Showing papers by "Takafumi Yao published in 2012"

Improvement of Light Extraction Efficiency and Reduction of Leakage Current in GaN-Based LED Via V-Pit Formation

Abstract: Four types of GaN-based light-emitting diodes (LEDs) with V-pits formed in different regions were grown by metal-organic chemical vapor deposition. The position of the V-pits embedded in the layers of the LED structures was controlled by varying the growth temperature. We achieved the highest output power and lowest leakage current values with the LED structures comprising V-pits embedded in active regions and the p-GaN textured surface. The V-pit formation enhances the light output power and reverse voltage values by 1.3 times the values of the conventional LED owing to the enhancement of the light scattering probability and the effective filtering of threading dislocations.

A facile method for patterned growth of ZnO nanowires using a black ink

Abstract: Herein we present a simple route to grow patterned ZnO nanowires on various substrates using black ink. Without any metal catalysts or templates, the nanowires are selectively grown on the ink covered surface of the substrate by a chemical vapor transport and condensation method. The crystal and optical properties of the nanowires are dependent on the substrate materials. Dot patterned ZnO nanowires grown on Si substrate are demonstrated as a template for the enzyme immunoassay. This facile method for growth and patterning of inorganic nanostructures would be beneficial for easy and low-cost fabrication of micro and nano electronic, optoelectronic devices and sensors.

Optical properties of edge dislocations on (11¯00) prismatic planes in wurtzite ZnO introduced at elevated temperatures

Abstract: An arbitrary number (3×108−1×1010 cm−2) of edge dislocations on (11¯00) prismatic planes, with the Burgers vector of (a/3)[112¯0] and with the dislocation lines nearly parallel to [0001], were introduced intentionally in wurtzite ZnO bulk single crystals at elevated temperatures of 923–1073 K, and the optical properties were examined. After the introduction of the dislocations, the intensity of the intrinsic emissions existing in pre-dislocated crystals, i.e., near-band edge emissions and deep level emissions, was almost unchanged, and donor-acceptor pair (DAP) emissions with photon energies of 2.20 and 2.50 eV at temperature of 12 K appeared. The intensity of the DAP emissions increased with increasing the dislocation density. The origin of the DAP emissions was determined as acceptor levels of 0.9 and 1.2 eV depth introduced with the dislocations.

Lattice Deformation in a-Plane ZnO Films Grown on r-Plane Al2O3 Substrates Grown by Plasma-Assisted Molecular-Beam Epitaxy

Abstract: The lattice deformation of a-ZnO films on r-plane Al2O3 substrates, grown by plasma-assisted molecular-beam epitaxy (PAMBE), is strongly dependent on growth temperature: i) the unit cell volume is smaller than that of a reference ZnO bulk and it increases with the increase in growth temperature; ii) a-plane lattice constants decrease and c-plane lattice constants increase with the increase in growth temperature; and iii) residual strain decreases with the increase in growth temperature, irrespective of the direction of a-, m-, and c-axes. It is proposed that the ZnO lattices in the a-ZnO/r-Al2O3 system are individually relaxed along in-plane axes by releasing the residual strain in terms of compensating thermal mismatch.

Heteroepitaxial growth of GaN on various powder compounds (AlN, LaN, TiN, NbN, ZrN, ZrB 2 , VN, BeO) by hydride vapor phase epitaxy

Abstract: We investigated the nucleation and growth behavior of GaN on various powders by hydride vapor phase epitaxy. In relative comparison, the nucleation tendency of GaN on each powder can be summarized as AlN > LaN, TiN, NbN > ZrN > ZrB2 > VN, BeO, indicating that the number of nucleation sites increased from right to left. LaN and NbN have not yet been reported as buffer materials for GaN growth. Of these, NbN is expected to be a good buffer material because the interatomic distance on the NbN (111) plane has only 2% difference from that on the GaN (0001) plane.

