Magnetoresistance effects in hybrid semiconductor devices

Abstract

Until recently, mainstream electronics was exclusively based on charge proper- ties. Apart from charge, an electron also possesses an intrinsic angular momen- tum (spin), and directly coupled to that a magnetic moment. A newly emerg- ing approach is to use this extra spin degree of freedom to provide additional performance and functionality in widely used commercial applications, although nowadays mostly in metallic structures. Semiconductors are of great interest for future spin-based electronics because of the predicted longer spin coherence. The ability to electrically inject, manipulate, and detect spin polarized carriers within the active semiconductor are essential components from the integration point of view for such a technology. Transport of spin polarized carriers is commonly demonstrated by a speci¯c resistance change as function of the external applied magnetic ¯eld (magnetoresistance). The realization of all electrical spin injection and detection in an inorganic (GaAs) and in an organic (Alq3) semiconductor will be the focus of this thesis. Besides, observed large magnetoresistance e®ects in silicon from di®erent origin will be extensively discussed. Before we address the experimental aspects of the hybrid semiconductor de- vices, a detailed theoretical analysis has been performed to the feasibility of all electrical spin injection and detection in semiconductors by means of ferro- magnetic electrodes and including spin-selective interface barriers to overcome the impedance mismatch. Based on the Poisson and the di®usion equation, in- cluding electric ¯eld e®ects, the expected resistance di®erence for parallel and anti-parallel magnetization alignment of the ferromagnetic electrodes has been analytically calculated. The in°uence of the sample and measurement geom- etry has been extensively investigated and the results will be used within our experimental design analysis. Moreover, we propose a new lateral measurement geometry for which the magnetoresistance closely approaches the magnetoresis- tance value obtained for a vertical device. Electric ¯elds created in di®erent regions via separate bias voltages minimizes the spin loss in the semiconductor side branches of the lateral device. Our ¯rst experimental device consists of electron-beam-lithography processed ferromagnetic electrodes crossing a GaAs transport channel. Although silicon is the industrially most relevant semiconductor, GaAs has been chosen for its di- rect band gap, which enables optical investigations of the spin polarization in our group. Control over the magnetization alignment of the electrodes is established via an external applied magnetic ¯eld. Di®erent switching ¯elds for di®erent electrode widths have been measured by magnetic force microscopy. Towards observation of magnetoresistance due to spin transport, we show that the dop- ing pro¯le under the ferromagnetic contacts and in the semiconductor transport channel is of critical importance for the e®ective detection and depolarization of the carriers. Organic semiconductors pro¯t from their °exibility and the ease of process- ing. While these synthetic organic materials are exploited for their tunability of charge-carrier transport properties, their spin transport properties form a less explored area. Spin transport through a hybrid Al2O3(1.5 nm)/Alq3 barrier has been studied as a function of the thickness of the organic (0-6 nm) layer at room temperature. From the current-voltage behavior we conclude that for hybrid bar- riers with more than 2 nm Alq3, transport by multiple step tunneling dominates over transport by direct tunneling. Because of weak spin-orbit interaction at the intermediate molecular site and a possible low hopping rate, spin should be sensitive to small local ¯elds, in particular the total hyper¯ne ¯eld originating from the local hydrogen nuclei. We suggest that in the regime of multiple step tunneling, hyper¯ne coupling could be a relevant source of spin randomization and could modify the characteristic hysteretic magnetoresistance by a symmetric contribution. A possible ¯ngerprint is indeed observed in our experiment. Silicon holds exceptional promise for spin-based electronics, by virtue of its compatibility with the current CMOS technology. As a possible implicit contri- bution to future silicon-based spintronics devices or as a magnetoresistive sensor, boron-doped Si-SiO2-Al structures are fabricated to study extremely large mag- netoresistance e®ects. Current-voltage characteristics show a strong non-linear behavior, dominated by an autocatalytic process of impact ionization. At low temperatures, the magnetic ¯eld postpones the onset of impact ionization to higher electric ¯elds. This results in a symmetric positive magnetoresistance of over 10,000% at magnetic ¯elds of 400 kA/m. A macroscopic transport model is introduced that is able to describe how the magnetoresistance is controlled by voltage, electrode spacing and oxide thickness. Utilizing that the e®ect of a magnetic ¯eld on an acceptor wave function becomes stronger with increas- ing distance from the impurity, we present a room temperature constant voltage magnetoresistance of over 1000% at 1 MA/m in low doped silicon.