菊池 昭彦

Abstract The fabrication technology for photonic crystals (PhCs) pertaining to the near-infrared region is mature, and the development of highly functional PhCs using low-symmetry nanoholes is rapidly progressing. In the visible region, InGaN/GaN systems that have good luminescent and electrical properties are the most promising candidate materials for such types of highly functional PhCs, but the development is not progressing. In this study, we report on the basic design parameters and a new fabrication method for InGaN/GaN-based PhC membranes by combining hydrogen environment anisotropic thermal etching based on hydrogen-assisted thermal decomposition and nitric acid wet etching of the AlInN sacrificial layer. Using this method, we fabricated high-quality InGaN/GaN multiple-quantum-well PhC membrane structures having six-membered rings of well-formed fine equilateral triangular nanoholes with a side length of 100 nm. Enhanced green room-temperature photoluminescence with an intensity nine times higher than that of as-grown wafers was observed for the PhC membrane.

A novel crystal growth technique is proposed that uses a thin ionic liquid film with an extremely low vapor pressure as the crystal growth field, and a solute supplied by electrospray deposition through the surface of the liquid film. The precipitation properties of small molecule Alq3 single crystals were investigated at 25 °C under atmospheric pressure in a nitrogen atmosphere. Using the proposed method, aggregated small crystals of less than 10 μm, rhombic plate-like crystals with diagonal lengths of 50 μm, and hexagonal plate-like crystals of 100 μm were precipitated when the solution supply rates (supply times) were 8.0 (90), 4.0 (180), and 2.0 μl min−1 (360 min), respectively. Alq3 is known to be a material difficult to grow into plate-like crystals, however, when solution was supplied to the ionic liquid at a rate of 2.0 μl min−1 for 16 h, large hexagonal plate-like crystals with a diagonal length of about 400 μm are obtained, which are among the largest plate-like Alq3-based crystals ever reported. The proposed crystal growth technique is expected to be beneficial for growing large size plate-like single crystals of various organic materials.

We report an organic single crystal growth technique, which uses a nonvolatile liquid thin film as a crystal growth field and supplies fine droplets containing solute from the surface of the liquid thin film uniformly and continuously by electrospray deposition. Here, we investigated the relationships between the solute concentration of the supplied solution and the morphology and size of precipitated crystals for four types of fluorescent organic low molecule material [tris(8-hydroxyquinoline)aluminum (Alq3), 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD), and N,N-bis(naphthalene-1-yl)-N,N-diphenyl-benzidine (NPB)] using an ionic liquid as the nonvolatile liquid. As the concentration of the supplied solution decreased, the morphology of precipitated crystals changed from dendritic or leaf shape to platelike one. At the solution concentration of 0.1 mg/ml, relatively large platelike single crystals with a diagonal length of over 100 μm were obtained for all types of material. In the experiment using ionic liquid and dioctyl sebacate as nonvolatile liquids, it was confirmed that there is a clear positive correlation between the maximum volume of the precipitated single crystal and the solubility of solute under the same solution supply conditions.

We study Raman scattering from surface phonon polaritons in regularly arrayed GaN nanowalls and nanocolumns. The surface phonon mode around 705 cm(-1) decreases in intensity with increasing wall width w under a constant interval period p = 355 nm in the nanowalls, and it disappears in the film. This suggests that the interaction between them on the adjacent walls works so as to satisfy the selection rule in the film. The Raman peak from the surface phonon polariton in the nanocolumns is much broader than that in the nanowalls. It decreases in intensity and increases in width with decreasing column diameter d, and it is not observed when d = 75 nm. This behavior is discussed in terms of the quantization of the azimuthal wave vector of a surface phonon polariton on the circumference of the columnar structure. The Raman spectrum of the surface phonon polariton in the regularly arrayed GaN nanocolumns is compared with that of the Frohlich mode in irregularly arrayed ones.

<title>Abstract</title>We demonstrated the fabrication of a peculiar GaN/InGaN-based high-density nanocrystal array on a nitrogen polarity GaN layer using a simple self-assembly process for the first time. The nanocrystals consist of bending InGaN nanoplates and supporting GaN nanocolumns. The nanocrystals are umbrella shaped with diameters of ∼200–700 nm; therefore, they are referred to as InGaN nanoumbrellas. Transmission electron microscopy revealed the crystal structures of the nanoumbrellas and provided information about their growth mechanism. The photoluminescence (PL) properties of the InGaN nanoumbrellas were also characterized, and an extremely wide range of optical emission wavelengths (∼360–800 nm) was obtained from a small excitation diameter of ∼10 μm. Multiple sharp peaks resembling lasing actions were also observed in the PL spectrum; the resonant mode was likely caused by the whispering gallery mode. These results indicate that the high-density GaN/InGaN-based nanoumbrella array can be used as a source of white light without phosphors.

