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Photon‐pair generation based on optical parametric down‐conversion has attracted for the application as a light source for quantum information. Highly efficient wavelength‐conversion devices require a polarity‐inversion structure when using nitride semiconductors. A transverse quasi‐phase‐matching (QPM) polarity‐inverted GaN bilayer channel waveguide device is suitable for efficient wavelength conversion. This study designed a cross‐section device to satisfy the modal dispersion phase‐matching condition between the TM02 mode pump light and the TM00 mode signal/idler light. Moreover, an AlN oxidation interlayer fabricates the Ga‐polar/N‐polar (+c/−c) GaN layers via metalorganic vapor‐phase epitaxy (MOVPE). A 145 nm thick film layer with a macro‐step‐free surface is grown by optimizing the −c‐GaN growth conditions and reducing the substrate off‐angle to 0.2°. Next, the AlN layer is oxidized in an electric furnace and MOVPE is used to regrow a 1500 nm thick +c‐GaN layer. A macrosteps‐free surface can be achieved by reducing the off‐angle to 0.2° and optimizing the −c‐GaN growth conditions to avoid hillock formation. These results pave the way for improving the efficiency of GaN transverse QPM wavelength‐conversion devices. [ABSTRACT FROM AUTHOR]

Selective area growth and epitaxial lateral overgrowth (ELOG) of semipolar (11¯$1 \left(\right. \macr \left.\right)$01) GaN stripes are demonstrated on a trench patterned vicinal (001) Si substrate fabricated by a maskless photolithography‐based process. High precision alignment enables selective mask formation to one sidewall of the trench. Selective area growth of GaN stripes is conducted from the (111) plane sidewall of Si, and ELOG region reaches ≈13 μm. The ELOG GaN crystal is dislocation‐free at most areas. The semipolar GaN stripes with atomically flat surface morphology are uniformly obtained. Light‐emitting diode structures with InGaN/GaN multiple quantum wells are grown on the 13‐μm ELOG (11¯$1 \left(\right. \macr \left.\right)$01) GaN stripes and a single photoluminescence emission peaked at 485 nm is obtained, suggesting potential for the cost‐effective semipolar micro light‐emitting diode fabrication technologies. [ABSTRACT FROM AUTHOR]

AlN is a promising candidate for photonic integrated circuits. In this study, horizontally stacked transverse quasi‐phase‐matched (QPM) waveguides with an AlN center core and Si3N4, Ta2O5, and TiO2 side cores were designed to generate squeezed light at 920 nm. The squeezing level was calculated by considering the propagation loss at the wavelengths of the squeezed light and pump light. Using TiO2 for the side cores enhanced the electric field amplitude of the pump light in the AlN center core, and a high nonlinear coupling coefficient was obtained. Owing to the small propagation loss of the AlN waveguide on sapphire and high nonlinear coupling coefficient of the transverse QPM with TiO2 side cores, a squeezing level of over 8.1 dB was expected with a 1 cm long waveguide and pump power of 70 mW. [ABSTRACT FROM AUTHOR]

GaN is an attractive material for integrating optical quantum devices. Adding a large optical nonlinearity of MgO doped congruent LiNbO3 (MgO:CLN) to GaN will improve the efficiency of quantum light sources. In this work, we proposed transverse quasi-phase-matched wavelength conversion devices with waveguide core materials of MgO:CLN and GaN. The waveguide core is formed by an adhesion-free surface activated bonding (SAB). A high thin film transfer yield was achieved with a high bonding strength of 4 MPa by optimizing the bonding conditions and reducing the surface roughness of the GaN film to be 0.5 nm in a 100 × 100 μ m2 area using chemical mechanical polishing. The MgO:CLN/GaN waveguide structure was successfully fabricated by MgO:CLN thin film transfer, lift-off and dry etching processes. This MgO:CLN/GaN adhesion-free SAB technique is expected to be applied to various devices, such as optical devices and electronic devices, to enhance their functionality. [ABSTRACT FROM AUTHOR]

