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Wurtzite BAlN alloy with large bandgap is an emerging material system for UV optoelectronic and power devices. In this study, the BAlN/GaN heterojunction with a sharp interface and uniform distribution of the elements was formed by careful control of epitaxial conditions to avoid GaN desorption and parasitic reactions whose band alignment was then measured for the first time. The valence band offset (VBO) was found to be nearly-zero, i.e. −0.2 ± 0.2 eV, in the B 0.14 Al 0.86 N/GaN heterojunction by X-ray photoelectron spectroscopy. Additionally, the conduction band offset (CBO) is estimated at 2.1 ± 0.2 eV, which is the largest reported CBO among epitaxial GaN-based heterojunctions. In comparison to the type-I Al 0.75 Ga 0.25 N/GaN heterojunction (both Al 0.75 Ga 0.25 N and B 0.14 Al 0.86 N alloy have the same bandgap of 5.7 eV), the CBO and VBO of the B 0.14 Al 0.86 N/GaN heterostructure are significantly larger and smaller, respectively. The nearly-zero VBO and the very large CBO of the B 0.14 Al 0.86 N/GaN heterojunction could potentially lead to considerable performance enhancement for GaN optoelectronics and power electronics devices. [ABSTRACT FROM AUTHOR]

Abstract: A dramatic reduction in threading dislocation density and stress-relaxation was simultaneously achieved in GaN epilayer using a silica nanosphere embedded structure on V-groove patterned sapphire substrate by metal-organic chemical vapor deposition. By depositing silica nanospheres at two different instances during a growth process, a two-step growth that included selective area growth and lateral overgrowth was initiated. This approach led to GaN template of high crystal quality, which was confirmed from x-ray diffraction rocking curve and micro-Raman measurements and further corroborated by transmission electron microscopy. GaN light-emitting diode fabricated by this strategy showed a significant enhancement in the light output power. [Copyright &y& Elsevier]

Abstract: Selective growth by metal-organic chemical vapor deposition (MOCVD), and electrochemical etching of a heavily Si-doped GaN (n+-GaN) interlayer were employed to obtain air-gaps embedded in a u-GaN layer. As confirmed by Raman spectroscopy, the introduction of an n+-GaN, which was later etched to obtain air-gaps, also enhanced the strain-compliance of GaN epilayer on sapphire substrate. An enhanced electroluminescence emission was observed from the light-emitting diodes (LEDs) fabricated on the air-gap embedding template. Using theoretical LED simulation, it was discerned that the increase in optical emission from the LED was caused predominantly by the redirection of photons at GaN/air-gap interface. Finite-difference time domain (FDTD) simulation method was employed to understand the mechanism of optical emission enhancement and its spatial variation over the LED surface. [Copyright &y& Elsevier]

Abstract: A two-step growth approach based on facet-controlled epitaxial lateral growth and the application of silica nanospheres was established to enhance the performance of GaN based light-emitting diodes (LEDs). In the first step, open inverted honeycomb cones (IHCs) were fabricated. These IHCs were filled with silica nanospheres and a second growth step was performed. As compared to LEDs fabricated on IHC templates, 2.7 fold electroluminescence (EL) intensity was obtained for silica nanospheres-stacked IHC due to improved crystal quality and light scattering at silica nanospheres. Simulation of emission intensity was carried out to determine the effect of dislocation density reduction on EL enhancement of the LEDs. [Copyright &y& Elsevier]

Abstract: Graphene oxide coated on GaN was simultaneously reduced and doped with nitrogen via excimer laser irradiation. Nitrogen dopant was originated from the GaN during the laser-induced dissociation at high energies. This phenomenon was confirmed by the absence of C–N bond formation in laser irradiated graphene oxide on SiO2. A top-gated field-effect transistor based on laser reduction of graphene oxide channel on GaN showed n-type behavior via the gate voltage modulation. The present findings indicate a paradigm for the formation of graphene-nitride semiconductor interfaces. [Copyright &y& Elsevier]

Abstract: We report on the formation and optimization of undercut-microholes (UM) generated by a wet etching process. GaN epilayers with 6μm and 15μm polygonal holes (PH) were grown by using selective metal organic chemical vapor deposition under identical growth conditions. The samples were wet etched with either a KOH solution or a mixed H3PO4:H2SO4 solution. Both kinds of etching solution produced the formation of UM. In the case of the etching produced with the mixed H3PO4:H2SO4 solution, the angle of UM was varied with an increase of H2SO4 in the solution. The etching produced by the KOH solution was very simple, and it formed a clear UM with an angle of 62°. This was achieved without etching the hard mask because of the selective etching and crystallographic characteristics of the GaN. UM were optimized through etching with PH structures, and the results showed formation of clear UM in a 15μm PH structure. [Copyright &y& Elsevier]

Abstract: This paper reports on the deflection effect of ellipsoidal air tunnel (EAT) array as an embedded deflector with low refractive index in InGaN/GaN light-emitting diodes (LEDs). Light extraction efficiency (LEE) for different height-to-width (h/w) ratios of air structure is analyzed by using an optical simulation program. Maximum LEE was obtained at a h/w ratio of 0.3, which corresponds to an ellipsoidal shape of the air structure. An LED structure with EAT array embedded between the sapphire substrate and the GaN epitaxial layer has been fabricated. The reflectance spectra measured over the entire visible spectral range showed reflectance modulations by the EAT structures. Moreover, the light-output power of the EAT-embedded LED was enhanced by 2.3times compared to a conventional LED without EAT arrays. This could be attributed to the enhanced light deflection from EAT structures. [Copyright &y& Elsevier]

Abstract: We fabricated GaN-based light-emitting diodes (LEDs) with a transparent ohmic contact made from nanoporous indium tin oxide (ITO). The nanoporous structures are easily made and controlled using a simple wet etching technique. The transmittance, sheet resistance, and root-mean-square surface roughness of the nanoporous ITO films are correlated strongly with the etch times. On the basis of the experimental values of these parameters, we choose an optimum etch time of 50s for the fabrication of LEDs. The wall-plug efficiency of the LEDs with nanoporous ITO is increased by 35% compared to conventional LEDs at an injection current of 20mA. This improvement is attributed to the increase in light scattering at the nanoporous ITO film-to-air interface. [Copyright &y& Elsevier]