Growth Of Polycrystalline Silicon Research Articles

Fabrication of Nanocrystalline Silicon Thin Films Utilized for Optoelectronic Devices Prepared by Thermal Vacuum Evaporation.

Metal-induced crystallization of amorphous silicon is a promising technique for developing high-quality and cheap optoelectronic devices. Many attempts tried to enhance the crystal growth of polycrystalline silicon via aluminum-induced crystallization at different annealing times and temperatures. In this research, thin films of aluminum/silicon (Al/Si) and aluminum/silicon/tin (Al/Si/Sn) layers were fabricated using the thermal evaporation technique with a designed wire tungsten boat. MIC of a:Si was detected at annealing temperature of 500 °C using X-ray diffraction, Raman spectroscopy, and field emission scanning electron microscopy. The crystallinity of the films is enhanced by increasing the annealing time. In the three-layer thin films, MIC occurs because of the existence of both Al and Sn metals forming highly oriented (111) silicon. Nanocrystalline silicon with dimensions ranged from 5 to 300 nm is produced depending on the structure and time duration. Low surface reflection and the variation of the optical energy gap were detected using UV–vis spectroscopy. Higher conductivities of Al/Si/Sn films than Al/Si films were observed because of the presence of both metals. Highly rectifying ideal diode manufactured from Al/Si/Sn on the FTO layer annealed for 24 h indicates that this device has a great opportunity for the optoelectronic device applications.

Microstructure evolution of polycrystalline silicon by molecular dynamics simulation

Polycrystalline silicon is the dominant material in solar cells and plays an important role in photovoltaic industry. It is important for not only the conventional production of silicon ingots but also the direct growth of silicon wafers to control crystallization for obtaining the desired polycrystalline silicon. To the best of our knowledge, few studies have systematically reported about the effects of crystalline planes on the solidification behavior of liquid silicon and the analysis of the microstructural features of the polysilicon structure. In this study, molecular dynamics simulations were employed to investigate the solidification and microstructure evolution of polysilicon, with focus on the effects of the seed distribution and cooling rate on the growth of polycrystalline silicon. The (110), (111), and (112) planes were extruded by the (100) plane and formed the inclusion shape. The crystallization of silicon consisted of diamond-type structures is relatively high at a low cooling rate. The simulations provide substantial information regarding microstructures and serve as guidance for the growth of polycrystalline silicon.

Modeling of poly-crystalline silicon ingot crystallization during casting and theoretical suggestion for ingot quality improvement

Finite element (FE) and cellular automaton (CA) models are used in order to predict the microstructure of Poly-Crystalline Silicon (PC-Si) ingots during casting in a directional solidification furnace. During solidification, the 3D-FE model is used to simulate thermal field inside the furnace, while the nucleation and growth of PC-Si are simulated by the modified version of CA model. In this model both nucleation at silicon-crucible interface and nucleation of equiaxed grains on impurities are modeled. The origin of impurity is considered SiC particles when carbon segregates during the solidification and precipitates as SiC particles if carbon solubility limit is reached. The model is evaluated with experimental data for different parameters, such as: the temperature profile at the top and bottom of the PC-Si, grain size, and the height of solidified silicon during solidification. A one-dimensional form of grain growth is observed at the crucible-silicon interface, while a more two-dimensional form of growth is predicted at higher silicon height. The microstructure of PC-Si shows that the grain size increases as a function of PC-Si ingot height. The effects of the cooling rate and competitive growth on the final microstructure of PC-Si are also investigated. The results show that less nucleation sites are formed at the bottom of the crucible with a slower cooling rate, which leads to a larger grain size. Furthermore, the grains at the crucible side walls tend grow inward at a lower cooling rate, which can significantly change the microstructure of PC-Si. This study shows that the competitive growth phenomena plays an important role in the final microstructure of silicon ingots. A simulated model is proposed which shows that by changing the density and location of nucleation sites it is possible to achieve more effective control on final PC-Si micro structure. This simulation helps to explain the crystal behavior observed in ingots cast under different conditions.

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 °C. 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 °C 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 × 1020 cm−3, which indicates that the epitaxial poly-Si film prepared by HWCVD has the possibility to be used in photovoltaic and TFT devices.

Tempered glass substrate effect on the growth of polycrystalline-silicon and its applications for reliable thin-film transistors

We investigated the unique growth behavior of metal-induced laterally crystallized polycrystalline-silicon under a tempered glass substrate and fabricated a stable thin-film transistor.