The polarity inversion of GaN films grown on vicinal sapphire substrates is demonstrated using the metal–organic vapor phase epitaxy (MOVPE) method via an AlN oxidation interlayer for application in wavelength conversion devices. By annealing the N‐polar AlN surface at 900 °C in air, the pit‐free Ga‐polar GaN layer can be regrown. Anisotropic etching in KOH and transmission electron microscopy confirms the formation of a GaN polarity‐inverted structure. X‐ray diffraction measurements reveal an increase in threading dislocations due to the nucleation island formation by low‐temperature GaN layer and strain relaxation at the regrown interface. The simple fabrication process of the GaN polarity‐inverted structure using an AlN oxidation interlayer will pave the way for applications in new devices that require polarity‐controlled and/or polarity‐inverted stacking structures. [ABSTRACT FROM AUTHOR]

Temperature-dependent radiative and non-radiative dynamics of photoexcited carriers were studied in In0.3Ga0.7N nanodisks (NDs) fabricated from quantum wells (QWs) by neutral-beam etching using bio-nano-templates. The NDs had a diameter of 5 nm, a thickness of 2 and 3 nm, and a sheet density of 2 × 1011 cm–2. The radiative decay time, reflecting the displacement between the electron and hole wavefunctions, is about 0.2 ns; this value is almost constant as a function of temperature in the NDs and not dependent on their thickness. We observed non-exponential decay curves of photoluminescence (PL) in the NDs, particularly at temperatures above 150 K. The thermal activation energies of PL quenching in the NDs are revealed to be about 110 meV, corresponding to the barrier heights of the valence bands in the disks. Therefore, hole escape is deemed responsible for the PL quenching, while thermal activation energies of 12 meV due to the trapping of carriers by defects were dominant in the mother QWs. The above-mentioned non-exponential PL decay curves can be attributed to variations in the rate of hole escape in the NDs because of fluctuations in the valence-band barrier height, which, in turn, is possibly due to compositional fluctuations in the QWs. We found that non-radiative trapping, characteristic of the original QW, also exists in about 1% of the NDs in a form that is not masked by other newly formable defects. Therefore, we suggest that additional defect formation is not significant during our ND fabrication process. [ABSTRACT FROM AUTHOR]

A GaN crystal comprises two polar structures along the c -axis direction, and functions as a quasi-phase-matching (QPM) crystal by fabricating a periodic inversion structure. We fabricated GaN-QPM crystals to design rib-waveguide-type devices for achieving highly efficient wavelength conversion. The QPM period required for wavelength conversion was calculated in the design phase of the device structure. GaN-QPM crystals with the obtained period were fabricated using double-polarity selective-area growth (DP-SAG). The GaN-QPM crystal was then used to fabricate a second-harmonic generation (SHG) device with a rib waveguide structure. Optical measurements revealed that the device achieved wavelength conversion from 840 to 420 nm. Further, the SHG device exhibited a wavelength conversion efficiency of 1.5 × 10–4% W−1. These results indicated that GaN-QPM crystals fabricated by DP-SAG can be used for wavelength conversion. [ABSTRACT FROM AUTHOR]

Watt-class wavelength conversion is expected by enlarging the mode size of the channel waveguides to dozens of micrometers to avoid optical damage with high-power pump lasers. In this work, uniform periodically-poled (PP) structures in MgO doped stoichiometric LiTaO3 were fabricated by applying voltage with a SiO2 insulation layer. Annealed proton-exchanged waveguides with a full width at 1/ e 2 maximum in a width of 43 ÎĽ m with an in-depth of 28 ÎĽ m were obtained by proton exchanging for 4 h and thermal diffusion for 75 h. This waveguide can be excited by pump waves with a power of 8.6 W considering the optical damage threshold. Second harmonic generation with a wavelength of 515 nm was conducted in the low power region of about 35 mW to investigate the basic characteristics. Normalized wavelength conversion efficiency in the 15 mm long PP structures was estimated to be 4.3% Wâˆ'1. [ABSTRACT FROM AUTHOR]

We investigated influences of mask pattern on the emission from InGaN multiple quantum wells through the differences in plane orientation appearing on the multifaceted islands in selective area metalorganic vapor phase epitaxy. Cathodoluminescence mapping confirmed that emission colors changed depending on the crystal plane. Photoluminescence spectroscopy showed that the emission wavelength red-shifted by increasing the mask width. By combining the difference of indium incorporation efficiency depending on the crystal plane and the lateral vapor phase diffusion effect, multiple quantum wells with different emission wavelengths of up to 106 nm were grown simultaneously in the microscale region. [ABSTRACT FROM AUTHOR]

