Active Layer Thickness-Dependent Reliability of Solution-Processed Indium-Gallium-Zinc Oxide Thin-Film Transistors Under Bias Stress
Authors: Im Hwarim , Kim Yong-Sang 
Published: December 2024

Amorphous indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs) have been widely investigated because of their high carrier mobility and excellent uniformity. The IGZO TFTs’ device performances have significantly advanced, and commercial products based on IGZO TFTs have been released on the display market. On the other hand, solution-processed IGZO TFTs have also been researched to reduce fabrication costs [1]. However, the electrical properties and reliability of solution-processed IGZO TFTs should be improved for display applications compared to vacuum-processed IGZO TFTs. Because the IGZO layer is deposited using different processes, the improvement method of device performance should be investigated considering the solution process characteristics compared to the vacuum process.

The thickness control method is one of the different points between the solution and vacuum process. In the vacuum process, the deposition time can control the layer thickness. However, the thickness of the solution-processed IGZO layer can be controlled by the layer stacking or the molarity of the precursor solution. Then, the thin film properties of the solution-processed IGZO layer can be varied depending on thickness. Therefore, the effect of thickness on the IGZO thin film properties should be considered when investigating the device performances of solution-processed IGZO TFTs with different active layer thicknesses.

In this study, we investigated the reliability of solution-processed IGZO TFTs under bias stress as active layer thickness varied. Solution-processed IGZO TFTs were fabricated on a heavily doped p-type silicon wafer and thermally grown silicon dioxide using the gate electrode and gate insulator, respectively. The IGZO layer was deposited via a spin coating process using the precursor solution, which was prepared by dissolving indium acetate, gallium nitrate, and zinc acetate into 2-methoxyethanol. After annealing and patterning the IGZO layer, aluminum source and drain electrodes were deposited using thermal evaporation. We controlled the thickness of the solution-processed IGZO layer by controlling the molarity of the precursor solution and fabricated IGZO TFTs with active layer thicknesses of 15 and 80 nm.

Solution-processed IGZO TFTs with a thick active layer (Device A) exhibited a negatively shifted threshold voltage and low field-effect mobility compared to devices with a thin active layer (Device B). Moreover, Device A was more significantly influenced by the constant bias stress than Device B. The threshold voltage shift (ΔV
th) of Device A was 29.6 V, and that of Device B was 19.8 V, as shown in Figure 1. In other words, the reliability of solution-processed IGZO TFTs was degraded as the active layer thickness increased. The results of electrical properties and reliability of solution-processed IGZO TFTs with different active layer thicknesses were closely related to the defect state level of the IGZO layer. We analyzed the density-of-states (DOS) of solution-processed IGZO TFTs with different active layer thicknesses using the field-effect method [2]. Devices A and B exhibited DOS levels of 1.8×1017 and 9.8×1015 cm-3eV-1, respectively. In other words, the defect states increased as the thickness of solution-processed IGZO TFTs increased. The DOS variations depending on the IGZO layer thickness were attributed to the pin holes or disorder formed during the IGZO layer deposition process. Consequently, we concluded that the thin-film properties, including the DOS level, should be carefully controlled when controlling the active layer thickness to obtain high-performance solution-processed IGZO TFTs.

Figure 1