Microcrystalline Si Films Research Articles

Structural analysis of polycrystalline silicon thin films produced by two different ICPCVD approaches

The structural properties of polycrystalline Si (poly-Si) thin films formed by using ICPCVD and LIA-ICPCVD methods under different H dilution ratios were investigated. Columnar-structured poly-Si films with 82% crystalline volume fraction were obtained by ICPCVD at a lower H dilution ratio of 50%. In the case of LIA-ICPCVD, to obtain nearly the same crystalline volume fraction (80%), a higher H dilution ratio (98%) was required and the poly-Si films obtained were mostly of cone-shaped columnar structure. The reverse effect is due to the difference in the antenna design in these systems, which changes the plasma characteristics. Multilayer deposition process, and subsequent H treatment and annealing were used in ICPCVD for producing highly crystallized poly-Si thin films from microcrystalline Si films. In the case of LIA-ICPCVD, annealing the ITO coated glass substrate in the deposition chamber prior to Si:H film deposition together with the optimization of process pressure yielded highly crystalline poly-Si film.

Effect of Temperature on the Microstructure and Electrical Conductivity of Microcrystalline Si Films

Micro-Si films were deposited using Ar diluted SiH4 gaseous mixture by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD). The effects of the substrate temperature on microstructure and electrical conductivity of micro-Si film were investigated. The results show that, with the increasing of substrate temperature, crystallinity and grain size increased monotonously, of which a competing balance would determine the electrical conductivity of micro-Si films. Based on these results, relatively small grain size and appropriate crystallinity would be beneficial to improve the electrical properties of micro-Si films.

Estimation of local built-in potential of amorphous silicon thin-film solar cells by Kelvin force microscopy

The local surface potential of pin-type hydrogenated amorphous silicon (a-Si:H) thin-film solar cells has been evaluated by Kelvin force microscopy (KFM). We have also estimated the local built-in potential of the solar cells by KFM. In the surface morphology image of the solar cells, large convex grains related to the textured structure of the substrate were found. The surface potential distribution related to the surface morphology was observed in the solar cells. A similar surface potential distribution was also found in an n-type hydrogenated microcrystalline Si (µc-Si:H) film. The surface potential of the solar cells was not the same as that of the n-type film. The difference in average surface potential between the n-type hydrogenated microcrystalline Si (µc-Si:H) film and the solar cells increased with increasing built-in potential. The difference in local surface potential on large convex grains was smaller than that in the region between the large convex grains.

Characterization of Si and SiOxfilms deposited in very high-frequency excited atmospheric-pressure plasma and their application to bottom-gate thin film transistors

Silicon (Si) and its alloys, such as silicon oxides (SiOx) and silicon nitrides, are indispensable thin film materials for the fabrication of large-area electronic devices. The goal of the present study is to develop a highly efficient deposition technology for good-quality hydrogenated amorphous Si (a-Si), microcrystalline Si (µc-Si), and SiOx films on flexible polymer substrates. For this purpose, we have been using atmospheric-pressure (AP) plasma excited by a 150 MHz very high-frequency (VHF) power. The changes of thickness and structure of the Si and SiOx films in the gas flow direction were studied as a function of distance from the plasma entrance, VHF power density, and gas flow rates. Based on the results, the performance of bottom-gate TFTs with the Si channel and SiOx gate insulator layers being deposited using AP-VHF plasma were examined to test the applicability of the Si and SiOx films to actual TFTs. A TFT with a field-effect mobility of ∼1.7 cm2 V−1s−1 was obtained at a temperature of 120 °C.

Influence of Power on the Microstructure and Optical Properties of Microcrystalline Si Films

Microcrystalline silicon films were deposited using Ar diluted SiH4 gaseous mixture by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD). The effects of power on microstrcture and optical properties of microcrystalline silicon films were investigated. The results show that, with the increasing of the power, the crystallinity increased, but the concentration of hydrogen decreased monotonously. Furthermore, the absorption coefficient of the films increased monotonously, and the optical bandgap changed from 1.89eV to 1.75eV with the microwave power ranging from 400 W to 650W.

