Haruo Iwasawa

Solution-processed silicon films and transistors.

The use of solution processes—as opposed to conventional vacuum processes and vapour-phase deposition—for the fabrication of electronic devices has received considerable attention for a wide range of applications, with a view to reducing processing costs. In particular, the ability to print semiconductor devices using liquid-phase materials could prove essential for some envisaged applications, such as large-area flexible displays. Recent research in this area has largely been focused on organic semiconductors, some of which have mobilities comparable to that of amorphous silicon (a-Si); but issues of reliability remain. Solution processing of metal chalcogenide semiconductors to fabricate stable and high-performance transistors has also been reported. This class of materials is being explored as a possible substitute for silicon, given the complex and expensive manufacturing processes required to fabricate devices from the latter. However, if high-quality silicon films could be prepared by a solution process, this situation might change drastically. Here we demonstrate the solution processing of silicon thin-film transistors (TFTs) using a silane-based liquid precursor. Using this precursor, we have prepared polycrystalline silicon (poly-Si) films by both spin-coating and ink-jet printing, from which we fabricate TFTs with mobilities of 108 cm2 V-1 s-1 and 6.5 cm2 V-1 s-1, respectively. Although the processing conditions have yet to be optimized, these mobilities are already greater than those that have been achieved in solution-processed organic TFTs, and they exceed those of a-Si TFTs (≤ 1 cm2 V-1 s-1).

Spin-on n-Type Silicon Films Using Phosphorous-doped Polysilanes

We have developed a liquid precursor that can be used in a solution process to form n-type doped silicon films. This precursor is based on phosphorus-doped hydrogenated polysilane synthesized by photo-copolymerizing cyclopentasilane and white phosphorus. By spin-coating this precursor, we have prepared n-type amorphous silicon films and polycrystalline silicon films with resistivities of 6.5–27 Ωcm and 2.0–10 mΩcm, respectively.

P-5: Solution-Processed SiO2 Films Using Hydrogenated Polysilane Based Liquid Materials

Polycrystalline silicon thin-film transistors (poly-Si TFTs) have been fabricated using solution-processed SiO2 films made from a polysilane-based liquid precursor. Spin-coating this precursor, which already had been reported as a precursor for silicon film [Nature, vol. 440, pp. 783–786, 2006], we have prepared gate silicon oxide films for the TFTs. The fabricated TFT has been operated with the mobility of 108.9 cm2/Vs and sufficient low gate leakage current.