Crystallization Of Silicon Films Research Articles

Crystallographic orientation dependence in creep deformation of micron-thick silicon films for 3-D microstructures

In this study, the steady-state creep deformation properties of 5 μm-thick single crystal silicon (Si) films at elevated temperatures were investigated using punch creep-forming tests for the design of three-dimensional (3-D) microstructured MEMS. The relationship between the creep strain rate and stress of the Si films with {100}, {110}, and {111} planes at temperatures of 1223–1323 K was derived using finite element analysis following punch creep-forming tests, and a power-law creep constitutive equation for Si films was determined. The steady-state creep properties of the Si film samples varied clearly with crystallographic orientations. Among the three crystallographic orientations, the creep strain rate of the micron-thick Si films was the fastest for the {011} plane, followed by the {111} and {001} planes, in the applied stress range of 110 MPa to 220 MPa. This is consistent with the increasing order of magnitude of the thermal activation energy. The crystallographic orientation dependence of the steady-state creep properties of Si films is discussed based on the mobility of dislocation gliding per unit lattice and scanning electron microscope observations. The sample-size dependence of the creep strain rate of Si with the {001} plane was clearly observed between the micron- and millimeter-thick samples, which was attributed to the difference in the frequency factor of the power-law creep. However, the thermal-activation energy remained constant. This study successfully revealed the steady-state creep properties of Si films and provided useful engineering data for the design of 3-D Si-MEMS fabrication by the plastic processing of Si films.

Influence of bending on the structural properties of crystallized silicon films on flexible substrates

Influence of bending on the structural properties of crystallized silicon films on flexible substrates

Investigation on characteristics of millisecond solid-phase crystallized silicon films annealed by atmospheric pressure DC arc discharge micro-thermal-plasma-jet and their application to bottom-gate thin film transistors fabrication

Millisecond solid phase crystallization (SPC) of silicon film induced by micro-thermal-plasma-jet is a potential method for small grain bottom-gate thin-film transistors (TFTs). The characteristics of millisecond SPC films under the conditions of scanning speed (v) from 500 to 900 mm s−1 were investigated. The films show good crystalline quality when formed at a slow v. The crystallinity estimated by micro-Raman spectroscopy is 61% when v was 500 mm s−1. Hall effect measurement results indicate that under v at 500 mm s−1, the resistivity is 1.36 × 10−3 Ωcm, the carrier concentration is 3.41 × 1020 cm−3, and the carrier mobility is 12.2 cm2 Vs−1. Millisecond SPC films surface reduce larger than 76% roughness compared to that of the melted film surface. Bottom-gate TFTs fabricated with millisecond SPC film, which annealed under conditions close to melted condition, show high field effect mobility of 28 cm2 Vs−1 and good uniformity.

Characterization in power and energy of two photovoltaic grid connected systems of different technologies (crystal silicon and thin film), operating in Lima

The Centre for Renewable Energies of the National University of Engineering of Peru (CER-UNI) and the Research and Development Group in Solar Energy (IDEA Group) of the University of Jaen - Spain have elaborated the present work that is based on the installation, instrumentation and monitoring of two Grid Connected Photovoltaic Systems at UNI, one with crystalline PV modules and the other with second generation PV modules of thin film, with amorphous microcrystalline tandem technology. An energy characterization of both systems has been made according to IEC 61724-1: 2017 norm, using two procedures: First, plotting the I-V curves of both PV generators and transferring the measured maximum power data to Standard Conditions, SC, using the methods of Evans Osterwald and Constant Form Factors. Then, the Power at SC of the PV generators and the complete systems was obtained under the protocols of the Solar Energy Institute of the Polytecnic University of Madrid, and validated by the IDEA group of the University of Jaen. Finally, the results obtained were analysed and the most relevant conclusions of the work are reported.

