Research Fields

Interface and surface have become the valuable sciences fields because they are important not only for the application fields such as devices but also for the basic physics such as understanding the crystal growth. In this view, our groups are studying the crystal structures, electronic structures, and optical properties of various exotic materials; bulk semiconductors, burried interfaces, growing surfaces etc., by using first principles and model calculations. Our recent interests are as follows;

Heterointerfaces;
When two materials touch with each other, the interface between them show different properties from the bulk materials. The surface is one of interfaces because the vacuum is "one material". Most phenomena in nature occurrs at interfaces. In this view, our group is studying the physics of interfaces from the total viewpoints of crystal structure, electronic structure (band offsets etc.), and conductive and optical properties, especially the physics of semiconductor hetero interfaces.

For example,
GaAs/AlAs (standard heterostructures)
strained interfaces (II-VI such as ZnSe/BeTe)
heterocrystalline interfaces (cubic/hexagonal interface etc.)
heterovalent interfaces (such as ZnSe/GaAs)
the origin and classification of band offset
semiconductor/insulator interfaces such as III-V/I-VII
semiconductor/metal interfaces
Surfaces;
Recently optical measurements such as RDS (Reflectance Difference Spectroscopy) and SPA (surface Photo Absorption) have received intensive attention to observe growing surfaces of various materials. Since the optical response reflects transitions between electronic states of system, one can know the bond dynamics of surface atoms from observed spectra. There is, however, no theoretical method to analyze such spectra. In this view, our group has developed the theoretical method to calculate the optical response spectra of surfaces and interfaces from the first principles, applied the method to various semiconductor surfaces/interfaces, and clarified the atomic structures, the electronic structures, and the dynamical processes during crystal growth.
For example,
GaAs, ZnSe (001), (110), (111) surfaces
N,Se,S,Te-terminated GaAs and ZnSe surfaces
GaAs epitaxial growth and dynamical spectra
spectra of steps and kinks
first principles evaluation of SHG spectra
crystal growth of heterovalent ZnSe/GaAs syetem and defect formation
heteroepitaxial growth simulation
h-GaN/c-GaN growth on GaAs, atomic hydrogen effects
lattice vibration of adsorbants
Ordered Vacancy Compounds;
Misvalent atom combinations of III-V atoms such as Ga-Se crystalize with vacancies inside. What is the physics in these materials ? In this view, we have studied the origin of vacancy and crystal polymorphism, the electronic structures, optical and conductive properties, and lattice viblations.

For example,
monoclinic, cubic and hexagonal Ga2Se3, layered In2Se3, mesoscopic Ga2Te3
monoclinic and cubic Ga2Se3
chalcopirite vacancy-ordering systems such as II-III2-VI4 compounds
topology of vacancy ordering
localized and coherent vibration and Raman spctra
Photonic Bands
Inhomogeneity of dielectric function changes the energy disperdion of the photonic field and produces the band structures, the mechanism of which is similar to that of the electron field. However, both fields are quite different in that the electron field is spinor and has the interaction between electrons while the photon field is vector and has the interaction between photons only in materials. We have investigated the phenomena induced by such fundamental difference between two fields.
for example,
many-body effects on photonic bands
bound states of photons
anisotropy and polarization effects
Developements of First Principles Methods;
First principles (ab initio) calculation is the ultimate theory in physics, where starting from the unique Hamiltonian/Lagrangean having no adjustable parameters all the physical properties and phenomena of all materials are quantatively explained. Namely, only the input data are physical constants such as the charge unit, e, and electrom mass, m. We have been developing these methods.
For example,
pseudopotential methods in density functional theory
molecular dynamics
GW-quasiparticle and excitonic calculation
Green's function method at surfaces and interfaces
pressure-induced phase trantion, lattice vibration and stress
scale-N method
time-dependent calculation of response spectra
nonlinear response calculation
electron-phonon interaction and self-trapped exciton
etc.
Sorry, recent research topics are as follows, but the details are under construction (Please contact/mail me!)
1.Crystal growth mechanism on surfaces
2.Mesoscopic current under optical excitation
3.Surfactant effects on epitaxial growth
4.Control of crystal structure
5.Ab initio Raman scattering calculation
6.Etching process of Si surfaces
7.Deconstruction theory of crystals under external fields
etc.

Our group cheerfully accept young students as master/doctor (PHD) course researchers as well as posdoc researchers!!!! Please mail me.

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