Carrier Concentration and Threshold Voltage Variability of Amorphous Oxide Semiconductors Using Vacuum Rapid Thermal Annealing

Abstract

This study evaluates the effect of rapid thermal annealing (RTA) in vacuum on the electrical properties of various amorphous oxide semiconductors (AOSs). We fabricated bottom-gate type TFTs using four types of a-IGZO (In2O3:Ga2O3:ZnO = 1:1:1, 1:1:2, 4:2:3, 4:2:4.1), a-ZTO (ZnO:SnO2 = 1:1), a-ZnO, or two types of a-AZTO (Al2O3:ZnO:SnO2 = 3:67:30, 2:10:10) thin films on the active channel and performed vacuum RTA at various temperatures. Measurement of the electrical characteristics of the thin film transistors (TFTs) revealed a unique behavior that the on-current of a-IGZO and a-ZnO TFTs increases with increasing vacuum RTA temperature, while threshold voltage (VTH) shifts significantly in the negative direction and increases the leakage current. As a result of analyzing the resistivity and carrier concentration of the AOS thin films using the Hall measurement, it was verified that the electron concentration increases as the vacuum RTA temperature increases, resulting in a decrease in resistivity. In addition, the increase in the carrier concentration of the IGZO and ZnO films is larger than that of the ZTO and AZTO films. In particular, the IGZO film with a composition of 4:2:3 showed the largest electron increase, while the AZTO film with a composition of 2:10:10 showed the smallest electron increase. XPS analysis has demonstrated that a reduction in lattice oxygen concentration is observed in vacuum RTA-processed AOS thin films. The reduction of lattice oxygen in the IGZO film of 4:2:3 was the largest, but in AZTO film 2:10:10 was the smallest. The reduction of lattice oxygen due to vacuum RTA results in an increase in oxygen vacancies, resulting in an increase in electron concentration, an increase in on-current, a negative shift in VTH, and an increase in leakage current. The loss of lattice oxygen in AOS thin films depends on the metal-oxygen binding energy. Since Al and Sn in the ZTO and AZTO films have a large binding energy with oxygen, the oxygen hardly diffuses out of the film and little change in VTH occurs. In comparison, the binding energy of In, Ga, and Zn of IGZO and ZnO films are small. Therefore, the higher the external diffusion of oxygen in IGZO and ZnO films by vacuum RTA, the variability of the carrier concentration and the VTH become larger.