Materials World Network: Terahertz Spectroscopy of Modulation Doped Si and SiGe Nanostructures

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This Materials World Network award to University of Delaware is for experimental and theoretical studies on shallow dopants in modulation-doped silicon and silicon-germanium nanostructures. Dopants in nanostructures have unusual behavior because of complex materials interactions between the composition, interfaces, and the method of synthesis. Strain in nanostructures can push the dopant levels into the band creating a resonant state with enhanced luminescence, and the modulation doping of nanostructures may produce novel functionality as it has done for bulk materials and thin films. The properties of dopants in nanostructures have not been well understood, however, and there are fundamental questions regarding the mechanisms that affect the energy states and transport of charge carriers. As a result, it has been difficult to distinguish between fundamental and process related influences. To achieve a greater understanding, the investigators will study radiative terahertz transitions between different localized and resonant states of impurities and continuum states, as well as carrier transport phenomena. The energy spectra, density of states, capture and ionization rates of two dimensional impurity states split by strain and space quantization, and the probabilities of optical transitions will be obtained experimentally and theoretically. The growth techniques will be molecular beam epitaxy and chemical vapor deposition, guided by experiments and theory to uncover fundamental principles. The characterization techniques will be temperature dependent terahertz electroluminescence spectroscopy and current versus voltage measurements. The results of experiments will be compared with simulations and modeling interactively, to refine theories and to guide the choice of growth conditions for creating doped nanostructures with specific parameters. The approaches planned here can yield new information on the spectral features of impurity transitions and states in SiGe nanostructures, and the materials parameters that affect the radiative emission frequency and intensity. A special objective is to investigate the conditions for, and origin of population inversion in impurities states, and to clarify these mechanisms in modulation doped Si and SiGe structures. Possible technological consequences for this research would be to guide the selection of dopants as device sizes shrink, and to identify phenomena for new families of devices based on radiative transitions in dopants. The project will be carried out in collaboration with Russian scientists at Ioffe Physico-Technical Institute (St. Petersburg), and the Institute of Radioengineering and Electronics (Moscow). The assembled team has complementary expertise and facilities to conduct the proposed research. The award will provide opportunities for education and discovery in this international collaboration by students and junior and senior researchers. This collaborative research between scientists at University of Delaware and Russia could lead to a greater understanding of the synthesis and properties of doped SiGe nanostructures, and the identification of mechanisms that underlie carrier transport and radiative transitions. The education and training of students in a technologically important materials project is a pivotal aspect of the proposed international collaboration. In addition the results of this program will be incorporated into courses at the University of Delaware, and highlights of research will be featured on a web page designed to provide educational information on nanostructures to students and the general public.