  Ultraviolet-C light-emitting diodes (UVC-LEDs) have great application in
pathogen inactivation under various kinds of situations, especially in the
fight against the COVID-19. Unfortunately, its epitaxial wafers are so far
limited to 2-inch size, which greatly increases the cost of massive production.
In this work, we report the 4-inch crack-free high-power UVC-LED wafer. This
achievement relies on a proposed strain-tailored strategy, where a
three-dimensional to two-dimensional (3D-2D) transition layer is introduced
during the homo-epitaxy of AlN on high temperature annealed (HTA)-AlN template,
which successfully drives the original compressive strain into tensile one and
thus solves the challenge of realizing high quality Al$_{0.6}$Ga$_{0.4}$N layer
with a flat surface. This smooth Al$_{0.6}$Ga$_{0.4}$N layer is nearly
pseudomorphically grown on the strain-tailored HTA-AlN template, leading to
4-inch UVC-LED wafers with outstanding performances. Our strategy succeeds in
compromising the bottlenecked contradictory in producing large-sized UVC-LED
wafer on pronounced crystalline AlN template: The compressive strain in HTA-AlN
allows for crack-free 4-inch wafer, but at the same time leads to a
deterioration of the AlGaN morphology and crystal quality. The launch of 4-inch
wafers makes the chip fabrication process of UVC-LEDs matches the mature blue
one, and will definitely speed up the universal of UVC-LED in daily life.
