Characteristics of Ga-doped ZnO films deposited by pulsed DC magnetron sputtering at low temperature

Characteristics of Ga-doped ZnO (GZO) transparent conductive oxide films have been investigated based on the absorption behavior and chemical states of dopant Ga in the film. GZO samples were prepared by pulsed DC magnetron sputtering at 423 K by varying the sputtering power from 0.6 to 2.4 kW and the Ga₂O₃ concentration in the targets from 0.6 to 5.7 wt%. Absorption spectra of the GZO films in the visible to ultraviolet range were characterized by long absorption tails and shoulders near the absorption edges indicating the presence of impurity states or bands that overlap with the conduction band. X-ray photoelectron spectroscopy and X-ray diffraction revealed that substantial portion of the dopant exists as finely dispersed or amorphous metallic Ga and oxide of Ga, which would be related to the formation of the impurity bands or states, especially in the samples with lower Ga content. Presence of these species is correlated to the limited doping efficiency observed in the GZO films.