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Defects in ammonothermal GaN have been studied using a monoenergetic positron beam. Through measurements of Doppler broadening spectra of the annihilation radiation, the major defect species was identified as a Ga vacancy coupled with impurities such as oxygen and/or hydrogen. Those defects were found to be stable even after annealing at 1000 °C. The shape parameter S for the Doppler broadening spectrum corresponding to positron annihilation at the surface was found to be decreased by illumination within energy ranges of 1.5–2.6 eV and 3.2–3.6 eV. This phenomenon is attributed to the suppression of recombinations between holes and electrons due to trapping centers, which can hold electrons for a long time, and a resultant accumulation of holes at the surface. Recovery of the S value required almost one day, but it was shortened by the annealing at 1000 °C.

InGaN single quantum wells (SQWs) grown on m -plane bulk GaN substrates show significant differences in peak emission wavelength when grown on substrates oriented nominally on-axis compared to substrates with small intentional misorientations (miscuts) towards the orthogonal − c -direction or a -direction. SQWs on substrates intentionally miscut toward the a -direction emit longer wavelengths than those with miscuts towards the − c- direction in a variety of identical growth conditions, while SQWs on nominally on-axis m- plane with pyramidal hillocks features display emission characteristics of both. These preliminary co-loaded growth studies may provide insight into broad or anomalous wavelength emission observed on nonpolar GaN-based visible light emitters and suggest opportunities for improving LED and laser diode device performance on this naturally occurring crystal plane.

A spatio-time-resolved cathodoluminescence (STRCL) system was constructed by replacing the electron beam (e-beam) gun of a conventional scanning electron microscope by the in-house manufactured pulsed e-beam gun, which is excited using femtosecond laser pulses. By using this system, STRCL measurements were carried out on low threading dislocation density freestanding GaN substrates grown by hydride vapor phase epitaxy. High-resolution cathodoluminescence imaging allows for visualization of nonradiative recombination channels in the vicinity of accidentally formed domain boundaries. Local cathodoluminescence lifetimes (τCL) for the near-band-edge (NBE) emission are shown to be sensitively position dependent. A linear relation between the equivalent internal quantum efficiency (ηint eq) and τCL for the NBE emission was observed at room temperature under a weak excitation condition, and spatially resolved excitation led to the observation of the highest ηint eq of 20 % with τCL of 3.3 ns.

The properties of Si-doped GaN (GaN:Si) thin films and InGaN/GaN light-emitting diodes (LEDs) grown by metalorganic chemical vapor deposition on free-standing {1 0 1 0} m -plane GaN substrates were investigated with regard to carrier gas and substrate misorientation toward the [0 0 0 1] c − direction. The surface morphology of the GaN:Si thin films and the LEDs was found to be strongly dependent on the choice of carrier gas and substrate misorientation. Growth of GaN:Si thin films on nominally on-axis substrates produced surfaces with a high density of four-sided pyramidal hillocks, regardless of the composition of the carrier gas. In contrast, growth of GaN:Si thin films on substrates with misorientation angles greater than 0.7° yielded moderately to severely faceted surfaces when grown with H 2 as the carrier gas and atomically smooth surfaces when grown with N 2 as the carrier gas. Although the surface morphology varied significantly with substrate misorientation, the average indium mole fraction in the InGaN-based quantum wells, the output power of the LEDs, the electroluminescence (EL) peak wavelength, and the EL linewidth showed little dependence on substrate misorientation. Substrates with misorientation angles of about 1° were successfully used to grow LEDs with improved surface morphology but similar optical properties, compared to LEDs grown on nominally on-axis substrates.

The nonradiative lifetime (τNR) of the near-band-edge emission in various quality GaN samples is compared with the results of positron annihilation measurement, in order to identify the origin and to determine the capture-cross-section of the major intrinsic nonradiative recombination centers (NRCs). The room-temperature τNR of various n-type GaN samples increased with decreasing the concentration of divacancies composed of a Ga vacancy (VGa) and a N vacancy (VN), namely, VGaVN. The τNR value also increased with increasing the diffusion length of positrons, which is almost proportional to the inverse third root of the gross concentration of all point defects. The results indicate that major intrinsic NRC in n-type GaN is VGaVN. From the relationship between its concentration and τNR, its hole capture-cross-section is estimated to be about 7 × 10−14 cm2. Different from the case of 4H-SiC, the major NRCs in p-type and n-type GaN are different: the major NRCs in Mg-doped p-type GaN epilayers are assigned to ...

