Improvement of Negative Bias Temperature Illumination Stability of Amorphous IGZO Thin-Film Transistors by Water Vapor-Assisted High-Pressure Oxygen Annealing

Abstract

The effect of high-pressure annealing on the performance and negative bias temperature illumination stress (NBTIS)-induced instability of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) is investigated. IGZO TFTs that were annealed in a H 2 O vapor-assisted high-pressure O 2 atmosphere exhibited significantly improved field-effect mobility and stability against NBTIS compared with those annealed in only high-pressure O 2 or air. Annealing under high-pressure O 2 in the presence of H 2 O vapor effectively reduces oxygen-related defects, which act as subgap states within the bandgap. This phenomenon affects band alignments, including the bandgap and conduction band offset [ (E C -E F )] of IGZO semiconductors, and is the basis for the improved performance and stability of the TFTs.Amorphous oxide semiconductor (AOS)-based thin-film transistors (TFTs) are promising structures that can be incorporated in a variety of electronic applications, including large-area display panels such as active matrix liquid crystal displays (AMLCDs) and organic light-emitting diodes (AMOLEDs). 1,2 Their major advantages include high field-effect mobility, excellent uniformity, low processing temperature, and transparency in visible light. 3 Considerable research has been carried out to simultaneously improve the performance and stability of AOS-based TFTs. Although their electrical performance is acceptable, their poor stability under bias thermal illumination stress still remains a crucial problem that inhibits commercialization. 1,2 Many studies have been carried out on AOS-based TFTs to better understand the deterioration of the device stability under negative bias temperature illumination stress (NBTIS). 4-10 Although the origin of the degradation remains unclear, it is generally believed that the presence of donor-like oxygen vacancies (V O ) located within the wide AOS bandgap leads to poor device stability by shifting the threshold voltage. 11,12 Two mechanisms have been proposed concerning the relationship between V O and NBTIS instability, i.e., the trapping of photo-generated hole carriers 5,6 and the creation of ionized oxygen vacancy defects. 7-9 Several groups have annealed AOS-based TFTs to suppress such oxygen-related defects. Those approaches have included atmospheric-pressure annealing in H 2 O or O 2 , or H 2 O + O 2 , and high-pressure annealing (HPA) in O 2 . 7,10,13-15 Nomura et al. 10 reported that annealing in wet O 2 effectively decreased electron trapping, but there was no direct evidence for enhancement of NBTIS stability. Ji et al. 15 suggested that O 2 HPA of IGZO TFTs could strongly suppress the generation of photo-generated hole carriers. However, there is still a lack of direct electronic structure data to explain the variation in TFT performance, such as TFT mobility and NBTIS instability. There have been few reported attempts to improve simultaneously the performance and NBTIS stability of AOS-TFTs that can be implemented in backplane devices. In this work, the effect of ambient HPA on the perfo...

Origin of light instability in amorphous IGZO thin-film transistors and its suppression

“…Fabrication process optimization to ensure a clean back interface is thus another way to minimize the light instability. The use of etch-stopper layers 51 , damage-free source/drain metal etchants 19 , high quality encapsulation layers 16 , high temperature or long-time annealing 12 , high pressure or water vapor-assisted annealing 17 , 18 and treatments 52 , 53 , are some of the optimization techniques that can be implemented.…”

Energy band offsets of dielectrics on InGaZnO4

“…11,14,17 This has been a major drawback for utilizing such material in optoelectronics and a-Si:H suffers from light-induced instabilities. [84][85][86] a-IGZO has a wide bandgap ($3.5 eV) and is highly transparent in the visible spectrum with transmittance over 90%. 7,9,10,23 The large difference between a-Si and a-IGZO allows for higher resolution, transparent displays at an equivalent transmittance.…”

“…[23][24][25][26][27] The progress on improving the control of instabilities during biased operation of IZGO has also been impressive. [86][87][88][89][90][91][92][93][94]…”

Self-Assembled Nanodielectrics for Solution-Processed Top-Gate Amorphous IGZO Thin-Film Transistors

“…Top-gate TFT devices offer many attractions over bottom-gate architectures, especially for metal oxide semiconductors. , Oxides are well known to react with atmospheric oxygen and water, and top-gate structures enable the dielectric to passivate the oxide surface by functioning as an encapsulation layer, thus making device response more uniform. − Top-gate IGZO TFTs have been reported with several classes of insulators including polymers and metal oxides. Polymer dielectrics offer mechanical flexibility and solution-processing at low temperatures; however, they typically exhibit low dielectric constants ( k ), excessive gas/H 2 O permeability, and limited thermal stability. ,, Inorganic dielectrics offer high k values and environmental stability but generally require either capital-intensive vacuum deposition or high annealing temperatures, which can compromise the underlying semiconductor. ,, While solution-processed SANDs combine many of the attractions of both inorganic and organic dielectrics and minimize many of their limitations, ,,, they have never been implemented in top-gate devices due to fabrication challenges.…”