Photoluminescence changes in n-type GaN samples after photoelectrochemical treatment

Abstract: Photoelectrochemical water splitting using semiconductors to produce hydrogen is a promising technique for converting sunlight to chemical energy. However, anodic photocorrosion occurs during the reaction even when n-type GaN, a chemically stable material, is used for the photo-illuminated working electrode. We previously demonstrated that the stability is related to the amount of Si doping used to make the GaN to be n-type. This means that, the surface damage after photoelectrochemical reaction depends on the amount of Si. We have now investigated the relationship between this surface change and the optical properties especially for the near band edge peaks. With the peak shift by the strain relaxation due to the surface change from smooth to rough, new peaks around 3.4 eV were observed after photoelectrochemical treatment. These new peaks are defect-related emission, which has been reported as Yi lines. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Comprehensive Study about the Effect of Heat Treatment on the Electrical Properties of Single-Crystalline ZnO Materials

Abstract: We have comprehensively studied about the effect of heat treatment on electrical properties in single-crystalline ZnO film, O-polar ZnO bulk, and Zn-polar ZnO bulk. We note that the electrical resistivity of ZnO materials is seriously fluctuated by the heat treatment: in the ZnO film grown by molecular-beam epitaxy, it slowly increases with the increase of annealing temperature, while in the ZnO bulk grown by flux-melt method, it abruptly increases and decreases below and above a critical temperature, respectively. These phenomena are systematically clarified from the relationship between annealing temperature and electron transport properties.

Comprehensive study of the surface morphology evolution induced by thermal annealing in single-crystalline ZnO Films and ZnO bulks

Abstract: We report on the evolution of the surface morphology induced by thermal annealing in N2 ambient over a wide temperature range of 500–1200 °C in single-crystalline ZnO films and ZnO bulks. The surface morphology is seriously changed by the annealing temperature, and the evolution can be categorized into three regions: island growth, island agglomeration, and pit formation. Island growth at low temperatures below 700 °C, is ascribed to the atomic migration to reduce surface energy, which causes surface roughening. Island agglomeration at intermediate temperatures of 700–900 °C is ascribed to the migration and the evaporation of surface atoms, which causes surface flattening. Pit formation at high temperatures above 900 °C is ascribed to the atomic evaporation by high vapor pressure, which causes surface destruction. On the other hand, the bulk lattice is continuously improved with increasing annealing temperature in the temperature regions before the surface-destruction region, which is attributed to the reduction in the numbers of point and line defects caused by recrystallization. As a result, the best surface morphology and the best bulk lattice are obtained at an annealing temperature of 900 °C. The common surface-morphology evolution of ZnO films and ZnO bulks with increasing annealing temperature can be summarized using the three steps of surface roughening by island growth, surface flattening by island agglomeration, and surface destruction by pit formation.

Properties of ultraviolet anti-Stokes photoluminescence in ZnO single crystals

Abstract: Ultraviolet (UV) anti-Stokes photoluminescence (ASPL) was observed in ZnO single crystals at low temperature. The ASPL spectrum was essentially the same as the normal photoluminescence (PL) spectrum under excitation in the band to band transition. By analogy to the ASPL of GaN, a two-step two-photon absorption process is suggested to occur in ZnO. The ratio of re-excited electrons in the intermediate state is, however, smaller than that for GaN from the excitation intensity dependence of the ASPL. This difference is probably due to the carrier lifetime of the intermediate state for ZnO being shorter than that for GaN.

Recombination activity of dislocations on (0 0 0 1) introduced in wurtzite ZnO at elevated temperatures

Abstract: The activity for non-radiative recombination at dislocations on (0 0 0 1) basal planes was examined in wurtzite ZnO bulk single crystals. In panchromatic cathodoluminescence intensity maps, the dislocations did not exhibit apparent contrast when they were introduced at elevated temperatures of 923–1073 K, while the dislocations introduced at low temperatures (below 623 K) were observed as dark bands. It was suggested that the dislocations formed complexes involving point defects, via the thermal migration of point defects at elevated temperatures, resulting in the suppression of the recombination activity. The complexes did not influence the existing emission lines in pre-dislocated crystals.

Effects of GaN Thin Layer on InGaN at Electrolyte-Semiconductor Interface for the Application of Photoelectrochemical Water Splitting

Abstract: Photoelectrochemical properties of nitride semiconductors are paid attention due to their possibilities of water splitting by visible light absorption. However, the photocurrent density of InxGa1-xN, which absorbs visible light, is usually lower than that of GaN, which has larger band-gap and absorbing only UV light. The reasons of this are thought to be the band-edge position at the semiconductor-electrolyte interface and the crystal quality. The conduction band-edge decreases with increasing of indium composition and across the hydrogen generation energy at around the indium composition of 0.2. This means that the hydrogen generation ability decreases with increasing of indium composition. Low crystal quality is obtained because the lower growth temperature of InxGa1-xN than that of GaN to achieve the indium incorporation. In order to improve the photocurrent density, band-edge energy control and quantum tunneling effect are tried using the structure of thin GaN layer on InxGa1-xN here. The effect for the photocurrent densities is also discussed.