We succeeded in preparing very thick c -plane bulk gallium nitride (GaN) crystals grown by hydride vapor phase epitaxy. Growth of the bulk GaN crystals was performed on templates with 3 μm GaN layer grown by metal organic chemical vapor deposition on (0 0 0 1) sapphire substrates. Colorless freestanding bulk GaN crystals were obtained through self-separation processes. The crystal's diameter and thickness were about 52 and 5.8 mm, respectively. No surface pits were observed within an area of 46 mm diameter of the bulk GaN crystal. The dislocation density decreased with growth direction (from N-face side to Ga-face side) and ranged from 5.1×10 6 cm −2 near the N-face surface to 1.2×10 6 cm −2 near the Ga-face. A major impurity was Si, and other impurities (O, C, Cl, H, Fe, Ni and Cr) were near or below the detection limits by SIMS measurements.

Recently, an innovative method of fabricating vertical light-emitting diode (LED) was introduced by the chemically lift-off (CLO) process. Such fabrication of the vertical LED could be realized by inserting the CrN interlayer between sapphire substrate and gallium nitride (GaN) layer. In this paper, by using off-cut sapphires, we tried to find out the influence of off-angles on the overgrown GaN films with respect to crystal quality and surface roughness. The CrN is formed by nitridizing the deposited Cr layer on the sapphire substrate at 1080 °C, and then GaN was grown on the nitrided Cr. By X-ray diffraction system (XRD) measurements, the crystal quality tendency of the GaN film on the variation of the off-angles was similar with that of the CrN interlayer. Through high-resolution transmission electron microscopy (HRTEM) measurement, it was found out that AlN layer with a thickness of a few nanometers also formed between sapphire and CrN during the nitridation treatment of Cr layer. Compared with the previous reports, such tendency of the crystal quality for the vicinal sapphires with CrN interlayer was well matched with the tendency of the previous results in the vicinal sapphire without the CrN interlayer. Therefore, it can be surmised that the AlN layer between sapphire and CrN was precedently influenced by the vicinal substrate and then, the AlN layer had an influence on the upper layers of GaN as well as CrN. In addition, it was found that the vicinal sapphires with the off-angles of 0.2–0.3° had the minimum full-width at half-maximum (FWHM) values by XRD measurements, which were lower by 20% than the others.

Effects of off-axis substrates on m-plane InGaN/GaN light-emitting diodes (LEDs) grown by metal organic chemical vapor deposition were investigated. The surface morphology of n-GaN was improved by increasing an off-axis angle from the m-plane toward the c-plane. The InGaN/GaN quantum wells (QWs) grown on the off-axis substrates toward the c−-direction (N-polar) emitted at a longer peak wavelength than the on-axis m-plane substrates, indicating that the off-axis substrates have impact on enhancement of the indium incorporation in the InGaN/GaN QWs. The LED on the c−-5° off-axis substrate emitted at 457 nm under DC current of 20 mA and showed 0.7 nm red-shift from 1 to 20 mA. The c−-5° off LED showed an optical polarization ratio of 0.91, which is comparable to the nonpolar material.

Relatively large size (about 10 mm × 10 mm) m -plane GaN substrates are grown by hydride vapor phase epitaxy (HVPE). The high crystalline quality of the substrates was observed by X-ray diffraction analysis. Typical full widths at half maximum values of X-ray rocking curves measured for the (100) and (102) reflections were 25–35 arcsec and 30–40 arcsec, respectively. The defect density distribution was analyzed from the [100] direction by the cathodeluminescence method. The dark spot density decreased in the growth direction and could be as low as 2.5 × 105 cm–2. No stacking faults could be seen. The surface roughness after the lapping, polishing and cleaning was measured by atomic force microscopy and the RMS roughness was 0.072 nm. All of the phonon modes that the selection rules allow have been observed in polarized Raman measurements. High-quality large-size m -plane GaN substrates will enable the realization of commercial production of nonpolar devices. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The seeded growth of GaN by the ammonothermal method is updated. By using a polycrystalline GaN nutrient together with higher concentration of NaNH 2, we have attained a long-term ammonothermal growth to 90 days. The as-grown crystal showed an improved structural quality to the previously reported GaN bulk crystals grown by the seeded ammonothermal growth, demonstrating the feasibility of the ammonothermal method for a high-quality bulk GaN growth. The threading dislocation density estimated by plan-view transmission electron microscopy (TEM) observations was < 1 × 10 6 cm -2 for the Ga-face and 1 x 10 7 cm -2 for the N-face. There was no dislocation generation observed at the interface on the Ga-face although a few dislocations were generated at the interface on the N-face. The chemical etching together with X-ray diffraction measurements revealed macroscopic grains in the grown crystals.

