Proceedings Article | 5 March 2021
KEYWORDS: Microscopes, Optical microscopes, Birefringence, Etching, Crystals, X-rays, Gallium nitride, X-ray imaging, Crystallography
Recently, quality of free-standing gallium nitride (GaN) substrate has been improved. For example, single crystal GaN substrate with threading dislocation density (TDD) of 104 cm-2 is available. Meanwhile, there is a progress in observation method of dislocation in GaN. In addition to conventional etch pits method, X-ray topography, and CL observation, recently, multiphoton PL microscope which can observe dislocation three-dimensionally is actively being carried out. However, these methods are not simple observation because these are destructive observation or require a lot of labor and time. In this study, we attempted crystal evaluation of GaN with a newly developed birefringence microscope, which can be used with the same convenience as the conventional optical microscope. GaN is a material having birefringence, so that strain around dislocation can be seen with birefringence microscope as retardation[1]. Therefore, we tried observing commercially available GaN substrate with TDD of 106, 105, 104 cm-2. As results, an useless random contrast image was obtained with the substrate with TDD of 106 cm-2. With the substrate with TDD of 105 cm-2, distinctive contrasts that seem to coincide with the dislocation cores were obtained, but it was an unclear image. On the other hand, with the substrate with TDD of 104 cm-2, we could obtain contrasts that seems to be independent dislocation cores and surrounding strain field. It seems that the strain field of each dislocation overlapped because the distance between dislocations was too close in substrate with TDD of 106 and 105 cm-2. For the substrate with TDD of 104 cm-2, comparison with the X-ray topography image taken at BL8S2 of Aichi SR was carried out, and it was confirmed that the dislocation was exist at the position of the contrast in the image taken by birefringence microscope. However, we also found that there is no contrast at some dislocation positions. This is considered to be caused by that no strain occurs in the planar direction when the threading dislocation is pure screw dislocation. In some of the dislocations, butterfly type contrast which is theoretically expected to appear around edge dislocation was observed. We consider that this novel birefringent method is very useful method to discriminate edge component and its direction of Burgers vector, and also to discriminate screw dislocation in combination with other methods.
