Masaya Ueda

Blue, Green, and Amber InGaN/GaN Light-Emitting Diodes on Semipolar {11-22} GaN Bulk Substrates

We demonstrate the fabrication of blue, green, and amber InGaN/GaN light-emitting diodes (LEDs) on semipolar {11-22} bulk GaN substrates. The {11-22}GaN substrates used in this study are produced by cutting out from a c-oriented GaN bulk crystal grown by hydride vapor epitaxy. The LEDs have a dimension of 320 ×320 µm2 and are packed in an epoxide resin. The output power and external quantum efficiency (EQE) at a driving current of 20 mA are 1.76 mW and 3.0%, respectively, for the blue LED, 1.91 mW and 4.1% for the green LED, and 0.54 mW and 1.3% for the amber LED. The maximum output powers obtained with a maximum current of 200 mA are 19.0 mW (blue), 13.4 mW (green), and 1.9 mW (amber), while the maximum EQEs are 4.0% at 140 mA (blue), 4.9% at 0.2 mA (green), and 1.6% at 1 mA (amber). It is confirmed that the emission light is polarized along the [1-100] direction, reflecting the low crystal symmetry of the {11-22} plane.

Monolithic Polychromatic Light-Emitting Diodes Based on InGaN Microfacet Quantum Wells toward Tailor-Made Solid-State Lighting

Monolithic polychromatic light-emitting diodes (LEDs) based on micro-structured InGaN/GaN quantum wells are demonstrated. The microstructure is created through regrowth on SiO2 mask stripes along the [1100] direction and consists of (0001) and {1122} facets. The LEDs exhibit polychromatic emission, including white, due to the additive color mixture of facet-dependent emission colors. Altering the growth conditions and mask geometry easily controls the apparent emission color. Furthermore, simulations predict high light extraction efficiencies due to their three-dimensional structures. Those observations suggest that the proposed phosphor-free LEDs may lead to highly efficient solid-state lighting in which the color spectra of light sources are synthesized to satisfy specific requirements for illuminations.

Epitaxial growth and optical properties of semipolar (112¯2) GaN and InGaN∕GaN quantum wells on GaN bulk substrates

GaN and InGaN∕GaN multiple quantum well (MQW) were grown on semipolar (112¯2) GaN bulk substrates by metal organic vapor phase epitaxy. The GaN homoepitaxial layer has an atomically flat surface. Optical reflection measurements reveal polarization anisotropy for the A, B, and C excitons. Free A excitons dominate the photoluminescence (PL) spectrum at 10K and are accompanied by a weaker, sharp doublet emission due to neutral donor-bound excitons. The InGaN∕GaN MQW grown on a GaN homoepitaxial layer involves fast radiative recombination processes. The PL decay monitored at 428nm can be fitted with a double exponential curve, which has lifetimes of 46 and 142ps at 10K. These values are two orders of magnitude shorter than those in conventional c-oriented QWs and are attributed to the weakened internal electric field. The emissions from GaN and MQW polarize along the [11¯00] direction with polarization degrees of 0.46 and 0.69, respectively, due to the low crystal symmetry.

Emission color tunable light-emitting diodes composed of InGaN multifacet quantum wells

We demonstrate that the apparent emission colors of InGaN-based light-emitting diodes using microstructured multifacet quantum wells as active layers can externally be controlled over a wide spectral range that encompasses green to blue or white at a color temperature of 4000K to blue along the Planckian locus. The controllability relies on facet-dependent polychromatic emissions. The pulsed current operation with the appropriate duties varied their relative intensities and the consequent apparent colors without seriously affecting the total number of emitted photons, particularly for the blue to green variation.

Growth of ZnO nanorods on A-plane (1120) sapphire by metal-organic vapor phase epitaxy

The growth mode of ZnO on a-plane (110) sapphire in metal-organic vapor phase epitaxy exhibited stronger tendency toward three-dimensional nucleation at lower temperature (≤700°C) and toward two-dimensional layered growth at higher temperature (900°C), resulting in the formation of rods and films, respectively. ZnO nanorods with diameters smaller than 10 nm were fabricated at the appropriate precursor flow rate and growth temperature. Blue shift of emission from free excitons in a photoluminescence spectrum was observed for the nanorods, particularly for those grown at 400°C. Because the diameters of those nanorods were sufficiently small for quantum confinement, the blue shift was reasonably attributed to quantum size effects.

