The Use of Synchrotron Radiation to study Overgrowth Phenomena in InAs/GaAs Nanostructures

This work focuses on the investigation of overgrowth phenomena in
InAs/GaAs nanostructures using synchrotron radiation.
Surface-sensitive grazing incidence small angle x-ray scattering
(GISAXS) and grazing incidence diffraction (GID) are applied to
study shape, strain, and interdiffusion in self-organised grown
nanostructures. The technique of anomalous x-ray diffraction at the
weak (200) superstructure reflection enhances the chemical
sensitivity of the measurements. For the investigation of
(partially) buried nanostructures finite-element simulations (FEM)
have been performed. The following sample systems were
investigated: ((1)) Free-standing and buried InGaAs quantum dots:
Free-standing In(x)Ga(1-x)As islands grown on GaAs (001) by
molecular beam epitaxy (MBE) with a nominal concentration of x=0.5
have been investigated. Contrast variation close to the K edge of
As by anomalous GID at the (200) superstructure reflection is used
for a direct determination of the InAs concentration as a function
of the lateral strain in the quantum dots (QDs). The evaluation of
intensity mappings recorded in reciprocal space close to the (200)
reflection together with atomic force micrographs (AFM) allows to
attribute the strain and the InAs concentration to a certain height
in the quantum dots. Thereby, a three-dimensional model of the
strain and interdiffusion profile of the InGaAs QDs can be
reconstructed. A discussion of measurements taken on buried InGaAs
QDs and free-standing islands grown on the strain modulated surface
of a buried QD layer shows the limits of this technique. ((2))
InGaAs quantum rings: The formation of nanoscopic InGaAs ring
structures on a GaAs (001) substrate takes place when InAs quantum
dots, grown by Stranski-Krastanov self-organisation, are covered by
a thin layer of GaAs. The shape transformation into rings is
governed by strain, diffusion and surface tension, quantities which
are of importance to understand magneto-optical and electronic
applications of the rings. GISAXS and GID is applied to
characterise morphology and structural properties such as strain
and chemical composition of the rings in three dimensions. From
GISAXS the shape is found to be of circular symmetry with an outer
radius of 26nm, a height of 1.5nm, and a hole in the middle, in
good agreement with AFM measurements. The most surprising results
are obtained from intensity mappings in reciprocal space close to
the (220) and (2-20) reflection done in surface sensitive GID
geometry. From a comparison of the intensity maps with FEM model
calculations the InGaAs interdiffusion profile in the ring is
determined. It strongly depends on the crystallographic
orientation. In the ring a maximum InAs concentration of more than
80% along [1-10] is found while along [110] it is below 20%. This
is explained by the preferred diffusion of In along [1-10]. ((3))
Quantum wires formed by cleaved edge overgrowth: Quantum wires
(QWRs) fabricated by the cleaved edge overgrowth (CEO) technique
use tensile strain to confine the charge carriers to one dimension.
The cleaved edge of a pseudomorphically strained
In0.1Al0.9As/Al0.33Ga0.67As superlattice (SL) is overgrown by a
GaAs layer of 10nm thickness. The lateral charge carrier
localisation in the overgrown layer is induced by the periodic
strain modulation of the SL. Using GID this strain state of the
system is determined. The strain modulation due to the overgrown
superlattice occurs only within 3micron of the total wafer
thickness of 150micron. The GID technique allows for a clear
separation of the strain modulation in the cap layer and the
superlattice underneath. It can be proved that the strain
modulation in the GaAs cap layer is not of compositional origin but
purely elastic with an average lattice parameter change of
(0.8+-0.1)% with respect to relaxed GaAs. The strain profile
obtained is confirmed by FEM model calculations.