We report a novel and facile method for the fabrication of various AlN nanostructures with Al polarity using polarity control and selective etching without a mask or metal catalyst. To investigate the polarity transitions of the AlN layers obtained with different growth parameters, AlN layers were grown by high-temperature metalorganic chemical vapor deposition with varying growth temperatures and trimethylaluminum (TMAl) preflow rates. The growth of Alpolar AlN was clearly supported by a lower growth temperatures and higher TMAl preflow rates. Transmission electron microscopy showed that the threading dislocations (TDs) generated at the AlN−sapphire interface were bent toward the boundary of the N-polar grain because of the threedimensional growth mode of the mixed-polarity AlN layer. Finally, defect-free nanopillars, nanorods, nanofurrows, and nanowalls were fabricated by etching mixed-polarity AlN layers with an aqueous KOH solution. ■ INTRODUCTIONAlN layers are interesting materials for optical and electronic devices because of their wide band gap energy, high thermal conductivity, and high electrical resistivity. 1,2 Although its lattice and thermal expansion coefficients are greatly different from those of AlN layers, c-plane sapphire (α-Al 2 O 3 ) is commonly employed as a substrate for these layers because of its relatively low cost. However, in previous studies of AlN layers grown on sapphire substrates, the layers exhibited a low crystal quality because of the lattice and thermal expansion coefficient mismatches between the AlN layer and the sapphire, as well as the small surface diffusion length of the Al adatoms. 3,4 A high density of threading dislocations (TDs) gives rise to a decrease in the internal quantum efficiency (IQE) of deepultraviolet (DUV) light-emitting diodes (LEDs) 5,6 and the output power density of high-electron mobility transistors (HEMTs). 7,8 Therefore, there have been several reports of improved methods for the growth of high-quality AlN layers on sapphire substrates for high-efficiency AlN-based applications. Fujimoto et al. 9 and Brunner et al. 10 reported on the growth of AlN layers by high-temperature metalorganic chemical vapor deposition (HT-MOCVD), which increases the surface diffusion length of the Al adatoms. Several methods have been employed to improve the crystal quality of the AlN layers, including epitaxial lateral overgrowth (ELO), 11 patterned sapphire substrate (PSS) methods, 12 the use of a nucleation layer, 13 prior-to-growth flow schemes, 14 and migrationenhanced epitaxy (MEE). 15 With the development of these methods, the crystal quality of AlN layers has improved gradually. Another way to fabricate high-efficiency AlN-based devices is to use three-dimensional (3D) nanostructures, which provide many advantages. These nanostructures can reduce the defect density, 16 helping to relieve the strain. 17 Moreover, their 3D shape provides an increased active volume of the optical devices with various planar geometries. 18 Xiang J. reported a high-performance field-e...

This study investigates the crystallographic polarity transition of AIN layers grown by high temperature metalorganic chemical vapor deposition (HT-MOCVD), with varying trimethylaluminum (TMAI) pre-flow rates. AIN layers grown without TMAI pre-flow had a mixed polarity, consisting of Al- and N-polarity, and exhibited a rough surface. With an increasing rate of TMAI pre-flow, the AIN layer was changed to an Al-polarity, with a smooth surface morphology. Finally, AIN nano-pillars and nano-rods of Al-polarity were fabricated by etching a mixed polarity AIN layer using an aqueous KOH solution.

In this study, we have intentionally grown novel types of (11-22)- and (1-10-3)-oriented(3) and self-assembled inclined GaN nanorods (NRs) on (10-10) m-sapphire substrates using metal organic chemical vapor deposition without catalysts and ex situ patterning. Nitridation of the m-sapphire surface was observed to be crucial to the inclined angle as well as the growth direction of the GaN NRs. Polarity-selective KOH etching confirmed that both (11-22) and (1-10-3) GaN NRs are nitrogen-polar. Using pole figure measurements and selective area electron diffraction patterns, the epitaxial relationship between the inclined (11-22) and (1-10-3) GaN NRs and m-sapphire substrates was systematically demonstrated. Furthermore, it was verified that the GaN NRs were single-crystalline wurtzite structures. We observed that stacking fault-related defects were generated during the initial growth stage using high-resolution transmission electron microscopy. The blue-shift of the near band edge (NBE) peak in the inclined angle-controlled GaN NRs can be explained by a band filling effect through carrier saturation of the conduction band, resulting from a high Si-doping concentration; in addition, the decay time of NBE emission in (11-22)- and (1-10-3)-oriented NRs was much shorter than that of stacking fault-related emission. These results suggest that defect-free inclined GaN NRs can be grown on m-sapphire without ex situ treatment.

No abstract

In this study, we suggest a polarity-selective in-situ thermal etching and re-growth process for the fabrication of high quality Al terminated AIN epilayers by high temperature metalorganic chemical vapor deposition. Mixed-polar AIN layers grown on a thin (5 nm) buffer layer at a high temperature (950 degrees C) exhibited high crystalline quality. Surface morphologies of in-situ thermally etched AIN layers depended on the grain size and distance between grains. Increasing the initial grain size and diminishing the space between grains increased etching depth and width. During re-growth, threading dislocations were bent and annihilated in the vicinity of voids, which were formed by lateral growth of Al-polar AIN regions after thermal etching. Finally, a high quality Al-polar AIN template, as verified by an aqueous KOH solution, was successfully fabricated.