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AlN‐based field‐effect transistors (FETs) enable high‐breakdown voltage, high drain current, and high‐temperature operation. To realize high‐frequency devices, N‐polar AlGaN/AlN heterostructure FETs are focused on. N‐polar Al0.1Ga0.9N/Al0.9Ga0.1N/AlN FET is fabricated using metal–organic vapor‐phase epitaxy, and its electrical characteristics are evaluated. An N‐polar AlN layer is grown on a sapphire substrate with a misorientation angle of 2.0° toward m‐axis, on which a 20 nm thick Al0.9Ga0.1N base layer and a 20 nm Al0.1Ga0.9N channel layer are grown. The static FET operation is confirmed to exhibit an n‐channel and pinch‐off. Normally, during operation with a turn‐on voltage of −3.2 V, a high operating breakdown voltage of 620 V and high operating temperature of 280 °C are also confirmed. [ABSTRACT FROM AUTHOR]

Herein, the operation of dopant‐free GaN‐based p‐n junctions formed by distributed polarization doping (DPD) is experimentally demonstrated and their space charge profiles and carrier transport properties are investigated. The device exhibits ideal space charge profiles explained by polarization effects and demonstrates the excellent controllability of DPD. In addition, it shows rectification and electroluminescence under forward‐biased conditions. The carrier transport properties could be explained by the conventional recombination/diffusion model used for impurity‐doped p‐n junctions. Repeatable breakdowns are also observed in all devices and the temperature‐dependent breakdown voltages reveal that the breakdowns are caused by avalanche multiplication, which is also the same as those reported in impurity‐doped GaN p‐n diodes. These results indicate that DPD is a promising doping technology for GaN‐based power devices overcoming any issues associated with conventional impurity doping. [ABSTRACT FROM AUTHOR]

The properties of phase‐separated MgZnO induced by heat treatment are investigated, because the electrical conductivity of MgZnO deposited by sputtering can be improved by heat treatment. The absorption edge shows a red shift, but the transmittance in the ultraviolet (UV)‐C region increases after heat treatment. This is due to the formation of wurtzite (WZ) MgZnO with a high Zn content and a rock salt (RS) structure with a low Zn content induced by phase separation. This suggests that the current path is formed by the WZ crystals with low resistivity, and the transmittance in the UV region is increased by the RS crystals with high transmittance. It is confirmed that the RS structure is partially mixed with the WZ crystal in the in‐plane direction. In contrast, different distributions of crystals are observed in the stacking direction. A WZ structure is formed after RS‐MgZnO is formed on the substrate interface. It is found that Mg is incorporated into the RS structure by heat treatment, and Mg‐rich MgZnO is formed. [ABSTRACT FROM AUTHOR]

Herein, gallium nitride (GaN) samples implanted with magnesium (Mg) and fluorine (F) ions are investigated by photoluminescence (PL) measurements. In low‐temperature PL measurements, the characteristic green luminescence (GL) band attributable to nitrogen vacancies (VN) is observed in Mg‐ion‐implanted GaN. As VN are likely to act as donors, suppressing their formation is essential to realizing p‐type conductivity. The energy required for a F impurity to replace VN in GaN and eventually form F on a N site decreases when the Fermi level approaches the valence band maximum, and therefore F is employed as a subsequent implantation element to compensate for VN. The GL band peak disappears upon implanting Mg and F ions at a high temperature and adjusting the F concentration to an appropriate value. This result suggests that VN generated by Mg ion implantation can be suppressed using an element with a lower formation energy than that of VN. [ABSTRACT FROM AUTHOR]

Herein, a newly developed birefringence microscope is used to observe dislocations in gallium nitride (GaN) substrates. The comparison results of the observation method using this microscope with other observation methods, such as X‐ray topography and Raman microscopy, confirms that dislocations in a GaN substrate can be detected with a birefringence microscope. In addition, the observation can be carried out as easily as with an optical microscope. It is also found that under certain observation conditions, the direction of the edge component of dislocations can be determined. [ABSTRACT FROM AUTHOR]

