The effects of oxygen partial pressure on local structural properties for Ga-doped ZnO thin films

We report on Raman scattering studies of Ga-doped ZnO thin films that were grown by intentionally changing oxygen partial pressure in order to study the influences of oxygen partial pressure on local structural properties of this material. Raman spectra of ZnO:Ga (3 wt.% Ga-doped) films revealed vibrational modes at 575 and 630-660 cm ^-1 in addition to the host phonons of ZnO. These additional modes correspond to local vibrational modes associated with oxygen vacancy (V _O) and Ga impurity (Ga _Zn), respectively. With increasing oxygen partial pressure (oxygen flow rate up to ∼ 10 sccm), the 575-cm ^-1 mode decreases in its intensity, indicating the reduced V _O concentration. Further increasing oxygen partial pressure (> 10 sccm), we find a substantial disorder apparent in host ZnO phonons and some additional modes. These results suggest that the oxygen-rich condition may cause the formation of compensating-defects such as oxygen interstitials (O _i), Zn vacancy (V _Zn) and their complexes (Ga _Zn-O _i, Ga _Zn-V _Zn), strongly reducing carrier concentration in this system.