We demonstrate high power blue and green InGaN/GaN multiple-quantum-well (MQW) light emitting diodes (LEDs) grown on low extended defect density semipolar (10 1 1) and (11 2 2) bulk GaN substrates by metal organic chemical vapor deposition. The output power and external quantum efficiency (EQE) at 20 mA under pulsed operation (10 % duty cycle) were 19.7 mW and 34.4 % for the blue LED and 9.0 mW and 18.9 % for the green LED, respectively. The blue LED showed

Two preliminary experiments are reviewed and seeded growth of GaN is updated in this paper. In a mineralizer study, it was shown that basic mineralizers promoted synthesis of wurtzite GaN. Through a dissolution study of polycrystalline GaN in basic supercritical ammonia, it was confirmed that GaN has retrograde solubility. A GaN film as thick as 45 μm was obtained in seeded growth from metallic Ga nutrient. Uniform growth of GaN over a 3 x 4 cm 2 oval-shaped seed crystal was also demonstrated. Seeded growth of GaN from polycrystalline GaN nutrient was attempted to avoid growth saturation problem in Ga nutrient. Extended growth up to 11 days was attained with increased mineralizer concentration. The cross-sectional TEM observation showed high density of voids and defects at the growth interface, although most of them did not propagate to the top surface. The estimated threading dislocation density was at the order of low-10 9 cm -2 and no evidence of basal plane stacking fault was observed.

A relatively large sample of gallium nitride (GaN) was grown as a single crystal using the hydride vapor phase epitaxy (HVPE) process. The thermal diffusivity of the single crystal has been measured using a vertical-type laser flash method. The thermal expansion was measured using a dilatometer in order to estimate the thermal diffusivity with sufficient reliability. The effect of sample thickness and temperature on thermal diffusivity was evaluated. The specific heat capacity of GaN was also measured by using a differential scanning calorimeter. The thermal properties of single-crystal GaN have been compared with the measured thermal properties of single-crystal silicon carbide (SiC). The thermal conductivity of single-crystal GaN at room temperature is found to be 253 � 8:8% W/mK, which is approximately 60% of the value obtained for SiC. The excellent thermal property that is obtained in this study clearly indicates that GaN crystals are one of the promising materials for use in high-power-switching devices. [doi:10.2320/matertrans.MRP2007109]

Microcrystalline GaN was synthesized in supercritical ammonia with acidic, neutral, and basic mineralizers. For both acidic and neutral mineralizers, a mixture of wurtzite GaN and zinc-blende GaN was obtained, whereas wurtzite GaN was preferentially synthesized with basic mineralizers. Gibbs free energy for wurtzite GaN and zinc-blende GaN was calculated to investigate phase stability of GaN. The free energy of formation for wurtzite GaN was smaller than that for zinc-blende GaN by 720 J/mol (8 meV) at 600 K and 1310 J/mol (14 meV) at 800 K. Given this small energy difference, the selection of phase in ammonothermal synthesis turned to depend more on kinetics than on thermodynamics. Water-dissolving powder which was occasionally synthesized in basic conditions suggested that the reaction path in basic conditions is different from that in acidic and neutral conditions. The difference in the reaction path is thought to affect the phase selection.

Ammonothermal growth of GaN was studied through three types of experiments: synthesis of powder GaN from metallic Ga with various kinds of mineralizers, a weight loss measurement of polycrystalline GaN, and growth of GaN via fluid transport. Hexagonal (wurtzite) GaN was exclusively synthesized with basic mineralizers, whereas neutral and acidic mineralizers resulted in a mixture of hexagonal and cubic (zinc blende) phases, confirming that basic mineralizers are suitable for growing hexagonal GaN. Through a dissolution study of GaN in basic supercritical ammonia, it was discovered that the solubility of GaN has negative temperature dependence. Based on these results, the Ga nutrient was placed in a lower temperature zone and GaN was precipitated on single crystalline GaN seeds. Seeds placed in a higher-temperature zone showed more weight gain than those in a lower-temperature zone. Spontaneously nucleated precipitates on the GaN seeds showed clear crystallographic facets. The maximum size obtained in 24-h growth was about 10 μ m. Deposition of planar GaN on seed surfaces was also confirmed.

