Momoko Deura

High Electron Mobility Metal–Insulator–Semiconductor Field-Effect Transistors Fabricated on (111)-Oriented InGaAs Channels

Metal–insulator–semiconductor field-effect transistors (MISFETs) were fabricated on the (111)A surface of In0.53Ga0.47As for the first time. Al2O3 gate dielectrics were formed by atomic layer deposition on sulfur-stabilized InGaAs surfaces. The MISFET on (111)A demonstrated channel mobility higher than that on (100), achieving more than 100% improvement with respect to Si even at a high surface carrier concentration.

High Stability and Efficiency of GaN Photocatalyst for Hydrogen Generation from Water

Continuous hydrogen generation from water was realized for 110 h using an n-type GaN film with a NiO cocatalyst. This GaN–NiO photocatalyst exhibits incident photon-to-current conversion efficiencies of 70 and 57% at wavelengths of 300 and 350 nm, respectively. These results indicate that the GaN–NiO photocatalyst has high stability and efficiency for hydrogen generation.

Dislocation-Free InGaAs on Si(111) Using Micro-Channel Selective-Area Metalorganic Vapor Phase Epitaxy

We have obtained a dislocation-free InGaAs layer on Si(111) using micro-channel selective-area metalorganic vapor phase epitaxy. By increasing the supply of the Ga precursor, we removed rotational twins, which have been observed in III–V layers on (111) substrates. From analysis of the atomic structure, the Ga content in the crystal was found to increase gradually as the growth proceeded. This increase resulted in growth in a tilted direction from [111] and extinction of twins. It was concluded that a twin-free InGaAs layer can be obtained when a Ga-rich layer is inserted in the middle of the growth.

Highly Stable GaN Photocatalyst for Producing H2 Gas from Water

Efficient production of H2 from water without the use of an extra bias or any sacrificial reagents is possible using a GaN photocatalyst with a NiO cocatalyst. The average energy conversion efficiency from light energy to H2 energy was approximately 1% for 500 h. The total amount of hydrogen was 184 mL/cm2 of the GaN surface. H2 production rate was as high as 0.37 mL/(cm2h).

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.

Three-dimensional evaluation of gettering ability for oxygen atoms at small-angle tilt boundaries in Czochralski-grown silicon crystals

Three-dimensional distribution of oxygen atoms at small-angle tilt boundaries (SATBs) in Czochralski-grown p-type silicon ingots was investigated by atom probe tomography combined with transmission electron microscopy. Oxygen gettering along edge dislocations composing SATBs, post crystal growth, was observed. The gettering ability of SATBs would depend both on the dislocation strain and on the dislocation density. Oxygen atoms would agglomerate in the atomic sites under the tensile hydrostatic stress larger than about 2.0u2009GPa induced by the dislocations. It was suggested that the density of the atomic sites, depending on the tilt angle of SATBs, determined the gettering ability of SATBs.

Dependences of Initial Nucleation on Growth Conditions of InAs on Si by Micro-Channel Selective-Area Metal–Organic Vapor Phase Epitaxy

In order to improve the uniformity of InGaAs lateral islands on Si grown by micro-channel selective area growth, we have investigated the dependences of initial InAs nucleation on the partial pressures of trimethylindium (PTMIn) and tertiary butylarsine (PTBAs) using the in situ monitoring of surface reflectivity. The high PTMIn resulted in a short incubation period, a high density of nuclei, and vertical growth, suggesting that a high PTMIn is suitable for obtaining single nuclei in each growth area, which is vital for the growth of single-domain crystals on Si. Laterally grown InAs nuclei, which are preferable for the lateral growth of InGaAs crystals that succeeds the initial nucleation of InAs, were obtained using either a low PTMIn or a high PTBAs, however, the effect of the latter was not significant. PTBAs did not affect the incubation period. The density, uniformity, and shape of InAs nuclei can be controlled effectively by adjusting PTMIn, but the uniformity and lateral shape could not be obtained simultaneously. We, therefore, devised a flow-modulated sequence and obtained InAs islands that grew in the lateral direction and almost filled the growth area with a single-crystal domain.

Kinetic Analysis of Surface Adsorption Layer in GaAs(001) Metalorganic Vapor Phase Epitaxy by In situ Reflectance Anisotropy Spectroscopy

A kinetic analysis of the surface adsorption layer of III/V semiconductors in metalorganic vapor phase epitaxy (MOVPE) was carried out by monitoring the temporal behavior of surface reconstruction that is assumed to be maintained during growth with the adsorption layers. A GaAs(001) surface was observed in situ using reflectance anisotropy spectroscopy (RAS), and both spectroscopic and kinetic analyses were performed when trimethylgallium (TMGa) was supplied intermittently while the supply of tertiarybutylarsine (TBAs) was continuous. When the flow of TMGa was switched on/off, the transient RA signal exhibited time constants of 0.2–20 s. By observing the dependence of these time constants on the partial pressure of TMGa and substrate temperature, we considered that the time constant (t0) at the start of TMGa supply corresponds to the adsorption of Ga from the gas phase to the adsorption layer, and two constants (i.e., t1 and t2) at the end of TMGa supply are associated with the crystallization of adsorbed Ga and the recovery of surface reconstruction, respectively. The equivalent thickness of the adsorption layer was estimated to be 0.19 monolayer thickness at 550 °C.

High-Temperature Annealing Effect of Si in Group-V Ambient Prior to Heteroepitaxy of InAs in Metal--Organic Vapor Phase Epitaxy

We investigated the state of the Si(111) surface and its effect on InAs growth after annealing at high temperature with and without an As or P source flow in H2 ambient in metal–organic vapor phase epitaxy (MOVPE). In multi-step growth of InGaAs by micro-channel selective-area growth, perfect coverage of Si growth areas by InAs, which is grown first, by controlling the state of the Si surface is critical for the following InGaAs lateral growth. Although both As and P sources protect the surface against contamination from inside the reactor, annealing with the P source at high temperature is optimal in terms of InAs nucleation and coverage of growth areas by InAs. The amount of O contamination after P annealing at high temperature was significantly lower than that under other annealing conditions. Therefore, O is the most critical contamination in InAs nucleation.

Impact of atomistic surface structure on macroscopic surface reaction rate in MOVPE of GaAs

In situ reflectance anisotropy spectroscopy was employed to interpret the surface reaction rate constant (k s ) of a Ga precursor on a GaAs(001) surface in metallorganic vapor-phase epitaxy (MOVPE), which was extracted by the analysis of selective-area growth. The activation energy of k s significantly decreased above 625-630°C. Correspondingly, the surface anisotropy spectrum changed around 600°C from that of surface reconstruction containing Ga dimers at lower temperatures to that of c(4 X 4)-like reconstruction with As dimers at higher temperatures. These observations suggest a step-flow growth mode at higher temperatures and an island growth mode at lower temperatures.