Imaging of Quantum Dots in 2D and 3D space

U.V. Desnica¹, M. Buljan¹, P. Dubcek¹, I.D. Desnica Frankovic¹, N. Radic¹, M. Ivanda¹, Z. Siketic¹, I. Bogdanovic-Radovic¹, G. Dražić², K. Salamon³, S. Bernstorff⁴

¹R. Boskovic Institute, Physics Department, Bijenicka 54, HR-10000 Zagreb, Croatia

²Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

³Institute of Physics, Bijenicka 56, HR-10000 Zagreb, Croatia

⁴Sincrotrone Trieste, SS 14 km 163.5, 34012 Basovizza (TS) Italy

Grazing incidence small angle x-ray scattering (GISAXS) and Transmission Electron Microscope (TEM) were applied to image Ge Quantum Dots (QDs) sinthesized in SiO2 amorphous matrix. GISAXS provides information (in inverse space, from the large area of the samples with excellent statistics) about size and shape of QDs as well as about their spatial correlations.

Ge QDs were formed in multilayered films deposited by magnetron sputtering in form of 20 bi-layers on either SiO2 or Si substrate. Each bi-layer consisted of a layer of co-sputtered mixture of 40%mol Ge and 60%mol SiO2 (‘active layer’), and a layer of pure SiO2, serving as the spacer between ‘active’ layers. Deposition temperature, Td, ranged from room temperature to 700 ^oC. As-deposited samples were subsequently thermally annealed up to Ta = 1000 ^oC. The 2D GISAXS patterns, besides detecting bi-layered structure, revealed the formation of Ge QDs in deposited films and their possible ordering. The shape and the size of Ge QDs were found to be strongly dependent on both Td and Ta. The average size of QDs can be controlled by the thickness of the Ge-SiO2 co-sputtered layer and Ta, while the ordering of QDs depended strongly on Td and on the thickness of the spacer layer. The synthesis of good quality spherical crystalline Ge QDs was achieved by Td up to 600 ^oC followed by post-deposition annealing in 700-800 ^oC range. In contrast, the deposition at Td=700 ^oC results in loss of most of Ge atomsl and formation of non-spherical, faceted Ge QDs. For Ta=500 ^oC the 2D ordering of Ge QDs occurs already during deposition, with no loss of Ge. For a very narrow range of deposition conditions (Ta=500 ^oC and spacer layer 7 nm thick) 3D ordering of Ge QDs into hexagonal superlattice was achieved.