Polycrystalline Si Films Research Articles

Defect elimination in solid-phase crystallised Si thin films by line-focus diode laser annealing

A high density of intragrain defects in solid-phase crystallised Si thin films results in poor electronic properties and impedes their use for thin-film solar cell or thin-film transistor applications. This paper demonstrates that a high-power line-focus diode laser can eliminate intragrain defects (microtwins and dislocations) in polycrystalline Si films while maintaining the smooth defect-free surface. Improved electronic properties of ultra-thin polycrystalline Si thin films are thus achieved. To alleviate crack formation during diode laser annealing, a rapid-thermal pre-treatment at 800°C for 60s is introduced since it effectively relieves the tensile stress in the films and thus generates a more stable precursor material for subsequent laser annealing. The film thickness plays an important role in diode laser annealing. The films thinner than 100nm show relatively smaller improvement due to the limited absorption of 808nm laser radiation.

Fabrication of a high-performance poly-Si thin-film transistor using a poly-Si film prepared by silicide-enhanced rapid thermal annealing process

A 50-nm thick polycrystalline Si film was fabricated by the crystallization of anamorphous Si film using silicide-enhanced rapid thermal annealing (SERTA). The amorphous Si film was deposited on a 5-nm thick polycrystalline Si seed layer containing nickel silicide precipitates in grain boundary areas. With the help of the silicide precipitates, the RTA temperature decreased from 730 to 680°C and the grain size of the crystallized polycrystalline Si film increased to 1.4 — 2.2 μm. Few defects were found within the grains and the Ni concentration in the polycrystalline film decreased to 1 × 1018 cm−3 due to the very-thin seed layer that contained nickel silicide precipitates. As a result, the field-effect hole mobility in the p-channel poly-Si thin film transistors (TFTs), fabricated employing the polycrystalline Si film, was as high as 169 cm2/V∙s at a drain voltage of V D = −0.1 V; the subthreshold swing was as small as 0.24 V/decade. The minimum leakage current at V D = 5V was 1.5 × 10−10 A with very good diode characteristics.

The control of the film stress of Cat-CVD a-Si films and its impact on explosive crystallization induced by flash lamp annealing

Catalytic chemical vapor deposition (Cat-CVD) can produce amorphous silicon (a-Si) films with low film stress, in general, compared to plasma-enhanced CVD, and is thus suited for the preparation of precursor a-Si films for thick poly-Si films applied for solar cells. The stress of a-Si films is known to sometimes play an important role for the crystallization of a-Si films and resulting grain size of polycrystalline Si (poly-Si) films formed. I investigate the impact of the stress of Cat-CVD a-Si films on the mechanism of explosive crystallization (EC) induced by flash lamp annealing (FLA). The stress of Cat-CVD a-Si films can be controlled by changing the temperatures of substrates and/or a catalyzing wire during film deposition. Cat-CVD a-Si films with tensile stress (~200MPa) can be deposited as well as films with compressive stress. The enlargement of grain size is observed in a part of flash-lamp-crystallized (FLC) poly-Si films formed from Cat-CVD films with tensile stress compared to those with compressive stress, which might be an indication of a certain degree of impact of film stress on poly-Si formation. The grain size is, however, much smaller than that of FLC poly-Si films formed from electron-beam- (EB-) evaporated a-Si films with similar tensile stress. This fact may indicate the existence of other critical determinants of EC mechanism.

Two-Step Annealing for Solution-Based Metal Induced Crystallization of Amorphous Silicon Films

Solution-based nickel (Ni)-induced crystallization (S-MIC) of amorphous silicon (a-Si) films has been studied with a two-step annealing process. We especially introduced Ethyl cellulose (EC) into the Ni-salt solution, so the viscous Ni-salt solution can be uniformly spin-coated on the a-Si film prepared by plasma enhanced chemical vapor deposition (PECVD). The annealing temperature can be first set from room temperature (RT) to 400°C and kept at 400°C for 2 h in nitrogen ambience. And then, it is increased from 500°C to 550°C and kept for several hours in the following annealing. The correlations among crystallization, the concentrations of Ni-salt solution and annealing conditions can be discussed. The experimental results show that with the help of the two-step annealing, the a-Si films can be crystallized at a low temperature of 500°C. The crystalline fraction gets up to 81.2% after annealing at 520°C for 2 h and the grain size of the polycrystalline Si film is approximately 0.2 μm. Energy dispersive spectroscopy (EDS) analysis shows that very little Ni metal atoms reside in the crystallized Si film for S-MIC.

