Effect of Thermal Annealing on Characteristics of Polycrystalline Silicon

Oxygen and carbon behaviors and minority-carrier lifetimes in multi-crystalline silicon （mc-Si） used for solar cells are investigated by FTIR and QSSPCD before and after annealing at 750～ 1150℃ in N2 and O2 ambient. For comparison, the annealing of CZ silicon with nearly the same oxygen and carbon concentrations is also carried out under the same conditions. The results reveal that the oxygen and carbon concentrations of mc-Si and CZ-Si have a lesser decrease,which means oxygen precipitates are not generated,and grain boundaries in mc-Si do not affect carbon behavior. Bulk lifetime of mc-Si increases in N2 and O2 ambient at 850,950,and 1150℃ ,and the lifetime of mc-Si wafers annealed in 02 are higher than those annealed in N2, which shows that a lot of impurities in mc-Si at high temperature annealing diffuse to grain boundaries,greatly reducing recombination centers. Interstitial Si atoms filling vacancies or recombination centers increases lifetime.

Effect of Boundary Layers on Polycrystalline Silicon Chemical Vapor Deposition in a Trichlorosilane and Hydrogen System

This paper presents the numerical investigation of the effects of momentum, thermal and species boundary layers on the characteristics of polycrystalline silicon deposition by comparing the deposition rates in three chemical vapor deposition （CVD） reactors. A two-dimensional model for the gas flow, heat transfer, and mass transfer was coupled to the gas-phase reaction and surface reaction mechanism for the deposition of polycrystalline silicon from trichlorosilane （TCS）-hydrogen system. The model was verified by comparing the simulated growth rate with the experimental and numerical data in the open literature. Computed results in the reactors indicate that the deposition characteristics are closely related to the momentum, thermal and mass boundary layer thickness. To yield higher deposition rate, there should be higher concentration of TCS gas on the substrate, and there should also be thinner boundary layer of HCl gas so that HCl gas could be pushed away from the surface of the substrate immediately.

Studies on the polycrystalline silicon/SiO2 stack as front surface field for IBC solar cells by two-dimensional simulations

Interdigitated back contact（IBC） solar cells can achieve a very high efficiency due to its less optical losses. But IBC solar cells demand for high quality passivation of the front surface. In this paper, a polycrystalline silicon/SiO_2 stack structure as front surface field to passivate the front surface of IBC solar cells is proposed. The passivation quality of this structure is investigated by two dimensional simulations. Polycrystalline silicon layer and SiO_2 layer are optimized to get the best passivation quality of the IBC solar cell. Simulation results indicate that the doping level of polycrystalline silicon should be high enough to allow a very thin polycrystalline silicon layer to ensure an effective passivation and small optical losses at the same time. The thickness of SiO_2 should be neither too thin nor too thick, and the optimal thickness is 1.2 nm.Furthermore, the lateral transport properties of electrons are investigated, and the simulation results indicate that a high doping level and conductivity of polycrystalline silicon can improve the lateral transportation of electrons and then the cell performance.

Carrier Transport Across Grain Boundaries in Polycrystalline Silicon Thin Film Transistors

We established a model for investigating polycrystalline silicon（poly-Si） thin film transistors（TFTs）.The effect of grain boundaries（GBs） on the transfer characteristics of TFT was analyzed by considering the number and the width of grain boundaries in the channel region,and the dominant transport mechanism of carrier across grain boundaries was subsequently determined.It is shown that the thermionic emission（TE） is dominant in the subthreshold operating region of TFT regardless of the number and the width of grain boundary.To a poly-Si TFT model with a 1 nm-width grain boundary,in the linear region,thermionic emission is similar to that of tunneling（TU）,however,with increasing grain boundary width and number,tunneling becomes dominant.

4H-SiC Schottky barrier diodes with semi-insulating polycrystalline silicon field plate termination

Based on the theoretical analysis of the 4H-SiC Schottky-barrier diodes （SBDs） with field plate termination, 4H-SiC SBD with semi-insulating polycrystalline silicon （SIPOS） FP termination has been fabricated. The relative dielectric con-stant of the SIPOS dielectric first used in 4H-SiC devices is 10.4, which is much higher than that of the SiO2 dielectric, leading to benefitting the performance of devices. The breakdown voltage of the fabricated SBD could reach 1200 V at leak-age current 20 μA, about 70% of the theoretical breakdown voltage. Meanwhile, both of the simulation and experimental results show that the length of the SIPOS FP termination is an important factor for structure design.