Plasma-Assisted Molecular Beam Epitaxy of In X Ga 1-X N Films on C-plane Sapphire Substrates

Abstract: We report plasma-assisted molecular beam epitaxy of films on c-plane sapphire substrates. Prior to the growth of films, GaN film was grown on the nitride c-plane sapphire substrate by two-dimensional (2D) growth mode. For the growth of GaN, Ga flux of torr as a beam equivalent pressure (BEP) and a plasma power of 150 W with a nitrogen flow rate of 0.76 sccm were fixed. The growth of 2D GaN growth was confirmed by reflection high-energy electron diffraction (RHEED) by observing a streaky RHEED pattern with a strong specular spot. InN films showed lower growth rates even with the same growth conditions (same growth temperature, same plasma condition, and same BEP value of III element) than those of GaN films. It was observed that the growth rate of GaN is 1.7 times higher than that of InN, which is probably caused by the higher vapor pressure of In. For the growth of films with different In compositions, total III-element flux (Ga plus In BEPs) was set to torr, which was the BEP value for the 2D growth of GaN. The In compositions of the films were determined to be 28, 41, 45, and 53% based on the peak position of (0002) reflection in x-ray measurements. The growth of films did not show a streaky RHEED pattern but showed spotty patterns with weak streaky lines. This means that the net sticking coefficients of In and Ga, considered based on the growth rates of GaN and InN, are not the only factor governing the growth mode; another factor such as migration velocity should be considered. The sample with an In composition of 41% showed the lowest full width at half maximum value of 0.20 degree from the x-ray (0002) omega rocking curve measurements and the lowest root mean square roughness value of 0.71 nm.

Comparative study of photoluminescences for the Ga-/N-faces of a free-standing GaN bulk fabricated by using hydride vapor-phase epitaxy and self-separation technique

Abstract: The authors report on a comparative study of photoluminescences for the Ga-/N-faces of a freestanding GaN bulk, fabricated by using hydride vapor-phase epitaxy and self-separation technique. It is found that the Ga-face and the N-face of the same bulk have apparently different optical properties: i) the Ga-face has well-defined excitionic emission lines while the N-face has just broad emission peaks at similar energy levels and ii) the Ga-face is characterized by a transition crossover between DoX (neutral donor-bound exciton) and FXA (A free exciton) in changing temperature while the N-face is characterized by changes in the dominant emission mechanism in the order of DoX, 3.43-eV emission, DoAo (neutral donor-acceptor) pair and eAo (neutral electron-acceptor), and FXA with increasing temperature from 10 K to room temperature. On the other hand, the N-face is shown to have a rougher surface morphology and a larger density of edge dislocations, which is due to structural imperfections generated during the lift-off process. Also, the N-face is shown to have a higher conductivity and a larger electron concentration, which indicates that the N face contains a larger density of donor-type defects. It is suggested that the different emission mechanism of the Ga-/N-faces in the free-standing GaN bulk is ascribed to the fact that the N-face contains a larger density of electrically active radiative and nonradiative recombination centers, which are generated during the self-separation process.

Formation and Evolution of Misoriented Grains in a-Plane Oriented Gallium Nitride Layers

Abstract: Annealed low-temperature GaN layers grown on r-plane sapphire substrates were examined by X-ray diffraction pole figure measurements. The GaN layers were mainly a-plane oriented, in which misoriented grains with four different orientations were detected. The c-axes of the misoriented grains are tilted from the surface normal by about 35°, which are along the bonds not parallel to the c-axis of the a-plane oriented layers. There was a difference among the peak intensities corresponding to the c-planes of the misoriented grains with the four different orientations. Considering the difference, the relative amount of misoriented grains with each orientation can be predicted. The evolution of the misoriented grains is expected to be controlled by the major polarity. [doi:10.2320/matertrans.MA201221]