Substrate Dependence of Microcrystalline Silicon Growth with SiH<sub>4</sub> Diluted by Ar

Microcrystalline Si films were deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) using Ar diluted SiH4gaseous mixture. The effects of the substrate on deposition rate, preferred orientation and roughness of the films were investigated. The results show that, the influence of the substrate surface chemical nature on the deposition rate is significant in the initial stage of the growth. And considering the crystallinity and roughness of the films, the substrate is favored in its preferred orientation with a rougher surface. Based on these results, it is confirmed that the combination of diffusion and etching is indispensable to describe the deposition of μc-Si with SiH4diluted by Ar, and the mechanism of μc-Si growth could be controlled by diffusion of Si and etching of the Ar+on the film surface.

Effects of crystalline nanoparticle incorporation on growth, structure, and properties of microcrystalline silicon films deposited by plasma chemical vapor deposition

We investigated effects of incorporation of Si crystalline nanoparticles into microcrystalline Si films during their deposition on their growth, structure and properties using multi-hollow discharge plasma chemical vapor deposition. The films with nanoparticles show a lower (220) orientation ratio than those without nanoparticles, whereas both the films have nearly the same film thickness and crystallinity. The films with nanoparticles have inverted conical growth, while the films without nanoparticles have columnar growth. Nucleation density of the films with nanoparticles is higher than that of the films without nanoparticles. Both photo and dark conductivities of the films with nanoparticles tend to be lower than those without nanoparticles. Thus, incorporation of small amount of nanoparticles into microcrystalline Si film affects growth, structure and properties of the films.

Effects of nanoparticle incorporation on properties of microcrystalline films deposited using multi-hollow discharge plasma CVD

We investigated the effects of incorporation of crystalline nanoparticles on properties of microcrystalline Si films deposited using multi-hollow discharge plasma CVD. Both photo and dark conductivity of films with nanoparticles are lower than the films without nanoparticles. Films with nanoparticles have higher photosensitivity than those of films without nanoparticles in a low crystallinity region. Microcrystalline films of high crystallinity and high photosensitivity are obtained only in a range of small volume fraction of nanoparticles under low dilution ratio, R=([SiH4]+[H2])/[SiH4]. Incorporation of small amount of nanoparticles into films strongly affects photo and dark conductivity of films.

EFFECTS OF INTERELECTRODE SPACING ON THE PROPERTIES OF MICROCRYSTALLINE SILICON ABSORBER AND SOLAR CELLS

ABSTRACTWe study the effect of the spacing between electrodes in very high frequency plasma enhanced chemical vapor deposition on the properties of microcrystalline silicon films and their related n-i-psolar cells. We vary the spacing from 0.2 to 1.0 cm to deposit microcrystalline silicon at 67.8 MHz while maintaining other growth parameters. The spacing between the electrodes significantly changes the plasma conditions, which govern film precursor chemistry as well as introduce etching and ion bombardment to the film; thereby, influencing nucleation and growth of the microcrystalline Si films. The resulting films were characterized by UV-Vis spectrometry, atomic force microscopy, X-ray diffraction, and transmission electron microscopy. We found that deposition rate decreases, while surface roughness and short circuit current density increase with smaller spacing.

Surface chemistry of the preferred (111) and (220) crystal oriented microcrystalline Si films by radio‐frequency plasma‐enhanced chemical vapor deposition

AbstractThe surface chemistry of the preferentially (111) and (220) crystal orientated chlorinated hydrogenated microcrystalline silicon (µc‐Si:H:Cl) films was studied using a rf PE‐CVD of a dichlorosilane (SiH2Cl2) and H2 mixture. The growing surface for the preferentially (220) crystal oriented µc‐Si:H:Cl films included much voids and dangling bonds, whereas the growing surface with the preferential (111) crystal orientation was chemically stable relatively. These findings suggest that the sticking process of deposition precursors and/or the reconstruction of Si clusters within the sub‐surface determine the preferential crystal orientation. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural properties of microcrystalline Si films prepared by hot-wire/catalytic chemical vapor deposition under conditions close to the transition from amorphous to microcrystalline growth

The structural properties of microcrystalline Si films prepared by hot-wire/catalytic chemical vapor deposition, with various dilution ratios of silane in hydrogen, were investigated as regards to the role of hydrogen. A large surface roughness correlated with a low crystalline nuclei density was observed for microcrystalline Si films deposited near the transition from amorphous to microcrystalline growth. Investigations of hydrogen-related properties suggest the presence of molecular hydrogen in these films. We tentatively propose that the diffusion of atomic hydrogen into the subsurface layer of growing films, which leads to the relaxation of amorphous Si network and to the generation of molecular hydrogen, plays an important role for determining the film properties, besides top surface reactions.

