The Experts below are selected from a list of 270 Experts worldwide ranked by ideXlab platform
Qi Wang - One of the best experts on this subject based on the ideXlab platform.
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Charge Transport in the Transition From Hydrogenated Amorphous Silicon to Microcrystalline Silicon
MRS Proceedings, 2011Co-Authors: A. Kattwinkel, R. Braunstein, Qi WangAbstract:The electronic transport properties of a series of samples prepared by hot-wire chemical vapor deposition with a transition from a-Si:H to {micro}c-Si:H were measured applying the photoconductive frequency mixing technique. The authors found both improved stability against light-soaking and different values for the photomixing electron lifetime and mobility close to the onset of Microcrystallinity as compared to the amorphous state. In particular, the mobility-lifetime product of charge carriers in some of the {micro}c-Si:H samples turns out to lie about two orders of magnitude higher than that of a-Si:H films. The mobility on the other hand, is shown rather to decrease in the transition to the {micro}c-region. Additional measurements of the range and the depth of long range potential fluctuations yield a possible explanation for the results in that grain boundaries may serve as scattering centers and barriers against recombination.
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optical and electronic properties of microcrystalline silicon as a function of Microcrystallinity
Journal of Applied Physics, 2000Co-Authors: J D Lorentzen, Hitoe Habuchi, Qi WangAbstract:Films were prepared by hot wire chemical vapor deposition at ∼240 °C with varied hydrogen dilution ratios R=H2:SiH4 from 1 to 20. The optical and electronic properties as a function of Microcrystallinity were studied. We found: (a) At low H dilution R⩽2, there is no measurable crystallinity by Raman spectroscopy and x-ray diffraction in the a-Si:H matrix, but an optical absorption peak at ∼1.25 eV appears; when R=2, the film shows the lowest subgap absorption, the highest photosensitivity, and the largest optical gap. (b) When R⩾3, the c-Si phase is measurable by Raman and a low-energy photoluminescence (PL) band (0.84–1.0 eV) appears in addition to the high-energy band (1.3–1.4 eV). Meanwhile, all the absorption spectra show a featureless line shape. (c) An energy redshift is observed for both PL peaks as the film grows thicker. Finally, (d) the conductivity activation energy first decreases from 0.68 to 0.12 eV, then increases with increasing Microcrystallinity. A mode of two sets of energy bands of ele...
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Efficient high-deposition-rate all-hot-wire hydrogenated amorphous silicon n-i-p solar cells
Conference Record of the Twenty-Eighth IEEE Photovoltaic Specialists Conference - 2000 (Cat. No.00CH37036), 2000Co-Authors: Qi Wang, A H Mahan, E. Iwaniczko, Yueqin Xu, B.p. Nelson, R.s. Crandall, H.m. BranzAbstract:Efficient all-hot-wire chemical vapor deposition (HWCVD) hydrogenated amorphous silicon (a-Si:H) n-i-p solar cells have been demonstrated. Using HWCVD, we made the intrinsic layer deposited at 17-20 /spl Aring//s and the doped layers at 5-10 /spl Aring//s. An initial efficiency of 7.1% and a stable efficiency of 5.4% after 1000 hours of 1-sun light-soaking have been achieved on untextured stainless-steel (SS) substrate. This efficiency improvement results primarily from incorporating into the p/i interface about 60 /spl Aring/ of on-the-edge a-Si:H material grown just below the condition that would lead to the transition to Microcrystallinity. We also optimize p-layers in solar cells by varying p-layer thickness, substrate temperature, trimethylboron gas flow rate, and chamber pressure. The cell's fill factor increases from 0.60 to 0.68, and the open-circuit voltage increases from 0.86 to 0.88 V. Recently, we improved the light-trapping by depositing the solar cell on textured Ag/ZnO-coated SS supplied by Uni-Solar, and we obtained an all-HWCVD record initial efficiency of 8.7%.
Guanglin Kong - One of the best experts on this subject based on the ideXlab platform.
