The goal is to study the SiGe heterostructure system with special emphasis on atomically thin layers deposited on an Si substrate by MBE. The project aims to exploit novel effects and material properties such as optical transitions in the near-infrared regime, strain effects, non-linear optical properties and change of carrier mobility. Simple demonstrator devices showing the performance and characteristics of Si-based optoelectronic devices are planned.
Silicon germanium microstructures grown on silicon (100) substrates offer the possibility of integrating optical devices with silicon (100) based conventional integrated circuits (IC). These microstructures have a variety of multifunctional device capabilities making use of novel bandstructure effects and transport phenomena in strained silicon-germanium heterostructures.
Major achievements so far are:
substantial improvement of material quality by applying the technique of surfactant growth in terms of interface sharpness, reduction of defect density and planarity of grown layers;
breakthrough in buffer quality in terms of reduction of threading dislocation (TD) density and improvement of material quality by growth of a germanium graded s
ilicon l-y (z) germanium y (z) buffer layer followed by a constant composition, fully relaxed silicon l-y germanium y buffer layer;
strong photoluminescence (PL) signals observed in the near infrared regime;
shift of PL wavelength as a function of SLS composition in agreement with expected behaviour of quasi direct transition;
first room temperature electroluminescence from a strain symmetrized silicon 6 germanium 4 SLS diode;
theoretical understanding of interface stability and ordering effects (antimony as surfactant);
theoretical investigation of novel silicon germanium quantum well (QW) structures with large optical oscillator strength in the near infrared and simpler growth structure;
study of nonlinear and electric field induced properties of silicon germanium QWs and silicon m germanium n SLS;
observation of intersubband transitions in the far infrared regime from pseudomporphically grown strain adjusted silicon/silicon l-x germanium x QW;
evaluation of strain and composition of a relaxed silicon l-y germanium y buffer layer by reciprocal space mapping in the analysis of a strain adjusted silicon m germanium n SLS.
APPROACH AND METHODS
The work is divided into the following major tasks:
- Growth of the SiGe microstructures by molecular beam epitaxy (MBE) methods on <001> Si substrates. The previous ESPRIT Action 3174 led to the growth of superlattices with a period length smaller than the lattice constant of silicon (2 monolayers period L=0.28nm a0=0.5431nm bulk lattice constant of Si).
- Structural characterisation of the SiGe microstructures by transmission electron microscopy (TEM), X-ray diffraction and Raman spectroscopy.
- Study of the electronic and optical properties of the Si/Ge microstructures by photoluminescence (PL), photoconductivity, absorption and Hall measurements. Also modulation techniques such as photoreflectance and electroreflectance as well as junction space charge techniques such as photocapacitance and short circuit current measurements will be applied.
- Fabrication and characterisation of mesa and waveguide diodes as test vehicles for novel SiGe devices. The electrical characteristic of Si/Gep-n junctions (I-U and C-U curves) will state the progress in the material growth, device technology and functions. Optical responsivities or electronic transport times will be measured and compared with existing Si devices.
- Achievement of the qualitative and quantitative understanding of the electronic bandstructure of the SimGen superlattices, Si/Ge and Si/Si1-xGex quantum well structures and Si1-xGex alloy layers based on ab initio and empirical pseudopotential calculations. Also an understanding of the microscopic mechanism of the SiGe growth and interface formation especially under MBE growth conditions. The influence of various parameters such as substrate temperature, growth rate, dopant incorporation and surface segregation on the grown layer quality will be studied theoretically and compared with experiment. Special emphasis will be given to ordering phenomena during growth.
- Search for novel effects and their possible applications. It is planned to study inter sub-band and miniband absorption to extend the possible wavelength range to the mid- infrared region. In addition we plan to examine nonlinear optical effects.
In general, the Si/Ge microstructure system can serve as model system to study strain effects, mismatch accomodation and growth techniques for other strained heterostructure systems. The results obtained here for growth parameters, characterisation techniques and numerical tools for calculating the bandstructure of SiGe microstructures are of broad usage and can be applied in other heterostructure systems such as for example in the III-V material area (egInGaAs/GaAs) as well.
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221 00 Lund
NE1 7RU Newcastle Upon Tyne