The crystal structures, thin-film properties, and field-effect transistor (FET) characteristics of tetrathiafulvalene (TTF) derivatives with two phenyl groups are systematically investigated. The highest mobility, 0.11 cm(2) V(-1) s(-1), is observed in biphenyl-substituted TTF (1). The correlation between the crystal structures and the FET properties demonstrates that good transistor properties are associated with two-dimensional intermolecular interaction, which is achieved when the molecules are standing nearly perpendicular to the substrate. Since these TTF derivatives are strong electron donors, the use of a metallic charge-transfer salt (TTF)(TCNQ) as the source and drain electrodes has resulted in a considerable reduction of the off current (TCNQ: tetracyanoquinodimethane).
Organic field-effect transistors based on alkyl-substituted dibenzotetrathiafulvalenes (DBTTF) are fabricated by solution process. The molecules with butyl or longer alkyl groups are standing perpendicular to the substrates in the thin films, and the transistors exhibit comparable performance to the vacuum-deposited DBTTF devices.
We developed an original method to measure the time-dependent vapor composition, and applied this method to vacuum evaporation of BaSi 2 thin films at three source heating currents. At high heating current, the vapor was found to change from Ba-rich to Si-rich, and the resultant film is of high surface crystallinity. On the other hand, Si-rich vapor did not appear at low heating current, and the film was more a-axis oriented. Based on these results, the formation mechanisms of BaSi 2 films and a guideline for further improvement are discussed.
BaSi2 homojunction diodes on Nb-doped TiO2 (TiO2:Nb) coated glass substrates were fabricated using aluminum-induced crystallization (AIC) and two-step evaporation method. From Raman scattering spectra, the growth of BaSi2 on TiO2:Nb was confirmed when the thickness of poly-Si grown by AIC (AIC-Si) was more than 150 nm. The partial formation of BaSi2 diodes was confirmed from the samples prepared at temperature during AIC TAIC=475-525 oC. The long-wavelength edge of photoresponsivity of the diodes was located around 950 nm, which corresponds to the bandgap of BaSi2 of 1.3 eV, suggesting that this photocurrent is derived from BaSi2 thin films. At TAIC =500 oC, the maximum value of photoresponsivity was obtained. Since the largest grains in AIC-Si were also obtained at TAIC=500 oC, these results suggest that larger grain of AIC-Si leads to the improvement of the quality of BaSi2 thin films themselves and the performance of BaSi2 diodes.
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