Origin of threshold voltage instability in indium-gallium-zinc oxide thin film transistors

Abstract

We investigated the impact of the passivation layer on the stability of indium-gallium-zinc oxide (IGZO) thin film transistors. While the device without any passivation layer showed a huge threshold voltage (Vth) shift under positive gate voltage stress, the suitably passivated device did not exhibit any Vth shift. The charge trapping model, which has been believed to be a plausible mechanism, cannot by itself explain this behavior. Instead, the Vth instability was attributed to the interaction between the exposed IGZO backsurface and oxygen and/or water in the ambient atmosphere during the gate voltage stress.

Material characteristics and applications of transparent amorphous oxide semiconductors

“…Our group has reported that thermal annealing under a wet O 2 atmosphere stabilizes the chemical bonds, leading to improved and stable TFT characteristics, and we have found that the true origin of the stability originates from the formation of acceptor-type electron traps formed under constant bias stress [77]. It has also been pointed that the instability issue in part comes from the adsorption and desorption of O-and H 2 O-related molecules, and the use of an appropriate passivation layer can improve the stability [71].…”

“…It was reported that the shortrange uniformity of a-IGZO TFTs is excellent and similar to that in a-Si:H TFTs owing to the amorphous structure [18], but scaling up to practical substrate sizes and examining the meter-scale uniformity by a production-compatible method such as RF/DC sputtering will be required. It has also been pointed that the operation characteristics of AOS TFTs are very sensitive to the atmosphere, such as oxygen, moisture and hydrogen content [54,[70][71][72]. These are explained by the fact that the formation of oxygen defi ciencies by thermal annealing under a reducing atmosphere and the incorporation of hydrogen by thermal annealing in an H 2 -containing atmosphere and by ion implantation easily increases the electrical conductivity even at low temperatures around 200 °C [54].…”

“…As seen in Figure 5(b), the oxygen deficiencies and incorporated hydrogen act as shallow donors and generate mobile electrons [68]. This problem is solved by employing a top protection layer (passivation layer) composed of SiO x , SiN x or similar, which blocks the penetration and diffusion of O-, H 2 O-and H-related molecules [71]. On the other hand, this sensitivity to a reducing atmosphere and hydrogen has been utilized to form improved source and drain contacts [72], and has been applied to self-alignment processes [73], which are expected to lead to a simpler process for fl at-panel display production.…”

Overview of electroceramic materials for oxide semiconductor thin film transistors

“…Therefore, recent efforts have been focused on understanding device instability and improving long-term stability. To more systematically understand the instability phenomena of oxide TFTs, many research groups have considered four different types of practical stress conditions: negative/positive gate-bias, temperature, illumination, and environment (e.g., humidity) [155,156]. Upon application of each stress, in general, the oxide TFT only exhibits a V th shift without a significant change in mobility, as shown in Fig.…”

Analog-to-digital converter with oxide thin-film transistors

“…Owing to their transparency and higher mobility compared to the amorphous silicon TFTs, a-IGZO TFTs have been studied widely, and various integrated circuits, such as the pixel circuit, scan drivers, inverter, ring oscillator, and logic gates, have been reported [5][6][7][8][9][10][11][12][13][14]. As p-channel operation is difficult due to the large density of states below the Fermi level, a-IGZO TFT circuits have been implemented with only n-channel TFTs.…”