We report epitaxial growth of AlN films with atomically flat surface on nano-patterned sapphire substrates (NPSS) prepared by nano-imprint lithography. The crystalline quality can be greatly improved by using the optimized 1-μm-period NPSS. The X-ray diffraction ω-scan full width at half maximum values for (0002) and (10"Equation missing"<!-- image only, no MathML or LaTex -->2) reflections are 171 and 205 arcsec, respectively. The optimized NPSS contribute to eliminating almost entirely the threading dislocations (TDs) originating from the AlN/sapphire interface via bending the dislocations by image force from the void sidewalls before coalescence. In addition, reducing the misorientations of the adjacent regions during coalescence adopting the low lateral growth rate is also essential for decreasing TDs in the upper AlN epilayer.

Hexagonal AlN (h-AlN) nanowires with an average diameter of around 15 nm have been prepared by an extended vapor–liquid–solid growth technique and characterized by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray analysis, Raman spectroscopy and field emission measurements. This preparation is a rather simple route for bulk fabrication of h-AlN nanowires. The promising field emission property observed for h-AlN nanowires points to the important application potential of this material.

Morphology and microstructure of dislocation etch pits in GaN epilayers etched by molten KOH have been investigated by atomic force microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). Three types of etch pits ($\alpha$ , $\beta$ , and $\gamma$ ) are observed. The $\alpha$ type etch pit shows an inversed trapezoidal shape, the $\beta$ one has a triangular shape, and the $\gamma$ type one has a combination of triangular and trapezoidal shapes. TEM observation shows that $\alpha$ , $\beta$ , and $\gamma$ types etch pits originate from screw, edge, and mixed-type threading dislocations (TDs), respectively. For the screw-type TD, it is easily etched along the steps that the dislocation terminates, and consequently, a small Ga-polar plane is formed to prevent further vertical etching. For the edge-type TD, it is easily etched along the dislocation line. Since the mixed-type TDs have both screw and edge components, the $\gamma$ type etch pit has a combination of $\alpha$ and $\beta$ type shapes. It is also found that the chemical stabilization of Ga-polar surface plays an important role in the formation of various types of dislocation etch pits.

We experimentally demonstrate the direct-to-indirect bandgap transition of monolayer MoS ₂ under hydrostatic pressure.