Structural properties of Ge nanocrystals embedded in sapphire

Isotopically pure Ge74 nanocrystals were formed in a sapphire matrix by the ion beam synthesis method. In contrast to those embedded in amorphous silica, sapphire-embedded nanocrystals are clearly faceted and are preferentially oriented with respect to the crystalline matrix. In situ transmission electron microscopy of heated samples reveals that the nanocrystals melt at 955±15°C, very near to the bulk Ge melting point. The Raman spectra indicate that the sapphire-embedded Ge nanocrystals are under compressive stress in the range of 3–4GPa. The magnitude of the stress is consistent with that expected for hydrostatic pressure arising from solidification. Stress relaxation was not observed for sapphire-embedded Ge nanocrystals; this is attributed to the slow self-diffusion rate of the alumina matrix atoms at temperatures below the nanocrystal melting point.