Shintaro Ogura

Characterization of an oxide semiconductor prepared by microwave sintering

We fabricated a thin-film transistor (TFT) using amorphous indium gallium zinc oxide (a-IGZO), which was formed through annealing of an IGZO precursor film with a single-mode cavity microwave at 2.45 GHz. The transisitor fabricated with the a-IGZO film prepared by microwave annealing for 15 min showed higher device performance, i.e., a field effect mobility of 5.75 × 10−2 cm2·V−1·s−1, an on/off ratio of 106, and a threshold voltage of 20 V, than that prepared by annealing with a conventional oven for 120 min. The Raman spectra confirm that the device improvement originates from the decrease in the number of –OH groups and removal of organic species for 15 min by microwave annealing. These results suggest that the microwave annealing method has an advantage as the annealing process of solution-processed oxide semiconductors to reduce the process time. It can be applied to the fabrication of TFTs.

Rapid preparation of solution-processed InGaZnO thin films by microwave annealing and photoirradiation

We fabricated solution-processed indium–gallium–zinc oxide (IGZO) thin-film transistors (TFTs) by microwave (MW) annealing an IGZO precursor film followed by irradiating with vacuum ultraviolet (VUV) light. MW annealing allows more rapid heating of the precursor film than conventional annealing processes using a hot plate or electric oven and promotes the crystallization of IGZO. VUV irradiation was used to reduce the duration and temperature of the post-annealing step. Consequently, the IGZO TFTs fabricated through MW annealing for 5 min and VUV irradiation for 1 min exhibited an on/off current ratio of 108 and a field-effect mobility of 0.3 cm2 V−1 s−1. These results indicate that MW annealing and photoirradiation is an effective combination for annealing solution processed IGZO precursor films to prepare the semiconductor layers of TFTs.

Solution-processed hybrid organic–inorganic complementary thin-film transistor inverter

We investigated hybrid organic–inorganic complementary inverters with a solution-processed indium–gallium–zinc-oxide (IGZO) n-channel thin-film transistor (TFT) and p-channel TFTs using the high-uniformity polymer poly[2,5-bis(alkyl)pyrrolo[3,4-c]pyrrolo-1,4(2H,5H)-dione-alt-5,5-di(thiophene-2-yl)-2,2-(E)-2-(2-(thiophen-2-yl)vinyl)thiophene] (PDVT-10). The IGZO TFT was fabricated at 150 °C for 1 min. It showed a high field-effect mobility of 0.9 cm2V−1s−1 and a high on/off current ratio of 107. A hybrid complementary inverter was fabricated by combining IGZO with a PDVT-10 thin-film transistor and its operation was confirmed.

Flexible InGaZnO TFT devices obtained via humid-UV irradiation with an aqueous-fluoroalcoholic precursor

We designed an aqueous-fluoroalcoholic InGaZnO precursor for obtaining thin-film transistors (TFTs) on a flexible plastic film by spin-coating and low-temperature annealing processes without inert gas conditions. The precursor shows a low surface tension (23.7 mN m−1), which is advantageous for homogeneous coating onto plastic film. Thermal analysis of the precursor indicates formation of metal oxides at less than 300 °C. InGaZnO TFTs were obtained from the precursor by annealing at 300 °C via UV irradiation under humid atmosphere on a transparent polyimide film as well as on a p–Si substrate. The bottom-gate top-contact TFTs on the p–Si show 5.1 cm2 V−1 s−1 of the average saturation mobility. The top-gate top-contact TFTs on the transparent polyimide film drive with 0.99 cm2 V−1 s−1 of the average saturation mobility. The transparent polyimide film maintains flexibility even after humid-UV irradiation and annealing processes. The InGaZnO TFTs on the transparent polyimide film show more than 80% transmittance in the visible light region between 400 and 780 nm.

Repairing of Pattern Defects on OTFT by Fountain Pen Nano-Lithography

The patterning by Fountain-pen nano-lithography (FPN) with silver nanoparticles ink was reported. The FPN, a kind of pen-type nano-lithography technique, enables on-demand patterning of micro-meter size at wide area. At first, preparation of silver nanoparticles ink and patterning condition by FPN with the ink were investigated. Then the printed wires with disconnection of OTFT were repaired by FPN with the ink. The repaired lines showed electrical conductivity. These results suggest the possibility that the FPN technique is able to become powerful tool for repairing of device patterning.

Reverse Offset Printing of Semidried Metal Acetylacetonate Layers and Its Application to a Solution-Processed IGZO TFT Fabrication

The submicrometer resolution printing of various metal acetylacetonate complex inks including Fe, V, Mn, Co, Ni, Zn, Zr, Mo, and In was enabled by a robust ink formulation scheme which adopted a ternary solvent system where solubility, surface wettability, and drying as well as absorption behavior on a polydimethylsiloxane sheet were optimized. Hydrogen plasma in heated conditions resulted in bombarded, resistive, or conductive state depending on the temperature and the metal species. With a conductivity-bestowed layer of MoO x and a plasma-protecting layer of ZrO x situated on the top of an IGZO layer, a solution-processed TFT exhibiting an average mobility of 0.17 cm2/(V s) is demonstrated.