Shape, size, strain and correlations in quantum dot systems studied by grazing incidence X-ray scattering methods

We present results obtained by grazing incidence X-ray scattering methods on three different semiconductor heterostructures containing 3D islands ('quantum dots'). We show that the combination of the depth sensitivity of these methods and the use of synchrotron radiation allows for a full structural characterisation of the quantum dots. All samples systems were grown by molecular beam epitaxy. (1) Ge (15 nm) deposited on boron terminated Si(111) surfaces is shown to form relaxed triangular Ge pyramids with no orientational dispersion. We demonstrate how the 3-fold symmetry is obtained. (2) In the case of coherent InAs islands grown on GaAs(100), grazing incidence diffraction between the (220) surface reflections of InAs and GaAs reveals that the quantum dots are pseudmorphically strained at the interface to the substrate while they become fully relaxed at the top of the islands. In a novel approach ('iso-strain scattering') we are able to determine the interdependence of radius and strain in the dots which turns out to be linear. (3) In the third example of coherent Ge islands embedded in a Si/Ge superlattice we find correlations both laterally and in the growth direction. On the surface the dots are organised in a short range order square lattice. By increasing the scattering depth to investigate the superlattice, we find the buried Ge dots to be strongly correlated in the growth direction. Surface sensitive X-ray techniques using grazing incidence and exit angles have been turned into a versatile tool to study structural properties of quantum dots which are essential for understanding their self-organised growth and quantum confinement effects, q 1998 Elsevier Science S.A. All rights reserved.