Mitsuda-lab, Institute of Industrial Science, The University of Tokyo
Mitsuda Lab

  - Introduction -

Main object in our research is to develop novel synthesis methods of inorganic materials. Much attention has been paid to the modeling of the spatial and surface reactions of diamond, metal oxides and related materials in order to control basic properties of them. Some of recent topics in our research are shown as below.

•   Depositions and electrical characterizations of semiconducting diamond thin films

Diamond is a promising semiconductor material for the electro- and electro optical- devices because of its wide band gap (5.47 eV), high thermal conductivity (2000 W/mK), and high break down field (10 MV/cm). It is necessary to control the electrical conductivity and conduction types in order to realize diamond devices, and many researchers are concentrating on the doping techniques to fabricate semiconducting diamond thin films with high carrier mobility. It this project, diamond thin films are deposited from methane through microwave plasma-enhanced chemical vapor depositions (CVD), and the electrical conductivity of them are controlled by doping boron and sulfur from gas phases. A novel process chamber for in-situ doping was newly designed. Our motivation is to clarify effects of incorporated dopant on the nucleation and growth of the diamond phases. Diamond pn diode is an additional landmark of our study, which demonstrates the overall success of our doping methods.

•   Fabrication of nano-structures with controllable electric conductivity by probe microscopies

It is widely know that diamond is a good insulating material. However, it was reported recently that the diamond surface can exhibit the electric conductivity in some situations. This phenomenon was reproduced by various research groups, and named as surface conductivity (SC) of diamond. It was gradually clarified that SC is caused by the p-type conduction originated from the hydrogen termination of the diamond surface. Now many researchers are studying the physical modeling of it as well as the method to control SC to achieve novel electronic devices. Some researchers are concentrating attention on the bio-sensors based on the SC mechanisms because diamond is stable in various chemicals even at high temperatures. Some other researchers are trying to fabricate nano-patterned conducting regions on the diamond surfaces using SC to fabricating extremely thin interconnections in ULSI devices. In our study, bulk and thin film diamond is exposed to the hydrogen ion and radical by using micro-wave plasma reactor or annealing chamber up to 1300 K. The surface structure and the composition are characterized by RHEED, LEED and AES, to clarify the relation between the surface condition and the conduction. These surfaces are analyzed by probing microscopes, i.e., AFM and STM, at pressures from 10^{-5} to 10^{6} Pa and temperatures from room temperature to 800 K. Challenging goals of this study are to control SC by applying stress and voltage using probes and to form patterned nano-scaled conductive regions on the surface of diamond.

•   Large-area hetero-epitaxy of diamond and in-situ characterizations of plasma parameters

Many researchers have been studying the growth of diamond thin films in two decades. gLargerh and ghigher@crystal qualityh are main purposes of this research field. In this subject, we are trying to generate large plasma with high stability to synthesize single-crystalline diamond thin films with 100 mm-diameter. In this study, diamond growth in noble gas-plasma such as helium and argon are also examined. Hydrogen plasma is widely used for the deposition of diamond, but the feasibility and the safety of H2-based process is poor. If we displace hydrogen with noble gasses, the cost of the total process is reduced drastically. However, the source gas of plasma affects strongly on all kinds of process parameters, e.g., ion density, electron and gas temperature, methane decomposition rate, momentum flux to the surface and so on, and a new reaction modeling and a design of the process chamber are necessary for the gnoble-gas processh.

•   Diamond Like Carbon (DLC) thin film growth by reactive sputtering

Diamond like carbon (DLC) has unique physical and chemical properties similar to diamond. Surface roughness of DLC is very small because of its amorphous structure, and tribological properties and hardness of DLC are ideal for the wear-resistance protective coatings. However, it is difficult at this stage to compose hardness, adhesion and transparency all together. In our study, chemical reactions are introduced to the physical sputtering process to control the DLC properties individually. Hydrogen and oxygen gases and metal dopant are introduced to the deposition atmosphere in order to control the surface reaction and the formation mechanisms of the carbon bondings. In this way, chemical bonding-states and compositions are modified to complete essential properties for applications. Langmuir probe method, optical spectroscopy and quadrupole mass spectroscopy are utlized to characterize the deposition condition, and deposited films are analyzed ex-situ by basic material characterization methods, e.g., SEM, EDS, AES, XPS, AFM and SIMS.

•   Carbon nanotube manipulations in transparent electron microscope (TEM)

Transparent electron microscopy is one of the most effective techniques to analyze nanometer-sized structures. One group of our collaborators is trying to characterize individual nanotubes by TEM. They can handle single carbon nanotube in TEM by using piezo-electric cantilevers. In-situ observations of nano-structures are performed under DC voltages and bending stresses, and the basic electric and mechanical properties of individual nanotubes and the effects of structures on them are being clarified in this study.

•   Transparent conductive thin films by laser ablation and plasma-assisted methods

Flat panel displays (FPD) are now widely used in our lives, and sizes of them are becoming larger in recent years. The top layer of the illuminant and the transistors of FPDs are covered by transparent conductive thin films, and indium tin oxide (ITO) has been widely used for a long time. Transparent conductive films are also essential for solar panels to transport carriers without blocking the sunlight to the diode region. It is necessary in this application to form large and uniform films in low-cost. The problem in these applications is that indium is one of a noble metal and the resource is very limited on the earth. Now many researchers are trying to substitute ITO by other oxides and nitrides. We are trying to fabricate halogen-doped tin oxide thin films by laser ablation and plasma-enhanced chemical vapor depositions. In this study, composition is an essential parameter to affect the conductivity and the optical property. Therefore, much attention has been paid to the control of the composition, atomic substitution and the formation of vacancies in our study. We analyze deposited films by van der Pauw method, spectroscopic ellipsometry and other basic material characterization method, and seek the new material and processing to realize high conductivity and transparency together.