Polycrystalline silicon films obtained by crystallization of amorphous silicon on aluminium based substrates for photovoltaic applications

The fabrication of crystalline silicon thin films on foreign substrates is an attractive and alternative approach to the ingot casting aiming to the reduction of the total costs of photovoltaic cells and modules. The purpose of this work is to describe the CRYSTALSI process which aims at forming polycrystalline silicon films thanks to the thermal crystallization of amorphous silicon layer deposited on aluminium based substrates. The latest are used as a catalyzer for silicon crystallization but also as a back metal contact and reflector for photovoltaic solar cells. Two types of aluminium substrates were applied in these studies: a pure aluminium substrate (99.7% purity) and a silicon rich aluminium substrate containing about 12% of silicon. Silicon thicknesses between 1 and 10μm were deposited and then annealed at temperatures of 490°C, 520°C and 550°C and for duration times from 5min to 12h. The crystallized silicon films were then characterized by Raman spectroscopy, by scanning electron microscopy and by electron backscatter diffraction. The analyses show that the resulting annealed film is composed of two distinct layers: a thin polycrystalline silicon film located just above the substrate and a thicker layer made of a mixture of silicon and aluminium. Contrary to the case of the pure aluminium substrate, the silicon rich aluminium substrate allow to obtain thick and continuous polycrystalline silicon layers due to a controlled diffusion of the silicon within the substrate. As a result, the crystallization at 550°C of 5μm thick amorphous silicon on silicon rich aluminium substrate led to the formation of a thick polycrystalline silicon layer composed of grains of few micrometers in size. A low activation energy of about 2eV is extracted suggesting that the silicon rich aluminium substrate is a catalyzer for the crystallization of amorphous silicon. As for the AIC process, it can be noticed that the limiting step of the CRYSTALSI process is the diffusion of the silicon in the aluminium. A chemical etching using a HNO3, HF, H2O (72.5ml/1.5ml/28ml) solution is found to be appropriate to remove the residual top layer, in order to have access to the polycrystalline silicon layer. This work demonstrates that the CRYSTALSI process can lead to the formation of polysilicon films that can serve as a seed layer for the growth of a thicker absorbing silicon film for photovoltaic applications.

Fully Crystallized Silicon Nanostructured Film Prepared at Low Temperatures by Plasma-Enhanced Chemical Vapor Deposition

I studied the nanocrystalline silicon thin films by means of photoluminescent spectroscopy, Raman spectroscopy and Fourier-transformed infrared spectroscopy technique. The chemical bonding properties was studied by using the Fourier-transformed infrared spectroscopy in the range from500 cm-1 to 2300 cm-1. Spectral peak at wave number of 1100 cm-1 is related to the Si-O-Si bonding configuration. Hydrogenation of film can be estimated by using spectral lines at 2100 cm-1 and 2080 cm-1. The Si-O dipoles which are located into silicon film play great role because of electron affinity for oxygen. Photoluminescent (PL) properties are significant for the films which were made by using hydrogenation of silicon. Fourier-transformed infrared spectra of film’s absorption show the changes in chemicals in the film: from the oxygen incorporation into silicon to the elimination the Si-O bonding by adding the silicon tetra fluoride into electrochemical reactor, and increasing the Si-F density of bonds. The dissociation of SiF4 molecule causes the appearance of fluorine on the film surface, but the hydrogen atoms react with fluorine and excess fluorine is removed. By increase of SiF4 flow rate, the Si-O-Si bridges were etched by HF acid. The size effect in PL from nanocrystalline silicon film can be explained by means of statistical method. We suggested that the ƒ(x) is distribution function of crystallites in their sizes, θ(x) is a quantum efficiency of nanocrystals. Accordingly, the quantity of emitted hydrogenizednanocrystals by band-to-band radiative transitions is important to estimate the PL linear optical response. The processing of grown hydrogeneited silicon nanocrystalline film by annealing and etching the oxides results in preparation of fully crystallized silicon film for manufacturingthe photonic devices with significant quantum yield.

