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

Abstract

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 A1g 2 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.

Correlation analysis of vibration modes in physical vapour deposited Bi2Se3 thin films probed by the Raman mapping technique

“…As mentioned by the authors of ref. 24 and 25 these values are already much larger than those reported for bulk materials, 33 which implies strong inverse dependence of PDP on thickness. 25 Therefore, our results for the 3 nm thick lm correspond to this tendency.…”

“…The Raman frequency vibrations in topological insulators such as Bi 2 Se 3 are governed by various thicknessdependent factors, including spin orbit coupling, 31 charge carrier concentration, 32 and strain. 24 The linear scattering of A 2…”

“…In ref. 24 the A 2 1g , E 2 g , and A 1 1g modes are red-shied with increase of applied strain and the Raman shi strain coefficient of the A 2 1g mode is À1.68 cm À1 /%, for the 79 nm thick lm. A similar value for a 100 nm thick nanoribbon value has been reported in ref.…”

“…Moreover, we evaluated the Raman shi strain coefficient of A 2 1g for the 2 nm Bi 2 Se 3 thick lm based on the estimated deformation given in ref. 24, which varies with the lm thickness, $À4.83 À1 /%. Considering the strong dependence of PDP (Raman shi strain coefficient) on the thickness together with 2 times difference in the out of plane strain for the same applied in-plane stress, 3 biaxial zz ¼ 2 Â 3 uniaxial zz , (please see the ESI of ref.…”

“…It strongly depends on the thickness of the material, and sophisticated experiments have been performed to extract this dependency for bismuth chalcogenides. 24,25 In these studies strain was introduced via either articial 24 or natural bending 25 in the transferred lm and nanoribbons, respectively; the Raman shi coefficients were evaluated for thickness in the range of 79-260 nm, limiting their applicability to the case of epitaxial ultrathin lms. The theoretically estimated value of PDP for a 2 nm thin chalcogenide lm is expected to be $5 cm À1 /%.…”