A challenge for the development of nano-electronics is to elaborate semiconductor quantum dots (QDs) that are homogeneous in shape, size, strain and composition, thus resulting in well-defined electronic and optical properties [1]. This is obtained by e.g. the growth of Ge islands on lithographically prepatterned Si(001) substrates in the form of ordered pits [2]. Such growth has been intensively studied by ex situ techniques like AFM, STM and ex situ X-Rays. As these techniques do not allow to determine the chemical composition and the variation of relaxation prior to and during island formation, in situ X-Ray scattering during growth is very well suited to understand and investigate the whole dynamic growth process.
The combination of Grazing Incidence Small Angle X-ray Scattering (GISAXS) [3], X-ray Diffraction (GIXD) and Anomalous Scattering using the MAD (Multiwavelength Anomalous Diffraction) [4] method, all performed in situ, during growth in an Ultra High Vacuum chamber, allowed for the characterization of the Ge QDs regarding their shape and organization (GISAXS) as well as their strain and composition (GIXD).
Then, in order to compare the growth on nanostructured substrates with that on nominal ones, ex-situ studies were performed. The prepatterned and nominal Si substrates were grown simultaneously under the same growth conditions in a MBE chamber. Using GIXD and anomalous scattering allowed us to determine the composition and strain profiles of the Ge islands. For instance, it is found that the in-plane lattice parameter is significantly reduced for islands grown on the nominal surface. By measuring basis-forbidden x-ray reflections [5], the existence of atomically ordered regions inside the Ge islands for both substrates is found. Scattered intensity was measured in the vicinity of the (200) reflection, revealing at least partially ordered SiGe alloy.
[1] Y. Arakawa and H. Sakaki, Appl. Phys. Lett. 40, 939 (1982).
[2] Z. Zhong and G. Bauer, Appl. Phys. Lett. 84, 1922 (2004).
[3] G. Renaud et al, Science 300, 1416 (2003).
[4] A. Letoublon, V. Fabre-Nicolin, H. Renevier, et al, Phys. Rev. Lett. 92, 186101 (2004).
[5] A. Malachias et al, Phys. Rev. B 72, 165315 (2005).
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