Effect of Negative Oxygen Ions on a-IGZO Thin Films by Superimposed RF/DC Magnetron Sputtering

During the magnetron plasma sputtering process, the bombardment effects of the negative oxygen ions (NOIs) on the thin film properties of amorphous indium-gallium-zinc oxide (a-IGZO) has been investigated. While the kinetic energies of NOIs effectively adjusting by using a superimposed rf/dc magnetron sputtering system, the characteristics of NOIs extracted by in situ analyses with ion energy analyzer are good agreement with the calculated values by the Kevin Meyer equation as shown in Figs 1(a) and 1(b). Furthermore, we find that the observed energies of NOIs corresponding to the applied dc bias are enough high to influence a lattice distortion and/or displacement in a-IGZO thin films. Figure 2(a) and 2(b) show the results of the Hall measurement and work function of a-IGZO films at RT with different dc biases. The σ and n_e of a-IGZO films monotonically increases with increasing the dc bias in logarithmic scale. This is due to the increment in the n_e as a function of dc bias. Interestingly, the hall mobility (μ_hall) drops rapidly with opposite direction against that of n_e, and σ until under the dc bias of 300V, and becomes flat over 300V. In general, μ_hall largely depends on n_e in AOSs & TCOs owing to their structural randomness; μ_hall of AOS thin film increases with n_e, while that of TCO decreases with n_e. However, the observed n_e (> 10¹⁹ cm^-3) of IGZO films with relatively higher dc bias is too high as well as that of TCOs. It is obvious that the variation of σ, n_e, and μ_hall above the dc bias of 300 V is quite different with that of the measured values at relatively lower dc bias (< 300 V). Furthermore, Fig. 2(b) shows the workfunction of a-IGZO films extracted by a kelvin probe method monotonically decreases from 4.986 to 4.940 eV with increasing the dc bias from 100 to 400 V, but it has also a knee point around the dc bias of 300 V. The overall analysis revealed that the bombardment of NOIs with high energy has a great influence on a-IGZO film properties. Our work demonstrates the bombardment of NOIs is one of a crucial issue to achieve the high quality a-IGZO films.