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 InSe and BiSe on c-plane sapphire substrates, the transverse-optical vibrational mode (A 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/% for the InSe A(LO + TO) mode and -1.68 cm/% for the BiSe A mode. InSe and BiSe 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 InSe, the strain accumulates with increasing film thickness, while a low strain is observed in thicker BiSe films.