We proposed a monolithic microcavity wavelength conversion device without a polarity-inverted structure. The device consists of a low birefringence paraelectric material and a dielectric material. A fundamental wave intensity is enhanced significantly in the microcavity with two distributed Bragg reflectors, and a second-harmonic wave is efficiently generated in a very short region close to a coherence length. As a first step of this study, we used GaN as a device material, and succeeded in the blue second harmonic generation with a wavelength of 428 nm. [ABSTRACT FROM AUTHOR]

Burgers vectors b of threading dislocations in freestanding GaN substrates were identified through statistical classification using the etch pit and multiphoton-excitation photoluminescence methods. The b = 1 a + 1 c threading mixed dislocations were propagated with large inclination angles more than four degrees from the c-axis to minimize elastic strain energy. The b = 1 c threading edge dislocations had a large contrast difference of more than 0.4 in the photoluminescence image due to their strong nonradiative properties. Based on these characteristics obtained through multiphoton-excitation photoluminescence measurements, major types of dislocations (b = 1 a , 1 a + 1 c , 1 c) could statistically be identified. [ABSTRACT FROM AUTHOR]

We report on the observation of biexciton luminescence from single quantum‐confined structures in N‐polar InGaN/GaN multiple quantum wells (MQWs) grown on c‐plane sapphire by metalorganic vapor phase epitaxy. Sharp emission lines are observed at different positions of the sample by micro‐photoluminescence (μ‐PL) mapping. The density of sharp emission lines is ≈0.3 μm−2, which is comparable with the inversion domain (ID) density found from transmission electron microscope observation, suggesting that the sharp emission lines originate from single quantum‐confined structures formed by the combination of quantum well and IDs in N‐polar InGaN/GaN MQWs. It is found from the excitation power dependence of the PL intensity of two adjacent sharp lines that the intensity of biexciton luminescence at the lower energy side shows a quadratic dependence on the excitation power while that of exciton luminescence at the higher energy side increased linearly with increasing excitation power. The biexciton binding energy is found to be 0.8 meV. [ABSTRACT FROM AUTHOR]

Cleaved ScAlMgO4 (SCAM) substrates with small lattice mismatch of 1.8% to GaN were used for metalorganic vapor phase epitaxial (MOVPE) growth of GaN. A single crystalline GaN film with a flat surface was obtained on a cleaved substrate without peeling off of the film, which is contrast to the growth on ZnO substrate with comparably small lattice mismatch to GaN. Polarity of the film was proven to be Ga polarity and an interface model was proposed based on the topmost surface structure of the cleaved SCAM. The effect of substrate-cleaning and growth conditions on the surface morphology, crystallographic orientation relationship with the substrate, threading dislocation densities, small residual strains, and small background carrier density of GaN have been studied. To discuss the residual strains, thermal expansion coefficient of SCAM has also been measured. [ABSTRACT FROM AUTHOR]

Unintentional impurity incorporation in N-polar GaN films grown by metalorganic vapor phase epitaxy (MOVPE) was investigated. The influences of growth conditions on the impurity concentration, electrical properties, and optical properties were discussed. Secondary-ion mass spectroscopy showed that both concentrations of carbon and oxygen were changed by growth conditions. With decreasing H2/(H2 + N2) ratio, the carbon concentration increased, while the oxygen concentration decreased. At higher V/III ratios, the oxygen concentration decreased. Nitrogen-rich carrier gas and a high V/III ratio can decrease oxygen incorporation in the N-polar GaN epitaxial layer. The growth parameters discussed in this study are concluded to affect the adsorption/desorption properties of impurities. Based on these results, the electrical properties of N-polar III-nitrides can also be controlled by choosing the appropriate MOVPE growth conditions. This technology will lead to improvements in N-polar III-nitride-based device performance. [ABSTRACT FROM AUTHOR]