Suppressing light reflection from polycrystalline silicon thin films through surface texturing and silver nanostructures

This work demonstrates a novel method combining ion implantation and silver nanostructures for suppressing light reflection from polycrystalline silicon thin films. Samples were implanted with 20-keV hydrogen ions to a dose of 1017/cm2, and some of them received an additional argon ion implant to a dose of 5 × 1015 /cm2 at an energy between 30 and 300 keV. Compared to the case with a single H implant, the processing involved both H and Ar implants and post-implantation annealing has created a much higher degree of surface texturing, leading to a more dramatic reduction of light reflection from polycrystalline Si films over a broadband range between 300 and 1200 nm, e.g., optical reflection from the air/Si interface in the AM1.5 sunlight condition decreasing from ∼30% with an untextured surface to below 5% for a highly textured surface after post-implantation annealing at 1000 °C. Formation of Ag nanostructures on these ion beam processed surfaces further reduces light reflection, and surface texturing is expected to have the benefit of diminishing light absorption losses within large-size (>100 nm) Ag nanoparticles, yielding an increased light trapping efficiency within Si as opposed to the case with Ag nanostructures on a smooth surface. A discussion of the effects of surface textures and Ag nanoparticles on light trapping within Si thin films is also presented with the aid of computer simulations.

Fabrication of Polycrystalline Si Films by Silicide-Enhanced Rapid Thermal Annealing and Their Application to Thin Film Transistors

Amorphous (a-Si) films were epitaxially crystallized on a very thin large-grained poly-Si seed layer by a silicide- enhanced rapid thermal annealing (SERTA) process. The poly-Si seed layer contained a small amount of nickel silicide which can enhance crystallization of the upper layer of the a-Si film at lower temperature. A 5-nm thick poly-Si seed layer was then prepared by the crystallization of an a-Si film using the vapor-induced crystallization process in a NiCl2 environment. After removing surface oxide on the seed layer, a 45-nm thick a-Si film was deposited on the poly-Si seed layer by hot-wire chemical vapor deposition at 200 o C. The epitaxial crystallization of the top a-Si layer was performed by the rapid thermal annealing (RTA) process at 730 o C for 5 min in Ar as an ambient atmosphere. Considering the needle-like grains as well as the crystallization temperature of the top layer as produced by the SERTA process, it was thought that the top a-Si layer was epitaxially crystallized with the help of NiSi2 precipitates that originated from the poly-Si seed layer. The crystallinity of the SERTA processed poly-Si thin films was better than the other crystallization process, due to the high-temperature RTA process. The Ni concentration in the poly-Si film fabricated by the SERTA process was reduced to 1 × 10 18 cm �3 . The maximum field- effect mobility and substrate swing of the p-channel poly-Si thin-film transistors (TFTs) using the poly-Si film prepared by the SERTA process were 85 cm 2 /V·s and 1.23 V/decade at Vds = �3 V, respectively. The off current was little increased under reverse bias from 1.0 × 10 �11 A. Our results showed that the SERTA process is a promising technology for high quality poly- Si film, which enables the fabrication of high mobility TFTs. In addition, it is expected that poly-Si TFTs with low leakage current can be fabricated with more precise experiments.