Optical design of nanowire array on silicon thin film solar cell

Light trapping has been considered as an important strategy to increase the conversion efficiency of silicon thin film solar cell. It shows that photonic crystal with feature size comparable to the wavelength, for example, the silicon nanowire array has a great potential to exceed the conventional Yablonovitch 4n2 limit. Silicon nanowire array has been designed and constructed on silicon thin film solar cell due to its excellent optical properties. Generally, silicon nanowire array is used as the antireflection coating, axial or radial p-n junction of solar cell. Different applications of the silicon nanowire arrays need different optical properties. Theoretical investigations show that the optical property is strongly dependent on the structural parameters. In this work, several structural parameters including period (P), diameter (D), height (H), and filling ratio (FR) are optimized when silicon nanowire array plays different roles. Here, by using the finite difference time domain (FDTD) method, we focus on the relations between the structural parameters and the optical properties including reflection and absorption from 300 to 1100 nm. In the FDTD simulation model, the substrate material is crystal silicon film, and the silicon nanowire array is on the surface of the substrate. In this calculation, the top and the bottom of the unit cell are air with perfectly matched layers, and with periodic boundary conditions at the side walls. When the silicon nanowire array is used as the antireflection coating, the silicon nanowire array shows a lowest reflection (7.9%) with H=1.5 m, P=300 nm, and FR=0.282. When silicon nanowire array acts as axial p-n junction solar cell (the p-n junction is formed by substrate and nanowire array), the absorption efficiency reaches a maximum value of 22.3% with H=1.5 m, P=500 nm, and FR=0.55. When the silicon nanowire array acts as the radial p-n junction solar cell, the absorption efficiency could obtain a maximum value of 32.4% with H=6 m, P=300 nm, FR=0.349. In addition, the optical properties of silicon nanowire array with random diameter and position are also analyzed here. The absorption efficiency of optimized random silicon nanowire array reaches 27.8% compared with a value of 19.9% from ordered silicon nanowire array. All of these results presented here can provide a theoretical support for the silicon thin film solar cell to increase the efficiency in the future application.

Understanding Phenomena of Thin Silicon Film Crystallization on Aluminium Substrates

The realization of crystalline silicon thin films on foreign substrates is an attractive alternative to the ingot casting aiming at a reduction in total costs. The purpose of this work is to form polycrystalline silicon films using the crystallization of amorphous silicon deposited on aluminum (Al) substrates. The Al-substrate is used as a catalyzer for silicon crystallization but also as a conductive substrate and as a back reflector for the photovoltaic cell. The crystallization of 1-5μm thick amorphous silicon films were carried out at a temperature of 550°C and for duration times from 10 to 80min. The crystallized silicon films were then characterized by Raman spectroscopy, scanning electron microscopy and by electron backscatter diffraction. The analysis show that the annealed layer is composed of two distinct layers: a thin polysilicon film located just above the Al substrate and on the top a thicker layer made of a mix of silicon and aluminum. The thickness of the polysilicon film is found to increase with the annealing time. The crystallization of 5μm thick amorphous silicon during 80min resulted in a 1μm thick polysilicon layer composed of grains of few micrometers in size. The mechanisms of accelerated crystallization are discussed. Such polysilicon films can be used as a seed layer for the growth of a thicker absorbing silicon film for photovoltaic applications.

Lifetime analysis of laser crystallized silicon films on glass

Only recently, the quality of liquid phase crystallized silicon directly on glass substrates made a huge leap towards the quality of multi-crystalline wafers with open circuit voltages well above 600 mV. In this paper, we investigate the material quality in order to identify the factors limiting further performance improvements. We employ photoluminescence imaging on a state of the art test structure with lifetime calibration by transient photoluminescence. The resulting lifetime map is converted into an effective diffusion length map and the origin of regions with short lifetimes is investigated with electron backscattering and transmission electron microscopy. High local dislocation densities in areas with dissociated coincidence site lattice boundaries were found to be responsible for the localised quenching of the photoluminescence signal.

Effects of the single and double (overlap) scanned excimer laser annealing on solid phase crystallized silicon films

Thin amorphous silicon (a-Si) films were crystallized into polycrystalline silicon (poly-Si) by combining solid phase crystallization (SPC) and subsequent excimer laser annealing (ELA). Then thin film transistors (TFTs) were fabricated by using the poly-Si formed in the single and double excimer laser scanned area. The device performance of the TFTs fabricated with the excimer laser energy density of 230mJ/cm2 is almost equal for the single and double scanned area. This observation indicates that the overlapping laser irradiation with the laser energy density below 230mJ/cm2 does not change the characteristics of TFTs. Based on this result, we discuss the correlation between performance of active matrix organic light emitting display (AMOLED) panels and excimer laser energy density during ELA for SPC treated and non-treated poly-Si films.

Impact of dislocations and dangling bond defects on the electrical performance of crystalline silicon thin films

A wide variety of liquid and solid phase crystallized silicon films are investigated in order to determine the performance limiting defect types in crystalline silicon thin-film solar cells. Complementary characterization methods, such as electron spin resonance, photoluminescence, and electron microscopy, yield the densities of dangling bond defects and dislocations which are correlated with the electronic material quality in terms of solar cell open circuit voltage. The results indicate that the strongly differing performance of small-grained solid and large-grain liquid phase crystallized silicon can be explained by intra-grain defects like dislocations rather than grain boundary dangling bonds. A numerical model is developed containing both defect types, dislocations and dangling bonds, describing the experimental results.

