Thermally Stimulated Evolution of Optical and Structural Properties of Germanium-Doped Alumina Films

Group IV semiconductor nanoclusters/nanocrystals attract a great interest because of substantial transformation of their optical, electrical and luminescent properties due to quantum confinement effects. During last decades significant attention was paid to the Si nanocrystals (ncs) embedded in Si oxide or nitride, and later, to the Ge-ncs embedded in these matrices. The mechanism of these 'ncs' formation was investigated in details. Recently we have reported on the Si-ncs formation in hafnia and alumina thin films. This work will present the results obtained for the Ge-rich Al2O3 films. The effect of Ge content and annealing conditions on the evolution of optical and structural properties of these materials and formation of Ge-ncs will be addressed. The films with different Ge content were grown on quarts substrates by magnetron co-sputtering of Ge and Al2O3 targets in argon plasma. After deposition, they were annealed in nitrogen atmosphere and analyzed by means of spectroscopic ellipsometry, FTIR, Raman scattering, XRD and photoluminescence methods. It is observed that suitable thermal treatment leads to the decomposition of nonstoichiometric films and results in the Ge-ncs formation. The formation of Ge-ncs via phase separantion requires lower temperatures (600-700°C) than that for Si-ncs production in alumina host (1050-1100ºC). An annealing at 800-950ºC results in the Ge out-diffusion from the films via formation of volatile GeO. This process results in the significant depletion of the near-surface regions in germanium that was confirmed by an appearance of gradient in the refractive index via film volume. Decomposition process stimulates also an appearance of visible and near-infrared photoluminescence. The shape of luminescent spectrum depends significantly on both Ge content and the excitation wavelength. It is observed that exciton recombination inside nanocrystals dominates at longer wavelength excitation, whereas the UV-blue excitation results in the radiative recombination via interface or host defects in the materials. This result was found to be caused by the competition of different radiative channels in the films. The evolution of optical and luminescent properties will be discussed in details.