Spectroscopic Ellipsometry Investigation of Temperature-Dependent Dielectric Functions and Critical Points of β-InSe

Two-dimensional layered materials, such as β-InSe, have attracted significant interest due to their unique optical, electronic, and mechanical properties. The sample in this study is a bulk β-phase InSe single crystal, which is grown using the temperature gradient method. The quality of this crystal is confirmed by X-ray diffraction, high-resolution transmission electron microscopy, and Raman spectroscopy. This investigation focuses on the temperature-dependent dielectric functions and critical points (CPs) of β-InSe, particularly at its cleavage plane. The spectral range is 0.74–6.42 eV, and the temperature range is 27–300 K. Measurements are done with a dual-rotating-compensator spectroscopic ellipsometer. At low temperatures, CPs are resolved that cannot be observed at room temperature, a consequence of sharper features due to reduced broadening from electron–phonon interactions. The CPs are identified through first-principles density functional theory calculations. These findings provide a deeper understanding of the optical properties of β-InSe, further supporting its application in optoelectronic devices.