Comprehensive study of the surface morphology evolution induced by thermal annealing in single-crystalline ZnO Films and ZnO bulks

We report on the evolution of the surface morphology induced by thermal annealing in N-2 ambient over a wide temperature range of 500-1200 A degrees C in single-crystalline ZnO films and ZnO bulks. The surface morphology is seriously changed by the annealing temperature, and the evolution can be categorized into three regions: island growth, island agglomeration, and pit formation. Island growth at low temperatures below 700 A degrees C, is ascribed to the atomic migration to reduce surface energy, which causes surface roughening. Island agglomeration at intermediate temperatures of 700-900 A degrees C is ascribed to the migration and the evaporation of surface atoms, which causes surface flattening. Pit formation at high temperatures above 900 A degrees C is ascribed to the atomic evaporation by high vapor pressure, which causes surface destruction. On the other hand, the bulk lattice is continuously improved with increasing annealing temperature in the temperature regions before the surface-destruction region, which is attributed to the reduction in the numbers of point and line defects caused by recrystallization. As a result, the best surface morphology and the best bulk lattice are obtained at an annealing temperature of 900 A degrees C. The common surface-morphology evolution of ZnO films and ZnO bulks with increasing annealing temperature can be summarized using the three steps of surface roughening by island growth, surface flattening by island agglomeration, and surface destruction by pit formation.