Microstructures of as-grown and annealed β-Ga2O3 single crystals were studied by electron microscopy. Energy filtered electron diffraction patterns exhibited weak diffuse scattering at 1/2 and 1/4 positions between the fundamental reflections along the q204̄ vector. Dark-field images observed with the 1/2 diffuse scattering showed a characteristic domain-like structure, where the size of each domain was less than 5nm. The diffused scattering was somewhat weakened by annealing the specimen. The observations indicate a non-homogeneous structural modulation, which is considered to be related to the distribution of oxygen vacancies.
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