The size of crystals estimated from SEM and optical microscopy images varies for different compositions not exceeding 400 µm but is generally below 100 µm. That, in contrast to undoped crystals often reaching 1 mm in length, is expected since in the presence of an impurity the nucleation rate increases over the growth rate. Such effect is particularly noticeable for P-doped samples due to intense chemical interaction between ZnO melt and phosphorus: