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.
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