Quantitative evaluation of probe response functions for Raman and fluorescence bands of single-crystalline and polycrystalline Al2O3

The probe response functions of Raman and fluorescence bands of single-crystalline sapphire and polycrystalline Al2O3 with different grain sizes were systematically evaluated and compared. As a general issue, it was found that the probe response function strongly depended on the selected signal and, more importantly, on grain size. From a comparison between the probe characteristics of Raman and fluorescence bands, the following characteristics could be found: (i) the probe depth of the Raman bands of sapphire single-crystal (the main band at 417 cm-1 was selected as a paradigm Raman sensor) was about a half that of the Cr3+ fluorescence doublet (a direct comparison was made with the R1 band located at 14 405 cm-1); (ii) the probe depth in polycrystalline Al2O3 decreased with decreasing grain size. In this latter case, the probe depth of the R1 band could be up to >10 times larger than that of the 417 cm-1 Raman band, the difference in depth increasing with decreasing grain size. These large differences in probe geometry need to be taken into consideration when applying spectroscopy methods as, for example, piezo-spectroscopic stress assessments. Nevertheless, the results of this investigation suggest that a noticeable improvement in the in-depth spatial resolution can be obtained using Raman rather than fluorescence bands as stress probes for assessing high property gradients present in the materials.