Masahiro Deguchi

Simulations and experiments of SiC heteroepitaxial growth on Si(001) surface

Mechanism of SiC heteroepitaxial growth by the carbonization of the Si(001) surface was studied at the atomic scale using molecular dynamics (MD) simulations and molecular beam epitaxy (MBE) experiments. Heteroepitaxial growth of single crystal 3c‐SiC on the Si(001) surface (3c‐SiC[001]∥Si[001] and 3c‐SiC[110]∥Si[110]) was observed in both the MD simulations and MBE experiments. Breaking of the Si—Si bonds and shrinkage of the [110] Si rows with C atoms are possible mechanisms for the heteroepitaxial growth of SiC on Si(001). Microscopic structures and mechanisms of the twin formations and pit formations are discussed. Ultraviolet light irradiation is proposed and confirmed to enhance the epitaxial growth of SiC in the MBE experiments.

Atomic level smoothing of CVD diamond films by gas cluster ion beam etching

Abstract Chemical vapor deposited diamond films on silicon substrates were etched by a gas cluster ion beam. We found that a gas cluster ion beam of 10 17 ions/cm 2 would be effective to smooth the surface of the CVD diamond films. It was confirmed that atomic level smooth surfaces ( R a = 1.9 nm by AFM measurements) were formed by Ar gas cluster ion beam (Ar 3000 + ) etching. We believe that the gas cluster ion beam etching technique will be a key technology for diamond device fabrication.

Photo-induced CO2 Reduction with GaN Electrode in Aqueous System

CO2 reduction with water and light illumination is realized using a gallium nitride (GaN) photoelectrode in which excited electrons induce CO2 conversion at the counter electrode. For the counter electrode, a copper (Cu) plate was chosen. The low affinity and wide gap of the nitride semiconductor enable us to create an electron–hole pair which has a sufficient energy for both CO2 reduction and water oxidation, in spite of the fact that a high energy for CO2 reduction is required. Within this system, the generation of formic acid (HCOOH) with 3% Faradic efficiency was confirmed by light illumination alone.

Enhanced CO2 reduction capability in an AlGaN/GaN photoelectrode

Light illumination of a gallium nitride photoelectrode creates separate electron-hole pairs that drive water oxidation and CO2 reduction reactions. Here, we show enhanced photocurrent in an AlGaN/GaN device that consists of an unintentionally doped (uid-) AlGaN photoabsorption layer and an n+-GaN electrical-conduction layer. The production rate of formic acid by CO2 conversion in the uid-AlGaN/n+-GaN photoelectrode is about double that in the uid-GaN/n+-GaN device. This improvement is most likely due to the effect of internal bias in the uid-AlGaN layer generated by the polarization effect, which improves electron-hole separation.

Piezoresistive property of CVD diamond films

Abstract Electrical and piezoresistive properties of chemical vapor-deposited boron-doped p-type polycrystalline diamond films were investigated. The p-type polycrystalline diamond films of about 2 μm thickness were grown on a flat insulating polycrystalline diamond substrate using a conventional microwave plasma CVD system. The optimized p-type polycrystalline CVD film exhibited similar quality to homoepitaxial single crystalline diamond film with activation energy of 0.31–0.33 eV and small effects of grain boundaries. Piezoresistors (500 μm long and 50 μm wide) constituted from the optimized p-type polycrystalline diamond films were fabricated on diaphragm structure using photolithography and reactive-ion etching in an oxygen plasma. Relative change of the electrical resistance (ΔR/R0) of the p-type diamond piezoresistor was almost proportional to the applied strain. Gauge factor K for the p-type diamond piezoresistor was derived to be ∼ 1000 at room temperature and > 700 even at 200°C.

Tandem photo-electrode of InGaN with two Si p-n junctions for CO2 conversion to HCOOH with the efficiency greater than biological photosynthesis

We report on a highly improved CO2 to HCOOH conversion system using a tandem photo-electrode (TPE) of InGaN and two Si p-n junctions. To improve its efficiency, narrow-band-gap InGaN was applied as the photo-absorption layer. In the TPE structure, the current matching between GaN-based photo-absorption layer and two Si p-n junctions is crucial for the improvement of the efficiency. The energy conversion efficiency for HCOOH production reached 0.97%, which is greater than average of global biological photosynthetic one.

Enhanced Capability of Photoelectrochemical CO2 Conversion System Using an AlGaN/GaN Photoelectrode

We report significantly improved photosynthesis system based on AlGaN/GaN photochemical process. The resultant energy conversion efficiency is 0.13% which is the same level as that of real plants. The capability of this system is enhanced by high cathode potential due to the reduction of energy loss while utilizing the piezoelectric effect in the AlGaN/GaN heterostructure. The Faradaic efficiency of the CO2 conversion to organic materials is enhanced, accompanied by an increment in photocurrent by modification of the AlGaN/GaN photoelectrode structure and electrolytes. Furthermore, reaction products such as C2H4 and C2H5OH are generated by light illumination alone.

CO2 Conversion with Light and Water by GaN Photoelectrode

Light illumination on a photoelectrode creates separate electron and hole pairs that lead to an oxidation and reduction reaction. Here, we show that CO2 reduction by means of water and light is realized by a gallium nitride (GaN) photoelectrode in which excited electrons drive CO2 conversion at the counterelectrode. A copper (Cu) plate was chosen as the counterelectrode. With this system, the generation of formic acid (HCOOH) with 9% Faradic efficiency was confirmed by light illumination alone with the help of NiO co-catalysts.

Highly efficient photochemical HCOOH production from CO2 and water using an inorganic system

We have constructed a system that uses solar energy to react CO2 with water to generate formic acid (HCOOH) at an energy conversion efficiency of 0.15%. It consists of an AlGaN/GaN anode photoelectrode and indium (In) cathode that are electrically connected outside of the reactor cell. High energy conversion efficiency is realized due to a high quantum efficiency of 28% at 300 nm, attributable to efficient electron-hole separation in the semiconductors heterostructure. The efficiency is close to that of natural photosynthesis in plants, and what is more, the reaction product (HCOOH) can be used as a renewable energy source.

Electron emitter device of NEA diamond thin film

A new type of cold cathode of chemical vapor deposited diamond thin film is proposed. The electron emitter consists of a hydrogenated diamond surface of negative electron affinity (NEA) which also is a p-type semiconducting layer, and a metal-insulator-semiconductor (MIS) structure for electron injection into the diamond film. The diamond film is deposited by microwave plasma chemical vapor deposition (CVD) and thinned by electron cyclotron resonance (ECR) plasma etching. The p-type conductivity of the hydrogenated diamond surface is taken into consideration as well as the NEA feature itself to design the emitter device. The emitter worked from a low driving voltage of 40 V with high cathode efficiency of 2%, which mean the applying voltage for the MIS diode and the ratio of the emission current to the diode current, respectively. The emission current of 20 pA was obtained at a driving voltage of 100 V for the fabricated cathode.