Yoshihiro Okumura

Bio: Yoshihiro Okumura is an academic researcher. The author has contributed to research in topics: Photodiode & Avalanche photodiode. The author has an hindex of 2, co-authored 3 publications receiving 1526 citations.

Transparent thin film transistors using ZnO as an active channel layer and their electrical properties

Abstract: Bottom-gate-type thin film transistors using ZnO as an active channel layer (ZnO–TFT) have been constructed. The ZnO layers were deposited using pulsed laser deposition at 450 °C at an oxygen pressure of 3 m Torr, and the material that was formed had a background carrier concentration of less than 5×1016 cm−3. A double layer gate insulator consisting of SiO2 and SiNx was effective in suppressing leakage current and enabling the ZnO–TFT to operate successfully. The Ion/Ioff ratio of ZnO–TFTs fabricated on Si wafers was more than 105 and the optical transmittance of ZnO–TFTs fabricated on glass was more than 80%. These results show that it is possible to fabricate a transparent TFT that can even be operated in the presence of visible light.

Photo-Current Multiplication Phenomenon of Amorphous Silicon-Based Multilayer Photodiodes Fabricated on Crystalline Silicon Substrate

Abstract: Recently, high-resolution and high-density solid-state imaging devices have been in high demand due to their high photogain because the smaller pixel size makes the smaller photo-current dependent on the reduced area. Multilayer photodiodes having a-SiC:H/a-Si:H/a-SiN:H structures fabricated on the crystalline silicon (c-Si) substrate were studied for their photo-multiplication effects which marked a gain of 6.6. These multiplication phenomenona arose in the wavelength region longer than 550 nm, while in the shorter-wavelength region, the photogain was less than unity. This is explained by the electron carrier process where the carriers photogenerated in the c-Si substrate are accumulated at the a-SiN:H/c-Si interface and tunneled via localized states in the a-SiN:H layer by the induced high electric field. From these results, it is considerably difficult to support the theory of avalanche effect to explain these photo-multiplication effects.

Amorphous Silicon Avalanche Photodiode Films Using a Functionally Graded Superlattice Structure.

Abstract: The photocurrent multiplication due to impact ionization was observed in an a-Si:H/a-SiC:H staircase photodiode. In the staircase photodiode with one band offset, the photocurrent was doubled and was saturated. It was confirmed that almost all the electrons multiplied after they crossed the band offset. On the staircase photodiode with 3 band offsets, a saturation multiplication gain of about 6 was obtained. The gamma values of the photocurrent characteristics were 1.0, indicating that no excess carriers entered from the electrode and no interband tunneling affected the photoinduced current. These results suggested that the impact-ionization at each conduction band step due to the conduction-band discontinuity could be the dominant mechanism of the photocurrent multiplication.

A comprehensive review of zno materials and devices

Abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

Oxide Semiconductor Thin‐Film Transistors: A Review of Recent Advances

Abstract: Transparent electronics is today one of the most advanced topics for a wide range of device applications. The key components are wide bandgap semiconductors, where oxides of different origins play an important role, not only as passive component but also as active component, similar to what is observed in conventional semiconductors like silicon. Transparent electronics has gained special attention during the last few years and is today established as one of the most promising technologies for leading the next generation of flat panel display due to its excellent electronic performance. In this paper the recent progress in n- and p-type oxide based thin-film transistors (TFT) is reviewed, with special emphasis on solution-processed and p-type, and the major milestones already achieved with this emerging and very promising technology are summarizeed. After a short introduction where the main advantages of these semiconductors are presented, as well as the industry expectations, the beautiful history of TFTs is revisited, including the main landmarks in the last 80 years, finishing by referring to some papers that have played an important role in shaping transparent electronics. Then, an overview is presented of state of the art n-type TFTs processed by physical vapour deposition methods, and finally one of the most exciting, promising, and low cost but powerful technologies is discussed: solution-processed oxide TFTs. Moreover, a more detailed focus analysis will be given concerning p-type oxide TFTs, mainly centred on two of the most promising semiconductor candidates: copper oxide and tin oxide. The most recent data related to the production of complementary metal oxide semiconductor (CMOS) devices based on n- and p-type oxide TFT is also be presented. The last topic of this review is devoted to some emerging applications, finalizing with the main conclusions. Related work that originated at CENIMAT|I3N during the last six years is included in more detail, which has led to the fabrication of high performance n- and p-type oxide transistors as well as the fabrication of CMOS devices with and on paper.

Semiconductor device, and manufacturing method thereof

Abstract: An object is to provide a semiconductor device of which a manufacturing process is not complicated and by which cost can be suppressed, by forming a thin film transistor using an oxide semiconductor film typified by zinc oxide, and a manufacturing method thereof. For the semiconductor device, a gate electrode is formed over a substrate; a gate insulating film is formed covering the gate electrode; an oxide semiconductor film is formed over the gate insulating film; and a first conductive film and a second conductive film are formed over the oxide semiconductor film. The oxide semiconductor film has at least a crystallized region in a channel region.

Wide-bandgap high-mobility ZnO thin-film transistors produced at room temperature

Abstract: We report high-performance ZnO thin-film transistor (ZnO-TFT) fabricated by rf magnetron sputtering at room temperature with a bottom gate configuration. The ZnO-TFT operates in the enhancement mode with a threshold voltage of 19V, a saturation mobility of 27cm2∕Vs, a gate voltage swing of 1.39V∕decade and an on/off ratio of 3×105. The ZnO-TFT presents an average optical transmission (including the glass substrate) of 80% in the visible part of the spectrum. The combination of transparency, high mobility, and room-temperature processing makes the ZnO-TFT a very promising low-cost optoelectronic device for the next generation of invisible and flexible electronics.