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This study proposes using double-layer wire-grid structures to create narrow-band, perfect plasmonic absorbers, which depend on polarization, for the short-wavelength visible and near-ultraviolet regions of the electromagnetic spectrum. A rigorous coupled-wave analysis reveals that the maximum absorption attained using Ag and Al is ~90% at 450 nm and 375 nm. Experiments using Ag yielded results similar to those predicted by simulations. These results demonstrate that narrow-band perfect plasmonic absorbers, which depend on the polarization, can be realized at 450 nm and 375 nm using Ag or Al.

We fabricated a binary diffractive lens to control focal distribution, such as intensity distribution, by controlling the focal length and depth of focus. The results revealed changes in the focal length and depth of focus as a function of changes in the ring zone interval Δ R M at the end of the lens. Similar results were obtained from experiments. The peak position on the optical axis shifts further away from the lens. The half-width in the propagation direction increases with the Δ R M . These results demonstrate the possibility of controlling the focal distribution using single flat lenses by changing the periodic structure.

This study proposes a plasmonic band-stop filter with surface plasmon resonance in a doublelayer wire grid structure targeting short-wavelength visible and near-ultraviolet regions for applications in ultraviolet photography. Using Ag and Al, the rigorous coupling wave of analysis method revealed that the maximum absorption was approximately 90% at 450 nm and 375 nm. The experiments using Ag produced similar results in a simulation. These results demonstrate that plasmonic band-stop filters in the visible and near-UV region can be realized at 450 nm and 375 nm using Ag or Al.

Selective area growth of GaN (SAG-GaN) films grown on nonpolar m-plane bulk GaN substrates using trench patterns was performed by metalorganic vapor phase epitaxy. We investigated the transformation of SAG-GaN facet structures by changing growth temperature, ambient, and miscut angle of substrates. Substrates with trench patterns along a -axis formed (10-11), (10-10), and (000-1) facet structures after SAG-GaN growth for growth conditions of 800 °C in N 2 +NH 3 and 1000 °C in H 2 +NH 3 ambience. Those with trench pattern along 45° off from a -axis contained (11-22), (10-10), (0-110) facet structures in substrates with a miscut angle of 5° whereas SAG-GaN films completely coalesced and formed smooth (10-10) surface in substrates with a miscut angle of 1°. Undesirable grains were formed for 800 °C in N 2 +NH 3 ambience regardless of the miscut angle of substrates whereas theses grains were annihilated using 1000 °C in H 2 +NH 3 ambience due to the intensively hydrogen etching.

High-crystallinity a-plane AlN(11 2 0) films containing a low-temperature AlN (LT-AlN) buffer layer and a high-temperature AlN (HT-AlN) film were grown on r-plane sapphire(1 1 02) substrates. We investigated the effect of the growth temperature and thermal annealing conditions for the LT-AlN buffer layers on the crystallinity and surface morphology. The surface roughness of the buffer layers became smooth with the decrease in growth temperature to 900 °C, and the crystallinity of the buffer layers was improved by thermal annealing at temperatures over 1600 °C. HT-AlN films were then grown on the annealed LT-AlN buffer layers at 1500 °C. The optimum crystallinity of HT-AlN films without any facet formation at the surfaces was obtained with full width at half maximum values of the X-ray rocking curves for AlN(11 2 0)//[1 1 00]AlN at 770 and (0002) at 640″.

In plant cultivation, the number of photons is more important than the light energy from the chemical reactions that occur during photosynthesis. In addition, the blue and red photon flux (B/R) ratio is an important parameter for plant cultivation. Here we discuss the effect of the spectral irradiance distribution and the B/R ratio on plant cultivation. We cultivated lettuce seedlings, Lactuca sativa L. Cv. Okayama, using a light-emitting diode illumination system that can precisely control the spectral irradiance distribution and B/R ratio. The B/R ratio varied from 0.36 to 2.06 according to the intensity of the blue light when the photosynthetic photon flux density values were sufficient to ensure the 150 - 200 μmol⋅m−2⋅s−1. High photon flux densities of blue light result in reduced plant length, plant height, and leaf area, thereby suggesting its role in the suppression of leaf growth. Therefore, we conclude that a lower photon flux of blue light (B/R Ratio) is optimal for lettuce cultivation.

