Comparison of the structural and optical properties of zinc oxide thin films deposited by d.c. and r.f. sputtering and spray pyrolysis

ZnO thin films were deposited using three processes: d.c. reactive sputtering of Zn, r.f. sputtering of ZnO, and spray pyrolysis. The influence of the temperature (substrate temperature or post-deposition annealing) on composition, structural, and optical properties of these thin films was investigated. All sputtered ZnO thin film exhibit a preferred orientation along the c-axis of the hexagonal structure if they have a crystallised phase. An increase of the substrate temperature to 498 K favours the crystallinity of the samples deposited by reactive sputtering of Zn. At 598 K the preferred orientation disappears and the deposit becomes almost amorphous. The optical transmittance of the ZnO films increases with the substrate temperature. The influence of the post-deposition thermal treatments of the samples obtained by sputtering of ZnO on the optical properties, composition and stress variations is discussed. The spray pyrolysis process using a ZnCl2 solution led to obtain ZnO deposits highly orientated along the c-axis at a temperature close to 573 K.