Broadband Ag/SiO2/Fe/TiO2 ultrathin planar absorber with a wide acceptance angle from visible to near-infrared regions


Investigation of V/III ratio dependencies for optimizing AlN growth during reduced parasitic reaction in metalorganic vapor phase epitaxy.


High growth temperature for AlN by jet stream gas flow metalorganic vapor phase epitaxy.


Ultra-thin deep ultraviolet perfect absorber using an Al/TiO2/AlN system


Major target for UV-induced complete loss of HIV-1 infectivity: A model study of single-stranded RNA enveloped viruses


Reduction of parasitic reaction in high temperature AlN growth by jet stream gas flow metal organic vapor phase epitaxy


Ultraviolet Light-Emitting Diode (UV-LED) Sterilization of Citrus Bacterial Canker Disease Targeted for Effective Decontamination of Citrus Sudachi Fruit


Design of AlN-subwavelength grating for deep ultraviolet wavelength reflector operating at 244 nm of wavelength


Quantitative evaluation of SARS-CoV-2 inactivation using a deep ultraviolet light-emitting diode.

for 300 nm are required to inactivate 99.9% of SARS-CoV-2. Our results provide quantitative antiviral effects of DUV irradiation on SARS-CoV-2, serving as basic knowledge of inactivation technologies against SARS-CoV-2.


Electrical properties of GaN metal-insulator-semiconductor field-effect transistors with Al2O3/GaN interfaces formed on vicinal Ga-polar and nonpolar surfaces


Low interface state densities at Al2O3/GaN interfaces formed on vicinal polar and non-polar surfaces


V-shaped dislocations in a GaN epitaxial layer on GaN substrate

In this study, V-shaped dislocations in a GaN epitaxial layer on a free-standing GaN substrate were observed. Our investigation further revealed that the V-shaped dislocations were newly generated at the interface in the epilayer rather than propagated from the GaN substrate. V-shaped dislocations consist of two straight parts. The straight parts of the V-shaped dislocations were separated from each other in the m-direction and tilted toward the step-flow direction of the GaN epitaxial layer. The V-shaped dislocations are continuous single dislocations having a Burgers vector component of 1a and an intrinsic stacking fault between their straight parts.


Ammonia decomposition and reaction by high-resolution mass spectrometry for group III Nitride epitaxial growth

The decomposition of ammonia (NH3) in nitrogen (N2) ambient was studied under non-equilibrium conditions similar to those in a metal organic vapor phase epitaxy (MOVPE) reactor during the epitaxial growth of group-III nitrides. The gas phase was sampled at different positions and analyzed using a time-of-flight mass spectrometry system with a high resolution (better than 0.002u). Our results expand earlier findings. Even at the high temperature of 1200°C, only 26% of NH3 decomposed in a clean metal-free reactor, whereas a higher ratio of NH3 decomposition was realized in the presence of stainless steel. The activation energy in the clean reactor was calculated to be 0.965±0.004eV. These results demonstrate the capability of our setup and shed new light on the elucidation of the vapor phase growth mechanism of group III-nitrides by MOVPE.


Effect of substrate misorientation on the concentration of impurities and surface morphology of an epitaxial GaN layer on N-polar GaN substrate by MOVPE

This study examines the effect of (0001) GaN substrate misorientation on the residual impurities and surface morphology of N-polar GaN grown by metalorganic vapor-phase epitaxy. Carbon, silicon, and oxygen concentrations decreased with increasing GaN substrate misorientation angle, with the lowest impurity concentration achieved for a misorientation angle of 2° toward the m-axis, with 6×1015cm3 carbon, 6×1015cm3 silicon, and 4×1017cm3 oxygen atoms. The oxygen concentration was measured at a depth of 0.5μm below the wafer surface, and the oxygen concentration decreased with increasing thickness. The incorporation of carbon and oxygen revealed a strong dependence on the misorientation angle. The step distance height of the steps parallel to the [1120] direction (or perpendicular to the [1100] m-direction) was confirmed to be a double-height layer step. This phenomenon indicated that m-direction steps are stable for N-polar growth in GaN. In cases of large misorientation toward the m-axis in of the GaN substrate it was difficult to control the misorientation perpendicular to the nominal direction leading to a-axis direction by wafer bowing at wafer manufacturing. Therefore, step-bunching was generated for each symmetric m-axis due to an increase in the compound's off-angle, thus causing the surface roughness to become large.


