Imaging Atomically Thin Semiconductors Beneath Dielectrics via Deep Ultraviolet Photoemission Electron Microscopy

Imaging of fabricated nanostructures or nanomaterials covered by dielectrics is highly sought after for diagnostics of optoelectronics components. We show imaging of atomically thin MoS2 flakes grown on SiO2-covered Si substrates and buried beneath HfO2 overlayers up to 120 nm in thickness using photoemission electron microscopy with deep-UV photoexcitation. Comparison of photoemission yield (PEY) to modeled optical absorption evinced the formation of optical standing waves in the dielectric stacks (i.e., cavity resonances of HfO2 and SiO2 layers on Si). The presence of atomically thin MoS2 flakes modifies the optical properties of the dielectric stack locally. Accordingly, the cavity resonance condition varies between the sample locations over buried MoS2 and surrounding areas, resulting in image contrast with submicron lateral resolution. This subsurface sensitivity underscores the role of optical effects in photoemission imaging with low-energy photons. This approach can be extended to nondestructive imaging of buried interfaces and subsurface features needed for analysis of microelectronic circuits and nanomaterial integration into optoelectronic devices.