Characterization of magnetically doped amorphous semiconductor and nanowires

In this dissertation, the characterization of magnetically doped amorphous semiconductors and self-assembly nanowires is described. The structure and morphology of Gd-Si, Gd-Ge, Gd-Al-Si, Gd-C, Nb-Si, Nb-C, and Co-Si magnetically doped amorphous semiconductors have been evaluated. A structural and magnetic characterization of thin FeSi2 nanowires is also described. Structural characterization was performed using transmission electron microscopy and chemical analysis by energy-dispersive x-ray spectroscopy (EDS) and other electron microscopy techniques. The samples studied were prepared both by e-beam evaporation and magnetron sputtering. Incorporation of Gd into amorphous matrix Si exhibits enormous MR at low temperature. Gd-Si films start to develop clustering at Gd concentrations of about 25 at.%. This clustering at high dopant concentrations leads to a peak in the DC conductivity as a function of Gd composition. Substitution of Ge for Si dramatically reduces MR and the characteristic temperature T*. Similar structural behavior of Gd-Si was observed for Gd-Ge systems and thus, it was not able to resolve this phenomenon. The decrease in MR in Gd-Ge is probably due to decreased band gap relative to Si. This leads to study of a-Gd-C alloys as a function of sp2/sp3 ratio, which controls the a -C band gap. Addition of non-magnetic elements which increase electron concentration causes a reduction in all effects (T*, magnetic interaction strength, and magnitude of MR). Similarly, possible acceptor Al dopants into Gd-Si were studied to control electron and magnetic moment separately. Addition of Al did not have any effect on lowering the electron concentration as Al was clustered in the film Substitution of Mn for Gd showed no large MR although a-Mn-Si was magnetic and undergoes a M-I transition at similar composition as Gd-Si and Gd-Ge. The Mn-Si films were also characterized to determine whether this behavior was related to the structure changes. Finally, Co-Si films were characterized as a function of growth temperature, and microstructure was correlated with magnetic and electrical properties. A complete transmission electron microscopy (TEM) characterization of thin FeSi2 nanowires is also described. It was found that the cubic gamma-FeSi2 phase was formed at growth temperature of 700°C. This phase transformed to beta-FeSi 2, which is semiconducting, after annealing for 30 minutes.