Fabrication of AlN templates on SiC substrates by sputtering-deposition and high-temperature annealing

High-quality AlN templates fabricated by sputtering-deposition and post-deposition high-temperature annealing have great potential for deep ultraviolet light-emitting device applications. In this work, we fabricated AlN films on 6H-SiC substrates by sputtering and face-to-face annealing and characterized the structural quality of the AlN films before and after annealing. As reported in previous studies, to accomplish high-quality AlN films on SiC substrates using conventional methods, such as molecular beam epitaxy or metalorganic vapor phase epitaxy (MOVPE), it is important to grow the AlN on the SiC coherently. However, in this work, although the annealed AlN films were fully relaxed from the SiC substrates, or even had tensile strain, the AlN films indicated high crystallinity. The X-ray rocking curve full width at half maximum (XRC-FWHM) values of the 200-nm-thick annealed AlN film were 17 and 246arcsec for the AlN (0002) and (1012) diffraction, respectively. Though the annealed AlN film indicated rough surfaces with bunched step structures, the surface morphology was remarkably improved by MOVPE growth and clear atomic step-and-terrace structures were formed. The XRC-FWHM values of the MOVPE-grown AlN were 90 and 239arcsec for the AlN (0002) and (1012) diffraction, respectively.


Improved emission intensity of UVC-LEDs from using strain relaxation layer on sputter-annealed AlN


Comparing high-purity c- and m-plane GaN layers for Schottky barrier diodes grown homoepitaxially by metalorganic vapor phase epitaxy

GaN vertical Schottky barrier diodes (SBDs) were grown on m-plane GaN substrates by metalorganic vapor phase epitaxy (MOVPE) using a quartz-free flow channel (FC). The use of the quartz-free FC reduced the impurity concentrations of silicon and carbon by factors of 2 and 10, respectively, compared with the concentrations obtained using a conventional reactor with a quartz FC. The oxygen concentration was found to decrease with increasing the layer thickness. We achieved the same impurity concentration for the epitaxial layers grown on m-plane GaN substrates as for those grown on c-plane GaN substrates under the same growth conditions. A high resistivity of unintentionally doped GaN was achieved by decreasing the impurity concentration. Additionally, for the further understanding of the low impurity concentration in the m-plane GaN, the n-type GaN was inserted between the m-plane GaN substrate and the drift layer. The results revealed that the c- and m-plane breakdown voltages and leakage currents have similar tendencies.


Interface amorphization in hexagonal boron nitride films on sapphire substrate grown by metalorganic vapor phase epitaxy

Hexagonal boron nitride (h-BN) films directly grown on c-plane sapphire substrates by pulsed-mode metalorganic vapor phase epitaxy exhibit an interlayer for growth temperatures above 1200 °C. Cross-sectional transmission electron microscopy shows that this interlayer is amorphous, while the crystalline h-BN layer above has a distinct orientational relationship with the sapphire substrate. Electron energy loss spectroscopy shows the energy-loss peaks of B and N in both the amorphous interlayer and the overlying crystalline h-BN layer, while Al and O signals are also seen in the amorphous interlayer. Thus, the interlayer forms during h-BN growth through the decomposition of the sapphire at elevated temperatures.


Correlation between dislocation and leakage current of p-n diodes on free standing GaN substrate

Dislocations that cause a reverse leakage current in vertical p-n diodes on a GaN free-standing substrate were investigated. Under a high reverse bias, dot-like leakage spots were observed using an emission microscope. Subsequent cathodoluminescence (CL) observations revealed that the leakage spots coincided with part of the CL dark spots, indicating that some types of dislocation cause reverse leakage. When etch pits were formed on the dislocations by KOH etching, three sizes of etch pits were obtained (large, medium, and small). Among these etch pits, only the medium pits coincided with leakage spots. Additionally, transmission electron microscopy observations revealed that pure screw dislocations are present under the leakage spots. The results revealed that 1c pure screw dislocations are related to the reverse leakage in vertical p-n diodes.