An aluminium nitride (AlN) buffer layer with 200 nm thickness was grown on (0001) sapphire substrate using the metal–organic vapour phase epitaxy (MOVPE) method in a low-pressure furnace, followed by a clean-up treatment of sapphire substrate at 1100 o C. Thereafter, the AlN buffer layer was annealed at a high temperature in the range of 1500 o C to 1700 o C for 2 hours under the atmosphere of N2+CO. The objective of this research is to determine the microstructure changes with different annealing temperatures. Cross-sectional TEM has revealed that, after annealing at 1500 o C, two types of defects remained in the AlN buffer layer: inverted cone shape domains and threading dislocations. The former domains were observed in an image taken with diffraction of g=0002, but not in an image with g=1010. The morphology and the diffraction condition for the image contrast strongly, suggesting that the domains are inversion domains. The threading dislocations were invisible in the image taken with the diffraction of g=0002, revealing that they were a-type dislocations. However, after annealing at 1600 o C, the inversion domains coalesced with each other to give a two-layer structure divided by a single inversion domain boundary at the centre of the AlN buffer layer. The density of threading dislocation was roughly estimated to be 5×10 9 cm -2 after annealing at 1500 o C, and to be reduced to 5×10 8 cm -2 after annealing at 1600 o C. These experimental results validate the fact that the annealing temperature around 1600 o C is high enough to remove the defects by the diffusion process. Therefore, it was discovered that high temperature annealing is an effective treatment to alter the microstructure of AlN thin films and remove defects by the diffusion process. Annealing at high temperature is recommended to increase the emission efficiency for fabrication of optoelectronic devices.

We experimentally detect high-refractive-index media (n > 1.5) using a surface plasmon resonance (SPR) sensor with a diffraction grating. While SPR sensors are generally based on the attenuated total reflection method using metal films, here, we focus on a method using a diffraction grating, which can detect relatively higher refractive-index media and is suitable for device miniaturization. In this study, we used the rigorous coupled-wave analysis method to simulate the dependence of the reflectance on an incident angle for media with refractive index values up to 1.700. In the experiment, a medium (n = 1.660 - 1.700) was successfully detected using this grating. Under the conditions of the grating (period: 600 nm, Au thickness: 40 nm) using a red laser (λ: 635 nm), a sharp decline in the reflectance and a rise in the transmittance at certain angles were confirmed, demonstrating the extraordinary transmission enabled by SPR. Because excitation angles changed with changes in the refractive index, we concluded that this method can be applied to sensors that detect high-refractive-index media.

To fabricate such deep UV devices, a high-quality underlying AlN film is desirable because of its wide bandgap. In this work, we studied the effects of thermally annealing sputtered AlN films grown on sapphire substrates on the crystallinity and surface morphology, where the thermal annealing was performed in nitrogen ambient. The effects of annealing were investigated as a function of the sputtered AlN film thickness and the thermal annealing temperature. The thicknesses of the sputtered AlN films were 170 and 340 nm. Subsequently, the sputtered AlN films were thermally annealed in N2 at 1600 - 1700 oC for 1 h. Both the (0002) and (10-12) XRCs of the AlN films had a single sharp peak after thermal annealing owing to the elimination of the tilt and twist components from the sputtered AlN films by high-temperature thermal annealing. The improved crystallinity is related to the solid-phase reactions that occur at high annealing temperatures. The FWHMs of the (0002) and (10-12) XRCs were markedly reduced from 532 to 49 and 6031 to 287 arcsec, respectively. Cross-sectional STEM images of sputtered AlN on sapphire substrate after annealing were observed. Columnar structures are observed in grown layer before annealing while they disappear after annealing. With annealing, hexagonal shaped voids pointed by white arrows form and the polarity invert from N-polar to Al-polar about 10 nm away from AlN/sapphire interface.

Surface plasmon resonance (SPR) is widely used in applications such as the detection of infinitesimal materials and high-efficiency photoelectric conversion. For a target medium with a high refractive index (n > 1.5), total reflection generally becomes impossible owing to a limitation in the incidence angle of light; therefore, such a medium cannot be detected. For this problem, we focused on a method that uses a diffraction grating as the structure of the sensor.

We fabricate a double-layer wire grid polarizer (WGP) and perform optical characterization to clarify the relationship between the structural and polarization characteristics. For normal incidence, the fabricated double-layer WGP exhibits an extinction ratio of 30.4 dB for a period of 400 nm. The transverse magnetic transmittance peak angle is found to vary with the period of the WGP. The peak shift can be explained on the basis of the extraordinary optical transmittance phenomena exhibited by the surface plasmon polaritons (SPPs) of metal slit structures according to the dispersion curve of the SPP. From the simulation of rigorous coupled-wave analysis, it is considered that the incident light passes through the resist layer, followed by the excitation of SPPs at the interface between the resist and Au. Subsequently, the SPPs combine with the transmitted light in the glass substrate, leading to strong transmitted light with transverse magnetic polarization. Therefore, we demonstrate the extraordinary optical transmittance phenomena exhibited by the SPPs by both experiment and simulation.