Total Ionizing Dose Radiation Effects in the P-Type Polycrystalline Silicon Thin Film Transistors

The total ionizing dose radiation effects in the polycrystalline silicon thin film transistors are studied. Transfer characteristics, high-frequency capacitance-voltage curves and low-frequency noises (LFN) are measured before and after radiation. The experimental results show that threshold voltage and hole-field-effect mobility decrease, while sub-threshold swing and low-frequency noise increase with the increase of the total dose. The contributions of radiation induced interface states and oxide trapped charges to the shift of threshold voltage are also estimated. Furthermore, spatial distributions of oxide trapped charges before and after radiation are extracted based on the LFN measurements.

Three Dimensional Study of Spectral Response of Polycrystalline Silicon Solar Cells： Vertical Junction Frequency Modulation Scheme

In this paper, the modeling ofa bifacial polycrystalline silicon solar cells vertical junction is presented. The study in dynamic frequency is limited to wavelengths from 400 nm to 1100 nm. The dependence of solar cell spectral response on wavelengths for several modulation frequencies was evaluated by using solar cell internal quantum efficiency.The objective is to characterize the polycrystalline silicon in 3D. The effect of frequency modulation pulsation on the phase of internal quantum efficiency was presented as well as values of shunt and series resistance for various grains size values. The results show that the value of maximum internal quantum efficiency is about 50% with a wavelength of 0,82 nm and a frequency of 103 rad/s under monochromatic illumination.

Selected area laser-crystallized polycrystalline silicon thin films by a pulsed Nd:YAG laser with 355 nm wavelength

Selected area laser-crystallized polycrystalline silicon （p-Si） thin films were prepared by the third harmonics （355 nm wavelength） generated by a solid-state pulsed Nd：YAG laser. Surface morphologies of 400 nm thick films after laser irradiation were analyzed. Raman spectra show that film crystallinity is improved with in- crease of laser energy. The optimum laser energy density is sensitive to the film thickness. The laser energy density for efficiently crystallizing amorphous silicon films is between 440-634 mJ/cm^2 for 300 nm thick films and between 777-993 mJ/cm^2 for 400 nm thick films. The optimized laser energy density is 634, 975 and 1571 mJ/cm^2 for 300, 400 and 500 nm thick films, respectively.

Aluminum induced crystallization of strongly (111) oriented polycrystalline silicon thin film and nucleation analysis

A polycrystalline silicon thin film was fabricated on glass substrate by means of aluminum induced crystallization (AIC). Al and α-Si layers were deposited by magnetron sputtering respectively and annealed at 480°C for 1 h to realize layer exchange. The polycrystalline silicon thin film was continuous and strongly (111) oriented. By analyzing the structure variation of the oxidation membrane and lattice mismatch between γ-Al2O3 and Si, it was concluded that aluminum promoted the formation of (111) oriented silicon nucleus by controlling the orientation of γ-Al2O3, which was formed at the early stage of annealing.

Preparation and properties of polycrystalline silicon seed layers on graphite substrate

Polycrystalline silicon（poly-Si） seed layers were fabricated on graphite substrates by magnetron sputtering. It was found that the substrate temperature in the process of magnetron sputtering had an important effect on the crystalline quality,and 700℃was the critical temperature in the formation of Si（220） preferred orientation. When the substrate temperature is higher than 700℃,the peak intensity of X-ray diffraction（XRD） from Si（220） increases distinctly with the increasing of substrate temperature.Moreover,the XRD measurements indicate that the structural property and crystalline quality of poly-Si seed layers are determined by the rapid thermal annealing （RTA） temperatures and time.Specifically,a higher annealing temperature and a longer annealing time could enhance the Si（220） preferred orientation of poly-Si seed layers.

Modeling of self-heating effects in polycrystalline silicon thin film transistors

An analytical DC model accounting for the self-heating effect of polycrystalline silicon thin-film transistors（poly-Si TFTs） is presented.In deriving the model for the self-heating effect, the temperature dependence of the effective mobility is studied in detail.Based on the mobility model and a first order approximation, a closed-form analytical drain current model considering the self-heating effect is derived.Compared with the available experimental data, the proposed model, which includes the self-heating and kink effects, provides an accurate description of the output characteristics over the linear, the saturation, and the kink regimes.

A Novel Maximum Resolved Shear Stress Gradient Criterion and Its Application for Crack Deflection Behavior in Polycrystalline Silicon Materials

In this paper,a development of new resolved shear stress gradient criteria is performed for the study of crack propagation behavior in polycrystalline materials with an emphasis on the effect of slip plane and slip direction.The prediction of crack deflection behavior by the maximum resolved shear stress gradient criterion shows good agreement with the experimental results.Comparison study for the prediction of crack propagation behavior in poly crystalline materials by other criteria demonstrates that the maximum resolved shear stress gradient criterion is superior to other fracture criteria.This suggests that the new criterion could be further applied for predicting the crack deflection behavior in other polycrystalline materials.

