Effect of gallium doping and ball milling process on the thermoelectric performance of n-type ZnO

We report a systematic investigation of the thermoelectric properties of n-type Ga-doped ZnO synthesized using different ball milling conditions. Samples fabricated by the high-energy ball milling resulted in a highly dense layered structure with randomly distributed voids. These samples measured the lowest room temperature thermal conductivity, i.e., 27 W/mK due to increased phonon scattering. Furthermore, the Ga:ZnO system showed a metal–semiconductor transition above 300 K with transition temperature decreasing with increasing doping level. Measurement of the activation energy revealed the presence of one donor level around 3.9–7.8 meV and a deeper donor level around 15.4–18.1 meV below the conduction band for the Ga-doped samples. For Ga-doped ZnO, Seebeck coefficient of −185 μV/K (at 1000 K) was achieved, which is ∼30–45% higher than the values previously reported for Zn:Ga system. Jonker plot analysis was used to analyze the scope of Ga:ZnO bulk system.