We exploit surface-plasmon polaritons (SPPs) at the GaP–Au interface to detect chemical species in high refractive index media and verify the device function by detecting 2,4-dichlorotoluene (C 7 H 6 Cl 2 , n =1.55) and 1-bromonaphthalene (C 10 H 7 Br, n =1.65). The calculated dispersion relationship shows that the SPP excited in GaP by 635 nm light incident at 31.6° is effective for detecting C 7 H 6 Cl 2 . Moreover, we analyze the reflectance as a function of the Au layer thickness and find that the illumination angle at which SPPs are excited is consistent with simulation results. When illuminated at 31.6°, the optical electric field is concentrated and transmitted along the Au–C 7 H 6 Cl 2 interface. Furthermore, we use the device to detect solvent concentrations of the solutions C 2 H 5 OH–C 7 H 6 Cl 2 and C 2 H 5 OH–C 10 H 7 Br. The SPP-excitation angle increases with increasing concentration of C 7 H 6 Cl 2 and C 10 H 7 Br. We conclude that chemical species in solutions with an index of refraction less than 1.62 can be detected with this approach. These results suggest that SPPs excited at the GaP–Au interface can be exploited to detect chemical species in high-refractive-index media.

The influences of three types of lattice-relaxation layer between AlGaN multiple quantum wells (MQWs) and AlN layer on sapphire substrate were investigated. For structure (a), two-high-Al-mole-fraction AlGaN layers grown at 1170 °C was prepared. For structure (b), an AlN interlayer with growth temperature at 1450 °C was inserted between AlN/sapphire substrate and two-high-Al-mole-fraction AlGaN layer. For structure (c), an Al-composition-graded AlGaN layer with decreasing Al composition was grown after an AlN interlayer at 1450 °C by lowering the temperature gradually from the growth temperature of AlN. The narrowest rocking curve X-ray diffraction at AlGaN MQWs occurs while using structure (b) as lattice-relaxation layer. The highest emission intensity of cathodoluminescence from AlGaN MQWs at room temperature was also shown by using structure (b). The ratio of integrated intensity of spectra at 300 K and 10 K (I300K/I10K) is 58%. High temperature AlN films as lattice-relaxation layer improved the crystal quality and emission intensity in AlGaN MQWs. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The combined temporally, spatially and spectrally-resolved optical techniques, namely the photoluminescence, light induced transient grating, and differential reflectivity were used for investigation of excitation-dependent PL efficiency, exciton lifetime, and diffusion coefficient in Si-doped Al-rich multiple quantum wells and epilayers at various temperatures. Novel features of carrier recombination and in-plane diffusion were observed. Low-excitation radiative lifetime of 1–2 ns was found temperature-independent in 80–150 K interval, while it sublinearly decreased with excitation at excess carrier densities above 1018 cm−3. The lifetime decrease correlated with the increase of diffusion coefficient, indicating excitation-enhanced delocalization of localized excitons and therefore enhanced capture to nonradiative centers. The droop of photoluminescence efficiency with excitation was the strongest at 80–150 K due to strong delocalisation at low-temperatures, while at higher temperatures the thermal activation prevailed in photoluminescence excitation dependence. The photoluminescence efficiency quenching at T > 200 K provided rather high activation energies of ∼100 and 160 meV for Al-rich multiple quantum wells and epilayers, correspondingly.

The microscopic crystalline structure (MCS) such as domain texturing, lattice tilting fluctuation and strain fluctuation in thick AlN films grown on trench-patterned AlN/α-Al2O3 templates was clarified by utilizing position-dependent X-ray microdiffraction measurements in combination with transmission electron microscopy observations. The results clearly demonstrate that the trench-patterned templates have a strong influence on the MCS in the thick AlN films. The MCS is anisotropic between the [ 11 2 ¯ 0 ] and [ 1 1 ¯ 00 ] directions. Periodic domain texturing was observed along the [ 11 2 ¯ 0 ] direction, corresponding to the periodicity of the patterning pitch of the template. Submicron crystal domains are tilted at different angles to the [ 11 2 ¯ 0 ] direction, and the crystal domain tilting becomes larger in the void-containing trench regions than in the terrace regions. This generates the periodicity in lattice tilting fluctuation and strain fluctuation along the [ 11 2 ¯ 0 ] direction. These were found to be caused primarily by two factors; one of which being the elastic strain relaxation that depends on the growth direction in the terrace regions, and the other being the inhomogeneity in distribution and morphology of dislocations that have Burgers vectors with a screw component in the void-containing trench regions.