The development of a solution-based technique for the deposition of small-molecule organic thin films is desirable for the advancement of organic devices. In this study, we investigated the deposition characteristics of an organic thin film deposited by modified electrospray deposition with three typical spraying modes: Taylor cone, convergent jet, and multijet. The results indicate that the multijet mode can generate the smallest droplets with the narrowest size distribution. Using the multijet mode, we deposited a pinhole-free, smooth CBP:PBD:TPD:Ir(mppy)<inf>3</inf>small-molecule organic film with a root-mean-square surface roughness of 2.5 nm.

Effects of Al doping on surface morphology, sheet resistance, optical transmission spectra, and thermal durability of a thin Ag layer and AZO/Ag/AZO dielectric/metal/dielectric (DMD) transparent conductive films (TCFs) were investigated. The 1.7 at. % Al doping suppressed the initial island growth of a thin Ag layer and the plasmon resonant absorption dip in the optical transmission spectra. The threshold thickness of percolation conductivity was reduced from 9–10 (pure Al layer) to 5–6 nm (1.7 at. % Al-doped Ag layer). Al doping in the Ag layer improved the thermal durability of AZO/Ag/AZO-DMD TCFs. The threshold temperature for Ag void formation increased from 400 °C (DMD with pure Ag layer) to 600 °C (DMD with a 10.5 at. % Al-doped Ag layer). The optimum annealing temperature increased from 300 °C (DMD with a pure Ag layer) to 500 °C (DMD with a 10.5 at. % Al-doped Ag layer). Maximum figures of merit (FOM) were 0.5 × 10⁻²and 1.1 × 10⁻²Ω⁻¹for the DMD with a pure Ag layer and that with a 10.5 at. % Al-doped Ag layer, respectively.

Abstract Transparent conductive films (TCFs) with dielectric/metal/dielectric (DMD) structure covering the UVA (λ = 315–380 nm) and visible wavelength region (380–780 nm) were fabricated using Al‐doped Mg_0.4Zn_0.6O (AMZO) dielectric and 1.7 at% Al‐doped Ag (Ag(Al)) metal layers. The AMZO/Ag(Al)/AMZO‐DMD film with a 7 nm thick Ag(Al) layer after annealing in vacuum at 400 °C showed superior performance as a TCF, i.e., average transmittance from UVA to visible region (315–780 nm) of 88.2% and sheet resistance of 7.6 Ω/sq. The effect of Al doping on optical transmission, sheet resistance, surface morphology, and thermal durability of the AMZO/Ag(Al)/AMZO‐DMD films were also investigated. The 1.7 at% Al doping suppressed the initial island growth of the thin Ag layer and the plasmon resonance absorption dipped in the optical transmission spectra. Al doping in the Ag layer also improved the thermal durability of AMZO/Ag/AMZO‐DMD films. The threshold temperature for Ag void formation caused by metal segregation increased from 300 °C (DMD with a pure Ag layer) to 400 °C (DMD with an Al‐doped Ag layer). (© 2016 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

We report a new GaN etching technique with high anisotropy involving a thermal decomposition reaction in a low-pressure H<inf>2</inf>environment. A GaN microridge stripe structure (5 µm in width and 1.2 µm in height) with extremely smooth sidewalls was fabricated at 1,050 °C and a H<inf>2</inf>pressure of 10 Pa for 15 min using a SiO<inf>2</inf>mask. The activation energy of the vertical etching was calculated to be 62–77 kcal/mol. In the GaN nanoridge stripe structure, the side etching under the SiO<inf>2</inf>mask was less than 5 nm in depth and showed top width and height of ∼40 and ∼180 nm, respectively. The sidewall was extremely smooth and tilted by ∼15° from the m-plane along the a-axis, while being slightly rough and tilted by ∼30° from the a-plane along the m-axis. The [Formula: see text] ([Formula: see text]) planes were relatively stable in this etching technique.

