Composition control of HIZO thin-film transistors by co-sputtering: Mobility-stability trade-off and optical-electrical correlation

초록

Amorphous oxide semiconductors (AOSs) are extensively investigated for transparent and flexible electronics, but their efficiency is often limited by electrical instability originating from oxygen vacancy-related defects. To address this challenge, IZO/HIZO thin-film transistors (TFTs) were fabricated by a co-sputtering method to precisely control the Hafnium (Hf) concentration while maintaining a constant In:Zn ratio. Structural, optical, and electrical characterizations revealed that increasing Hf incorporation suppressed oxygen-vacancy-related defects, leading to smoother surfaces (RMS roughness decreased from 0.223 nm to 0.132 nm), widened bandgaps (from 3.37 eV to 3.55 eV), and reduced Urbach energy (from 0.771 eV to 0.634 eV). Transfer characteristics demonstrated that higher Hf content improved threshold voltage stability and lowered trap density, but simultaneously reduced field-effect mobility due to decreased carrier density, indicating a clear mobility-stability trade-off. Transmission line method analysis confirmed this reduction in carrier transport through increased sheet and contact resistance. Furthermore, negative bias temperature stress tests showed that Delta Vth was significantly suppressed from 11.81 V to 1.15 V with higher Hf incorporation, confirming enhanced reliability. These findings highlight co-sputtering as an effective approach for continuous compositional tuning and provide practical guidelines for optimizing dopant concentration in oxide semiconductors to realize high-performance and reliable thin-film transistors.

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