Study of optical property variations in MBE-grown 2L, 4L, and 6L 1T′-WTe2 using ellipsometry

Abstract

This study focuses on the optical properties of MBE-grown 1T′-WTe₂ using spectroscopic ellipsometry (SE). Bi-layer (2L), 4L and bulk (6L) WTe₂ have been grown over a c-plane sapphire substrate using molecular beam epitaxy (MBE). Raman spectroscopy confirms the semi-metallic 1T′ phase of WTe₂. Atomic force microscopy showed a uniform growth over the substrate with minimal roughness. X-ray photoelectron spectroscopy is used to calculate the chemical composition of the films, confirming nearly stoichiometric composition. X-ray diffraction of WTe₂ displays peaks only corresponding to the (002) family of planes, revealing the out-of-plane epitaxial growth. Spectroscopic Ellipsometry (SE) measurement is carried out, and the obtained results are analyzed to find out the refractive index (n), extinction coefficient (k) and complex dielectric constant for all three WTe₂ samples. A comparative study between the 2L, 4L and 6L WTe₂ shows the variation of different optical constants with thickness. Subsequent analysis allowed the determination of absorption coefficient and band gap values from the extinction coefficient. The results revealed a zero-band gap for all the samples, corroborating with its semi-metallic 1T′ phase. These findings contribute to future applications in the field of optoelectronics.