A challenge for the development of nano-electronics is to elaborate semiconductor quantum dots that are homogeneous in shape, size, strain and composition, thus resulting in well-defined electronic and optical properties. Recently the growth of highly monodisperse Ge islands on prepatterned Si substrates has been obtained by a combination of lithography and self-assembly techniques [1,2]. As the structures required for functional devices are getting more complex, non-destructive in situ studies are gaining ground as characterization tools in the development and optimization of the growth parameters [3,4]. The aim of our work is to compare the growth of Ge QDs on prestructured and nominal Si surfaces and to optimize the preparation of nanopatterned Si surfaces as templates for island or wire growth. The combination of Grazing Incidence Small Angle X-ray Scattering (GISAXS), X-ray Diffraction (GIXD) and Anomalous Scattering (AD) performed in situ, during growth in an Ultra High Vaccuum chamber, allows for the characterization of of the Si surfaces and the Ge QDs regarding their faceting, shape and organization (GISAXS) as well as their strain and composition (GIXD).
The measurements were performed using the MBE growth chamber of the BM32 beamline at the ESRF. The deposition was followed in situ by X-rays, monolayer by monolayer. GISAXS measurements provide the detailed evolution of the morphology such as the facetted pits on the Si surface, as well as the shape of the grown QDs. GIXD adds information on the atomic structure of the surface and on the onset of island nucleation after the beginning of lattice relaxation. Moreover, AD was performed in order to determine the Ge content inside the islands.
It is found that a much higher relaxation inside Ge-islands is obtained on the patterned surfaces as compared to nominal Si(001). Furthermore, our methods prove that the Ge content in both cases is similar and depends on the temperature and deposition rate rather then on the surface structure.
References:
[1] Z. Zhong et al., Appl. Phys. Lett. 84, 1922 (2004).
[2]G. Chen et al., Phys. Rev. B 74, 035302 (2006).
[3] T.U. Schülli et al., Appl. Phys. Lett. 89, 143114 (2006).
[4] M.-I. Richard et al., submitted to Phys Rev. B (2006). |