Diplomarbeit Diplom-Ingenieur - Institut für Halbleiter

Abstract
Nano-structuring allows the manipulation of electronic and optical properties of
semiconductors, which is useful for various applications. So-called quantumdots (QD) reveal
optical properties comparable with atoms. They are a promising method for the production of
material systems for opto-electronic devices in frequency ranges, which are not accessible
with the standard materials.
A novel approach of QD formation was realized by decomposition of two immiscible
semiconductors with different lattice structures. The experiments were carried out at
CdTe/PbTe heterostructures. PbTe possesses the structure of rock salt (rs), CdTe has
zincblende (zb) structure. A PbTe layer, embedded in CdTe, disintegrates into highly
symmetrical nano-crystals with atomically sharp interfaces during an annealing step. The QDs
show intense photoluminescence in the mid-infrared, an energy region, where the excitation
energies of molecules are found. No efficient semiconductor lasers exist for this frequency
range. The aim of this work was a concise characterisation of these QDs with transmission
electron microscopy (TEM), for which different techniques were used.
As will be shown, the PbTe nanocrystals have a thermodynamic equilibrium shape of
small rhombi-cubo-octahedrons with {001}, {110} and {111} PbTe/CdTe interfaces. An
insight into the processes during the annealing step is given. The size of the QDs can be
controlled by the epi-layer thickness. The density of the dots depends strongly on their
average size.
The main part of this thesis is the atomic characterisation of the PbTe/CdTe interfaces and
TEM image simulations, which are necessary for the interpretation. The different lattice
structures of the two materials induce strong atomic displacement of single atoms or groups of
atoms at the interfaces. The displacements end up in bonding configurations for the zb or rs
lattice close to their respective bulk configuration. It was shown that two different {001}
interfaces exist at the same QD, depending on the termination of the CdTe crystal half. The
most remarkable effects can be found at the {110} interfaces, where the displacements lead to
a shift of more than 10% of the two crystal halves against each other. The interface problem
was also treated by R. Leitsman et al. with ab-initio calculations. The theoretical
displacements and those obtained by high resolution TEM images show excellent agreement.
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