The Germanium "halo": Visualizing Ge interstitial dynamics in nanocrystallite formation

We report the ability to image distributions of Ge interstitials within amorphous SiO₂ matrices using conventional transmission electron microscopy and energy dispersive X-ray spectroscopy mapping. While previous, established methods have only been able to indirectly infer the presence of interstitials within materials, our direct "visualization" approach allows the ability to study and better understand the dynamics of several interesting phenomena occurring within the Ge-Si-O system during processing at temperatures greater than 700 °C which has not been described in any previously published work. Experimental observations by TEM of the Ge interstitial dynamics has allowed us to clarify the various operating mechanisms for Ge nanocrystallite growth within these SiO₂ matrices. These phenomena include Ostwald ripening, migration of Ge nanocrystallites through amorphous SiO₂ and Si₃N₄ matrices, Ge incorporation in Si-rich materials such as Si₃N₄ and Si, and finally, morphological changes in the Ge nanocrystallites themselves.