Quantitative TEM of nanostructures for MIR applications

The mid infrared frequency range is crucial for many existing and future technologies. Most excitation energies of organic and inorganic molecules and radiation from objects at room temperature can be found in this frequency range, which can thus be used for the detection of pollutants, drugs, cancer and thermal images. Up to now, no efficient optoelectronic device exist for this frequency range, because of the lack of semiconductors with suitable properties. One possibility to overcome this material problem is structuring of semiconductors on the nanometer scale, and the exploitation of quantum effects for an adjustment of the optical properties. A promising and novel material system is based on PbTe nanocrystals embedded in CdTe. But also strained Si-based structures are of interest. For full control of these nanostructures, one has to understand them down to the atomic level. The main goal of this project is the investigation of such nanostructures with different methods of the transmission electron microscopy (TEM), the technique of choice for the characterizations on the nanoscale. The planned investigations during the proposed stay at the Laboratory of Electron Microscopy (LEM) at the Karlsruhe Institute of Technology comprises challenging tasks regarding the characterization of the structural, compositional and electrostatic properties of semiconductor nanostructures. Modern TEM techniques, such as electron energy loss spectroscopy and electron holography will be used to better understand the formation, the structure and the optical properties of the investigated structures. The gained know-how of analytic TEM methods will be transferred to Linz and will help to develop the microscopy expertise at the Johannes Kepler University in Linz. A project at the LEM, the quantitative analysis of nanocrystals, is used during the initial phase as a means for becoming involved in modern TEM techniques. The developed methods will be also used for the investigation of the PbTe dots. The fabrication of symmetric quantum dots by decomposition of epitaxially grown PbTe layers is a novel method of nanostructuring of semiconductors. The dots develop during an annealing step. In-situ annealing experiments reveal remarkably fast diffusion processes in these structures during this formation. With a chemical analysis of traces of the formation process one can better understand the formation of the structures. They exhibit differently terminated interfaces on opposite sides. These can induce electrostatic fields into the quantum structures, which influences the optical properties. These potentials will be investigated by electron holography. Silicon, the most prominent material of the semiconductor industry will also be investigated. SiGe structures grown on Si substrates will be investigated concerning their strain and compositional state. The growth of SiGe structures on pre-patterned substrates avoids disadvantages of the Stranski-Krastanow growth mode and produces ordered islands with a narrow size distribution. Lattice-fringe image analysis will help to reveal the strain state of such structures. The findings will be connected to compositional analysis of the islands and finite element calculations to fully reveal the strain-composition state. This is crucial to understand and control the formation, and thus the final properties, of the SiGe material.

Many substances, organic and inorganic molecules, can be detected with infrared light. This can be exploited for present and future technologies to monitor or detect toxics, explosives, drugs or cancer markers. But no efficient and cheap electronic components for these applications exist, because of the lack of suitable semiconductors with the right optical properties. Structuring of semiconductors in the nanometer region can solve this problem. During the Schrödinger fellowship Quantitative transmission electron microscopy of nanostructures for mid-infrared application different promising material systems were investigated with transmission electron microscopy techniques to gain a better understanding of semiconductor nanostructures. Achievements were reached with the research of embedded lead telluride (PbTe) nanocrystals. These high-symmetric structures develop during an annealing step from PbTe layer which are embedded in a matrix material. With quantitative methods the chemical composition of the final structures was investigated and a new model was applied to reveal possible layer structure of the nanocrystals. Also silicon-germanium structures were investigated in the frame of this project. With high resolution lattice fringe images the strain state of Ge islands was determined. Additionally, defects of SiGe structures were analysed, which can results from such strained structures and influence the optical properties. The gained results help to understand and to better control the nanostructuring of semiconductors for optical applications.