Investigation of silicon grain structure and electrical characteristics of TFTs fabricated using different crystallized silicon films by atmospheric pressure micro-thermal-plasma-jet irradiation

Amorphous silicon (a-Si) films were crystallized using three grain growth modes induced by micro-thermal-plasma-jet (µ-TPJ) irradiation and applied to the channel regions of thin-film transistors (TFTs). Solid phase crystallization (SPC) formed microcrystalline grains and showed a lower crystallinity of 70%, whereas leading wave crystallization (LWC) and high-speed lateral crystallization (HSLC) formed significantly larger grains than the TFT channel region. The SPC-TFT showed a lower field-effect mobility (μFE) due to the small grain size and the existence of many grain boundaries, whereas LWC- and HSLC-TFT channels were formed by only single grains and showed a μFE higher than 300 cm2 V−1 s−1 in the n-channel. The defect density of HSLC was smaller than that of LWC; consequently, the HSLC-TFT performed better than the LWC-TFT. The maximum μFE values of n- and p-channel HSLC-TFTs were 418 and 224 cm2 V−1 s−1, respectively.

Crystallization induced by thermal annealing with millisecond pulses in silicon-on-insulator films implanted with high doses of hydrogen ions

The crystallization of silicon-on-insulator films, implanted with high doses of hydrogen ions, upon annealing with millisecond pulses is studied. Immediately after hydrogen-ion implantation, the formation of a three-phase structure composed of silicon nanocrystals, amorphous silicon, and hydrogen bubbles is detected. It is shown that the nanocrystalline structure of the films is retained upon pulsed annealing at temperatures of up to ∼1000°C. As the temperature of the millisecond annealing is increased, the nanocrystal dimensions increase from 2 to 5 nm and the fraction of the nanocrystalline phase increases to ∼70%. From an analysis of the activation energy of crystal phase growth, it is inferred that the process of the crystallization of silicon films with a high (∼50 at %) hydrogen content is limited by atomic-hydrogen diffusion.

Ionic nucleated crystallized silicon thin-film transistor fabricated at 130 °C

An ionic nucleation was proposed to deposit crystallized silicon film fast at 130°C. Initially, ionic argon was introduced into the deposition of amorphous silicon film for a short time. The energetic argon particles bombarded the grown surface slightly and reduced the nucleation. Then the argon flow was turned off and the film deposition remained proceeding. The low temperature deposited film was crystallized well. When this processed silicon film acted as a channel of bottom-gate thin-film transistor, it possessed current drivability superior to that of thin-film transistor using hydrogen diluted crystallized silicon channel.

Thickening of thin laser crystallized silicon films by solid phase epitaxy for photovoltaic applications

Multicrystalline silicon films up to 2μm thick with grain sizes up to 100μm were prepared on glass substrates by laser crystallization followed by solid phase epitaxy of electron beam deposited amorphous silicon (a-Si) at 600°C. The dependence of the epitaxial growth rate on the crystallographic orientation was investigated. While grains with <100> orientation with respect to the surface normal show the highest growth rate, <111>-grains tend to grow the slowest. Furthermore, we studied the kinetics of the solid phase growth depending on the deposition conditions of a-Si. For this purpose we implemented a simple measurement system that determines the transmittance of the c-Si/a-Si layer stack during furnace annealing at a wavelength of 808nm. Fastest growth is obtained for a-Si deposited at highest rates at a temperature of 300°C. Further increase of the deposition temperature prevents epitaxy. Interface cleaning deserves particular care since contaminations at the interface lead to a retardation time for solid phase epitaxy.

