Research

-We develop useful things using oxides-

In our laboratory, we focus on functional oxides, so called ceramics. We fabricate high-quality thin films with atomically flat surface. We extract the intrinsic performance of functional oxides. We challenge to develop novel devices. Especially, we are trying to develop “Optic, electric, and magnetic memory devices”, “Thermoelectric materials”, “Spintronic devices” based on functional oxides. Further, we develop “Special epitaxial growth technique” to fabricate high-quality thin films of functional oxides.

Thermoelectric materials

After “Giant Thermoelectric Seebeck coefficient of a Two-dimensional Electron Gas in SrTiO3”, Nature Mater. 6, 129–134 (2007).

 

thermoelectric

Thermoelectric energy conversion technology attracts great attention to convert the waste heat into electricity. Recently, metal oxides attract much attention as thermoelectric power generation material operating at high temperatures on the basis of their potential advantages over heavy metallic alloys in chemical and thermal robustness. We have fabricated high quality epitaxial films of oxide thermoelectric materials, which are suitable to clarify the intrinsic “real” properties. Now we are trying to clarify the origin of giant thermopower of extremely thin conducting oxide toward realization of truly practical oxide thermoelectric materials. (Hiromichi Ohta)

Reviews

[1] H. Ohta, Mater. Today 10, 44 (2007). [2] H. Ohta et al., Inorg. Chem. 47, 8429 (2008). [3] H. Ohta, J. Mater. Sci. 48, 2797 (2013).

Original Papers

[1] S. Ohta, H. Ohta et al., J. Appl. Phys. 97, 034106 (2005). [2] S. Ohta, H. Ohta et al., Appl. Phys. Lett. 87, 092108 (2005). [3] H. Ohta et al., Nature Mater. 6, 129 (2007). [4] Y. Mune, H. Ohta et al., Appl. Phys. Lett. 91, 192105 (2007). [5] K-H. Lee, H. Ohta et al., Appl. Phys. Express 1, 015007 (2008). [6] W-S. Choi, H. Ohta et al., Phys. Rev. B 82, 024301 (2010). [5] H. Ohta et al., Nature Commun. 1, 118 (2010). [6] H. Ohta et al., Adv. Mater. 24, 740 (2012). [7] W-S. Choi, H. Ohta et al., Adv. Mater. 26, 6701 (2014). [8] W. S. Choi, H. Ohta et al., Adv. Funct. Mater. 25, 799 (2015).

Optic, electric, and magnetic memory devices

“A Transparent Electrochromic Metal-Insulator Switching Device with Three-Terminal Transistor Geometry”, Scientific Reports 6, 25819 (2016).

 

electrochemicalmodulation

The optic, electric, and magnetic properties of many transition metal oxides can be switched by their non-stoichiometry i.e. oxygen excess or deficiency and protonation. For example, WO3, known as an electro-chromic material, is basically transparent insulator, but it becomes blue colored metal by electrochemical protonation (HxWO3). SrCoO2.5 with Brownmillerite structure is known as insulating non-magnet, but it can be changed into SrCoO3 with Perovskite structure, which is ferromagnetic metal. Thus, memory devices, which can switch optical transmission and electrical conductivity or magnetism and electrical conductivity, can be realized by using transition metal oxide appropriately. For transition metal oxides, water is a strong reductant (H+) as well as an oxidant (OH). Although such memory devices can be realized by using liquid electrolytes for electrochemical reaction, there is liquid leakage problem. We have developed “liquid-leakage-free water”, in which water molecules are infiltrated in a nano-porous glass. By using “liquid-leakage-free water”, we can switch optic, electric, and magnetic properties of transition metal oxides. (Hiromichi Ohta)

Reviews

[1] H. Ohta, J. Mater. Sci. 48, 2797 (2013).

Original Papers

[1] H. Ohta et al., Nature Commun. 1, 118 (2010). [2] H. Ohta et al., Adv. Mater. 24, 740 (2012). [3] T. Katase, H. Ohta et al., Adv. Electron. Mater. 1, 1500063 (2015). [4] T. Katase, H. Ohta et al., Adv. Electron. Mater. (2016). [5] T. Katase, H. Ohta et al., Sci. Rep. 6, 25819 (2016).

Special epitaxial growth technique

“Surface Modification of Glass Substrate for Oxide Heteroepitaxy: Pastable Three-dimensionally Oriented Layered Oxide Thin Film”, Advanced Materials 18, 1649–1652 (2006).

 

 

reactivesolidphaseepitaxy

As represented by YBa2Cu3O7, which is known as a high Tc superconductor, most complex oxides are considered to have a complicated, layered crystal structure. Since such complex layered oxides are regarded as superlattices in the atomic/molecular layer laminates, one may expect that they show a variety of interesting physical properties. Single-crystalline films are indispensable to fabricate thin-film devices, but it is not easy to fabricate single-crystalline films of complex layered oxides. For example, if you try to fabricate single-crystalline films of InGaO3(ZnO)m (m: integer) by standard vapor deposition method, you will fail because the vapor pressure differential is so large. Even a single phase of InGaO3(ZnO)m may not be fabricated. A perfect solution in such a case is namely “Reactive Solid-Phase Epitaxy (R-SPE)” (Fig.) [1]. The single-crystalline InGaO3(ZnO)m films, which can be fabricated by our R-SPE method, are transparent in the entire visible light region, showing high field-effect mobility equivalent to that of polycrystalline Si-TFT [2]. In addition, an amorphous InGaZnO4 thin film has been applied for mobile phone recently as a thin-film transistor (IGZO-TFT) material though it is not a single-crystalline film [3]. (Hiromichi Ohta)

Reviews

[1] H. Ohta and H. Hosono, Mater. Today 7, 42 (2004). [2] H. Ohta, J. Ceram. Soc. Jpn. 114, 147 (2006).

Original Papers

[1] H. Ohta et al., Adv. Funct. Mater. 13, 139 (2003). [2] K. Nomura, H. Ohta et al., Science 300, 1269 (2003). [3] K. Nomura, H. Ohta et al., Nature 432, 488 (2004). [4] H. Ohta et al., Cryst. Growth Des. 5, 25 (2005). [5] K. Sugiura, H. Ohta et al., Appl. Phys. Lett. 88, 082109 (2006). [6] K. Sugiura, H. Ohta et al., Appl. Phys. Lett. 89, 032111 (2006). [7] A. Mizutani, H. Ohta et al., Cryst. Growth Des. 8, 755 (2008). [8] K. Sugiura, H. Ohta et al., Appl. Phys. Lett. 94, 152105 (2009).

Spintronic devices

spintronics

Spintroncs devices, utilizing the electron spin and charge degrees of freedom, are attracted much attention because of their potential for reducing power consumption of electronics. We are focusing on functional oxides for realizing them. In particular, in oxide half metals such as La0.67Sr0.33MnO3, direction of conduction electron spin is polarized. By using them, realization of high perfomance nonvolatile memory, nonvolatile logic and magnetic sensor is expected. Moreover, in oxide heterostructures such as SrTiO3/LaAlO3, it has been reported that the Rashba effective magnetic field acts on the conduction electron spin. Now we are working on developing new functional spintronics  devices based on these electronic and spin states in functional oxides. (Michihiko Yamanouchi)