We compare facet morphology, device characteristics, and far field patterns (FFPs) for semipolar (1122) laser diodes fabricated with mechanically polished and dry etched mirror facets. Facets formed by Cl2-based dry etching produced inclined and heavily striated facets. Mechanically polished facets, in contrast, provided vertical and smooth facets (rms roughness=5.2 nm). The threshold currents of polished facet devices were on average ~ 100 and ~ 200 mA lower than etched facet devices (2 × 1200 μm2 and 4 × 1200 μm2 dimension devices, respectively). FFPs from etched facets were also shown to be obscured due to substrate reflections.

Growth of A1N on patterned sapphire substrates was studied. Deep grooves were made on sapphire substrates by a laser-induced backside wet etching technique. KrF excimer laser was irradiated at an interface between sapphire and phenolphthalein/ N -methyl-2-pyrrolidone solution, which acts as a super-heating liquid. An etch rate of sapphire was as high as 6 μm/min for laser power of 1.0 J/cm 2 . The A1N layer was grown on the patterned sapphire substrate by chemical vapor reaction process at 1190 °C for 1 h. On the inclined sidewall of the etched grooves, A1N grew with its c -axis perpendicular to the sidewall. This produced m -plane surfaces which were inclined around 20° to the basal plane of sapphire. A threading dislocation density of m -plane A1N domains was 8×10 8 cm −2 .

For rating unambiguous performance of a light-emitting semiconductor material, determination of the absolute quantum efficiency (AQE) of radiation, which is basically a product of internal quantum efficiency (IQE) and light-extraction efficiency, is the most delightful way. Here, we propose the use of omnidirectional photoluminescence (ODPL) spectroscopy for quantifying AQE of the near-band-edge (NBE) emission, in order to evaluate bulk GaN crystals and wafers. When the measurement was carried out in the air, the AQE showed a continuous decrease most likely due to the formation of extrinsic nonradiative recombination channels at the surface by photo-pumping. However, such an influence was suppressed by measuring ODPL in an inert ambient such as nitrogen or in vacuum. Consequently, AQE was revealed to depend on the photo-pumping density. The increase in AQE of the NBE emission caused by the increase in the excess carrier concentration was significant, indicating gradual saturation of nonradiative recombina...

The effect of thick n-type Al0.12Ga0.88N cleave assistance layers (CALs) on the morphology of c -plane cleaved facets was investigated for m -plane InGaN/GaN laser diode (LD) ridge waveguide structures. Five LD structures with AlGaN CAL thicknesses ranging from 0 to 800 nm were thinned, scribed, cleaved, and characterized via plan view optical microscopy to evaluate the morphology of the cleaved facets. The fraction of facets that appeared straight across the entire width of the ridge waveguides increased with increasing AlGaN CAL thickness, nearly doubling from a minimum of 43% in samples with no AlGaN CAL to a maximum of 85% in samples with a 600 nm AlGaN CAL. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

微小な偏位を呈する後天性上下斜視12例ついて、その特徴、発症機序について検討した。偏位量は近見および遠見とも2Δ～10Δであり平均4.7Δであった。Hess coordimeterまたはsynoptophoreによる9方向眼位測定において全例共同性斜視のパターンを示した。また、全例ともこのようなわずかな眼位ずれではあるが、鋭敏に複視を自覚し、プリズムによる中和が可能であった。今回検討した症例の特徴は、後天性であり共同性のパターンを示した。このことから今回検討した偏位は、Skew deviationの1つの型ではないかと考えられた。

