Origin of positive Vth shift and mobility effects in amorphous GaInZnO thin films

We investigated the relationship between band alignments and threshold voltage shifts of GaInZnO (GIZO) thin films grown on SiO2/p++-Si by the RF sputtering method via utilizing X-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy. The band gap energies of the GIZO thin films are increased from 3.03 eV to 3.43 eV with the increase in their Gallium (Ga)/Indium (In) ratios of 0.83, 1.05, and 1.34. The barrier height of GIZO/Mo is also increased by increasing in the Ga/In ratio, and then the threshold voltages positively shift. From the result of DLTS, it is found that the D defect is located at 0.56 eV below the conduction band, and its defect density shows the increasing tendency by the increase in the ratio of Ga/In (i.e. the decrease in In content). From the transfer curves of the GIZO thin films, the mobility shows the decreasing tendency by the increase in the ratio of Ga/In. As a result, it is suggested that the increase in the density of D defect density in the GIZO thin film plays a dominant role in the decrease in its mobility.