Enhancement of thermoelectric properties of MBE grown un-doped ZnO by thermal annealing

In this paper, we have reported an enhancement in thermoelectric properties of un-doped zinc oxide (ZnO) grown by molecular beam epitaxy (MBE) on silicon (001) substrate by annealing treatment. The grown ZnO thin films were annealed in oxygen environment at 500oC-800oC, keeping a step of 100oC for one hour. Room temperature Seekbeck measurements showed that Seebeck coefficient and power factor increased from 222 to 510 uV/K and 8.8x10-6 to 2.6x10-4 Wm-1K-2 as annealing temperature increased from 500 to 800oC respectively. This observation was related with the improvement of crystal structure of grown films with annealing temperature. X-ray diffraction (XRD) results demonstrated that full width half maximum (FWHM) of ZnO (002) plane decreased and crystalline size increased as the annealing temperature increased. Photoluminescence study revealed that the intensity of band edge emission increased and defect emission decreased as annealing temperature increased because the density of oxygen vacancy related donor defects decreased with annealing temperature. This argument was further justified by the Hall measurements which showed a decreasing trend of carrier concentration with annealing temperature.