Characterization of WSe$_2$ films using reflection Kikuchi diffraction in the scanning electron microscope and multivariate statistical analyses

TL;DR

This paper demonstrates that reflection Kikuchi diffraction combined with multivariate statistical analysis in SEM effectively characterizes the thickness and crystallographic orientation of WSe$_2$ thin films, validated by AFM measurements.

Contribution

It introduces a novel application of SEM-based reflection Kikuchi diffraction with multivariate analysis for detailed 2D material characterization.

Findings

RKD with MSA effectively differentiates thickness and orientation.

Thickness dependence linked to inelastic electron scattering.

Validated thickness measurements with atomic force microscopy.

Abstract

The study of thin films and 2D materials, including transition metal dichalcogenides such as WSe$_2$ offers opportunities to leverage their properties in advanced sensors, quantum technologies, and device to optimize functional performance. In this work, we characterize thin WSe$_2$ samples with variable thicknesses using scanning electron microscope (SEM)-based techniques focused on analysis of backscattered electron signal and Kikuchi diffraction patterns. These data were collected via a pixelated electron-counting direct electron detector positioned below the pole piece primarily configured for reflection Kikuchi diffraction (RKD), and a similar detector placed in the more conventional electron backscatter diffraction geometry. In addition to conventional pattern analysis for orientation microscopy, multivariate statistical methods (MSA) based on principal component analysis were applied to analyze diffraction patterns and differentiate thickness variations and crystal orientations within the thin films through data clustering. These results were compared with atomic force microscopy to validate thickness measurements. Our findings indicate that RKD combined with MSA is highly effective for characterizing 2D materials, enabling simultaneous assessment of thickness and crystallographic orientation. Systematic acceleration voltage variations in RKD experiments and comparisons with EBSD data suggest that the thickness dependency arises from inelastic scattering of diffracted electrons, which affects pattern contrast in the thin film regime. Collection and analysis of patterns obtained from monolayer, bilayer and tri-layer of WSe$_2$ are also demonstrated. This work reinforces the utility of SEM-based techniques, such as RKD, as valuable tools for the materials characterization toolkit, particularly for thin films and 2D materials.