Abstract
Using in situ high-resolution transmission electron microscopy, we have explored migration mechanism of a grain boundary in a GaN bicrystal as a model system. During annealing at 500 °C, the grain-boundary region underwent a decrease in thickness, which occurred by decomposition or sublimation of GaN during annealing at 500 °C coupled with electron-beam sputtering. The decrease in thickness corresponds to an increase in the driving force for migration, because the migration of the grain boundary was driven by the surface energy difference. As the driving force increased with annealing time, the grain-boundary morphology turned from atomically smooth to rough, which is characterized by kinetic roughening. The observations indicate that a grain boundary exhibits a nonlinear relationship between driving force for migration and migration velocity, in discord with the general presumption that a grain boundary follows a linear relationship.
Highlights
A number of practical applications ranging from heat treatments of metal and ceramic polycrystals to the fabrication of nanocrystalline materials require the control of grain-boundary (GB) migration
The GB structure is strongly expected to vary with the driving force for migration as in the crystalline surfaces, unambiguous and comprehensive information regarding the atomic process of kinetic roughening in a GB has been missing
As will be discussed below, the atomistic observations well prove that GBs exhibit a nonlinear relationship between driving force for migration and migration velocity, in contrast to the general notion that the GB migration velocity is linear with the driving force for migration[22]
Summary
A number of practical applications ranging from heat treatments of metal and ceramic polycrystals to the fabrication of nanocrystalline materials require the control of grain-boundary (GB) migration. In the present study, using in situ high-resolution transmission electron microscopy (HRTEM) at 500 °C of a GaN bicrystalline GB as a model material, we have successfully captured a detailed kinetic roughening process in a GB region at the atomic scale. This was possible, because the driving force for GB migration was being increased during observation at 500 °C and we could observe a change in migration behavior with increasing. As will be discussed below, the atomistic observations well prove that GBs exhibit a nonlinear relationship between driving force for migration and migration velocity, in contrast to the general notion that the GB migration velocity is linear with the driving force for migration[22]
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