Effects of thermal annealing on the structural and electrical properties of ZnO thin films for boosting their piezoelectric response

The growth process and post-deposition treatment have strong effects on the structure of ZnO thin films, which can also affect significantly their electrical and piezoelectric properties. In this report, the effect of high temperature annealing under oxygen atmosphere on ZnO stacked thin films grown on silicon by pulsed-liquid injection metal-organic chemical vapor deposition is investigated in detail. By varying the annealing temperature from 600 to 1000 degrees C, the structural analysis reveals the occurrence of c-axis oriented grain coarsening and growth processes associated with the improvement of crystallinity and removal of carbon impurities. The electrical resistivity also varies strongly with thermal annealing. Those variations are accompanied by an enhancement of the piezoelectric amplitude measured by piezo-response force microscopy (PFM). Interestingly, the PFM measurements reveal the predominant development of O-and Zn-polar domains at low and high temperature annealing, respectively, originating from the anisotropic grain boundary velocity. The present findings show that the high temperature annealing under oxygen atmosphere in ZnO thin films can enhance not only its piezoelectric amplitude but also its Zn-polarity uniformity, which unambiguously leads to the improvement of its piezoelectric performance. (C) 2021 Elsevier B.V. All rights reserved.