Three-Dimensional Integrated X-ray Diffraction Imaging of a Native Strain in Multi-Layered WSe₂

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Two-dimensional (2-D) materials, such as transition-metal dichalcogenides, are promising silicon alternatives for advanced semiconductor and optoelectronic devices.
• Performance variability and stochasticity in these devices are major challenges, with native strain from substrate interactions being a key suspected factor.
• Direct 3-D mapping of nanoscopic strain over micron scales is crucial for understanding interfacial effects but has been elusive.

Purpose of the Study:

• To directly image and quantify 3-D native strain in multilayered WSe2 on a silicon substrate.
• To understand the origins of substrate-induced strain and its relationship with material properties and device performance.
• To investigate the impact of strain on the electronic properties and device variability of WSe2.

Main Methods:

• Coherent X-ray diffraction imaging was used for direct 3-D visualization of native strain along the (002) direction in multilayered WSe2.
• Experimentally informed continuum models and machine-learned atomistic models were employed to analyze strain origins and behavior.
• First-principles calculations were performed to determine the effect of strain on the band gap.
• WSe2 transistors were fabricated and their performance characteristics measured.

Main Results:

• Significant localized out-of-plane strains of approximately 0.2% were observed in multilayered WSe2.
• Strain originates from non-uniform substrate contact, exacerbated by nanometer-scale asperities, and is localized to contact regions.
• Strain effects become more pronounced in few- to single-layer WSe2.
• A band gap shift of up to 125 meV per percent of strain was calculated.
• Fabricated WSe2 transistors exhibited significant variability in threshold voltage and off-current, partly attributed to substrate-induced strain.

Conclusions:

• Direct 3-D strain mapping using coherent X-ray diffraction imaging is feasible and reveals significant substrate-induced strain in 2-D materials.
• Localized strain due to substrate asperities is a critical factor influencing the electronic properties and device performance variability of WSe2.
• The findings have broad implications for the design and fabrication of reliable devices based on layered materials and heterostructures.