Effects of Mg doping rate on physical properties of Mg and Al co-doped Zn1−x−0.02MgxAl0.02O transparent conducting oxide films prepared by rf magnetron sputtering

The transparent and conductive Mg-Al co-doped ZnMgAlO films with different Mg contents have been sputter grown on quartz substrates at room temperature. Modulation of the band-gap and conductivity was tried by varying the Mg doping rate from 5 at.% to 20 at.% at a fixed Al content of 2 at.%. Optical characterization of the deposited ZnMgAlO films confirmed that the average optical transmittance in the visible region was ∼85% and the transparency of the near infra-red (NIR) region expanded to further IR region with increasing the Mg doping level, which can be profitable for the application where higher NIR transmittance is required. At a low Mg doping level of 5 at.%, the best quality of electrical properties were achieved with the resistivity of 3.7 × 10−4 Ω cm, the electron concentration of 7 × 1020/cm3, and the Hall mobility of 25.66 cm2/V s, whereas with further Mg doping up to 20 at.%, the resistivity increased to 1.9 × 10−2 Ω cm, which is accompanied with a degradation of crystal quality. Optical band-gap reached to 3.87 eV at a high Mg doping level of 20 at.%. Additional phonon modes at 275 cm−1 and 648 cm−1 found in the Raman spectra of the ZnMgAlO films were assigned to the host lattice defect vibrational complexes related with Al dopants and it was suggested that the broadening, weakening, and shift of phonon peaks with increasing the Mg doping level may be related with the enhanced lattice distortion in ZnO crystal and the reduced mass change induced by substitution of Mg in Zn sites.