Study of Sputtered ZnO:Ga2O3 Films for Energy Harvesting Applications

Thin films of ZnO:Ga2O3 (ZGO) were deposited by radio frequency (RF) sputtering at voltages of 0.5, 0.9 and 1.1 kV. The films were studied with respect to their suitability in flexible piezoelectric nanogenerators. The analysis of the spectroscopic and microscopic results showed that piezoelectric features were revealed for the films grown at all sputtering voltages, but the most favorable morphology in terms of low roughness was achieved at 1.1 kV. The effect of the sputtering voltage on the films crystallinity and lattice strain was studied. It was found that the increasing sputtering voltage promoted the films crystallization. Additionally, the presence of oxygen vacancies in the piezoelectric films was negligible as it is not a major factor affecting their performance. The electrical measurements of the Ag/ZnO:Ga2O3/Ag harvester on a flexible substrate in the low-frequency range showed a piezoelectric voltage of 414 mV, a current of 10.4 µA and an electric power output of 1.4 µW at a mass load of 100 g. These results were achieved by a simple architecture of a single piezoelectric layer with a relatively small size of 3 cm2 and small piezoelectric film thickness (600 nm) containing lead-free material. It was proven that the sputtered ZGO films are suitable for energy harvesting elements and their performance could be tuned by the sputtering voltage. Another possible application of the proposed device, excluding low-frequency vibrational harvesting, could be a pressure sensor or strain gauge, due to the good linearity of the electrical parameter dependences on the strain.