Measuring the Thickness of Flakes of Hexagonal Boron Nitride Using the Change in Zero-Contrast Wavelength of Optical Contrast

Using the reflectivity mode of an optical microscope, we analyzed the optical contrast to identify the layer number of flakes of hexagonal boron nitride on a $SiO_2$ /Si substrate. Overall optical contrast in the visible range varies with the thickness of flakes. However, the wavelength of zero contrast exhibits a linear redshift of 0.53 nm per layer, independent of the $SiO_2$ thickness, and increases proportionally with $SiO_2$ thickness. Experiments show good agreement with calculations and the results of AFM measurements. These results show that this zero-contrast approach is more accurate and easier than the reflectivity-contrast approach using the overall optical contrast.