The first semipolar nitride laser diodes (LDs) have been realized on low extended defect density semipolar (10) GaN bulk substrates. The LDs were grown by conventional metal organic chemical vapor deposition (MOCVD). Broad area lasers with uncoated etched facets were fabricated and tested under pulsed conditions. Lasing was observed at a duty cycle of 0.025% with the threshold current density (Jth) being 24.5 kA/cm2 for a 5 µm × 800 µm device. An output power of 65 mW was measured from a single facet of the device at a drive current of 1.5 A, with the calculated differential quantum efficiency (ηdiff) being 8%. Stimulated emission was observed at 405.9 nm with a narrow full width half maximum (FWHM) of less than 0.3 nm. The lowest measured Jth was 16.5 kA/cm2 for a 10 µm × 800 µm device. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

InGaN/GaN-based light emitting diodes (LEDs) grown on m -plane, a -plane and off-axis between m - and a -plane GaN bulk substrates were investigated. A smooth surface was obtained when a -plane substrate was applied; however, large amounts of defects were observed. Photoluminescence measurements of the LEDs with a well thickness of 2.5 nm revealed that all the LEDs showed the peak emission wavelength at 389 nm. The PL intensity of the a -plane LED is one order of magnitude lower than that of the m -plane LED. The a -plane LEDs showed significant lower electroluminescence output powers than m -plane LEDs, suggesting that excitons are trapped by the defects, which act as non-radiative recombination centers. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of m-plane GaN substrate miscut was investigated for InGaN/GaN multi quantum wells (MQWs) light emitting diodes (LEDs). The n-GaN and InGaN/GaN LEDs were grown by metal organic chemical vapor deposition on m-plane (1100)GaN substrates with miscut angles toward the [0001] direction of 0.01 (nominally on-axis m-plane), 0.45, 0.75, 5.4, and 9.6°. The surface morphology improved with increasing the miscut angles toward the [0001] direction. The peak emission wavelength of the electroluminescence grown on the on-axis m-plane and the miscut angle of 0.45° was 391–396 nm, while the miscut angle of 5.4 and 9.6° showed 440–454 nm. These results demonstrate that the surface morphology of GaN and In incorporation in the MQWs are strongly impacted by the miscut angle of GaN substrate.

Improved nonpolar m-plane (1100) light emitting diode (LED) with a thick InGaN active layer of 8 nm and a thick GaN barrier layer of 37.5 nm for multi-quantum-well (MQW) structure have been fabricated on low extended defect bulk m-plane GaN substrates using metal organic chemical vapor deposition (MOCVD). The peak wavelength of the electroluminescence (EL) emission from the packaged LED was 468 nm. The output power and external quantum efficiency (EQE) were 8.9 mW and 16.8%, respectively, at a DC driving current of 20 mA.

We demonstrate continuous-wave (CW) operation of nonpolar m-plane InGaN/GaN laser diodes without Al-containing waveguide cladding layers. Thick InGaN quantum wells (QWs) are used to generate effective transverse optical mode confinement, eliminating the need for Al-containing waveguide cladding layers. Peak output powers of more than 25 mW are demonstrated with threshold current densities and voltages of 6.8 kA/cm2 and 5.6 V, respectively. The unpackaged and uncoated laser diodes operated under CW conditions for more than 15 h.

We demonstrate a high power green InGaN/GaN multiple-quantum-well (MQW) light emitting diode (LED) with a peak emission wavelength of 516 nm grown on low extended defect density semipolar (112) bulk GaN substrate by metal organic chemical vapor deposition. The output power and external quantum efficiency (EQE) at drive currents of 20 and 100 mA under direct current (DC) operation were 5.0 mW, 10.5% and 15.6 mW, 6.3%, respectively. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Improved nonpolar m -plane light emitting diodes (LEDs) with a thick InGaN multi-quantum-well (MQW) structure have been fabricated on low extended defect bulk m -plane GaN substrates using metal organic chemical vapor deposition (MOCVD). The peak wavelength of the electroluminescence emission from the packaged LEDs was 402 nm, which is in the blue-violet region. The output power and EQE were 28 mW and 45.4%, respectively, at a pulsed driving current of 20 mA. With increasing current, the output power increased linearly, and fairly flat EQE was observed with increasing drive current. At 200 mA, the power and EQE were 250 mW and 41%, respectively. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrate highly manufacturable nonpolar (m-plane) InGaN/GaN laser diodes without any Al-containing waveguide cladding layers. These devices utilize thick InGaN quantum wells to generate transverse optical mode confinement and can be grown and fabricated in a manner analogous to InGaN/GaN light emitting diodes. Pulsed lasing operation was demonstrated, with threshold voltages and current densities of 6.7 V and 3.7 kA/cm2, respectively.

