Localized Strain Measurement in Molecular Beam Epitaxially Grown Chalcogenide Thin Films by Micro-Raman Spectroscopy

We developed an experimental metrology for measuring local strain in molecular beam epitaxially (MBE) grown crystalline chalcogenide thin films through micro-Raman spectroscopy. For In₂Se₃ and Bi₂Se₃ on c-plane sapphire substrates, the transverse-optical vibrational mode (A₁ phonon) is most sensitive to strain. We first calibrated the phonon frequency-strain relationship in each material by introducing strain in flexible substrates. The Raman shift-strain coefficient is -1.97 cm^-1/% for the In₂Se₃ A₁(LO + TO) mode and -1.68 cm^-1/% for the Bi₂Se₃ A1g2 mode. In₂Se₃ and Bi₂Se₃ samples exhibit compressive strain and tensile strain, respectively. The observations are compliant with predictions from the opposite relative thermal expansion coefficient between the sample and the substrate. We also map strain cartography near the edge of as-grown MBE samples. In In₂Se₃, the strain accumulates with increasing film thickness, while a low strain is observed in thicker Bi₂Se₃ films.