Controlled aluminum-induced crystallization of an amorphous silicon thin film by using an oxide-layer diffusion barrier

Aluminum-induced crystallization (AIC) of amorphous silicon with an Al2O3 diffusion barrier was investigated for controlling Si crystallization and preventing layer exchange during the annealing process. An Al2O3 layer was deposited between the a-Si and the Al films (a-Si/Al2O3/Al/Glass) and was blasted with an air spray gun with alumina beads to form diffusion channels between the Si and the Al layers. During the annealing process, small grain SixAl seeds were formed at the channels. Then, the Al2O3 diffusion barrier was restructured to close the channels and prevent further diffusion of Al atoms into the a-Si layer. A polycrystalline Si film with (111), (220) and (311) crystallization peaks in the X-ray diffraction pattern was formed by annealing at 560 °C in a conventional furnace. That film showed a p-type semiconducting behavior with good crystallinity and a large grain size of up to 14.8 µm. No layer conversion occurred between the Si and the Al layers, which had been the fundamental obstacle to the applications in the crystallization of a-Si films by using the AIC method.

Fabrication and electrical properties of polycrystalline Si films on glass substrates

Al-induced a-Si crystallization process has been used to prepare polycrystalline Si (pc-Si) thin films on glass substrates. It has been found that the glass/Al/Al2O3/a-Si multilayer could be transformed into the structure of glass/Si/Al2O3/Al via a thermal treatment at 500°C for 5h. The Si layer in the glass/Si/Al2O3/Al system is in the polycrystalline state and exhibits a high crystallographic quality, a dense and continuous surface morphology, an average grain size of ∼18μm, a ∼2.6×1019cm−3 hole concentration and a ∼24.2cm2/Vs hole mobility. The crystallographic quality and electrical performance of the pc-Si film can be further improved by increasing crystallization time and temperature. The obtained pc-Si material may be a suitable candidate for the solar cells.

Identification of intra‐grain and grain boundary defects in polycrystalline Si thin films by electron paramagnetic resonance

AbstractWe investigate the characteristics of intra‐grain and grain boundary defects in polycrystalline Si films, by employing quantitative electron paramagnetic resonance measurements on liquid phase crystallized layers with an average grain size of 200 µm and tailored solid phase crystallized Si layers with similar intra‐grain morphology but systematically varied grain sizes between 0.25 µm and 1 µm. The defect characteristics are found to be composed of two distinctive g ‐values of g = 2.0055 and 2.0032, which are attributed to grain boundary defects and intra‐grain defects, respectively. Additional hydrogenation leads to a reduction of the overall defect concentration, while a rapid thermal annealing process primarily heals intra‐grain defects.magnified image(© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Large-grained (111)-oriented Si/Al/SiO2 structures formed by diffusion-controlled Al-induced layer exchange

We investigated inverted Al-induced crystallization (AIC) technique of amorphous Si films (thickness: 50–100nm) for the formation of polycrystalline Si films on Al coated glass substrates at low-temperature (<500°C). A SiO2 interlayer was inserted in between Al and Si layers in order to control the Al-Si diffusion rate. As a result, the crystal orientation of the AIC-Si layer strongly depends on the thickness of the SiO2 interlayer: thin (1nm) interlayer provided (100) orientation while thick (10nm) interlayer provided (111) orientation. Meanwhile, the thicker the SiO2 interlayer, the larger the grain size of the AIC-Si layer. In particular, for a sample with 10-nm-thick SiO2 interlayer, the (111) orientation fraction reached 99% and the average grain size over 50-μm diameters. This AIC-Si layer holds promise as epitaxial templates for light-absorption layers of thin-film solar cells, as well as for functional silicide materials.

Orientation Control of Large-Grained Si Films on Insulators by Thickness-Modulated Al-Induced Crystallization

The thickness-modulated Al-induced crystallization technique enables us to control the orientation of polycrystalline Si films on insulators. The amorphous Si film on a SiO2 substrate was crystallized at a low temperature (425 °C) through the layer exchange between Si and catalytic Al layers. Electron backscattering diffraction (EBSD) measurements revealed that the crystal orientation of the grown Si layer varied significantly depending on the Al/Si thickness: The [111]-orientation fraction reached 97% for the 50-nm-thick sample and the [100]-orientation fraction 88% for the 200-nm-thick sample. This mechanism was discussed in terms of the heterogeneous nucleation energy. The 50-nm-thick sample provided the maximum Si grain-size as large as 68-μm diameters. These large-grained Si films with controlled-orientations are promising epitaxial templates for advanced Si-based thin-film solar cells.

