Infrared spectroscopic ellipsometry study for narrow-band semiconductors

Narrow-band semiconductors (0.1–2 eV band gap) are core materials for mid- and long-wave infrared optoelectronics. Infrared spectroscopic ellipsometry (IRSE) serves as a powerful non-destructive metrology tool to characterize their optical, electronic and structural properties. We summarize the principles, instrumentation, data analysis and typical applications of IRSE for narrow-band semiconductors. IRSE enables accurate extraction of band gap, carrier concentration, mobility, film thickness, interface roughness and strain, supporting band-gap engineering, carrier dynamics and heterostructure characterization. Recent advances in broadband light sources, Mueller-matrix detection, extreme-condition setups and machine learning greatly improve measurement capability and analytical efficiency. IRSE has been widely applied in mercury cadmium telluride (Hg_1-xCd_xTe), two-dimensional materials, quantum structures and phase-change semiconductors. Challenges including far-infrared noise, model generalization, and industrial translation are discussed, and future directions are prospected. This article provides a practical reference for IRSE-based characterization and device optimization of narrow-band semiconductors.