Effect of quantum confinement on optical properties of Ge nanocrystals in GeO2 films

Germanium dioxide films containing Ge nanocrystals are studied. The films have been prepared by two methods: (i) deposition from supersaturated GeO vapors with subsequent decomposition of metastable germanium monoxide into a heterophase Ge:GeO2 system, and (ii) formation of anomalously thick native germanium oxides with a GeO2(H2O) chemical composition by a catalyst-accelerated oxidation of germanium. The films, which have been prepared on various substrates, are studied using the photoluminescence technique, Raman spectroscopy, spectral ellipsometry, and high-resolution electron microscopy. In the GeO2 films with built-in Ge nanoclusters, intense photoluminescence is detected at room temperature. The nanocluster sizes are estimated from the position of the Raman peak related to localized optical phonons. The correlation between a decrease in the nanocluster size and the shift of the photoluminescence peaks to the blue spectral region as the relative Ge content decreases is revealed. The presence of nanoclusters is confirmed by the data obtained from high-resolution electron microscopy. The correlation of the optical gap calculated taking into account the quantum confinement of electrons and holes in the nanoclusters with the experimentally observed luminescence peak is established. It can be concluded from the data obtained that the Ge nanoclusters constructed in the GeO2 matrix represent type I quantum dots. (c) 2005 Pleiades Publishing, Inc.