Characterization of structrual and optical properties of Si1 -xGex nanocrystal produced by laser-induced pyrolysis and by ion implantation

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The first objective of the present project is to produce Si1 ¿ xGex nanocrystals (Si1 ¿ xGex -nc), or quantum dots covering the entire range from x = 0 to x = 1, under original conditions and by means which have not yet been exploited: (1) in the gas phase by laser-induced pyrolysis of a mixture of silane (SiH4) and germane (GeH4), and (2) in matrices by sequential ion (Si+ and Ge+) implantation and postannealing. The nanocrystals prepared in process (1) will be selected in size and deposited on substrates in the form of thin films, whereas those prepared by method (2) will be embedded in solid matrices (mainly silica). Applying both techniques and varying the process parameters, Si1 - xGex nanocrystals with different Si/Ge relative concentrations and with various surroundings, passivating oxide shell in (1) or matrix in (2), will be produced. In the second step, the structure and composition of the quantum dots will be characterized by electron microscopy as well as IR and Raman spectroscopy. We will explore how the crystal structure varies with size and composition and whether phase transitions can be observed. Finally, constituting the most important objective of the proposal, we will investigate the photoluminescence (PL) properties of the Si1 - xGex quantum dots as a function of size and composition. The studies will address the question whether the PL results from radiative recombination of charge carriers in quantum-confined systems and to what extent other mechanisms such as interface defects are operative. Quantum-confined PL appears most interesting since it allows to vary the wavelength by varying the size. As far as optoelectronic applications are concerned, it is our aim to verify the improved PL properties of Si nanocrystals upon Ge-doping as predicted by theory.