High mobility zinc oxynitride-TFT with operation stability under light-illuminated bias-stress conditions for large area and high resolution display applications

Abstract

Introduction:In spite of the successful achievement of oxide-semiconductor (OS) technology in recent years, stability degradation especially at high mobility regime limits the application of oxide semiconductors in next generation displays. According to previous works, the instability is closely related to oxygen vacancies (V o ) causing persistent photoconductivity (PPC) [1,2]. From this point of view, zinc oxynitride (ZnON)[3] with small bandgap (1.3 eV) and high intrinsic mobility is attractive to overcome the performance issues of OS. In this paper, we report on ZnON-thin film transistors (TFTs) with field effect mobility near 100 cm 2 /Vs and operation stability(< 3 V) under light-illumination biasstress. Our results demonstrate that ZnON-TFTs are strong candidates for pixel switching devices in ultra-high definition and large area displays. ZnON film deposition and electrical properties: ZnON films were deposited by reactive rf magnetron sputtering using Zn target and Ar/O 2 /N 2 mixture gas. By varying the O 2 flow rate and deposition pressure at a fixed N 2 and Ar flow rate, the anion ratio (O/N) in ZnON was modulated, while maintaining Zn concentration at ~55 at%. As-deposited and annealed films are composed of amorphous ZnON and nanocrystallites of ZnO and Zn 3 N 2 (Fig. 1). Density functional theory (DFT)-based calculations (Fig. 2) revealed that ZnON has a smaller effective mass (0.19 m e ) than conventional OSs (0.27 m e for ZnO, 0.32~0.34 m e for GIZO[4], and 0.22 m e for In 2 O 3 ), indicating high mobility characteristics of ZnON. Hall measurements confirmed its high mobility values of 35~118 cm 2 /Vs with carrier concentrations of 7.7x10 16~3 .1x10 18 /cm 3 [Fig 3(b) and (c)]. As shown in Fig. 3, N incorporation is directly related with the enhancement of mobility and carrier concentration. ZnON-TFT fabrication and basic characteristics:We have fabricated etch-stopper (ES) type ZnON-TFTs (N/O=37/6) on 150 mm x 150 mm glass substrates by photolithography and etching processes. After patterning a sputter-deposited Mo layer for gate electrodes, plasma-enhanced chemical vapor deposition (PECVD) was used to deposit gate dielectrics of SiN x /SiO x (350 nm/50 nm). ZnON (50 nm) was subsequently sputter-deposited as the active layer. Then, 100 nm-thick PECVD SiO x was deposited and patterned to form the ES and contact regions. Mo or AlNd source/drain (S/D) electrodes were formed by sputtering and dry etching process. TFT structures are illustrated in Fig. 4. After annealing in air at 250 o C for 1 hr, we have measured I-V characteristics, including operation stabilities, in vacuum (<10 -6 Torr). Fabricated ZnON-TFTs operated in depletion mode. Thus, TFT current includes fringe component. TFTs still have mobility as high as ~100 cm 2 /Vs even after fringe current correction by channel width (W) dependent I ds -V gs analysis (Fig. 5). Transfer (I ds -V gs ) and output characteristics are given in Fig. 6(a) and (b),

P‐2: High Mobility Zinc Oxynitride TFT for AMOLED

“…As reported, the oxygen vacancies, which could causing persistent photoconductivity and degrade the stability under light illumination, are mostly existed near valence band maximum of IGZO semiconductor [2]. Thus, the smaller bandgap (1.3 eV) of ZnON could prevent from creation of the oxygen vacancy and increase the stability under light illumination.…”

“…V th shifts (△V th ) of ZnON TFTs after PBS-linear and PBS-saturation tests are 0.014V and -0.029V, which is much better than the reported results of other oxide TFTs. The superior PBS test results may come from the smaller bandgap (~1.3eV) of ZnON, which can overcome the creation of oxygen vacancies [2]. However, V th shift of ZnON TFT after NBS test is -1.869V, which is much worse than that of IGZO TFT.…”

“…Therefore, increasing demands for high field effect mobility and high switching or driving performance in the flat panel display are urgently needed. Additionally, according to previous works, the stability of IGZO TFT would be degraded under light illumination, which is possible due to the oxygen vacancies causing persistent photoconductivity [1,2], and more development efforts are needed for the actually application of panel driven by IGZO TFT.…”

P-11: Development of IGZO ESL Type TFTs at G8.5 for 55” AMOLED TVs

“…Among several candidates with both a high stability and a high scalability, oxide backplanes using IGZO show a high performance and can exploit existing amorphous silicon process lines, which may be the most promising choice for large size OLED TVs However, for the next generation displays, the electrical stability of IGZO TFT(~10cm 2 /Vs) is not sufficient to satisfy the needs of large size AMOLED TVs. The Hysteresis stability and mass-product yield are still the main hindrances for large size AMOLED TVs mass product [7][8] [9].…”

Fabrication and Characterization of Film Profile Engineered ZnO TFTs With Discrete Gates

“…These merits could satisfy the requirements of most high-resolution large-area displays [3], [4], and flexible electronics [5]- [7]. More recently, attention was also put on the application of back-end-of-line (BEOL) active device technology integrated on advanced large-scale-integration (LSI) chips [8]- [11].…”