In contrast, for higher doping the substituted Sn ions at zinc
sites deteriorated the crystallinity of the films, which may be due
to the formation of stress induced by ion size difference between
zinc and tin and the segregation of tin in grain boundaries for high
doping concentrations. Therefore, Sn ions may act as inhibitors for
the growth of ZnO along (002) plane during the deposition process
for higher concentrations.