Probing the thermally driven response of Raman-active phonon modes in sapphire single crystals by in situ Raman spectroscopy

Given the practical application of sapphire as a promising transparent crystal in extreme environments, understanding the fundamental physical response of its optical properties to temperature is urgently needed. Herein, the vibrational characteristics of Raman-active modes of the strain-free sapphire and the strained sapphire substrate under the AlN epilayer were systematically investigated by Raman scattering measurements in the temperature range of 95-870 K. The observed nonlinear temperature-dependent frequency redshift for all phonon modes over the entire temperature range can be fitted with some empirical formulas. Interestingly, the frequency of these phonon modes varies linearly with temperature at high temperatures and can be explained by first-order temperature coefficients. The linear temperature coefficients of A1g,1 modes for strain-free sapphire and strained sapphire determined at high temperatures are -0.02224 cm- 1/K and -0.02189 cm-1/K, respectively. The strong temperature dependence in sapphire is expected to portray the local temperature distribution of sapphire-based devices by in situ Raman spectroscopy as a non-contact thermal probe. Furthermore, the frequency downshift and linewidth broadening observed in strain-free sapphire can be well quantified by a theoretical model involving thermal expansion and anharmonic decay. The epilayer-substrate interaction, however, needs to be additionally considered in strained sapphire. This theoretical analysis process can also be radically extended to other emerging ceramic and semiconductor materials. Our quantitative analysis may pave the way for a deeper understanding of the behavior of optical phonons in sapphire under extra perturbation effects.

Automated summary

This study systematically investigated the vibrational characteristics of sapphire under temperature variation using Raman scattering measurements. The results revealed the linear temperature dependence of phonon mode frequencies at high temperatures and provided corresponding temperature coefficients. Additionally, the study quantified the frequency downshift and linewidth broadening in strain-free sapphire and applied a theoretical model to analyze these phenomena. The findings can contribute to a deeper understanding of the behavior of optical phonons in sapphire and have potential applications in other materials.