Characterization of microcrystalline Si films deposited at low temperatures with high rates by atmospheric‐pressure plasma CVD

AbstractLow‐temperature and high‐rate depositions of hydrogenated microcrystalline silicon (μc ‐Si:H) films are investigated using stable plasma excited at atmospheric pressure (AP) by supplying 150‐MHz very high‐frequency (VHF) power. VHF power density, H2/SiH4 ratio and plasma gap are varied as parameters under a fixed substrate temperature (Tsub) of 220 ΩC. Increasing VHF power density and H2/SiH4 ratio is primarily important for the sufficient decomposition of source gas molecules, improving both deposition rate and film property. In addition, the plasma gap is found to be another crucial parameter in the deposition process using AP‐VHF plasma. Numerical analyses on the temperature distribution around the plasma region have revealed that the steady‐state surface temperature of the glass substrate becomes approximately 100 ΩC higher than the back surface temperature (Tsub), which is caused by the moderate surface heating effect of the AP‐VHF plasma. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of Microcrystalline Silicon Thin Film Transistors Fabricated by Thermal Plasma Jet Crystallization Technique

The electrical characteristics of thin-film transistors (TFTs) fabricated by thermal plasma jet (TPJ)-crystallized microcrystalline Si (µc-Si) films have been investigated. Amorphous Si (a-Si) films were crystallized with the TPJ under the scanning speed (v) of 350 to 550 mm/s, and µc-Si TFTs were successfully fabricated with a 300 °C process. By reducing v, µFE increases from 3.2 to 17.1 cm2 V-1 s-1, and Vth and S decrease from 9.2 to 5.2 V and 1.3 to 0.6 V/decade, respectively. The variations of µFE, Vth, and S were kept within small values of 1.06 (±4.4%), 0.14 (±1.1%), and 0.04 (±4.0%), respectively. The µc-Si is formed with ∼20-nm-sized randomly oriented small grains, and this isotropic nature results in very small variation of TFT performance. With decreasing v, the fraction of nano sized grains and disordered bonds at the grain boundary decreases, which results in improved TFT performance.

Inductively coupled plasma-assisted RF magnetron sputtering deposition of boron-doped microcrystalline Si films

An innovative custom-designed inductively coupled plasma-assisted RF magnetron sputtering deposition system has been developed to synthesize B-doped microcrystalline silicon thin films using a pure boron sputtering target in a reactive silane and argon gas mixture. Films were deposited using different boron target powers ranging from 0 to 350 W at a substrate temperature of 250 °C. The effect of the boron target power on the structural and electrical properties of the synthesized films was extensively investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and Hall-effect system. It is shown that, with an initial increase of the boron target power from 0 to 300 W, the structural and electrical properties of the B-doped microcrystalline films are improved. However, when the target power is increased too much (e.g. to 350 W), these properties become slightly worse. The variation of the structural and electrical properties of the synthesized B-doped microcrystalline thin films is related to the incorporation of boron atoms during the crystallization and doping of silicon in the inductively coupled plasma-based process. This work is particularly relevant to the microcrystalline silicon-based p– i– n junction solar cells.

Fast deposition of microcrystalline Si films from SiH 2Cl 2 using a high-density microwave plasma source for Si thin-film solar cells

Fast deposition rate of 27 Å/s for microcrystalline (μc-) Si films was investigated using a high-density and low-temperature microwave plasma source with a spoke antenna of SiH 4–H 2 and SiH 2Cl 2–H 2 mixtures. The film crystallization was promoted efficiently when film was fabricated using SiH 4 with an aid of large amount of hydrogen atom, while the volume fraction of void in the film was relatively the larger of 10–20%. On the other hand, the volume fraction of amorphous silicon phase in the μc-Si films were 20–30% with no significant void fractions in films fabricated from SiH 2Cl 2, although the volume fraction of the crystalline silicon phase was 75–85%. The role of chlorine in the growth of μc-Si films is discussed in terms of the chemistry of the H and Cl terminated growing surface. The preliminary result of p–i–n Si thin-film solar cell is demonstrated using μc-Si:H:Cl films as an intrinsic layer.

