Effect of Exposure to Ultraviolet-Activated Oxygen on the Electrical Characteristics of Amorphous Indium Gallium Zinc Oxide Thin Film Transistors

Abstract

An exposure of the back channel of an indium gallium zinc oxide (IGZO) thin film transistor (TFT) to ultraviolet (UV)-activated oxygen can effectively shift the threshold voltage (Vth )o f the TFT. The Vth decreases linearly with the exposure time while the on-state current greatly increase with the exposure time. The exposure doesn't have a strong impact on other device parameters. The effect of the exposure on the Vth is attributed to the increase in the electron concentration of the channel layer as a result of the Transparent oxide semiconductors (TOS) have attracted much at- tention for the application of flat-panel displays due to their high car- rier mobility (> 1c m 2 /V s) and high transparency in visible range. 1-3 Amorphous indium gallium zinc oxide (a-IGZO) is one of the most attractive TOS that can be used in the next-generation active-matrix flat panel displays, because it has a few superior properties including relatively high mobility, good uniformity, low temperature process, high optical transparency, high threshold voltage stability, etc. 4,5 It has been recently reported that the threshold voltage (Vth) of a-IGZO thin film transistors (TFTs) is affected by the process parameters such as deposition pressure, target composition, channel thickness, post annealing temperature. 6,7 The oxygen content and the thickness of the a-IGZO channel layer are also reported to play an important role in the electrical characteristics including the Vth of the TFTs. 8,9 The Vth is determined by the electron concentration in the channel layer of the TFT, thus it is related to the oxygen vacancies which provide the free charge carriers for electrical conduction in the oxide layer. Control- ling the Vth is required in the TFT application; however, it is not an easy task as the oxygen vacancies can be easily generated during the fabrication processes. In the present work, we have demonstrated that the Vth can be easily changed by exposing the back channel of the IGZO TFT to ultraviolet (UV)-activated oxygen. The Vth of the TFT is found to be highly sensitive to the exposure to UV-activated oxygen, and a linear relationship between the Vth shift and the exposure time is observed. However, the exposure does not have a large impact on other device parametersincludingthefield-effectmobilityandsubthresholdswing. The IGZO TFTs were fabricated on a heavily-doped n-type Si substrate with resistivity lower than 0.001 � ·cm, which served as the bottom gate of the TFTs. Firstly, a 30 nm Al2O3 layer was deposited onto the Si substrate by an atomic layer deposition (ALD) process to form the gate insulator. A 50 nm IGZO thin film was subsequently deposited on the Al2O3 thin film layer by radio-frequency sputtering in Ar ambient at room temperature using a IGZO target (In:Ga:Zn mole ratio = 1:1:1). The IGZO thin film was then subject to the expo- sure to UV-activated oxygen for different durations. Photolithography process followed by wet etching was used to pattern the active region. Finally, 200 nm thick Al source and drain electrodes were deposited by electron-beam evaporation and patterned through the standard lift- off processes to form the TFT structures. The channel length (L )a nd channel width (W) of the a-IGZO TFTs were 20 μm and 20 μm, respectively. The device structure of the bottom-gated TFTs device structure and the experimental setup for the UV-activated oxygen treatment are shown in Fig. 1. An UV lamp with the light wavelength