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The 7th (2007) Yamazaki-Teiichi Prize Winner Material

Exploration of ZnO Based Semiconductor Technology

winner Winner
Masashi Kawasaki
History
Mar. 1989 Doctorate Degree (Dr. of Engineering), Department of Chemical Energy Engineering, School of Engineering, The University of Tokyo,
Apr. 1989 Research Fellow, Japan Society for the Promotion of Science
Sep. 1989 Postdoctoral Research Fellow, IBM Thomas J. Watson Research Center
Jul. 1991 Research Associate, Materials and Structures Laboratory, Tokyo Institute of Technology
Apr. 1997 Associate Professor, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology
Apr. 2001 Professor, Institute for Materials Research, Tohoku University
May 2001 Concurrently acted as Team Leader, Correlated Electron Research Center (CERC), National Institute for Advanced Industrial Science and Technology (AIST)
Oct. 2007 Concurrently acted as Team Leader, Cross-Correlate Materials Group, Frontier Research System, RIKEN
Oct. 2007 Professor, WPI Advanced Institute for Materials Research, Tohoku University
Present

Reason for award

Also known as "flower of zinc," zinc oxide (ZnO) has been frequently used in recent years, instead of white lead, as a white pigment or powder. Its powder sintered compact is an electronic ceramic widely used in the form of varistors, gas sensors, surface elastic-wave filters and the like. In addition, ZnO has a wide band gap (3.37 eV; 368 nm) and is a transparent semiconductor that transmits visible light. Thin films doped with various elements are attracting attention as new transparent conductive films.
Masashi Kawasaki focused on a variety of functions besides high-temperature super- conductivity of oxides and undertook research on sophisticated thin film crystal growth technology [laser molecular beam epitaxy (MBE)] to functionalize various oxides for promoting device research. This research included the selection of ZnO as a research subject in terms of its hidden possibilities as an optical functional material.

Background of research and development

In the practical electronic functions of oxide semiconductors, transparent conductive films, varistors and other passive functions have been primary, while materials-infrastructure to demonstrate active electronic functions had remained undeveloped. Masashi Kawasaki, the prizewinner, and his colleagues achieved technological innovation for fabricating high quality zinc oxide (ZnO) epitaxial thin films, and then demonstrated ultrahigh-efficiency, room-temperature ultraviolet (UV) lasing through exciton recombination with photoexcitation in 1996 to clearly show the possibilities for developing optical devices with oxide semiconductors. However, the tough issue toward practical application had been the polarity control of oxides that readily become n-type in nature.

Achievements

As the prizewinner's success in lasing clearly showed the possibilities of ZnO as a UV solid state lighting source, active research was conducted worldwide on the development of p-type ZnO and light-emitting diodes (LEDs). Immediately thereafter, announcements asserting success in p-type formation followed in succession, but problems were indicated with reproducibility and evaluation technology, owing to the Hall-effect and the like.In addition, there were no cases of success in manufacturing LEDs with p-n junctions, and the entire field was clouded by pessimism. In order to achieve valence control for the p-type, the prizewinner did not slavishly rely on acceptor doping. Instead, he suppressed donors originating from point defects, which brought ZnO into n-type, to the utmost and focused on the extent to which properties as intrinsic semiconductors could be determined. As a result, residual electron concentration below 1015cm-3 and electron mobility over 400cm2/Vs were achieved, realizing electronic properties that far surpassed those of gas-phase synthesized bulk single-crystal ZnO, which had been the most intrinsic semiconductor at that time.
In addition, he developed a new method for doping acceptors. Although substituting the oxygen in ZnO with nitrogen had been considered optimal for the p-type, because nitrogen readily evaporates, it had been difficult to mix it into crystalline thin films. Lowering thin-film growth temperature makes mixing somewhat possible, but crystallinity substantially deteriorates.The attempt to improve crystallinity by raising growth temperature introduces the dilemma of being unable to mix nitrogen in at all. The prizewinner conceived of a process involving the repeated high-speed raising and lowering of the growth temperature so that not only would nitrogen be sufficiently mixed in, but high crystallinity would be maintained, as well. As a result, p-type ZnO was actually synthesized for the first time. The emission of blue light from the ZnO pn junction could be confirmed with the naked eye, and UV emission was also demonstrated from the spectroscopic characteristics.

Meaning of the achievements

The results of this research deeply impressed semiconductor researchers in terms of the possibilities for oxides. His paper in Nature Materials, which was posted on the Web around 2004, has been frequently cited and ranked first in the world in the field of material science according to Thomson's Hot Paper statistics. From the perspective of material science, the demonstration, for the first time, of p-type conductivity with oxides not involving d-orbits is highly significant.Although the valence band is composed mainly of 2p-orbits, as hole conduction was demonstrated as possible for the first time through combination with metal's s-orbits, it can be said that Masashi Kawasaki has led research geared to the application of oxide semiconductors. In addition, his strategy to suppress defect formation in oxide semiconductors to the utmost and his proposal of a new method of impurity doping and its activation for valence control, will serve as valuable guidelines for the practical research to be conducted henceforth.
From a practical perspective, the demonstration of one more option for UV LEDs for solid-state lighting is also significant. Blue LEDs involving gallium nitride and white LEDs that have been comprised of blue LED and yellow light emitting phosphors have both been put to practical use and have penetrated society extensively: market scale for 2010 is slated to reach one trillion yen. Although development appears to be solid, there are problems with the supply and stable pricing of indium and gallium, which are used as raw materials. In addition, the red and green components for the current white LEDs are limited and inappropriate for comfortable lighting. Also the luminous efficiency of gallium nitride LED rapidly deteriorates in the UV region that makes pure-white LED possible. This problem originates from the defects introduced by the use of lattice mismatched sapphire substrates. The ZnO UV LED is attracting attention as a material that will make it possible to solve these problems all at once, owing to the fact that other material technologies other than p-type are in order, such as large ZnO single crystals for substrates, high-efficiency UV emission and the shortening of emission wavelength by the quantum effect in heteroepitaxial structures with using (MgZn)O.As a result of the prizewinner's research, emerging technical challenges are identified as an increase of hole concentration by an order of magnitude from what it is at present, 1017cm-3, and the establishment of p-type formation technology for (MgZn) O.

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