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 o n three different semiconductor heterostructures containing 3D islands ('qu antum dots'). We show that the combination of the depth sensitivity of thes e methods and the use of synchrotron radiation allows for a full structural characterisation of the quantum dots. All samples systems were grown by mo lecular beam epitaxy. (1) Ge (15 nm) deposited on boron terminated Si(111) surfaces is shown to form relaxed triangular Ge pyramids with no orientatio nal dispersion. We demonstrate how the 3-fold symmetry is obtained. (2) In the case of coherent InAs islands grown on GaAs(100), grazing incidence dif fraction between the (220) surface reflections of InAs and GaAs reveals tha t the quantum dots are pseudmorphically strained at the interface to the su bstrate while they become fully relaxed at the top of the islands. In a nov el approach ('iso-strain scattering') we are able to determine the interdep endence 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 superlattic e we find correlations both laterally and in the growth direction. On the s urface the dots are organised in a short range order square lattice. By inc reasing the scattering depth to investigate the superlattice, we find the b uried Ge dots to be strongly correlated in the growth direction. Surface se nsitive 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 quan tum confinement effects. (C) 1998 Elsevier Science S.A. All rights reserved .