabricated by very high frequency PECVD at 250 ℃

Amorphous-layer-free nanocrystalline silicon films were prepared by a very high frequency plasma enhanced chem-ical vapor deposition （PECVD） technique using hydrogen-diluted Sill4 at 250 ℃. The dependence of the crystallinity of the film on the hydrogen dilution ratio and the film thickness was investigated. Raman spectra show that the thickness of the initial amorphous incubation layer on silicon oxide gradually decreases with increasing hydrogen dilution ratio. High-resolution transmission electron microscopy reveals that the initial amorphous incubation layer can be completely eliminated at a hydrogen dilution ratio of 98%, which is lower than that needed for the growth of amorphous-layer-free nanocrystalline silicon using an excitation frequency of 13.56 MHz. More studies on the microstructure evolution of the initial amorphous incubation layer with hydrogen dilution ratios were performed using Fourier-transform infrared spectroscopy. It is suggested that the high hydrogen dilution, as well as the higher plasma excitation frequency, plays an important role in the formation of amorphous-layer-free nanocrystalline silicon films.

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 ℃ .

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.

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.

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.

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

The structural un-uniformity of microcrystalline silicon, thin film, amorphous incubation layerc-Si：H films prepared using very high frequency plasma-enhanced chemical vapour deposition method has been investigated by Raman spectroscopy, spectroscopic ellipsometer and atomic force mi- croscopy. It was found that the formation of amorphous incubation layer was caused by the back diffusion of SiH4 and the amorphous induction of glass surface during the initial ignition process, and growth of the incubation layer can be suppressed and uniform μc-Si：H phase is generated by the application of delayed initial SiH4 density and silane profiling methods.

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.

Preparation and Characterization of Amorphous Silicon Oxide Nanowires

Large-scale amorphous silicon nanowires (SiNWs) with a diameter about 100 nm and a length of dozens of micrometers on silicon wafers were synthesized by thermal evaporation of silicon monoxide (SiO). Scanning electron microscope (SEM) and transmission electron microscope (TEM) observations show that the silicon nanowires are smooth. Selected area electron diffraction (SAED) shows that the silicon nanowires are amorphous and en-ergy-dispersive X-ray spectroscopy (EDS) indicates that the nanowires have the composition of Si and O elements in an atomic ratio of 1:2,their composition approximates that of SiO2. SiO is considered to be used as a Si sources to produce SiNWs. We conclude that the growth mechanism is closely related to the defect structure and silicon monoxide followed by growth through an oxide-assisted vapor-solid reaction.

Concise Modeling of Amorphous Dual-Gate In-Ga-Zn-O Thin-Film Transistors for Integrated Circuit Designs

An analytical model for current-voltage behavior of amorphous In-Ga-Zn-O thin-film transistors（a-IGZO TFTs）with dual-gate structures is developed.The unified expressions for synchronous and asynchronous operating modes are derived on the basis of channel charges,which are controlled by gate voltage.It is proven that the threshold voltage of asynchronous dual-gate IGZO TFTs is adjusted in proportion to the ratio of top insulating capacitance to the bottom insulating capacitance（C_（TI）/C_（BI））.Incorporating the proposed model with Verilog-A,a touch-sensing circuit using dual-gate structure is investigated by SPICE simulations.Comparison shows that the touch sensitivity is increased by the dual-gate IGZO TFT structure.

Solid-phase Crystallization of Amorphous Silicon Films by Rapid Thermal Annealing

The morphous silicon films prepared by PECVD at substrate temperatures of 30℃ have been crystallized by rapid thermal annealing method, the budget of time-temperature in the annealing process is 600℃ for 120s, 850℃ for 120s, and 950℃ for 120s. The results indicate the crystallization at 850℃ and 950℃ are better as shown in micro-Raman scattering and scanning electronic microscope.

Temperature-dependent bias-stress-induced electrical instability of amorphous indium-gallium-zinc-oxide thin-film transistors

The time and temperature dependence of threshold voltage shift under positive-bias stress（PBS） and the following recovery process are investigated in amorphous indium-gallium-zinc-oxide（a-IGZO） thin-film transistors. It is found that the time dependence of threshold voltage shift can be well described by a stretched exponential equation in which the time constant τ is found to be temperature dependent. Based on Arrhenius plots, an average effective energy barrier Eτ stress= 0.72 eV for the PBS process and an average effective energy barrier Eτ recovery= 0.58 eV for the recovery process are extracted respectively. A charge trapping/detrapping model is used to explain the threshold voltage shift in both the PBS and the recovery process. The influence of gate bias stress on transistor performance is one of the most critical issues for practical device development.

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.

2H-SiC Dendritic Nanocrystals In Situ Formation from Amorphous Silicon Carbide under Electron Beam Irradiation

Under electron beam irradiation, the in-situ formation of 2H-SiC dentritic nanocrystals from amorphous silicon carbide at room temperature was observed. The homogenous transition mainly occurs at the thin edge and on the surface of specimen where the energy obtained from electron beam irradiation is high enough to cause the amorphous crystallizing into 2H-SiC.