Conditions on chemical surface treatment and thermal treatment of sublimation-freestanding AlN substrates were investigated to remove damage layers on surfaces for high-quality and crack-free AlN films grown by hydride vapour phase epitaxy (HVPE). By wet etching, the residue on the surface was removed and the polishing scratches were reduced. Atomic steps were formed on the surface by the subsequent thermal treatment at 1450 °C for 10 min, and surface layer of 220 nm in thickness was removed, Homoepitaxial growth on freestanding AlN substrate with trench pattern was also performed. A crack-free AlN film with atomic steps was obtained on the trench-patterned bulk AlN substrate, and the emission at the band edge near 206.9 nm was dominant. The wavelength of luminescence from cross-sectional and surface of the HVPE-AlN layer was maintained. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We characterize the optical properties of a double-layer wire grid structure and investigate in detail the excitation mechanism of surface plasmon polaritons (SPPs). Angular spectra for the transmittance of the transverse magnetic polarized light that are obtained through the experiment reveal two peaks. In addition, simulated mapping of the transmittance and the magnetic field distribution indicate that SPPs are excited in two areas of the wire grid structures: at the interface between the Au layer and the resist layer or the glass substrate and at the interface between the Au layer and air. The experimental data are consistent with the transmittance mapping result and the distribution of the magnetic field. Accordingly, we constructed a model of SPPs propagation. We consider that SPPs excited at the interface between the Au layer and the resist layer or the glass substrate strongly contribute to the extraordinary transmission observed in the wire grid structures.

We report on the fabrication and analysis of a diffractive lens with binary structure that allows the control of focal length and depth of focus. Beam propagation measurements show that focal length and depth of focus depend upon the structural parameters of prototype lenses.

Principle and analysis for ellipsometers usable in the vacuum ultraviolet (VUV) region up to about 25 eV, which are consisting of a sample mirror and an inclined photodiode, are described. Our ellipsometer has two degrees of freedom in rotation; the main chamber can rotate around the axis of incidence and, the analyzer unit can also rotate around the axis of reflection. All the unknown parameters (Stokes parameters of the source, ellipsometric parameters of the sample and the polarization sensitivity of the inclined detector) are obtained by rotating the main chamber and analyzer independently and solving the simultaneous equations with the measurands. Measurement results on polarization characteristics of photodiodes, Stokes parameters of the beam emerging from a synchrotron radiation beamline, and optical constants of AlN film are given. It is concluded that the ellipsometer is advantageous in the simplicity and worked as intended.

We investigated local strain distribution in a cross-sectional area throughout the thickness of a thick aluminum nitride (AlN) film epitaxially grown on a trench-patterned AlN/α-Al2O3 template using X-ray microdiffraction measurements for AlN and Bragg reflections. The results show that the presence of voids caused by the trench pattern strongly influences on the distribution of the strain components in the and directions, which are perpendicular to the trench lines. Discrepancy between strain values obtained from the two Bragg reflections was shown to be the result of twisting of the crystal domains about the axis in the thick AlN film.

With 100-nm-thick 3C-SiC intermediate layer, GaN layer of relative high quality was obtained on Si substrate by using a maskless epitaxial lateral overgrowth (ELO). The three-dimensional growth of maskless ELO process was investigated with wafer reflectance collected during growth. Low temperature cathodoluminescence observation confirmed the improvement of the GaN quality by ELO. An AlN interlayer was used to prevent crack of the GaN epilayer. As a result, the as-grown GaN layer shows a crack-free surface with long atomic steps. The threading dislocation density is lower than 1 × 109 cm−2 in GaN layer underneath AlN interlayer.

Light-emitting devices based on high-Al content (Al,Ga)N suffer from high densities of dislocations or other point defects, counterbalanced by the carrier localization arising from disorder in the ternary alloy. One way to improve the internal quantum efficiency is the controlled Si-doping of AlxGa1−xN/AlyGa1−yN multi-quantum wells, which was assigned to the reduction of point defects by reduction of internal strains, for some ideal concentration of Si. Time-resolved photoluminescence (TR-PL) can be used [S. F. Chichibu et al., Appl. Phys. Lett. 99, 051902 (2011)] to try and correlate the observed PL time-decays with the density of defects, in relation with the nonradiative recombination probability of photoexcited carriers. We present TR-PL studies of MOVPE-grown Si-doped AlxGa1−xN/AlyGa1−yN multi-quantum wells with typical values of x = 0.6 and y = 0.7 and of AlxGa1−xN epilayers with x up to 0.86. High-Al content MQWs and epilayers exhibit similar bi-exponential PL decay dynamics, with the slower component rapidly quenched when T is increased. The fast decay component remains in the nanosecond range at all temperatures and the PL intensity loss is limited by carrier localization.

