Silicon Nanowires by Metal Assisted Etching

Group-IV semiconductor nanowires (NWs) are attracting interest among the scientific community as building blocks for future nanoscaled devices. Different techniques are currently used for Si NWs preparation, the bottom-up vapor-liquid-solid (VLS) mechanism or the top-down approach which uses the electron beam lithography (EBL). Moreover, literature shows that in the last few years Metal-assisted chemical etching (MACEtch) has become a powerful technique to obtain high density and low-cost Si NWs with high and controllable aspect ratio. It consists of an etching of a Si substrate in a solution containing dihydrogen peroxide, hydrogen fluoride and a metallic salt. Instead of using metallic salts as catalysts (which leave metallic dendrites over the NWs after the etching process), ultrathin films of gold or silver have been evaporated at room temperature on a Si surface, and then etching has been performed. By using for the first time ultra thin films of gold or silver as catalysts for the etch, their main size becomes less than 10 nm, allowing quantum confinement effects. A Si core - SiO2 shell structure is obtained and it is possible to tune the core of the NWs scaling them down to 5 nm. Both energy filtered TEM analyses and Raman analyses strictly confirm these data. Also, a more complex system has been realized, indeed by etching a multi quantum well made by stacks of 1 nm thick Ge and 54 nm thick Si it is possible to fabricate Si/Ge MQW NWs. In this way a structure made of Si NWs which confines carriers in two dimensions (leaving them free on the third one), and a structure of Ge dots (which confines carriers in three dimensions) can be obtained. In literature, a Si NWs system which is natively suitable for photonics is still lacking. All the attempts made by oxidizing VLS grown or EBL made silicon nanowires reported in literature gave poor results. Obtaining light from Si NWs at room temperature under optical and electrical pumping is still a big challenge and would have a tremendous impact on silicon photonics. This thesis demonstrates that both MACetch Si NWs and Si/Ge MQW NWs are suitable for photonic applications. It will be shown a detailed and complete study of the excitation and de-excitation properties as a function of the temperature and of the pump power, determining the excitation cross section, and both presence and origin of possible non-radiative phenomena. A light emitting device based on Si NWs has been realized, showing the efficient electroluminescence emission at room temperature in the red (700 nm) under low voltage pumping. Finally, we realized Si/Ge NWs by the same synthesis approach, in order to obtain different confined structures of both Si and Ge inside each NW. Photoluminescence emission properties of Si/Ge NWs have been studied at room temperature. The last part of the thesis deals with the optical trapping of the single MACetch Si NW. Optical trapping (OT) of nanostructures has acquired tremendous momentum in the past few years. Manipulating nanoparticles with OT is generally difficult because radiation forces scale approximately with particle volume and thermal fluctuations can easily overwhelm trapping forces at the nanoscale. Hence, the role of size-scaling is crucial for understanding the interplay between optical forces and hydrodynamic interactions that change dramatically with size, therefore much affecting both force-sensing and spatial resolution in precision applications. A detailed study on how optical trapping and Brownian motion of very thin Si NWs depending on their size has been performed. The NWs length is the key parameter that regulates forces, torques, and hydrodynamics. The core of the last chapter fully characterizes the three-dimensional translational and angular Brownian motion, deals with the measure of the root-mean-square displacements and shows the different size-scaling due to the interplay between radiation forces defining the trapping potential and hydrodynamics.