| Ion beam synthesis is utilized to fabricate isotopically pure 70Ge and 74Ge nanocrystals in silica and sapphire matrices and a selective etching technique for obtaining exposed, stable nanocrystals is developed. The structural and optical properties of these nanocrystals are characterized using a variety of experimental techniques and quantitative theoretical models of the observed behaviors are presented. In the as-grown state, embedded nanocrystals exhibit considerable compressive stresses that are determined using Raman spectroscopy. Post-growth thermal annealing of silica-embedded nanocrystals allows for controllable stress relaxation that may be quantitatively explained by a theoretical model for diffusive stress relaxation. Stresses on sapphire-embedded nanocrystals, which arise due to the volume solidification expansion of Ge upon rapid cooling from the growth temperature, are evaluated by comparison of a phonon confinement model to the observed spectra. Raman spectroscopy is also used to probe the surface reconstructions of exposed nanocrystals that produce amorphous-like contributions to the total scattering intensity arising from increased bond length and angle distributions. In situ Raman spectroscopy of exposed nanocrystals is used to characterize the kinetics of photo-oxidation under visible laser irradiation and a qualitative mechanism for this process is presented. The melting behaviors of nanocrystals are explored via in situ transmission electron microscopy. Thermal hysteresis of ∼470 K between the melting and solidification temperatures, located approximately symmetrically around the bulk Ge melting temperature, is observed. This unusual behavior, which is believed to be impossible in bulk systems, is explained using a quantitative kinetic model that predicts both supercooling and superheating. The effect of oxygen and germanium (and silicon) co-implantation is examined and is utilized to achieve patterned growth of nanocrystals. The theoretically predicted luminescence from the quantum confined optical gap of Ge nanocrystals is not observed due to a high interface state density. However, future experiments are suggested to determine the effect of passivation treatments on Ge nanocrystal interface states. The wide size distribution that is characteristic of ion beam synthesized nanocrystals is found to be a consequence of athermal clustering during the implantation process and experimental techniques to narrow the size distribution are proposed. |
