3.1.2. Step II
After the sol–gel solution was aged for more than 24 h and spincoated,
the substrates were preheated at 200, 250 and 300 °C to evaporate
the solvent and remove organic residuals, since the boiling point
of MEA was known to be 170 °C [21]. The morphologies of the
preheated films are shown in Fig. 3. From the SEM images of the films,
the surfaces of the preheated films at 200 and 300 °C look rough and ravine,
while the film at 350 °C has rather smooth surfaces. Thus, the
preheating temperature of spin-coated films for smooth surfaces was
chosen to be 250 °C.
The morphologies of ZnO seed films obtained at different annealing
temperatures are shown by the SEMimages in Fig. 4. It is observed that
the ZnO film surfaces have higher surface roughness with the increase
of temperature. Up to 500 °C, the grain size (~10 nm) stayed almost
the same as that of preheated films shown in Fig. 3. However, the
grain size was increased to ~50 nm in the annealing temperature
range from 550 to 650 °C. Starting from the annealing temperature of
700 °C, the average grain sizes of ZnO thin films were increased to
about 150 nm, possibly due to the coalescence of the next lying ZnO
nanoparticles. Thus, vacancies in the ZnO films became larger as the
annealing temperature was increased. Whereas the ZnO films grown
between 550 and 650 °C exhibited rather smooth surfaces among the
films annealed at different temperatures. Therefore, we have chosen
600 °C as the annealing temperature of ZnO seed films.