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

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 cm 2 /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 cm 2 /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 cm 2 /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 (V N ) in ZnON act as carrier trap sites, so that degrade the electrical characteristics of ZnON TFTs. Moreover, the zinc nitride (Zn 3 N 2 ) is unstable to air due to its low heat of formation, thus the Zn 3 N 2 into ZnON is easily converted to zinc oxide. Therefore, reducing V N s and preventing the degradation of Zn 3 N 2 is necessary for obtaining high-performance ZnON TFTs. In this study, the effects of a thin ZnF 2 interlayer between source/drain electrode and ZnON active layer are investigated. The properties of ZnON TFTs were evaluated by the different thicknesses of ZnF 2 and the change of deposition conditions. The inserted ZnF 2 interlayer acts as i) carrier modulation between source/drain and active layer due to high resistive ZnF 2 , and ii) passivation layer which prevents Zn 3 N 2 volatilization. Furthermore, in our previous study demonstrated that the incorporated fluorine in ZnON acts as a structural stabilizer, so that reduces V N 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 V N s near the interface between ZnF 2 and ZnON would be reduced, therefore the net density of V N s is decreased. Experimental results showed that an optimized ZnON/ZnF 2 TFTs showed high field-effect mobility (> 50 cm 2 /Vs) and good transfer characteristics (high on/off ratio and low subthreshold swing), whereas ZnON TFTs without ZnF 2 interlayer showed completely conductive behavior. In addition, the air stability of ZnON TFTs was improved via the ZnF 2 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.