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

The growth behaviors and electrical performances of semiconducting ZnO, SnO2, and (Zn,Sn)O-x thin films, grown by atomic layer deposition (ALD) using O-3 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 O-3 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)O-x ALD was facilitated with an increasing concentration ratio of Sn to Zn. The electrical performance of the (Zn,Sn)O-x 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)O-x 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 cm(2 )V(-1) s(-1), and saturation mobility of 6.20 cm(2) V-1 s(-1). The amorphous structure of the films could be retained up to 600 degrees 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.