Effect of Sputtered ZnO Layers on Behavior of Thin-Film Transistors Deposited at Room Temperature in a Nonreactive Atmosphere

Abstract

In this work we present the electrical characterization of ZnO-based thin-film transistors fabricated at room temperature. The ZnO films were deposited by radiofrequency magnetron sputtering at variable argon pressure (3 mTorr to 10 mTorr) at room temperature. The sputtered ZnO films were polycrystalline with hexagonal structure and electrical resistivity ranging from 101 Ω cm to 108 Ω cm for films deposited from 3 mTorr to 10 mTorr. The trend in the electrical behavior of the devices was found to be due to the variation of the electron concentration of the ZnO films. The devices with better performance showed a field-effect mobility of 2.9 cm2/Vs, threshold voltage of 20 V, Ion/Ioff ≈ 106, and electrical resistivity of ~108 Ω cm. In addition, linear behavior of Ion/Ioff with deposition pressure was observed. The lowest Ion/Ioff ratio (~2) was calculated for devices with ZnO layers deposited at 3 mTorr, and the highest ratio (~106) for devices processed at 10 mTorr. Hall-effect measurements were performed on ZnO films showing the lowest resistivity. The layer grown at 3 mTorr showed a Hall mobility of μH = 8.9 cm2/Vs and carrier concentration of n = 4.2 × 1016 cm−3 with resistivity of ρ = 31.8 Ω cm. For films deposited at 5 mTorr, the Hall mobility, carrier concentration, and resistivity were μH = 7.9 cm2/Vs, n = 3.4 × 1016 cm−3, and ρ = 38.4 Ω cm, respectively. Films deposited at 8 mTorr and 10 mTorr could not be measured due to their high resistance.