The reduction of unintentional impurities in m‐plane ( 10 1 ¯ 0 ) GaN homoepitaxial layers is demonstrated by using nitrogen (N2), as opposed to hydrogen (H2), as carrier gas in metalorganic vapor phase epitaxy (MOVPE). Secondary ion mass spectrometry (SIMS) analysis shows that the impurity levels of residual oxygen (O), carbon (C), and silicon (Si) are decreased by nearly one order of magnitude in N2‐grown samples. Although the full width at half maximum (FWHM) values for the on‐axis m‐plane X‐ray rocking curves of all specimens are quite similar (around 50 arcsec), plan‐view scanning transmission electron microscopy (STEM) measurements reveal a clear reduction of dislocation densities in N2‐grown films. Their origin is likely related to an initial surface roughening with H2 carrier gas, which also causes surface faceting resulting in the formation of large four‐sided pyramidal hillocks, while using N2 results in smoother surfaces. Hence, MOVPE growth with N2 carrier gas is an effective method to lower the impurity incorporation in m‐plane GaN materials in addition to reducing the formation of defects and improving the surface morphology, which can enable the development of high‐performance GaN‐based devices on non‐polar surfaces. [ABSTRACT FROM AUTHOR]

The main chain structure of optically isotropic amorphous fluorine resin for encapsulating AlGaN‐based deep‐ultraviolet LEDs (λ < 300 nm) is studied. One type of amorphous fluorine resin has a five‐membered ring with a single oxygen atom and the other has that one with two oxygen atoms. Reliability testing is performed using AlGaN‐based LEDs of 262 and 289 nm. For the resin with two oxygen atoms in the ring, visible damage to the electrode and significantly increased leakage current are considered to been caused by the photolysis of the ring induced by irradiation with deep‐ultraviolet light. In contrast, electrode damage and increased leakage are not observed in the case of the resin with the single‐oxygen ring. [ABSTRACT FROM AUTHOR]

A gallium nitride (GaN) epitaxial layer with a low density of threading dislocations is successfully grown on a silicon substrate by using in situ gas etching. Silicon nitride (SiNx) film is used as a mask, and ammonia is intermittently supplied in hydrogen ambient during the etching. After etching, high‐density deep pits appeared on the surface of a GaN template layer and corresponded to the threading dislocations in the layer. In this novel method, before growing an additional GaN layer on the template GaN layer, a second SiNx layer is deposited after the etching process, and this layer prevents GaN nuclei from growing on the upper side‐walls of the pits. By using this method, the density of threading dislocations of the GaN surface is reduced to 6.7 × 107 cm−2. This method is cost effective, completing all the necessary processes in one growth run without taking samples out from a metalorganic chemical vapor deposition (MOCVD) reactor. [ABSTRACT FROM AUTHOR]

In this study, GaN m‐plane Schottky barrier diodes are fabricated by metalorganic vapor‐phase epitaxy (MOVPE) on several off‐angle gallium nitride (GaN) substrates, and the off‐cut angle dependence of impurity incorporation is investigated. We show that the MOVPE layer on the substrate inclined 5° toward the [000–1] direction has extremely low impurity incorporation. These results provide important suggestions for the fabrication of m‐plane power devices. [ABSTRACT FROM AUTHOR]

In this study, by using time-of-flight (TOF) high-resolution and high-sensitivity mass spectrometry measurements, the decomposition of trimethylgallium into dimethylgallium and monomethylgallium along with its adduct formation with NHx in a commercially available horizontal metalorganic vapor-phase 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. [ABSTRACT FROM AUTHOR]

