Low interface trap density in scaled bilayer gate oxides on 2D materials via nanofog low temperature atomic layer deposition

Al₂O₃ and Al₂O₃/HfO₂ bilayer gate stacks were directly deposited on the surface of 2D materials via low temperature ALD/CVD of Al₂O₃ and high temperature ALD of HfO₂ without any surface functionalization. The process is self-nucleating even on inert surfaces because a chemical vapor deposition (CVD) component was intentionally produced in the Al₂O₃ deposition by controlling the purge time between TMA and H₂O precursor pulses at 50 °C. The CVD growth component induces formation of sub-1 nm AlOx particles (nanofog) on the surface, providing uniform nucleation centers. The ALD process is consistent with the generation of sub-1 nm gas phase particles which stick to all surfaces and is thus denoted as nanofog ALD. To prove the ALD/CVD Al₂O₃ nucleation layer has the conformality of a self-limiting process, the nanofog was deposited on a high aspect ratio Si₃N₄/SiO₂/Si pattern surface; conformality of >90% was observed for a sub 2 nm film consistent with a self-limiting process. MoS₂ and HOPG (highly oriented pyrolytic graphite) metal oxide semiconductor capacitors (MOSCAPs) were fabricated with single layer Al₂O₃ ALD at 50 °C and with the bilayer Al₂O₃/HfO₂ stacks having C_max of ∼1.1 µF/cm² and 2.2 µF/cm² respectively. In addition, Pd/Ti/TiN gates were used to increase C_max by scavenging oxygen from the oxide layer which demonstrated C_max of ∼2.7 µF/cm². This is the highest reported C_max and C_max/Leakage of any top gated 2D semiconductor MOSCAP or MOSFET. The gate oxide prepared on a MoS₂ substrate results in more than an 80% reduction in D_it compared to a Si_0.7Ge_0.3(0 0 1) substrate. This is attributed to a Van der Waals interaction between the oxide layer and MoS₂ surface instead of a covalent bonding allowing gate oxide deposition without the generation of dangling bonds.