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Create new high-performance electron devices
using functional materials

TOKUMITSU Lab.
Professor:TOKUMITSU Eisuke

E-mail:
[Research areas]Semiconductor devices, Electronic materials
[Keywords]thin film transistor, oxide semiconductor, ferroelectric material, transition metal dichalcogenide

Skills and background we are looking for in prospective students 

Basic knowledge of physics and chemistry, and electrical engineering. Intellectual curiosity and challenging spirit to try to make something new. Logical thinking ability to analyze experimental results.

What you can expect to learn in this laboratory

The objective of the laboratory is to create novel electron devices by utilizing functional materials such as oxide semiconductors, transition metal chalcogenide 2D materials, and ferroelectric materials. In addition, new concepts and structures of electron devices have been investigated. The members of this laboratory learn basic knowledge of device physics, thin film deposition and device fabrication process along with evaluation technologies of thin films and electron devices.

【Job category of graduates】
Manufacture, development and research work of electronics and materials related companies

Research outline

We create new electron devices using functional materials. Recent research activities include large-charge-controlled thin film transistors, solution process of oxide and transition metal chalcogenide materials, and device fabrication using direct nanoimprint of functional materials.

1. Large-charge-controlled thin film transistor

Since ferroelectric materials have non-volatile memory function in its nature, they are used for non-volatile memory integrated circuits. In addition, ferroelectric materials have one more interesting feature, that is a fact they can induce very large charge density compared to conventional dielectric materials. For instance, maximum induced charge density of SiO2 which is common insulator of silicon MOSFET, is limited to 3.5 μC/cm2 at a breakdown electric field of 10 MV/cm, whereas ferroelectric materials such as PZT can induce as large as 50 μC/cm2 at even 0.5 MV/cm. We have proposed a new concept thin film transistor (TFT) with conductive channel controlled by the anomalously large charge density of the ferroelectric gate insulator. Until now, we have developed indium-tin-oxide (ITO) channel TFTs, transparent nonvolatile memory TFTs, and flexible oxide channel TFTs using this concept.

2. Oxide channel thin film transistor by solution process

Silicon integrated circuits have achieved advanced development due to the miniaturization of MOSFET. However, high-cost complicated equipment is required to fabricate advanced integrated circuits at present. On the other hand, solution process is one of the techniques which can fabricate various kinds of thin films with low energy at low cost. In addition, printing technologies can be used in the solution process, which makes it possible to fabricate electron devices without using conventional lithography. In this laboratory, we are developing new fabrication techniques based on solution process and nanoimprint technology for oxide-channel TFTs. We have demonstrated ITO-channel ferroelectric-gate TFT fabricated by a single nanoimprint step [1].

3. Solution process for metal chalcogenides


Fig.1 TEM cross section of solution-derived MoS2 film

Two dimensional transition metal dichalcogenide (TMDC) materials such as MoS2 and WSe2 are promising for future electronics, because they exhibit high carrier mobility. At present, the TMDC films have been fabricated by mechanical exfoliation from single crystals or chemical vapor deposition. In this laboratory, we have investigated chemical solution process of TMDC materials. We have obtained MoS2 formation as shown in Fig.1. In addition, we succeeded TFT operation with solution-derived MoS2 channel directly fabricated on a high-k gate insulator.

Key publications

  1. Ken-ichi Haga, Yuusuke Kamiya, and Eisuke Tokumitsu, “Direct imprinting of indium-tin-oxide precursor gel and simultaneous formation of channel and source/drain in thin film transistor”, Jpn. J. Appl. Phys., Vol.57, 02CB14 (2018)
  2. Joonam Kim, Koichi Higashimine, Ken-ichi Haga, and Eisuke Tokumitsu, “Fabrication of MoS2 thin films on oxide-dielectric-covered substrates by chemical solution process”, Phys. Status Solidi B, Vol.254, 1600536 (2017).
  3. Ken-ichi Haga, Yuuki Nakada, Dan Ricinschi, and Eisuke Tokumitsu, “Relationship between source/drain-contact structures and switching characteristics in oxide-channel ferroelectric-gate Thin-Film Transistors”, Jpn. J. Appl. Phys., Vol.53, 09PA07 (2014)

Equipment

Rapid thermal annealing (RTA)
Sputtering system
Nanoimprint system

Teaching policy

In our laboratory, in order to create a new electronic device, we would like to know "feelings" of functional materials and use them properly for devices applications. To realize this purpose, we should have basic knowledge of physics and chemistry to understand material physical properties, along with operation principles of devices. We also need flexible thinking, Innovative ideas, a crucial decision. Let’s consider these factors together through experiments and create a new device that made use of your idea by the end of the course.

[Website] URL:http://www.jaist.ac.jp/ms/labs/tokumitsu/

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