Overview of Quantum Electron Physics Research

Yoshiro Hirayama
Physical Science Laboratory

 Our research in the fields of quantum physics and electronics is based on semiconductor nano-structures fabricated by high-quality semiconductor crystal growth and advanced device fabrication techniques. We use these nanostructures to investigate quantum coherent control, carrier interactions, and wide-bandgap semiconductor physics. Our aim is the development of innovative semiconductor devices. The 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 low-dimensional semiconductor heterostructures (carrier interactions in bilayer systems; interactions between nuclear-spin and conduction electrons).
(2) Quantum electronic state control in quantum dot systems (spin control & carrier dynamics of quantum dots; 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) Developing new semiconductor materials such as InN and diamond.

 Major results obtained fiscal-year 2002 are reported in the following pages.
 We have successfully carried out electrical pump and probe measurements of quantum dot systems. We precisely measured electron relaxation time from the excited state to the ground state and found an extremely long relaxation time when electron relaxation is accompanied by spin flip. This means that electron spin is well separated from circumstances in a quantum dot system. We have also succeeded in observing coherent oscillation of an electron in a coupled quantum dot. This is the first demonstration of all-electrical control of a semiconductor qubit.
 We have studied interaction between a two-dimensional electron system and nuclear spins. In a certain condition, a driving current flowing in the electron layer polarizes nuclear spins. We have successfully demonstrated all-electrical control of nuclear spin polarization and relaxation. In nanomechanical systems, we have fabricated InAs based mesoscopic-scale cantilevers and have found high sensitivity arising from quantum effects.
 We have successfully fabricated an npn-type GaN/InGaN heterojunction bipolar transistor (HBT) using the base regrowth technique to improve ohmic characteristics. The HBT characteristics were drastically improved. This demonstrates that nitride HBTs are promising for the future high-power electronic devices.
 We have clarified the bandgap energy of InN, which was believed to be about 1.9 eV for more than a quarter century. We discovered that our experimental data did not support the previous value, so we grew a high-quality InN epitaxial layer to investigate its bandgap energy. Then, we showed that the real bandgap energy of InN is about 0.9 eV, which indicates that InN is a promising material for optical devices for communications.

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