Effect of hydrogen on low temperature epitaxial growth of polycrystalline silicon by hot wire chemical vapor deposition

Polycrystalline silicon （poly-Si） films were prepared by hot-wire chemical vapor deposition （HWCVD） at a low substrate temperature of 525 ℃. The influence of hydrogen on the epitaxial growth of ploy-Si films was investigated. Raman spectra show that the poly-Si films are fully crystallized at 525 ℃ with a different hydrogen dilution ratio （50%-91.7%）. X-ray diffraction, grazing incidence X-ray diffraction and SEM images show that the poly-Si thin films present （100） preferred orientation on （100） c-Si substrate in the high hydrogen dilution condition. The P-type poly-Si film prepared with a hydrogen dilution ratio of 91.7% shows a hall mobility of 8.78 cm2/（V-s） with a carrier concentration of 1.3 × 10^20 cm^-3, which indicates that the epitaxial poly-Si film prepared by HWCVD has the possibility to be used in photovoltaic and TFT devices.

Low Energy H^+ Effects on the Photovoltaic and Optical Properties of Polycrystalline Silicon Solar Cells

Low energy hydrogen ion was used to passivate the electrically active defects existing in grains and grain boundaries of polycrystalline silicon solar cells.Short circuit current of H + implanted cells remarkably increased before and after preparing TiO 2AR(antireflective)coating.The measurements(at λ=6328) of the optical properties of H + implanted silicon samples show that:the value of absorption coefficient reached the level of a Si;refractive index n and reflectivity R significantly decreased;the optical band gap increased from 1.1 eV to 1.3 eV.The results indicate that Si H bonds have been formed after H + implantation.The calculation shows that the optical thickness cycle of TiO 2 AR coating will reduce correspondingly in order to obtain the optimum optical match between AR coating and implanted silicon since refractive index decreases after H + implantation.

Corrective Effect of the Angle of Incidence of the Magnetic Field Intensity on the Performance (Series and Shunt Resistances) of a Bifacial Silicon Solar Cell

This article presents a three-dimensional analysis of the impact of the angle of incidence of the magnetic field intensity on the electrical performance (series resistance, shunt resistance) of a bifacial polycrystalline silicon solar cell. The cell is illuminated simultaneously from both sides. The continuity equation for the excess minority carriers is solved at the emitter and at the depth of the base respectively. The analytical expressions for photocurrent density, photovoltage, series resistance and shunt resistance were deduced. Using these expressions, the values of the series and shunt resistances were extracted for different values of the angle of incidence of the magnetic field intensity. The study shows that as the angle of incidence increases, the slopes of the minority carrier density for the two modes of operation of the solar cell decrease. This is explained by a drop in the accumulation of carriers in the area close to the junction due to the fact that the Lorentz force is unable to drive the carriers towards the lateral surfaces due to the weak action of the magnetic field, which tends to cancel out as the incidence angle increases, and consequently a drop in the open circuit photovoltage. This, in turn, reduces the Lorentz force. These results predict that the p-n junction of the solar cell will not heat up. The study also showed a decrease in series resistance as the incidence angle of the magnetic field intensity increased from 0 rad to π/2 rad and an increase in shunt resistance as the incidence angle increased. His behaviour of the electrical parameters when the angle of incidence of the field from 0 rad to π/2 rad shows that the decreasing magnetic field vector tends to be collinear with the electron trajectory. This allows them to cross the junction and participate in the external current. The best orientation for the Lorentz force is zero, in which case the carriers can move easily towards the junction.

Nickel-disilicide-assisted excimer laser crystallization of amorphous silicon

Polycrystalline silicon （poly-Si） thin film has been prepared by means of nickel-disilicide （NiSi2） assisted excimer laser crystallization （ELC）. The process to prepare a sample includes two steps. One step consists of the formation of NiSi2 precipitates by heat-treating the dehydrogenated amorphous silicon （a-Si） coated with a thin layer of Ni. And the other step consists of the formation of poly-Si grains by means of ELC. According to the test results of scanning electron microscopy （SEM）, another grain growth model named two-interface grain growth has been proposed to contrast with the conventional Ni-metal-induced lateral crystallization （Ni-MILC） model and the ELC model. That is, an additional grain growth interface other than that in conventional ELC is formed, which consists of NiSi2 precipitates and a-Si. The processes for grain growth according to various excimer laser energy densities delivered to the a-Si film have been discussed. It is discovered that grains with needle shape and most of a uniform orientation are formed which grow up with NiSi2 precipitates as seeds. The reason for the formation of such grains which are different from that of Ni-MILC without migration of Ni atoms is not clear. Our model and analysis point out a method to prepare grains with needle shape and mostly of a uniform orientation. If such grains are utilized to make thin-film transistor, its characteristics may be improved.