We have investigated crystalline morphology such as wafer curvature, mosaicity, and lattice tilting of an AlN film grown on a triangular-striped AlN/α-Al2O3 template by X-ray rocking curve (XRC) and reciprocal space map (RSM) measurements. The result of XRC for the AlN (0002) plane showed the difference of the value in the wafer curvature between the [11–20] and [1–100] directions. This indicates the anisotropic strain relaxation preferentially along [11–20] direction due to the triangular-striped structure and the voids in the AlN film. From the results of asymmetric RSMs for AlN 11–24 and 1–104 diffractions, the broadening of the reciprocal lattice spots in elliptical shape was observed. These results reflect anisotropic crystalline morphology in the AlN film. The elliptical diffraction spot for AlN 11–24 predominantly indicates the lattice tilting fluctuation around the [1–100] axes while that for AlN 1–104 indicates the lattice spacing fluctuation. Anisotropic crystalline morphology in the AlN film grown on the triangular-striped template has been clarified in combination with XRC and asymmetric RSMs.

X-ray microdiffraction (XRMD) measurements using the AlN 2 2 ¯ 01 and 1 2 ¯ 10 Bragg reflections were performed to determine the cross-sectional distribution of local residual strain and twisting of crystal domains for a thick AlN film grown on a trench-patterned AlN/α-Al 2 O 3 template. The distribution of the strain components in the [0001] and [ 11 2 ¯ 0 ] directions, which are perpendicular to the trench lines, was strongly influenced by the presence of voids caused by the trench pattern. The strains in the [ 1 1 ¯ 00 ] and [ 11 2 ¯ 0 ] directions determined using the two Bragg reflections were found to be significantly different, and this was shown to be the result of twisting of the crystal domains about the [0001] axis under the influence of anisotropic shear stress in the prismatic planes.

For epitaxy of GaN on 3C-SiC/Si substrates, optimization of growth temperature of AlN interlayers (ILs) was performed. With a proper growth condition of AlN IL, crack-free 3.5 μm thick GaN layer was realized by multiple AlN ILs on 3C-SiC/Si (1 1 1) substrate. The distribution of D 0 X peak position in low temperature cathodoluminescence spectra was mapped to investigate the stress in as-grown wafer. An increase of tensile stress was found in the top GaN layers above AlN ILs. Cross-sectional transmission electron microscopy images confirmed that AlN ILs could induce compressive stress and reduce threading dislocations in the GaN epilayer grown on 3C-SiC/Si. The reduction of dislocations should account for the part of incremental tensile stress revealed by the inhomogeneous distribution of luminescence.

The effects of Si doping on the structural properties of a high-quality AlN layer grown on a trench-patterned AlN/sapphire by metalorganic vapor phase epitaxy (MOVPE) were systematically studied. With a high Si doping concentration, void formation became much slower than that in undoped AlN film. This phenomenon was attributed to the anti-surfactant effect of Si in the growth process. Moreover, as the Si doping concentration increased, compressive stress was significantly relaxed in Si-doped AlN films. A high Si doping concentration of 1×1018 cm−3 was successfully achieved for AlN grown at 1500 °C.

We designed and fabricated two types of binary diffractive lenses using Electron beam lithography (EBL) on optical films film for controlling LED light. In the case of the binary diffractive convex lens with 2-mm focal length, it is possible to control the luminous intensity distribution. To improve the diffraction efficiency and realize a thin LED light source, the binary diffractive lenses with 100-μm-order focal length are effective. Furthermore we fabricated and characterized the binary diffractive concave lenses for application in LED lighting. It is found that white-light LEDs are strongly diffused by using the binary diffractive concave lenses.