Linear polarized electroluminescence was investigated for semipolar (3031) and (3031) InGaN light-emitting diodes (LEDs) with various indium compositions. A high degree of optical polarization was observed for devices on both planes, ranging from 0.37 at 438 nm to 0.79 at 519 nm. The extracted valence band energy separation was consistent with the optical polarization ratios. The effect of anisotropic strain on the valance band structure was studied using k?p method for the above two planes. The theoretical calculations are consistent with the experimental results.

Electroluminescence measurements were performed on GaN-based micro-cavity light-emitting diodes (MCLEDs) which had different cavity thickness. The MCLEDs were fabricated by laser lift off (LLO) technique to remove the sapphire substrate and were thinned with reactive ion etching (RIE) followed by chemical-mechanical polishing (CMP). The number of multiple peaks in the electroluminescence spectrum caused by interference effects decreased as the cavity thickness decreased. A far-field pattern measurement of the thin cavity sample showed a "rabbit ears" pattern, specific of microcavity emission.

Omnidirectional photoluminescence (ODPL) measurement using an integrating sphere was carried out to absolutely quantify the quantum efficiency of radiation ( η) in high quality GaN single crystals. The total numbers of photons belonging to photoluminescence (PL photons) and photons belonging to an excitation source (excitation photons) were simultaneously counted in the measurement, and η was defined as a ratio of the number of PL photons to the number of absorbed excitation photons. The ODPL spectra near the band edge commonly showed a two-peak structure, which originates from the sharp absorption edge of GaN. A methodology for quantifying internal quantum efficiency ( ηint) from such experimentally obtained η is derived. A record high ηint of typically 15% is obtained for a freestanding GaN crystal grown by hydride vapor phase epitaxy on a GaN seed crystal synthesized by the ammonothermal method using an acidic mineralizer, when the excitation photon energy and power density were 3.81 eV and 60 W/cm2, r...

Dispersion relationships of the refractive index and extinction coefficient of m-plane Al1− x In x N epitaxial films (x = 0.00, 0.23, and 0.30) grown on a freestanding m-plane GaN substrate were determined by spectroscopic ellipsometry measurement. The experimentally obtained ellipsometric parameters tan Ψ and cos Δ, which represent the differences in the p- and s-polarized amplitudes and phases of the incident light, respectively, were well fitted using the standard analytical functions. As the measurement was carried out at photon energies between 1.55 and 5.40 eV, the dispersion curves of the extinction coefficient k exhibited local maxima at approximately the Al1− x In x N bandgap energies of x = 0.23 and 0.30, and the sample with x = 0.00 showed an ordinal absorption spectrum with a bandtail formed owing to high-concentration residual impurities. A large and x-dependent energy difference between the absorption and emission spectra (Stokes’ shift) was observed for the Al1− x In x N films, suggesting the presence of carrier localization phenomena.

As-grown surfaces of (110) oriented laser-ablated YBa2Cu3Oy films have been investigated by scanning tunneling microscopy and spectroscopy, for the first time, at room temperature and 4.2 K. We found semi-macro scale surface structures along the c-axis, which were composed of (110) top planes and (100)/(010) lateral planes. The orthorhombic atomic lattice image, corresponding to Cu atom arrangement of both CuO2 double layers and CuO chain layers alternately, was observed on the (110) top plane at 4.2 K. Furthermore, broad swell-and-depression structures were observed as stripes perpendicular to the c-axis on the top plane. The STS results indicated superconducting nature on the swells and semiconducting nature on the depressions.

Fundamental electronic and optical properties of a low-resistivity m-plane GaN single crystal, which was grown by hydride vapor phase epitaxy on a bulk GaN seed crystal synthesized by the ammonothermal method in supercritical ammonia using an acidic mineralizer, were investigated. The threading dislocation and basal-plane staking-fault densities of the crystal were around 104 cm−2 and less than 100 cm−1, respectively. Oxygen doping achieved a high electron concentration of 4 × 1018 cm−3 at room temperature. Accordingly, a photoluminescence (PL) band originating from the recombination of hot carriers was observed at low temperatures, even under weak excitation conditions. The simultaneous realization of low-level incorporation of Ga vacancies (VGa) less than 1016 cm−3 was confirmed by using the positron annihilation technique. Consistent with our long-standing claim that VGa complexes are the major nonradiative recombination centers in GaN, the fast-component PL lifetime of the near-band-edge emission at room temperature longer than 2 ns was achieved.

