Overview of Quantum Physics and Electronics Research
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 electronic state 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 the 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 (two dimensional carrier transport and correlation effects in high-mobility semiconductors, carrier interactions in bilayer (electron-electron and electron-hole) systems, and interactions between nuclear-spin and conduction electrons in semiconductors). (2) Quantum electronic state control in quantum dot systems (electronic properties of vertical and lateral semiconductor quantum dots (artificial atoms and molecules), spin control in quantum dots, carrier dynamics of quantum dots, and fundamental properties of solid-state quantum computers using artificial molecules). (3) Semiconductor nano-mechanical systems (fabrication of nano-mechanical structures and their characterization) and nano-probing (direct nano-scale imaging of electronic states by low-temperature scanning-tunneling-microscopy (STM) technique).
Wide-Bandgap Semiconductor Research Group
(1) High-quality GaN crystal growth (mechanism of GaN and facet-controlled crystal growth by MOCVD, high-concentration p-type doping, and device processing technology). (2) GaN semiconductor device physics (electronic and optical properties of GaN quantum well structures, electronic devices suitable for high-temperature operation, and short-wavelength light emitting devices). (3) Electron field emission in AlN cold cathode materials (field emission characteristics, such as threshold field and available emission current, as a function of Si-dopant density).
Major results obtained this fiscal year 2000 are reported in the following pages. We experimentally investigated Kondo effect caused by spin correlated transport in well-defined semiconductor quantum dots. We successfully demonstrated the unitary limit Kondo effect, i.e., theoretical maximum in conductance. We also discovered novel aspect in Kondo effect that appears when even number of electrons are involved. These results promise controllable electron-spin correlation in quantum dots, leading to future "quantum correlated electronics".
We have successfully imaged electron waves (Friedel oscillations) in real space by applying low-temperature STM technique. Different from localized electron waves at surface boundaries so far observed, our direct observations were carried out for conducting electrons in two-dimensional electron gas accumulated in near-surface region of InAs. We believe that this technique will be indispensable for fully understanding ultimately small devices.
Besides conventional electronic and photonic device applications of nitride-based semiconductors, we demonstrated novel property favored in wide bandgap materials, i.e., highly efficient electron-field-emission in heavily Si-doped AlN. Available emission current density obtained in our AlN has already exceeded the maximum value realized so far in diamond, which is exciting enough for future applications to field-emission display and so on.
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