Fractal study of surface nanostructures of microcrystalline silicon films: From growth kinetics to electronic transport

We studied surface nanostructures of microcrystalline Si (μc-Si) films and their relationship to electronic transport, based on fractal concepts. Two distinctive nanostructures, i.e. columnar and granular structures, are presented together with the crystallographic preferential orientations. The topological evolution of μc-Si surfaces monitored by atomic force microscope measurements is also discussed. The scaling exponents derived from fractal concepts quantitatively revealed a difference in the topological features of surface evolution related to the nanostructures of μc-Si films, thus providing a new insight into the growth kinetics. Growth kinetics were further evaluated using Monte Carlo simulations. Finally, the electronic transport of μc-Si is discussed using the lateral size and fractal boundary determined by fractal concepts.

Characterization of High-Rate Deposited Microcrystalline Si Films Prepared Using Atmospheric-Pressure Very High Frequency Plasma

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Microcrystalline Si films grown at low temperatures (90–220 °C) with high rates in atmospheric-pressure VHF plasma

This work deals with the structural properties of microcrystalline silicon (μc-Si:H) films grown at low temperatures (90–220 °C) with high rates in atmospheric-pressure He/H2/SiH4 plasma, which is excited by a 150 MHz very high frequency power using a porous carbon electrode. This plasma permits to enhance the chemical reactions both in gas phase and on the film-growing surface, while suppressing ion impingement upon the surface. Raman crystalline volume fractions of the μc-Si:H films are studied in detail as functions of film thickness and substrate temperature (Tsub). The results show that the μc-Si:H film deposited with 50 (SCCM) (SCCM denotes standard cubic centimeters per minute at STP) SiH4 has no amorphous transition layers at the film/substrate interface in spite of the high deposition rate of 6.4 nm/s, which is verified by the cross sectional observations with a transmission electron microscope. In addition, the Tsub dependence of Raman crystallinity of the μc-Si:H films indicates that a highly crystallized μc-Si:H film grows even when Tsub is reduced to 90 °C. Further systematic studies are needed for both device applications and deposition on thermally sensitive plastic materials.

Rapid crystallization of amorphous silicon utilizing a very-high-frequency microplasma jet for Si thin-film solar cells

The rapid crystallization of amorphous silicon utilizing a very-high-frequency thermal microplasma jet of argon is demonstrated. Highly crystallized microcrystalline Si films were fabricated on textured a-Si:H:B/SnO 2/glass by adjusting the translational velocity of the substrate stage. The crystallization of amorphous silicon was promoted from the bottom surface with no significant inter diffusions of B and Sn from Si:H:B/SnO 2 layers to intrinsic crystallized Si layer. The preliminary result of p–i–n Si thin-film solar cell is demonstrated using the microcrystalline Si films fabricated by the plasma annealing.

Fabrication of selective-emitter silicon heterojunction solar cells using hot-wire chemical vapor deposition and laser doping

The Si heterojunction (HJ) solar cells were fabricated on the textured p-type mono-crystalline Si (c-Si) substrates using hot-wire chemical vapor deposition (HWCVD). In view of the potential for the bottom cell in a hybrid junction structure, the microcrystalline Si (μc-Si) film was used as the emitter with various PH 3 dilution ratios. Prior to the n-μc-Si emitter deposition, a 5 nm-thick intrinsic amorphous Si layer (i-a-Si) was grown to passivate the c-Si surface. In order to improve the indium-tin oxide (ITO)/emitter front contact without using the higher PH 3 doping concentration, a laser doping technique was employed to improve the ITO/n-μc-Si contact via the formation of the selective emitter structure. For a cell structure of Ag grid/ITO/n-μc-Si emitter/i-a-Si/textured p-c-Si/Al-electrode, the conversion efficiency (AM1.5) can be improved from 13.25% to 14.31% (cell area: 2 cm × 2 cm) via a suitable selective laser doping process.