Injecting current with a uniform carrier concentration is important for applications with three-dimensional architectures such as vertical power devices or displays. In III-nitride nanostructures, dopants not only incorporate differently depending on the surface orientation but can also seriously affect the kinetic equilibrium shapes of the nanorods. Herein, we report selective-area growth of doped GaN nanorods grown by pulsed-mode metalorganic chemical vapor deposition. Two dopants, Si and Mg, were employed as donor and acceptor atoms, respectively, for a mono-doping approach. Furthermore, a mixed flow of Si and Mg was supplied for a co-doping approach. We compared the morphological effects and growth rates of each doped GaN nanorod array. Then, we proposed appropriate growth mechanisms for the doped GaN nanorods on the basis of our structural characterizations. These results might extend the morphological functionality of GaN nanorods by including doping and may also provide an appropriate foundation for the design of nanostructure-based electronic or photonic devices. [ABSTRACT FROM AUTHOR]

We investigate the influence of crystal defects on p-type GaN grown by metalorganic vapor phase epitaxy. Sets of p-type GaN films were grown on sapphire substrates and on free-standing GaN (F-GaN) substrates simultaneously using various Et-Cp2Mg flow rates. Although there is a difference of two orders of magnitude between the threading dislocation densities of p-type GaN grown on sapphire and F-GaN substrates, there is no significant difference in hole concentration. However, there are problems with the surface morphology of p-type GaN grown on sapphire. The deterioration of the surface was caused by the difference in nanopipe density. The electrical properties of a p-n junction diode formed on sapphire with a high density of nanopipes were observed using emission microscopy under both forward- and reverse-bias conditions. Our results demonstrate that the nanopipes are electrically inactive, and that other types of threading dislocation have more influence on the current-voltage characteristics. [ABSTRACT FROM AUTHOR]

In this study, GaN m-plane Schottky barrier diodes fabricated with a metalorganic vapor-phase epitaxy on a GaN substrate were investigated using emission microscope, photoluminescence, and cathodoluminescence. In addition, facet dependence of leakage current under reverse-biased condition was observed. We showed that the leakage-current distribution was caused by the facet dependence of the carrier concentration and oxygen concentration. These results can provide important suggestions for the fabrication of m-plane devices. (a) four-faceted hillocks on m-plane GaN MOVPE sample, facet dependence of (b) leakage current and (c) PL peak intensity of the m-plane GaN Schottky barrier diode. [ABSTRACT FROM AUTHOR]

The optical properties of InGaN/GaN multiple quantum wells (MQWs) have been studied by means of photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy. The radiative and nonradiative recombination lifetimes were evaluated as a function of the excitation energy density. The radiative recombination lifetime decreased and subsequently reached a nearly constant value with increasing excitation energy density, which was attributed to screening of internal electric fields by photoexcited carriers. On the other hand, the nonradiative recombination lifetime increased and subsequently decreased with increasing excitation energy density. The initial increase in the nonradiative recombination lifetime could be attributed to saturation of nonradiative recombination centers with photoexcited carriers. The nonradiative recombination lifetime was found to decrease to a considerably weaker extent than that expected for the Auger recombination process of free carriers. This indicated that the decrease in the internal quantum efficiency (IQE) at high carrier densities could not be explained only by the Auger recombination process of free carriers. [ABSTRACT FROM AUTHOR]

Recombination dynamics and internal quantum efficiency (IQE) of green luminescent InGaN nanowires with different crystalline qualities have been studied by means photoluminescence (PL) and time-resolved PL spectroscopy. Temperature- and excitation-power-density-dependent PL spectroscopy enabled to evaluate the IQE as a function of excitation power density. The shape of the efficiency curves at low temperature strongly depended on the magnitude of nonradiative recombination processes. This leads to the misestimation of the IQE in the lower quality nanowire. In addition, the PL decay curves were well described by a double exponential function both at 6 and 300 K. The PL decay time of the faster component was affected by nonradiative recombination processes even at low temperature. This indicated that the radiative recombination lifetime cannot be estimated from the PL decay time in the lower quality nanowire. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

A set of GaN/GaInN single quantum wells (SQWs) were grown on c-plane GaN/sapphire templates using metal organic vapor phase epitaxy (MOVPE) and atomic layer epitaxy (ALE). The structural information on the interfaces and surfaces was obtained by fitting the experimental X-ray spectra and was compared between MOVPE and ALE mode. The X-ray crystal truncation rod (CTR) scattering measurement results showed that the samples grown by ALE had sharper interfaces than those grown by MOVPE. Both the X-ray reflectivity (XRR) curve-fitting results and atomic force microscope (AFM) results indicated that the surfaces of the ALE-grown SQWs were smoother than those of MOVPE-grown SQWs. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

