Site: RIKEN (Institute of Physical and Chemical Research)
Frontier Materials Research
Semiconductor Laboratory
2-1 Hirosawa Wako-shi
Saitama 315-01, Japan
Fax: (81) 48-462 4659

Date Visited: 22 July 1997

WTEC: E. Hu (report author)

Hosts:

BACKGROUND

The Frontier Research Program was set up within the Institute of Physical and Chemical Research (RIKEN) within the Science and Technology Administration to be a more flexible program emphasizing

All programs have a fixed lifetime of eight years, extendible, with a mid-program review. The Frontier Materials Research program that the WTEC team visited was in Phase II of its activities.

Within the Frontier Materials Research Program were three subareas: (1) the Laboratory for Nano-Electronics Materials (Sugano), (2) the Laboratory for Nano-Photonics Materials (Sasabe), and (3) The Laboratory for Exotic Nano Materials (Knoll). The emphasis is on basic research, rather than on applications (this was explicitly stated).

RESEARCH AND DEVELOPMENT HIGHLIGHTS

Dr. Aoyagi gave the WTEC team an introduction to some of his team's research activities.

Quantum wire growth: they used a technique similar to that developed by Kapon for the growth of (primarily) GaAs/AlGaAs and GaP/AlGaP quantum wires. The attempt was to improve that process and gain better control of the growth process, with a higher selectivity of incorporation, using the fact that the growth rates on the (111)A plane is minimal to zero. Measurements were done in a 40 T magnetic field; the researchers expected to observe a diamagnetic shift in the luminescence peak under the high field conditions, and obtained 123 eV/T2 for the LH transition and 210 eV/T2 for the HH transition. These are far larger shifts than expected (110 eV/T2 for bulk and 20 eV/T2 for 2-D systems). Aoyagi attributes the discrepancy to the influence of the interaction of the electrons in the wire with the adjacent impurities.

Si nanostructure formation: these studies began with amorphous silicon deposited onto Si substrates and annealed in a hydrogen or nitrogen ambient. The result was the formation of Si nanocrystals, ~ 7 nm in size, embedded within an amorphous matrix. Emission in the blue was observed, with about 10-5 stated efficiency. Emission at 420 nm and 380 nm was observed. Simulations have been carried out to look at the effects of confinement on the relative regions of the amorphous and crystalline areas (Figure D.8).

 


Figure D.8. Effects of confinement on the relative regions of the amorphous and crystalline areas.

GaN dot formation: dot formation is attempted on a nearly lattice-matched substrate: GaN on AlGaN (thin buffer layer, grown on SiC, ~ 10-7 defects/cm2). Growth is believed to proceed by step-flow. In order to promote dot formation (without the influence of strain), researchers chose to control the surface energy, by using a monolayer of silicon as a surfactant (this has been published in APL). They have achieved stimulated emission in these dots, with a density of greater than 109/cm2.

Transport: Dr. Aoyagi showed the WTEC team quantum dots formed by split gate structures, with a separate gate that allowed coupling between 2 dots. He observed interference fringes in the I-V, indicative of coupling, measurements taken at 10 mK. He also showed magnetoCoulomb oscillations, using the magnetic field rather than a gate to alter the interactions (Figure D.9).


Figure D.9. MagnetoCoulomb oscillations using the magnetic field.

Dr. Aoyagi then took us for a brief tour of various labs in his area: a transport lab with three cryostats, an He 3/4 dilution refrigerator, and high field magnets (8 T); a JEOL e-beam writer, MOCVD capabilities (MBE was elsewhere), analysis lab with SEM, FIB, and PL.

The WTEC team was then provided with some overviews of the research in the Exotic Nano Materials group. Dr. Katsuhiko Fujita discussed some of the projects within this group, headed by Dr. Wolfgang Knoll. He described a supramolecular architecture, building from a substrate to a metal layer, to a biological interface, to proteins. Dr. Fujita showed poster projects of

Dr. Takashi Isoshima, a researcher in the Biopolymer Physics Lab (degree from Tokyo University), then described some of the experiments involving polymers for optical devices: ultrafast optical switching, low power consumption photorefractives. This group carries out its own polymer synthesis, molecular design, and modification, in order to enhance optical properties such as nonlinearity and absorption. The goal is to synthesize multicomponent, photorefractive materials with capabilities in electrooptic coefficient, photoconductivity, etc. He showed us an impressive optical characterization lab: subpicosecond systems, a time-resolved fluorescence setup (100 fs), three optical benches, and optical fiber devices for ultrafast multiplexing.

Dr. Hideo Yabuki then gave us a general overview of RIKEN, its history and current mission. RIKEN is a semipublic corporation, receiving 95% of its funding from the Science and Technology Agency. It is believed that this semipublic status gives it more autonomy. There is extensive collaboration with universities (University of Tokyo, Tokyo Institute of Technology, and others), with a number of dual appointments and joint doctoral courses. RIKEN collaborates extensively on an international scale and has established a number of laboratories outside of Japan; the Rutherford Appleton Laboratory was the first overseas lab to be established.

Yoshiro Miki, the Director of the Frontier Research Program Division and the Brain Science Planning Office (he has been at STA, in Materials Research, and then at MITI on an excimer laser project), spoke of a new priority project : a Brain Research Institute. Citing a research effort that is only 10% of that carried out in the United States, promoters of the Brain Research Institute hope that this will help to bring "brain science" in Japan up to world-class levels. Miki also spoke of the disconnect between RIKEN and the universities in terms of identifying important research priorities; there is a 10-15 years lag between initiation of RIKEN priority programs and observation of changes in university programs.

EQUIPMENT

Various laboratory equipment included a synthesis lab, with Langmuir Trough, Brewster angle microscope, an analysis laboratory with two STMs, a UHV STM, and AFM. He described the development of a surface plasmon resonance microscope (magnification limited), and a near field optical microscopy facility that is being built under the coordination of Dr. Ruggiero Micheletto (current resolution is 100 m).


Published: September 1999; WTEC Hyper-Librarian