How to distinguish between opposite faces of an a-plane sapphire wafer

A method to distinguish between two symmetrically equivalent opposite and faces of an a-plane sapphire wafer is described. It is shown that use of conventional X-ray diffraction analysis makes it possible to determine the `sign' of the sapphire a face in contrast to the `sign' of the c, m or r faces. Correct determination of the a-plane wafer orientation is important for further growth and processing of heteroepitaxial structures.