Growth of hexagonal boron nitride on sapphire substrate by pulsed-mode metal-organic vapor phase epitaxy

Hexagonal boron nitride (h-BN) was directly grown on sapphire substrate using alternating ammonia (NH3) and triethylboron (TEB) supply (pulsed mode) in metalorganic vapor phase epitaxy. The best condition is when just enough NH3 is supplied to fully convert the TEB within one cycle. Excess NH3 caused islands on h-BN film surface while a lack of NH3 does not form h-BN at all. The epitaxial relationship between grown h-BN layer and c-plane sapphire was confirmed to be [0001]h-BN[0001]sapphire and [10-10]h-BN[11-20]sapphire. It is known that, compared to AlN, BN requires higher V/III ratios for good crystallinity, which due to severe gas-phase reactions is difficult to achieve using continuous supply. Thus using pulsed mode the FWHM of the symmetric (0002) diffraction was almost halved and the growth rate was several times faster.


m-Plane GaN Schottky barrier diodes fabricated with MOVPE layer on several off-angled m-plane GaN substrate

In this study, GaN mplane Schottky barrier diodes are fabricated by metalorganic vaporphase epitaxy (MOVPE) on several offangle gallium nitride (GaN) substrates, and the offcut angle dependence of impurity incorporation is investigated. We show that the MOVPE layer on the substrate inclined 5° toward the [0001] direction has extremely low impurity incorporation. These results provide important suggestions for the fabrication of mplane power devices.

Physica Status Solidi (A) Applications and Materials Science


Facet dependence of leakage current and carrier concentration in m-plane GaN Schottky barrier diode fabricated with MOVPE

The background of this cover page shows a superimposed emission microscope image observed from backside of a GaN mplane Schottky barrier diode with reverse biased condition. It can be observed that the epitaxial layer of mplane GaN has pyramidal hillocks at the surface, and only facets inclined toward the [0001] direction (+c facet) have emission due to leakage current. Three images at the corner are differential interference contrast microscopy image (upper left), photoluminescence intensity mapping image (PL, upper right), and cathode luminescence image (CL, lower left), respectively, observing the same pyramidal hillocks to investigate the characteristics of each facet. The PL image indicates the intensity of nearbandedge emission (around 363 nm wavelength) of the facets. Details are discussed in the article by Tanaka et al. (no. 1600829). The +c facets have high emission intensity, indicating that there is a high impurity concentration in the +c facets. Furthermore, we consider this is the reason that +c facets have high leakage current. The CL image is a panchromatic image, and facets inclined toward [0001] direction (c facet) have lowest emission. Together with PL intensity mapping it can be considered that c facets have less yellow emission and less impurity concentration.


In this study, GaN mplane Schottky barrier diodes fabricated with a metalorganic vaporphase epitaxy on a GaN substrate were investigated using emission microscope, photoluminescence, and cathodoluminescence. In addition, facet dependence of leakage current under reversebiased condition was observed. We showed that the leakagecurrent distribution was caused by the facet dependence of the carrier concentration and oxygen concentration. These results can provide important suggestions for the fabrication of mplane devices.


Decomposition of trimethylgalliumand adduct formation in a metalorganic vapor phase epitaxy reactor analyzed by high-resolution gas monitoring system

In this study, by using timeofflight (TOF) highresolution and highsensitivity mass spectrometry measurements, the decomposition of trimethylgallium into dimethylgallium and monomethylgallium along with its adduct formation with NHx in a commercially available horizontal metalorganic vaporphase epitaxy (MOVPE) reactor with a resistive heater was investigated and observed in more detail than previously reported. The results confirmed the use of the TOF monitoring system in analyzing the elementary reaction process in actual MOVPE systems.

Physica Status Solidi (B) Basic Solid State Physics : PSS


Effect of V/III ratio on the surface morphology and electrical properties of m-plane (1010) GaN homoepitaxial layers

We have investigated the effect of V/III ratio on the surface morphology, impurity concentration and electrical properties of mplane Gallium Nitride (GaN) homoepitaxial layers. Four-sided pyramidal hillocks are observed on the nominally on-axis mplane GaN films. Hillocks sizes relatively increase by increasing the V/III ratio. All facets of pyramidal hillocks exhibit well-defined step-terrace features. Secondary ion mass spectrometry depth profiles reveal that carbon impurities decrease by increasing the V/III ratio while the lowest oxygen content is found at an optimized V/III ratio of 900. Vertical Schottky barrier diodes fabricated on the mGaN samples were characterized. Low leakage current densities of the order of 1010 A/cm2 at 5 V are obtained at the optimum V/III ratio. Oxygen impurities and screw-component dislocations around hillocks are found to have more detrimental impact on the leakage current mechanism.


