Size controlled germanium nanocrystals in dielectrics : structural and optical analysis and stress evolution

Gülseren, Oğuz
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Bilkent University
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Group IV semiconductor nanocrystals, namely silicon and germanium have attracted much interest in the past two decades due to their broad applications in photovoltaic, memory, optoelectronic, medical imaging and photodetection devices. Generally, there are two major features of semiconducor nanocrystals: First, spatial confinement of charge carriers which leads to the significant changes in optical and electronic properties of materials as a function of size. This effect gives the possibility to use the size and shape of the nanocrystals to tune the energy of electronic energy states. Second feature of nanocrystals, is the increased of surface area to volume ratio of the nanocrystal with reducing size. This leads to an enhanced role of the effects related to surface and interface of the nanocrystal. Furthermore, stress on the nanocrystals can lead modification of the band structure as well as in uencing the crystallization of the nanomaterials. Recent works show that measurement and control of the stress can open the way for strain engineering of the electronic band structure, thereby opening the way for new physics and applications. In this thesis, we first carry out a study on the synthesis of germanium embedded in silicon nitride and oxide matrices. In uence of the annealing method as well as germanium concentration on the formation of nanocrystals is discussed. It was found that Ge concentration and annealing play important roles in the formation of the Ge nanocrystals. With crystallographic data obtained from high resolution transmission electron microscopy, quantitative analysis of stress state of germanium nanocrystals have been done by analyzing Raman peak shift of embedded nanocrystals taking into account the phonon confinement effect. Finally, using stressors as buffer layers, superlattices of Ge nanosheets were studied to understand the effects of the stressors on the stress state of Ge nanocrystals. We demonstrate that it is possible to tune the stress on the Ge nanocrystals from compressive to tensile. Finally we showed a three dimensional Ge quantum solid that can be used in optoelectronic applications.

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Germanium Nanocrystals, Dielectric Matrices, Quantum Confinement Effect, Phonon Confinement Model, Phonon Raman Spectroscopy, High Resolution Transmission Electron Microscopy, Strain Engineering, Photoluminescence Spectroscopy, Superlattices
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