Overview of Quantum Physics and Electronics Research

Yoshiro Hirayama
Physical Science Laboratory

 Our research in the fields of quantum physics and electronics, which is based on semiconductor nano-structures fabricated by high-quality semiconductor crystal growth and advanced device fabrication techniques, focuses on quantum coherent control, carrier interactions and wide-bandgap semiconductor physics. Our aim is the development of innovative semiconductor devices. Quantum Solid State Physics Research Group and Wide-Bandgap Semiconductor Research Group are working in the following areas.

Quantum Solid State Physics Research Group
(1) Carrier interactions in semiconductor heterostructures (carrier interractions in bilayer systems, interactions between nuclear-spin and conduction electrons).
(2) Quantum electronic state control in quantum dot systems (spin/charge control, carrier dynamics of quantum nanostructures, fundamental properties of solid-state quantum computers).
(3) Semiconductor nano-mechanical systems (fabrication and characterization).
(4) Direct nano-scale imaging of electronic states by low-temperature STM.

Wide-Bandgap Semiconductor Research Group
(1) Optical device physics in ultra-violet LEDs and optical devices using micro-facets.
(2) Electronic device physics, such as carrier transport in nitride FETs and HBTs.
(3) Impurity doping into wide-bandgap semiconductors and its characterization.
(4) High-quality diamond epitaxial growth and its application to electronics devices.
(5) Developing new semiconductor materials such as InN.

 Major results obtained this fiscal year 2003 are reported in the following pages.
 We have successfully applied electrical pump and probe technique to control a single electron motion in a coupled quantum dot coherently. We have demonstrated one qubit operation, i.e. arbitrary rotation on the Bloch sphere, for the semiconductor charge qubit. This is the first step towards physical realization of electrically-controlled semiconductor quantum computer.
 In a high magnetic field, electron interactions in the two-dimensional electron gases result in the fractional quantum Hall effects. In this regime, two different electron spin states energetically degenerate in a certain condition. We have found that a novel interaction between electron and nuclear spins occurs in such condition and this interaction is very sensitive to the electron spin state. We have also shown a possibility to control nuclear spin polarization in a mesoscopic scale.
 By using high-quality diamond films grown by MOCVD, we have fabricated a diamond field-effect transistor (FET) in collaboration with Ulm University. The fabricated FET exhibited the highest cut-off frequencies among diamond FETs and showed the first amplification in the millimeter-wave range (30〜300 GHz).
 We have also successfully fabricated an npn-type GaN/InGaN heterojunction bipolar transistor (HBT) using the base regrowth technique. The fabricated HBTs show high current gains and super-high-power density of 230,000 W/cm2.

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