Protected Ion Beam Fabrication of Two-Dimensional Transition Metal Dichalcogenides based Photonic Devices

TL;DR

This work tackles the problem of preserving the intrinsic optical properties of 2D TMDCs during focused ion beam patterning, a key step for integrating these materials into photonic devices. It demonstrates that conventional dielectric encapsulation fails to prevent Ga-ion–induced damage, while PMMA encapsulation acts as a sacrificial layer that mitigates collateral damage. The authors further show that XeF2-assisted Ga-ion FIB patterning reduces Ga implantation and enables anisotropic etching with ultra-smooth sidewalls, facilitating high-quality photonic resonators from 2D TMDCs. The combined PMMA encapsulation and XeF2-assisted patterning offer a robust, scalable single-step fabrication route that preserves optical functionality, with significant implications for robust TMDC-based quantum photonics and compact optical circuits.

Abstract

Two-dimensional (2D) transition metal dichalcogenides are pivotal for next-generation photonic devices due to their exceptional optical properties and strong light-matter interactions. However, their atomic thinness renders them susceptible to damage during nanoscale fabrication. Focused ion beam technology, while offering precise defect engineering for tailoring optoelectronic properties, often induces collateral damage far beyond the target region, compromising device performance. This study addresses the critical challenge of preserving the intrinsic optical characteristics of 2D TMDCs during FIB patterning. We demonstrate that conventional dielectric encapsulation fails to protect 2D TMDCs from gallium ion-induced damage, leading to persistent defects and quenched optical responses in patterned microstructures. In contrast, polymeric encapsulation with PMMA (polymethyl methacrylate) effectively mitigates damage by acting as a sacrificial layer that absorbs ion impact, thereby preserving the optical properties of the underlying TMDC. Furthermore, we leverage XeF2-assisted Ga ion beam direct patterning, which significantly reduces collateral damage, minimizes Ga ion implantation, and enables precise anisotropic material removal, yielding ultra-smooth sidewalls critical for high-quality photonic resonators. This combined approach of PMMA encapsulation and XeF2-assisted FIB patterning offers a robust, cost-effective, and scalable single-step fabrication route for integrating 2D TMDCs into high-performance photonic devices, thereby maintaining their intrinsic optical functionality essential for advancing quantum technologies and compact optical circuits.

Figures (6)

• Figure 1: Schematic of Ga-Focused Ion Beam (FIB) patterning on a dielectric encapsulated 2D TMDC layer
• Figure 2: Radial extent of optical damage in $\text{Al}_2\text{O}_3$ encapsulated 2D $\text{MoS}_2$ following Ga-FIB patterning.(a) Schematic showing photoluminescence (PL) measurement points ($\text{P}1$ to $\text{P}4$) at increasing radial distances from the ion-milled region (b) Corresponding PL spectra demonstrating a severely quenched exciton signal ($\text{P}1$, $\text{P}2$) that only recovers to the intrinsic level ($\text{P}4$) far from the patterned region, quantifying the non-local damage.
• Figure 3: Effect of dielectric encapsulation: Raman spectra of $\text{Al}_2\text{O}_3$ encapsulated $\text{MoS}_2$ show a significant peak shift and broadening closer to the site of ion bombardment. The inset shows the optical micrograph detailing the patterned area and the specific measurement points ($\text{P}1$ to $\text{P}4$).
• Figure 4: PMMA encapsulation and $\text{XeF}_2$-assisted FIB patterning: Illustration of Gas Injection System (GIS)-assisted FIB patterning using $\text{XeF}_2$, which enhances chemical etching to minimize Ga-ion implantation, yielding ultra-smooth sidewalls. Schematic cross-section shows the protective PMMA layer over the 2D TMDC, acting as a sacrificial layer to absorb ion energy and prevent non-local damage
• Figure 5: SEM image of a PMMA-capped 2D $\text{MoS}_2$ micro-resonator: Patterned using $\text{XeF}_2$-assisted Ga-FIB, the structure demonstrates precise material removal and ultra-smooth sidewalls critical for achieving low-loss coupling and high-quality factor (Q) in integrated photonic devices