Damage-Free Atomic Layer Etch of WSe: A Platform for Fabricating Clean Two-Dimensional Devices.

The development of a controllable, selective, and repeatable etch process is crucial for controlling the layer thickness and patterning of two-dimensional (2D) materials. However, the atomically thin dimensions and high structural similarity of different 2D materials make it difficult to adapt conventional thin-film etch processes. In this work, we propose a selective, damage-free atomic layer etch (ALE) that enables layer-by-layer removal of monolayer WSe without altering the physical, optical, and electronic properties of the underlying layers. The etch uses a top-down approach where the topmost layer is oxidized in a self-limited manner and then removed using a selective etch. Using a comprehensive set of material, optical, and electrical characterization, we show that the quality of our ALE processed layers is comparable to that of pristine layers of similar thickness. The ALE processed WSe layers preserve their bright photoluminescence characteristics and possess high room-temperature hole mobilities of 515 cm/V·s, essential for fabricating high-performance 2D devices. Further, using graphene as a testbed, we demonstrate the fabrication of ultra-clean 2D devices using a sacrificial monolayer WSe layer to protect the channel during processing, which is etched in the final process step in a technique we call sacrificial WSe with ALE processing (SWAP). The graphene transistors made using the SWAP technique demonstrate high room-temperature field-effect mobilities, up to 200,000 cm/V·s, better than previously reported unencapsulated graphene devices.