Evolution of morphology of surface during film growth of polycrystalline silicon with hemispherical grains

Evolution of surface morphology during film growth of polycrystalline silicon with hemispherical grains (HSG-Si films) is studied using scaling analysis of surface images obtained by atomic-force microscopy. It has been found from the height-height correlation functions that roughness exponent α = 0.92 ± 0.03 and does not depend on the thickness of a film. The dependences of interface width W(t), correlation length ξ(t) and wavelength λ(t) on deposition time, as well as the scaling coefficients β, 1/z, and p, are found.

Growth of poly-crystalline silicon–germanium on silicon by aluminum-induced crystallization

The formation of poly-crystalline silicon–germanium films on single-crystalline silicon substrates by the method of aluminum-induced crystallization was investigated. The aluminum and germanium films were evaporated onto the single-crystalline silicon substrate to form an amorphous-germanium/aluminum/single-crystalline silicon structure that was annealed at 450°C–550°C for 0–3h. The structural properties of the films were examined using x-ray diffraction, Raman spectroscopy and Auger electron spectroscopy. The x-ray diffraction patterns confirmed that the initial transition from an amorphous to a poly-crystalline structure occurs after 20min of aluminum-induced crystallization annealing process at 450°C. The micro-Raman spectral analysis showed that the aluminum-induced crystallization process yields a better poly-crystalline SiGe film when the film is annealed at 450°C for 40min. The growth mechanism of the poly-crystalline silicon–germanium by aluminum-induced crystallization was also studied and is discussed.

Characterization of granular silicon, powders, and agglomerates from a fluidized bed reactor

Growth of polycrystalline silicon from fluidized bed reactors (FBR) produces two general types of silicon products: granular material (diameters on the order of mm) and homogeneously nucleated material often called nanopowder (diameters in the range 10–100 nm). Nanopowder particles tend to be amorphous and have a spherical morphology with an average particle diameter of ~80 nm. Granular material is generally spherical, highly twinned, polycrystalline with crystallite sizes that can reach 200 nm, and includes regions of porosity. The porosity is ~1–4 volume percent, and only the smallest pores exhibit evidence of amorphous regions along the pore surface. The amount of nanopowder that agglomerates on the granular material has been identified using transmission electron microscopy, but agglomeration plays only a minor role in the overall growth process. Therefore, it is proposed that the primary mechanism for granular formation in commercial FBR is chemical vapor deposition, and the pores are associated with nanopowder agglomeration and incomplete sintering.

Modelling the Effects of Material Property and Dimension on the Heating of Silicon with Induction Directional Casting Furnace

Directional Casting of silicon is a cost effective process to grow multi-crystalline Si ingots for wafers of solar cells. An appropriate melting process of polycrystalline silicon is closely related to the material properties and the size of graphite susceptors. These parameters have great influence not only on the melting temperature of silicon melt but also on the efficiency of induction heating, impurity distribution, dendrite and the direction of crystalline grains, which ultimately affect the properties of the solar cells. Therefore, in order to obtain good quality and energy efficiency of growth of polycrystalline silicon, one needs to know how the temperature fields relate to the processing parameters such as different sizes and properties of graphite susceptors in the furnace. In this paper, the influences of different properties such as density, electrical conductivity, thickness of graphite susceptor and cooling base-plate on the temperature of silicon with induction heating have been studied. To have an optimized control of processing parameters, a finite element-based software was used to simulate the temperature distribution of silicon melt in a polycrystalline vacuum induction refining furnace. The simulation takes into account the interaction of the induced eddy current and the heat transfer coupling in the vacuum induction furnace. Some of the modelling results are summarized as follows: 1. The material properties of the graphite susceptor have great influence on the temperature distribution. 2. The higher the operating frequency of the current, the sooner it reaches the melting temperature. 3. Base-plate made of stainless steel 304 performs better than the copper base-plate for the control of temperature distribution. 4. There exists an optimal thickness of the graphite susceptor, and the rise of temperature is not linearly proportional to the thickness of the graphite susceptor.

