Takeshi Ohwaki

Band-Gap Narrowing of Titanium Dioxide by Nitrogen Doping

TiO2-based powder, including 0.1 at% of N doped in the rutile lattice, has been synthesized by oxidation of TiN. As a result, a significant shift of the absorption edge to a lower energy in the visible-light region has been observed. The substitutional doping of N into the TiO2 lattice is found to be effective; its 2p states contribute to the band-gap narrowing by mixing with O 2p as shown in ab initio electronic structure calculations.

Schottky barriers and contact resistances on p‐type GaN

We measured the Schottky barrier heights and specific contact resistivities of four different metals on p‐type GaN. The Schottky barrier heights of Pt, Ni, Au, and Ti were obtained from the current‐voltage characteristics to be 0.50, 0.50, 0.57, and 0.65 eV, respectively. The specific contact resistivities were 0.013, 0.015, 0.026, and 0.035 Ω⋅cm2, respectively. Our experimental results proved that the Schottky barrier heights and specific contact resistivities decrease with increase in metal work function as expected theoretically.

Deep-level optical spectroscopy investigation of N-doped TiO2 films

N-doped TiO2 films were deposited on n+-GaN∕Al2O3 substrates by reactive magnetron sputtering and subsequently crystallized by annealing at 550 °C in flowing N2 gas. The N-doping concentration was ∼8.8%, as determined from x-ray photoelectron spectroscopy measurements. Deep-level optical spectroscopy measurements revealed two characteristic deep levels located at ∼1.18 and ∼2.48eV below the conduction band. The 1.18 eV level is probably attributable to the O vacancy state and can be active as an efficient generation-recombination center. Additionally, the 2.48 eV band is newly introduced by the N doping and contributes to band-gap narrowing by mixing with the O2p valence band.

Electrical characterization of p-type N-doped ZnO films prepared by thermal oxidation of sputtered Zn3N2 films

We report on p-type conduction in N-doped ZnO (ZnO:N) films that were prepared by oxidative annealing of sputtered Zn3N2 films at temperatures between 500 and 800°C in flowing O2 gas. Room-temperature Hall-effect measurements show a significant improvement of p-type doping characteristics by the oxidative annealing at temperatures between 700 and 800°C, where more N acceptors are activated and the oxidation state is enhanced, as confirmed by deep-level optical spectroscopy and secondary ion mass spectrometry measurements. Therefore, the high-temperature oxidation of Zn3N2 is effective in moderating the self-compensation effect in ZnO:N from the viewpoint of oxygen vacancy annihilation.

Photodegradation of toluene over TiO2−xNx under visible light irradiation

We report the photooxidation of toluene over nitrogen doped TiO(2) (TiO(2-x)N(x)) under visible light irradiation. The photocatalytic oxidation of toluene in air over TiO(2-x)N(x) powders was studied using diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), gas chromatography (GC), ion chromatography (IC), and gas chromatography mass spectrometry (GC-MS), focusing on the photocatalytic decomposition processes of toluene. Results obtained indicate that toluene, weakly adsorbed on the catalyst surface, is initially photooxidized to benzaldehyde which adsorbs onto the TiO(2-x)N(x) surface more strongly, leading to the formation of ring-opening products such as carboxylic acids and aldehydes. No gaseous intermediates were detected during the photooxidation. Major intermediates adsorbed at the catalyst surface were oxalic acid, (COOH)(2), acetic acid, CH(3)COOH, formic acid, HCOOH, and pyruvic acid, CH(3)COCOOH, whereas more complicated carboxylic species, including propionic acid, CH(3)CH(2)COOH, isovaleric acid, (CH(3))(2)CHCH(2)COOH, and succinic acid, (CH(2)COOH)(2), were also found in the early stage of the photooxidation. These intermediate products were gradually photodegraded to CO(2) and H(2)O under visible light irradiation.

Electrical characterization of band gap states in C-doped TiO2 films

We report on band gap states in C-doped TiO2 films that were prepared by oxidative annealing of sputtered TiC films at 550°C in flowing O2 gas. Deep-level optical spectroscopy measurements revealed three deep levels located at ∼0.86, ∼1.30, and ∼2.34eV below the conduction band. The first level is probably attributable to the intrinsic nature of TiO2, whereas the latter two levels are newly introduced by the C-doping. In particular, the pronounced 2.34eV band contributes to band gap narrowing by mixing with the O 2p valence band. Additionally, the 0.86 and 1.30eV levels can be active as an efficient generation-recombination center.

Dislocation etch pits in GaN epitaxial layers grown on sapphire substrates

Dislocations in GaN epitaxial layers grown on sapphire substrates have been studied by chemical etching. The authors have examined molten KOH as a defect etchant and characterized the etch pits on GaN layers. By use of molten KOH etching, etch pits were revealed on the surface of the GaN layer. All pits were hexagonal pyramids, which reflect the crystal symmetry of GaN. Results showed that molten KOH etching might be a useful method for the evaluation of the dislocations in GaN layers. The etch pit density (EPD) was typically 2 {times} 10{sup 7} cm{sup {minus}2}.

Characterization of Silicon Native Oxide Formed in SC-1, H2O2 and Wet Ozone Processes

The structures of silicon native oxides formed in the SC-1, H2O2 and wet ozone processes were characterized using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR). Spectral simulation was performed to clarify the FT-IR spectra, assuming that the native oxide was pure silicon dioxide. Effective medium theories were applied to understand deviations of the observed spectra from the calculated ones. The deviations between the native oxide thickness evaluated by XPS and the absolute thickness obtained by TEM were also discussed. These deviations can be explained if the void is incorporated in the native oxides and the interface between the native oxide and the basal silicon obtained by the wet ozone process has a relatively smooth surface and a structure more similar to that of pure silicon dioxide, compared with that obtained by SC-1 or H2O2 treatment.

FIELD EMISSION STUDY OF GATED GAN AND AL0.1GA0.9N/GAN PYRAMIDAL FIELD EMITTER ARRAYS

A self-aligned process has been proposed for fabrications of gated GaN and Al0.1Ga0.9N/GaN pyramidal field emitters with a small and precise distance between the gate electrode and the emitter tip. The sharp pyramidal emitters were obtained by a selective area growth technique. SiO2 and polyimide were used as the insulating layers between the emitters and the gate electrode. The gate-tip spacing and the tip protrusion through the gate openings were precisely controlled by adjusting the thickness of the SiO2 and polyimide layers. The turn-on voltage was reduced by narrowing the gate-tip spacing and by using Al0.1Ga0.9N/GaN emitter instead of GaN; the low onset gate voltage of 42 V was obtained for a gate-tip spacing of 0.4 μm.

Deep-level characterization of N-doped ZnO films prepared by reactive magnetron sputtering

We report on band gap states in N-doped ZnO (ZnO:N) films that were deposited on indium tin oxide/quartz substrates by reactive magnetron sputtering. Colored ZnO:N samples showed enhanced polycrystalline with increasing N-doping concentration, as determined by x-ray diffraction patterns. Deep-level optical spectroscopy measurements revealed three characteristic deep levels located at ∼0.98, ∼1.20, and ∼2.21eV below the conduction band. In particular, the pronounced 2.21eV band is newly introduced by the N doping and behaves as part of the valence band, resulting in band gap narrowing of ZnO. Therefore, this deep level is probably one origin of visible-light sensitivity in ZnO:N.