To achieve both the reduction of threading dislocation density (TDD) and the enhancement of light extraction efficiency (LEE) of light-emitting diodes (LEDs), a two-directional stripe-patterned sapphire substrate (TPSS) was fabricated. First, a stripe pattern parallel to the a-axis of Al2O3 was formed on an as-received c -sapphire wafer and then a second stripe pattern rotated around 3° from the m-axis of Al2O3 was formed on the first stripe PSS. The light output power of TPSS LEDs was around 1.25 times higher than that of LEDs on a flat sapphire substrate. This enhancement is caused by the increase of both the internal quantum efficiency (IQE) and the LEE. On the other hand, although the LED on TPSS with the second stripe just parallel to the m-axis of Al2O3 showed a similar enhancement ratio of the light output power, the current leakage of this LED increased with the concentration of threading dislocations on the surface of LEDs. Stripes rotated 3° from the m-axis of Al2O3 changed the growth mode and realized GaN films with a smooth surface without pit formation. Therefore, the misalignment of the stripe from the m-axis of Al2O3 will become a new design parameter for realizing high-efficiency reliable LEDs. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Low-temperature-deposited (LT-) AlN layers and the initial growth of GaN on a- and c-sapphire have been investigated and compared. The as-deposited LT-AlN layers on both a- and c-sapphire are composed of high-density islands. By annealing, the islands become large. The density of islands on a- and c-sapphire are 5.6 × 1010 and 7.9 × 1010 cm-2, respectively, indicating that the evolution of the solid phase epitaxy of AlN islands induced by annealing is different on a- and c-sapphire. The moiré spacings obtained by plan-view electron microscopy reveal that the strain of the AlN islands on a-sapphire is larger than that of islands on c-sapphire. These results imply that development of the solid phase epitaxy is influenced by the strain. The coalescence of GaN islands grown on a-sapphire, which has larger AlN islands, is advanced compared with that of islands on c-sapphire. The relationship between the annealed AlN island density and the threading dislocation (TD) density in the GaN films is investigated on a- and c-sapphire. The TD density of the GaN films is small when the density of annealed AlN islands is low. These results indicate that decreasing the island density of the annealed AlN layers is essential for improving the crystalline quality of GaN films. It is therefore important to control the solid phase epitaxy of LT-AlN layers by annealing. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Clinical evidence has shown that chronic otitis media with cholestentoma causes greater bone resorption than otitis media without cholesteatoma. What is the role of the epithelium and its products on the morbidity of cholesteatoma? We have studied the mechanism of collagenase production in cultures of rat epidermal cells and skin fibroblast-like cells under various conditions. The epithelial cells significantly induced mesenchymal cells to produce collagenase. We conclude that interaction between epithelial cells and fibroblast-like cells enhances production of collagenase. This enhancement of proteolytic activity may be crucial in the series of molecular events resulting in bone resorption associated with cholesteatoma. [ABSTRACT FROM AUTHOR]

The suppression of the temporal bone's pneumatized cavities is commonly observed in chronic middle ear inflammatory disease, but this fact has not been explained in respect to its etiology, i.e., whether the poorly pneumatized cavity induces chronic middle ear infection or middle ear inflammatory condition suppresses the pneumatization of cellulae. We studied the effect of middle ear infection and eustachian tubal dysfunction in the early stage of life upon the development of pneumatization of the mastoid using 13 pigs, whose mastoid is comparable to that of man. We used two methods, the eustachian tube stenosis test and the tympanic membrane paraffin infusion test. The present study indicated that in six pigs infection of the middle ear in the early stage of life suppressed the normal development of pneumatization of the mastoid bone in pigs. [ABSTRACT FROM AUTHOR]