Re-Evaluating the Impact of Stacked Electrodes in Metal–Semiconductor Contacts Based on a-IGO Thin-Film Transistors

Low-resistance ohmic contact is a critical issue that must be addressed for the practical application of oxide thin-film transistors (TFTs). In stacked electrodes, it was conventionally believed that the bottom metal layer in direct contact with the semiconductor (typically 5-20 nm) played a decisive role in device performance. Here, a significant influence of the top metal layer on device performance has been demonstrated based on amorphous indium-gallium-oxide (a-IGO) TFTs. By engineering In/X (X: Ag, Au, Cu, Al) stacked electrodes, the field-effect mobility (μFE) of a-IGO TFTs varies between 18.53 and 33.66 cm²/(V s). Kelvin probe microscopy measurements reveal that the effective work functions of different metal combinations can differ by up to 0.33 eV. This finding suggests that the traditional approach of analyzing contact characteristics based solely on the bottom metal's work function should be reconsidered, as the effective work function of stacked metal requires reevaluation. By optimizing stacked electrodes and bottom metal thickness, a-IGO TFTs with 15 nm In/Cu contacts achieve a μFE exceeding 33.66 cm²/V·s and contact resistance of 92.7 kΩ·μm. Our experiments provide valuable references for the design of subsequent oxide TFTs.