Fabrication and characterization of semiconductor Nanowires based on Silicon and Germanium

Low-Dimensional semiconducting material specially Silicon and Germanium are of the vastly studied systems due to their interesting properties and also their abundance which makes them economically favored. Germanium, in particular, has the advantage of enhanced electrical properties compared to silicon while compatible with conventional silicon technology. In the middle, the 1-D systems due to their unique properties can be tuned for different band gaps because of the quantum confinement which will be applied in the other two dimensions. Hence the electrons in these systems act like they are bounded in a two dimensional quantum well. This tuning happens to be a useful property for fabrication of the advanced devices. For instance, the transport mechanism in these structures can lead to a ballistic regime, i.e. the scattering happens only at the boundaries. This property opens up vast variety of the application of the 1-D semiconductors system. However, this property strongly depends on the surface properties of the synthesized structures. The possibility to control the surface roughness can lead to an enhanced electronic transport in these structures. Furthermore, doping of these structures with a magnetic element can lead to a 1-D diluted magnetic semiconductor system with an enhanced ferromagnetic transition temperature (up to 400 K) due to the confinement as it has been demonstrated for Ge nanodots doped with Mn. This will allow another way of the control over the energy band level tuning as well as ferromagnetic behavior with possibility to be utilized in spintronic applications. The enhanced electrical and optical properties of Si and Ge nanowires make them an interesting subject in a frame of study. Although there has been many studies in last decade on these system, the mechanism behind the fabrication processes and the properties of the fabricated structures are not clearly understood yet. Hence, in this work I have tried to go further in comprehension of the mist that still remains on top of the knowledge of the fabrication and properties of 1-D Si and Ge systems. In this thesis I have considered two approaches, so called ''top down'' and ''bottom up''. The ''top down'' approach includes the self assembly of semiconductor nanowires by vapor liquid solid growth and the ''endotaxial'' growth. The ''bottom up'' approach will be focused on metal assisted etching fabrication of silicon nanowires. Amongst the different mechanism available for the self assembly of nanowires (bottom up), Vapor Liquid Solid (VLS), is the well studied method to grow semiconducting nanowires. This method can be used in a molecular beam epitaxy (MBE) system with few modifications of the original idea. MBE technique, allowing low temperature growth, low deposition rate and fine control over ultra high vacuum level, it is considered one of the most reliable and sensitive deposition techniques in nanostructure fabrication. VLS growth in MBE differs from the chemical deposition methods in the sense that the liquid droplet does not act as a catalyst but as a seed, i.e. nucleation center, and the growth is strongly dependent on the diffusion of the adatom rather than direct impinging on the liquid droplet. This growth mechanism of nanowires is called diffusion induced VLS (DI-VLS). In this framework the growth of the wires are strongly affected by their diffusion. There are models concerning the DI-VLS mechanism including models concerning the diffusion of adatoms or the models based on the mass transfer in the process. However, these models explaining this mechanism do not clarify certain aspects of the growth. Here, I have carried out a systematic experimental work to have a more clear understanding of the above mentioned mechanisms which is reported in the first chapter. I have shown that the geometry of the wire can be affected by the growth condition which has not been considered previously in the models. I have also found that the DI-VLS model is size dependent and discussed the effects that controls the growth at high temperatures. I have studied ''endotaxial'' growth of the Mn-Ge nanowires which can be categorized as a bottom up method (second chapter). Due to the novelty and importance of the work I preferred to report the experimental results in a separate chapter, rather than include them as a section in the first chapter. This work is done based on the idea proposed by Tromp and Tersoff about the possibility of the nanowire growth by strain relaxation of the wetting layers deposited on a substrate. This method can have more control over the dimension of the grown structure, particularly the diameter of the nanowires which is a crucial parameter in the quantum confinement. Since these wires are in the plane of the substrate they are easier to be utilized as a device compared to the VLS grown wires which are in vertical or tilted direction with respect to the substrate. Nonetheless the engineering of the electrical contacts may reveal several problems due to their superposition in the substrate matrix. Regarding the top down approach frame, wires are fabricated rather than synthesized. For silicon nanowire fabrication, metal assisted chemical etching (MAcE) is one of the studied methods which is relatively simple and economically favored. However, the process mechanism is in its infancy and clear understanding of the roles played by the different parameters, involved in the process, is still lacking. In this thesis I have tried to clear out these roles by a deep and systematic study. I propose a model which can explain up to some extent the effect of doping and metal catalyst on the growth and structure of the fabricated wires. For both of the two methods mentioned above the level of control of the different parameters may allow device fabrication. In VLS system, I have engineered the dimension of the droplets to have a very narrow distribution of the diameter which resulted in a narrow distribution of the grown Ge wires. In MAcE, I used two stage polymer and plasma etching lithography which resulted in a sub 100 nm large uniform area of the silicon nanowires. Both single silicon and germanium wires have been contacted in order to characterize their electrical properties. Unique electrical properties of the DI-VLS synthesized germanium nanowire are reported at the end of the first chapter, while the complicated properties of semi porous silicon nanowire, as well as its optical properties are reported at the end of the third chapter. Finally, I desire to thank all the colleagues and technicians of other research groups who helped me to carry on the experimental work. The results presented in this thesis are in collaboration of groups, listed below : Transmission electron microscopy at University of Marseilles, France and IIT Turin Scanning electron microscopy in INRiM, Turin Nanolithography in INRiM, Turin Photoluminescence spectroscopy at Polytechnic of Turin.