ATOMIC SCALE ANALYSIS OF SEMICONDUCTOR NANOSTRUCTURES WITH
CROSS-SECTIONAL SCANNING TUNNELING MICROSCOPY
P.M.Koenraad
COBRA Inter-university Research Institute, Department of Physics,
Eindhoven University of Physics, P.O. Box 513, 5600 MB Eindhoven, the Netherlands
In the last decade we have seen the successful development of several growth
techniques to produce low-dimensional III/V semiconductor nanostructures. Growth
techniques like Molecular Beam Epitaxy (MBE) and Metal Organic Chemical Vapour
Deposition (MOCVD) are extensively used to produce the 2D, 1D and 0D semiconductor
nanostructures. Composition profiling of these semiconductor nanostructures is essential for
the study of the growth processes forming them and the understanding of their optical and
electronic behaviour.
With cross-sectional scanning tunnelling microscopy (X-STM) a sample is cleaved
under ultra high vacuum conditions and the cleaved surface reveals a cross-sectional image of
the grown layer structure. As the STM is only sensitive to the top layers of the cleaved surface
we can nicely resolve alloy fluctuations, interfaces, doping atoms, segregation effects, etc.
Using cross-sectional STM we have studied the local composition in III/V quantum
wells and quantum dots. We are able to determine the local composition by either simply
counting the constituent atoms, measuring the local lattice constant or measuring the
relaxation of the cleaved surface due to the elastic field of the buried strained structures. We
will show that with our analysis technique composition details can be assessed that are
unattainable by any other technique and that the application of our analysis leads to a new and
better understanding of the formation and the optical and electronic properties of III/V
nanostructures. As an example in Fig. 1 an example of a stack of InAs quantum dots in GaAs
is shown.
In this talk I will also present our latest results on the imaging of magnetic Mnimpurities in GaAs which are of special interest in the field of spintronics.
Fig. 1: Cross-sectional STM image of a stack of
5 InAs quantum dots in a GaAs matrix. The
growth direction runs from the lower bottom
right corner to the upper left corner. The InAs
quantum dots appear as lens shaped features.
Individual In atoms can be observed in-between
the wetting layers and the quantum dots. Each
quantum dot layer was formed by growing 2.4
monolayers of InAs. The intended distance
between the quantum dot layers was 10 nm.
Image size 55 by 55 nm. The image is high pass
filtered to reveal the fine structure more clearly.