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gap states of hydrogenated amorphous silicon near and above the threshold of Microcrystallinity with subtle boron compensation
Applied Physics Letters, 2001Co-Authors: Shuran Sheng, Xianbo Liao, Guanglin KongAbstract:The effects of hydrogen dilution, subtle boron compensation, and light-soaking on the gap states of hydrogenated amorphous silicon films (a-Si:H) near and above the threshold of Microcrystallinity have been investigated in detail by the constant photocurrent method and the improved phase-shift analysis of modulated photocurrent technique. It is shown that high hydrogen dilution near the threshold of Microcrystallinity leads to a more ordered network structure and to the redistribution of gap states; it gives rise to a small peak at about 0.55 eV and a shoulder at about 1.2 eV below the conduction band edge, which are associated with the formation of microcrystallites embedded in the amorphous silicon host matrix. A concurrent subtle boron compensation is demonstrated to prevent excessive formation of Microcrystallinity, and to help promote the growth of the ordered regions and reduce the density of gap defect states, particularly those associated with microcrystallites. Hydrogen-diluted and appropriately boron-compensated a-Si:H films deposited near the threshold of Microcrystallinity show the lowest density of the defects in both the annealed and light-soaked states, and hence, the highest performance and stability. (C) 2001 American Institute of Physics.
Ming-shiann Feng - One of the best experts on this subject based on the ideXlab platform.
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Microcrystallinity of undoped amorphous silicon films and its effects on the transfer characteristics of thin film transistors
Japanese Journal of Applied Physics, 1995Co-Authors: Chia-wen Liang, Wen-chuan Chiang, Ming-shiann FengAbstract:The Microcrystallinity of hydrogenated amorphous silicon films deposited by the conventional radio-frequency plasma-enhanced chemical vapor deposition (rf-PECVD) method and its dependence on chamber pressure are discussed. In a wide range of pressure at which the microcrystalline film can be formed, a critical pressure (500 mT) is found. Films deposited at this critical pressure possess the highest crystalline volume fraction and the smallest grain size. An ion-bombardment-assisted model is proposed to explain the experimental results. Concerning the applications of microcrystalline films to thin-film transistors (TFTs), the subthreshold swing and the field effect mobility are studied, both of which are found to be smaller than those of the hydrogenated amorphous silicon (a-Si:H) TFTs.
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Microcrystallinity of undoped amorphous silicon films and its effects on the transfer characteristics of thin film transistors
Proceedings of 4th International Conference on Solid-State and IC Technology, 1995Co-Authors: Chia-wen Liang, Wen-chuan Chiang, Ming-shiann FengAbstract:The Microcrystallinity of the hydrogenated amorphous silicon films deposited by the usual radio-frequency plasma-enhanced chemical vapor deposition (rf-PECVD) method and its dependence on the chamber pressure are discussed. Films deposited at the critical pressure posses the highest crystalline volume fraction and the smallest grain size. An ion-bombardment assisted model is proposed to explain the experimental results. Concerning the applications of microcrystalline films to thin film transistors (TFTs), the subthreshold swing and the field effect mobility are also studied.
Maria Losurdo - One of the best experts on this subject based on the ideXlab platform.
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Enhancement of the Amorphous to Microcrystalline Phase Transition in Silicon Films Deposited by Sif 4 -H 2 -He Plasmas
MRS Proceedings, 2011Co-Authors: G. Cicalai, Maria Losurdo, Pio Capezzuto, Giovanni Bruno, Teresa Ligonzo, Luigi Schiavulli, C. Minarini, Mariacristina RossiAbstract:AbstractHydrogenated microcrystalline silicon (μc-Si:H) thin films have been obtained by plasma decomposition of SiF4-H2-He mixtures at low temperature (120 °C). The size of crystalline grain and their volume fraction with respect to the amorphous phase have been found dependent on the r.f. power as evaluated by grazing incidence X-ray diffraction, microRaman and ellipsometry measurements. Chemical and electrical properties change according to the Microcrystallinity. Pure and/or highly microcrystalline silicon has been obtained at temperature and r.f. power as low as 120 °C and 15 Watt.