We observed crossed transitions and anti-Stokes emissions in single quantum-dot-like objects embedded in the active layer of InGaN/GaN quantum disks by two-photon absorption techniques. We proposed a phenomenological model based on the interplay between Auger effect and crossed transitions to explain the origin of anti-Stokes emissions and the preferential excitation of 0D objects at the expense of their surroundings. ((c) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have studied phonon modes in GaN/AlN multiple quantum disk nanocolumns, which are self-organized on Al2O3 and Si substrates by rf-plasma-assisted molecular beam epitaxy, by means of Raman scattering. The Raman spectrum drastically changes with decreasing thickness of the quantum disks. In thick quantum disks, the broad peaks reflect the density of states of the pristine phonon modes in GaN and AlN. They are activated by the folding of the Brillouin zone owing to the multiple quantum disk structure. Several peaks of the confined, quasi-confined, and interface phonon modes are observed in thin quantum disks, which are peculiar modes in the superlattice structure. Moreover, the pristine optical phonons are entirely confined to one of the constituents in the case of very thin quantum disks, where the dielectric continuum model is not applicable. © 2013 The Physical Society of Japan.

The characteristics of MoO3/F8BT/ZnO inorganic/organic hybrid light emitting diodes (IO-HyLEDs) fabricated on ZnO/Ag/ZnO dielectric/metal/ dielectric (DMD) and conventional ZnO/ITO were investigated. The DMD had a low sheet resistance of 9 Ω/sq and a high transmittance of 90.7%. The device fabricated on DMD showed similar current density-voltage (J-V) and luminance-current density (L-J) characteristics to that on ZnO/ITO, indicating the possibility of DMD as a promising transparent conductive layer for IO-HyLEDs. The maximum luminous intensity of 237,000 cd/m2 was demonstrated under pulsed condition for the DMD device. We also investigated the effect of the combination interlayer (CIL) at the F8BT/ZnO interface on the IO-HyLEDs. The CIL was composed of a Mg0.52Zn0.48O/Mg 0.25Zn0.75O bilayer and a self-assembled dipole molecule (SADM) of BA-CH3. The devices with CIL exhibited an approximately threefold enhancement of the luminous intensity and efficiency in comparison with the devices without CIL. This improvement was considered to be brought about by the enhancement of the electron injection efficiency by CIL. © 2012 Elsevier B.V. All rights reserved.

We conducted numerical and experimental studies on Anderson localization of light in two-dimensional random systems of semiconductor columns. We investigated finite size scaling of the localization effect as a function of localization length and system size from the calculation. We therefore obtained a localization parameter map. We also report observations of random lasing in the semiconductor samples. We show that the occurrence of the laser action have a strong relationship to the localization length by comparisons of the map with the experimental results

We study Raman scattering of GaN regularly-arrayed nanowalls, nanocolumns and nanorings, and observe a peak from surface phonons around 705 cm -1. The peak from the surface phonons in the nanocolumns is much broader than those in the nanowalls and nanorings, but is not observed when the diameter of column is thinner than 100 nm. This is explained in terms of quantization of the azimuthal wave vector on the circumference of columnar structure. In the nanorings, we observe a very weak peak from a surface phonon. © 2013 AIP Publishing LLC.

Optical properties of InN grown by the epitaxial lateral overgrowth (ELO) method have been studied using photoluminescence (PL) and excitation-correlation (EC) measurements. The PL spectrum is analyzed by free-electron recombination band (FERB) model, which shows that the ELO sample has a very low background carrier concentration (n=5.5*10(16)[cm(-3)]). EC measurements show that the dependences of the band gap renormalization and Auger effect on the carrier concentrations are similar in spite of the different physical origins.

We study electric conduction of single GaN nanocolumns, which are synthesized by rf-plasma-assisted molecular beam epitaxy. We attach Ti/Al electrodes to a single GaN nanocolumn on SiO2/Si substrate by using photolithography, and we succeed in creating the ohmic contacts. We observe the nonlinear conduction at large currents and low temperatures. The temperature dependence of resistivity represents the activation-type one with an activation energy of about 11 meV due to donor levels at high temperatures. At low temperatures, it shows the Mott variable-range hopping conduction. We find a strong positive magnetoresistance effect at low temperatures. © 2012 The Surface Science Society of Japan.

We study micro-Raman spectroscopy in free-standing single GaN nanocolumns with 0.45-2.1 ìm diameters, which are synthesized by rf-plasma-assisted molecular beam epitaxy. We clearly observe the breakdown of the polarization selection rule of Raman spectra from E H 2 phonon, when the diameter of nanocolumn is thinner than four times the wavelength of incident light inside the sample. The selection rule gets recovered with increasing diameter of nanocolumn. Moreover, we observe quasi-TO phonon modes, which strongly affects the Raman spectra from the A 1(TO) and E 1(TO) phonons. © 2012 The Surface Science Society of Japan.

