Thermal Atomic Layer Etching Process for 2D van der Waals Material CrPS4

Preparing two-dimensional (2D) van der Waals materials with atomic-level precision remains a major hurdle, preventing both a number of fundamental explorations of quantum phenomena and a wider range of applications that can be based on a variety of their properties. It is especially challenging for tertiary materials, such as CrPS4, which cannot be produced by controlled deposition but could be managed by etching or thinning in a layer-by-layer approach. Thin flakes of this material can display ferromagnetic or antiferromagnetic behavior depending on the number of layers since the crystal exhibits A-type antiferromagnetic ordering. In order to understand the magnetism down to the monolayer limit and the dynamic excitations in magnons and excitons, and to eventually make devices based on this and similar materials viable, well-controlled layered structures must be produced. The existing methods for controlling CrPS4 thickness, such as mechanical and liquid exfoliation, are not well controlled and are prone to introducing damage to the crystal structure. In this study, we show that thermal atomic layer etching (ALE) can be used to controllably etch the 2D crystals of this material without noticeable contamination. As a starting point, CrPS4 flakes were mechanically exfoliated onto a solid substrate and mounted in an ultra-high vacuum chamber. ALE process consisted of a chlorine gas dose from a solid-state halogen doser followed by exposure to gas-phase acetylacetone (acacH). This ALE approach showed an etch rate of approximately 0.10 nm/cycle at 450 K, confirmed by atomic force microscopy. The etch rate is noticeably faster for the flakes with a large number of defects. The overall process is highly temperature-dependent with a narrow window for successful application.