Concept, equipment, and exploitation of spatio-time-resolved cathodoluminescence (STRCL) spectroscopy will be presented. As one of its advantages is unlimited bandgap energy of the sample, we show STRCL data of high AlN mole fraction Al x Ga 1−x N structures. For understanding the mechanisms of the improvement in the luminescence efficiency of Al 0.68 Ga 0.32 N / Al 0.77 Ga 0.23 N multiple quantum wells by proper Si-doping in the wells, STRCL measurements and self-consistent Schrodinger-Poisson calculations were carried out, and positron annihilation measurement was carried out on Al 0.6 Ga 0.4 N films with various Si-doping concentrations. The increase in the luminescence lifetime at room temperature, which reflects the decrease in the concentration of nonradiative recombination centers (NRCs), is correlated with increased terrace width of Si-doped wells. We will discuss the importance of H 3 SiNH 2 doping-reactant formation that gives rise to enhanced decomposition of NH 3 and provides wetting conditions by surface Si-N bonds, which reduce the total energy and concentration of NRCs composed of Al vacancies.

We experimentally detect high-refractive-index media using a surface plasmon resonance (SPR) sensor with a diffraction grating. In this study, we used the rigorous coupled-wave analysis method to simulate the dependence of the reflectance on an incident angle for media with refractive index values up to 1.700. In the experiment, a medium (n = 1.660-1.700) was successfully detected using this grating. Under the conditions of the grating (period: 600 nm, Au thickness: 40 nm) using a red laser (: 635 nm), the extraordinary transmission enabled by SPR is occurred at the interface between Au and a medium. We concluded that this method can be applied to sensors that detect high-refractive-index media.

The periodic trench patterning of sapphire substrates is useful for the growth of crack-free AlN thick films. However, the in-plane epitaxial relationship between AlN and a-plane sapphire depends on the initial growth temperature. Therefore, two trench directions, the a-plane sapphire and a-plane sapphire directions are investigated in this study. We investigated the control of in-plane epitaxial relationship between AlN and a-plane sapphire by low-pressure HVPE and the effect of the trench direction of AlN, which results in two different in-plane epitaxial relationships on a-plane sapphire (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The strong demand for high-efficiency light-emitting diodes in the deep-ultraviolet region and for sensors has led to renewed interest in the growth of high-quality AlN. However, the growth of high-quality AlN is extremely difficult owing to the low surface migration rate of Al adatoms. In this study, the effects of the carrier gas ratio and growth temperature on the growth rate of AlN are examined with the aim of optimizing the growth conditions of high-temperature AlN. AlN epilayers were grown on sapphire substrates by metal-organic vapor phase epitaxy (MOVPE) at a growth temperature of 1430 °C with a medium-temperature AlN layer grown at 1100 °C. A combination of H2 and N2 was used as the carrier gas in the growth of AlN, and the ratio of H2 to N2 in the carrier gas was changed to examine its effect on the growth of AlN. Under all growth conditions, crack-free and smooth surfaces at the atomic level with an extremely small tilt were obtained. The growth rate decreased with increasing growth temperature and ratio of H2 in the carrier gas. This suggests that the high temperature and the H2 carrier gas inhibit the adsorption of atoms. Also, with increasing ratio of N2 in the carrier gas, the radius of curvature of the AlN layer increased and the compressive strain decreased. The reduction of compressive strain and increase in curvature of the AlN layer were due to an increase in the number of dislocations. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Cold filters, which simultaneously reflect infrared light and transmit visible light, prevent overheating in charge-coupled device cameras, microscopes, and other heat-sensitive equipment. This study proposes a cold filter based on a two dimensional (2D) metal periodic grating structure. Conventional dielectric multilayer films with abrupt filtering characteristics are undesirably affected by incident angle, temperature, and polarization. To solve these problems, a 2D metal periodic grating structure, which does not depend on the polarization, was applied. The grating structure comprises an Au layer and an electron beam resist layer, and was fabricated by electron beam lithography. The optical characteristics of this structure in the visible light region were measured by a spectrometer, and the optical properties were related to structural parameters of the double-layer, 2D grating structure. In particular, the reflectance over the entire visible light spectrum decreased at periods of 800 nm and 1 μm. The wavelengths of minimum and maximum reflectance were shifted by changing the spacing between the upper and lower metal layers from 270 to 370 nm. Simulation results suggested that the interference between the upper and lower layers and the surface plasmon resonance between the metal and resist layers occur simultaneously. Therefore, in the visible light region, the reflectance and transmission spectra were controlled by altering the structure of the 2D metal periodic grating.

