Control of Interfacial Defect States in the Flexible InGaZnO Thin Film Transistor with a 6FDA-MDA Gate Insulator by Using an Al2O3 Interlayer

Abstract

Recently, the high-mobility flexible thin film transistor (TFT) has been reported by using an organic-inorganic heterojunction structure to enhance the flexibility. This study investigates the origin of enhancement in an In-Ga-Zn-O (IGZO)-based TFT, utilizing 6FDA-based polyimide (6FDA-MDA) as the gate insulator (GI) to achieve a high-mobility flexible TFT. 6FDA-MDA has great advantages to apply to a flexible TFT as a gate insulator, such as a low leakage current, high mechanical stability, a room-temperature process, and smooth surface topographies. Through qualitative/quantitative defect analysis of the IGZO/GI interface, we find the origin of enhancement in the IGZO/6FDA-MDA TFT and suggest a facile method by applying an Al2O3 interlayer. By analyzing the physical properties of the IGZO TFT with a 6FDA-MDA GI, we find that diffused fluorine atoms from the 6FDA-MDA in the active area modulate the Fermi level toward the valence band in the interface. To prevent the diffusion of fluorine, we used the Al2O3 layer as a blocking layer, and it is confirmed by quantitative defect analysis that the defect states in the active/GI interface region are reduced by inserting the Al2O3 interlayer. By applying an Al2O3 interlayer between the GI and the active layer, the performance has been improved in terms of mobility, bias stability, and on/off ratio. Precisely, the field-effect mobility increased about 5 times from 0.05 to 14.57 cm(2)/(V s), the positive biased threshold voltage decreased to 8.0 from 14.42 V, and the on/off ratio increased to 2.62 x 10(8) from 3.77 x 10(6). Moreover, the TFT device with Al2O3 blocking can operate over 2000 times. Due to sputtering damage, fluorine could be diffused from 6FDA-MDA to the IGZO active area and generate defects in a channel. The formation energy and energy level in the band gap of the IGZO film with fluorine are obtained by density functional theory calculations. Fluorine causes the defect states in the IGZO/6FDA-MDA interface, and the energy level of each defect state is lowered to the conduction band by fluorine diffusion.