FTIR Analysis of SiC Crystals

At Anderson Materials Evaluation, Inc., we specialize in addressing unusual materials problems with advanced analytical techniques. We were asked by a Moissanite merchant to find a way to distinguish between the two crystal structures or polytypes that artificial silicon carbide (SiC) crystals are made in, 4H and 6H.  To achieve this, we utilized FTIR spectroscopy, a technique typically used almost exclusively for analyzing organic materials, for our analysis.

SiC single crystals, also known as Moissanite, are a cost-effective substitute for diamonds. They are harder and more chemically inert than cubic zirconia, and possess a higher degree of brilliance (even more than diamonds). The crystals are artificially produced with bilayers of atoms arranged in three different configurations: A, B, and C. The two polytypes of SiC, 4H and 6H, differ in the sequence in which these bilayers are stacked.

Our report on how we learned to distinguish and identify these crystal structures utilizing FTIR Spectroscopy is presented here with the permission of our customer: FTIR to Distinguish the 4H and 6H Polytypes of SiC Crystals. This report outlines how we were able to differentiate the 4H and 6H structures using FTIR analysis and address issues of disorder in these structures as well.

By utilizing FTIR spectroscopy for this unique application, we were able to offer a solution that exceeded our customer’s expectations. Contact us today to learn more about our advanced materials analysis services.

This graph shows the analysis of a sample of Moissanite. Three FTIR transmission spectra of the Princess-cut SiC crystal are shown. The spectra are very similar to those of the known 4H polytype SiC crystal. The There are minor differences, such as the doublet nature of the major absorption minimum near 1148 cm-1, with the slightly deeper minimum of absorption at 1154 cm-1 and another local minimum of 1112 cm-1, in two of the three spectra . The third spectrum in green has a minimum at 1154 cm-1, very close to the minimum of 1148 cm-1 observed in the 4H polytype. The absorption minimum at 978 cm-1 found in the 4H polytype is also found here to be similar, but slightly altered with a small, sharp absorption peak at 974 cm-1 in the midst of the broader minimum. The green spectrum looks very much like the 4H polytype spectra, while there are still other differences in the blue and black spectra, such as the suppression of the local absorption maximum at 1035 cm-1 relative to the one at 1017 cm-1. There is relatively more absorption at about 850 cm-1 in the unknown crystal than in the 4H crystal. All of the spectra of the unknown crystal have absorption maxima at about 570 cm-1 similar to the 4H crystal and a minimum of absorption of about 506 cm-1.
Three FTIR transmission spectra of the Princess-cut SiC crystal are shown. The spectra are very similar to those of the known 4H polytype SiC crystal (Moissanite).