GaN layers were grown on 3C-SiC/Si substrates with AlGaN intermediate layers with various thickness and AlN mole fraction by low-pressure metal-organic vapor phase epitaxy (LP-MOVPE). The effects of the interlayer on the crystal quality and strain-status of GaN epilayer were investigated systematically by varing the thickness and the Al mole fraction of the AlGaN. Raman scattering and X-ray diffraction results show that the AlGaN with thinner thickness and higher Al mole fractions can improve the crystalline quality of the GaN epilayer and the redue the strain at the same time. This phenomenon should be attributed to three dimensional (3D) growth at the initial stage of (the) GaN epitaxy. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Cross-sectional transmission electron microscope (TEM) observation was performed in detail to analyze the morphology of threading dislocations (TDs) in GaN thin layers with various thicknesses. The GaN layers were overgrown on an Al0.28Ga0.72N layer by the metal-organic vapor-phase epitaxy (MOVPE) method. In a GaN layer about 50 nm in thickness, TDs running up in the AlGaN layer pass into the GaN layer and most of them reach the top surface without bending. In thicker GaN layers, on the other hand, many of TDs form a hairpin-configuration on or above the interface of GaN and AlGaN to be annihilated. This difference in morphology of TDs indicates that the TDs have moved down inside the GaN layer. Since the formation of hairpins is attributed to a stress-relief, there should be an extra half-plane between the paired TDs. Therefore, the movement of TDs should be of “climb motion”. Another example of possible TD movement inside a GaN layer is also described. It is emphasized that the possibility of TD-movements inside the thin film crystal during the growth should be taken into account in analysis of thin-layer growth through the behavior of TDs (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The growth of a-plane (11-20) AlN and AlGaN on r-plane (1-102) sapphire has been investigated with the aim of application to LEDs and LDs in the deep ultra-violet region. a-plane AlN and AlGaN were grown on r-plane sapphire by low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). The crystalline quality and surface morphology of a-plane AlN were optimized at a growth temperature of 1250 °C with the off-angle of r-plane sapphire between -1° and +1°, and a-plane AlGaN grown on the AlN layer using -0.45°-off r-plane sapphire as a substrate exhibited high crystalline quality. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A microstructure in an AlGaN/GaN layer was analyzed in detail by means of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) with special attention to the formation of steps on the surface. The AlGaN layer was grown by MOVPE on a GaN template with periodic trenches. It was revealed that there formed were Al-depleted bands in the AlGaN layer. These bands were generated from rather lower regions in the AlGaN layer or those above the trenches, and run upwards. Some of them reached the top surface to connect a macro step. The formation mechanism of the Al-depleted region is discussed in terms of thermodynamics. If the total bonding energy of atoms on the macro step of surface is assumed to be smaller than that of atoms on a flat surface, the Al-depletion can be explained provided that the local equilibrium in concentration is conserved during the growth of AlGaN layer. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigated the spatial distribution of luminescence near threading dislocations in AlGaN/AlGaN multiple quantum wells (MQWs) by cathodoluminescence mapping. Emission at the higher-energy side of the AlGaN MQW peak was locally observed near the threading dislocations, which were not accompanied by any surface V-pits. Such higher-energy emission was not observed in the AlGaN epilayers. The energy difference between the AlGaN MQW peak and the higher-energy emission peak increased with increasing barrier-layer Al composition. These results suggest that the origin of the higher-energy emission is likely local thickness fluctuation around dislocations in very thin AlGaN MQWs.

Nonpolar a -plane and m -plane AlN layers were grown on a -plane and m -plane 6H-SiC substrates by low-pressure hydride vapor phase epitaxy (LP-HVPE), respectively. The effects of growth temperature were investigated. Results showed that surface roughness was reduced by increasing the temperature for both a -plane and m -plane AlN layers. In-plane morphological anisotropy was revealed by scanning electron microscopy and atomic force microscopy, which was used to image the morphological and structural transitions with temperature. Anisotropy in on-axis X-ray rocking curves was also detected by high-resolution X-ray diffraction. However, compared with the a -plane AlN layer, a smooth surface was easily obtained for the m -plane AlN layer with good crystalline quality. The optimal temperature was lower for the m -plane AlN layer than that for the a -plane AlN layer. The stress characteristics of nonpolar AlN layers were studied using polarized Raman spectra. Results showed the presence of anisotropic in-plane stresses within the epitaxial nonpolar AlN layers.

