Enhanced piezoelectric coefficient and the piezoelectric nanogenerator output performance in Y-doped ZnO thin films

We report a significant improvement of the piezoelectric coefficient of the ZnO thin film (12.4 p.m./V) by yttrium (Y)-doping (49.6 p.m./V @1.6 at % Y), demonstrating further to enhance piezoelectric nanogenerator (PENG) performance, generating output voltage (0.012 V) and output current (45 nA) more than the undoped ZnO thin film by 6 and 1.8 times, respectively. Y doping (1.6 at %) led to the formation of permanent dipoles as ferroelectric phase in ZnO evidenced by a clear, distinct hysteresis loop with remnant polarization of 0.0423 μC/cm² and a coercive field of 0.142 V/cm. All the films were grown by radio frequency magnetron co-sputtering with the maximum Y doping concentration up to 3.5 at%. The films maintain highly oriented along the c-axis (0002) at low Y concentrations (∼1.6 at%), but become more randomly oriented with the emergence of both the (10 1‾ 1) and (112‾ 0) peaks at high Y concentrations (>1.6 at%). The morphology varies from granular microstructure with smaller grain size at low Y doping to larger elongated crystals at the Y > 1.3 at%. Concurrently, cross-sectional images revealed the conversion of columnar nanorod like microstructure into film like (thickness: 650–750 nm) with increased Y doping concentration.