Natural band alignment of MgO1−xSx\rm MgO_{1-x}S_{x} alloys

We have calculated formation enthalpies, band gaps, and natural band alignment for $\rm MgO_{1-x}S_{x}$ alloys by first principles calculation based on density functional theory. The calculated formation enthalpies show that the $\rm MgO_{1-x}S_{x}$ alloys exhibit a large miscibilitygap, and a metastable region was found to occur when the S content was below 18% or over 87%. Effect of S incorporation for band gaps of $\rm MgO_{1-x}S_{x}$ alloys shows large bowing parameter (b $\simeq$ 13 eV) induced. The dependence of the band lineup of $\rm MgO_{1-x}S_{x}$ alloys on the S content by using two different methods, and the change in the energy position of valence band maximum (VBM) was larger than that of conduction band minimum. Based on the calculated VBM positions, we predicted that $\rm MgO_{1-x}S_{x}$ with S content 10 to 18% can be surface charge transfer doping by high electron affinity materials. The present work provides an example to design for p-type oxysulfide materials.Comment: 22 pages, 6 figure

Blue light-emitting diode based on ZnO

A near-band-edge bluish electroluminescence (EL) band centered at around 440 nm was observed from ZnO p-i-n homojunction diodes through a semi-transparent electrode deposited on the p-type ZnO top layer. The EL peak energy coincided with the photoluminescence peak energy of an equivalent p-type ZnO layer, indicating that the electron injection from the n-type layer to the p-type layer dominates the current, giving rise to the radiative recombination in the p-type layer. The imbalance in charge injection is considered to originate from the lower majority carrier concentration in the p-type layer, which is one or two orders of magnitude lower than that in the n-type one. The current-voltage characteristics showed the presence of series resistance of several hundreds ohms, corresponding to the current spread resistance within the bottom n-type ZnO. The employment of conducting ZnO substrates may solve the latter problem.Comment: 13 pages, 4 figures. Jpn. J. Appl. Phys. in pres

Microstructural evolution in m-plane GaN growth on m-plane SiC

This letter presents a study on the nucleation and microstructural evolution of m-plane GaN epilayers on m-plane SiC substrates using high-temperature AlN buffer layers. Controlled growth interruptions were carried out to render snapshots of heteroepitaxial dynamics. It was discovered that island coalescence results in an inhomogeneous mosaic tilt along the c-axis. Mesoscopic study of nucleation evolution helps elucidate the origin of commonly observed surface undulation and striation, which is attributed to concave growth due to the coalescence of trapezoidal islands upon contact. A model correlating microstructural defects with optical properties is proposed to explain the observed pattern in spatially resolved cathodoluminescence mapping.open221

Structural evaluation of ions-implanted GaN films by photothermal deflection spectroscopy

We characterize the behavior caused by thermal annealing for C, O, Si and Mg ions implanted in GaN films by photothermal deflection spectroscopy (PDS) with respect to structural disorder and defect levels related to yellow luminescence. Although the implanted region damaged by ion bombardment is recovered by annealing, the FWHM values of ω(0002) evaluated by x-ray diffraction are almost independent of the temperature. However, the Urbach energy, as an index of structural disorder, evaluated by PDS is improved. Recovery of the structural disorder is likely to depend on the dose quantity rather than the acceleration voltage. Defect states in the band gap are reduced as well, though featured PDS signals related to the kind of implanted ion are hardly detected except for the carbon ion. The intensity of yellow luminescence at room temperature is enhanced according to the improvement of the Urbach energy and reduction of defect states in the band gap. PDS is useful for defect analysis of III-V nitride semiconductors that are electrically and optically inactive, such as ion-implantation samples, especially Mg-implanted GaN for achieving reliable p-type conduction

Fabrication of Transparent and Conductive SWCNT/SiO₂ Composite Thin-Film by Photo-Irradiation of Molecular Precursor Films

