Vertically grown metal nanosheets integrated with atomic-layer-deposited dielectrics for transistors with subnanometre capacitance-equivalent thicknesses

Abstract

Integrating thin atomic-layer-deposited dielectrics with two-dimensional (2D) semiconductors could be used to fabricate 2D transistors with sub-1 nm capacitance-equivalent thicknesses. However, non-uniform nucleation from atomic-layer deposition on inert surfaces and subsequent high-energy metal evaporation can make atomically thin dielectrics non-insulating. Here, we report a bismuth-oxide-assisted chemical vapour deposition method to synthesize single-crystalline metal nanosheets with atomically flat surfaces. The nanosheets grow vertically on a substrate and can be easily transferred to a target substrate through polymer-free mechanical pressing. We show that palladium nanosheets offer an excellent surface for atomic-layer deposition of flat aluminium oxide (Al2O3) and hafnium oxide (HfO2) dielectrics with sub-3 nm thicknesses. These can then be laminated onto few-layer molybdenum disulfide (MoS2) as a gate stack with a capacitance-equivalent thickness of 0.9 nm and a capacitance density of around 3.9 μF cm−2. Our MoS2 top-gated transistors with a 2-nm-thick Al2O3 or HfO2 dielectric exhibit leakage currents of 10−6 A cm−2, low operating voltages of around 0.45 V and a hysteresis of less than 1 mV. Vertical metal nanosheets with atomically flat surfaces grown with a bismuth-oxide-assisted chemical vapour deposition method can be used to make metal–oxide dielectric stacks and laminated onto two-dimensional semiconductors to create transistors with sub-1 nm capacitance-equivalent thicknesses.