Thermoelectric effect of silicon films prepared by aluminum-induced crystallization

Aluminum-induced crystallized silicon films were prepared on glass substrates by magnetron sputtering. Aluminum was added in the silicon films intermittently by the regular pulse sputtering of an aluminum target. The amount of aluminum in the silicon films can be controlled by regulating the aluminum sputtering power and the sputtering time of the undoped silicon layer; thus, the Seebeck coefficient and electrical resistivity of the polycrystalline silicon films can be adjusted. It is found that, when the sputtering power ratio of aluminum to silicon is 16%, both the Seebeck coefficient and the electrical resistivity decrease with the increasing amount of aluminum as expected; the Seebeck coefficient and the electrical resistivity at room temperature are 0.185–0.285 mV/K and 0.30–2.4 ω·cm, respectively. By reducing the sputtering power ratio to 7%, however, the Seebeck coefficient does not change much, though the electrical resistivity still decreases with the amount of aluminum increasing; the Seebeck coefficient and electrical resistivity at room temperature are 0.219–0.263 mV/K and 0.26–0.80 ω·cm, respectively.

Fast Deposition of Low-Temperature-Crystallized Silicon Films by Hybrid Nucleation

A new method involving hybrid nucleation and ionic-argon-assisted dissociation was proposed to achieve the high-speed deposition of crystallized silicon at 150 °C. At the initial stage of deposition, the nucleation process was reduced by in situ physical and chemical etchings. In a mixture of various plasmas, ionic argon removed some nuclei. Meanwhile, atomic hydrogen passivated dangling bonds and improved the crystallinity of the processed film. When a seed layer formed, the presence of ionic argon accelerated the dissociation of the source gases. Thus, a high deposition rate of crystallized silicon films was achieved. The deposition rate was as high as 15.6 nm/min. The optimum crystallized film in this study had a crystalline fraction of 72%. In addition, a thin-film transistor adopted the proposed crystallized film as a channel showing robust reliability to electrical stressing.

Biaxially-textured photovoltaic film crystal silicon on ion beam assisted deposition CaF2 seed layers on glass

We grow biaxially textured heteroepitaxial crystal silicon (c-Si) films on display glass as a low-cost photovoltaic material. We first fabricate textured CaF2 seed layers using ion-beam assisted deposition, then coat the CaF2 with a thin, evaporated epitaxial Ge buffer and finally deposit heteroepitaxial silicon on the Ge. The silicon is grown by hot-wire chemical vapor deposition, a high-rate, scalable epitaxy technology. Electron and X-ray diffraction confirm the biaxial texture of the CaF2 and epitaxial growth of the subsequent layers. Transmission electron microscopy reveals columnar silicon grains about 500 nm across. We fabricate a proof-of-concept epitaxial film c-Si solar cell with an open circuit voltage of 375 mV that is limited by minority carrier lifetime.

Pyramidal light trapping and hydrogen passivation for high-efficiency heteroepitaxial (100) crystal silicon solar cells

We report growth and characterization of heteroepitaxial silicon solar cells on sapphire to demonstrate the promise of heteroepitaxial crystal silicon (c-Si) film photovoltaics on inexpensive substrates coated with chemically inert crystalline buffer layers such as Al2O3. Our work isolates and addresses critical material and light-trapping issues that must be solved to develop film c-Si solar cells. Microscopy reveals high dislocation densities and other crystalline defects in the silicon layers, and these defects limit the unhydrogenated devices with a 1.5 μm absorber layer to below 1% sunlight-to-electricity conversion efficiency. By exposing an identical device to atomic H from a remote plasma, we demonstrate a 5.2% efficient device with dramatically improved quantum efficiency (QE) and open circuit voltage, as the minority carrier diffusion length increases from ∼1 μm to ∼4.5 μm. When we incorporate both hydrogen passivation and top surface pyramidal light trapping we further improve the QE and achieve 6.8% efficiency.

Crystallization of silicon films of submicron thickness by blue-multi-laser-diode annealing

Blue-Multi-Laser-Diode Annealing (BLDA) was performed in the continuous wave (CW) mode on Si films as thick as 0.5 μm and 1 μm deposited by rf sputtering. As a result of controlling the laser power from 4.0 to 4.8 W, a whole Si layer of 0.5 μm in thickness was completely crystallized and consisted of a columnar structure of fine grains beneath a partially melted Si surface owing to the high temperature gradient along the depth in the Si layer. After additional hydrogenation in a furnace ambient, the ratio of the photo/dark current under AM1.5 illumination distinctly improved to 6 times higher than that of as-deposited condition. The BLDA is expected to be applied to thin-film solar cells and/or to thin film transistor (TFT) photo-sensor systems on panels as a new low-temperature poly-silicon (LTPS) fabrication technique.