Evaluation of the high mobility and stability of InGaZnO/InSnZnO bilayer thin-film transistors via quantitative defect analysis

This study presents bilayer (BL) InGaZnO (IGZO)/InSnZnO (ITZO) thin-film transistors (TFTs) with different ITZO layer thicknesses. A BL-TFT with an optimized thickness of 5 nm for ITZO and 35 nm for IGZO demonstrated high mobility (μ_FE) of ∼36 cm²/Vs, less negative threshold voltage (V_th) as well as minimal V_th shifts of 1.15 V and -0.25 V under positive-bias stress (PBS) and negative-bias stress (NBS), respectively. Such performance improvements were unattainable when layers were used independently. Photoinduced Current Transient Spectroscopy (PICTS) was employed to determine the defects inside the bandgap quantitatively. The PICTS analysis showed that integrating ITZO and IGZO in a single device leads to an optimal number of shallow donor defects that are neither too high nor too low, thus achieving high μ_FE and less negative V_th values simultaneously. Furthermore, the remarkable stability of the 5 nm ITZO/35 nm IGZO BL-TFT under both NBS and PBS was attributed to its lower defect concentrations measured at 4.1 × 10¹⁸ #/cm³ below the Fermi level and 9.0 × 10¹⁶ #/cm³ above the Fermi level. In contrast, reduced NBS and PBS stabilities of single-layer ITZO and IGZO TFTs were associated with their elevated defect densities at 6.7 × 10¹⁸ #/cm³ below the Fermi level and 3.9 × 10¹⁷ #/cm³ above the Fermi level. This quantitative defect analysis is crucial for understanding the performance and stability degradation of oxide semiconductors, ultimately aiding in addressing these challenges.