Dependence of oxygen flow rate on piezoelectric photothermal spectra of ZnO thin films grown by a reactive plasma deposition

Undoped and Ga-doped (3 wt.%) n-type ZnO thin films were grown by a reactive plasma deposition method on glass substrates at 200 °C under oxygen flow rate from 0 to 50 sccm. Piezoelectric photothermal spectroscopy (PPTS) was measured for characterizing the ZnO films from the viewpoint of nonradiative recombination processes. Strong peak due to the band edge transition was observed around 3.3 eV in all the samples at room temperature. Furthermore, a broad band of the PPT signal at 2.5 eV was observed for the undoped ZnO samples grown under low oxygen flow rate. This signal disappeared with increasing the oxygen flow rates and did not appear in the Ga-doped samples. Therefore, it is considered that this PPT signal at 2.5 eV was due to the electron transition from the oxygen vacancies.