Doping effects of group-IIIA elements on physical properties of ZnMgXO (X: Al, Ga, In, Tl) transparent conducting oxide films prepared by RF magnetron sputtering

ZnMgXO films alloyed with different group-IIIA elements (X: Al, Ga, In, Tl) were sputter grown on quartz and the effects of dopants on the structural, optical, and electrical properties of the resultant films were estimated and compared with each other. X-ray investigation and Hall measurement results confirmed that the crystal quality and the electrical properties of the films are strongly affected by the doped elements. It was observed that films with high crystal quality and a low resistivity of ∼10−4 Ω cm were obtained with Al or Ga doping, whereas films with a deteriorated crystal quality and a higher resistivity of ∼1.6 × 10−2 Ω cm were obtained with the In incorporation. Optical characterization results indicated that all the films were highly transparent with an average transmittance of ∼90% in the visible region between 400 nm and 800 nm. Films doped with Al or Ga had wide band-gaps of 3.75 eV or 3.7 eV, respectively, whereas In or Tl doped films had a lower band-gaps of 3.57 eV. It was suggested that the observed dependence on the dopant species is related to the discrepancy in the ionic radii between the alloying elements and the Zn host atom. It was also proposed that the band-gap difference in the films can be due to the combined effect of the Burstein-Moss shift and the many-body effect (the electron-electron scattering, electron-impurity scattering), and band-tailing.