The alumina boat was then loaded into the centre
of the horizontal quartz reactor and the reactor was
flushed with nitrogen gas for about 20 min to clean
the growth zone. The reactor was then heated to the
designated temperature at a rate of 30◦C/min under
the continuous flow of 1 SLPM (standard litres per
minute) of nitrogen gas. The oxygen gas was allowed
into the growth zone once the set growth temperature
was reached