Articles you may be interested inHigh-resolution X-ray diffraction analysis of AlxGa1−xN/InxGa1−xN/GaN on sapphire multilayer structures: Theoretical, simulations, and experimental observations
Optical properties of nearly stacking-fault-free m -plane GaN homoepitaxial films grown by metal organic vapor phase epitaxy on low defect density freestanding GaN substrates
The optical properties of Ga-rich AlxGa1−xN (x=0.019, 0.038, 0.057, 0.077, and 0.092) ternary alloy epitaxial layers have been studied by means of temperature-dependent photoluminescence (PL) and time-resolved PL spectroscopy. The luminescence intensity of excitons in five epitaxial layers indicated a thermal quenching process with two activation energies. The two quenching activation energies were attributed to the delocalization of excitons and thermal dissociation of excitons. Anomalous temperature dependence of the PL peak energy was also observed in the epitaxial layers, which enabled the evaluation of the localization energy of the excitons. The localization energy increased as the 1.7th power of the PL linewidth, which reflected a broadening of the density of localized exciton states. In addition, the luminescence decay of the localized excitons for the five epitaxial layers became longer with decreasing emission energy. These observations suggest that the decay of excitons is caused not only by radiative recombination, but also by transfer to lower energy states.
A high quality AlGaN layer with low dislocation density and low c-axis tilt angle in wing regions was demonstrated by the advanced ELO technique, namely air-bridged lateral epitaxial growth. An underlying GaN seed layer was grooved along the 〈1100〉 GaN direction to the sapphire substrate, whose sidewalls and etched bottoms were covered with silicon nitride masks, and regrowth of AlGaN was carried out by a low-pressure metalorganic vapor phase epitaxy system. Fabrication of air-bridged structures suppresses interference of nucleated poly-crystals on the silicon nitride masks during lateral growth, and the low dislocation density AlGaN layer was realized. The threading dislocation density in the wing regions was reduced to 2 × 10 7 cm -2 and the c-axis tilt angle was 0.19°.1 Introduction III -V nitrides have shown potential for use in shorter wavelength optical devices. These nitride devices are usually fabricated on sapphire substrates because of a lack of large-size GaN substrates. The threading dislocation (TD) density has been known between 10 8 cm -2 and 10 10 cm -2 due to the large lattice mismatch between GaN and sapphire. In order to reduce the TD density, epitaxial lateral overgrowth (ELO) techniques have been performed [1,2]. The ELO techniques have achieved to considerable success in reducing the TD density to 10 6 cm -2 range in wing regions. A c-axis tilting in the wing regions, however, was observed with a different direction on both sides of seed regions in an azimuth perpendicular to the stripe direction. The tilt angle of the wing regions is commonly 0.2°-1.0° for a thin GaN layer by using (0002) X-ray diffraction (XRD) rocking curve measurements [3,4]. In our previous work, we have reported an advanced ELO technique, namely "air-bridged lateral epitaxial growth (ABLEG)" [5,6]. By using the ABLEG technique, the TD density at the surface of the wing regions is reduced to less than 5 × 10 6 cm -2 and the tilt angle of the wing regions is lowered to be 0.1°. In conventional nitride based device structures, AlGaN layers are grown on underlying GaN layers. It is necessary that high power and low noise optical devices have crack-free and thick AlGaN cladding layers. It is not, however, easy to grow high-crystalline quality AlGaN/GaN heterostructures due to the lattice mismatch. In order to suppress the crack formation, growth of an AlGaN layer on an underlying AlGaN layer instead of the underlying GaN layer is effective, because the lattice mismatch is relatively small. But it is quite difficult to grow a low TD density AlGaN layer on sapphire substrate, and to reduce the TD density by ELO techniques because of nucleation of poly-crystals on the mask surface [7]. Several apporaches have reported a TD density reduction of AlGaN layer [8,9]. The ABLEG technique not only reduces the interfacial stress between the wing regions and the masks, but also suppresses interference of nucleated poly-crystals during the lateral growth. In this study, we have performed ABLEG of AlGaN and achieved a TD density reductio...
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