Thermal Atomic\nLayer Etching Process for 2D van der\nWaals Material CrPS<sub>4</sub>

Preparing two-dimensional (2D) van der Waals materials\nwith atomic-level\nprecision remains a major hurdle, preventing both a number of fundamental\nexplorations of quantum phenomena and a wider range of applications\nthat can be based on a variety of their properties. It is especially\nchallenging for tertiary materials, such as CrPS<sub>4</sub>, which\ncannot be produced by controlled deposition but could be managed by\netching or thinning in a layer-by-layer approach. Thin flakes of this\nmaterial can display ferromagnetic or antiferromagnetic behavior depending\non the number of layers since the crystal exhibits A-type antiferromagnetic\nordering. In order to understand the magnetism down to the monolayer\nlimit and the dynamic excitations in magnons and excitons, and to\neventually make devices based on this and similar materials viable,\nwell-controlled layered structures must be produced. The existing\nmethods for controlling CrPS<sub>4</sub> thickness, such as mechanical\nand liquid exfoliation, are not well controlled and are prone to introducing\ndamage to the crystal structure. In this study, we show that thermal\natomic layer etching (ALE) can be used to controllably etch the 2D\ncrystals of this material without noticeable contamination. As a starting\npoint, CrPS<sub>4</sub> flakes were mechanically exfoliated onto a\nsolid substrate and mounted in an ultra-high vacuum chamber. ALE process\nconsisted of a chlorine gas dose from a solid-state halogen doser\nfollowed by exposure to gas-phase acetylacetone (acacH). This ALE\napproach showed an etch rate of approximately 0.10 nm/cycle at 450\nK, confirmed by atomic force microscopy. The etch rate is noticeably\nfaster for the flakes with a large number of defects. The overall\nprocess is highly temperature-dependent with a narrow window for successful\napplication.