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)­O x 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)­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 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.

Recent progress and perspectives on atomic‐layer‐deposited semiconducting oxides for transistor applications

“…Although binary metal‐oxide channel systems offer simple composition and processing, securing the high mobility and controllability of V TH and excellent I ON/OFF ratio simultaneously can be difficult. To overcome this limit of binary oxides, multi‐component oxide materials such as ternary (indium gallium oxide [IGO], 55–57 indium zinc oxide [IZO], 58–60 and zinc tin oxide [ZTO] 61,62 ) and quaternary oxide (indium gallium zinc oxide [IGZO], 63–68 and indium zinc tin oxide [IZTO] 69,70 ) species have been examined as active‐material candidates for high‐performance TFTs. The design rationale should balance mobility enhancement and carrier suppression.…”

“…A ternary ZTO system has the advantages of low material costs due to indium‐ and gallium‐free composition and reasonable stability compared with IGO and IZO systems. Kim et al 62 reported the impact of cation composition on the electrical characteristics of ZTO TFTs. The ALD growth mechanism of binary ZnO and SnO 2 films was compared using substrates of bare silicon and SiO 2 /Si.…”

“…(I) The variations of μ FE and μ sat with different tin concentration. Reprinted with permission 62 . Copyright 2020, American Chemical Society…”

Zinc–Tin Oxide Film as an Earth-Abundant Material and Its Versatile Applications to Electronic and Energy Materials

“…Zinc–tin oxide (ZTO) is a wide bandgap semiconductor (Eg = ~3.6 eV) [ 1 , 2 , 3 , 4 ] that has been studied as a potential material for electronic and energy devices, such as thin-film transistors [ 5 , 6 , 7 , 8 ], versatile sensors [ 9 , 10 ], and solar cells [ 11 , 12 , 13 ]. The ZTO film has promising properties, including transparency [ 14 , 15 , 16 ], flexibility [ 14 , 15 ], and semiconducting ability [ 6 ], with high electron charge transport behavior that offers desirable electronic and energy materials.…”

“…ZTO films can be synthesized using solution- and vacuum-based processes [ 17 ]. Various synthesis approaches have been developed for ZTO films, owing to their compatibility, including sputtering [ 18 , 19 , 20 , 21 ], atomic layer deposition (ALD) [ 3 , 8 , 22 , 23 ], chemical vapor deposition (CVD) [ 24 , 25 , 26 , 27 ], spin-coating [ 28 , 29 , 30 , 31 , 32 ], and printing techniques [ 6 , 7 , 33 , 34 ].…”