Precursor Engineering of Atomic Layer Deposition for Top-Gate Insulators on Monolayer MoS₂ Transistors

Integration of ultrathin, high-quality gate insulators is critical to the success of two-dimensional (2D) semiconductor transistors in next-generation nanoelectronics. Here, we investigate the impact of atomic layer deposition (ALD) precursor choice on the nucleation and growth of insulators on monolayer MoS₂. Surveying a series of aluminum (AlO_x) precursors, we observe that increasing the length of the ligands reduces the nucleation delay of alumina on monolayer MoS₂, a phenomenon that we attribute to improved van der Waals dispersion interactions with the 2D material. Using the precursor triisobutylaluminum (TIBA), we achieve uniform coverage of ∼3 nm AlO_x on MoS₂ after just 30 cycles. We also build top-gated transistors with alumina seed layers grown by different precursors, demonstrating how the nucleation behavior of the seed layer influences the device behavior. With a bilayer stack of TIBA-AlO_x and HfO₂ as the top-gate insulator, we achieve n-type MoS₂ transistors with negligible hysteresis, small and positive threshold voltage, ∼80 mV/dec subthreshold swing at room temperature, and a top-gate equivalent oxide thickness of 0.95 nm. Through this work, we develop a simple, industry-compatible, all-ALD process for depositing a top-gate insulator directly on monolayer MoS₂, and we elucidate critical insights into how the ALD chemistry can be tuned to improve insulator deposition.