Manipulation And Optimization Of Magnetotransport In Inhomogeneous Semiconductors

Magnetoresistance (MR) has attracted considerable interest since its discovery. Thiseffect is defined as change in resistance at applied magnetic fields, which is particularlylarge in certain natural or artificial materials. In the initial stage the extensively studied MRmaterials were divided into two types: one is the artificial granular composites andmultilayered films based on magnetic materials, and the other doped perovskite manganites.In all these materials the resistance decreases with applied magnetic fields, which is theso-called negative magneoresistance. Recently, in inhomogeneous nonmagneticsemiconductors, dramatic positive magnetoresistance were reported when properly dopedor made in special hybrid structures. Such a special MR effect, called “extraordinarymagnetoresistance”, is different from what we have known and cannot be explained withany traditional magnetotransport mechanisms. Since this MR effect is based onnonmagnetic materials, with the behaviors related with the underlying inhomogeneousstructures, it is possible to achieve great MR enhancement by tuning geometric factors,which is particularly advantageous in applications.Nonmagnetic semiconductors which exhibit extraordinary MR can be divided intotwo types according to their geometric characteristics: one is the type with regularinhomogeneity and the other with disordered structures. The former is represented by themetal-semiconductor hybrid structures. Many efforts have been devoted to theinvestigations on the influence of a variety of geometrical factors on this type of MR,including the composition, shape, electrode positions etc. While for disordered materialssuch as doped semiconductors, the way and degree of the effects of the geometrical factorson the magnetotransport behaviors is still ambiguous due to the difficulty in the analysis of these disordered structures, and no sufficient studies were reported to investigate whetherthe magnetoresistance can be enhanced geometrically in these systems.In this thesis we propose an effective medium model according to the structurecharacteristics of nonmagnetic semiconductors with disordered inhomogeneity. Byintroducing shape factors of granules, analysis of the effects of component configurationsand asymmetry of the phases are performed. The difficulties in the calculation ofanisotropic-shape effects are addressed by taking advantage of the duality of thetwo-dimensional binary disordered systems,. This new model offers insight into therelation between the magnetotransport behaviors and the underlying inhomogeneity inthese systems. Particularly the effects of the formation of percolating paths revealed in thisstudy are beyond the results from the preceding models. The main results from oursystematic investigations are listed as follows.First, by establishing the new effective medium model, we have revealed theparticular relation beween the formation of percolating paths and magnetotransportbehaviors in nonmagnetic semiconductors with disordered inhomogeneity. In this modelthe percolation transition of the two phases can be separated when the orientation of thegranules is fixed. The calculated results show that the occurrence of the MR peaks isaccompanied by the percolation transition of the semiconducting phase instead of themetallic phase, which provides new viewpoints of the mechanism of magnetoresistance inthese materials.Secondly, geometrical factors which contribute to the MR enhancement in thesematerials are discussed. Accordingly, we propose a scheme to improve the MR effect in thelightly-doped range. Since the asymmetric phases in our model can possess differentgranular shapes, we can calculate the effects of the geometrical factors of the phasesrespectively. It is shown that for randomly-oriented systems, the elongated semiconductorgranules lead to obvious increase of the MR peaks while the elongation of metallic phasewill have negative results. When the orientation of the granules is fixed, the MR peaksarise when the particles of either phase are elongated in the direction of the external electric field, although more sensitive to the change of the shape of semiconductingcomponents. We also consider how to improve the MR effect of the nonmagneticsemiconductors when only a small portion of metal is introduced. It is found that when thegranules of the two phases are elongated simultaneously, the enhancement in this range isoptimized. And this effect can be further promoted by enlarging the mismatch of the Ohmreceptivity of the two components.