Takaaki Miyasako

Ferroelectric-gate thin-film transistors using indium-tin-oxide channel with large charge controllability

We have fabricated and characterized ferroelectric-gate thin-film transistors (TFTs) using indium-tin-oxide (ITO) as a channel and ferroelectric Bi4−xLaxTi3O12 (BLT) as a gate insulator. We have obtained a typical n-channel transistor property with clear current saturation in drain current and drain voltage (ID–VD) characteristics. The obtained on∕off current ratio is more than 104 and the field-effect mobility is estimated 9.1cm2∕Vs. In particular, we demonstrate a large “on”-current of 2.5 mA in ITO∕BLT structure TFT in spite of the low channel mobility. This is because the ferroelectric film can induce large charge density due to the spontaneous polarization.

Impact of UV/O3 treatment on solution-processed amorphous InGaZnO4 thin-film transistors

Ultraviolet–ozone (UV/O3) treatment was adopted to the fabrication of solution-processed amorphous In–Ga–Zn–O thin-film transistors (TFTs), with metal composition of In:Ga:Zn = 1:1:1 represented by InGaZnO4. By applying UV/O3 treatment In–Ga–Zn–O gel films, their condensation was notably enhanced through decomposition of organic- and hydrogen-based elements, which drastically improved the quality of the amorphous InGaZnO4 films. As a result, high TFT performance, with values of on/off ratio, 108; subthreshold swing, 150 mV/decade; threshold voltage, 9.2 V; and field-effect mobility, 5.1 cm2 V−1 s−1, was achieved.

High-Performance Solution-Processed ZrInZnO Thin-Film Transistors

We report on the high performance and high stability of thin-film transistors (TFTs) using solution-processed Zr-In-Zn-O (ZIZO) as an active layer. The effects of adding Zr to In-Zn-O, particularly the electrical characteristics of their thin films and TFTs, were systematically investigated. The Zr effectively controlled the oxygen vacancies because of its low standard electrode potential, which was confirmed by modifications in the optical bandgap energy, carrier concentration, and oxygen-vacancy density of the ZIZO thin films. Consequently, we found that the “off” current decreased and the threshold voltage increased with the increasing Zr content. The optimal ZIZO TFT was obtained at a Zr/In/Zn mole ratio of 0.05 : 2 : 1, and its “on/off” ratio, channel mobility, and subthreshold swing voltage were ~ 109 , 6.23 cm2·V-1·s-1 , and 0.19 V/dec, respectively, which are comparable to those of vacuum-processed oxide TFTs. Furthermore, the performance and bias-stress stability of the ZIZO TFTs were improved as a result of the reduced interface charge trapping.

A low-temperature crystallization path for device-quality ferroelectric films

We show a path for low-temperature crystallization of device-quality solution-processed lead zirconate titanate films. The essential aspect of the path is to circumvent pyrochlore formation at around 300 °C during temperature increase up to 400 °C. By maintaining enough carbon via pyrolysis at 210 °C, well below the temperature for pyrochlore formation, Pb2+ can be reduced to Pb0. This leads to the lack of Pb2+ in the film to suppress the development of pyrochlore, which accounts for the usual high-temperature conversion to perovskite. Films on metal, metal/oxide hybrid, and oxide bottom electrodes were successfully crystallized at 400–450 °C.

Totally solution-processed ferroelectric-gate thin-film transistor

We have fabricated inorganic ferroelectric-gate thin film transistors (FGTs) using only a chemical solution deposition (CSD) process. All layers, including the channel [indium-tin-oxide (ITO)], ferroelectric-gate insulator [Pb(Zr,Ti)O3], gate electrode (LaNiO3) and source/drain electrodes (ITO), were formed by the CSD process. The fabricated FGT exhibited typical n-channel transistor operation with good saturation in drain current and drain voltage (ID-VD) characteristics. The obtained on/off current ratio, memory window, and subthreshold voltage swing were about 107, 2.5 V, and 420 mV/decade, respectively.

