Research

1. Optical study of semiconductors, electronic materials, and their nano-structures by using spectroscopic ellipsometry

  Spectroscopic ellipsometry is a versatile and powerfull optical spectroscopy which estimates the dielectric functions
and the thicknesses of multilayers. It measures the ellipsometric angles ( Psi, Delta) which are related to the ratio of the reflectivity
of p- and s-polarization. Using multi-layer modeling analysis on the ellipsometric angles, we can estimate the complex dielectric functions 
which are the square of the complex refractive indexes. From the dielectric functions, we can determine the critical points
which are the symmetry points in the electronic band structure where the conduction band and the valence band is parallel.

2. Real-time spectroscopic ellipsometry including temperature dependence and in situ etching etc.

Real time spectroscopic ellipsometry usually uses white light source and a very fast detection system such
as charge coupled detector (CCD) because we measure the samples which are under growth or under rapid etching.
They are usually attached to a vacuum chamber or plasma etching chamber. Therefore, we can study a crystal growth mode
or surface morphology (states) during etching.

3. Materials and Devices of non-volatile  memories- PRAM and RRAM

Non-volatile random access memory (NVRAM) device does not lose memory even when the applied voltage is turned-off.
Presentl, flash memory made of silicon is widely used. However, it has a slow speed and a limit to device size. In order to circumvent
the disadvantages, other NVRAMs are intensively investigated in the leading world laboratories. PRAM (Phase change RAM) utilizes
high  resistance (amorphous) and low resistance state (crystalline phase) of chalcogenides materials such as Ge2Sb2Te5,
where phase change is induced by Joule heat. RRAM (resistance switching RAM) devices uses the resistance switching of oxides materials
induced by a sort of filmentation induced by the applied electric field. The resistance switching does not involve phase change,
but  creation of current path in the medium. The mechanism of RRAM is not established and is believed due to both
the mobile oxygen ions and inetrface properties with electrodes. Using ellipsometry and I-V measurement systems, we can investigae
the physical properties of the NVRAMs.

4. Electronic band structure of organic semiconductors e.g. functionalized DNA thin films

Bio-molecules and polymers are of increasing interest in particular in the field of nanotechnology and the development of new functional materials.
Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions. Chemically, double strand DNA(dsDNA) consists of
two long polymer chains of simple units called nucleotides, with backbones made of sugars and phosphate groups joined by ester bonds.
These two strands run in opposite directions to each other and are therefore anti-parallel. Attached to each sugar is one of four types of
molecules called bases. In addition to its size in the nanometer scale range, the DNA assembles spontaneously. It is believed therefore
to be a promising candidate for designing new nanostructured materials. The possibility to use the dsDNA as a programmable material
for nanotechnology applications has sparked intensive research in the field. Structural modifications of water-soluble natural DNA
and synthesis of organic-soluble DNA are of high technological interest in diverse fields such as development of bio-sensors
or new types of electronic and optoelectronic devices. A central issue for designing such organic hybrid devices is the determination of structural,
electronic and optical properties of organic films. By using ellipsometry and Raman spectroscopy, we investigated the electronic propeerties
of functionalized DNAs.

LED made with CTMA DNA as hole transport layer.