Electrical Properties of High Mobility Ultrathin Amorphous Indium Zinc Oxide Deposited by Atomic Layer Deposition

High mobility ultrathin (thinner than 10nm) amorphous Indium Zinc Oxide (a-IZO) films are deposited by Atomic Layer Deposition (ALD). Thickness is set as the main parameter, which is varied between 5nm ~ 50nm by changing the cycle numbers. It is easily predicted that the electrical conductivity shows serious degradation with decreasing thickness of the films because of the increase of the fraction of the surface. However, it is found that a-IZO shows less rapid decrease of the electrical conductivity than crystalline conducting oxides. Aluminum doped Zinc Oxide (AZO) is deposited by ALD to compare the various electrical properties such as mobility, carrier concentration, and resistivity with the a-IZO films. In addition, the resistivity data of Tin doped Indium Oxide (ITO) deposited by ALD is accepted from a literature for more appropriate comparison, because the resistivity of AZO film is one order of magnitude higher than those of IZO and ITO. The results say that with decreasing thickness, the resistivity of a-IZO decreases more slowly than those of crystalline AZO and ITO. As thickness varies from ~50nm to ~10nm, the resistivity of a-IZO goes up from 3.9 ˣ 10^-4 Ω cm to 8.2 ˣ 10^-4 Ω cm while that of AZO increases from 1.6 ˣ 10^-3 Ω cm to 2.0 ˣ 10^-2 Ω cm, and that of ITO increases from 3 ˣ 10^-4 Ω cm to 9 ˣ 10^-4 Ω cm. Moreover, for ~5nm, the resistivity of ITO is about an order of magnitude higher than that of a-IZO, so the conductivity of a-IZO overtakes that of ITO at ~10nm film thickness. This phenomenon is attributed to the electron mobility degradation due to the grain boundary scattering in polycrystalline oxides. As film thickness gets thinner to or below the dimension of grain size, grain size becomes smaller because of the constraint of the dimension in the direction of film normal. Therefore, as thickness varies from ~50nm to ~10nm, the electron mobility of AZO film changes from 11.2 cm² / V s to 5.6 cm² / V s while that of a-IZO changes from 52.1 cm² / V s to 40.6 cm² / V s. The models of the mobility from grain boundary scattering and surface scattering are explained well in this work, and the absence of grain boundary scattering in amorphous oxides explains the excellent electrical properties of a-IZO.