Undoped and Ga-doped (3wt%) n-type ZnO thin films were grown by using a reactive plasma deposition method on glass substrates at 200C under an O flow rate of 0 to 50sccm. The defect and band-edge related signals in the optical absorption spectra for ZnO thin films were studied by using piezoelectric photothermal spectroscopy, which was effective for observing a non-radiative transition process. A piezo-electric photothermal peak at around 2.5eV was observed only for undoped ZnO samples grown under a low O flow-rate. This signal was considered to be related to O vacancies, because it disappeared with an increase in the O flow rate. No corresponding peak was found for Ga-doped samples. This indicated that Ga doping inhibited the generation of O vacancies, in agreement with first-principles electronic band structure calculations. Theoretical calculation were also made of the optical absorption edge of degenerate ZnO as a function of the carrier concentration. It was concluded that the Burstein-Moss effect and a band-gap narrowing effect in ZnO should be considered in the case of high carrier concentrations. Upon comparing the experimental results with theoretical predictions, it was found that the proposed piezo-electric photothermal edge energy coincided well with the Fermi level.

Defect Centers and Optical Absorption Edge of Degenerated Semiconductor ZnO Thin Films Grown by a Reactive Plasma Deposition by Means of Piezoelectric Photothermal Spectroscopy. Sakai, K., Kakeno, T., Ikari, T., Shirakata, S., Sakemi, T., Awai, K., Yamamoto, T.: Journal of Applied Physics, 2006, 99[4], 043508