Growth mechanism and optical properties of Ge nanocrystals embedded in a GeO x matrix

Quantum confined germanium (Ge) nanocrystals were synthesized by a thermal annealing of germanium oxide thin films fabricated by an e-beam evaporation method. The nanocrystal formation, structure, and sizes upon annealing are evaluated by a combination of X-ray diffraction, transmission electron microscopy, and Raman spectroscopy studies. Spherical nanocrystals with sizes of around 3 nm and having both diamond and tetragonal phases are formed for an annealing temperature of 500 A degrees C. The thermally induced transformation and phase separation of amorphous germanium oxide to nanocrystalline Ge is discussed using X-ray photoelectron spectroscopy. The bandgap agrees with the calculated values using effective mass approximation. The visible photoluminescence is investigated as a function of the annealing temperature. This disproportionation mechanism of germanium oxide films showing favorable optical properties suggests its scope for forming quantum confined Ge nanocrystals embedded in its amorphous matrix relevant for both electronic and optoelectronic applications.