Analysis of Semiconductor Surfaces via FT-IR: New Insights with Infrared Reflection Anisotropy Spectroscopy

Technician inspecting semiconductor wafer: © ryanking999 - stock.adobe.com

Researchers at Saint Petersburg Electrotechnical University (LETI) introduce a groundbreaking method, published in Applied Spectroscopy, leveraging Fourier Transform Infrared Reflection Anisotropy Spectroscopy (FT-IR RAS) to analyze semiconductor materials. By extending the spectral range into the mid-infrared, this technique offers unprecedented capabilities in studying surface phenomena and anisotropic properties. FT-IR RAS is used to analyze the directional variations in optical properties of materials (1).

FT-IR RAS is an advanced analytical technique that combines the principles of reflection anisotropy spectroscopy (RAS) with the capabilities of Fourier transform infrared (FT-IR) spectroscopy. By integrating a photoelastic modulator (PEM) into an FT-IR spectrometer setup, FT-IR RAS allows for the modulation of linear polarization direction, enabling precise measurements of surface-associated optical properties across a wide spectral range, from visible to mid-infrared wavelengths.

This innovative approach provides valuable insights into semiconductor surfaces, offering enhanced sensitivity and accuracy in characterizing anisotropic materials, surface phenomena, and thin film structures, thereby facilitating advancements in materials science, surface chemistry, and device engineering. Optical anisotropy refers to directional variations in optical properties within a material, arising from structural asymmetry or molecular alignment, with significant implications for device performance and materials science applications (2).

Semiconductor materials are a class of materials with electrical conductivity properties lying between those of conductors and insulators. These materials are characterized by the ability to conduct electricity under certain conditions, such as exposure to light, heat, or voltage, while also exhibiting insulating properties under different circumstances. Common semiconductor materials include silicon, germanium, and gallium arsenide, which play fundamental roles in electronic devices like transistors, diodes, and integrated circuits. Semiconductor materials form the foundation of modern electronics, enabling the development of advanced technologies in telecommunications, computing, and renewable energy systems.

Semiconductor surfaces hold crucial insights into material behavior, with reflection anisotropy spectroscopy (RAS) emerging as a powerful tool for non-destructive characterization. Led by Dmitrii D. Firsov, Semyon A. Khakhulin, and Oleg S. Komkov, researchers at Saint Petersburg Electrotechnical University "LETI" have pioneered advancements in this field. In their recent study, published in Applied Spectroscopy, they unveil FT-IR RAS, which extends the spectral range into the mid-infrared, revolutionizing semiconductor surface analysis (1).

RAS has long been instrumental in probing semiconductor surface phenomena, offering insights into growth processes, thin film formation, and surface reconstruction. However, existing RAS setups have been limited to visible and ultraviolet spectral regions, restricting their applicability. The team's innovation lies in harnessing the efficiency of FT-IR spectroscopy to expand RAS capabilities into the mid-infrared range, unlocking new possibilities for surface characterization (1).

By integrating a photoelastic modulator (PEM) into an FT-IR spectrometer setup, the researchers devised a method to modulate the direction of linear polarization, crucial for RAS measurements (1). They developed a sophisticated measurement algorithm to mitigate spectral inhomogeneity of the PEM efficiency, ensuring accurate and reliable data acquisition across the extended spectral range. Furthermore, they devised a specialized phase correction procedure to preserve the sign of the RAS signal, essential for interpreting surface anisotropy.

In validating their approach, the researchers achieved agreement between FT-IR RAS spectra and conventional RAS measurements in the visible range. Additionally, they demonstrated the method's efficacy in probing anisotropic interband transitions in layered black phosphorus, showcasing its potential for diverse semiconductor materials and structures (1).

The introduction of FT-IR RAS marks a significant milestone in semiconductor surface analysis, offering unparalleled capabilities in the near- and mid-infrared spectral regions. With the ability to probe surface anisotropy and optical properties across a wide range of materials, from III–V semiconductor crystals to plasmonic nanostructures, this innovative technique opens new avenues for fundamental research and technological advancements (1).

The study by Firsov, Khakhulin, and Komkov not only expands the spectral range of RAS but also enhances its sensitivity and accuracy, laying the groundwork for future investigations in semiconductor science and engineering. Through meticulous experimental design and sophisticated signal processing techniques, the researchers have established FT-IR RAS as a powerful tool for unraveling the intricacies of semiconductor surfaces, paving the way for innovations in materials science and technology.

References

• Firsov, D. D.; Khakhulin, S. A.; Komkov, O. S. Fourier Transform Infrared Reflection Anisotropy Spectroscopy of Semiconductor Crystals and Structures: Development and Application in the Mid-Infrared. Appl. Spectrosc. 2023, 77 (5), 470–481. DOI: 10.1177/00037028231153421
• Feng, Y.; Chen, R.; He, J.; Qi, L.; Zhang, Y.; Sun, T.; Zhu, X.; Liu, W.; Ma, W.; Shen, W.; Hu, C. Visible to Mid-Infrared Giant In-Plane Optical Anisotropy in Ternary Van der Waals Crystals. Nat. Commun. 2023, 14 (1), 6739. DOI: 10.1038/s41467-023-42567-x