Optimisation of amorphous zinc tin oxide thin film transistors by remote-plasma reactive sputtering

K M Niang,W I Milne,J Cho,A J Flewitt,S Heffernan

doi:10.1063/1.4961608

Abstract

The influence of the stoichiometry of amorphous zinc tin oxide (a-ZTO) thin films used as the semiconducting channel in thin film transistors (TFTs) is investigated. A-ZTO has been deposited using remote-plasma reactive sputtering from zinc:tin metal alloy targets with 10%, 33%, and 50% Sn at. %. Optimisations of thin films are performed by varying the oxygen flow, which is used as the reactive gas. The structural, optical, and electrical properties are investigated for the optimised films, which, after a post-deposition annealing at 500 °C in air, are also incorporated as the channel layer in TFTs. The optical band gap of a-ZTO films slightly increases from 3.5 to 3.8 eV with increasing tin content, with an average transmission ∼90% in the visible range. The surface roughness and crystallographic properties of the films are very similar before and after annealing. An a-ZTO TFT produced from the 10% Sn target shows a threshold voltage of 8 V, a switching ratio of 108, a sub-threshold slope of 0.55 V dec−1, and a field effect mobility of 15 cm2 V−1 s−1, which is a sharp increase from 0.8 cm2 V−1 s−1 obtained in a reference ZnO TFT. For TFTs produced from the 33% Sn target, the mobility is further increased to 21 cm2 V−1 s−1, but the sub-threshold slope is slightly deteriorated to 0.65 V dec−1. For TFTs produced from the 50% Sn target, the devices can no longer be switched off (i.e., there is no channel depletion). The effect of tin content on the TFT electrical performance is explained in the light of preferential sputtering encountered in reactive sputtering, which resulted in films sputtered from 10% and 33% Sn to be stoichiometrically close to the common Zn2SnO4 and ZnSnO3 phases.

Highlights

We explore the use of a remote reactive High Target Utilisation Sputtering (HiTUS) for the deposition of Zinc Tin Oxide (ZTO) channel layers for thin film transistors (TFTs)
While ZnO TFTs exhibited a poor device performance due to grain boundaries, amorphous zinc tin oxide (a-ZTO) TFTs exhibited a significant increase in mobility and the mobility increases with tin content up to 50 at. % of Sn in the film
Device performances are explained with the increased tin content in the films brought about by the preferential sputtering, such that films sputtered from 10% and 33% Sn targets are very close to stoichiometric ZnSnO3 and Zn2SnO4

Summary

INTRODUCTION

For over two decades, hydrogenated amorphous silicon (a-Si:H) has dominated as the material of choice for the channel semiconductor in thin film transistors (TFTs) for display backplanes. Recently, amorphous oxide semiconductors (AOSs) have been identified as a promising alternative to a-Si:H, due to their higher field effect mobility, high transparency, scalability to large substrate areas, and possibility of processing at low temperatures, making them very attractive for display backplanes and future flexible electronics. While the quaternary oxide semiconductors such as indiumgallium-zinc oxide (IGZO) have been leading the way in oxide TFTs, a simpler ternary compound, such as Zinc Tin Oxide (ZTO), is very favourable from an economic point of view as this material system does not contain expensive and/ or resource-scarce elements like indium and gallium.. Have investigated the stability of a-ZTO TFTs under gate bias stress and showed a small threshold voltage shift of 30 mV after 1000 min stressing, thereby demonstrating that ZTO TFTs are suitable as current drivers for transparent active matrix OLED displays.22 Various deposition techniques, such as sol-gel, atomic layer deposition (ALD), metal organic chemical vapour deposition (MOCVD), and pulsed laser deposition (PLD), have been reported for ZTO, rf or dc magnetron sputtering is most commonly used.. The separation of the substrate from the plasma reduces damage from ion and electron bombardment which is typically encountered in rf magnetron sputtering.34 These advantages have been demonstrated previously in producing high quality dielectric films such as amorphous hafnium oxide (dielectric constant $ 30).. The effect of tin content on the TFT electrical performance is explained in the light of preferential sputtering encountered in reactive sputtering

EXPERIMENTAL DETAILS

Growth rate and Hall resistivity

Thin film transistors

CONCLUSIONS