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structural and optical investigation of plasma deposited silicon carbon alloys insights on si c bond configuration using spectroscopic ellipsometry
Journal of Applied Physics, 2005Co-Authors: Maria Losurdo, Giovanni Bruno, Maria M Giangregorio, P Capezzuto, Fabrizio GiorgisAbstract:Amorphous (a–Si1−xCx:H) and microcrystalline (μc–Si1−xCx:H) thin films have been deposited by plasma-enhanced chemical-vapor deposition using SiF4–CH4−H2 rf plasmas. Gas flow rates have been varied to deposit films with different carbon content and microstructure. The microstructure and optical properties have been investigated by IR/Raman spectroscopy and by spectroscopic ellipsometry in the energy range of 1.5–5.0 eV. Ellipsometric spectra have been analyzed in terms of the tetrahedron model combined with the Bruggeman effective-medium approximation to determine the film microstructure and silicon–carbon-bond configurations. Correlation between Si–C bond configurations and optical properties of films has been studied as a function of carbon content and microstructure. It is found that the optical properties and the band gap value depend not only on the carbon content, but also on the Si–C bond configuration and Microcrystallinity. The films consist of Si-centered Si–Si4−vCv(v=0–3) Si-centered tetrahedra...
Subhendu Guha - One of the best experts on this subject based on the ideXlab platform.
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Effects of Hydrogen Dilution on a-Si:H and its Solar Cells Studied by Raman and Photoluminescence Spectroscopy
MRS Proceedings, 2011Co-Authors: Jessica M. Owens, J.r. Weinberg-wolf, Jeffrey Yang, Kenneth Lord, Subhendu GuhaAbstract:a-Si:H films and their n-i-p solar cells were prepared using plasma-enhanced CVD. The samples were prepared with no-, low-, standard, and high-H dilution. Raman and photoluminescence (PL) were used to characterize the i-layer. The main results are (a) Raman shows typical a-Si:H mode except for a c-Si peak in the 450 nm-thick film with high-H dilution, and (b) PL shows two regimes. (I) Below the onset of Microcrystallinity characterized by x-ray diffraction, a blue-shift of the 1.4 eV PL peak energy and a decrease of the band width occur. (II) Above the onset of Microcrystallinity, the PL efficiency decreases by a factor of 4-5, and the PL peak energy is red-shifted toward 1.2 eV as the μc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to-microcrystalline structural transition in thin film solar cells.
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Electronic states of intrinsic layers in n-i-p solar cells near amorphous to microcrystalline silicon transition studied by photoluminescence spectroscopy
Applied Physics Letters, 2000Co-Authors: D.l Williamson, Jeffrey Yang, Kenneth Lord, Subhendu GuhaAbstract:Thin film n-i-p solar cells were prepared using decomposition of disilane-hydrogen mixtures by plasma-enhanced chemical vapor deposition. By increasing either the H dilution ratio or the thickness, the i-layer structure showed a transition from amorphous to microcrystalline silicon characterized by x-ray diffraction. The electronic states of the i layer were examined by photoluminescence (PL) spectroscopy, which showed that: (a) below the onset of Microcrystallinity, a blueshift of the 1.4 eV PL peak energy along with a decrease of the band width occur as the structural order is improved; (b) above the onset of Microcrystallinity, the PL efficiency decreases by a factor of 4–5 and the PL peak energy is redshifted toward 1.2 eV as the μc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to...
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structural defect and device behavior of hydrogenated amorphous si near and above the onset of Microcrystallinity
Applied Physics Letters, 1999Co-Authors: Subhendu Guha, D.l Williamson, J Yang, Yoram Lubianiker, J. D. Cohen, A H MahanAbstract:High-hydrogen-diluted films of hydrogenated amorphous Si (a-Si:H) 0.5 μm in thickness and optimized for solar cell efficiency and stability, are found to be partially microcrystalline (μc) if deposited directly on stainless steel (SS) substrates but are fully amorphous if a thin n layer of a-Si:H or μc-Si:H is first deposited on the SS. In these latter cases, partial Microcrystallinity develops as the films are grown thicker (1.5–2.5 μm) and this is accompanied by sharp drops in solar cell open circuit voltage. For the fully amorphous films, x-ray diffraction (XRD) shows improved medium-range order compared to undiluted films and this correlates with better light stability. Capacitance profiling shows a decrease in deep defect density as growth proceeds further from the substrate, consistent with the XRD evidence of improved order for thicker films.