Modulation of electrical and optical properties of gallium-doped ZnO films by radio frequency magnetron sputtering

Ga-doped ZnO (GZO) films are prepared on amorphous glass substrates at room temperature by radio frequency magnetron sputtering. The results reveal that the gallium doping efficiency, which will have an important influence on the electrical and optical properties of the film, can be governed greatly by the deposition pressure and film thickness. The position shifts of the ZnO (002) peaks in X-ray diffraction (XRD) measurements and the varied Hall mobility and carrier concentration confirms this result. The low Hall mobility is attributed to the grain boundary barrier scattering. The estimated height of barrier decreases with the increase of carrier concentration, and the trapping state density is nearly constant. According to defect formation energies and relevant chemical reactions, the photoluminescence (PL) peaks at 2.46 eV and 3.07 eV are attributed to oxygen vacancies and zinc vacancies, respectively. The substitution of more Ga atoms for Zn vacancies with the increase in film thickness is also confirmed by the PL spectrum. The obvious blueshift of the optical bandgap with an increase of carrier concentration is explained well by the Burstein-Moss (BM) effect. The bandgap difference between 3.18 eV and 3.37 eV, about 0.2 eV, is attributed to the metal-semiconductor transition.