Overview of Quantum Physics and Electronics Research
Takaaki Mukai
Physical Science LaboratoryOur 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. 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 inter-actions 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) and nano-probing (direct nano-scale imaging of electronic states by low-temperature STM). Wide-Bandgap Semiconductor Research Group
(１) High-quality GaN crystal growth by MOCVD and device processing technology. (２) GaN semiconductor electronic/optical device physics (FET, HBT, LED and LD). (３) Electron field emission in AlN cold cathode materials. (４) High-quality diamond epitaxial growth and its characterization. Major results obtained this fiscal year 2001 are reported in the following pages.
We have successfully measured the local density of states (LDOS) distribution by differential conductance measurement using scanning tunneling microscopy (STM) at low temperature. It is confirmed that the measured LDOS distributions of a semiconductor nanostructure coincides with the probability distributions of the zero-dimensional eigenstates. This clearly demonstrates that we can now directly observe the nano-scale phenomena predicted by quantum mechanics, as if we looked through a microscope.
Applying perpendicular magnetic field to two parallel layers of two-dimensional electron gases separated by a thin tunnel barrier (electron bilayer system), two sets of Landau levels, i.e., originating from the two subbands energetically separated by the tunneling gap, give rise to many level crossings. We found there exists a finite energy gap even at the level crossing point. This is a new class of integer quantum Hall effect which relies solely on interactions.
We fabricated light emitting diodes (LEDs) having AlGaN active layer whose possible emission wavelength covers from 200 to 360 nm, and demonstrated high output power of 10 mW, i.e., one order of magnitude larger output than previous reports, at 352 nm wavelength. Present external quantum efficiency is still low around 1 %, however, the internal quantum efficiency has already been high enough as large as 80 %. This demonstrates that nitride materials containing Al have strong potential for highly-efficient light emitting devices.
We fabricated Npn-type heterojunction bipolar transistor (HBT) applying our originally developed p-InGaN layer with high hole concentration to a base layer. The measured maximum current gain was as high as 20, which is a world-record for nitride-based HBT. This demonstrates that the crystal quality of our p-InGaN layer is good enough and wide-bandgap nitride semiconductors show promise for high-power electronic devices.
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