The Instability of a-Si:H TFT under Mechanical Strain with High Frequency ac Bias Stress

Abstract

The instability of amorphous Si thin film transistors under uniaxial strain has been studied. Compared to the effect of tensile bias stress, larger threshold voltage shift is observed under compressive bias stress. These results are related to the damage of weak Si-Si bonds during the ac bias stress. However, the V th shift of devices on the re-flattened substrate is larger after tensile strain than that of compressive strain. In addition, the defeat diminished effect of tensile situation is decreased after re-flattening the device. Therefore, after re-flattening the substrate the V th shift resulting from tensile bias stress is larger than that of the compressive one.The amorphous Si thin film transistor ͑a-Si:H TFT͒ has been widely used as a switching device of AM-LCD. Recently, a portable communication system is spotlighted such as electronic paper, smart labels, displays for vehicles and portable electron devices. 1 For these applications, the traditional glass substrate of large-area electronics must be replaced with a flexible and lightweight substrate. 2 Except for the substrates, the electrical instability is also a critical issue for a-Si:H TFTs, as it can degrade the electrical characteristics and affects the lifetime of devices. Many reports indicated that the electrical instability is composed of charge trapping and state creation. 3,4 The charge trapping in the slow state, which is dominant at higher voltage and long stress time, relates to trap sites in the gate insulator. 5 Conversely, the state creation in the fast state, which is dominant at lower voltage, is associated with the breaking of Si-Si weak bonds in the a-Si:H layer or at the interface between the gate insulator and a-Si:H layer. 6,7 The charge trapping shows logarithmic and weak temperature dependence while the state creation shows power-law time and thermally activated temperature dependence. 8 The charge trapping leads to the threshold voltage shift, and less degradation of subthreshold swing. 9 On the other hand, the state creation can lead to positive shifts of threshold voltage, and degrade the subthreshold swing. Furthermore, the defeat pool model has been proposed to explain the instability mechanisms of a-Si:H TFTs. 10 For flexible display application, display panels are required to suffer a certain degree of bending. Bending operation will induce strain in the electronic circuits and would influence the TFT device characteristics. Another consideration for TFT device characteristics is the device operation modes. The dc and ac operation has been extensively studied, but not for flexible display applications in which the real operation mode is under the mechanical strain with ac operation. Therefore, the knowledge of the electrical performance of TFTs under the mechanical strain with the ac operation becomes more important for flexible display application. There are many studies on the relationship between mechanical strain and a-Si:H TFT characteristics. 11-17 The simple stress for a-Si:H TFT has already been reported by Won ...

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