We investigated the photoluminescence (PL) properties of regularly arranged N-polar InN microcrystals with m -plane sidewall facets. We observed narrow PL emission at 0.678 eV with a linewidth of similar to 14 meV at 4 K and a clear band-filling effect with increasing excitation power. We also observed a normal red shift of the PL peak energy as large as 51 meV (similar to 150 nm) with increasing temperature from 4 to 300 K, similar to that observed for non-degenerated semiconductors. The integrated PL intensity ratio I300K/I4K was measured to be 6.1%. These results indicate that InN microcrystals have a low residual carrier density and excellent optical properties without being adversely affected by surface electron accumulation, despite their relatively high surface area. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High-quality Inx Al1xN (0.71 = xIn = 1.00) nanocolumns (NCs) have been grown on Si(111) substrates by rf-plasma-assisted molecular-beam epitaxy (rf-MBE). Low-temperature photoluminescence (LT-PL) spectra of various In-rich InAlN NCs were measured at 4 K and single peak PL emissions were observed in the wavelength region from 0.89 mu m to 1.79 mu m. Temperature-dependent PL spectra of In0.92Al0.08N NCs were studied and the so-called S-shape (decreaseincreasedecrease) PL peak energy shift was observed with increasing temperature. This shift indicates the carrier localization induced by the In segregation effect and is different from the anomalous blue shift frequently observed in InN films and nanowires with high residual carrier concentra- tions. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The optical properties of InGaN/GaN multiple-quantum-well (MQW) nanocolumns whose emission wavelength is in the yellow-to-red region (2.1 similar to 2.3 eV) have been investigated using time-resolved photoluminescence (PL) measurements. The analysis of the PL decay curves using a stretched exponential function shows that the radiative and nonradiative rates in the red region are almost the same as those in the yellow region. Long-lived migrating carriers also contribute the strong PL intensity in the red region. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The orientation-dependent lateral growth of InN was studied and the epitaxial lateral overgrowth (ELO) of InN by rf-plasma-assisted molecular-beam epitaxy was demonstrated for the first time using stripe molybdenum (Mo)-mask-patterned sapphire (0001) substrates. Transmission electron microscopy observation revealed a high dislocation density of similar to 5x10(-9) cm(-2) in the window region. By contrast, very few threading dislocations were observed in the wing region. In particular, there were no threading dislocations in the superficial layer of up to 3 mu m width. An InN ELO sample exhibited narrow near-IR emission with a peak photon energy of 0.677 eV and a linewidth of 16.7 meV at 4 K. Copyright 2011 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [doi: 10.1063/1.3664138]

We propose an array of submicrometer mirrors to assess luminescent nano-objects. Micromirror arrays (MMAs) are fabricated on Si (001) wafers via selectively doping Ga using the focused ion beam technique to form p-type etch stop regions, subsequent anisotropic chemical etching, and A1 deposition. MMAs provide two benefits: reflection of luminescence from nano-objects within MMAs toward the Si (001) surface normal and nano-object labeling. The former increases the probability of optics collecting luminescence and is demonstrated by simulations based on the ray-tracing and finite-difference time-domain methods as well as by experiments. The latter enables different measurements to be repeatedly performed on a single nano-object located at a certain micromirror. For example, a single InGaN/GaN nanocolumn is assessed by scanning electron microscopy and microphotoluminescence spectroscopy. (C) 2011 American Institute of Physics. [doi:10.1063/1.3589855]

We report on optically pumped green stimulated emissions from regular nanocolumn arrays. Various InGaN-based triangular-lattice nanocolumn arrays with different lattice constants and nanocolumn diameters were prepared on a (0001) GaN/c-sapphire template using selective-area growth by rf-plasma-assisted molecular-beam epitaxy. The nanocolumn arrays were optically pumped at room temperature, and sharp peaks of stimulated emission were observed at green color wavelengths from 530 to 560 nm. The wavelengths varied with the photonic band edge, which was determined by the structural parameters of the nanocolumn arrays. The photonic crystal effect occurred, with the periodic arrangement of the nanocolumns contributing to the stimulated emission. (C) 2011 The Japan Society of Applied Physics

The possibility of Anderson localization of light in random dielectric systems has been discussed over the last three decades [1]. However, theoretical and experimental studies for the light localization in random media are much more difficult than those in photonic crystals. Therefore, observation of the light localization has been realized only recently [2]. As a result, it is fair to say the wave localization phenomenon is not well understood. © 2011 IEEE.