In this paper, we discusse the origin of basal-plane stacking faults (BSFs) generated in the homoepitaxial hydride vapor phase epitaxy (HVPE) growth of m-plane gallium nitride (GaN). We investigated the effects of seed quality, especially dislocation density, on BSF generation during homoepitaxy. The results clearly identify basal-plane dislocation in the seed as a cause of BSF generation. We realized high-quality m-plane GaN substrates with a 2-in. diameter using HVPE on low-dislocation-density m-plane seeds.

GaN-based semipolar (2021) laser diodes (λ=505 nm) with a 4 nm wavelength blueshift from spontaneous emission to lasing are demonstrated. The minimal blueshift is attributed to the stable energy profile in the quantum wells.

Semipolar (2021) light-emitting diodes are demonstrated with high optical polarization ratio, high indium incorporation rate, small wavelength shift and narrow spectrum width. These advantages resulted in LEDs with high efficiency and low droop.

We have investigated systematically the particle formation on the surface of high-quality ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (YBCO) epitaxial films grown by pulsed laser deposition. Scanning-Auger-electron spectroscopy identified two types of precipitates, i.e., ${\mathrm{BaCuO}}_{2}$ and CuO, on the films deposited from a stoichiometric YBCO target. The coexistence of these two phases with a YBCO matrix film is well explained by the pseudoternary phase diagram of ${\mathrm{YO}}_{1.5}$-BaO-CuO, assuming a slight Y deficiency in the film. The precipitate formation was reduced when the films were grown under conditions slightly deviated from the optimum for the formation of well-equilibrated, highly crystalline, and high-${\mathit{T}}_{\mathit{c}}$ and high-${\mathit{J}}_{\mathit{c}}$ films. Under such thermodynamically off-equilibrated conditions, the phase separation into YBCO and secondary phases is presumed to be kinetically prevented to give solid-soluted films and off-stoichiometric compositions and inferior crystallinity and superconductivity. The film-thickness dependences of the precipitate density and size were quantitatively analyzed by scanning electron microscopy to indicate that the nucleation of precipitates occurred by the initial 50 nm growth of the film to be followed by the increase of the precipitate's size. The use of off-axis polished substrates could successfully suppress the nucleation of the precipitates. A possible mechanism of precipitate formation is proposed.

The topographic and electronic properties of the surfaces of (001) and (110) oriented YBa2Cu3Oy, epitaxial films have been probed by atomic resolution STM/STS at 4.2 K. The STM image on the (001) surface clearly revealed the atomic corrugation of the tetragonal lattice with an average spacing of 0.4 nm. while on the (110) surface the orthorhombic atomic lattice, corresponding to the Cu atoms of both CuO2 and CuO chain planes, was observed. The STS result on the (001) surface indicated the semiconducting nature of the terminating layer. As the tunneling tip came closer to the surface, however, the shape of the tunneling spectrum became more metallic and showed a superconducting energy gap, which seems to arise from the underlying superconducting layer. On the other hand, the tunneling spectra on the (110) surface indicated superconducting gap structures, independent of the tip-sample distance.

We have previously demonstrated the AlGaN-cladding-free (ACF) laser diode (LD) concept over a wide visible spectral range and various nonpolar/semipolar crystallographic orientations. The benefits of this ACF epitaxial design include lower operating voltages and higher production yields. Nonpolar LDs have been demonstrated out to 500nm1 with semipolar reaching as long as 534nm2. No results have been published on nonpolar/semipolar orientations for short wavelength LDs. In this paper we report on the short wavelength limits of this epitaxial design on nonpolar bulk m-plane GaN substrates.

Violet InGaN/GaN light emitting diodes (LEDs) was fabricated on semipolar GaN bulk substrates. The output power and external quantum efficiency at a driving current of 20 mA were measured. The first nonpolar m-plane nitride laser diodes (LDs) were realized on low extended defect bulk m-plane GaN substrates.