Solution-Based Metal Induced Crystallization of Amorphous Silicon Films

A viscous Nickel (Ni) solution was applied on amorphous Si films by spin coating and its effect on the crystallization of amorphous Si films was investigated with a two-step annealing process. The experimental results show that with the help of the two-step annealing, the crystallization of the film can take place at 500oC. At the same time, the crystalline fraction gets up to 79.4% after annealing at a high temperature of 520oC and the grain size of the polycrystalline Si films is approximately 200 nm.

Epitaxial growth of heavily boron-doped Si by Al(B)-induced crystallisation at low temperature for back surface field manufacturing

P-type polycrystalline Si film on a foreign substrate can be fabricated at temperatures lower than 773 K by an aluminium-induced crystallisation process. However, the ultimate carrier concentration of the Si film is limited to approximately 3 × 1018 cm−3 because of the low solid solubility of Al in Si at temperatures below 773 K. In this study, a process called B-AIC is developed in which boron is co-doped with Al to increase the carrier concentration in Si films to ∼1019 cm−3 at temperatures as low as 673 K. The carrier concentration can be tuned by the initial thickness of a-Si layer in the B-AIC process. Beside the fabrication of polycrystalline Si film on glass, the epitaxial growth of this heavily doped p++-Si film can also be realized on a single crystalline Si wafer via a solid phase epitaxy mechanism.

Polycrystalline silicon films prepared by metal-induced crystallisation using pre- and post-deposited aluminium on amorphous silicon

We report on the successful fabrication of polycrystalline silicon films by aluminium-induced crystallisation (AIC) of Radio frequency (rf) plasma-enhanced chemical vapour deposited (PECVD) a-Si films. The effects of annealing at different temperatures (300 and 400°C), below the eutectic temperature of the Si–Al binary system, on the crystallisation process have been studied. This work emphasises the important role of the position of the Al layer with respect to the Si layer on the crystallisation process. The properties of the crystallised films were characterised using X-ray diffraction, Raman spectroscopy, ellipsometry, field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). With an increase in the annealing temperature, it was found that the degree of crystallisation of annealed a-Si/Al and Al/a-Si films increased. The results showed that the arrangement where the Al was on top of the a-Si had a more prominent effect on crystallisation enhancement than when Al was below the a-Si. The interfacial layer between the Al and a-Si film is crucial because it influences the layer-exchange process during annealing. The oxide layer formed between the Al and the a-Si layers greatly retards the crystallisation process in the case of the Al/Si arrangement. Our investigations suggest that polycrystalline Si films formed by AIC can be used as a seed layer in solar cell fabrication.

Improving Crystallinity of Thin Si Film for Low-Energy-Loss Micro-/Nano-Electromechanical Systems Devices by Metal-Induced Lateral Crystallization Using Biomineralized Ni Nanoparticles

The characteristics of thin Si films were investigated in terms of crystallization for low-energy-loss micro-/nano-electromechanical systems (MEMS/NEMS) devices. Metal-induced lateral crystallization (MILC) using Ni nanoparticles accommodated within cage-shaped protein, apoferritin, was applied to an amorphous Si film to obtain a polycrystalline Si (poly-Si) film. The poly-Si film with MILC had crystallized domains of 50–60 µm, whereas the poly-Si film without MILC had grains smaller than 1 µm. Crystallized domains in the poly-Si film with MILC showed almost the same crystalline orientations, whereas those without MILC showed random crystalline orientations. Crystallization-induced tensile stress in the poly-Si film with MILC was increased to 461 MPa (without MILC: 363 MPa). The poly-Si film with MILC was applied to an electrostatically driven MEMS resonator. In the frequency responses, resonant frequency was shifted higher and the Q factor was increased by 20%.