Surface morphology and impurity distribution of electron beam recrystallized silicon films on low cost substrates for solar cell absorber

A line shaped electron beam recrystallised polycrystalline silicon film on the low cost substrate was investigated for the use of the solar cell absorber. The applied EB energy density strongly influences the surface morphology of the film system. Lower EB energy density results in droplet morphology and the rougher SiO2 capping layer due to the low fluidity. With the energy increasing, the capping layer becomes smooth and continuous and less and small pinholes form in the silicon film. Tungstendisilicide (WSi2) is formed at the interface tungsten/silicon but also at the grain boundaries of the silicon. Because of the fast melting and cooling of the silicon film, the eutectic of silicon and tungstendisilicide mainly forms at the grain boundary of the primary silicon dendrites. The SEM-EDX analysis shows that there are no chlorine and hydrogen in the area surrounding a pinhole after recrystallization because of outgassing during the solidification.

Migration behavior and aggregation mechanism of SiC particles in silicon melt during directional solidification process

SiC inclusions in a multicrystalline silicon ingot have a negative effect on the performance of solar cells.The migration behavior and aggregation mechanism of SiC particles in the silicon melt during the directional solidification process was studied.Results show that SiC particles collide and aggregate in the melt due to the effect of melt flow.Larger aggregation of SiC particles is easily deposited at the bottom of the melt,whereas smaller SiC particles are pushed to the top of melt.Meanwhile,the particles migrate to the edge of melt under the effect of electromagnetic force.Furthermore,the enrichment region of SiC particles can be controlled by adjusting the temperature field distribution of the melt.With an increase of the melt temperature,the SiC particles are enriched at the top of the silicon ingot,indicating that SiC particles can be effectively separated from silicon.

Thermodynamic behaviors of SiCl_2 in silicon deposition by gas phase zinc reduction of silicon tetrachloride

The modified Siemens process,which is the major process of producing polycrystalline silicon through current technologies,is a high temperature,slow,semi-batch process and the product is expensive primarily due to the large energy consumption.Therefore,the zinc reduction process,which can produce solar-grade silicon in a cost effective manner,should be redeveloped for these conditions.The SiCl2 generation ratio,which stands for the degree of the side reactions,can be decomposed to SiCl4 and ZnCl2 in gas phase zinc atmosphere in the exit where the temperature is very low.Therefore,the lower SiCl2 generation ratio is profitable with lower power consumption.Based on the thermodynamic data for the related pure substances,the relations of the SiCl2 generation ratio and pressure,temperature and the feed molar ratio（n（Zn）/n（SiCl4） are investigated and the graphs thereof are plotted.And the diagrams of Kpθ-T at standard atmosphere pressure have been plotted to account for the influence of temperature on the SiCl2 generation ratio.Furthermore,the diagram of Kpθ-T at different pressures have also been plotted to give an interpretation of the influence of pressure on the SiCl2 generation ratio.The results show that SiCl2 generation ratio increases with increasing temperature,and the higher pressure and excess gas phase zinc can restrict SiCl2 generation ratio.Finally,suitable operational conditions in the practical process of polycrystalline silicon manufacture by gas phase zinc reduction of SiCl4 have been established with 1200 K,0.2 MPa and the feed molar ratio（n（Zn） /n（SiCl4）） of 4 at the entrance.Under these conditions,SiCl2 generation ratio is very low,which indicates that the side reactions can be restricted and the energy consumption is reasonable.

Dynamic Ni gettered by PSG from S-MIC poly-Si and its TFTs

A dynamic phosphor-silicate glass （PSG） gettering method is proposed in which the processes of the gettering of Ni by PSC and the crystallizing of α-Si into poly-Si by Ni take place simultaneously. The effects of PSC gettering process on the performances of solution-based metal induced crystallized （S-MIC） poly-Si materials and their thin film transistors （TFTs） are discussed. The crystallization rate is much reduced due to the fact that the Ni as a medium source of crystallization is extracted by the PSC during crystallization at the same time. The boundary between two neighbouring grains in S-MIC poly-Si with PSG looks blurrier than without PSG. Compared with the TFTs made from S-MIC poly-Si without PSC gettering, the TFTs made with PSC gettering has a reduced gate induced leakage current.