Characterization of Amorphous and Microcrystalline Silicon using Raman Spectroscopy

Importance of the Topic

Photovoltaic devices built from thin film silicon require precise control of material phases to achieve high energy conversion efficiency and long term stability. The relative amounts and spatial distribution of amorphous and microcrystalline silicon directly impact cell performance. Raman spectroscopy provides a non invasive, rapid and quantitative tool for phase analysis, making it essential for quality control in solar cell manufacturing.

Objectives and Study Overview

This application note outlines how Raman spectroscopy coupled with chemical imaging can be employed to:

• Quantify the proportion of amorphous versus crystalline silicon using Beer Law based peak height ratios.
• Generate two dimensional maps of thin film samples to monitor spatial uniformity of phases.
• Identify practical considerations and pitfalls when applying Raman methods across different instruments.

Methodology and Instrumentation

Raman spectra were acquired on a DXR Raman microscope equipped with:

• 532 nm excitation laser with a built in laser power regulator.
• Full range grating and high sensitivity detector.
• Motorized stage for point by point mapping.
• OMNIC 8 software for spectral acquisition.
• OMNIC Atlas mapping software to generate chemical images from peak height ratios.

Measurements focused on the sharp crystalline silicon band at 521 cm⁻¹ and the broad amorphous silicon band centered at 480 cm⁻¹. Laser power was carefully controlled to avoid phase conversion.

Main Results and Discussion

Raman spectra from pure crystalline and predominantly amorphous samples displayed clearly resolved peaks at 521 cm⁻¹ and 480 cm⁻¹ respectively. Beer Law calculations based on the ratio of these peaks enabled accurate phase quantification. Mapping experiments revealed:

• Line scans across 30 µm showed localized microcrystalline regions embedded in an amorphous matrix.
• Two dimensional maps over 750 × 2250 µm areas highlighted gradients and heterogeneities in phase distribution.

Laser power studies demonstrated that excitation levels above 4 mW can induce conversion of amorphous to crystalline silicon. Below this threshold no measurable phase change occurred. The built in laser power regulator ensured reproducible conditions across multiple measurements.

Benefits and Practical Applications

Raman mapping of thin film silicon provides:

• Quantitative phase analysis that supports process optimization in photovoltaic manufacturing.
• Spatially resolved chemical images to detect defects or non uniform deposition.
• Non destructive testing compatible with in line quality control.

Future Trends and Applications

Advances in Raman instrumentation and data processing are expected to:

• Enable faster large area mapping with higher spatial resolution.
• Integrate machine learning algorithms for automated phase classification.
• Extend the technique to other emerging thin film materials and multilayer architectures.

Conclusion

Raman spectroscopy with controlled laser power and mapping capabilities offers a robust approach for characterizing amorphous and crystalline silicon in thin film solar cells. The technique delivers quantitative phase ratios and high resolution spatial information, supporting quality assurance and process development in photovoltaic manufacturing.