High Mobility and Air-Stable Zinc Oxynitride Thin-Film Transistors Via ZnF2 Interlayer Insertion

Abstract

Increasing demands for high-performance thin-film transistors (TFT) in the display application have led to the development of amorphous oxide semiconductors (AOS) such as In-Ga-Zn-O (IGZO), In-Zn-O (IZO), etc. The AOS based TFTs show high field-effect mobility as high as 10 cm2/Vs, and are suitable for the fabrication of large size liquid crystal display (LCD) with high resolution. However, the displays based on organic light-emitting diodes (OLEDs) or micro light-emitting diodes (micro-LED) with very high resolution (super-high vision, 8000 X 4000) require relatively high driving currents compared to the LCD display. Therefore, the AOS based TFTs with higher field-effect mobility (> 50 cm2/Vs) are strongly required into the display backplane array.Recently, zinc oxynitride (ZnON) have researched intensively as an active material for high-performance TFTs due to its high field-effect mobility (> 50 cm2/Vs), excellent stability under bias or light irradiation, and easily tunable bandgap energy. However, some studies reported that the nitrogen-related defects such as nitrogen-deficient site (VN) in ZnON act as carrier trap sites, so that degrade the electrical characteristics of ZnON TFTs. Moreover, the zinc nitride (Zn3N2) is unstable to air due to its low heat of formation, thus the Zn3N2 into ZnON is easily converted to zinc oxide. Therefore, reducing VNs and preventing the degradation of Zn3N2 is necessary for obtaining high-performance ZnON TFTs.In this study, the effects of a thin ZnF2 interlayer between source/drain electrode and ZnON active layer are investigated. The properties of ZnON TFTs were evaluated by the different thicknesses of ZnF2 and the change of deposition conditions. The inserted ZnF2 interlayer acts as i) carrier modulation between source/drain and active layer due to high resistive ZnF2, and ii) passivation layer which prevents Zn3N2 volatilization. Furthermore, in our previous study demonstrated that the incorporated fluorine in ZnON acts as a structural stabilizer, so that reduces VN sites by increasing the effective coordination number of zinc. Moreover, incorporated fluorine which substitutes nitrogen also shown acceptor-like behavior rather than carrier donor. Thus, the VNs near the interface between ZnF2 and ZnON would be reduced, therefore the net density of VNs is decreased.Experimental results showed that an optimized ZnON/ZnF2 TFTs showed high field-effect mobility (> 50 cm2/Vs) and good transfer characteristics (high on/off ratio and low subthreshold swing), whereas ZnON TFTs without ZnF2 interlayer showed completely conductive behavior. In addition, the air stability of ZnON TFTs was improved via the ZnF2 interlayer, the transfer characteristics were almost unchanged even a few months later. Further analysis such as bias and illumination stability and charge transport mechanisms were discussed in detail.