Rheology printing for metal-oxide patterns and devices

Technologies of device printing have been widely explored, but existing printing techniques still cannot produce well-defined patterns required by fine electronic devices. Here, a new printing method is proposed and the printing of metal-oxide patterns with well-defined shapes was demonstrated. Excellent thin-film transistors with channel lengths around 500 nm were completely printed by this method in an air atmosphere. This printing utilizes a viscoelastic transformation of the precursor gel when imprinted; it softens at a certain temperature during thermal-imprinting so that the gel can be rheologically imprinted. The imprinted pattern shows very small shrinkage during post-annealing, thereby achieving a high shape fidelity to the mould; this results from metal-oxide condensation at imprinting. The viscoelastic transformation and metal-oxide condensation at imprinting constitute the basis for this printing method, which is closely related to the cluster structure in the precursor gel. This method has worked for patterns down to several tens of nanometers.

Ferroelectric-Gate Thin-Film Transistor Fabricated by Total Solution Deposition Process

We have fabricated inorganic ferroelectric-gate thin-film transistors (FGTs) using only a chemical solution deposition (CSD) process. All layers, including the LaNiO3 (LNO) gate electrode, Pb(Zr,Ti)O3 (PZT) ferroelectric-gate insulator, indium–tin-oxide (ITO) source/drain electrodes, and ITO channel, were formed on a SrTiO3 (STO) substrate by the CSD process. We obtained a local epitaxially grown PZT/LNO perovskite heterostructure with good crystalline quality and no interfacial layer. The fabricated FGT exhibited typical n-channel transistor operation, with a counterclockwise hysteresis loop due to the ferroelectric nature of the PZT-gate insulator, and also exhibited good drain current saturation in output characteristics. These properties are equivalent to or better than those obtained with FGTs fabricated by means of conventional vacuum processes. The obtained on/off current ratio, memory window, and subthreshold voltage swing were about 106, 2.5 V, and 357 mV/decade, respectively.

Optimization of Pt and PZT Films for Ferroelectric-Gate Thin Film Transistors

The polycrystalline Pt film with an excellent (111) orientation and a small grain size of about 30 nm was successfully prepared on a SiO2/Si substrate by the new structured-sputtering system. By optimizing annealing process and using the highly (111)-oriented Pt film as a bottom electrode, an epitaxial-grade (111)-oriented PZT film was successfully prepared by the sol-gel method. Operation of the ferroelectric-gate thin film transistor (FGT) with indium-tin-oxide (ITO) channel, which was based on the optimum Pt and PZT films, has been verified. The FGT device exhibited good properties and performance with high “on/off” current ratio (∼105), adequate memory window (1.2 V) and small swing factor (∼88 mV/decade).

Low-Temperature All-Solution-Derived Amorphous Oxide Thin-Film Transistors

We prepared thin-film transistors (TFTs) in which all the layers were fabricated using simple chemical solution-processed, vacuum-free routes, followed by thermal annealing at 400°C. A ruthenium oxide film prepared via low-temperature processing was used for both gate and source/drain electrodes. Amorphous lanthanum-zirconium oxide and zirconium-indium-zinc oxide films were used as the gate insulator and channel layer, respectively, which enabled the fabrication of a TFT with the desired performance at a sufficiently low temperature. The ultraviolet-ozone treatment was adopted to channel layer to facilitate precursor decomposition and condensation processes. As a result, the obtained ON/OFF ratio, subthreshold swing voltage, and channel mobility were ~ 6×105, 250 mV/decade, and 5.80 cm2V1 s-1, respectively. This result contributes to the development of sustainable completely printed inorganic electronics.

All solution-processed amorphous oxide thin-film transistors using UV/O3 treatment

In the fabrication of amorphous oxide thin-film transistors (TFTs) by all-solution process, an ultraviolet–ozone (UV/O3) treatment and solution materials were adopted. By applying the UV/O3 treatment for solution-processed In2−xGaxZnO4 channel layers, enhancement of TFT characteristics was achieved. In particular, the most appropriate metal composition for the In2−xGaxZnO4 system with UV/O3 treatment was found to be x = 1.0. In addition to the channel layers, solution-processed LaNiO3, Bi–Nb–O/La–Ta–O stacked layer, and ITO films were formed as the gate electrode, gate insulator, and source and drain electrodes, respectively, for TFT fabrication. Using UV/O3 treatment and solution materials, all-solution-processed amorphous oxide TFTs were successfully fabricated, and superior TFT properties, including an on–off current ratio of 107, a threshold voltage of 1.6 V, a subthreshold swing of 200 mV/decade, and a field-effect mobility of 0.49 cm2 V−1 s−1, were achieved.