A single-walled carbon nanotube (SWCNT)-silica composite thin film on a quartz glass was formed by ultraviolet irradiation (20–40 °C) onto a spin-coated precursor film. With 7.4 mass% SWCNTs, the electrical resistivity reached 7.7 × 10−3 Ω·cm after UV-irradiation. The transmittance was >80% at 178–2600 nm, and 79%–73% at 220–352 nm. Heat treatment increased the transparency and pencil hardness, without affecting the low electrical resistivity. Raman spectroscopy and microscopic analyses revealed the excellent film morphology with good SWCNT dispersal. The low refractive index (1.49) and haze value (<1.5%) are invaluable for transparent windows for novel optoelectronic devices

In Situ Synchrotron X-ray Diffraction Reciprocal Space Mapping Measurements in the RF-MBE Growth of GaInN on GaN and InN

In this work, in situ synchrotron X-ray diffraction reciprocal space mapping (RSM) measurements were carried out for the radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) growth of GaInN on GaN and InN layers, which were also grown by RF-MBE on commercialized GaN/c-sapphire templates. In situ XRD RSM measurements were performed using an MBE apparatus directly coupled to an X-ray diffractometer at the beamline of the synchrotron radiation facility SPring-8. It was observed in situ that both lattice relaxation and compositional pulling occurred during the initial growth stage, reducing the strain of GaInN on GaN and InN. Different initial growth behaviors of GaInN on GaN and InN were also observed from the results of the evolution of GaInN integrated peak intensities

MOCVD Growth and Investigation of InGaN/GaN Heterostructure Grown on AlGaN/GaN-on-Si Template

The investigation of the III-V nitride-based driving circuits is in demand for the development of GaN-based power electronic devices. In this work, we aim to grow high-quality InGaN/GaN heterojunctions on the n-channel AlGaN/GaN-on-Si high electron mobility transistor (HEMT) templates to pursue the complementary p-channel conductivity to realize the monolithic integrated circuits. As the initial step, the epitaxial growth is optimized and the structure properties are investigated by comparing with the InGaN/GaN heterojunctions grown on GaN/sapphire templates. It is found that both the In composition and relaxation degree are higher for the InGaN/GaN on the HEMT template than that on the sapphire substrate. The crystalline quality is deteriorated for the InGaN grown on the HEMT template, which is attributed to the poor-quality GaN channel in the HEMT template. Further analysis indicates that the higher In incorporation in the InGaN layer on the HEMT template may be caused by the higher relaxation degree due to the compositional pulling effect. An increase in the growth temperature by 20 &deg;C with optimized growth condition improves the crystalline quality of the InGaN, which is comparable to that on GaN/sapphire even if it is grown on a poor-quality GaN channel

Hydrophilic Titania Thin Films from a Molecular Precursor Film Formed via Electrospray Deposition on a Quartz Glass Substrate Precoated with Carbon Nanotubes

Titania precursor films were electrosprayed on a quartz glass substrate, which was pre-modified with an ultra-thin film obtained by spin-coating a single-walled carbon nanotube (SWCNT) dispersed solution. The X-ray diffraction patterns of the thin films obtained by heat-treating the precursor films at 500 &deg;C in air for 1 h indicated that the formed crystals were anatase. A new route to fabricate transparent thin films on the insulating substrate via electrospray deposition (ESD) was thus attained. The photoluminescence spectrum of the thin film showed a peak at 2.23 eV, assignable to the self-trapped exciton of anatase. The Raman spectrum of the thin film demonstrated that heat treatment is useful for removing SWCNTs. The thin film showed a water contact angle of 14 &plusmn; 2&deg; even after being kept under dark conditions for 1 h, indicating a high level of hydrophilicity. Additionally, the thin film had a super-hydrophilic surface with a water contact angle of 1 &plusmn; 1&deg; after ultraviolet light irradiation with an intensity of 4.5 mW cm&minus;2 at 365 nm for 1 h. The importance of Ti3+ ions in the co-present amorphous phase, which was dominantly formed via the ESD process, for hydrophilicity was also clarified by means of X-ray photoelectron spectroscopy