In disordered materials, the combination of multiple light scattering and optical interference induces the localization of light. This phenomenon is called Anderson localization which is widely observed in electron systems with random potentials. Experimental studies of Anderson localization of light have been performed over the last three decades [1]. However, most of those studies attempted to secure the evidence of light localization by macroscopic observation of the light scattered by disordered materials. Recently, we proposed the direct observation of light localization using GaN nanocolumn samples by near-field scanning optical microscopy (NSOM), and presented the evidence of the light localization by a histogram analysis of NSOM image and by showing the wavelength dependence of the position of the localized spots in NSOM images [2,3]. We also observed random lasing in GaN nanocolumn samples and discussed the lasing property from a viewpoint of Anderson localization [4]. In this study, we extracted the two-dimensional (2D) spatial dependence from the NSOM image to estimate the localization length in GaN nanocolumns. © 2011 IEEE.

The relaxation dynamics of photo-excited carriers of indium nitride (InN) films and nanocolumns were examined using degenerate pump-probe measurements at room temperature. We measured two InN films and nanocolumns with different background carrier densities, and performed numerical calculations incorporating band-filling and bandgap-renormalization effects, as well as LO phonon scattering. We found that the intrinsic relaxation properties of InN can be understood by considering the density of states and electron occupation number of the conduction band. It was also revealed that the decay dynamics of InN are not affected by the carrier recombination time under the appropriate conditions. In addition, we examined the differences in carrier relaxation properties between films and nanocolumns.

Inorganic/organic hybrid light-emitting diodes (LEDs) (IO-HyLEDs) composed of p-type GaN/n-type Tris-(8-hydoroxyquinoline) aluminum (Alq3) were fabricated with and without thin MgO electron-blocking layer (EBL) at the inorganic/organic interface. These LEDs showed clear and stable current rectifying diode characteristics and electroluminescence (EL) peaked at UV region at room temperature. For the sample with MgO-EBL, obvious enhancement of green emission from Alq3 layer was observed. This result suggests that due to effective suppression of electron transport from Alq3 to p-GaN by MgO-EBL, radiative recombination of electrons and holes in Alq3 layer was enhanced. It was indicated that the band engineering technique can be applied to control the emission property of inorganic/organic hybrid LED. © 2011 Materials Research Society.

In the two-dimensional random system composed of a disordered array of a dielectric cylindrical column ensemble, Anderson localization of light is possible. We show localization parameter maps for the light localization adopting parameters of gallium nitride nanocolumn samples, which consist of random arrays of parallel nanosized columnar semiconductor crystals. The maps indicate parametric dependence of the localization characteristics on the light frequency, the radius of the columns, and the filling fraction of the columns. To obtain the maps, we have simulated temporal light diffusion in random media using the two-dimensional finite-difference time-domain method and analyzed the simulation results by Fourier transformation. We conclude that the main mechanism for localization varies continuously with the column filling fraction from Mie resonance of single column to Bragg-like diffraction of the column ensemble.

We study Raman scattering of GaN nanowalls and regularly-arrayed GaN nanocolumns, and observe a peak from surface phonon (SP) around 705 cm(-1). The Raman intensity of SP decreases with increasing wall width in GaN nanowalls, while it increases with increasing column diameter in GaN nanocolumns. The Raman peak of SP has different polarization characteristics between GaN nanowalls and nanocolumns.

In the two-dimensional random system composed of a disordered array of dielectric cylindrical columns, Anderson localization of light occurs. To obtain frequency dependence of the light localization characteristics, we have simulated temporal diffusion of electromagnetic waves in such a random system adopting parameters of actual nanosized semiconductor samples with a high filling fraction of the columns, using the finite-difference time-domain method. We have investigated diffusion length, autocorrelation function of light energy density, and time variation in total energy within the system at several frequencies. We obtain universal behavior of light localization phenomenon as a function of the light localization length and system size, from which we estimate frequency dependence of the localization length. In addition, we show that the frequency dependence of the localization effect depends on the degree of wave interference due to Bragg-like diffraction, rather than on the magnitude of the light scattering cross section of a single scatterer.

We investigated the selective-area growth (SAG) of InN by rf-plasma-assisted molecular-beam epitaxy using molybdenum (Mo)-mask-patterned sapphire (0001) substrates, which resulted in the formation of regularly arranged N-polar InN microcrystals. Transmission electron microscopy observation confirmed that the laterally grown side areas were nearly dislocation-free, although many threading dislocations (10(9)-10(10) cm(-2)) were generated at the InN/sapphire interface and propagated into the center of the InN microcrystals along the crystal c-axis. The laterally grown InN microcrystals exhibited narrow near-IR emission spectra with a peak photon energy of 0.627 eV and a linewidth of 39 meV at room temperature. (C) 2010 American Institute of Physics. [doi:10.1063/1.3488824]