X-ray diffraction studies of pristine and heavily-doped polyacenic materials

Abstract An X-ray diffraction study was performed for pristine and heavily-doped polyacenic materials prepared by pyrolytic treatment of phenolformaldehyde (PF) resin. Seven kinds of pristine samples prepared under different pyrolysis temperatures were studied, as well as the moulded PF resin itself. The result of the analyses shows that the structural change of the samples in the heavily-doped regime differs in a peculiar manner from what is expected in conventional intercalated graphite. An attempt is made to interpret the obtained results in terms of the amorphous structure of the present polyacenic materials.

Selective formation of ZnO nanodots on nanopatterned substrates by metalorganic chemical vapor deposition

Selective formation of ZnO nanodots was accomplished by metalorganic chemical vapor deposition on nanopatterned SiO2/Si substrates. Self-organized ZnO nanodots were selectively formed in nanopatterned lines of Si created by etching of SiO2 with focused ion beam (FIB), whereas any nanodots were hardly observed on the SiO2 surface in the vicinity of the FIB-sputtered Si areas. The mechanism of the selective formation of ZnO nanodots on FIB-nanopatterned lines is mainly attributed to the effective migration of Zn adatoms diffusing on the SiO2 surface into the Si lines followed by the nucleation at surface atomic steps and kinks created by Ga+ ion sputtering. Cathodoluminescence measurements confirmed that the emission originated from the selectively grown ZnO nanodots.Selective formation of ZnO nanodots was accomplished by metalorganic chemical vapor deposition on nanopatterned SiO2/Si substrates. Self-organized ZnO nanodots were selectively formed in nanopatterned lines of Si created by etching of SiO2 with focused ion beam (FIB), whereas any nanodots were hardly observed on the SiO2 surface in the vicinity of the FIB-sputtered Si areas. The mechanism of the selective formation of ZnO nanodots on FIB-nanopatterned lines is mainly attributed to the effective migration of Zn adatoms diffusing on the SiO2 surface into the Si lines followed by the nucleation at surface atomic steps and kinks created by Ga+ ion sputtering. Cathodoluminescence measurements confirmed that the emission originated from the selectively grown ZnO nanodots.

Additive color mixture of emission from InGaN∕GaN quantum wells on structure-controlled GaN microfacets

Altering the mask geometry controls the apparent emission color from InGaN∕GaN quantum wells (QWs) grown on GaN microfacets formed by regrowth on SiO2 mask stripes over a wide spectral range, including white. The mask stripes are along the ⟨11¯00⟩ direction and the microfacet structure is composed of the (0001) and {112¯2} planes. With a large occupancy of the mask opening within a period, both facets simultaneously appear and emit different colors. For example, the {112¯2} facet QWs emit blue and the (0001) facet QWs emit green. On the other hand, with a small occupancy of the mask opening, the {112¯2} facets become dominant and a greenish-blue light is emitted. To synthesize these spectra, the mask patterns are designed so that two different microfacet structures are included within a period. Hence, the macroscopically observed emission color, which depends on the pattern design, can change from green to purple through white due to the additive color mixture.

Strain states in semipolar III-nitride semiconductor quantum wells

Strain states in wurtzite III-nitride semiconductor quantum wells (QWs) are investigated. X-ray diffraction (XRD) reciprocal space mapping using semipolar (112¯2) and (11¯01) InGaN/GaN QWs as test samples demonstrates that the projections of reciprocal lattice vectors of unstrained GaN and pseudomorphically strained InGaN on the interface agrees, indicating continuity of layers across the interface. High resolution transmission electron microscopy supports the XRD analysis. Based on the experimental results, strain tensor elements are extracted for arbitrary crystalline orientation. Furthermore, expansion of the model to arbitrary crystal structures is suggested.

Experimental and Theoretical Considerations of Polarization Field Direction in Semipolar InGaN/GaN Quantum Wells

The polarization-related electric field in semipolar {1122} InGaN/GaN quantum wells (QWs) is investigated. Experimentally, the direction and magnitude of the electric field are determined by bias-dependent photoluminescence. Using both (1122) and the reverse (1122) QWs enables the electric field direction, which indicates that the polarization direction flips between the polar (0001) plane and semipolar (1122) plane, to be reliably determined. Lattice distortion due to lattice mismatch, which is the origin of the polarization effect, is theoretically assessed by the valence force field model, and the analysis supports the experimental result.