Influence of sputter-etching of substrate on the microstructural and optical properties of ZnO films deposited by RF magnetron sputtering

ZnO films were prepared using radio frequency magnetron sputtering on Si(1 1 1) substrates that were sputter-etched for different times ranging from 10 to 30 min. As the sputter-etching time of the substrate increases, both the size of ZnO grains and the root-mean-square (RMS) roughness decrease while the thickness of the ZnO films shows no obvious change. Meanwhile, the crystallinity and c-axis orientation are improved by increasing the sputter-etching time of the substrate. The major peaks at 99 and 438 cm−1 are observed in Raman spectra of all prepared films and are identified as E2(low) and E2(high) modes, respectively. The Raman peak at 583 cm−1 appears only in the films whose substrates were sputter-etched for 20 min and is assigned to E1(LO) mode. Typical ZnO infrared vibration peak located at 410 cm−1 is found in all FTIR spectra and is attributed to E1(TO) phonon mode. The shoulder at about 382 cm−1 appearing in the films whose substrates were sputter-etched for shorter time (10–20 min) originates from A1(TO) phonon mode. The results of photoluminescence (PL) spectra reveal that the optical band gap (Eg) of the ZnO films increases from 3.10 eV to 3.23 eV with the increase of the sputter-etching time of the substrate.