An m-plane freestanding GaN substrate satisfying both low resistivity (ρ = 8.5 × 10−3 Ωcm) and a low point-defect concentration, being applicable to vertically conducting power-switching devices, was grown by hydride vapor phase epitaxy on a nearly bowing-free bulk GaN seed wafer synthesized by the ammonothermal method in supercritical ammonia using an acidic mineralizer. Its threading dislocation and basal-plane staking-fault densities were approximately 104 cm−2 and lower than 100 cm−1, respectively. A record-long fast-component photoluminescence lifetime of 2.07 ns at room temperature was obtained for the near-band-edge emission, reflecting a significantly low concentration of nonradiative recombination centers composed of Ga vacancies.

Local excitation and emission dynamics of an isolated “Type-I1” basal-plane stacking-fault (BSF) in very low dislocation density GaN were studied using spatio-time-resolved cathodoluminescence. The low temperature lifetime of the BSF emission was quantified to be 640 ps. The carrier diffusion length was estimated by observing the temporal delay of the BSF peak relative to the free-exciton signal as a function of distance from the BSF. The results indicate that the near-band-edge emission leads to subsequent optical excitation of the BSF that increases the apparent diffusion length. Limiting the observation volume can improve the spatial resolution.

Ammonothermal growth of GaN was studied to determine its eventual utility for mass production of GaN bulk crystals. Dissolution of GaN in supercritical ammonia with 1 M NaNH2 was investigated through a weight loss method. The time dependence of the weight loss was examined at 500°C and 525°C. Since the weight loss did not reach saturation as a function of time, the solubility limit was not realized. However, experiments demonstrate that GaN has a negative temperature dependence of solubility in supercritical ammonobasic solutions. Based on this result, GaN was grown via fluid transport from metallic Ga to a free-standing GaN single crystal seed by placing the seed crystal in a higher temperature zone and the nutrient in a lower temperature zone. GaN films with thickness of 5 μm (Ga face) and 4 μm (N face) were simultaneously grown on the seed in three days. The surface morphology, optical property, and defect density were found to be different for films on Ga face and N face.

Structural and optical qualities of half-a-μm-thick m-plane Al1−xInxN epilayers grown by metalorganic vapor phase epitaxy were remarkably improved via coherent growth on a low defect density m-plane freestanding GaN substrate prepared by hydride vapor phase epitaxy. All the epilayers unexceptionally suffer from uniaxial or biaxial anisotropic in-plane stress. However, full-width at half-maximum values of the x-ray ω-rocking curves were nearly unchanged as the underlayer values being 80 ∼ 150 arc sec for ( 101¯0) and ( 101¯2) diffractions with both ⟨0001⟩ and ⟨112¯0⟩ azimuths, as long as pseudomorphic structure was maintained. Such Al1−xInxN epilayers commonly exhibited a broad but predominant luminescence peak in ultraviolet (x ≤ 0.14) to green (x = 0.30) wavelengths. Its equivalent value of the internal quantum efficiency at room temperature was as high as 67% for x = 0.14 and 44% for x = 0.30. Because its high-energy cutoff commonly converged with the bandgap energy, the emission peak is assigned to ori...

Devices grown on nonpolar and semipolar planes of GaN offer key performance advantages over devices grown on the conventional c-plane, including reduced polarization fields. This allows for a wider design space on semipolar planes for light emitting diodes (LEDs) to address the problem of efficiency droop at high current densities. LED structures with very thick (10–100 nm) InGaN single-quantum-well/double heterostructure active regions were grown using conventional metal organic chemical vapor deposition on semipolar (303¯1¯) free-standing GaN substrates and processed and packaged using conventional techniques. Simulated band diagrams showed reduced polarization fields on the (303¯1¯) plane. The calculated critical thickness for misfit dislocation formation is higher on the (303¯1¯) plane than on other semipolar planes, such as (202¯1¯), allowing for thicker active regions than our previous work to further reduce droop. The higher critical thickness was confirmed with defect characterization via cathodol...

The first semipolar nitride laser diodes (LDs) have been realized on low extended defect density semipolar (1011) GaN bulk substrates. The LDs were grown by conventional metal organic chemical vapor deposition (MOCVD). Broad area lasers were fabricated and tested under pulsed conditions. Lasing was observed at a duty cycle of 0.025% with a threshold current density (Jth) of 18 kA/cm2. Stimulated emission was observed at 405.9 nm with a full width at half maximum (FWHM) of less than 0.3 nm.