Effect of dislocations on the growth of p-type GaN and on the characteristics of p-n diodes

We investigate the influence of crystal defects on ptype GaN grown by metalorganic vapor phase epitaxy. Sets of ptype GaN films were grown on sapphire substrates and on freestanding GaN (FGaN) substrates simultaneously using various EtCp2Mg flow rates. Although there is a difference of two orders of magnitude between the threading dislocation densities of ptype GaN grown on sapphire and FGaN substrates, there is no significant difference in hole concentration. However, there are problems with the surface morphology of ptype GaN grown on sapphire. The deterioration of the surface was caused by the difference in nanopipe density. The electrical properties of a pn junction diode formed on sapphire with a high density of nanopipes were observed using emission microscopy under both forward and reversebias conditions. Our results demonstrate that the nanopipes are electrically inactive, and that other types of threading dislocation have more influence on the currentvoltage characteristics.


Preflow trimethylaluminum treatment effect on GaN growth on SiC with an ultrathin interlayer

Previously, we reported a growth method for GaN on SiC by metalorganic vapor phase epitaxy. By using a preflow trimethylaluminum treatment, the poor wetting problem of gallium on the SiC surface was alleviated, resulting in a 1.2-µm-thick crack-free GaN grown on an on-axis 6H-SiC(0001) substrate via an ultrathin AlGaN interlayer. In this study, the impact of the preflow trimethylaluminum treatment time is investigated to understand why a crack-free epilayer was realized. To demonstrate the electrical performance of devices formed by our technique, GaN/SiC vertical Schottky barrier diodes were fabricated and compared with GaN/AlN/SiC and GaN/GaN vertical Schottky barrier diodes. Compared with diodes including a high-resistance AlN interlayer, the series resistance of GaN/SiC Schottky barrier diodes incorporating the ultrathin interlayer with 5 s of TMAl treatment showed a marked reduction from 4.0 × 107 to 2.0 × 101 Ωcenterdotcm2. The ultrathin interlayer growth technique is expected to be applied in future GaN/SiC hybrid high-power and high-frequency devices.


Evaluation of excess in during metal organic vapor-phase epitaxy growth of InGaN by monitoring via in situ laser scattering

Using an in situ laser absorption and scattering method, the surface roughness and incorporation of In in InGaN layers grown by metal organic vapor-phase epitaxy (MOVPE) were monitored. We observed that the laser light with energy higher than the GaN bandgap was fully absorbed in a GaN layer with a smooth film surface. On the other hand, we observed that the scattering laser light from the surface when the roughness of the InGaN surface increased owing to the formation of In droplets. Laser light with energy lower than the GaN bandgap was weakly absorbed by the GaN layer and was scattered at the back surface of the wafer. Furthermore, laser light intensity decreased during InGaN growth because of In incorporation. The threshold of trimethyl-In (TMIn) for the formation of In droplets as a function of growth temperature was determined using our in situ system. Moreover, we observed that the In droplets were removed by thermal or H2 treatment. The results indicate that multiwavelength laser absorption and scattering enable the optimization of the growth conditions for In-rich InGaN.


The interface analysis of GaN grown on 0° off 6H-SiC with an ultra-thin buffer layer

Previously, we reported a growth method by metalorganic vapor phase epitaxy using a single two-dimensional growth step, resulting in 1.2-µm crack-free GaN directly grown on 6H-SiC substrate. The introduction of Al-treatment prior to the standard GaN growth step resulted in improved surface wetting of gallium on the SiC substrate. Transmission electron microscope and energy dispersive spectrometer analysis of the epitaxial interface to the SiC determined that an ultra-thin AlGaN interlayer had formed measuring around 23 nm. We expect our growth technique can be applied to the fabrication of GaN/SiC high frequency and high power devices.