InGaN films on nitride-sapphire substrates were grown by metalorganic vapor phase epitaxy. Reciprocal space mappings of X-ray diffraction around the $(10\bar15)$ asymmetric plane were used to measure the accurate indium composition and in-plane strain of InGaN films. InGaN films with nitridation of sapphire substrates shows higher indium incorporation than without nitridation samples at low molar fraction ratio, TMIn/(TMIn+TEGa), of group-III MO source. However, the high molar fraction ratio region shows relatively low indium composition of InGaN. Most of the InGaN on nitrided sapphire substrates showed narrow FWHM of 2 θ - ω and X-ray rocking curves and the surface flatness is improved, which indicates the improvement of crystal quality. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

With respect to N-polar GaN grown on a sapphire substrate, the effects of Mg doping on the surface morphology, and the optical, and electrical properties are precisely investigated. By doping Mg, hillocks observed on the surface of GaN can be suppressed, while step bunching becomes severe. The atomic terrace width is extended with increasing Mg/Ga precursor ratio. Mg doping can promote the surface migration of Ga adatoms on a GaN surface during growth. In the case of heavily Mg-doped GaN, atomic steps become wavy. From photoluminescence spectra, the dominant transition was found to change from near-band-edge transition to donor–acceptor-pair transition. Hall-effect measurement shows p-type conduction at room temperature for a sample grown with the Mg/Ga precursor ratio of 4.5 × 10−3. The activation energy is 143 meV, which is comparable to that of Mg in the conventional Ga-polar (0001) GaN. [ABSTRACT FROM AUTHOR]

InGaN/GaN multiple quantum wells (MQWs) on semipolar (11̄01) GaN microstripes on a Si substrate were fabricated and their optical properties were investigated. From cathodoluminescence (CL) analysis, strong CL emission was obtained in an MQW emitting at 433 nm. However, dark lines appeared in MQWs emitting at longer wavelengths. These dark lines are attributed to lattice relaxation and the generation of misfit dislocations and stacking faults in an MQW, resulting in nonradiative centers. The internal quantum efficiency (IQE) was estimated from excitation-power-dependent photoluminescence analysis. The (11̄01) InGaN/GaN MQW had a high IQE owing to the high crystalline quality of the underlying GaN and the reduced piezoelectric field. The IQE at a carrier concentration of 1×1018 cm-3 in a sample emitting at 490 nm was as high as 90%. The efficiency decreased in a sample with a higher indium content in the MQW due to the generation of misfit dislocations and stacking faults. [ABSTRACT FROM AUTHOR]

GaN films were successfully grown on Si(111) substrates using a reactive-sputter-deposited AlN intermediate layer (sp-AlN) at 350 °C. Uniform GaN growth was obtained with the pre sputtering of a thin Al film before the deposition of sp-AlN. Atomic force microscopy (AFM) measurements revealed that the density of pits originating from the ends of mixed/screw-type dislocations is about 109 cm-2, which is much lower than that of GaN grown on a metalorganic vapor phase epitaxy (MOVPE) deposited AlN intermediate layer. The decrease in the pit density might have originated from the small number of initial nuclei of GaN. [ABSTRACT FROM AUTHOR]

A drastic reduction of the dislocation density in a semipolar (1122) GaN stripe on a patterned Si substrate was achieved by the two-step selective growth of a GaN stripe. After depositing a SiO2 mask on the (1122) and (0001) faces of a GaN stripe grown on a (113) Si substrate, GaN was regrown only on the (1122) face. The dislocation density estimated from the dark-spot density in a cathodoluminescence (CL) image greatly decreased from 4.0×108 to 1.0×105/cm2 in the regrowth region. A transmission electron microscopy (TEM) image also verified that there were no dislocations at the regrowth interfaces. [ABSTRACT FROM AUTHOR]

We present a study of semi-polar (1101) InGaN-based light emitting diodes (LEDs) grown on patterned (001) Si substrates by atmospheric-pressure metal organic chemical vapor deposition. A transmission electron microscopy image of the semi-polar template shows that the threading dislocation density was decreased significantly. From electroluminescence measurement, semi-polar LEDs exhibit little blue-shift and low efficiency droop at a high injection current because the reduction of the polarization field not only made the band diagram smoother but also restricted electron overflow to the p-GaN layer as shown in simulations. These results indicate that semi-polar InGaN-based LEDs can possess a high radiative recombination rate and low efficiency droop at a high injection current. [ABSTRACT FROM AUTHOR]