Applied Surface Science, Vol.463, 758-766, 2019
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 degrees 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 degrees C and with the bilayer Al2O3/HfO2 stacks having C-max of similar to 1.1 mu F/cm(2) and 2.2 mu F/cm(2) 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 similar to 2.7 mu F/cm(2). 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 Si0.7Ge0.3(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.