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The Ohkawa Laboratory has been introduced
in the US goverment Web site below. (This is a copy of the article in Vol. 30.) The Science University of Tokyo offers one of the oldest physics curricula in Japan - 120 years old. The University, split between 2 campuses, graduates approximately 20% of all Japanese educated physics students. The total student population is 18,000, with 500 professors, and about 550 students graduated from 4 physics curricula every year. Two years ago the University got into Nitrides.
Prof. Kazuhiro Ohkawa -- formerly of Matsushita
Central Research Laboratories, Honorary Professor
at the University of Bremen, and inventor
of p-type doping in ZnSe-based systems --
heads the new nitrides laboratory (http://www.rs.kagu.tus.ac.jp/~okwlab/signature.htm). Dr. Ohkawa invented not only p-type doping
by nitrogen plasma source but also n-type
doping for ZnSe-system by using Cl donors.
These remain, to date, the only way to realize
II-VI blue-green laser diodes. Initially
involved in MBE II-VI compounds, he found
growth techniques to realize high quality
II-VI semiconductor compounds, and now he's
into MOVPE Nitrides. Besides extensive state-of-the-art
characterization equipment, the Lab has unusual
capabilities: both a new NIPPON SANSO MOVPE
reactor and the software ability to understand
the metal-organic-vapor-phase-epitaxy (MOVPE)
growth process and reactions. In MOVPE, achieving
both good Hall (carrier) mobility and efficient
growth is very difficult, so this software-reactor
combination is important for optimizing growth
and attaining high mobility. To optimize
growth conditions, they currently do not
use a low-temperature buffer layer. High-quality
GaN has been obtained, and the next plan
is to obtain high-quality layers with the
use of a low-temperature buffer layer. Prof.
Ohkawa can perform 1-, 2-, and 3-flow MOVPE
simulations with varying concentrations.
(Maurice) Contract awarded: "Studies of the interface between nitride semiconductors and a solution," Dr. Kazuhiro Ohkawa, Department of Applied Physics, Science University of Tokyo, Tokyo, Japan, February 2001: Nitride semiconductors are attracting special attention in many applications due to astounding achievements of one of the collaborators on this project, Professor Shuji Nakamura, now of UCSB. While the means for growing high quality GaN heteroepitaxial layers had been successfully established by Dr. Nakamura while in Japan, the chemical characteristics of nitrides remain unclear. How nitrides behave in chemical reactions, in particular, is not well understood. Suitable techniques for wet etching thus remain undeveloped. Because defects in the material (imperfections in crystallinity) create dangling chemical bonds that react chemically, an improved understanding of their role and the interfacial physics involved will be valuable for developing wet-etching techniques. Preferable to dry etching in order to realize atomically flat surfaces, wet etching is fundamental for realizing precisely controlled structures and to subsequently yield efficient devices. Such techniques are lacking due to the same chemically stable feature of nitrides that makes them so attractive and ideally suited for many applications. That is, the stability of nitrides obstructs etching by chemical solution. This project investigates 1) the growth of GaN and AlInGaN alloys and heterostructures with p- and n-type doping grown by metal organic vapor phase epitaxy (MOVPE) and, additionally, 2) the role of light on nitride-solution reaction rates, an area in which no work has been done to date. In advancing an understanding of surface states and passivation of GaN, surface states can be reduced, in turn cutting the background leakage current and thus allowing increased sensitivity in, e.g., UV photodetectors. Dr. Ohkawa's expertise starts in MBE II-VI compounds. He invented not only p-type doping by nitrogen plasma source but also n-type doping for ZnSe-system by using Cl donors. These remain, to date, the only way to realize II-VI blue-green laser diodes. Besides extensive state-of-the-art characterization equipment, Dr. Ohkawa's lab has an unusual capability: both a new NIPPON SENSO MOVPE reactor and the software ability to simulate the complex 2-flow MOVPE process and reactions. These (the MOVPE growth process and the reactions involved) are very geometry dependent. An understanding of the reactor and its boundaries is required in order to optimize the growth conditions and is accomplished by computational fluid dynamics. Using this combination and their ample experience in materials growth and characterization, Dr. Ohkawa and co-investigator Dr. Karasawa (USHIO Inc. R&D Center, formerly of Matsushita and ATR), can perform 1-, 2-, and 3-flow MOVPE with varying concentrations. Together they have achieved high-quality growth of nitrides with and without the use of a low-temperature buffer layer. (See related article in ASL 30.) POC: Dr. Dan Johnstone, AFOSR/NE. (Maurice) |
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