This work quantitatively assessed the three-dimensional distribution of crystal lattice distortions in an epitaxial AlN thick film grown on a trench-patterned template, using nanobeam X-ray diffraction. Position-dependent ω-2θ-φ mapping clearly demonstrated local tilting, spacing and twisting of lattice planes as well as fluctuations in these phenomena on a sub-micrometer scale comparable to the pitch of the trench-and-terrace patterning. Analysis of the crystal lattice distortion in the depth direction was performed using a newly developed method in which the X-ray nanobeam diffracted from the sample surface to specific depths can be selectively detected by employing a Pt wire profiler. This technique generated depth-resolved ω-2θ-φ maps confirming that fluctuations in lattice plane tilting and spacing greatly depend on the dislocation distribution and the history of the AlN epitaxial growth on the trench-patterned structure. It was also found that both fluctuations were reduced on approaching the AlN su...

To obtain low-dislocation-density c-plane AlN on sapphire, we have studied the annealing of AlN films at 1650 °C. We prepared an AlN film with a thickness of 20 nm by radio-frequency (RF) sputtering, on which an AlN epilayer with a thickness of 280 nm was grown by metalorganic vapor phase epitaxy (MOVPE) at different growth temperatures. Finally, the AlN film on sapphire was annealed at 1650 °C. The full width at half maximum (FWHM) of the X-ray rocking curve (XRC) for AlN(0002) was approximately 50 arcsec for all samples, whereas that for AlN() significantly decreased from 6500 to 350 arcsec after thermal annealing. A surface morphology with atomic steps and a terrace structure was clearly observed after thermal annealing. From these results, we concluded that a high-quality AlN film can be obtained by annealing a stacking structure of MOVPE-grown AlN (buffer layer)/sputtered AlN (nucleation layer)/sapphire substrate.

Al0.25Ga0.75N/AlN/GaN high electron mobility transistor (HEMT) structures were grown on (0 0 0 1) sapphire substrates with vicinal angles of 0 . 0 ∘ , 0.2 5 ∘ , 0 . 5 ∘ and 1 . 0 ∘ by metalorganic vapor phase epitaxy (MOVPE). Vicinal sapphire was demonstrated to enhance the step-flow growth to improve morphology, crystal and optical qualities, which eventually suppressed interface scattering and dislocation scattering to enhance the mobility of 2-dimension-electron-gas (2DEG). The optimum vicinal degree was determined to be 0 . 5 ∘ , and the corresponding 300 K Hall mobility and sheet resistance were 1720 cm2/V s and 301 Ω/sq, respectively. Furthermore, the temperature dependence of Hall measurements proved good high-temperature performance of the 0.5-off sample.

The effect of the off-cut angle of an r -plane sapphire substrate has been investigated on the growth of a -plane AlN thick layer by low-pressure hydride vapor phase epitaxy (LP-HVPE). The off-cut angle ( θ ) was changed from +5.0° (close to c -axis) to −5.0° (close to m -axis). Results show that the crystalline quality and surface morphology are very sensitive to the sign of θ off-angle. The plus θ off-angle is found to be dramatically reduce the full-widths at half-maximum (FWHM) of X-ray rocking curves (XRC), compared with the minus θ off-angle. In-plane FWHM anisotropic feature marked as M- or W-shape dependence on azimuth angle was observed for a -plane AlN. The shape and degree of anisotropy depend on the sign of θ off-angle, while the plus of θ off-angle will leads to the W-shape and the decreased anisotropy. The minimum crystal tilts and twists of the films are observed for the vicinal sapphires with the plus off-angles of +0.2° to +1.0°.

A 4–6 μm thick a -plane (1 1 2¯ 0) AlN was grown on r -plane sapphire substrate by low-pressure hydride vapor phase epitaxy (LP-HVPE), using a direct growth without any nitridation and buffer layer, a single-step nitridation growth, a two-step nitridation growth and a two-step buffer growth method. For the two-step buffer growth procedure, smoother surface is observed with the lower full widths at half maximum (FWHM) of X-ray rocking curves (XRC) compared with the other two kinds of nitridation procedures. A smaller FWHM of in-plane XRC peak anisotropy features are reversed, which is consistent with the smaller in-plane stress anisotropic distribution in a -plane AlN, when the two-step nitridation or buffer growth method is used. In four kinds of initial growth procedures, the two-step buffer method is the suitable method for the growth of a -plane AlN by HVPE with the high crystal quality and more isotropic distribution.

Si-doped a -plane GaN films with different doping concentrations were grown by metal-organic vapor phase epitaxy. A mirrorlike surface without pits or anisotropic stripes was observed by optical microscopy. Detailed optical properties of the samples were characterized by temperature- and excitation-intensity-dependent PL measurements. A series of emission peaks at 3.487, 3.440, 3.375–3.350, 3.290 and 3.197 eV were observed in the low-temperature PL spectra of all samples. The origin of these emissions is discussed in detail.