Abstract. We report on the formation of Ge/Si quantum dots with core/shell structure that are arranged in a three-dimensional body centered tetragonal quantum dot lattice in an amorphous alumina matrix. The material is prepared by magnetron sputtering deposition of Al 2 O 3 /Ge/Si multilayer. The inversion of Ge and Si in the deposition sequence results in the formation of thin Si/Ge layers instead of the dots. Both materials show an atomically sharp interface between the Ge and Si parts of the dots and layers. They have an amorphous internal structure that can be crystallized by an annealing treatment. The light absorption properties of these complex materials are significantly different compared to films that form quantum dot lattices of the pure Ge, Si or a solid solution of GeSi. They show a strong narrow absorption peak that characterizes a type II confinement in accordance with theoretical predictions. The prepared materials are promising for application in quantum dot solar cells.
The effects of annealing in reactive gases (H 2 , N 2 , O 2 ) upon the optoelectric properties of nanophased titanium dioxide (TiO 2 ) prepared by chemical vapour deposition (CVD) were investigated. The nanocrystalline structure containing nanosize grains and pores was analyzed by grazing-incidence small-angle scattering of synchrotron radiation (GISAX). The annealing (up to 1073 K) in H 2 and N 2 generally proved detrimental to photoconductivity due to the overall increase of the electrical conductivity of the samples. In this work, the result of UV (248-404 nm) photoconductivity measurements on TiO 2 films annealed in O 2 at 773 K and 1073 K are presented. A rather long illumination time (typically 2 h) enabled us to clearly distinguish two types of nonequilibrium photoconductivity variations with time. A fast exponential photoconductivity increase occurred during the initial stage of irradiation, while a slow power-type increase was observed in the later stage. A nonlinear combination of both functions was used in a numerical fitting procedure, which allowed precise determination of the asymptotic value of exponential photoconductivity increase. A relative quantum efficiency for both as-prepared and annealed samples exhibits a nonmonotonic variation with photon energy. Such wavelength-dependence variation might be due to the electronic density function structure at the valence-band edge, or near-valence-band levels in the gap. Generally, the samples annealed at higher temperatures exhibit a higher quantum efficiency and shorter time constants of the excitation processes, in the examined UV range.
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