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Single-crystalline films of superconducting Sr1-xKxFe2As and Ba1-xKxFe2As2 were grown by molecular beam epitaxy (MBE). The most crucial problem in MBE growth of these compounds is the high volatility of elemental K. The key to incorporating K into films is low-temperature growth (≤ 350 ºC) in reduced As flux. We performed a systematic study of the doping dependence of Tc in Ba1-xKxFe2As2 for x = 0.0 to 1.0. The highest Tcon (Tcend) so far attained for Ba1-xKxFe2As2 is 38.3 K (35.5 K) at x ~ 0.3.
Production technologies of a-Si/a-SiGe flexible film solar cells have been established. The film solar cells having a unique monolithic device structure using through-hole contacts are continuously fabricated on flexible plastic films. Production technologies, such as (1) low temperature and high speed textured electrode deposition method, (2) high speed amorphous silicon deposition method, (3) modified roll-to-roll deposition apparatus technology, (4) large-area high quality transparent electrode deposition method, and (5) very high speed laser patterning method were developed. Increasing deposition rate of textured metal electrode was developed for improving production capacity. We developed and proposed low-temperature textured electrode deposition technique named “selective reactive sputtering”. In the deposition method, Ag-based electrode is deposited using small amount of Al containing Ag alloy targets. Ar/O2 mixed gas is used for reactive sputtering. In the deposition conditions, Ag is not oxidized and maintains low resistivity, whereas Al is oxidized selectively and changes the surface morphology drastically. The oxide incorporation affects on the surface reaction and enhances surface roughness formation. We also developed an amorphous silicon deposition control method which can increase deposition rate without deteriorating film properties. We found peak-to-peak voltage at the cathode (Vpp) had information of the plasma and could be used for optimizing the deposition conditions. Vpp has a strong relationship with stabilized efficiency of solar cells. Very high laser patterning method was also developed. The laser patterning method using a galvano scanner improves the pattering speed remarkably. Our new factory in Kumamoto started commercial production in the last year. We can produce 12MW/Y of solar cells using the new production line that can apply 1m-wide and 2000m-long film substrates at this time. We are planning to install more production lines and have 40MW/Y production capacity in FY 2008.
Light-weight, large-area and flexible solar cells and modules were developed. Roll-to-roll processes including an improved film deposition process named “Stepping-roll process” are used to fabricate large-area hydrogenated amorphous silicon-based solar cells in succession on plastic film substrates. A unique device having through-hole contacts was developed and applied to simplify the production processes and to generate high voltage using connection in series. Production technologies, such as (1) light-weight and large area module fabrication, (2) plasma condition controlled chemical vapor deposition, (3) low substrate temperature selective reactive sputtering, and (4) large-area uniform transparent conductive oxide film deposition are reviewed.
Recent photoemission studies on heavily boron-doped superconducting diamond films, reporting the electronic structure evolution as a function of boron concentrations, are reviewed. From soft X-ray angle-resolved photoemission spectroscopy, which directly measures electronic band dispersions, depopulation of electrons (or formation of hole pockets) at the top of the valence band were clearly observed. This indicates that the holes at the top of the valence bands are responsible for the metallic properties and hence superconductivity at lower temperatures. Hard X-ray photoemission spectroscopy observed shift of the main C 1s core level and intensity evolution of a lower binding energy additional structure, suggesting chemical potential shift, carrier doping efficiency by boron doping, and possibility of boron-related cluster formations.
HCl was added to SiH4 containing plasmas to grow a-Si:H(Cl) films with dangling bonds terminated with Cl instead of H. Bulk and surface infrared spectra, film thickness and optical band gap were examined by in situ multiple total internal reflection Fourier transform infrared spectroscopy and in situ spectroscopic ellipsometry. SiH2Cl2 was also used as a conventional Cl source for reference a-Si:H(Cl) film deposition experiments. The introduction of HCl does not affect the deposition rate significantly, and the deposited a-Si:H(Cl) films contain over 1021cm-3 Cl atoms. HCl addition to the gas phase changes the surface compositions of the growing films drastically from higher silicon hydride to chlorinated lower hydride. The surface reaction control eliminates unfavorable hydride bonding structures such as SiH2 and/or SiH in voids in the deposited films. The a-Si:H(Cl) films deposited from mixtures of SiH4 and HCl do not show significant optical band gap widening in spite of containing over 1021cm-3 Cl atoms, a concentration that is comparable to that of hydrogen. In contrast, a conventional chlorine source of SiH2Cl2 increases the deposition rate significantly compared to HCl. The increase in the deposition rate results in monotonic decrease of the refractive index and the optical band gap widening.
Hydrogenated amorphous silicon thin films deposited from SiH4 containing plasmas are used in solar cells and thin film transistors for flat panel displays. Understanding the fundamental microscopic surface processes that lead to Si deposition and H incorporation is important for controlling the film properties. An in situ method based on attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy was developed and used to determine the surface coverage of silicon mono-, di-, and tri-hydrides as a function of deposition temperature and ion bombardment flux. Key reactions that take place on the surface during deposition are hypothesized based on the evolution of the surface hydride composition as a function of temperature and ion flux. In conjunction with the experiments, the growth of a-Si:H on H-terminated Si(001)-(2×1) surfaces was simulated through molecular dynamics. The simulation results were compared with experimental measurements to validate the simulations and to provide supporting evidence for radical-surface interaction mechanisms hypothesized based on the infrared spectroscopy data. Experimental measurements of the surface silicon hydride coverage and atomistic simulations are used synergistically to elucidate elementary processes occurring on the surface during a-Si:H deposition.
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