Local charge diffusion in hafnia on MoS₂ using AFM

The semiconductor industry will eventually run up against the limits of silicon field-effect transistors. Ultrathin transition metal dichalcogenides (TMDs) are promising candidates to enable continued scaling of microelectronics, beyond the limits of silicon. This will require methods to integrate TMDs with high-performance dielectric materials. Hafnium oxide (HfO2), when formed through atomic layer deposition (ALD) on silicon, yields a high performance, ultra-thin dielectric layer with predictable control of phase and morphology. There is not yet an equivalent process to integrate HfO2 and related oxide dielectrics with TMDs, due to challenges arising from the dissimilar chemistry of TMD surfaces and ALD oxides. Among other challenges, HfO2 often forms discontinuous islands on MoS2, instead of continuous thin films - highlighting the need for dielectric testing with nanometer lateral spatial resolution. Here, we compare the properties of HfO2 grown by Plasma-Enhanced ALD (PEALD) on Si to the same film grown on MoS2. We characterize the electrical properties of HfO2 using atomic force microscopy (AFM) and Kelvin Probe Force Microscopy (KPFM), which enables localized spatial measurements of surface potential. To understand the dielectric performance of HfO2, we inject charge onto the exposed oxide surface using the AFM tip, and then we use KPFM to measure changes in surface potential over time as the injected charge leaks through the thin oxide layer. This approach enables quantitative comparison of dielectric isolation performance of oxides on the nanoscale, akin to industry-established methods of wafer-scale oxide integrity testing.