Probing Crystal Dislocations in a Micrometer-Thick GaN Film by Modern High-Voltage Electron Microscopy.

We report on extreme penetration power of relativistic electrons in a micrometer-thick gallium nitride epitaxial film and its application to probing threading dislocations, which were introduced during crystal growth. Maximum usable thickness of the specimen was quantitatively evaluated using high-voltage transmission electron microscopy (TEM) operating at 1 MV. The width of dislocation images was used as a measure for the evaluation of usable thickness. Superior maximum usable thickness was obtained in scanning transmission electron microscopy (STEM) than in TEM mode; the results were 6.9 μm for STEM and 4.4 μm for TEM. In STEM, dislocations can be imaged with an almost constant width of 15-20 nm in a wide thickness range 1-4 μm. The latest high-voltage STEM is thus useful for observing dislocations in micrometer-thick inorganic materials.