Hot-wire chemical vapor deposition and characterization of polycrystalline silicon thin films using a two-step growth method

A two-step growth method was proposed to reduce the amorphous incubation layer in the initial growth of polycrystalline silicon (poly-Si) films prepared by hot-wire chemical vapor deposition (HWCVD). In the two-step growth process, a thin seed layer was first grown on the glass substrate under high hydrogen dilution ratios ( φ ≥ 0.9), and then a thick overlayer was subsequently deposited upon the seed layer at a lower φ value. The effect of various deposition parameters on the structural properties of poly-Si films was investigated by Raman spectroscopy and transmission electron microscopy. Moreover, the electrical properties, such as dark and photo conductivities, of poly-Si films were also measured. It was found that the Si incubation layer could be suppressed greatly in the initial growth of poly-Si with the two-step growth method. In the subsequent poly-Si film thickening, a lower φ value of the reactant gases can be applied to enhance the deposition rate. Therefore, a high-quality poly-Si film can be fabricated via a two-step growth method with a sufficient growth rate using HWCVD.

Adaptive three-dimensional phase-field modeling of dendritic crystal growth with high anisotropy

The modeling of dendritic crystal growth of highly anisotropic materials is important in understanding the physics of nature phenomena, such as snow growth, and industrial processes, such as polycrystalline silicon growth. Phase-field modeling has emerged as a powerful tool in such a simulation, but the description of the anisotropic function for three-dimensional (3D) interfacial energy without miss-orientation remains quite difficult. In this paper, a simple method is proposed in our 3D adaptive phase field model. The simulated equilibrium shapes with different anisotropic strengths are first compared with the exact solutions, and they are in good agreement. The dendritic growth of a pure material with different anisotropic strengths for various supercoolings is further simulated, and the morphologies are discussed.

Thermally Stable NiSi Gate Electrode with TiN Barrier Metal for High-Density NAND Flash Memory Devices

As a gate word line, Ni silicide with a TiN barrier metal has been investigated and demonstrated for NAND flash memory devices with a 24 nm technology node. In addition, physical vapor deposition (PVD) and atomic layer deposition (ALD) TiN layers are compared as diffusion barrier metals. Results show that the carbon impurity in the TiN barrier layer may be one of the factors that affect the quality of the barrier layer at 900 °C. It is also found that Ni diffusion and the discontinuity of Ni silicide induced by the grain growth of polycrystalline silicon (poly-Si) are effectively suppressed by the PVD TiN layer of 20 nm inserted into the control gate. As a result, no significant sheet resistance increase is observed even at a narrow gate line of 24 nm width, and its thermal stability is maintained up to 900 °C.

Lateral Growth of Polycrystalline Silicon–Germanium Thin Films Enhanced by Continuous-Wave Laser Crystallization

The lateral crystallization of silicon–germanium (Si–Ge) thin films on glass substrates was carried out by using a continuous-wave laser. The Ge composition x and its distribution in the films were characterized by micro-Raman spectroscopy. Average x values in two films were observed to be 0.38 and 0.06, which were nearly equal to 0.41 and 0.05 obtained by X-ray photoelectron spectroscopy. Cellular structures with ridges were observed in certain parts of both films, many of which oriented along the laser scanning direction. The lengths of especially long cells were >100 µm. Twin and grain boundaries were observed along the top of a ridge and the bottom of a valley, respectively. The mapping of the Raman spectra indicated that Ge segregates toward the grain boundaries. The formation of directed cellular structures can be explained by the constitutional undercooling model that is characteristic of alloys. The addition of Ge to Si thin films was found to be useful for enhancing lateral growth in Si-related thin films on glass.

Growth of polycrystalline silicon on glass for thin-film solar cells

Polycrystalline Si (poly-Si) thin-film solar cells on glass feature the potential to reach high single-junction efficiencies at low costs. However, the preparation is challenging because the process temperatures are limited by the glass to about 600 °C. There are several methods to prepare poly-Si films on glass. So far, the best poly-Si thin-film solar cells on glass have been prepared by solid phase crystallization (SPC) of amorphous Si (a-Si). In this paper, we give an overview on the formation of poly-Si films by both SPC and the aluminum-induced layer exchange (ALILE) process (which is based on aluminum-induced crystallization (AIC) of a-Si). Due to the fact that the utilization of ZnO:Al-coated glass is an attractive option for future poly-Si thin-film solar cells, we discuss the influence of an additional ZnO:Al layer on the formation of poly-Si films. Such an additional ZnO:Al layer leads to enhanced nucleation. Thus, the time necessary to form the poly-Si film (process time) and the resulting grain size of the poly-Si film are reduced.

Modelling of convective processes during the Bridgman growth of poly-silicon

An original 3D model was used to numerically examine convective heat-and-mass transfer processes in the melt during the growth of polycrystalline silicon in vertical Bridgman configuration. The flow in the liquid was modelled using the Navier — Stokes equations in the Boussinesq approximation. The distribution of dissolved impurities was determined by solving the convective diffusion equation. The effects due to non-uniform heating of the lateral wall of the vessel and due to the shape of the crystallization front on the structure of melt flows and on the distribution of dissolved impurities in the liquid are examined.