Growth of low-dislocation-density AlGaN using Mg-doped AlN underlying layer

We report on a new technology for growing lowdislocationdensity AlGaN in which a Mgdoped AlN underlying layer is used. By growing AlxGa1xN on AlN:Mg with AlN molar fractions x of 0.3, 0.5 and 0.7, the density of misfit dislocations is much reduced compared with that in the case of growing AlGaN on undoped AlN. In addition, the surface becomes atomically flat.

Physica Status Solidi (C) Current Topics in Solid State Physics


Atomic layer epitaxy of AlGaN with an average growth rate of as high as 4.5 mm/h has been achieved by the highspeed switchingvalves (HSSVs) technique in a raisedpressure metalorganic vapor phase epitaxy system. Regarding the duration of group III metalorganics and NH3 gases input, 0.1 second for each injection time is found to be sufficient to realize AlGaN growth of one monolayer per cycle over a 2inch wafer with hydrogen and nitrogen carrier gases at a raised pressure. Lowtemperature growth of highquality AlGaN has been achieved using HSSVs.


Microstructural analysis was conducted to observe Mgrelated defects in highly Mgdoped AlN on an undoped AlN layer grown by metalorganic vapor phase epitaxy. Pyramidal defects at a density of 1015cm3 were observed in the Mgdoped AlN layer with a Mg concentration of 2.5×1019cm3. The size and density of the pyramidal defects decreased at a low Mgdoped AlN layer with a Mg concentration. When the Mg concentration is high, defects of different shapes were formed in the Mgdoped AlN layer.

Physica Status Solidi (A) Applications and Materials Science


High-output-power AlGaN/GaN ultraviolet-light-emitting diodes by activation of Mg-doped p-type AlGaN in oxygen ambient

We demonstrated activation annealing of Mgdoped ptype Al0.17Ga0.83N in different gases. The hole concentration of Al0.17Ga0.83N annealed in oxygen is higher than that annealed in nitrogen or air. A hole concentration of 1.3×1016cm3 at room temperature was achieved by annealing in oxygen flow at 900°C. Secondary ion mass spectroscopy shows that hydrogen dissociation from Mgdoped Al0.17Ga0.83N is found to be enhanced by annealing in a flow of oxygen, compared with annealing in a flow of nitrogen. We confirmed the effect of activation annealing in oxygen flow on the performance of UV lightemitting diode (LED). At a DC current of 100mA, the output power of the LED annealed in oxygen flow at 900°C is four times higher than that of the LED annealed in nitrogen flow at 800°C.


Crystal growth and p-type conductivity control of AlGaN for high-efficiency nitride-based UV emitters

Microstructural analysis was carried out to clarify the compositional dependence of the generation of dislocations in AlxGa1xN on an underlying AlN layer grown by metalorganic vapor phase epitaxy. When the film thickness is less than 1.5 μm, the threading dislocation density (TDD) increases with decreasing AlN molar fraction. However, when the film thickness exceeds 1.5 μm, TDD becomes maximum at x around 0.5. The growth of AlGaN on a grooved AlN template is effective in reducing TDD for all AlN molar fractions. TDD in AlGaN, which is close to binaries such as GaN and AlN, is a few 107 cm2, while for the intermediate composition with x around 0.5, TDD is still at mid 108 cm2. The activation energy of Mg in AlGaN is found to show a strong Mg concentration dependence with a negative onethird power law in Al0.25Ga0.75N and Al0.5Ga0.5N as well as in GaN. Overdoping of Mg causes an increase in the activation energy for every composition; from this, the optimum Mg concentration for realizing the highest hole concentration can be deduced.


Activation of Mg-Doped p-type Al0.17Ga0.83N in Oxygen Ambient

Annealing in oxygen ambient was found to be effective for realizing a high hole concentration in p-type Al0.17Ga0.83N:Mg. The maximum hole concentration obtained was 1.3×1016 cm-3 at room temperature. Hydrogen dissociation from Mg-doped Al0.17Ga0.83N is found to be enhanced by annealing in a flow of O2 compared with annealing in a flow of N2.


