Shedding light on epitaxial SiGeSn alloys with Raman spectroscopy: local order and thermomechanical properties

Thanks to recent breakthroughs in their epitaxy compatible with industrial standard, (Si)GeSn alloys are now emerging as material platform for monolithic integration of next-generation electronics, optoelectronics, thermoelectrics, and photonics. In order to support the rapid evolution of the (Si)GeSn material system, different advanced characterization methods have to be developed to investigate its many interesting physical properties. This work provides an overview of Raman spectroscopy investigation of binary GeSn and ternary SiGeSn alloys. After reviewing the fundamental principles of Raman spectroscopy, the typical features of (Si)GeSn alloy Raman spectra are introduced. These features are then attributed to vibrational modes of atom pairs, high-order scattering, or disorder-induced scattering. The discussion also covers the details of polarization-resolved Raman spectroscopy applied specifically to the group-IV alloys case, with a focus on probing the local atomic ordering. Different applications of the Raman spectroscopy to the (Si)GeSn material system are covered. Initially, micro-Raman measurements are complemented by ancillary techniques, such as x-ray diffraction. This allows the spatially resolved composition and strain of the investigated epitaxial alloys to be determined based on calibrated coefficients of vibrational mode shifts. Raman hyperspectral imaging, along with its integration with numerical simulations for strain mapping in microstructures and devices, is also presented. Subsequently, temperature-dependent Raman spectra are employed to investigate the crystal anharmonicity and the lattice expansion, with a detailed analysis of the multiple-phonon scattering coefficients, that are then compared with data present in the literature. The temperature dependence of Raman modes is also used to develop Raman thermometry to measure lattice thermal conductivity, in view of the recently proposed use of these materials in integrated opto-thermo-electronic devices. In summary, Raman spectroscopy is demonstrated as an essential tool for metrology and physical insight of the technologically relevant and complex SiGeSn ternary alloy system.