Substrate-Dependent Growth Behavior of Atomic-Layer-Deposited Zinc Oxide and Zinc Tin Oxide Thin Films for Thin-Film Transistor Applications

Abstract

The growth behaviors and electrical performances of semiconducting ZnO, SnO2, and (Zn,Sn)Ox thin films, grown by atomic layer deposition (ALD) using O3 as the oxygen source, were studied. A significant incubation stage was observed for ZnO ALD on the Si substrate, but not for the SnO2 thin-film substrate. The incubation cycles, along with the grain size, were increased with O3 feeding time, implying that the reactivity of the Zn-precursor varied with the degree of oxidation of the Si surface. The adsorption of the Zn-precursor in the early stage of (Zn,Sn)Ox ALD was facilitated with an increasing concentration ratio of Sn to Zn. The electrical performance of the (Zn,Sn)Ox film as a channel layer was estimated by fabricating bottom-gate thin-film transistors (TFTs). The TFT transfer curves showed an evident negative shift of threshold voltage as the Sn-concentration increased in (Zn,Sn)Ox films. The best electrical performance of the oxide TFTs was observed when the Sn-concentration was 40 at % with a threshold voltage of −0.12 V, subthreshold swing of 0.33 V decade–1, field-effect mobility of 13.6 cm2 V–1 s–1, and saturation mobility of 6.20 cm2 V–1 s–1. The amorphous structure of the films could be retained up to 600 °C of post-annealing. These performances are promising for the next-generation TFT for a vertical NAND flash or cell-stacked dynamic random access memory.