Superlateral Growth of Silicon by Artificially Designed Spatial Intensity Laser Beam Profile

A location-controlled superlateral grain growth of polycrystalline silicon was achieved by double irradiation with the excimer laser using an artificially designed spatial intensity beam profile with periodic maximum and minimum intensities. With the energy density corresponding to the partial melting regime at the location of minimum intensity, the first irradiation formed the large polycrystalline silicon grains at controlled locations through explosive crystallization and lateral growth. The second irradiation was performed using the same spatial intensity profile and energy density as the first irradiation but was shifted spatially from the first irradiation. Superlateral growth occurred from the unmolten crystalline seeds formed from a single grain at the location of minimum intensity, which had been formed by the first irradiation. The array of polycrystalline grains with a size of one period of minimum intensity was obtained uniformly over the entire irradiated area. The double-irradiation process confirmed that superlateral growth occurred uniformly with a wide range of energy densities from 0.80 to , whereas superlateral growth occurred randomly at a singular energy density of by the single irradiation.

Microstructure and grain growth of polycrystalline silicon grown in fluidized bed reactors

Fluidized bed technology is being implemented commercially to produce polycrystalline silicon that is used as a precursor for silicon ingot growth for the photovoltaic industry. The fluidized bed reactor produces polysilicon in a granular form, usually at lower temperatures than the traditional Siemens process. This current study documents for the first time grain growth and mechanical properties of polysilicon grown via a fluidized bed. In the as-grown state the granules produced by the fluidized bed reactor consist of equiaxed grains that are approximately 30 nm in diameter. Annealing at temperatures above 1000 °C causes significant grain growth to occur resulting in grains up to 300 nm. The hardness of the granular material was 10% less than that of single crystal silicon, which can be attributed to grain boundary sliding. Understanding the effect of annealing on microstructure, grain growth, and mechanical properties of the granules is critical for establishing appropriate techniques for handling the material.

Role of halogen in low-temperature growth of polycrystalline thin films by reactive thermal CVD

Low-temperature growth of polycrystalline silicon by reactive thermal CVD, in which a set of reactive gasses are selected to promote film growth and/or crystallization at low-temperatures in thermal CVD, was studied with disilane and halogens such as fluorine and chlorine. High quality polycrystalline Si films were grown on glass substrates in Si2H6–F2 system at 450°C, while polycrystalline films were hardly deposited when Si2H6–Cl2 system was adapted in the similar condition. We examined the major factors that govern the crystal growth at low-temperature by comparing the experimental results from these two systems. We suggest that the chemical processes on the growing surface in which Si-network is formed while the terminators are eliminated play a significant role for nucleation and growth in the low-temperature deposition by reactive thermal CVD.

A Novel Method for the growth of Low Temperature Silicon Structures for 3-D Flash Memory Devices

AbstractLow temperature (≤400°C) growth of polycrystalline silicon (poly-Si) is carried out using plasma-enhanced chemical vapour deposition. After an initial preparation step poly-Si was grown on the substrates. Optical band gap studies of the poly-Si films have been correlated to hydrogen content of the films as well as to their photoconductivity. Furthermore, the suitability of these films for use as information storage materials for future generation 3-D flash memory devices is investigated using capacitance-voltage (C-V) measurements via metal-insulator-semiconductor device structures. C-V analysis indicates strong charge storage behavior for the poly-Si films.

Excimer Laser Crystallization of Amorphous Silicon Film with Artificially Designed Spatial Intensity Beam Profile

A method of forming polycrystalline silicon film with large grains at a controlled location using a single irradiation of an excimer laser beam is proposed. The excimer laser beam is modified to have a specific spatial intensity profile, periodic spatial variation of intensity maxima and minima , by means of a specially designed mask composed of transparent and opaque patterns. It is noted that the evolution of polycrystalline silicon microstructure is critically dependent on the melt depth of amorphous silicon at , which we categorized as “partial melting,” “near-complete melting,” and “complete melting” regimes. While the lateral grain growth of polycrystalline silicon increases in proportion to the fluence gradient as the energy density is increased, surprisingly we find that it decreases in the complete melting regime due to the “secondary nucleation” occurring at . By judicious selection of optimum intensity modulation and the spacing between and , we have obtained the maximum lateral grain size of at controlled position.