Large-Grain Polycrystalline Silicon Films Formed through Flash-Lamp-Induced Explosive Crystallization

The flash lamp annealing (FLA) of electron-beam- (EB-) evaporated amorphous silicon (a-Si) films results in the formation of polycrystalline Si (poly-Si) films with at least a few µm long grains stretching along lateral crystallization directions. Unlike the case of using chemical-vapor-deposited (CVD) hydrogenated a-Si films as precursors, no peeling of Si films occurs even in the absence of Cr adhesion layers. Such a flash-lamp-induced crystallization occurs also in doped EB-evaporated a-Si films as in the case of undoped films. The p+/p-/n+ stacked structure is sufficiently kept even after crystallization, although the profiles of dopants are slightly modified. This fact clearly indicates that the crystallization observed is not based on liquid-phase epitaxy (LPE) after the complete melting of the whole a-Si precursor during millisecond-order treatment but through LPE-based explosive crystallization (EC), self-catalytic lateral crystallization driven by the release of latent heat. The formation of poly-Si films with large grains and the sufficient preservation of dopant profiles would lead to the utilization of the poly-Si films formed for solar cell devices.

Solid Phase Growth of Nickel Silicides in Polycrystalline Si Film on SiO2 with Cl Plasma Containing NiCl

not Available.

Control of the azimuthal orientation of grains in polycrystalline Si films

Multi-directional ion beam irradiation is applied to control the crystal orientation of Si thin films during growth via plasma-enhanced chemical vapor deposition. The primary ion beam is generated by electron cyclotron resonance using an SiH4/H2 mixture, and the auxiliary beam is obtained by redirecting a portion of the primary beam with a metal reflector. These two beams simultaneously hit the surface of the growing film from different directions. With this arrangement, we succeeded in controlling the orientation of the growth of deposited Si grains in virtually the same direction.

Effects of annealing temperature on crystallisation kinetics, film properties and cell performance of silicon thin-film solar cells on glass

Solid-phase crystallisation (SPC) of Si films on glass prepared by three deposition methods, plasma enhanced chemical vapour deposition (PECVD), electron-beam evaporation, and a combination of both, are compared for different annealing temperatures. Three independent techniques, optical transmission microscopy, UV reflectance spectroscopy, and X-ray diffraction, are used to characterise the crystallisation kinetics and film properties. The activation energy for the incubation is estimated to be 2.7–3.0eV. The scanning electron microscopy images of polycrystalline Si films after Secco etching show a gradually decreasing average grain size in each film type for higher SPC temperatures. The crystal quality of all film types degrades at higher crystallisation temperatures. Solar cells fabricated from these polycrystalline Si films were characterised by Suns-Voc and spectral response measurements. According to both the resulting open-circuit voltage and the short-circuit current, the electronic quality of all polycrystalline Si film types and the corresponding cell performance degrade for higher crystallisation temperatures but to a different extent depending on the film deposition method.

Impact of Rapid Crystallization of Si Using Nickel-Metal-Induced Lateral Crystallization on Thin-Film Transistor Characteristics

Metal-induced lateral crystallization (MILC) of amorphous Si using a nickel disilicide catalyst at temperatures up to 770 °C is investigated to produce high-quality polycrystalline Si films in a short period, while 670 °C is the maximum temperature allowed for processing using non alkaline glass substrates. Investigation of crystallization kinetics by isothermal annealing experiments provides activation energy values of 2.3 and 3.6 eV for MILC growth and spontaneous nucleation of amorphous Si, respectively. These values indicate that a MILC region of about 30 µm, which is large enough to place transistor circuits, can be grown by annealing for 15 min at 670 °C, which almost agrees with experimental results. N-channel thin-film transistors are fabricated on MILC films. An average carrier mobility of about 200 cm2·V-1·s-1 is obtained from the MILC film crystallized at 670 °C.