Investigation of the effects of drying process on microstructural and luminescence properties of Al-doped ZnO thin films

Al-doped ZnO (AZO) thin films are deposited on glass substrates by sol-gel method and dip-coating technique using different number of coating layers (6, 9 and 12). The effects of two different drying processes on structural, morphological, electrical and optical properties of the synthesized thin films are investigated by means of X-ray diffraction (XRD), atomic force microscopy (AFM), Hall-effect measurements, UV-Vis-NIR spectrometry and photoluminescence (PL) spectroscopy. In the first drying process, temperature is gradually increased with an increment of 10 degrees/min from room temperature to 150 degrees C . While in the second drying process, temperature is maintained at 250 degrees C. XRD results show that all deposited films crystallized in a hexagonal wurtzite structure. It is observed that the second drying process yields better crystal quality and films orientation. The AFM images reveal that both surface morphology and roughness are dependent on the drying process. Low roughness of 1.33 nm is obtained for the sample dried with the second process. Hall-effect measurements demonstrate high carrier concentration of 4.58 x 10(21) cm(-3) and low resistivity of 0.003 Omega. cm for the films which were found to be dependent on the drying process and film thickness. According to UV-visible-NIR optical measurements, all films exhibit an average transmission ranging from 70 to 86% and the optical band gap is sensitive for both the drying process and film thickness. Room temperature photoluminescence spectra show the dependence of the AZO thin films emission on the drying process. Higher emissions are obtained for the films dried at 250 degrees C, however the emission decreases with increasing thickness and this for both drying processes. The obtained results confirm that crystal quality and PL properties of the AZO films can be enhanced by the type of drying method.