Atomic plane misorientation assisted crystalline quality improvement in epitaxial growth of AlN on a nanopatterned sapphire (0001) surface for deep ultraviolet photoelectric devices

The atomic-layer misorientation during the growth of a 5 μm thick AlN thin film on a patterned (0001) sapphire substrate was investigated by the scan rotation approach using a probe aberration-corrected scanning transmission electron microscope at a nanometer scale. Through the geometrical phase analysis of the resulting twisted atomic structure at different depths below the top surface, it is shown that over 10% of local tensile and compressive strain is balanced in a 1.6° twist of the c-planes within the first micron of AlN growth. As a consequence, the formation of threading dislocations is reduced. The in-plane twist is seen to decrease toward the layer surface down to 0.5°. Finally, growth has adopted the conventional step flow mechanism with a reduced density of emerging dislocations by the thickness of 5 μm. Our finding forecasts the possibility of understanding the relationship between atomic bilayer twist and local strain accommodation at a nanometer scale, which could provide guidance for achieving better crystal quality of AlN thin films on patterned substrates during epitaxy.