Activation energy of Mg in Al0.25Ga0.75N and Al0.5Ga0.5N

The Mg concentration dependence of the acceptor activation energy in Al0.25Ga0.75N and Al0.5Ga0.5N was investigated. Temperaturedependent Hall effect measurements show that effective acceptor activation energy changes with a negative 1/3 power of Mg concentration, which means that the activation ratio increases with increasing Mg concentration. Similar to the case of GaN, however, overdoping of Mg causes serious compensation and a steep decrease of the hole concentration.


Photoluminescence and excitation spectrum of Mg-doped p-type AlxGa1-xN

We have been measured the near band edge photoluminescence (PL) and photoluminescence excitation (PLE) spectra of Mg doped AlGaN. PL and PLE peak energies were almost corresponding to those of undoped AlGaN. From the temperature dependent measurement of the PL spectra, the PL peak energy shift as a function of temperature showed S shaped behaviour. These results suggest that the mechanisms of the near band edge PL of AlGaN:Mg are basically similar to those of undoped AlGaN. Using the known spectral feature of undoped AlGaN which consists of main and LO phonon replica emission bands at even interval energy and same line width, the spectral decomposition of the PL spectrum reveals an additional band which is expected as the transition to the acceptor level arising from Mg doping. The temperature dependences of its peak energy and intensity suggest relative temperature shift of the acceptor level and also may imply the existence of another relaxation pathway.


Relaxation and recovery processes of AlxGa1-xN grown on AlN underlying layer

The processes as in title of relaxation of the lattice mismatch and the recovery of crystalline quality in thick AlxGa1xN on high-temperature-grown AlN were investigated. When x=0.3, rapid lattice relaxation occurred over a few microns, then the crystalline quality gradually recovered over 10 μm. In contrast, when x=0.7, relaxation of the lattice mismatch gradually occurred over 5 μm.


Effect of c-plane sapphire misorientation on the growth of AIN by high-temperature MOVPE

AlN layers were directly grown on sapphire substrates with slightly different miscut angles from the cplane by metalorganic vapor phase epitaxy at 1300 °C. By Xray diffraction analysis, the structural quality was found to improve with the increase in substrate offangle. When the substrate offangle was 0.5°, the Xray rocking curve (XRC) full widths at half maximum (FWHMs) of symmetric and asymmetric planes were 170 and 550 arcsec, respectively. At smaller offangle, XRCFWHMs were wider. When the substrate offangle was zero, the XRCFWHM was considerably inferior to that on 0.5ooffangle sapphire. The surface morphology was investigated. Atomic steps were observed on the substrates with a small misorientation. In contrast, macro steps appeared on largeroffangle sapphire substrates. (© 2008 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim)

Physica Status Solidi (C) Current Topics in Solid State Physics


High-efficiency AlGaN-based UV light-emitting diode on laterally overgrown AlN

Ultraviolet light-emitting diodes (UV-LEDs) with a peak wavelength of 335 nm were fabricated on AlN/sapphire templates. As templates for the fabrication of UV-LEDs, planar AlN and epitaxial laterally overgrown (ELO) AlN on sapphire (0 0 0 1) substrates were compared. The output power of UV-LEDs grown on ELO-AlN was 27 times higher than that of UV-LEDs on the planar AlN template.


High-efficiency AlGaN based UV emitters grown on high-crystalline-quality AlGaN using grooved AlN layer on sapphire substrate

Crackfree and lowdislocationdensity AlGaN was successfully grown on grooved AlN layer. UV lightemitting diodes (LEDs) with a peak wavelength of 330 nm on these AlGaN films were fabricated. The efficiency of the UV LEDs is found to be strongly affected by the crystalline quality of the underlying AlN. (© 2007 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim)

Physica Status Solidi (A) Applications and Materials Science


Light extraction process in moth-eye structure

We theoretically calculated the light extraction efficiency of the lightemitting diodes (LEDs) with a motheye structure by rigorous coupled wave analysis (RCWA). The dependences of light extraction efficiency on aspect ratio and period were measured. Light extraction efficiency was almost constant until the period of 170 nm and then gradually decreased with increasing of period. (© 2006 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim)


Low-Leakage-Current Enhancement-Mode AlGaN/GaN Heterostructure Field-Effect Transistor Using p-Type Gate Contact


次世代光による細胞光応答の解明

次世代光による細胞光応答の解明

LED総合フォーラム 2020 in 徳島 論文集

LED総合フォーラム 2020 in 徳島 論文集